THE BIOLOGY AND POPULATION ECOLOGY OF flACROSTELES SEXNOTATUS (FALLEN) (CICADELLIDAE, HEnIPTERA)
by
Miriam Becker, M.Sc. (Brazil)
December, 1974
A thesis submitted for the degree of Doctor of Philosophy of the University of London and for the Diploma of Imperial College
Imperial College of Science and Technology, Silwood Park, Ascot, Berkshire. 2
ABSTRACT
Studies in the laboratory and under field conditions
were made on the biology and population ecology of Macrosteles
sexnotatus (Fall6n) (Cicadellidae, Hemiptera).
Laboratory studies on the biology were carried out
under a set of constant temperature conditions. The rela-
tionship between temperature and rates of egg and nymphal
development are presented and discussed. Effects of tempera-
ture on fecundity and longevity were also studied, and choice
of oviposition sites under laboratory and field conditions
were investigated. Studies were carried out to induce
hatching of diapausing eggs and also to induce diapause in
the eggs.
The internal reproductive organs of males and
females are described and illustrated. Illustrated descrip-
tions are also given of the five nymphal stages and sexes
are distinguished from third instar onwards. Descriptions
and illustrations are given of a short winged form which
occurred in the laboratory cultures.
Population studies_of M. sexnotatus in an catfield
• were carried out from 1972 to 1974. Adults and nymphs were
sampled regularly with a D-vac suction sampler and occa-
sionally with a sweep net. Weekly population estimates
were made from June to late September for 1973 and 1974 and
for August and September of 1972. Population budgets are
presented and causes of mortality are discussed. Losses
caused by parasitism in the nymphal and adult stages are 3 shown to be smaller than those within the egg stage. Predation studies on nymphal and adult stages are also included.
Dispersal studies on immigration to and emigration from the oatfield were carried out. Dispersing insects were sampled by means of water traps, sticky traps and specially by aerial suction traps. The mode of coloni- zation of the oatfield was analysed and the following period of trivial flight by males was determined. The state of sexual maturity of the females was used as an index of migratory activity. The emigration from the oatfield by the young second generation adults throughout August and September was followed. The relation of flight to weather conditions was analysed. 4
CONTENTS
Page INTRODUCTION 9 SECTION I. Biology of Macrosteles sexnotatus 11 1. Life history 11 2. Host plants 12 3. The habitats of M. sexnotatus and of some of its closely related species 14 4. Methods of rearing 19 A. Stock cultures 19 B. Eggs 20 C. Nymphal stages 22 D. Adults 22 E. Parasites 23 (i) Egg parasites 23 (ii) Nymphal and adult parasites 23 F. Predators 24 •5. The nymphal stages 24 A. Description of the nymphal stages 24 (i)Introduction 24 (ii)Descriptions 26 B. The rate of growth 33 • 6. The adult stage ,35
A. Taxonomic literature 35 B. A short winged form 38 7. The internal reproductive organs and sexual maturation 48 A. The male organs 48 B. The female organs 50 C. Mating 54 t 5
Page 8. Laboratory studies on the rates of egg development 56 A. Introduction 56 B. The direct effect of temperature on developmental rates 61 , (i) Material and methods 61 (ii) Results 62 (iii) Discussion 64 C. The effect of acclimatization to temperature on developmental rates 66 D. The 'developmental zero' 68 E. The effect of humidity 69 (i) Introduction 69 (ii) Material and methods 72 (iii) Results 74 (iv) Discussion 74 9. Laboratory studies on diapause 75 A. Introduction 75 B. Induction of diapause in the eggs 79 C. The hatching of diapausing eggs subjected to a period at low temperatures 81 (i) Introduction 81 (ii) Material and methods 82 (iii) Results and discussion 83 10. Laboratory studies on the rate of development of the nymphal stages 84 A. Rearing under constant temperatures 84 (i)Introduction 84 (ii)Material and methods 84
(iii)Results 85 (iv) Discussion 94 4 B. The 'developmental zero'. 95 6
Page 11. Laboratory studies on fecundity 102 A. Fecundity under constant temperatures 102 (i)Introduction 102 (ii)Materials and methods 104, (iii)Results 106 (iv)Discussion 115 B. Fecundity and weight changes in the females 118 C. Ageing effects 122 12. Oviposition sites 126 A. On oat seedlings under laboratory conditions 126 B. On field oats in an outdoor insectary 131 (i)Material and methods 131 (ii)Results 133 (iii)Discussion 135 13. Longevity of the adults 136
SECTION II. Population studies 1. The study area 143 A. Natural grassland 143 B. An oatfield 144 2. Meteorological records 151 • 3. Methods of sampling the population 155 A. Introduction 155 B. Suction sampling 156 C. Sweep net sampling 161 D. Efficiency of suction sampling 164 E. Effectiveness of sweeping: meter equivalent sweeps 174 4. Weekly population estimates 177 • 7
Page
5. The total number of eggs laid 188 A. Estimation of the daily fecundity 188 (i)Materials and methods 188 (ii)Results 189 (iii)Discussion 192 B. Estimation of the numbers of ovipositing females and natality 197
6. Estimation of incubation rates and nymphal duration in the field 197
7. Integration of population estimates 198
8. The population budgets 213
9. The analysis of budget data 219
10. Parasitism in the egg stage 220
A. In natural grasses 220
B. In the oatfield 223
(i)Material and methods 223
(ii)Results 225
(iii)Discussion 229 11. Parasitism in the nymphal and adult stages 231
A. Introduction 231
B. Parasitism by Pipunculidae 233
C. Parasitism by Dryihidae 236
(i)In field populations 236
(ii)Infection tests 238 12. Predation in the nymphal and adult stages 243 A. Predator feeding trials 243 B. Detection of predation by the precipitin test 246 (i)Introduction 246 (ii)Preparation of antigen 247
• (iii) Preparation of the antiserum 248 8
Page
(iv) Titre of antiserum 248
(v) Specificity of antiserum 249
(vi) Tests on predators from the field 251
SECTION III. Studies on dispersal 256
1. Introduction 256
2. Methods of sampling dispersing insects 258
A. Aerial suction traps 258
B. Sticky traps 260
C. Water traps 263
D. Regular netting and suction sampling 264
3. The invasion of the oatfield 264
A. Macrosteles spp. as invaders 264
B. The mode of colonization 268
(i) Introduction 268
(ii) Local immigrants 269
(iii) Immigrants from other sources 275
4. The trivial flights of males 281
5. The state of maturation of female ovaries as an indication of migratory activity 284
6. The emigration from the crop by second generation adults 291 • 7. Flight and weather conditions 302
GENERAL DISCUSSION AND SUMMARY 305
ACKNOWLEDGEMENTS 315
REFERENCES 317
S 9
INTRODUCTION
The genus Macrosteles Fieber occurs in the Palearctic and Nearctic regions and about fifty valid species have been described within it. Some of the species- occur in natural grasses only while others invade culti- vated crops.
All the species of the genus consist of macropterous forms. Cicadellidae in general are slow fliers, easily blown off course by weak winds and are particularly prone to high altitude transport (Johnson, 1969). In North America, M. fascifrons (Stal)engages in long-distance displacements from the breeding grounds.
Both in terms of numbers of individuals and in numbers of species, leafhoppers form a most important component of the phytophagous fauna of temperate grass- lands (Waloff & Solomon, 1973). Leafhoppers can be important pests of agricultural crops either by direct feeding and oviposition on green tissues or by their role as vectors of plant diseases caused by mycoplasma-like organisms. M. fascifrons (Stal) is, the principal vector of the aster-yellows disease to plants on a number of • economically important crops in the United States and Canada. In Europe, some of the species of Macrosteles have been detected as vectors of diseases and to cause economic losses to cultivated crops. In England, as other leaf- hoppers, no economic importance is attached to Macrosteles species.
• 10
M. sexnotatus is a highly mobile species. In any Population system there are three pathways through which numbers may change: natality, mortality and migration. Most arable crops are initially 'ecological vacuums' (Southwood, 1971) where the whole area is open to coloni- zation and once the insect has invaded it, settled and commenced reproduction population growth in the young crop may be exponential. In the present study, this system was followed in an annual crop which was invaded each year. Laboratory studies supplied and complemented infor- mation on the biology of the species and its relations with natural enemies.
• 11
SECTION I
BIOLOGY OF MACROSTELES SEXNOTATUS (FALLEN)
1. Life History
Maerosteles sexnotatus is a bi-voltine species in Britain and overwinters in the egg stage.
It is a widespread species in the Palearctic region and is rather common in grasslands throughout the British Isles. It has polyphagous habits and some of its related species are economically important as vectors of mycoplasmas of the 'aster-yellows' group to cultivated crops.
M. sexnotatus is a mobile species though not as much as some other species within the genus. In southern England the overwintered eggs, laid in grasses, begin to hatch in late April. The first generation nymphal £nstars are spent in grassland.
During the spring and summer of 1972 to 1974 M. sexnotatus occurred in very low densities in the acidic
• grasses of Silwood Park. An experimental plot planted with oats was available for the study of invasion of crops by this species. Invasion commences at the beginning of June and may extend over a few weeks. The immigrants are young adults and soon after settling down shot•: a period of intense flitting, probably trivial flights of males in search of females. Mating was not
• 12
observed in the field as the density of adults was low in the oat field. in captivity, at 20°C constant temperature, the first mating takes place at approxi- mately 72 hours after the final moult of the female.
The oviposition period is long, and lasts six to eight weeks in the first generation in field condi- tions. The pre-oviposition period is short in relation to oviposition and there is no obligatory pest-oviposition period. The eggs are inserted into the plant tissues. In the field, the proportion laid in the leaf blade and in the leaf sheath depends on the developmental condition of the maturing oat plants.
There are five nymphal stages. As the first generation nymphs are subjected to rather low tempera- tures their development takes longer than that in the summer generation. First generation nymphs are notice- - ably darker than the second generation ones. A total overlap of nymphal stages occurs within three or four weeks after the appearance of the first instar.
Second generation adults are present in the oat- • field by mid-August and leave the wilting crop soon after their emergence. They move to natural grasses and ovi- posit diapausing eggs that hatch in the next spring.
2. Host Plants
M. sexnotatus is a polyphagous species. There are no data on definite host-plant preferences. However • 13
its presence on natural grasses and on agricultural crops has been mentioned in several papers.
in Silwood Park M. sexnotatus is found in acidic
grasslands in areas with a predominance of Agrostis
tenuis and white clover (Waloff & Solomon, 1973) and in grasses dominated by Agrostis tenuis together with Holcus lanatus, Festuca arundinacea, F. rubra, Anthoxantum
odoratum, Luzula campestris, Ranunculus repens and
Trifolium spp (personal observations). It was the
dominant leafhopper in an oatfield during this three-
year study and was also found on a Lolium perenne area
adjoining the oatfield.
M. sexnotatus was reported in small numbers in
chalk grasslands by Whittaker (1969) and Morris (1973)
and a large population of M. sexnotatus was found on
white clover seed crop undersown with oats (Prior, 1965).
In Italy, it is referred to as one- of the three
most common cicadellids on rice, while in natural habitats
it is said to feed on herbaceous plants, chiefly on
Gramineae(Olmi, 1968). • Ribaut (1952) says that in France it is found.-
in humid places dominated by Gramineaeand Holcus.
According to Prior (1967), in the British Isles it
prefers_ lowland humid grasslands; Le Quesne (1969) re-
lates its presence to grasses and clover fields. 14
Waloff « Solomon (1973) report that M. sexno- tatus invaded in small numbers plots seeded with Agrostis tenDis, Festuca rubra, Dactylis glornerata and Poa annua but became more abundant when the vegetation within the plots became denser and the paths and surrounding areas became overgrown with weeds.
Prior (1965) reared two generations of M. sexnotatus on Festuca rubra, while Waloff and Solomon
(1973) bred several generations on oats at 20°C, but their females failed to mature and oviposit on Agrostis tenuis.
3. The Habitats of Macrosteles sexnotatus and of Some of Its Closely Related Species
Several entomologists have been concerned with
the heterogeneity in the distribution of the meadow leaf-
hoppers. According to Kontkanen (1950) a factor of
importance in the distribution of the polyphagous leaf-
hoppers of meadows and swamps are their specific moisture requirements. He refers to fauna of dry, fresh, wet and
intermediate habitats according to the degree of moisture within them.
Waloff and Solomon (1973) say that the species of
grasses are not of primary importance in the distribution
of leafhoppers and that the structure of the microhabitats
should be looked into in more detail. That is, the struc-
ture of the grassland habitat and microclimatic conditions
within the grass tufts may be as important as the plant • 15
composition in determining the distribution of the polyphagous leafhoppers.
Within the habitat, heterogeneous spatial distri- bution of leafhoppers has been noted by several authors. Romney (1945), Fewkes (1961) and Jiirisoo (1964) report a diel periodicity in vertical distribution. Heikinheimo and Raatikainen (1962) describe a differential vertical distribution of the nymphal and adult stages of Javesella pellucida on Tymothy grass. Andrzejewska (1965) stresses the importance of the layer distribution of insects in grass associations in her investigations of intrapopula- tion structures and structures of animal communities forming the meadow biocenosis. She concludes that the populations forming the community of meadow Auchenorrhyncha are unevenly distributed in the habitat as a result of the variety of microclimatic conditions and plant struc- ture found in a vertical cross-section of such a meadow bionosis. At the same time, a continual movement by the species between the layers takes place as a result of the development of the insect. It is also suggested that the position of a species within the vertical layer may be • related to density.
The genus Macrosteles comprizes 30 European species of which 15 are found in Britain (Le Quesne, 1969). Of these 15 species, three commonly breed in Silwood Park (Waloff & Solomon, 1973).
• 16
During 1972 to 1974, M. sexnotatus was by far
the most widespread and abundant species within its
genus in Silwood Park, both on the ground and in aerial
traps. Also present were M. laevis, M. viridigriseus and M. cristatus.
Linnavouri (1952) reports that M. sexnotatus and M. viridigriseus are common in all types of damp meadows
in south-western Finland, while M. cristatus is frequently found in cultivated fields.
Razviaskina (1960) classifies M. sexnotatus
amongst the mesohygrophilous and hygrophilous species of
Macrosteles, occurring in marshes and wet meadows of the
USSR. The distribution of this group of species that feed
on grasses and reeds is related to the ecological condi-
tions supplied by damp associations rather than to the
food plants. On the other hand, M. laevis together with
M. cristatus show a wide tolerance to moisture and their
distribution is related to the food plants rather than
to the ecological conditions.
Andrzejewska (1962) says that 14 laevis tolerates
a wide range of environmental conditions, and refers to
this species as an index of disturbance. It was abundant
in the cultivated areas and on those that had suffered
some sort of disturbance like cultivation, repeated
mowing etc.
Whittaker (1969) lists the habitats of leafhopper 14. • species of chalk grasslands. sexnotatus is recorded • 17
in the 'field layer' and in the 'ground zone' (Elton & Miller, 1954) of the rides of deciduous woodlands, and M. laevis in the 'ground zone' when it forms an open formation type and also in the 'ground zone' covered by a field layer of vegetation in a field formation type. deciduous woodlands it is found in the 'field layer' and 'ground zones' of glades.
Morris (1971, 1973) studied the abundance and diversity of the leafhopper faunas of grazed and ungrazed chalk grasslands, respectively 'open ground' and 'field layer'. Of the 45 species recorded only two occurred more frequently on the grazed grasslands, one of them being M. laevis. This species was also found in the ungrazed grasses but always in larger numbers in the grazed situation.
M. cristatus and in smaller numbers M. laevis are reported in cereal crops, which are 'fresh' biotopes (Raatikainen, 1971) in Finland, S•7eden and Norway (Raatikainen, 1971, 1972; Raatikainen & Vasarainen, 1971). M. sexnotatus is only occasionally found in cereal crops
• in Finland (Raatikainen & Vasarainen, 1973).
Waloff and Solomon (1973) point out that leaf- hopper species that are abundant in acidic grasslands can be absent or rare in chalk grasslands. This is true of M. sexnotatus and M. viridigriseus, while M. laevis is found in grasses of both types of soil. During 1968 to 1970 M. laevis, M. sexnotatus and M. viridigriseus were • 18
breeding regularly in Silwood Park in areas characterized
by low and uneven vegetation with numerous patches of
moss or bare soil. These areas were drier than many
other acidic grasslands in Silwood Park. The areas were
dominanted either by low Agrostis with patches of bare
soil or by tall, dense stands of Agrostis. These two habitats can be classified respectively as 'fresh' and 'fresh-wet'. M. laevis and M. sexnotatus were found in
the first type and M. viridigriseus in the second. M.
laevis was one of the 10 most common species of leaf-
hoppers in the acidic grasslands while M. viridigriseus
was fairly common and M. sexnotatus was amongst the less
abundant species.
In Silwood Park, 1972, a low density population of
M. laevis was located at a place called Nash's Field of
several hundred square meters in area covered by low
grasses on an exposed hill side. This area is mowed at
the end of each summer. The patches where M. laevis were found were dominated by low Agrostis. The colonies studied
by Waloff and Solomon (1973). no longer existed when the
present work was initiated. An even smaller population
of M. viridigriseus was found in a small area of grasses
near Ashurt's Lodge. This patch of grass was no larger
than a few dozen square meters and was covered by a dense
mixture of common grasses on acidic soils. This very small
population was heavily infected by Dryinidae and was
sampled only in the summer of 1972. M. sexnotatus was
found also in very low densities in a restricted area • 19
dominated by Agrostis tenuis. This area, unmown for several years, covered 200 square meters and was sur- rounded by several hundred square meters of lawn. Simultaneously, M. sexnotatus was breeding and abundant in an oatfield from 1972 to 1974.
Even when occupying similar habitats the three species of Macrosteles seem to occupy different micro- habitats. M. laevis is common in 'disturbed' areas and is the species that readily colonizes areas with uneven vegetation with bare patches of soil (Razviazkina, 1960; Andrzejewska, 1962; Morris, 1971, 1973; Waloff & Solomon, 1973) and can live in drier habitats than M. sexnotatus and M. viridigriseus. The latter is common in marshy areas (Ribaut, 1952; Le Quesne, 1969). There seems to be a gradient of humidity preferences in the three species considered; M. viridigriseus being found in the most humid conditions, M. sexnotatus in less humid, and M. laevis being able to withstand a wide range of conditions, including dry ones.
4. Methods of Rearing
• A. Stock culture
Continuous breeding of M. sexnotatus was maintained throughout the whole period of this work so that insects were available for experimentation at any time.
Mass cultures on oats variety Condor were kept in a 20°C constant temperature room, at 16 light hours per • 20
day. Each culture was set up with 10 females and 10
males, as a rule not older than 10-15 days. The insects
were placed in a pot with young oat seedlings so that the host plant remained fresh as long as possible and
provided a suitable environment for the incubating eggs and, eventually, for feeding by the newly hatched first
instar nymphs.
A plastic flower pot 13.5 cm in diameter and
•12.5 cm high containing oats with a cylinder adjusted to
its distal end constituted the breeding cage. The
cylinder was made of cellulose acetate, ventilated by
two opposing windows of muslin. It was 32 cm high and
13.5 cm in diameter. Its basal part was adjusted
tightly to the outer rim of the flower pot. Its flat
top had an opening in the centre closed by a cork, so
that insects could be tipped in through the 2.5 cm hole
without the necessity of removing the whole cylinder.
The ventilation inside the cage was secured by the two
muslin squares 9 cm x 9 cm cut off at opposite sides and
at different levels so that the base of one was level
with the top of the opposite one (Fig. 1A). • B. Eggs
Amongst the several attempts the most successful
rearing proved to be in pill-boxes. was 5.2
cm in diameter and 2.6 cm high and was covered by a tightly
fitting lid, transparent enough to allow one to see through
it without removing.it in routine examinations. • • •
Fig. 1. Rearing cages (A) stock cultures; (B) individual nymphs; (C) individual t.) or paired adults; (D) nymphal and adult parasites. 22
The bottom of the box was covered by an even laver.
of absorbent cotton-wool and its surface smoothed by a
neatly adjusted sheet of filter paper. This was then moistened to saturation with distilled water. The oat
seedling or an oat fragment was placed horizontally over the filter paper. The tightly closed pill-box prevented
moisture from escaping so that the moisture was retained even at high temperatures.
C. Nymphal stages
Individual nymphs were reared in flat bottomed glass tubes 7.5 cm high and 2.4 cm in diameter.
The bottom of a tube was covered with a compacted
layer of absorbent cotton wool topped by two closely fitting filter paper discs. This was kept moist through-
out the experiment by dripping water directly over the
paper by means of a pipette so as to avoid drops of water
from falling on the tube itself and trapping small nymphs.
The distal, open end of the tube was closed by a terylene
square held by an elastic band (Fig. lB).
• One or two oat seedlings held with a forceps were placed inside the tube with the seed in direct contact
with its moistened bottom part. The individual nymphs
were released directly into the tube from a hand aspirator.
D. Adults
Individual or paired. adults were kept in cages
similar to those used for stock cultures but proportionally 23
One, two, or a few oat seedlings were planted in a plastic flower pot containing the usual mixture of 50% peat to 50% sand.
The flower pot, 8.2 cm in diameter and 7.7 cm high, was topped by a cellulose acetate cylinder adjusted internally to its distal rim. The cylinder, 8.0 cm in diameter and 23.0 cm high was ventilated by two 5 cm x 5 cm opposing windows covered with muslin and its top had a central 2.5 cm hole closed by a cork (Fig. 1C).
E. Parasites
(i) Egg parasites
Experiments with egg parasites were set up in the same way as the egg incubation ones.
According to the extent of the area of the oat plant containing eggs, larger plastic containers were used instead of the usual 5.2 cm x 2.6 cm pill-boxes.
(ii) Nymphal and adult parasites
Parasitized specimens were set up individually. • The same set up was adopted both for nymphs and para- sitized adults.
The cage consisted of an inverted 7.5 cm long by 2.5 cm wide glass tube containing one or two oat seedlings to serve as food for the parasitized individuals. The open bottom of the inverted tube was closed by a cork covered by a square of muslin. The cork was bored in the 24
centre and a glass tube 5.0 cm long by 0.9 cm in diameter filled with water was adjasted to it so as to provide adequate humidity in the rearing chamber (Fig. 1D).
F. Predators
Predators used in laboratory feeding trials were kept in a plastic container 10.5 cm in diameter and 4.2 cm high. A 2.5 cm hole was made in the centre of the lid so that predator or prey could be tipped into the container without interference, for example, with the web-building spiders. The hole was closed by a lid and was the only aperture of the container.
The base of the box was lined with a layer of cotton-wool and covered by one or two discs of filter paper. It was kept moist with water and one or two oat seedlings were kept on the moistened area to provide food for the prey.
5. The Nymphal Stages
A. Description of the nymphal stages
(i) Introduction
There are only a few descriptions in literature of cicadellid nymphs and also of the characters for the separation of the sexes (Kath irithamby, 1974).
Osborn (1916) briefly described the immature stages of Cicadula sexnotata but it is not known to which species of Macrosteles he refers. Immature stages of 25
M. laevis are illustrated by Bollow (1950). Prior (1965) illustrates mounted primordial genital capsule and the ovipositor of the fourth and fifth instars of M. sexnotatus, and the hind tibiae of first to third instar nymphs. He also gives a drawing of a cleared and mounted specimen of a fifth instar M. sexnotatus female.
Kathirithamby(1971) studied the chaetotaxy of eight species of Cicadellidae from first instar onwards. The study was based on the pattern of distribution of bristles on the dorsal surface of the abdomen on abdominal segments III to VIII. Such bristles in M. sexnotatus are described as of medium size and of the same colour as the abdomen. Their pattern of distribution was found to remain constant in M. sexnotatus from first to fifth instar, except for the IX segment where the presence of four bristles characterizes the first instar nymph. M. sexnotatus was distinguished from the remaining cicadel- lids by the low number of bristles and the fifth instar nymph was diagnosed as yellow with two black streaks and spots on the vertex and by the presence of two bristles on segment VII and four on segment VIII.
Kathirithamby(1971, 1974) studied the development of the external male and female genitalia in the immature stages of Cicadellidae. The rudiments of the female genitalia are already differentiated in the third instar nymph. In the males, the subgenital plate is differen- tiated on the third instar as a triangular piece fused to the ninth sternite. 26
(ii) Descriptions
Creamy yellow, with numerous fuscous spots and
markings of variable intensity and distribution on head,
thorax and abdomen. Specimens reared at low temperature
(15°C) or spring generation in the field with denser and darker pigmentation. Lighter and less extensive pigmen-
tation in specimens reared at higher temperatures and in
the summer generation in the field. Abdominal. tergites
always more pigmented than the sternites. Pigmentation
normally absent on a longitudinal line on thorax and
abdomen. Two characteristic black dots on head, each half way between the mid longitudinal line and the eye; remaining markings rather variable but usually restricted
to the vertex or adjacent to it. In heavily pigmented
specimens these areas are more extensive and sometimes
fuse.
The data on measurements of the five instars of
M. sexnotatus nymphs is given in Table 1. The measure-
ments and illustrations are based on laboratory reared
nymphs at 20°C constant temperature and at 16 light hours.
First instar: overall colour darkened by fuscous patches
of varying extent on head, on each side of pronetum and
wing pads and on anterior two thirds of each abdominal
tergites except for an irregular mid longitudinal line.
Apex of ninth tergite provided with four distinct setae.
Visible lateral margin of mesothoracic wing pads approxi-
mately one third of the corresponding area in the 27
Table 1: Measurements of M. sexnotatus nymphs (mean and standard error based on measurements of 10 individuals of each ins tar)
Total body Hind femur Hind tibia Instars - Head width length length length
I 0.338=0.0029 1.107=0.0347 0.219=0.0041 0.362=0.0042 II 0.433=0.0347 1.524=0.0358 0.337=0.0055 0.533=0.0063 III 0.549=0.0031 1.898=0.0243 0.467=0.0105 0.724=0.0045 IV 0.689=0.0078 2.498=0.0457 0.624=0.0065 1.007=0.0148 V 0.868=0.0116 3.094=0.047 0.816=0.0065 1.336=0.0159
metathoracic wing pads. A minute bristle at the posterior angle of the metathorax. First abdominal segment not covered by the wing pads (Fig. 2A).
Second instar: overall body colour less darkened by
fuscous patches than in the first instar nymph. Posterior margins of meso and metathorax moderately undulate. Visible external margin of mesothoracic wing pads approxi- mately two thirds of the corresponding area in the meta- thorax. Postero-lateral angle of the metathorax with a minute bristle. First abdominal segment visible. Ninth tergite with 6 + 6 bristles visible from dorsal view (Fig. 2B).
Third instar: visible external margins of mesothorax as long as or slightly longer than the same area in the meta- thorax. Posterior margin of mesothorax clearly bisinuate, that of the metathorax moderately convex each side of a central concavity. Postero-external angles of metathorax devoid of bristles. Posterior margin of metathoracic wing pads extending over base of second abdominal tergite(Fig. 2C).
• Male: subgenital plate small, in an isosceles triangle, less • •
N.) CO
Fig. 2. Y. sexnotatus nymphal stages. A, instar I, Br instar II. 29
i•••■•••••a....II=MO+I•*m 1 MM
0.5 M FT,
Fig. 2 (Cont.) C„ instar III; D, in9tar IV; E. instal: V.
• 30 than half the length of the J.X segment (Fig. 3A). Female: first pair of outgrowths small, second pair more than twice as long as first pair (Fig. 4A).
Fourth instar: mesothoracic wing pads well pronounced, extending for most of the length of the metathorax; visible external margins about five times as long as visible external margin of metathoracic wing pads. Posterior margin of meta- thorax broadly V-shaped as a result of the prolongation of the wing pads; bristle at postero-external angle absent..
Metathoracic wing pads partially obscuring first and second abdominal tergites (Fig. 2D). Male: subgenital plate more than half the length of segment IX (Fig. 3B). Female: first pair of outgrowths almost as long as second pair; third pair rudimentary and concealed by second pair (Fig. 4B).
Fifth instar: mesothoracic wing pads extending well over the metathoracic ones so that lateral margi f metathorax are concealed by the lateral margins of mesothorax. Posterior margin of mesothorax sub-rectangular; that of metathorax deeply concave. First abdominal tergite visible on the mid longitudinal line. Extremity of both wing pads extending to the fourth abdominal tergite (Fig. 2E). Male: subgenital plate in a triangle rectangle extending for more than two thirds of the length of segment IX (Fig. 3C). Female: first pair of outgrowths long, narrowly triangular, longer than the second pair and as long as the third; second pair visible distally between the two halves of the first pair of outgrowths (Fig. 4C). 31
VII I
Fig. 3. M. sexnotatus, male, ventral view of last two abaominal segments: (A) III instar, (3) IV instar; (C) V instar; VIII: eighth segmnt; IX, ninth segment; X, tenth segment (anal tube); sp. suhcjeniLai plate. :3
yin
0.5 MM
Fig. 4. M. seznotatus, female, ventral view of last two abdominal segments: (A) III instar; (B) IV instar; (C) V instar. 1, first pair of outgrowths; - 2, second pair of outgrowths; 3, third pair of outgrowths; VIII, eighth segment; IX, ninth segment; X, tenth segment (anal tube). 33
D. The rate of growth
Richards (1949) proposed that there is a regular relation between the measurement of successive instars if account is made of their durations. Therefore, the longer - the instar duration the greater the amount of growth. He fitted linear regression equations to measurements against accumulated days for various insects and found that the relation could well be described by the straight line if first instars were excluded from the computation. This implied that the/ growth increment per day is constant. The point corresponding to the first instar always lies above the line on which the other instars lie and he suggested that in this instar there is an initial period where no growth takes place.
Equivalent results were obtained for the cicadellid Psammotettix confinis (Solomon, 1973) and for the delphacid Stenocranus minutus (May, 1971).
Linear regression equations were fitted to measure- ments in mm of head width, hind femur length and hind tibia length of M. sexnotatus nymphs against accumulated duration- in days of instars at 20°C (Fig. 5).
The regression equations obtained were: (1)Y = 0.27556 -I- 0.03374X for head width, with r = 0.9985 and 0.002 < P < 0.001. (2)Y = 0.164803 + 0.037219X for hind femur length with r = 0.9988 and 0.002 < P < 0.001. (3)Y =0.23086 + 0.06286X for hind tibia length with r = 0.9966 and 0.01 < P < 0.002. 34
14
A/
0 5 10 15 20 ACCUMULATED DAYS
ig. 5. Relation of measurements of M. sexnotatus nymphs (omitting first instar), against accumulated days at 20°C (Head Tirath (0) , length of hind femur (*), length of hind. tibia Ca) • 35
The points fall close 1-e, the straight line for
head width and hind femur length suggesting that growth
proceeds at a constant rate. The relation is less well
described by a straight line for tibiae measurements and
perhaps a better it could be obtained if a larger number
of measurements were made for each instar. Otherwise, as
the poorer fit has been consistently obtained for the third instars, it may be supposed that there is proportionately
less growth in preceding moult from the second to the third stage.
6. The Adult Stage
A. Taxonomic literature
M. sexnotatus belongs to a group of species within
the genus that can not be separated in terms of external
morphological characters only. The extent of the variation
in colour, markings and structure in all the species makes
specific identifications particularly difficult.
M. sexnotatus (Fallen) is the genotype of
Macrosteles and in early papers several species used to be
placed under its name. Therefore, much of the references
• prior to the more recent findings of diagnostic features
should be looked at with reserve since they may be dealing
with species other than. M. sexnotatus.
In recent years increasing use has been made of the
shape of the aedeagus in the male genitalia as a taxonomic character. • 36
Ribaut (1952) distinguishes the :ilacrosteles species
on the basis of the shape of the aedeagus while the females
are not diagnosed at species level. illustrations are pro- vided for the aedeagus, head, pronotum and scuteilum of M.
sexnotatus and for other species from France.
Beirne (1952, 1956) states that the intraspecific
variations can even affect the aedeagus and the seventh
sternite of the females. Also, that males tend to be darker
and more variable than the females. The seventh sternite
of the female can often be used to separate species into
groups but it lacks specific significance due to its vari- ability. M. sexnotatus is confirmed as an exclusively
European species and the records from the Nearctic region
are interpreted as misidentifications. The closely related
species M. laevis (Ribaut) and M. cristatus Ribaut occur
both in Europe and in North-America, the latter species con-
sisting of a complex of forms, poorly defined morphologi-
cally, in the United States. The structure of the aedeagus
is considered as the most reliable specific character and
illustrations are given for the Nearctic species including
a lateral view of that of M. sexnotatus for comparison
(Beirne, 1952).
Ossiannilsson (1949) points out the presence in
several genera of the Auchenorrhyncha of apodemes of the
second abdominal sternite of the males already in use as a
specific character within the genus Empoasca.
In 1951 Ossiannilsson described and illustrated the highly
specific abdominal apodemes of the Swedish species of • Macrosteles including M. sexnotatus whose identifications 37
so far were based exclusively on the male aedeagus.
Muller (1956) demonstrates that seasonal form-
cycles occur in the genus Euscelis where form and size of
the aedeagus is induced by photoperiod during nymphal,
development. Such findings provided a warning to the
exclusive use of the male aedeagus in taxonomic work.
The aedeagus in conjunction with the first and second pairs of abdominal apodemes are illustrated and
described by Prier (1965) for M. sexnotatus, M. viridig-
riseus (Edwards), M. laevis (Ribaut) and M. cristatus
Ribaut. A key based on these characters is presented and
also illustrations of head and thorax of these species
showing a range of variation in the extent of pigmentation.
Prior (1967) illustrates atypical aedeagus and
apodemes of some M. sexnotatus specimens parasitized by
Dryinidae and also the typical forms of M. sexnotatus
(Fallen), M. cristatus Ribaut and M. alpinus (Zetterstedt).
Le Quesne (1968a) illustrates the aedeagus and
apodemes of M. sexnotatus (Fall6n) and of M. ossiannilssoni
Lindberg (Ossiannilssoni Le Quesne, Le Quesne, 1968b). • Le Quesne (1969) gives a key to the British species
of the genus Macrosteles together with illustrations of the
aedeagus and apodemes for each of the species.
In the present work , all male specimens have been
identified using the aedeagus characters togPther with the
abdominal apodemes, As at present it is impossible to
• identify females to species level, females in field samples 38 were taken in proportion to the males. This did not constitute a major problem since in the study areas - M. sexnotatus was by far the most common and abundant species throughout this three-year study period.
B. A short winged form
Beirne (1952) states that the insects of the tribe Macrostelini are all macropterous with long slender fore wings.
Severin (1940) reports the presence of a variety or Physiological race of M. fascifrons occurring in the canyons of the Montara Mountains, California. This variety had longer wings than the common aster leaf-hopper M. fascifrons, and according to Severin (1940) the two forms did not inter- breed. No measurements or illustrations were given for this extra macropterous form. This is one of the forms of the Macrosteles fascifrons complex which extends from Mexico and PUerto Rico to Alaska and is composed of a large number of varieties oS even sub-species which differ biologically, physiologically and ecologically but that cannot be isolated in terms of external morphological characters (Beirne, 1952; De Long, 1971).
Wing polymorphism is known in many species of Auchenovrhyncha and is considered to be affected by factors like heredity, photoperiod, weather conditions, population density and nutrition (Kisimoto, 1956; Raatikainen, 1967; May; 1971; Mochida, 1973).
Brachypterous and macropterous forms of the same species are found in many of the Delphacidae. Within the Cicadellidae, brachypterous of submacropterous forms occur 39
in the subfamily Deltocephaiinae. They are present in the
tribe Doraturini in the genera Doratura,,Turrutus,
Arocephalus, Errastunus and Psaymotettix. In the tribe
Athysanini they occur in Rhytistylus, Granhocraerus,. Euscelis
and Macustus. On the other hand, the species of the four
genera of the tribe Macrostelini occurring in the British
Isles, namely Erotettix, Sagatus, Sonsorius and Macrosteles all have fully developed wings (Le Quesne, 1969).
Waloff (1973) reports the presence of a few rela-
tively short bodied and short winged M. sexnotatus males in
field samples in Silwood Park in 1970.
A laboratory culture of M. sexnotatus was set up in
August 1972 with young nymphs obtained in samples from the
experimental oatfield. The cultures- were kept bn oats
variety Condor at 20°C and 16 light hours. The first two
stock culture cages were set up in late September. First
generation laboratory bred nymphs were available by mid-
October and in mid-November two stock culture cages were
set up with first generation laboratory bred female and
male adults. A large number of second generation laboratory
bred nymphs was obtained in both•cages. Amongst the normal
• long winged adults, seven females and eight males were found to be short winged.
A cage was set up with three females and three males all short winged to observe the inheritance of the wing
length character. 14. sexnotatus females at 20°C will mate
within a few days of final moult. When the short winged
females and males were isolated in a cage for mating and
• oviposition it was possible that the females had already 40 mated and since the normal winged males were in a vast majority there was a high probability that the females had already mated with such males. All the nymphs obtained from this cage were allowed to reach the final moult and 177 adults were obtained, of which only three were short winged and all were males. Therefore only 1.7% of the total adults were short winged. No further culturing was attem- pted from this stock. Fecundity and fertility of the short winged females seem to be normal with an average of 60 offspring reaching adult stage per female.
As a routine procedure, nymphs from parental culture cages were transferred to fresh oats. The total offspring of the two cages set up with second generation laboratory bred M. sexnotatus, each cage with 10 pairs of normal winged adults, was maintained to final moult to see whether short winged forms would occur. There were 943 normal winged forms and five short winged forms were present, of which two were males and three were females corresponding to 0.53% f the total offspring in the third generation of laboratory bred M. sexnotatus.
Only normal macropterous forms were obtained in the two subsequent generations. A single short winged male was found amongst 41 males and 53 females normal winged in a cage containing the sixth generation.
No further short winged forms were obtained in the subsequent generations though not all the nymphs were allowed to reach final moult and therefore this character could not be properly assessed,
41
The mean and its standard error of the measurements of normal winged females and males and of the short winged specimens are given in Table 2. - The total body length though included, was not regarded as a reliable index of size since both males and females were preserved in 70% alcohol and had varied degrees of distension and contrac- tion specially of the abdomen. The head width across the eyes was taken as a better character for body size.
Table 2: Measurements of two winged forms of liacrosteles sexnotatus (measurements in mm; number of individuals in brackets)
Short winged Normal winged Normal winged males (6) males (15) females (15) x ± s.e. x ± s.e. x ± s.e.
Total body length 2.9 ±0.056 3.13 ±0.036 4.046±0.043 Fore wing length 1.52 ±0.104 3.05 ±0.0416 3.58 ±0.035 Hind wing length 1.84 ±0.052 2.65 ±0.029 3.04 ±0.031 Head width 0.882±0.29 0.876±0.009 1.01 ±0.012
Ratio of fore 1:0.85 to hind wing 1:1.21 1:0.87 Ratio of fore wing to head 1:0.58 1:0.29 1:0,28 width Ratio of hind wing to head 1:0.48 1:0.33 1:0.33 width
Both in the females and males with short wings the reduction in length was variable ranging from a little shorter than the abdomen to half as long as the abdomen. Proportionately the reduction w,,Is more accentuated in the fore wings. The hind wings were also reduced but not to such an extent. The average fore wing was reduced to 49.8% 42 of its normal length and the hind wing to 69.4%. In the normal winged males the average length of the hind wing is not more than 87% than that of the fore wing, while in the short winged form the situation is reversed: the average fore wing was not more than 83% of the length of the hind wing.
The description of the wing follows Ribaut's (1952) terminology for the Cicadellidae.
The normal fore wing (Fig. 6A)
The radial and medial veins arise from a rather long common branch. The subcostal and cubital veins are simple, as is characteristic for the Cicadellidae. The radial vein is simple, while the medial bifurcates, as is common for the Macrostelini, and its external branch coalesces with the radial vein for the middle third of the wing length (subapical region).
Regular transverse veins in the sense that their presence and position is constant for the species: discal vein or anterior medial-cubital, subapical vein or posterior medial-cubital, and the radial-medial vein. There are also apical cross-veins delimiting the apical cells.
The four discal cells are present. The discal cubital extends from the base of the elytra to the apical region.
Of the three subapical cells only the subapical central is present. The subapical internal (medial) cell 43 •
)
• •Fig. 6. The left fore wing of Macrosteles sexnotatus (A) normal form; (B) short form. CU = cubital vein CCS = clavo corial suture ER = extremity of radial vein ECU = extremity of the cubital vein EME = extremity of external branch of medial vein EMI = extremity of internal branch of medial vein ME = medial vein ME + R = fusion of radial vein and external branch of medial vein MEA = membranous appendix PE = peripheric vein R = radial vein SC = subcostal vein
1 = anterior medial-cubital or discal cross-vein 2 =-posterior medial-crhital or subapical cross-vein 3 = radial-medial cross-vein 4 = basal cell 5 = discal cubital cell 6 = discal medial cell 7 = discal radial cell 8 = discal subcostal cell 9 = subapical central cell 10 = apical internal or cubital cell. 11 = apical subinternal or medial cell 12 = apical intermediate cell • 13 = apical subexternal or radial cell 14 = apical external or subcostal cell WI • •
discal region apical region subapical region
-$ C cx) m e r :73 rn m C)
u
• CCs
C)
me a) NN me -- io)
C LI s • 45
is missing, fusing with the discal. The subapical external cell is also missing in the Macrostelini.
The five apical cells are present.
The fore wing in the short winged form (Fig. 6B)
The remigium is strongly reduced while the vannal and jugal areas remain unaltered. The membranous appendix is altogether absent and so is the peripheric vein. The five apical cells are absent and even the posterior medial cubital cross-vein is missing. Therefore, the discal medial cell is not defined as such. The subapical central cell results in an extreme distal external position and is not closed externally and posteriorly since the external branch of the medial extends only down to the radial vein but is absent posteriorly, the radial itself beyond this point is also absent, so that they do not occur in coalescence. The discal radial cell is present, neatly defined and lying externally to it is an ill-defined narrow extension of the discal subcostal cell. Therefore, the apical region of the fore wing is altogether supressed and the subapical region reduced.
The normal hind wing (Fig. 7A)
The venation of the hind wings is remarkably uniform in the Cicadellidae. Variation is restricted to the exten- sion of the peripheric vein and to the coalescences of the extremities of the subcostal vein and of the external branch of the radial vein (Ribaut, 1952). 46 I
)
• Fig. 7. The left hind wing of Macr'osteles Sexnotatus (A) normal form; (B) short form.
CU = cubital vein FH = frenulum hook J = jugal vein JF = jugal fold ME = medial vein MEA = membranous appendix PV = peripheric vein R = radial vein SC = subcostal vein SCB = branch of subcostal vein V = vannal veins VF = vannal fold
•
• 47
2 1 • 48
The two vannal veins are coalescent on the anterior
region; the cubital and medial veins are simple; the radial is bifurcate. The internal branch of the radial vein is
coalescent or united to the medial by a short cross-vein.
The external branch of the radial coalesces for a short
length with the subcostal vein. Adjacent to the frenulum
hook the subcostal emits a short external branch. The
peripheric vein extends from the jugum-vannus limit up to the frenulum hook and so does the membranous appendix.
The hind wing in the short winged form (Fig.. 7B)
The jugum and vannus are not altered except for the absence of the membranous appendix. It is the remigium that
is more severely affected. The membranous appendix is
totally absent, the peripheric vein is vestigial as well as
the frenulum hook. The area posterior to the contact of
both branches of the radial is noticeably reduced in size.
As in the normal wing, the contact of the internal branch
of the radial with the medial vein is either by coalescence
or by a short cross-vein. The contact of the external
branch of the radial with the subcostal vein is by coales-
cence. The remigium is reduced in such a fashion that both
branches of the subcostal and radial veins are altogether
supressed after the coalescence.
7. The Internal Reproductive Organs and Sexual Maturation
A. The male organs (Figs. 8A and 13)
The testis consists of six lobular follicles arranged
over a flat and elongate common base. The follicles are not
• enclosed within a common testicular sheath. —testis
—vas deferens
vesicula seminalis
accessory gland
buibus ejaculatorius Ft-, Q.) fig. 8. Male reproductive organs of M. sexnotatus: A, on emergence; B, mature. • 50
The vasa deferentia are very narrow between the testis and the vesicula seminalis, less so when immediately leaving the testicular base. They are rather wide when leaving the vesicula seminalis then taper towards the bulbus ejaculatorius.
The vesicula seminalis are elongate and stout. The bulbus ejaculatorius is delicate and pear-shaped.
A pair of elongate and convoluted accessory glands joins the vas deferens at the base of the vesicula seminalis.
Sexual maturation is accompanied by an increase in size of the vesicula seminalis and by a lengthening of the two sections of the vasa deferentia, notably between the bulbus ejaculatorius and the vesicula seminalis.
B. The female organs (Figs. 9A and B)
The ovaries are of the meroistic type each consisting of six ovarioles. The ovariole number per ovary is constaht as shown by the dissection of several hundreds of females.
The anterior part of the ovariole is prolonged into a terminal filament and posteriorly forms a pedicel that
• opens at the lateral oviduct.
The common oviduct is thick and elongate. A pouch- like spermatheca opens dorsally into the anterior end of the ill-defined vagina and a little further a single acces- sory gland opens at the posterior end of the vagina. This accessory gland is fingerlike and sometimes can be recurved
at its distal end. S • 51
\ --terminal f i lament
—ovary
ped icel sperrnatheca
‘ lateral oviduct-- 1
median oviduct
, -
-va gina •
—accessory gland
1117604111910t1BANEVIMEIV31
0.5 mm
Fig. 9. Female reproductive organs of 2,1. sexnotatus: A, on emergence. • 52 .
acrminel filament
I__ovarint ligament
ge r mar um
"r"ra
\\lute<3\11 A
lc ‘1 -"/ / '*#
- -pecti ce I
torol oviduct
_medium av iduct
vng in a,
---spermothec a
__accessory g land
mrn
Pig. 9 (cant) B, mature. 53
In the immature, recently emerged female, the ovarioles are small and are virtually coalescent. In unmated females the transparent walls of the spermatheca are close together and it projects like a spoon on the dorsal curvature of the common oviduct.
As development proceeds, the reproductive organs gradually increase in size. In a sexually mature female there is a considerable increase both in length and in diameter of the very elastic lateral oviduct.
In mature females oocytes in various stages of development are found in both ovaries. As a rule, only one fully developed oocyte is present per ovariole. In an ovi- positing female the average .number of oocytes with chorion present at any one time will be approximately six. Fully mature oocytes can be easily distinguished from the remaining ones by their shiny chorion. In ovarioles in which ovulation has taken place several times whitish nodular remains of corpora lutea are present in the anterior end of the pedicel. Eggs are laid daily during most of the ovipositing period. Therefore, normally there is no accumulation of large numbers of fully mature oocytes in the oviducts and pedicels.
• The length of the pre-oviposition period is a function of the rearing temperature. Under constant temperature conditions it ranges from an average of 3.1 days at 30°C to 13.7 days at 15°C.
On emergence the abdominal cavity is filled with the fat body which progressively decline in volume but a certain amount of it is maintained throughout the female life span. • 54
In a few instances the fat body was reduced to a thin, transparent and finely granular layer.
The occurrence of developed oocvtes devoid of yolk has never been detected either in the field or in laboratory cultures in females of any age.
Senescent ovaries are seldom present. In them there are no developing oocytes in the vitellarium or in oviducts. The ovarioles are thin and nodular. Even in extremely long lived females the occurrence of senescent ovaries was rarely seen. Females as old as 120 days still oviposited at 15°C.
In three of the four rearing temperatures in which fecundity experiments were carried out (15°, 25° and 30°C) it was noticed that an increase in weight takes place in the majority of the long lived females reaching a maximum before death. The dead females were dissected in several instances and in the majority of cases there was an accumu- lation of several eggs with chorion that had not been oviposited. This aspect will be treated later in part 11, B and C.
• C. Mating
Females and males were set up soon after emergence in the cages described in part 4 D. These newly emerged adults were obtained from stock cultures kept at 20°C constant temperature room at 16 light hours per day. The
cage was maintained at the 20°C constant temperature room and females were removed periodically for dissection and examination of the reproductive organs. Living females 55
were dissected in Ringer solution and the spermatheca was squashed to check for the sperm content. Males were also dissected occasionally for the presence of mobile sperm.
Six females and four males were set up as described above. After 48 hours three females were dissected and none had sperm in the genital ducts. The remaining three females were dissected 24 hours later, therefore 72 hours after emergence and none had been inseminated. Another set of newly emerged adults, this time seven females and five males was set up. Ninety six hours after final moult three females were dissected two of which did not have any sperm in the genital ducts while the third one had a dis- tended and milky spermatheca and mobile sperm was observed under the microscope. Two of the five males were also dissected and both had mobile sperm. Twenty four hodrs later two females were dissected and both were fertilized with 120 hours after emergence. The same was true of the two remaining females dissected 144 hours after final moult. The three remaining males were dissected and all contained mobile sperm.
Another set of nine females was set up just after final moult, this time with five males all of them already five days old. Three males of the same age were dissected and all of them had mobile sperm. Three of the females were dissected 24 hours after emergence and none had been inseminated. Another set of three was dissected 48 hours after final moult with the same result. The remaining three females were dissected 72 hours after emergence and one of the females had already been inseminated. • 56
The experiment was not repeated at other tempera- tures. Though not looked for, earlier mating was observed
both at 25° and 30°C constant temperature in the fecundity experiments. At 30°C a pair was seen in copulation within 48 hours from emergence and also on the subsequent day. At 25°C experiment three pairs were seen in copulation within 48 hours after final moult. Therefore, mating will take place in young M. sexnotatus females either if males are also young or older. The low numbers tested do not enable further conclusions to be drawn but suggest that at 20°C mating may take place as early as on the second or third day after emergence.
In several instances it has been shown that mating can take place before the female ovaries are fully developed. Rose (1971) reports that Cicadulina mbila will mate 48 hours after the imaginal ecdysis before maturation of the ovaries at 25°C. Mating also takes place within 48 hours of emer- gence in Empoasca fabae (Medler et al., 1966). Johansson (1958) reports that in the heteropteran Oncopeltus fasciatus females will accept mating one or two days after emergence but males will normally not mate before five to six days
i after emergence. Thus females can copulate soon after emergence, but for successful mating sufficient time has to elapse for hardening of cuticle, that of the genitalia in particular.
8. Laboratory Studies on the Rates of Egg Development
A. Introduction
The importance of temperature on the activity of
• poikilothermic animals has been long realized. Vast • 57
literature has accumulated over the years reporting the effects of temperature on rates of growth. Experiments are generally carried out at a number of constant tempera- tures and the results are presented as temperature-time curves and as temperature-velocity curves.
The developmental rate of insect eggs in relation to temperature has been reviewed by Howe (1967).
Mathematical descriptions of temperature effect on developmental rates have been discussed by Wigglesworth (1972), Andrewartha and Birch (1954), Howe (1967) and Watt (1968). This subject has been rather controversial and the authors express the same view point that a mathematical model that would faithfully describe the relationship between temperature and activity is not yet available. Every mathematical model proposed, so far, lends itself to criticism either because the theoretical basis is invalid or because the model fails to fit the type of data it is intended to describe, or both.
According to Wiggiesworth (1972) the attempts made to describe by a simple equation the curve representing the resultant of all the chemical and physical reactions, many of which are differently affected by temperature changes, are unsatisfactory, since no generalised formula is able to cover the diversity of events.
Andrewartha and Birch (1954) state that "...even a seemingly uniform stage like the embryogenesis is hardly likely to consist either of one uniform process or even of
S a series of successive processes so similar that the curves 58 describing them- • will have the same constants a and b."
LTowe (1967) remarks that it is extremely improbable
that the reactions constituting the development process would all be similarly affected by temperature or have the
same minimum and maximum thresholds. At the same time
...successive processes in an efficient system are unlikely
to have very different temperature limits and optima except in special circumstances like diapause."
Amongst the available curves, Howe (1967) recommends
the use of the asyrde'tric catenary. He also states that at
least 10 values spaced at intervals not greater than 2.5°C
are needed to determine the position of developmental period
curve and of the rate of developMent curve and that experi-
ments spaced at 1°C or 0.5°C intervals should be added for
about 3°C on either side of the optimum limits. These
requirements, for practical reasons, are seldom met with.
Besides, the asymPLric catenary cannot be easily computed
because of its four parameters and as pointed out by
Messenger and Flitters (1958) it gives a poor fit for low
temperatures. Wigalesworth (1972) remarks that if experi-
mental values were obtained over the entire range, by
various modifications of the eauation, the experimental
points might be fairly well fitted by the curve. Watt
(1968) in his analysis of the mathematical models available
does not take the asymetric catenary into much consideration-
and refers to criticisms made as early as 1928 doubting
that catenaries would give a better fit than even the simple
hyperbola. • 59
Andrewartha and Birch (1954) recommend the use of Davidson's (1944) logistic equation on the basis that it can be calculated from the empirical data and that it is the most adequate empirical description of the relationship between temperature and the speed of development. At the same time, they remark that owing to complexities of the developmental processes it is unlikely that any but the shortest and the simplest stage of morphogenesis can be truly represented by the logistic equation and even this is • not yet proved. Even so, they conclude by regarding the logistic equation as a useful empirical description of the relationship of temperature and the rate of development. Wigglesworth (1972) states that this curve gives "...a faithful representation of the speed of development-in many insects over 85-90% of the complete temperature range over which development can occur. Above a certain temperature retarding effects supervene so that the 'peak' temperature is usually well below the temperature at which K is attained." Browning (1952) analysed the statistical significance of the departure from the calculated value for the rate of develop- ment of eggs in examples that agree quite well with the logistic equation and found the departures to be statis- tically significant. Howe (1967) raised objection to the use of the logistic equation because it lacks solid logical justification and especially because it is inadequate above the optimum. Watt (1968) questions the use of Davidson's logistic formula both on theoretical grounds and on the grounds that the data can be fitted to the logistic curve only by leaving off points from one or both ends of the
• empirically obtained curve. • 60
Watt (1968) raises objection to the application of the asymmetric catenary, Davidson's logistic equation and
also Pradhan's equation (1945, 1946) for being empirical
formulae without a physicochemical basis. Pradhan's
equation is questioned because it describes a symmetrical
curve when it is already quite clear that the descent from
the maximum development rate is steeper than the ascent to it. Watt recommends the use of the empirical formula ver-
sion of Eyring's model to account for the rates of
temperature-reactions and to have a generally acceptable
theoretical foundation and also because extensive use of
this formula by ecologists might lead to an improved model
linking ecology to what has been achieved for reaction-rates
on enzymes.
Within the range of 15° to 35°C the rate of development
data of various Heteroptera and other insects has been
adequately described by a linear regression equation (Eguagie,
1972). The use of the linear regression is discussed later
in D.
As stated by Howe (1967) there seems to be little
basis, so far, for the choice between the various mathe- • matical relationships.
Davidson's logistic equation was applied to the
Present set of data obtained in the experiments carried out
at four constant temperatures for the reasons given by
Andrewartha and Birch (1954) and in the light of the views
outlined in the above text. • 61
B. The direct effect of temperature on developmental rates
(i) Material and methods
A set of females was taken at random from a culture of M. sexnotatus -adults of known age. These females were given numbers and set up individually in the tubes described in part 4C.
For each temperature tested, eggs were laid and incubated at the same temperature. The ovipositing females were kept at each constant temperature at least from their first nymphal instar. From emergence to the setting up of the experiments the females were kept with males to ensure fertilization.
For the four temperatures tested, the age of the females was equivalent physiologically and was within the first quarter of the oviposition period. The same ageing procedure was adopted to avoid heterogeneity that could result from the non-investigated potential maternal age- effect.
One oat seedling was introduced daily into the individual oviposition tubes. The seedling was removed and replaced within 24 hours until enough eggs were obtained. This took at most three consecutive days at the lowest temperature. The seedlings were examined under a binocular microscope and the number of eggs per female was recorded.
The eggs were set up as described in part 4B and the pill-boxes were labelled with the female number and date of oviposition. 62
After a satisfactory number of eggs was obtained, each living female was dissected under Ringer solution and the spermatheca was squashed to check for sperm content. The eggs of a female in which spermatheca lacked mobile sperm were to be discarded, but this never took place.
The eggs were checked daily at approximately the same hour (17.00). A twenty-four hour interval between routine observations was considered as adequate for an efficient estimation of the mean and variance of the incubation period.
(ii) Results
Table 3 summarizes the data obtained for the four temperatures. It also includes the values calculated from Davidson's logistic equation.
-Table 3: The direct effect of temperature on the rate of development of eggs of Macrosteles sexnotatus
100/Y Mean development in days Average % development x ± s.e. in one day Temperature in °C Observed Calculated Observed Calculated
15 26.66 ±1.81 26.86 3.751 3.724 • 20 11.88 ±0.89 11.94 8.417 8.378 25 7.645±0.636 7.67 13.072 13.045 30 8.359±0.625 6.45 11.963 15.512
The fitting of the observed points on the temperature- time curve and temperature-velocity curve calculated by the logistic equation are presented in Fig. 10.
• • • IW •
40 — 20
a- 30 4 0 m z 0 0 20
10
0 1+ e5.0087 —0.2502 100 6.79 5.0037-0.2502 0 = 0.1679 z 1 e I Y1 0 0 Z 10 — 0
< 6 10
0
10 15 20 25 30' 35
TEMPERATURE oc
Fig. 10. Observed points on the temperature time curve (open circles) and the temperature velocity curve (closed circles) for the incubation period of M. sexnotatus eggs. • 64
The relation of the spread of hatching to the incubation temperatures is represented in Fig. 11. The data on the hatchability of eggs at each constant tempera- ture is summarized in Table 4.
Table 4: Hatchability of eggs of Macrosteles sexnotatus at constant temperature • Temperature Number of eggs Number of eggs % Hatched in °C incubated hatched
15 105 86 81.9 20 133 116 87.2 25 87 81 93.1 30 68 64 94.0
(iii) Discussion
From Table 3 and Fig. 10 it can be seen that there is a close agreement between the observed means and the values calculated by the logistic equation for the three lower temperatures. As already stated elsewhere, the logistic equation gives a poor fit for points above the optimum where retarding effects supervene and it has con- spicuously overestimated the value for the point at 30°C.
The optimum velocity of development lies somewhere between 25°C and 30°C perhaps not much far below the latter, since it is known that the bend of the curve above the optimum is not symmetric, since the descent from the otimum is steeper than the ascent to it. There are no intermediate temperatures checked between 20° and 25°C but it seems clear that the optimum does not 'lie within this range for the ft" • • S
100
25°C 30 o c
9 -na- 40
.•11.311 PE= asam
1
=es.Plomern. ...1.1.1E.P.MIN.a..1•11.01•111'** .01047•0•111111116 ..M.111•Aa. MIMINImILIMMIN.L0117111•60.21= LL 23 25 27 29 31 11 13 15 7 8 9 9 10 INCUBATION (IN DAYS) Ui
:ig. 11. Spread of hatching in M. sexnotatus eggs at various constant temperatures. • 66
reason mentioned above and also because there is a very good agreement between the observed points and those esti- mated by the logistic equation. Furthermore, as will be seen later in part 8D a highly significant straight line can be fitted to the three lower temperatures (100/Y) indicating that within this range velocity is proportional to temperature.
The spread of hatching also reflects the effect of the incubating temperature. At the most favourable tempera- tures, 25° and 30°C, hatching takes place over three days. In the former, 90% of the hatching is equally divided in the first two days, while in the latter more than 70% of the total hatching takes place on the first day. This also suggests that the optimum temperature for incubation lies closer to 30°C, considering that a closer synchronization of _ hatching would take place at the most favourable temperature. The spread of hatching increases at lower temperatures indi- cating greater variability in the development in more adverse conditions.
The - 'optimum' temperature is regarded both as that at which the time of development is shortest and as that at which the greatest percentage of individuals complete their development. The hatchability figures also correspond to the results obtained for speed of development and spread of hatch- ing indicating that the 'optimum' lies between 25° and 30°C.
C. Effect of acclimatization to temperature on developmental rates
The aim of this experiment was to see whether there would be an effect on developmental rates when eggs were • 67
oviposited at a temperature different from that of incu-
bation as compared with the direct effect.
A common temperature of 20°C was adopted for the
three sets of experiments of incubation at 15', 25° and
30°C. The experiment otherwise was set up and carried out p as described in part 8B(i).
The results on developmental time and the percen- tage hatch are summarized in Table 5.
Table 5: The effect of temperature on the rate of development and hatchability of eggs of Macrosteles sexnotatus oviposited at 20°C
Y Mean 100/Y development Average % Number of eggs Temperature in days development in °C x ± s.e. in one day Incubated Hatched Hatched
15 25.68±1.37 3.89 143 108 75.5
25 7.75±0.602 12.9 127 116 91.3
30 7.94±1.216 1 2.59 73 67 91.8
A comparison between Tables 3 and 5 suggests that
the developmental time decreases in eggs incubated at 15°
and 30°C and remains virtually the same for incubation at
25°C. The results were analyzed statistically by comparing
the difference between the means and the respective standard
deviations. The ratio d obtained by the difference between
the means over the standard deviations taken together was
not significant for any of the three comparisons. Therefore,
at least under these experimental conditions, there was no
effect of acclimatisation per se on developmental rates of
• eggs. • 68
D. The 'developmental zero'
If all the developmental points at different tem- peratures fall on a straight line the implication is that the velocity of development is proportional to temperature, or, that there is a linear relation between rate and tempera- ture. This assumption is not true, since the line is a curve resulting from a bend of a straight line upwards at lower temperatures and downwards at higher.
If the reciprocal of the developmental period is plotted as 'rate of development', a shallow sigmoid curve that turns down above the optimum can be obtained. For most of the insect species this sigmoid curve is almost straight over a range of some 10°C just below the optimum and a straight line can be fitted to selected points. This has a practical application for it permits iha calculation of the 'developmental zero' point.
This theoretical 'developmental zero', obtained from a prolongation of the straight line over the temperature axis is the temperature at which development would cease if the curve did not in fact turn upwards in this region. This theoretical point is not in fact the true threshold of • development since the temperature at which development ceases lies much lower (Howe, 1967; Wigglesworth, 1972).
If interpreted as the minimum for the completion of a stage, the predicted developmental zero for constant temperatures can be adequately used, since there will be no hatching although development might proceed- at temperatures some degrees lower. 69
As stated above in 8B (iii) the 'optimum' for incu- bation of sexnotatus eggs lies somewhere between 25° and
30°C. At 30°C the developmental rate already exhibits retardation. Therefore, the temperatures below 25°C inclusive lie below the 'peak' of the velocity curve.
A linear regression of the form of
1/Y = 0.009329X - 0.102422 was fitted to the points of the inverse of incubation period
(1/Y) in days and temperature. From this equation a pre- diction formula has been calculated, thus: x = a/b, which is the point obtained by the prolongation of the regression line over the temperature axis.
The regression line was fitted to the data obtained from constant temperature experiments where incubation took place at the same temperatures as oviposition. The correla- tion coefficient obtained was 0.99. A t--test rejected the null hypothesis at 0.001 level.
The developmental zero for M. sexnotatus eggs is reached at 10.99°C. The regression line is illustrated in Fig. 12.
E. The effect of humidity
(i) Introduction
sexnotatus eggs, as is typical for leafhoppers, are deposited within the tender plant tissue wherein the incubation period is passed at saturation. • "RY •
0.15
0
- 0.10 0 y 0.0093 3 X - 0,10242 u-I
0 0.05
z F O RSE E NV
I ramoc.maux.....rem...”r.s.ataasua sremmteancarsasasessmammumgassmsnaralgenarmscaaartmussesawswasamameenaufflosaielemusRmuummialreasouramcssnostannzamorzAtssrseatun...../ 4 /// sourz.zeszimarammesoalsawatassaamuncimommornam te.a. 0 5 10 15 20 25 3 TEMPERATURE 0C
Fig. 12. Relationship between temperature and the inverse o the incubation period of In. sexnotatus eggs. 71
Humidity, together with temperature, have been long
acknowledged as the most important factors in the environ- ment influencing the physiology of insects (Wigglesworth,
1972). Also, diminution in the body water contents usually
depresses metabolism and retards development (Wigglesworth, 1972)_ In the egg stage, protective coverings are avail-
able to prevent the losS of water by evaporation (Beament,
1961; Solithwood, 1956). Al- the same time, absorption of water during embryonic development takes place in insect
species that oviposit in green plant tissues (Kayumbo, 1963;
Eguagie, 1972), An uptake of water can be detected either
by weighing the eggs during the embryonic development or
by observing if eggs become larger in size.
High moisture requirements by eggs of Cicadellidae
are reported in literature. Palmiter et al (1960) incubated
eggs of the nearctic cicadellid Scaphytopius acutus in apple
leaf cuttings at 20.5°C (69°F) over a range of relative
humidities. Hatching was 24, 48 and 88% respectively for
80, 85 and 100% RH. Hundred percent hatching was obtained
when the leaf cuttings in the vials at 100% RH were main-
tained permanently moistened by addition of water. Coupe i and Schulz (1968) report that eggs of the nearctic species Endria inimica fail to hatch when laid in parts of the host
plant susceptible to desiccation. Tay (1972) reports that
-below 98% relative humidity eggs of Cicadela viridis would
not hatch. The humidity requirements of the nearctic species
I1acrosteles fascifrons are given by Hagel and Landis (1967).
According to these authors M. fascifrons embryo requires a
moist environment, especially in the beginning of 0 • 72
embryogenesis and eggs will not survive if the leaf tissue surroundihg them wilt of dry up. Claridge and Reynolds (1972) report that eggs of Oncopsis swell and change shape after oviposition. Also, that dissected eggs unless kept at very high relative humidity will rapidly desiccate.
(ii) Material and methods
The eggs of Macrosteles sexnotatus both due to their small size and to oviposition habits are particularly difficult to isolate from the plant tissue without being damaged. Therefore, oat seedlings containing the eggs were set up and exposed to the humidities tested.
The effect of humidity on the developmental rates of eggs was studied in desiccators in which relative humi- dities were controlled with saturated solutions of different inorganic salts. These solutions were prepared according to Winston and Bates (1960) and Solomon (1951). The salts were selected from a list by Sokes and Robinson's, 1949, "Standard solutions for humidity control at 25°C", included
in Solomon's paper. BaC122H20 was adopted for a relative hymidit of 90%, KBr for 81% and SrC16H20 for 71% RH.
Besides these three relative humidities, eggs were also • REL. AT NE tested in 100%lhumidity, over water.
The first attempt to study the effect of this range of constant humidities was made by setting up the eggs in desiccators. The desiccators were 32.5 cm in diameter and 16.0 cm high. The saturated solutions, or water, covered the narrow bottom of the desiccator and a perforated zinc floor was placed on top of the rim made by the widening of • 73
the desiccator. Instead of the standard lid, a flat square of transparent glass was used to cover the desiccator. This was made air tight by a spread of vaseline. The oat seed- lings were set up in pill-boxes. These rested on the zinc floor and were not covered by a lid. The bottom of the pill-box was covered by a disc of filter paper to provide a contrasting background to the dark first instar nymphs. This set up was discarded later, as it proved inconvenient for handling: firstly, the emerging nymphs could easily escape from the uncovered pill-boxes and get lost; secondly, it was obviously difficult to dislodge the glass cover for routine examinations and by opening the desiccators the hymidity inside was altered and it took some time for the equilibrium to be reached after the lid was replaced.
A second set up, this time in 'micro-desiccators'
proved convenient for safe handling. Each 'micro-desiccator' consisted essentially of two glass tubes arranged one above the other with the open ends in contact. A lower tube, 10.0 cm long and 2.1 cm in diameter, contained the solution which covered approximately its basal fifth. The open 2 distal end was covered by a square of 3 cm terylene, held in place by a tightly fitting 1.0 cm wide external ring . made of thin and flexible cellulose acetate. The open end of the upper tube fitted tightly around the distal end of the basal tube. To ensure the maintenance of this position the tubes were fastened together with a self-adhesive clear tape. The oat seedling containing the eggs was placed upright in the upper tube, resting on the terylene.
The tubes were numbered and checked daily at 24 hour • intervals. The hatched nymphs were counted and it was not • 74
necessary to open the tubes to remove the nymphs because
they invariably died within the 24 hours between two con-
secutive observations.
The eggs were obtained in the same way as described
in part 8B(i). Oviposition took place at 20°C and the set
up eggs were transferred to a 25°C constant temperature room
for incubation.
(iii) Results
No hatching took place in any of the desiccators
at 90, 81 and 71% RH. Ninety, 119 and 99 eggs were set up
in each of these relative humidities, respectively.
Hatching took place only in the desiccators containing
water which orovided 100% RH.
The mean developmental time was 9.35 days. The
hatching period was spread over 5 days and the percentage
hatch was 67.
(iv) Discussion
No hatching was observed in relative humidities
below 100% but even then the proportions hatched were always
lower than when the eggs were in direct contact with moist
surfaces as in plant tissues, or in contact with moist
surfaces. The percentage hatch of eggs exposed to saturated
atmosphere was only 67 instead of the 91.3% obtained in
contact with moist surfaces. Similarly the mean develop-
mental time was almost two days more than the mean of 7.7
days obtained in saturated conditions. • 75
These results suggest that M. sexnotatus eggs to complete their development, either absorb a certain amount of water by imbibition through direct contact with a free moist surface or from a saturated atmosphere or, that a saturated atmosphere is necessary to prevent desiccation by evaporation.
Observations made on several other occasions •did not show any substantial, noticeable increase in size of eggs. They virtually maintained the same size and shape throughout development.
In this experiment, when it was considered that no more hatching would take place, the unhatched eggs were examined under a binocular microscope and there were no signs of shrinking due to a significant loss of water.
As stated above, the eggs were not isolated from the plant tissue and therefore were not weighed and under these circumstances the water relations remain obscure.
9. Laboratory Studies on Diapause
A. Introduction
Several reviews of literature on diapause in insects have been produced in recent years.. The ecological approach is presented by Andrewartha (1952), physiological aspects are discussed by Hinton (1954, 1957), Lees (1955, 1956) and Harvey (1962) and - the relation of diapause to photoperiod by De Wilde (1962) , Danilevsky (1961) and Danilevsky et al (1970).
A new classification of dormancy in insects was proposed recently by Mansingh (1971). 76
It has been demonstrated in several instances that there are all degrees of dormancy between quiescence and true diapause. Quiescence is a type of arrested development controlled directly by external conditions and brought to an end when temperature, food or water conditions are favour- able again. This results in a continuous succession of generations so long as conditions are favourable. Growth is arrested only by the direct action of adverse circum- stances, such as low temperature, drought or lack of food. On the other hand, in a true diapause, a prolonged arrest of growth supervenes at some stage of the life history irres- pective of the environment. In the first case diapause is 'facultative' and in the second case diapause is called 'obligate'. Diapause may take place in any stage of
•ontogenesis and generally occurs in the stage of the life- cycle which is more apt to withstand the seasonal adverse conditions (Andrewartha, 1952).
The voltinism and hibernation stage of 35 species of AuchenOirhyncha breeding in acidic grasslands in Silwood Park at 51° 24,-N are listed by Waloff and Solomon (1973). With the exception of Zygina scutellaris which is multivol-
tine, all the other species are either bi or univoltine. 0 Overwintering takes place in the egg stage in 59% of the species examined, in the nymphal stage in 31% and in the adult stage in only 10%. Amongst the cicadellid species listed, 74% overwinter as eggs, 15% as nymphs and 11% as adults.
Macrosteles sexnotatus has been referred to in
• literature as overwintering in the egg stage. It is • 77
reported to be univoltine in East Finland (62°N and 66°N)
and bivoltine in Stolzenau, Germany (52°N) (Kontkanen,
1954) , and England (Waloff and Solomon, 1973) and either
bi or trivoltine in northern Italy at 45°N (01mi, 1968).
Like M. sexnotatus, M. laevis is reported by the same
authors to be univoltine in Finland and bivoltine both in
Germany and England. Jiirisoo (1964) reports M. laevis as bivoltine in Halsingland, Sweden (61° 41'N) and overwin- tering in the egg stage.
According to Raatikainen (1971) M. cristatus is
normally univoltine in Lahia, Finland at 63°N, but in warm
years it can produce two generations.
The nearctic species M. fascifrons is a typical
migratory insect and overwinters in the adult stage in the
southern states of the U.S.A. remote from its summer
breeding areas in northern U.S.A. and southern_ Canada where
it normally does not overwinter. Nevertheless, hibernation
in the egg stage takes place in Washington (Hagel and Landis,
1967).
Miller and De Lyzer (1960) suggest that depending
on the weather conditions M. fascifrons could overwinter in
the adult stage in southwestern Ontario, Canada, since
several adults of an extremely concentrated population from
a previously caged area were found in the winter in a rye
sod frozen to a depth of 7.5 cm and were able to recover
after one day at normal temperatures in the laboratory.
Nevertheless this seems to have been an exceptional occur-
rence since no adults have been found overwintering in 78 either Canada or northern U.S.A. Furthermore, Saini (1967) demonstrated that brief exposures to subzero temperatures
like -12°C and -15°C can cause a high degree of mortality
in M. fascifrons adults, and prolonged exposures to such
temperatures would kill 100% of the insects. Temperatures
of 5°C or less can affect both the activities and mobility of adults but experiments were not carried out to see
whether M. fascifrons adults would be able to remain active after acclimatization to temperatures below 10°C which are
common during spring and autumn in its summer breeding
areas. On the other hand De Long (1970) reports the occur-
rence in Alaska of leafhopper populations of the M.
fascifrons complex living on alkaline grass covered twice
daily by tidal waters at temperatures of approximately 1°C.
M. fascifrons is reported to be able to produce as
many as five generations a year in Chatham, Ontario, at
43°N, when reared on oats in cages under field conditions
(Miller and De Lyz_er, 1960). Three to four generations a
year were produced in field cages containing a mixture of
wheat and barley in Yakima, eastern Washington, U.S.A. at
47°N (bagel and Landis, 1967). Three complete generations
and a partial fourth in the field are reported from
Manitoba, Canada (Westdal et al., 1961), and Anoka County,
east central Minnesota, U.S.A. at 45°N (Meade and Peterson,
1q64).
From the ecological point of view, photoperiod is
the most reliable signal for diapause initiation since day-
length, contrary to various other factors like temperature,
humidity and composition of food vegetation, is not subject • 79
to chance fluctuations and is in itself the essential cause of seasonal climatic variation (Danilevsky et al., 1970). According to Danilevsky (1961) the ecological regulation of diapause in polyvoltine species is always influenced by photoperiod. The reaction is of the 'long-day' type in the sense that higher temperatures and longer days promote development without diapause while lower temperatures stimulate diapause initiation and cause a rise in the cri- tical day length. Therefore, the time of onset of diapause in nature and th.e generation number may vary depending on the weather conditions during the year (Danilevsky, et al.: 1970). The reactivation ('physiogenesis') is always based on the effect of the low temperatures of autumn and winter followed by the higher temperatures of spring exceeding the threshold for morphogenesis (Andrewartha, 1952; Lees, 1955).
This seems to be true in the species of Macrosteles referred to above where both latitude reflected in day-length and temperature will determine the number of generations that can develop per year.
B. Induction of diapause in the eggs
M. sexnotatus exhibits facilitative diapause in the sense that continuous generations can be bred out in the laboratory provided that temperature and photoperiod are satisfactory.
In Silwood Park, two generations take place a year,
a spring and a summer one. Second generation females lay diapausing eggs that hatch in the next spring. First genera-
• tion females oviposit during June and July and lay non- • 80
diapausing eggs.
Waloff and Solomon (1973) bred out continuous generations of M. sexnotatus under laboratory conditions at 16 light-hours day at 20°C from first generation nymphs collected in the field in May.
First to third instars of second generation nymphs were brought in from the field in mid-August and were set up on oats in a constant temperature room in a long day photoperiod of 16 hours. Most of the resulting females produced non-diapausing eggs and it was thus possible to maintain a continuous culture.
Conversely, nymphs from continuous generation cultures in the laboratory were transferred to a short-day regime.
Sixty-five first instar nymphs were set up in a stock cage culture described in part 4A. The cages were placed in a cabinet with a time-switch supplying a short
photoperiod regime of 12 hours of light alternating with 12 hour's of darkness. The cabinet was housed in a 20°C constant temperature room and was provided with a fan for 0 circulation of the it to the inside and the outside.
The resulting adults were set up in the usual way in two cages each containing 10 females and 10 males. The whole experiment was repeated two times and on both occasions the females produced diapausing eggs.
It was not attempted to establish either the duration 0. of the sensitive period or the sensitive period itself which
• 81
could have been done by setting up cages containing nymphs according to instars all of them maintained previously in a long day regime. Also, experiments were not performed to establish the threshold region of the photoperiodic reaction, or the critical photoperiod, considered as causing diapause in 50% of the individuals.
C. The hatching of diapausing eggs subjected to a period at low temperature
(i) Introduction
Diapause is a physiological mechanism for survival during an adverse period and seems to consist in the separa- tion of physiological processes some of which require a low temperature for their completion and some requiring a high temperature. Each of these two sets of processes has a temperature velocity curve of the usual type and the curves may overlap at their extreme ranges but their individual 'optima' lie several degrees apart. The first can be called the reactivation process or 'diapause development' of Andrewartha (1952) and as shown for many species of winter diapause in temperate climates the most rapid reactivation usually lies between 0° and 12°C. The second set of pro-
• cesses takes place at higher temperatures and is the resump- tion of active morphological growth and differentiation. This stage was termed 'morphogenesis' by Andrewartha (1952) and is considered as the indicator of the completion of diapause.
In species like M. sexnotatus were diapause takes place in the egg stage, development may be arrested in • 82
diverse stages of embryonic development. The stage Of
embryogenesis where diapause supervenes is notwithstanding
usually characteristic of each species.
(ii) Material and methods
A number of M. sexnotatus females and males were
taken at random from adults obtained in the weekly field
samples. These were young adults collected on the 14th of August, 1973.
These adults were set up in a stock culture cage
described in part 4A, on oat seedlings. The pot was housed in an outdoor insectary where the females remained under
the natural photoperiod. The adults were kept for two weeks
to mature, mate and adjust to the new diet of young oats.
The females were numbered and set up individually in the
tubes described in part 4C. Each female was provided with
two oat seedlings and was allowed to oviposit during four
days. On the fourth day the oat seedlings were removed
and examined. The number of eggs was recorded and the
females were dissected alive in Ringer solution to check
for the presence of mobile sperm in the genital ducts.
The eggs were set up as described in part 4B and
the pill-boxes marked with the female number were trans-
ferred to a 25°C constant temperature room at 16 light hours.
The eggs were kept in the 25°C constant temperature room for
three weeks and no hatching took place. The pill-boxes
were then transferred to a refrigerator at 4.5°C after
checking for a satisfactory moisture inside the boxes. • 83
The eggs were kept in the refrigerator for three months and were then taken back to the 25°C constant temperature room.
Observations were made at approximately the same hour each day so that hatching could be observed at 24 hour intervals.
(iii) Results and discussion
A total of 58 eggs was incubated under the above conditions and 43 hatched. The percentage hatch obtained was 74.1 but included eggs from each experimental. female.
Day zero was the day when eggs were returned to 25°C after exposure to low temperatures. Hatching spread over five days. On the eighth day 30.2% of the total hatch took place, on the ninth day 27.9%, on the tenth 23.2% and
- 9.3% both on the eleventh and twelfth days.
The average developmental time in days was 9.4 ±1.26.
By comparing the results above with those obtained
from non2diapausing eggs oviposited at 20°C and incubated at 25°C it is seen that it takes longer for diapausing eggs s to complete morphogenesis, the average developmental time in days being respectively 7.75 and-9.4 days. The percentage hatching was poorer for diapausing eggs being 74.1% as compared with 91.3% for the non-diapausing eggs. The hatch spread over five days i.e. it was one day less synchronized than in the non-diapausing eggs.
Therefore, M. sexnotatus diapausing eggs are less a successful in their incubation rates, hatchability and 84
synchronization of eclosion as compared with non-diapausing
eggs incubated at the same humidity and temperature. If
percentage hatching is taken as Percentage survival then
diapausing eggs are less viable than the eggs that are not
subjected to a state of dormancy followed by chilling and exposure to higher temperatures.
This lack of coincidence is not surprising, since normally diapausing eggs experience lower temperatures in
spring than the non-diapausing summer brood.
10. Laboratory Studies on the Rate of Development o# Nymphal Stages
A. Rearing under constant temperatures
(i) introduction
Data on duration of larval instars of Macrosteles species at constant temperatures is available only for M.
fascifrons. Miller and De Lyzex (1960) reared nymphs on oats under constant illumination at 26.6°C (80°F). Total
nymphal duration obtained was 13.2 days and duration for
instars I to V was respectively 2.7, 2.3, 2.4, 2.9 and 3.1 days.
Hagel and Landis (1967) report a total duration of
26 days and 4, 5 6 and 7 days for instars I to V when reared on barley at fluctuating temperatures between 21° to 24°C.
The analysis of the data under constant temperature has been discussed in part 8A. • 85
(ii) Material and methods
Newly emerged first instar nymphs were obtained from eggs incubated in 20°C constant temperature room.
Oat leaves from stock cultures were taken at random and selected for developing eggs already with eye-spot. These were set up in the usual way (as described in part 4B). The eggs were checked twice a day and only those nymphs that hatched between two consecutive observations were used in the experiment. For each experiment, 17 to 20 nymphs were set up.
The newly emerged nymphs were set up individually in numbered tubes as described in part 4C. Small oat seedlings were selected and washed in tap water to remove soil particles from the roots. Excess water was removed I with absorbent paper and the seedlings were placed in the rearing tube.
The food was maintained fresh and succulent through- out the experiment at all temperatures tested to standardize food quality. The seedlings were easily changed without disturbing the nymphs. Observations were. made at 24 hour
• intervals. The moult takes place while the rostrum is firmly inserted into the plant tissue. The exuvium remains in place held by the rostrum and provides a reliable check of the moult.
(iii) Results
Nymphs were successfully reared in this set up and a
• high survival was obtained at the four constant temperatures.
• 36
Data an developmental rates of M. sexnotatus nymphs
are given in Tables 6 to 10 for the individual stages and
in Tahle 11 for the total nymphal period. These tables
include the observed values from the experimental data and
the calculated values obtained from the logistic equation both for mean development in days (Y) and for average
percentage development in one day (100/Y).
Table 6: The effect of temperature on the rate of development of i instar nymphs of Macrosteles sexnotatus
Y 100/Y Mean development Average % development in days in one day Temperature Observed in °C • x ± s.e. Calculated Observed Calculated
,..,"i 7.1 ±0,377 7.09 14.08 14.08
20 4.37±0.133 4.37 22.88 22.88
25 2.86±0.09 2.86 34.96 34.96
30 2.06±0.058 2.03 48.54 49.39
Table 7: The effect of temperature on the rate of development of II instar nymphs of M. sexnotatus (observed values only) I
y 100/Y Temperature Mean development Average % development in °C in days in one day x ± s.e.
15 7.5 ±0.292 13;13
20 4.1 ±0.348 24.39
25 1.92±0.137 52.08
30 1.7571- 0.1 12 57.14 • 87
Table 3: The effect of temperature on the rate of development of III instar nymphs of Macrosteles sexnotatus
100/Y Mean development Average % development in days in one day Temperature Observed in °C x ± s.e. Calculated Observed Calculated 15 8.5 ±0.335 3.5 11.76 11.76 20 3.9 ±0.286 3.9 25.64 25.64 25 2.2310.122 2.23 44.84 44.84 30 1.63±0.125 1.63 61.58 61.58
Table 9: The effect of temperature on the rate of development of IV instar nymphs of Macrosteles sexnotatus
Y 100/Y Mean development Average % development in days in one day Temperature Observed in °C x ± s.e. Calculated Observed Calculated 15 10.82±0.563 10.82 9.24 9.24 20 5.0 ±0.378 5.0 20.0 20.0 25 2.61±0.14 2.61 38.31 38.31 30 1.87±0.155 1.63 53.47 61.40
Table 10: The effect of temperature on the rate of development of V instar nymphs of Macrosteles sexnotatus
• Y 100/Y Mean development Average % development in days in one day Temperature Observed in °C x ± s.e. Calculated Observed Calculated 15 16.71±0-474 16.71 5.98 5.98 20 6.71±0.244 6.71 14.90 14.90 25 3.54±0.183 3.54 28.25 28.25 30 2.75±0.112 2.53 39.44 36.36
• • 88
Table 11: The effect of temperature on the rate of development to final moult of nymphs of .acrosteles sexnotatus
Y 100/Y Mean development Average % development in days in one day Temperature in °C Observed Calculated Observed Calculated
15 50.63 50.65 1.975 1.974 20 24.08 24.08 4.153 4.153 25 13.16 13.16 7.598 7.598 30 10.06 8.67 9.94 11.53
The observed cumulative duration of instars at each constant temperature is given in Table 12.
Table 12: Observed cumulative duration of instars of M. sexnotatus at constant temperatures
Rearing temperatures in °C Trstal- 1 20 25 30
I 7.1 4.37 2.86 2.06 II 14.6 8.47 4.78 3.81 III 23.1 12.37 7.01 5.44 IV 33.92 17.37 9.62 7.31 V 50.63 24.08 13.16 10.06
• The logistic equation was calculated for each nymphal stage and for the total nymphal period. The logistic curves. and the observed points on the temperature time curve and the temperature velocity curve are represented in Figs. 13 to 16 for instars I, III, IV and V respectively, and for stages I to V of nymphal development in Fig. 17. The calculation could not be carried out to its end for the • 89
70 10 1
60 AV 3
50 V4 30
0
0 0 0 1 40 K a. I 3 594-011851X 100 - 100 71 z 6.1 Y = 14 3.594-011851 X 10071 1+ C T
0 PMEN O 3
VEL a DE
20
•
10 0 35 10 15 20 25 30 TEMPERATURE 9C
Fig. 13. Observed points on the temperature time curve (open circles) and the temperature velocity curve (closed circles) for development of M. sexnotatus first instar nymphs.
— 70
10 m-
k 60 0 a
50
YCn \ 0 .Q 6 r \ / 0 , \ 40 -0 / z 0 1 \ •-• ,.,; .. \ / \ / 0 a 4 w .:771315 - 0 2 02648 x i I.e 108 0 - 78 23711 r_ Y = \ / 4 77136- 0 2026411 30 o 0782378 11,1 / c) 0
r- 20
▪ 10 0 10 15 20 25 30 35
TEMPERATURE °C
Fig. 1/',. Observed points on the temperature time curve (open circles) and the temperature velocity cve (closed circles) for development of I. ,c_,17notatus third instar nymphs. • V •
80
12
10 65 IN iT 30VZ
0 50 74'
0
z fl5 z
0 0 105 8446 Y 100 5 016802 +0178 X PIA 1+0 36 0 T
0 PMEN LO E V DE
10
2
0 L. 1 15 20 TEMPERATURE
2ig 15. Observed points on the temperature time curve (open circles) and the temperature velocity curve (closed circles) for development of M. sexnotatus fifth instar nymphs. 42 20 I
36 >
rn .-1 30.?)0,
IT 24
111 5.4037 -0 22979 x 100 48 33 1+e, 7. 0 48331 • 1+e 5 4037 - 0.22979 x 18 z
T C EN n11 PM O L 12 VE E D
I _ I 0 10 15 20 25 30 35 TEMPERATURE 0C
Tic_j 16. Observed points on the temperature time curve (open circles) and the temperature velocity curve (closed circles) for development of M. sexnotatus fifth instar nymphs.
• •
14 60
12
50
< 40 13A30 N3Wd0
a I 0 4.7644 -0.1779 X 100 18.03 1 +.e Y= 4.7644 -0.1779 X 0.1803 -re
30
A. 0
a
20
10
L -L 10 15 20 25 30 35 TEMPERATURE °C
17 Obsc,i-ved points on the temperature time curve (open circles) an the temperature velocity curve (closed circles) for M. sexnota4-ns total nymphal development. • 94
second instar because negative values were obtained for K P/P. This takes place whenever Yi + v /2 > Y > Y . - 2 2 1 Y3
(iv) Discussion
The relationship between temperature and nymphal development in M. sexnotatus is well described by the logistic equation.
Apart from second instars where calculations could not be carried out it can be seen in Tables 6, 8 to 11 that there is virtually a perfect agreement between the observed values and the calculated ones from the logistic equation for the three lower temperatures tested (15°, 20° and 25°C).
Discrepancies between observed and calculated values occur only for the highest temperature (30°C) for instars I, IV and V. For the third instar, the fit is virtually perfect throughout the temperature range. The second best fit for 30°C is in the first instar nymphs, followed by that in fifth instars and lastly by fourth instars, where duration is more visibly underestimated. Duration at 30°C is underestimated, accordingly, for the pooled results of all stages. • In values of average percentage development per day (100/Y) the discrepancies at 30°C between those observed and those calculated range from as little as 0.84% for first instars to as much as 7.93% for fourth instars. For the total period 1.06% discrepancy is found and 3.08% for the fifth instar nymphs.
In the set of experimental temperatures the greatest • developmental speed was at 30°C in all the five instars. • 95
B. The 'developmental zero'
The implications of this concept have already been- discussed in part 8D for rates of egg development. The same principles apply here to nymphal development.
The five instars were treated separately: a regres- 1 sion line was fitted to the points of the inverse of each developmental period (1/Y) in days and temperature. A regression line was also fitted to the inverse of develop- mental time in days to the final moult (instars I to V) and temperature. These are illustrated in Figs. 18 to 23.
The data relevant to the regression line and also to the developmental zero point are summarized in Table 13.
Table 13: The relation between nymphal developmental time (days) and temperature.
Y = 0 'developmental Instar Regression equation Correlation zero' point Y = a + bx Coefficient in °C
I Y = 0.2184 + 0.0231x 0.996 9.458 II Y = 0.3487 + 0.0318x 0.968 10.956 III Y = 0.3994 + 0.0337x 0.998 11.84 IV Y = 0.3769 + 0.0302x 0.995 12.48 V Y = 0.2565 + 0.0209x 0.996 12.27 I-V Y = 0.0639 + 0.0055x 0.996 11.68
The values of the theoretical thresholds of nymphal development systematically increase from the first to the fourth instar. That is to say, the temperature requirements •
a NIP •
0.5
0.4
C
0.3
a 0 0.2
a
0
0.1
0.0 8 12 16 20 24 28 32
TEMPERATURE
Fig. 1:;3. Relationship between temperature and the inverse of the developmental period of M. sexnotatus first instar nymphs. • • 0-) C ›- 0 U. c) DEV E LOPMENTA L T IMEIN 0.5 Fig. 19.Relationship betweentemperatureandthe inverse 0.5 OA 0.3 0.2 00 0.1 12 of thedevelopmental periodof second instarnymphs Y —0.03182x034869 1 16 TEMPERATURE °C 20 A. 0 sexnotatus 98
0.6 r
0.5 r
3 03373 - 0.30942
0.4
-J 0.3
l . 0 —J LU
LU 0
U. 0 UI 0.2
LU
0.0
12 16 20 24 28 32
TEMPERATURE "C
Fig. 20. Relationship het:een temperature and the inverse of the developmental period of M. sexnotatus third instar nymphs. • 99
0.6 r
0
0.5
0.03019x - 0.3769-j > 0.4
0 z ME TI 0.3 .. NTAL ME OP VEL DE
U- 0 0.2 RSE E NV I
• 0.1 r 0/
0.0 —/ 12 15 20 24 28 32
TEMPERATURE "C
Fig. 2i. Relationship between temperature and the inverse of the developmental period of - . sexnotatus fou-rth .ins tar nymphs. 0 0
0.4 r
0.3 1 Y - 0.02080x - 0.25640 0 z ME TI
0.2 NTAL E PM ELO DEV F O
E 0.1 RS E V IN
0.0
12 16 20 24 28 32
TEMPERATURE '-)C
Fig. 22. Relationship between temperature and the inverse of the developmental period of M. sexnotatus fifth instar nymphs.
0.11 r
0.09 .
-- 0.005463 x - 0.063E65
ft> >- 4 0.07 t. a z
w
•-•
4 z tai 0.051.
0
w
LIS a
0 0UI = 0.03 Ui
1
I I( 0.00L—L 12 16 20 24 28 32 TEMPERATURE "C
Fig. 23. Relationship between temPerature and the inverse of the total developmental period of ii. sexnotatus nymphs (instars I to V). tit • 102
for morphological development are less stringent in the
younger stages. A t-test of the null hypothesis of no
differences between the 'developmental zero' temperatures
predicted by the linear regression equation was made. The
null-hypothesis was rejected since a difference as small as
0.21°C between fourth and fifth instars was significant at 1% level.
The results obtained are suggestive in terms of
ecological requirements since M. sexnotatus overwinters in the egg stage and hatching of first instar nymphs takes
place early in April when mean temperatures are still rather
low. Thus, the increase in minimum temperature required for
development follows the pattern of seasonal increase in
temperature.
Obviously, the threshold of development is hardly
an essential parameter for second generation nymphs for
this is the summer brood and temperatures normally exceed
the thresholds for all the instars.
11. Laboratory Studies on Fecundity
A. Fecundity under constant temperatures
(i) Introduction
Sexual maturation, rates of egg production and
oviposition rates in the female are directly affected by
environmental factors such as temperature and food
(Johansson, 1964; Norris, 1964; Nochida, 1964; Metcalfe,
1970). Fecundity can also be affected by other factors a like moisture (Hamilton, 1950) , population density (Kiritani 103
& Kimura, 1965), mating (Kiritani & Kawahara, 1963; Surtees, 1964; Rose, 1973a; Mochida, 1973) and larval nutrition (Mochida, 1964).
In the present set of experiments only the effect of constant temperature on fecundity was studied. Other factors likely to affect fecundity were optimized as far as possible and kept standardized in the four sets of temperature experiments.
Differences in total fecundity as a response to external stimuli and also resulting from individual varia- tion within a species have been reported for Auchenorrhyncha species e.g. by De Long, 1971; Metcalfe, 1970 and Mochida, 1973 .
According to De Long (1971) the total fecundity for leafhopper species does not usually exceed 200 to 300 eggs per female, the lowest records available being around 15. Stoner and Gustin (1967) report that the North American cicadellid Graminella nigrifons when kept on sweet corn under temperatures fluctuating between 21° and 24°C (70° to 75°F) laid 10 to 20 eggs over a 30 day period but the authors consider this figure as an underestimation since in nature and in undisturbed stock cultures the reproduc- tive potential is higher. Gustin and Stoner (1968) report an average fecundity of 27 eggs per female for the nearctid cicadellid Deltocephalus sonorus kept on sweetcorn at 21°C. Macrosteles fascifrons is reported to have a total fecundity of not less than 40 eggs per female when reared on barley in a greenhouse with temperature fluctuating between 18° • 104
and 22°C (Hagel & Landis, 1967). Miller and De Lyzer
(1960) report that M. fascifrons is successfully bred out
on oats, barley and rye at temperatures around 24°C and
high humidities but data on fecundity are not included.
Tay (1972) examined the fecundity of a population of
Cicadella viridis throughout a ten week period of ovi-
position. Field females were changed every week and were
kept in containers with Juncus and Holcus at out of doors
temperature. An average of 178 eggs per female was obtained
but the author suggests that this figure is below the actual
fecundity in nature due to rearing difficulties in the
laboratory. Solomon (1973) reports an average of 34.5
and 36.8 eggs per female for Psammotettix confinis reared
on Agrostis tenuis at constant temperatures of 15° and 20°C
respectively. Rose (1973a) says that Cicadulina mbila
reared on wheat seedlings at constant temperatures of 20°,
22°, 26° and 28°C has the highest fecundity at 28°C with an
average of 130 eggs per female.
(ii) Material and methods
Experiments were carried out in four constant
temperature rooms all with 16 hours/day of illumination.
The temperatures adopted were 15°, 20°, 25° and 30°C.
Females and males set up in these experiments were
obtained from the stock cultures maintained on fresh oats
in a 20°C constant temperature room, so that during the
nymphal stages all specimens were subjected to the same
diet of fresh oats. Therefore, heterogeneity in fecundity
or in pre-oviposition period resulting from a differential • nutrition during the late instars was avoided.
• 105
Fifth instar nymphs were selected from stock cultures and isolated in a cage with oats in the 20°C • constant temperature room. The cage was sorted daily at 24 hour intervals and the newly emerged adults were removed. After each daily check the pot of oats was replaced by a new one to avoid confusion between new adults and or newly k moulted specimens of previous day hidden amongst the host plant.
The newly emerged adults were sexed and the experi- ment was commenced when an adequate number of both sexes was obtained on any one day. The females were weighed individually and set up with a male in a numbered cage as described in part 4D.
The cages permanently stood on a tray with water so that the soil in the pots was moist and provided adequate humidity in the rearing chamber. Each cage was provided with two oat seedlings usually not more than 3.0 to 4.0 cm long for food and oviposition. The seedlings were changed every third day and the females were weighed individually and then returned to their respective cages.
The four sets of experiments were carried out in • conditions allowing continuous pairing. A male was replaced whenever found dead or missing.
The females were weighed and the oviposition was recorded throughout the life span of each individual.
All the experiments were started with 20 pairs of insects. Females prematurely dead during the pre-. • • 106
ovirositional period or from accidental causes were excluded from the computations.
Oviposition was assessed by examining the oat seedlings in transmitted light under a binocular microscope.
The daily egg laying was estimated as the mean number of eggs obtained from a three-day period.
Except for the 30°C experiment, the pre-oviposition period extended over the first three days of female emer- gence so that no extra daily checks were necessary to establish its duration. At 30°C many females had oviposited between the time of setting up and the first routine examina- tion. Therefore, a parallel experiment was carried out where eight females were set up in the usual way but the oat seedlings were changed daily until the first oviposition took place. After that, this experiment was discarded.
(iii) Results
The pre-oviposition period
The mean duration of this period at each constant temperature is given in Table 1A.
For the lower temperatures there is a higher vari- ability in the duration of this period while at higher temperatures both the duration and the variability are decreased. For the 25°C constant temperature experiment the first oviposition in all the females tested took place on the fourth, fifth and perhaps even on the sixth day after emergence_ A four-day pre-oviposition period was considered representative for this temperature. • 107
Table 14: The effect of temperature on the duration of the reproductive performances of Macrosteles sexnotatus females
Rearing Periods in days temperature in °C Pre-oviposition Oviposition Postlovinosition x s.e. x = s.e. x = s.e.
15 13.7 ±1.16 68.76=4.63 16.17=3.73 20 7.69=1.18 47.92=7.22 6.92=2.4 25 4.0 33.89=5.62 8.0 ±2.55 30 3.3 ±0.33 35.8 ±3.3 5.5 ±2.0
The extreme durations were as short as 2 days at 30°C and as long as 28 days at 15°C in a female whose oviposition period lasted 69 days.
The oviposition period
The mean duration of this period at each constant temperature is given in Table 14.
The results can be given either as total fecundity (total number of eggs laid) or as the rate of egg production (daily number of eggs laid during the oviposition period).
The average for total fecundity and the daily rate of egg production obtained at each constant temperature are given in Table 15.
The average fecundity was obtained by taking the arithmetic mean of the fecundity of each female for the complete oviposition period. Similarly, the average rate of egg production is the arithmetic mean of the individual • 108
Table 15: The effect of temperature on the fecundity of Macrosteles- sexnotatus.
T),eal- ncr Total Daily rate temperature fecundity egg_production in °C x ± s.e. x ± s.e.
15 121.06±12.71 1.72±0.144 20 142.38±25.09 3.04±0.46 . 25 165.89±43.77 4.31±0.68 30 188.94±29.9 5.3 ±0.85
rates obtained by dividing the total fecundity by the respective number of days in the oviposition period.
The oviposition curves for the four constant temperatures are compared in Fig. 24. Each curve represents the average daily rate per female from day one of ovi- position period to the last day..
Total fecundity and the rate of oviposition at the four constant temperatures are illustrated in Fig. 25. The ordinate is the cumulative average number of eggs per female for each third day and the abcyssa is the oviposition period in days.
• A linear regression equation of the form of
Y = 52.35 4.543X was fitted to the points of total fecundity and temperature. The correlation coefficient obtained was 0.9998 and the t- test rejected the null hypothesis at 0.001 level (Fig. 26).
A linear regression equation of the form of
• Y = 1.812 4- 0.2402X
tr, • • •
1 0
0 rl I
6 ti r t I rf I I 9t I, i 0t 0-.Q 1/ / I ' r I 0 % , r / , 6 t . 1 e I /. iI I 0 , ■ I \ I 1 \" I I 01 0' I I o a 0.... I \ 1 1 / , " 0 I % x / e I \ I / ‘..b---IDI ii t e '17:1. I % 1 e° - ' "'a-.v., P1 ___04...... <)_0/1-■ s 0 I I I \ ' I I I ./.5,. f?- - fid . / 1 59 \ /0 0t ,0' 0 _ --0,
100 0 20 40 60 80
OVIPOSITION PERIOD (DAYS) cJ 24, Oviposition curves of U. sexnotatus at different constant temperatures (o---o 15°C; (11- -- 20°C; o•--o 25°C; a--A-J, 30°C) • • • •
CUMULATIVE N.E Ii] C) L.C:4 0 C C) 0 0 ROTI ▪ orgratmemisilsomsnirmrmiwner.lainwer. onomplaws....17cokr.o.AMMTommausower....avAarmlornerronairaelamnsanns...... orwaissw mar. t•.) Ui
O 0 C) • H; H • e'r; -0- o...„ 00 , ... • G1 .".7). 0- ... CD 0 4'ot o 1,-:S ct 0 P. - --"- a- ::------.,,..„ 0 - - -
0 0 O 0 iA 0,
d . ' ----,, U) *0, 0, • c1 ct ..„,,...„ CD CD ISO N.) I
UI (-t- 0 OI o hi •0 \ ... (-t• N C) i. /. ** \ • to CD CD ft 0 (A) tct- Q CD o u) '0• ---- in • --. •0 00
CD CD }-1 Ui0
CD • 111
200 -
180 L
Y = 52.35 4.543 x
-J N 0
140 -
120 -
12 16 20 24 23 32 TEMPERATURE °C
Fig, 26. Relationship. between temperature and total fecundity of M. sexnotatus • 112
was fitted to the points representing rate of egg production and temperature. The correlation coefficient was 0.9979 and. the t-test rejected the null hypothesis at 0.01 level (Fig, 27).
The lowest individual fecundity obtained was that of a female at 15°C where only 18 eggs were laid over a period of 33 days, total life span of the female being 84 days. This female was dissected at death and both ovaries were senescent.
The lowest rate of egg production was also observed at 15°C. The female laid at a rate of only 0.5 eggs per day over a 69 day period, in its total life span of 108 days.
The highest individual fecundity was observed at 30°C. A female oviposited 415 eggs over a 35 day ovi- position period in a total life span of 39 days. This female was dissected at death an eight oocytes with fully formed chorion were found in its ovaries. Therefore, the total egg production of this female was as high as 423 eggs.
The next highest individual fecundity was observed at 25°C where a female oviposited 392 eggs over a 52 day period in a total life span of 68 days. At 20°C the highest individual fecundity was 283 eggs in 83 days in a total life span of 93 days. The maximum observed at 15°C was 216 eggs oviposited during 105 days in a total life sPan of 126 days. This female was dissected at death and 12 fully developed oocytes with chorion were found in its ovaries which gives a total of 138 eggs produced,
0 1:L3
6.0
5.0
4.0 TION
POSI VI O
0 3.0
CL
2.0
0.-
0 32 12 16 20 24 26 TEMPERATURE ,C
Fig. 27. Relationship between temperature and rate of egg' production of M. sexnotatus. • 114
The highest daily rate of egg production was also
observed at 30°C. A female laid an average of 13.2 eggs
per day over a 27 day period. It failed to oviposit during its last two days of life and when dissected after death
10 fully mature oocytes with chorion were found in its
ovaries making a total of 366 eggs produced in a life span ) of 30 days. The next highest daily rate of egg production
was observed at 25°C where a female oviposited an average of 7.5 eggs per day over a 52 day oviposition period. At
20°C the highest daily rate of egg production was 6.8 eggs over a 32 day period. At 15°C the maximum rate was 2.5
eggs per day over a period of 54 days. This female failed
to oviposit in its last two days of life and dissection
after death showed four fully mature oocytes with chorion
making a total of 139 eggs produced in a life span of 69
days.
The post-oviposition period
The mean .duration of this period at each constant
temperature is given in Table 14.
This post reproductive stage was the most variable
period in the life span of a female. At all the tempera-
tures tested some females did not have a post-oviposition
period while others ceased to oviposit for a variable number
of days before death.
The longest post-oviposition period was seen at
15°C. A female with a life span of 123 days, did not
oviposit the last 53, following an oviposition period
0 that lasted 60 days. The female was dissected at death and • 115
had four fully mature oocytes with chorion, three in one ovary and one in the other. The remaining eight ovariolcs were senescent.
At 15°C, out of the 17 females whose life was observed from final moult to death, only one female- did
• not have a post reproductive period. Therefore, at 15°C this stage ranged from 53 days to zero, with a mean of 16.17 days.
As a contrast, at 30°C eleven females had a con- siderable post-oviposition period, whereas seven continued to lay until the last day of their- lives.
(iv) Discussion
The pre-oviposition period
This period corresponds with the stage of maturation of the reproductive organs and precedes oviposition.
The duration of this period is inversely affected by temperature.
A higher variability in its duration occurs at the lower temperature tested, while at the higher ones the 0 individual females present a more homogeneous response to the temperature stimulus. The results from the four constant temperatures tested suggest that a synchronization of response takes place at the most 'favourable' range of temperature while in the lower range of less favourable temperature conditions the response will be more hetero- geneous. 'Favourable' is here defined as temperature of maximum fecundity. • 116
The oviposition period
As said, fecundity is directly affected by tempera-
ture. The influence of temperature is reflected both in
terms of total fecundity and daily rate of egg production.
There is a gradient in the average total fecundity and
average egg rate between the four constant temperatures
tested.
This result indicates that the four experiments
were made within a range of temperatures favourable for
oviposition where the average response to the temperature
stimulus is linear. Fecundity and egg production within
this range are proportional to temperature.
Fecundity iS still high at the lowest temperature tested and the oviposition period extends over a long period.
The average egg rate is near to two eggs per day. Still
lower temperatures would tend to decrease this rate to a
lower limit where the rate would be virtually zero.
The same would be true for the other extreme of the
temperature scale.
The highest fecundity and also the rate of egg
production was attained at the highest temperature tested.
Therefore, the optimum might be very close to 33°C or might
be at an even higher temperature, above which deleterious
effects would supervene and the rate of egg production
would decrease accordingly.
The oviposition curves in Fig. 24 illustrate the
'contraction' of the oviposition period at 25° and 30°C and the
compensatory higher daily egg rate. • 117
The cumulative oviposition curves (shown in Fig. 25) for the four constant temperatures do not show any significant diversion in their directions such as would occur if the daily oviposition rate fluctuated strongly. Furthermore, the oviposition period of each individual female was divided arbitrarily into four sections and the egg rate was calcu- lated for each section. A t-test was carried out on the values so obtained for each section within the same tempera- ture and the differences observed were not significantly different. The oviposition curves illustrated in Fig. 24 also show that there is no marked peak in this activity at any of the temperatures tested and a moderate decline in the last part of the oviposition period takes place when females have achieved old age.
The oviposition curve is particularly flattened down and smoothed at 15°C, i.e. at the lowest temperature tested. The oviposition period at 15° is almost twice as long as that at 30°C. The total fecundity, however, is only.64% of that achieved at 30°C_and, accordingly, the egg rate is only 32% of that at 30°C. At higher temperatures, oviposition is concentrated in a shorter active period and the high fecun-
t dity achieved is a result of a much higher rate of egg production. The oviposition period at 20° is 25% longer than that of 30°C but the total fecundity is no more than 75% of that at 30°C. Similarly, the egg rate is only 57% of that at 30°C. The duration of the oviposition period is virtually the same in females at 25° and 30°C. However, at 25° the total fecundity is only 88% of that attained at 30° and the rate of egg production is 20% less than that of r females at 30°C. • 118
The post-oviposition period
The effect of contant temperatures on the post- oviposition period is more clearly manifested at the extreme of the temperature range tested. Lower temperatures increase the length of the post-oviposition period while at higher temperatures it may be supressed altogether.
post reproductive period does not seem to be an obligatory feature in M. sexnotatus females. Furthermore, the length of the post-oviposition period is not correlated. with that of the oviposition. Females with long periods of oviposition may have long, short or no post-oviposition period. The same is true of females with a short period of oviposition. Mochida (1964) also found that in rather long lived normal females of the delphacid Nilaparvata • lugens the duration of the pre-oviposition and of the post- oviposition periods were not closely related to each other.
B. Fecundity and weight changes of the females
The weight changes of a set of females was recorded every third day from day of adult moult to death.
The weight changes of the females were followed at • each of the four sets of constant temperatures used in the fecundity experiments.
At emergence all the females had nearly the same weight. The mean and standard error of the weight in milligrames of the females in each experiment at constant
temperatures was: at 15°C: 1.69±0.029; at 30°C, 1.7±0.038;
• at 25°C, 1.66±0.03 and at 30°C, 1.6±0038. • 119
The weight changes of the females from emergence to death are illustrated in Fig. 28. The day of emergence was regarded as day .zero of female life and within 24 hours of emergence the individual females were weighed, set up in cages as described in 11A(ii) and transferred to a constant temperature room where its life was followed to death.
The pattern of weight changes is not the same in all the constant temperatures as is illustrated in Fig. 23. At 20°C there is the initial increase in weight basically resulting from the maturation of the ovaries. This is followed by a•peak in weight in the first quarter of the female life and a moderate decline afterwards. The weight thereafter oscilating around 2.2 mg to 2.3 mg and rising slightly again between 100 and 120 days after emergence. At 15°C and also at 25°C and 30°C constant temperatures the weight progressively increases with age.
The mean weight with its standard error for the female before the first eggs are laid, its maximum weight attained throughout its life and the last weight recorded before its death are given in Table 16. The last column of this table gives the percentage increase in weight • attained between emergence and the observed maximum.
The adult weight changes in females of M. sexnotatus are basically the result of maturation of ovaries. Both the fat body and the accessory glands do not change during the adult life as much as to cause the fluctuations observed in the female body weight. The variations in weight are mainly resultant from the number of developed eggs in the
a • •
3.5
3.0
I t
z /
25 /4 P'I 0 Is 0 d 0 1 9 , \ / 1 0 d ,0 , / V i 0 ea / ` 0.0 .. , 0-(1 ,- 1 I 1 , .... , )::,.. .,0 ' ts 0 ■ , % V 6 2.0
...... a...1 agnamsawirsalsoo■megoassamomeamenrmjenaasuom.r.dayampancs am*"... 1.5 —_—..J_ -ansolomm 120 150 0 30 60 90 DAYS AFTER EMERGENCE rig. 28. The weight changes of M. sexnotatus females at different constant temperatures (o---o 15°C; o--o 20°C; o--o 25°C; 30°C) 1?'
Table 16: The effect of temperature on the weight changes (in mg) of Macrosteles sexnotatus females.
Average of % increase Weight Maximum between Rearing prior to weight Weight prior emergence temperature oviposition attained to death and maximum in °C x ± s.e. x. ± s.e. x ± s.e. x ± s.e.
15 2.1210.060 2.79±0.082 2.67±0.101 65.1 ±3.92 20 2.31±0.077 2.82±0.062 2.42±0.077 65.09±3.66 25 2.03±0.053 2.8810.153 2.7810.192 75.95±7.63 30 1.83±0.065 2.7310.109 3.3 ±0.33 76.0315.95
female reproductive organs. Development of musculature and cuticular thickening must also contribute to this increase.
Mature oceytes do not normally accumulate in the pedicels or in the lateral oviducts to be oviposited at intervals. Oviposition is regular, eggs being laid daily during this period. This rhythm was also observed in the egg experiments described in part 8.
The daily rate of egg production is thus fairly regular throughout the oviposition period except in the very old females. Similarly, So is the daily rate of egg laying.
The weight of a female fluctuates with the rate of egg development and the rate of oviposition. Whenever the rate of oviposition lags behind that of egg development, eggs accumulate and the weight of the female increases.
These two rates seem to be at equilibrium in the females reared at 20°C. For the other three temperatures • tested there is a progressive increase in body weight • 122
towards senility. This is specially pronounced at 25° and
30°C. The maximumindividual percentage increase in body
weigh was 94.4% at 15', 30.0% at 20°, 112,5% at 25° and
as =oh as 123.6% at 30°C.
These results suggest that in ageing females
specially where the metabolic rates are depressed or
accelerated by temperature, the rate of egg layingwill be lower than that of egg production.
C. Ageing effects
As seen in 11B above, differences in the egg pro-
duction rate and egg laying rate increase with age in M.
sexnotatus females so that eggs may accumulate in the
reproductive organs of old individuals.
This accumulation of eggs in the female body has
also been found by Mochida (1964, 1970) for Nilaparvata
lugens where females which present an approximately constant
rate of egg laying ceased oviposition several days before
dying. The same is reported for both macropterous and
brachypterous forms of Javesella pellucida by Mochida
(1973). May (1971) reports that some Stenocranus minutus
• females accumulated mature oocytes in old age without ovi-
positing them, though S. minutus usually shows a post-
oviposition period accompanied by senescence of .ovaries.
Tay (1972) reports that females of Cicadella viridis under
field conditions during the last four weeks of the ovi-
position period show an increase in weight and a decrease
in oviposition 7-ate, The failure to oviposit was also
• observed in young females by Johansson (1958) where two • 123
females of Oneopeitus fasciatus died with their abdomen
completely filled with mature eggs. The large number of
eggs blocked the intestine of these females and according
to Johansson death was probably caused by starvation.
Johansson (1958) artificially prevented oviposition in
normal females by blocking the ovipositor with paraffin
wax and egg production proceeded irrespective of the
mechanical impossibility of laying the mature eggs. In
some of the females the ovipositor was forced open by the
inner pressure while in others some of the mature eggs
had ruptured the oviducts and were found in the abdominal hoemocoele.
The reproductive organs of M. sexnotatus female do
not normally show a post-reproductive change where ovaries
become senescent. Similarly degenerative processes like
resorption of oocytes do not take place, at least under
conditions of adequate food supply. Starvation experiments
were not carried out.
Nearly all the females in these experiments carried
out at the extremes of the temperatures tested (15° and
30°C) were dissected at death. At the 15°C, out of 17 0 females dissected at death only two had thread--like senes-
cent ovarioles. At 30°C only one of the 16 females dissected
at death had senescent ovarioles. The number of fully
mature oocytes with chorion in the female reproductive organs
at death was also recorded. These figures are summarized in
Table 17. At 15°C, 76.5% of the females had mature oocytes
with chorion at death, while at 30°C the percentage was
as high as 94. The maximum number ever observed was 37 • 124
Table 17: The number of mature oocytes with chorion in the abdomens of ilacrosteles sexnotatus dissected at death.
Number of mature oocytes with chorion Total Rearing number of temperature 11- 16- 21- 26- 31- 36- females in °C 0 1-5 6-10 15 20 25 30 35 40 dissected
15 4 2 4 4 2 - 1 17
30 1 3 6 2 2 1 - 1 16
fully mature occytes with chorion in a single female at 30°C.
The abdomen was greatly distended and it is likely that the
eggs had ruptured the oviduct walls and arranged themselves
in the abdominal hoemocoele. This female had already ovi-
posited 20 eggs over a nine day period and had not ovi-
posited during the five last days of its life.
Old females specially those with greatly distended
abdomens move very slowly and awkwardly.
Nochida (1973) found that both brachypterous and
macropterous females of Javesella pellucida sometimes
deposited eggs outside the plant tissue at one to four days
before death. • In order to check whether old females laid eggs
outside the plant tissue a black paper disc with a slit
only large enough to let the oat seedlings through was
placed on top of the soil to give a contrasting background
for.any egg deposited out of the plant tissue.
Two of the longest lived females at 15°C constant
• temperature were observed. One female had not oviposited 125
in the plant tissue for 38 days after an 87 day ovipositing period with 165 eggs laid. It had stopped ovipositing at day 102 after emergence. The black paper disc was placed in its cage when the female was 120 days old. The cage was checked three days later and three eggs were found respectively at 3.0 cm, 1.5 cm and 1.0 cm from the food plant. From days 123 to 126 seven eggs were deposited outside the food plant. Four of the eggs were 0.3 cm away from the food plant, one egg 0.5 cm, one egg 1.0 cm and one egg 3.0 cm away. The last oviposition took place between 126 and 129 days when 6 eggs were deposited on the black paper disc. Four eggs were 3.5 cm away from the food plant, one egg at 3.0 cm and another one 2.5 cm away. The female died when 138 days old. Between 129 days and 138 days no eggs were laid. At death, this female had 12 eggs with chorion in its reproductive organs plus one egg between its gonapophysis.
The second female examined was already 120 days old when oviposition in the plant tissue ceased and the black paper disc was then placed in its cage. Two eggs were deposited on the paper at 1.0 cm and 1.5 cm away from the oat seedlings between 120 days and 123 days. This female died two days later without further oviposition and its reproductive organs contained12 fully mature occytes with chorion.
Since there was no disturbance this suggests that a senescent female may release eggs not in direct contact with the host plant. Perhaps this might take place more
• 196
frequently than expected but eggs so deposited are diffi-
cult to detect among soil particles.
12. Oviposition Sites
A. On oat seedlings under laboratory conditions
The data were obtained from laboratory experiments
on fecundity where females were confined with oat seedlings for oviposition.
The seedlings were removed every third day and ovi-
position sites and egg numbers were recorded. The seedlings
were usually not more than 3 cm to 4 cm long at the beginning
and in this three day period they did not exceed 10 to 11
cm in length at any temperature adopted.
The seedlings were examined under the binocular
microscope with transmitted light. The leaf sheath was
opened longitudinally to facilitate the observation of the
eggs.
A 12 cm long ruler was placed parallel to the seed-
lings. The length of each individual plant was measured
and recorded and the position of the eggs was read off from
• the adjacent ruler at each 0.5 cm interval.
Ground level was considered as the starting point
or zero cm. A record was taken of the oviposition sites
in relation to the height on the plant, whether in the leaf
sheath of in leaf blade, whether isolated or in cluster,
also the number of eggs constituting each cluster. Eggs
were regarded as oviposited in a cluster when the distance • 127
between two adjacent eggs was approximately equal to the width of an egg. Otherwise, eggs were considered as laid singly.
Attempts at oviposition were also recorded. These could easily be identified in the leaf blade as charac- • teristic longitudinal slits. The nature of the plant tissue, in the leaf sheath prevented a reliable identification of such attempts. Therefore data only for the leaf blade is available.
Eggs were laid along the longitudinal axis both of the leaf sheath and leaf blade beneath the plant epidermis. Each slit made corresponded to a single egg. The eggs in a cluster were disposed either irregularly or as a somewhat diagonal row. • Hagel and Landis (1967) report that on barley, oats and sweet corn, M. fascifrons eggs were deposited between the epidermis and the mesophyll, singly or in rows of as many as four eggs along the leaf margin. Eggs were also inserted in stems at or between the stem nodes with the anterior end pointed towards the top of the plant. On aster, horseweed and carrot the eggs were laid in the petioles and branches. If isolated on potato plants, M. fascifrons laid eggs in the leaf petioles.
Fig. 29 illustrates the selection of oviposition sites by M. sexnotatus females on oat seedlings. The data is based on 1052 oat seedlings containing 1750 eggs ovi- posited by 17 females over the complete oviposition period
• at 15°C. The length of the oat plant is that of the day • •
11.0
10.0
9.0
•
80
•
• • 7.0 • •
•
6.0
•
• 5.0 •
• • • • • • • • 4.0 0/0 0vipOsition • • • 1 • • • • • 0 -5 11! • • • I I I I 3.0 6 -10 .. . •• • 11-13 . . 2.0 I • • ! 16-20 . . • . . • 21-25 • • • 10 26-30
31-40 LJ
CL) Fig. 29. Oviposition sites in oat seedlings hy sexnotatus at 15°C constant temperature (height preferences represented as a percentage o: the total oviposition for each category of plant length). 3.29
when it was removed from the cages and when the number of eggs and egg sites were recorded. The figure does not discriminate between single eggs and egg clusters. The height preference is represented as a percentage of the total oviposition for each category of plant length.
The bulk of oviposition takes place in the basal part of the oat seedling, corresponding to the leaf sheath. In nearly all cases the highest percentage of oviposition took place between 0.6 and 1.0 cm height. Less eggs were laid on the leaf blade and a higher percentage was concen-
trated near the - extremity of the blade in nearly all the categories.
Fig. 30 illustrates the relative frequency of the different sizes of egg groups oviposited at 15°C. The frequency of the different sizes is similar both in the leaf sheath and leaf blade. In both, there is a prepon- derance of two-egg clusters. In the leaf sheath there were clusters with six to seven eggs, while in the blade the highest number per cluster was five.
The oviposition sites were also analysed at 30°C. In this instance the average rate of egg deposition was three times as great as at 15°C and on the same plant surface there was a higher number of eggs deposited close to each other so as to form clusters. A total of 3705 eggs oviposited by 18 females over the whole of the period was analysed. There was a higher percentage of incidences of egg clusters both in the leaf sheath and in the leaf blade, specially in the preferred area which is the leaf sheath. 130 •
80
0 2 3 4 5 6 • No OF EGGS/ EGG GROUP
Fig. 30. Sizes and frequencies of egg groups oviposited in oat seedlings by M. sexnotatus at 15°C constant temperature. Open and barred columns represent eggs laid in leaf sheath and leaf blade respectively.
• 0 131
There the high rate of oviposition resulted in 67.6% of
eggs in clusters to 32.4% in isolation. In the blade eggs
laid singly were still in higher proportion to those in
clusters, their percentages respectively being 53.2 and 46.8.
Attempts at oviposition in the leaf blade formed
only 2.4% of the total of incisions. At 15°C the attempts
represented 6.7% of the total number of incisions.
The size of the egg clusters at 15°C ranged from two
to seven per cluster. At 30°C the size ranged from two to
21 eggs. Again, the higher incidence was that of two eggs
per cluster. Occurrence of clusters with more than nine
eggs was always lower than 2%. Clusters with two, three and
four eggs accounted for 71% of the total in the leaf sheath
and as much as 90% in the leaf blade. In the leaf blade
the number of eggs per cluster did not exceed eight and
its frequency was as low as 0.7%.
B. On field oats in an outdoor insectary
(i) Material and methods
Complete oat plants were brought in from the oat-
field to assess oviposition sites by field collected M.
sexnotatus.
A few such plants were planted in an adaptation of a
Watkins and Doncaster cage. The metal base of the cage was
7.5 cm high and 21.5 cm in diameter. It contained soil and
had a cellulose acetate cylinder adjusted to its distal end.
The cylinder was 60 cm high and was ventilated by two 12 c1:1
• 132 per i2 cm opposing windows of muslin. The top had an opening in the centre closed by a cork.
Four oat plants were planted in the cage; they were selected in the field not to exceed 60 cm in height.
The cage was housed in an outdoor insectary where it was exposed to the fluctuation of the outdoor air tempera- ture and to natural day length.
The females were obtained from the experimental oatfield. The adults were examined in the laboratory and injured specimens were discarded.
The experiment was carried out twice. In the first instance nine females and seven males were isolated in a cage with four oat plants. The adults were undisturbed from the 15th to the 30th of June when the cage was opened to assess the oviposition sites.
The experiment was repeated from the 30th of June to the 13th of July. Seven females and two males were isolated in a cage with three oat plants.
Each plant was measured and subdivided into sections at 3.0 cm intervals, Ground level was taken as zero cm.
The leaves were numbered as one onwards from the base to the top. The height of the individual leaf was that of the corresponding ligula. The length of the leaf was not taken into consideration. The leaves were classified as fresh, wilting and dry. 133
(ii) Results
In both experiments oviposition took place in the leaf sheath as well as in the leaf blade.
No significant difference was found between ovi- position sites in the leaf sheath and ligula and in the leaf blade (P < 0.2). This was true for each experiment individually and for both taken together. The analysis was based on a total of 568 eggs of which 283 and 285 were deposited respectively in the leaf sheath and ligula and in the leaf blade.
Oviposition in the leaf sheath was recorded for every 5.0 cm interval but since only seven cat plants were used in the experiment and only 283 eggs were laid, the oviposition sites were gropped for the basal half of the plant and for the distal half. As each plant was measured individually, the total number of eggs deposited in the basal 50% and in the apical 50% of the plants was easily calculated. The figures were respectively 51.24% (145 eggs) and 48.76% (138 eggs).
Oviposition in the leaf blade took place at all the leaf heights from those close to the ground level up to those originated at a ligula level 57 cm above the soil.
Twenty nine leaves were available for oviposition of which eight were considered as fresh, 12 as wilting and nine as dry. A total of 285 eggs were oviposited in the 29 leaves: 101 eggs in the eight fresh leaves, 143 eggs in the 12 wilting leaves and 41 eggs in the nine dry leaves. • 134
was made for the hypothesis that each egg had
the same probability of being laid in any of the leaf
categories and the hypothesis was rejected at 0.001 2 probability level with a y, of 37.15 for two degrees of freedom.
The hypothesis of equal probability of eggs being
laid on wilting and fresh leaves was tested by the normal approximation of the binomial distribution. The null
hypothesis was accepted since P > 0.10.
Evidence of differential preferences in'oviposition was obtained for old oats in an experiment with caged
females under field conditions. The material and methods
are described in section II, under parasitism in the egg
stage. The experiment was carried out from the 7th to 11th ik of July, 1973 on fully grown, three months old oats.
Females were offered to oviposit on a 30 cm long segment
of an oatplant containing two consecutive leaves. There
were two females per cage and 20 replicates. Oviposition
took place under field conditions during four consecutive
days immediately after the estimated oviposition peak in
the field. The segment of each oat plant enclosed by a
cage was examined in the laboratory under the binocular
microscope and the number of eggs in each cage was recorded
separately for the leaf blades and for the leaf sheath.
A total of 670 eggs were obtained: 601 eggs were oviposited
in the leaf blades and 69 in the leaf sheaths. Therefore
only 10.3% of the eggs were oviposited in the leaf sheath.
Furthermore, these eggs were usually inserted in the softer • 1:35
sheath tissue close to the ligula. In six of the 20 cages, all eggs were oviposited exclusively in the leaf blade.
(iii) Discussion
M. sexnotatus females at least under this set of experimental conditions lay eggs as frequently in the leaf sheath and ligula as in the leaf blade. The oat plants used for oviposition were selected by height not to exceed 60 cm excluding the panicle where no oviposition takes place. The experiment took place when the oat plants were not more than 60 days old.
More mature, older oats with more rigid leaf sheath provide oviposition sites favouring the leaf blades.
Soft-stalked oats were more favourable for ovi- position than hard stalked ones for Javesella pellucida (Raatikainen & Tinnila., 1959). Raatikainen (1967) found that J. pellucida oviposited mainly in the oat leaf sheath and occasionally in the leaf blade and that smaller egg groups were found in thick walled stems and larger egg groups in thin walled ones. On the other hand the del- phacid - Nilaparvata lugens oviposited indiscriminately both
• in hard parts and soft parts of the rice plants (Mochida, 1964).
No significant difference was found in the selection of oviposition between fresh oat leaves and those drying up, whereas dry leaves were significantly less favoured.
The number of functional green leaves decreases as a cereal plant approaches maturity. After a leaf is fully • 136
expanded it exports assimilates to the younger distal leaves which besides receiving these supplies retain all their assimilates. The rate of assimilation of the fully expanded leaf declines progressively, followed by senes- cence and death (Langer, 1972).
Since oviposition took place in both experiments for a period of 14 days the wilting of the leaves might have been accentuated over this period. Therefore, leaves considered as dry when eggs were counted might have been still acceptable for oviposition when the experiment was set up. The same could be true for wilting leaves.
Oviposition took place in nearly equal proportions both in the basal half of the oat plant and in the distal half. A much higher number of plants needs to be tested to assess a real trend in any gradation of oviposition according to height. In the present set up it was demon- strated at least that no particular height is systematically avoided for oviposition.
13. Longevity of the Adults
Data on longevity was obtained from the fecundity experiments where the survival of the adults was followed from day of emergence to death.
Therefore, data is available for comparison from experiments carried out at four constant temperatures. For experiments at 15°, 25° and 30°C longevity was followed for both males and females while at 20°C only the female survival was recorded. 0 • 137
The rearing methods are described in part 11A(ii). The food plant was changed at a week or 10 day interval for the long lived males that remained alone in a cage after the death of the female.
Life span can be defined in terms of extreme or maximum longevity recorded for one of the individuals and also by the average or mean life span of the population from following the day-to-day progress of death (Rockstein & Miguel, 1973).
Table 18 summarizes the data on longevity. The mean and its standard error for the life span in days of males and females in relation to rearing temperature is presented.
• Table 18: Longevity of Macrosteles sexnotatus adults under different constant temperatures
Rearing temperature Females Males in °C x i s.e. x ± s.e.
15 98.65±12.71 110.63±12.8 20 63.0 ±9.93 not followed 25 45.8 ±7.66 39.64±9.36 30 43.7 ±3.95 41.47±6.7
Fig. 31 illustrates the survivorship curve for females and males at 15° and 30° and for females only at 20°C. The curve is based on 17 females and 16 males for
15°, 18 females and 19 males for 30° and 13 females for the 20°C constant temperatures. The curve for the 25°C M • L VA RVI SU
‘0
'0\
■■.* 200 0 20 40 60 80 100 120 140 160 180 DAYS AFTER EMERGENCE ;-' Fig. 31. Survivorship curve of M. sexnotatus adults at different constant temperatures. cu (o) 15°C, (A) 20°C, (0) 30°C, (--) , (---)d . • 139
was not included because almost 50% of the individuals were lost accidentally on a single day during the experiment and the remaining specimens were only nine females and 11 males.
A large variability around a mean value was found for the duration of the life span.
The curve for 15°C is similar to the 'intermediate curve' of Pearl's survivorship curves (Rockstein & Miguel, 1973) where the death rate is slow in the young population, gradually increases during the middle part of life and slows down at the end and the last few individuals tend to attain a life span which is about twice as long as the mean. At 15°C one male lived almost twice as long as the average and two others exceeded the mean by 71% and 66%.
Death may occur at any time during adult life in both sexes at the higher temperature. The curve for the 30°C is similar to the 'diagonal type' of Pearl's survivor- ship curves specially for the males where the rate of death is constant for the greater part of time. The longest lived male deviated by +126.5% of the average longevity and the next longest lived deviated by 100%.
The 'intermediate type' curve of Pearl is said to represent the situation in which populations die of senes- cence while the 'diagonal curve' is said not to involve senescence per se and may represent a situation where a specific adverse environmental factor may be acting through- out the adult life of the population (Rockstein & Niguel, 1973). • 140
The maximum longevity observed was that of a male
which lived for 195 days at 15°C constant temperature.
Also at this temperature there were males with life spans
of 139, 177, 183 and 188 days all these values exceeding
those of the females at the same temperature. At 25°C
there were two males that lived for 101 days. Both males
spent a long time in isolation. One was isolated for 53 days and the other for 29 days.
Also at 30°C there were seven males which lived
longer than the longest lived females. The maximum male
longevity at this temperature was 94 followed by 83 days,
two with 77 days and others with 74, 72 and 70 days. The maximum longevity achieved by a female was 138 days at
15°C. At 20°C there were two very long lived females
which reached 123 and 126 days of life. At 25°C the maximum
female longevity was 68 days and at 30°C it was 66.
No significant difference was found between female
and male longevity at 25°C (0.1 < P < 0.2) and also at
30°C (0.1 < P < 0.2). However a significant difference
was found at 15°C (0.001 < P < 0.005).
The females were subjected to a potentially more
traumatic handling than the males by confining each
individual in a small tube every third day for weighing.
On the other hand, the males were not weighed. Besides,
some of the males survived their paired female and there-
after the cage was handled only once a week or once per 10
days. This might be the reason for the greater longevity
of a few males in all temperatures tested. This is specially • 141
noticeable at 15°C where even the mean value for longevity was higher than that of the females.
More conclusive results as to the female and male longevity of M. sexnotatus could be obtained by having a large number of replicates of both sexes at the same treatment throughout the lives.
Longevity of M. fascifrons is reported by Miller and De Lyzer (1960) for 26.6°C (80°F) as 42.2 days for females and 29.6 days for males.
Rockstein and Miguel (1973) reviewed the literature on differential longevities in relation to sex and conclu- ded that duration of life cannot be explained in terms of the current hypothesis on sex-related longevity based on differences in chromosome number o8 in differences in rates of metabolism.
Significant differences were found in the comparison of longevities of females from 15° and 20°C (P < 0.001) and also between females reared at 20° and 25°C (P < 0.001). No significant differences were found between females reared at 25° and 30°C (0.2 < P < 0.4) and also between males reared • at 25° and 30°C (P > 0.5).
At 30° almost twice as many observations on longevity were made than at 25°C. Perhaps with a higher number of individuals tested at 25°C significant differences would arise.
The interpretations of the influence of temperature on life span of insects is still under discussion (Rockstein 142
& Miguel, 1973). Some authors (Clarke & Maynard-Smith,
1961; Hollingsorth, 1968) propose a 'threshold theory' of ageing where ageing is considered as temperature inde- pendent and dying as temperature dependent. Ageing proceeds until the 'vitality' of an individual has fallen to a threshold level below which the process of dying begins. There is an increased threshold level at higher temperatures so that individuals able to maintain them- selves at a low temperature may be unable to do so at some higher temperature if they have passed the threshold characteristic of that temperature.
However, Sacher (1967) suggests that the processes involved in ageing in relation to temperature are rather more complex and suggests that the mortality rate curve is positively temperature dependent and that anomalous physiological processes and ageing result as temperature is displaced from an optimum value.
• 143
SECTION II
POPULATION STUDIES
1. The Study Areas
A. Natural grassland
The site chosen was a quadrangular patch of grass surrounded by an extensive 10 year old lawn on the areas known as Silwood Bottom and Drive Hill, in Silwood Park, Imperial College Field Station, Ascot, Berkshire. This patch of grass was cut by hand twice a year because of the divergent supporting metal cables of the 13 m high suction trap that stands there.
The suction trap tower with a rectangular base of 1.23 x 0.92 m occupied the centre of the quadrate of 14.0 x 13.20 x 13.60 x 13.80 m of sides covering an area 2 of 187.6 m . Early in May 1972 the area was extended as to form a quadrate of25, m of sides. The plot was divided 2 across into eight sub-areas of 78 m . The total area was 2 then 624 m2 with the inclusion of 436.4 m of cut grass. Each sub-area was regarded independently as a and b corres-
• ponding respectively to the inner long grass and the external mown grass thereafter preserved uncut.
A quantitative survey of the area was carried out in late July of 1972 when the external area was sufficiently grown to permit identification of the plant species. Samples of the vegetation were recorded by analyzing the content delineated by a wire quadrate frame of side 30 cm.
Four random samples were recorded for each sub-area, two • 144
on the old grass patch and two on the newly extended area.
The average vegetation height as well as the maximum height
was recorded for each quadrate. Every plant species within the wire quadrate was recorded and also the percentage area
covered by bare ground, stone, dead leaves and mosses.
The plant species were classified as dominant and
co-dominant. In the first case it covered more than 50% of the sample area and in the second case two species covered
each approximately 30% of the sample area. The estimation
of percentage area was made easier by the use of an addi-
tional frame of the same size but subdivided into 25 squares of 6 cm of sides.
All sub-areas were dominated either - by Agrostis
tenuis or Folcus lanatus. The results are summarized in
Table 19.
B. An oatfield
For the three year study period oats were sown over 2 an area of approximately 1,400 m on a gentle slope of east-
west aspect known as Pond Field, in the grounds of Silwood
Park.
2 In ]972 the oatfield covered an area of 1,424.62 m
in a skewed t with irregular contours. The area was not
subdivided into sub-areas. The oatfield was treated with
nitrogen pellets. A northwards projecting trapezoid of 2 298.74 m was left uncut at the season's end and the dry
oats were preserved in place. The vegetation around the
oatfield had not significantly altered during these three •
Table 19. Survey of grassland study area (624 m2) on 24th of July 1972. (a = old patch of long grass; b = newly grown grass; D = dominant species; C = co-dominant species; P = present in smaller proportions) Sub-areas 1 2 3 4 6 7 8 a b a •b a b a b a b a b a b a
Average height in cm 37.5 4.0 32.5 4.5 39.0 5.5 30.0 6.5 27.5 5.5 37.5 7.5 32.5 5.5 37.0 4.0 Maximum height in cm 52.5 21.5 62.5 28.0 59.0 29.0 53.5 37.5 55.0 32.0 54.0 32.5 52.5 42.5 58.5 30.5 % of bare area etc. 0 0 0 0 0 0 0 0 4.0 7.0 0 0 0 0 0 2.0
Agrostis----__ tennis C D C C C C D C D D D D C C C D Hoicus lanatus C C C C C P C P P P P. C C C P Festuca spp. P P Anthoxantum odoratum P P P P P P P P P P P P P P Luzula campestris P P P P P P P P P Ranunculum repens P P P P P P P P P
Trifolium sp- p. P P P P P
• 146
years, apart from some cultivated areas. Area III illus-
trated in Fig. 32, a broad longitudinal band parallel to 2 the oatfield covering approximately 1,700 m , was spring
sown in 1972 with Lolium perenne (Rye Grass). This late
seeding favoured the development of weeds since Lolium developed rather poorly in that season. The most abundant
weed was Spergula arvensis L. (Caryophyleaceae) followed
by several other weeds common in that area and listed under weeds in the oatfield in 1973.
In 1973 the oatfield was sown on 13 March and
subdivided into 10 sub-areas soon after. One week later
the area was delimited by a wire fence to avoid the attack
by rabbits and other seed eating rodents. The area so de- 2 limited covered 1,200 m and was subdivided into 10 sub- 2 areas of equal size, each covering 120 m . Areas one to
seven lay serially in a east-west direction measuring 10 m
along the E-W axis and 12 m along the N-S border. The
three remaining areas projected northwards but still along
the E-W axis. Area nine was rectangular and measured
8 x 15 m and areas eight and 10 were trapezoidal and
measured 6 x 10 x 15 x 15.5 m. On 3 April the oats were
• already more than 1.5 cm high and were treated with nitro- gen pellets. Those outside the wire fence were heavily
attacked by rodents and were not treated with nitrogen; the
area was soon dominated by various weeds and the oats that
succeeded to grow did not exceed 40 to 50 cm in height.
The surroundings of the oatfield are indicated by
Roman numerals in Fig. 32. Area I was an open-ground
S • S
ati Nashs field Jashs field V V
0 0 0
VII
VI I
-
0 1 1 I V 1 1 0 I V 1 III IV 111 1 0 I .1 1 9 1 1 DI — -r - 0
X I 0
XII 0 o I) —1 11 II I) (1 20 111 Mertens acres 9 Mertens acres
Fig. 32, The oatfield and its immediate surroundings (see text"for further explanations) (A) 1973; (B) 1974; (0) sticky traps; (0) water trap; (0) aerial suction trap; (---) boundaries of the 10 subareas; ( ) approximate limits of surrounding vegetation types • 148
grassland dominated by Agrostis sop., Festuca spp., and
Eolcus spp. This area covers a smooth slope and was mown
short every autumn. Area II was dominated by dense tufts
of Holcus spp. and extended over a steeper slope. Area II
was kept unc114-. Area III was treated with nitrogen pellets
in 1973 and the Rye Grass developed intensely. No weeds were present in any significant number. Adjoining this
area there was a barley field (area IV) of approximately the same dimensions as area III. The dominant weed in this
crop was Spergula arvensis. Area V was on the top of the
hillside and was covered by long grasses. It was dominated
by Dactylis glomerate and Phleum pratense with an E-W
gradient of Phleum and the Dactylis west to area IV, with
a predominance of Phleum pratense west to area III and to
the oatfield. Area VI was originally sown with oats and
remained outside the wire fence resulting in a mixture of
dense weeds and sparse short oats. Area VII was a mixed
woodland, ranging from short shrubs to tall trees. Area 2 VIII was the 298.74 m of oats left uncut in 1972 and was
invaded and completely replaced by Holcus spp. in 1973.
Area XI was similar to area II and was kept uncut. Area
XII was dominated by Holcus and in its damper parts by • Juncus tufts, the latter being cut in the autumn together
with area I.
The most frequent weeds in the oatfield were
Ra hanus sativus, Hesperis matronalis, Capsella bursa-
pastoris, Stellaria media, Spergula arvensis, Sinapsis
arvensis, Vicea spp., Chenopodium album, Polygonum
convolvulus, Convolvulus arvensis, Matricaria recutita, • Senecio vulqaris and Lolium perenne. The same weeds were • 149
dominant in area VI.
The density of the oat plants was assessed by taking two random samples per sub-area' and counting the numbers of plants inside a wire quadrate of side 30 cm. Twenty such samples were taken and the mean and its standard error 2 for a 0.09 m area was 56.75±1.64 oat plants. Converted to 2 a square metre the density was 630.55 oat plant/m .
On 6 July 1973 the oatfield had already reached its peak of growth with maximum height of 1.20 m, and was affected by an unusually heavy rain storm which caused the collapse of the plants over most of the area. The extension of the damage was assessed by direct measurement, estimated by eye and by photographs of the area. The less affected areas were those where the oat plants were shorter and less luxuriant as those of sub-area 10 which presents soil deficiency. Sixty per cent of the plants of the total area of the oatfield collapsed and 40% remained. erect. Table 20 gives the figures for the collapsed and erect square metres area for the total oatfield.
In 1974 the oatfield was again subdivided into 10 2 sub-areas of equal size with a total of 1,200 m . Sub- areas one to eight were in the same situation as sub-areas one to seven in 1972. Areas 9 and 10 were 6 x 20 m and were adjacent to areas 6 and 5 and to areas 4 and 3, respectively. The oatfield was sown on 29 March and on 8 April the oat seedlings were already more than one cm high. The oatfield was not treated with nitrogen pellets. The overall growth in all sub-areas was irregular. Fourty • 150
Table 20: The cal:field after 6 July 1973
Area with Sub--areas Area with collapsed (of 120 m2) erect plants plants % of area with number (in m2) (in m2) collapsed plants
1 75 45 37.5 2 20 100 83.3 3 5 115 95.8 4 10 110 91.7 5 75 95 79.2 6 25 95 79.2 7 95 25 20.8 8 60 60 50.0 9 50 70 58.3 10 115 5 4.2
2 Totals 1,200 m2 480 m2 720 m 60.0%
random samples were taken after the oats were fully grown
and the average height was estimated as 0.75 m±0.29. This
poorer growth favoured the establishment of weeds, the two
dominant species being Raphanus sativus L. and Spergula
arvenis L. followed in much smaller numbers by Sinapsis
arvensis and occasionally by the same weeds listed for 1973.
The density of the oat plants was assessed as already des-
• 'cribed for 1973. The mean and its standard error for a 2 0.09 m area was 57.65±3.12. The density was 640.55 oat plants/m2.
The areas referred to as VI and VIII in 1973 had •
also been cut in the autumn of 1973 and sown with oats in
1974. A 15 x 20 m area treated with nitrogen pellets was
delimited where area VIII had stood in 1972 (here named
• area TX). Its south eastern corner was 5 m away from • 151
sub-area 10 and its south western corner was 10 m away from sub-area 9. Oats did not develop outside the 10 sub-areas of the oatfield (area VI) but were preserved in a trapezium of 20 x 5 x 10 m between area IX and sub-areas
9 and 10 opposite to it. The 300 m2 of area IX received full treatment of nitrogen pellets. Areas I, II, IV, V, VII, XI and XII remained much the same as in 1973. Area III was not cut in the autumn of 1973 and in 1974 the Lolium was largely replaced by Holcus spp. Area X was kept fallow and was invaded by Holcus and a small mustard plot was planted north and adjacent to it (Fig. 32B).
The situation of the experimental areas, traps, grasses and wooded areas etc. in Silwood Park are illus- treated in Fig. 33.
Other cereal crops besides the barley indicated as area IV in Fig. 32 were experimental barley crops covering an extension of 3,000 m2 in 1972, 1973 and 1974. The position of these crops are illustrated in Fig. 33 and the respective years are also indicated. Otherwise these areas were occupied by experimental crops such as brussel sprouts, beans, potatoes, etc. In 1974 an experimental wheat crop • 2 of approximately 1,000 m was kept in the area known as Four Acre Field, and its position is also indicated in Fig. 33.
2. Meteorological records
Data on temperature, relative humidity and rainfall were obtained from a Stevenson screen in Silwood Park
• (Fig. 33). A thermograph provided air temperature readings rs 2
0 0 0 - 0 ° 0 0 0 0 0 o 0 0
0 CHE APSIDE 0 0 0 0 ° 0 HELD 0 WATER 0 0 ° ° 0 ME A DOW o " '0 ° Irn ° 0 0 0 0 0 0 "
:::_ :_ __-__: --- :--- - 1: __ __ / OUTSIDE ______:= i siftwoou I / :::----....---- / / 1::::- - - - -
® SUCTION TRAPS A MET. SCREEN
BUILDINGS 500 M
• STUDY AREAS EXPERIMENTAL CROPS
0001n UNCUT GRASSES
1 1 GRAZED AND CUT GRASSES
WOODED AREAS
Fig. 33. Sketch of Silwood Park.
• • 153
at approximately 1.20 m in a temperature recorder. Humidity
was measured on a hygrogralph and data on rainfall was taken
using a Dines tilting syphon rain gauge.
Data on hours of sunshine were obtained from Bracknell Meteorological Station.
Wind speed record was obtained from the Meteorolo-
gical Department, Silwood Farm, by a tribble cup anemometer placed 2.0 m high on a mown lawn.
Temperature records in the oatfield were taken over
a 24 day period during 1973. This was considered as a more
realistic estimate of the actual temperature experienced by
the leafhoppers than the use of the untransformed data
obtained from air temperature from the Stevenson screen.
The data obtained in the oatfield was later related to that Of the screen.
The temperature in the oatfield was measured with
thermistors connected to an eight channel Grant recording
Potentiometer. The stainless steel thermistors probe 4.5
cm long and 0.2 cm in diameter was covered with several
coats of matte white paint. The extremity of the probe • was peeled off over an area approximately as big as an adult
Macrosteles and painted with a greenish cream colour similar
to that of nymphs and adults of M. sexnotatus so that direct
radiation, if any, would have a similar effect on the probe.
The Grant recorder was kept in the field for nearly
a month in a waterproof box and five thermistors were
connected to it. These thermistors have a long lead so
that they can be placed at convenient sites. The required
• 154
height for the individual thermistors was obtained by
fastening the probe horizontally to a thin wood stick with
the tip freely projecting outwards. The probes were
arranged at 2.5, 10, 30 and 60 cm above ground level on
the same wood stick to provide a comparison between tempera-
tures at different heights amongst the plants in the oat-
field. A fifth thermistor was introduced amongst collapsed
oats. The recording potentiometer marked the temperature
reading for each thermistor at 15 minute intervals.
Selected hours were adopted for the daily readings.
These were the same six standard times adopted for the
calculation of the daily mean air temperatures by Silwood
Park Meteorological Station, i.e. readings were taken at
02.00, 06.00, 10.00, 14.00, 18.00 and 22.00 hours over the
whole period for each probe. The reading for each probe was
then compared with the reading from the Stevenson screen.
The difference between the two readings was recorded and
the average daily difference was calculated for each probe.
Finally the overall average difference was calculated for
the 24 day period for each probe.
For all the five probes the temperatures were con- • sistently lower than those obtained from the meteorological
screen. Also, the daily curve in the oatfield was smoother,
so the extreme fluctuations were not registered. This was
particularly true for the readings obtained amongst the
collapsed oats. There was a gradient of average difference,
the highest figure obtained in the collapsed oats and the
least difference at the highest level recorded. The mean
and the standard error of the differences were • 155
-1.07±0.136°C, -0.97±0.189°C, -0.84±0.121°C and -0.67±0.216°C respectively for 2.5, 10, 30 and 60 cm above ground level. As much as -1.59±0.271°C was obtained for the temperature amongst the collapsed oats. No significant difference between the average differences was obtained between 2.5 and 10 cm in height, 10 and 30 cm, 10 and 60 cm and 30 and 60 cm. The null hypothesis was rejected at 0.001 for differences between 2.5 and 30 cm and also for 2.5 and 60 cm. The value obtained for collapsed oats was significantly different at P 0.001 from that of 2.5, 10, 30 and 60 cm. An average difference of -0.89°C was adopted for the erect oats.
3. Methods of Sampling the Population
A. Introduction
M. sexnotatus occurred in moderate numbers in the acidic grasses of Silwood Park from 1968 to 1970 (Waloff & Solomon, 1973). In 1971, habitats where colonies were
breeding in 1970 were no longer occupied by this species. After an extensive survey of the grasses in Silwood Park a few first generation M. sexnotatus nymphs were found in
• the plot deScribed in lA above. The scarcity of M. sexnotatus in grasses was a persistent feature throughout the three years of study. Sampling was carried out both by sweep netting and by vacuum sampler but the results were consis- tently unrewarding due to the very low population densities.
Population studies were carried out in the oatficld described in 1B above where young first generation m. sexnotatus adults congregated and bred and a large second
generation built up in the summer. • 156
The low density of eggs in the plant tissues of
both grases and oats made the direct egg count impossible.
Eggs are inserted in the plant tissue and no evidences of
oviposition can be detected by the naked eye although they
can be seen by holding the plant tissue against strong light.
• B. Suction sampling
The wide hose type suction apparatus of Dietrick
et al (1959, 1960) and Dietrick (1961) was adopted to sample populations of adults and nymphs. The commercial model
manufactured by D-vac Ltd., California, U.S.A. was used.
This has the advantages over the narrow suction-hose type of having a portable motor carried on the back. Also the
area to be sampled is automatically delimited by the head
of the suction tube thus avoiding insect escapes; it can
be used efficiently on damp foliage; also, different •attach-
ments can be made according to the particular needs and
finally it samples instantaneously a larger unit of the habitat.
112 A 0.09 (one square foot) suction head fitted to
a 21 cm diameter hose was adopted. As pointed out by
• Dietrick (1961) when the suction head is pressed against
the ground, the screen-covered openings of the collecting
head allow for a continuous air flow that promotes the
sucking un of the insects and the maintenance of insects
and debris firmly against the back of the collecting net.
This must be large enough and tapered to ensure an efficient
air flow after it begins to fill with insects and plant debris. • • 157
In sampling it was considered that a large volume of vegetation inside the organdie collecting bag was dele- terious to the efficiency of the catch. A good air flow is necessary for an effective suction and a dense mat of plant material prevents both the necessary air flow and the effective sieving and screening of the insect material. This probably occurs in oats as high as 80 or 90 cm even with a low density of stands per surface area. The standard cylinder provided by the manufacturers is a 33.7 cm in diameter and 27.5 cm long fibre glass piece adjusted to a 45 cm long slightly tapering intermediate section connecting to the suction hose. This section is kept open by five metal rings and two longitudinal metal supports and is covered by a resistant nylon material. The standard organdie insect collection net is a slightly tapered 48 cm long bag. The bag has a 10 cm long base of resistant material that is tightly stretched and folded over the border of the fibre glass cylinder. It is held in place by a narrow fibre glass. ring. The 11 cm long,collecting head is fitted to it and firmly held in place by strong rubber bands connected to the main cylinder.
Dietrick (1961) recommends the use of different attachments according to the particular needs and reports that cotton has been sampled by placing cylinders of dif- ferent sizes over the plants, increasing the height of the cylinders as the plant grows.
An adaptation was made to cope with the height of
oats. A 50 cm long cylinder made of flexible aluminium sheet replaced the standard 27.5 cm long fibre glass cylinder.
• 158
The original 48 cm long collecting net was replaced by a
78 cm long net The net tapered progressively and after
having the basal ring folded over the border of the cylinder
the actual collecting length was 70-72 cm. The top of the
net extended into the basal half of the intermediate section
of the hose and the distal half of the hose connected to the air duct acted solely as an air chamber. This 72 cm long
effective collecting area was lengthened to approximately
83 cm by the fitted 11 cm long collecting ring.
The D-vac model adopted for sampling the oatfield
had the air duct connected to the upper part of the machine
so that the hose bent over the shoulder of the sampler. The hose and cylinders were held by the longitudinal metal
supporters of the intermediate section. The collecting
cylinders could then be placed quickly and quietly over the
plants to be sampled and the collecting head could be easily
pressed against the ground. The long available area to
accommodate the plant material permitted sampling with less
disturbance to the insects since the oat plants did not have
to fold inside the bag but were covered by it. Also, a
better air flow was ensured, since no dense mat of plant e material accumulated into the collecting net. The sampling time was one minute per sample and the collecting cylinders
were slightly moved up and down a few times per sample to
increase the air flow and flush out specimens that might
cling on to the vegetation. The sample was then transferred
to a polythene bag containing a label.
The sampling programme was carried out in the form
of stratified random sampling with two samples. taken for each • 159
sub-area making a total of 20 samples for each occasion. The sites to be sampled were determined by pairs of computer generated pseudo-random numbers. The pairs were uniformly distributed numbers ranging from zero to 15 and from zero to 18 and corresponded to maximum number of steps necessary to cover any of the sub-areas. The position of the sampling spot was defined by two sets of steps. The steps were firstly counted along the external border of the sub-area from east to west. When the numbers of - steps required by the first random number of the pair had been counted, the sampler turned at a right angle therefore moving into the sub-area and counted the necessary steps corresponding to the second number of the pair of random numbers. A sample was than taken immediately afterwards and the process was repeated twice for each sub-area. The pairs of random numbers were found before each sampling date and the com- bination of steps was determined for each sub-area. This method had the further advantage of giving the approximate position of each sample unit provided the standard size of the step was known.
The system of allocation of samples had to be some-
0 what adapted in 1973 after the partial collapse of the oat- field. The samples were thereafter stratified since the oatfield no longer represented a homogeneous universe. The number of samples taken on collapsed and erect oats was taken in proportion to their respective areas. The architecture of the areas with collapsed plants prevented a straight forward random sampling per sub-area for collapsed and erect oats. Sub-area 10 was sampled twice for erect oats since • 160
only 4.2% of the area was flattened down. Sub-areas 2,
3 and 4, respectively with 83.3, 95.8 and 91.7 of collapsed
oats, were systematically sampled for flattened oats. The remaining sub-areas were sampled once for erect and once
for flattened oats. A sketch of the distribution of the
patterns within the sub-areas helped to eliminate before-
hand the pairs of random numbers that would necessarily lead
to sampling in the wrong pattern. Referring to Table 20, 60% of the total area had collapsed plants and 40% erect
plants. Therefore 12 samples were taken from then on from
the flattened oats and eight from the erect oats to maintain
20 samples per sampling occasion.
The samples were examined in the laboratory. The
polythene bags containing the individual samples were
immediately stored in a refrigerator to retard predation
and general activities as well as to avoid desiccation. Each
sample at a time was opened inside a white sorting frame.
A light stood behind the transparent cellulose acetate back
wall of the frame to attract both adults and nymphs. The
top and lateral walls of the frame were muslin covered
and the bottom was a sheet of white plastic to provide a
• contrasting background to the insect. A fine brush helped to sort the smaller nymphs from the plant and soil debris, since the older nymphs actively and rapidly moved towards
the light. Later in the season a large amount of oat grains
accumulated in the samples and were easily separated from the
sample with the help of an adeauate mesh sieve. Each indi-
vidual Macrosteles specimen was collected directly by
inducing the specimen to jump into a 5.0 x 0.8 cm sample • s 161
tube which was closed by a cork. Each specimen was examined and identified under a binocular microscope immediately after the sorting of the sample and was either killed in 70% alcohol or transferred to a pot or tube with oats according to further needs.
C. Sweep net sampling
This method of sampling has been widely used for the ease of manipulation and the low cost involved.
However, only relative estimates are obtained and several factors affect catch efficiency. De Long (1932) pointed out the impossibility for the same person to dupli- cate the results by sweeping. Factors influencing sampling were temperature, humidity, wind, position of the sun, plant size and condition and insect infestation. Further- more, the sweep net sampled only the upper 25-38 cm of the plant. Carpenter and Ford (1936) also found that personal error was an important factor. The number of sweeps neces- sary to obtain a representative sample of invertebrate popu- lations varied with the plant community. Fewkes (1961) remarked that sweep netting only removes the insect on the upper strata of the vegetation and that fluctuations of sample size can be due to the vertical movement of the insects; higher catches of nymphs and adults of Nabidae in grassy areas were obtained by night. Westdal et al (1961) reported that catches of nymphs and adults of Macrosteles fascifrons in crops were affected by temperature, precipi- tation, wind velocity, cloudiness and relative humidity and that the highest catches were obtained from late afternoon • 162
to dusk, when the air was still, warm and humid. Jfirisoo
(1964) reported that the size of the catches of Auchenor-
rhvncha in leys and cereal crops varied considerably at
different times of the day associated with the insects diel
vertical movements. Menhinick (1963) reported that such
jumping forms as Membracidae, Cicadellidae, etc.. were less easily captured by sweep net in herbaceous vegetation 60-90 cm high. Also, that the variation in the catch for
a given .species on different days was less than the varia-
tions between species on the same day. Catch was affected
by the sampler's technique and weather conditions.
Rose (1973b) was unable to sample Cicadulina spp.
in grasses with a sweep net. No Cicadulina were captured
with 500 sweeps and in the same spot 104 specimens were 2 collected over 12 m of grass with a Dietrick suction
sampler.
The efficiency of the sweeping method has been
reported by several authors. Emsley (1957) estimated that
he was able to sample 38% of the Miridae population in the
field with the first sweep sample; the estimation of
efficiency was calculated by removal trapping by sweeping.
Fenton and Howell (1957) sampled alfalfa field with sweep
net and say that they were able to collect 44 to 77% of the
insects resting on the vegetation. This method was con-
sidered effective for winged and very active species and
convenient for surveying areas with light infestation.
Banks and Brown (1962) compared sweep netting, counting in
quadrates and marking-release-recapture to estimate popu- • lation density of the invading E rvgaster integriceps in • 163
thickly sown, fully grown wheat fields. By sweeping only 10 to 20% of the bugs above ground were caught and it sampled only those at the tops of the plants. Even so, the estimates obtained from sweep counts alone reflected the differences in population sizes shown by the other two methods and the percentage sampling error of the estimates was small, ranging from seven to 10%. Dietrick et al (1960) compared sweep net catches to D-vac samples in alfalfa fields. When catches were adjusted to equal areas the suction samples were always more effective for sampling more than 100 different species of arthropods. Heikinheimo and Raatikainen (1962) compared the efficiency of sampling with a suction apparatus and with sweep net. Much higher numbers of nymphs were obtained by suction sampling and differences in extraction between the two methods were more marked in samples from Timothy leys than in those taken from spring cereal fields. Takai et al (1965) estimated the efficiency of sampling with a sweep net the leafhopper Nephotettix cincticeps in fully grown paddy fields by com- paring with population estimates based on mark and recapture. The efficiency of sweeping was estimated as 10 to 30% with a small sample variance. Callahan et al (1966) compared sweeping and D-vac sampling in alfalfa fields and found that the latter was more efficient for leafhopper sampling since the same number collected in five square feet by D-vac needed 60 square feet by sweeping.
The sweep net adopted to sample M. sexnotatus popu- lations had a D-shaped frame, the straight border was 37 cm long and 24 cm apart from the opposite semicircle border at • 164
its mid-lerluth. The handle was 52 cm long. The net was 52 cm long and 45 cm wide at the bottom. The handle was held with both hands, one close to its distal end and the other near the metal frame. The strokes were done rather strongly and rapidly in a right-left, left-right direction. •The sweeps were done at every step, at arms lengths, and each stroke was approximately 1.30 m long. This procedure was maintained throughout the sampling season to minimize error due to the sampler.
The sweep net samples were either taken to the laboratory still inside the sweep net or in polythene bags. In the latter case, immediately following the last sweep stroke the net was held closed next to the frame and a 60 x 45 cm polythene bag was adjusted to the mouth of the net. When the polythene bag was properly set in place the content of the sweep net was inverted into it and the bag was closed with a rubber band. After some practice these procedures could be carried out safely so that escapes - during transfer were minimal. The advantage of transferring the content to a polythene bag is that samples can be individualized and taken separately without it being neces- sary to carry a large number of nets to the field for each set of netting. The samples were sorted in the laboratory as described for the suction samples.
D. Efficiency of suction samplinq-
Tests were carried out for nymphs and adults of M. sexnotatus to determine the sampling efficiency in the oatfield. Sampling on long and medium-high oat plants was a • 165
tested separately. Also, tests were carried out on areas with collapsed oat plants.
The sampling procedure was evaluated by sampling the same areas three times in succession. The efficiency of sampling once was estimated by the removal trapping or collecting principle where the rate at which trap catches fall off will be directly related to the size of the total population (to be estimated) and number removed (that is counted after each trapping occasion). The conditions put forward by Moran (1951) for the application of this method to insects were satisfied: (a) the catching or trapping procedure must not lower or increase the probability of an animal being caught and the chance of being caught must be equal for all the animals. The Dietrick machine is unselec- tive as regards to plant strata. (b) the population must remain stable during the trapping or catching period; there must not be any significant natality, mortality (other than by the trapping) or migration. The one minute sampling time and the immediate successive samples prevents natality,
mortality or migration to take place.
The first condition is of particular importance, since an underestimation will arise if the probability of capture falls off with time and, conversely the population will be overestimated if the insects become progressively more susceptible to capture (Zippin, 1956, 1958). The removal trapping data was analyzed by Kono's 'time unit' collecting method (in Southwood, 1966, p.183) and by Zippin's (1956, 1958) procedure based on maximum likelihood. 0 • 166
Sa rpling on loner and medium-high oat plants was
tested in 1974 and on collapsed oats in 1973.
In 1974 tests were carried out when peak numbers of
nymahe were in the field. To maximize the efficiency of
the estimates, large numbers must be trapped and nymphs
of all instars were released in the oats to be sampled. These nymphs had been field collected and examined in the
laboratory before the release. The release spots were
identified with small labels. The nymphs were released as
quietly as possible on the vegetations and were allowed
one hour to settle down. The spots were then sampled with
the suction machine in the usual way, using one minute for
each sampling occasion. The sample was then transferred to
a labelled polythene bag and the procedure was repeated
• twice. Therefore, three consecutive Dietrick samples were taken on the very same spot. Special attention was taken
to place the collecting head of the machine precisely on
the same spot as previously sampled. The whole procedure
was carried out as quietly as possible so as to cause
minimal disturbance during and between the sampling occa-
sions. The average time interval between the consecutive
• set of samples was less than one and a half minutes.
The first set of tests was carried out on 0.65 m to
0.75 m high oat plants, on 2 August 1974, between 11.00 a.m.
and 1.00 p.m. The weather was dry, sunny, still and the
average temperature for the day was 14.8°C. Five sites
were sampled and the pooled results are given in Table 21.
The second set of tests was carried out on 1.00 to
1.20 m high oat plants, on 11 August 1974, between 11.00 a.m.
167 -
Table 21: The numbers of M. sexnotatus nymphs removed in three consecutive samples from the same area of oats 0.65 to 0.75m high, on 2 August 1974. (Pooled results from five sets of Dietrick samples; total area sampled 0.45 m2)
Nymphal instars Sampling Total occasions I II III IV V I to V
First 81 153 226 175 119 754 Second 18 34 35 16 11 114 Third 4 7 9 5 3 28
and 1.00 p.m. The weather conditions were similar to those of the previous tests and the average air temperature for the day was 14.7°C. Smaller numbers of first and second instar nymphs were then available for the test, and numbers had also declined in the field. Nine sites were sampled and the pooled results are given in Table 22.
Table 22: The number of M. sexnotatus nymphs removed in three consecutive samples from the same area of oats 1 to 1.20 m high, on 11 August 1974. (Pooled results from nine sets of Dietrick samples; total area sampled 0.81 m2)
Nymphal instars Sampling Total occasions -r II III IV V I to V
• First 31 35 77 129 124 396 Second 6 6 27 26 28 93 Third 4 5 6 11 12 38
The tests to estimate the efficiency of sampling adults on long and medium oats were also carried out when peak numbers were in the field .
S • 168
The tests on 0.65 m to 0.75 m high oats were carried
out on the 9 September_ and on 1.00 m to 1.20 in oats on
10 Sepi-cipber 1974. The average daily temperature was
respectively 13.0 and 12.1°C. Ten sites were sampled in
each test. The pooled results are given in Tables 23 and 24.
Table 23: The numbers of M. sexnotatus adults removed in three consecutive samples from the same area of oats, 0.65 to 0.75 m in height, on 9 September .1974. (Pooled results from 10 sets of Dietrick samples; total area sampled 0.9 m2)
Sampling occasion Males Females Total
First 51 85 136
Second 13 21 34
Third 5 7 12
Table 24: The numbers of M. sexnotatus adults removed in three consecutive samples from the same area of oats, 1.0 to 1.20 m in height, on 10 September 1974. (Pooled results from 10 sets of Dietrick samples; total area sampled 0.9 m2)
Sampling occasion Males Females Total
First 50 51 101
Second 10 11 21 S Third 5 2 7
The efficiency of sampling on collapsed oats in
1973 was tested on three different days with four, six
and three spots sampled on 19 and 23 August and 7 September.
The average temperature for each day was 17.8, 17.0 and
16,6cC respectively. These dates correspond to weeks 10, S 11 and 13 of 1973 sampling and were after the peak numbers • 169
of nymphs. Hence the low numbers sampled, especially of
the first three instars. The pooled results for nymphs.
and adults for the 11 sets of tests are given in Table 25.
Table 25: The numbers of M. sexnotatus nymphs and adults removed on three consecutive samples from the same area of collapsed oats, on the 19 and 23 August and 7 September 1973. (Pooled results from 11 sets of Dietrick samples; total area. sampled 0.99 m2)
Nymphal instars Sampling Total Total occasion I II III IV V I to V Males Females adults
First 31 44 10 30 62 177 56 38 94
Second 2 7 2 5 13 29 4 4 8 Third 0 0 0 3 2 5 3 1 4
The original population in each sampling spot was
estimated by the approximate method of Kono using the
equation: 2 n3 - nin2 (1) P - 2n - (n + n ) 3 1 2
where P is the estimated total population, and n1, n2 and n 3 are accumulated total catches after first, third and second sampling occasions with equal time intervals between
sampling occasions. Tables 26 to 30 summarize the results
for the tests and gives the efficiency of sampling deter-
mined as the percentage of the estimated original population collected on the first sampling occasion.
Zippins's procedure based on maximum likelihood is
regarded as the most accurate method for the analysis of
the removal collecting data (Southwood, 1966). According
to Zippin (1956, 1958) the estimates of population size 170
Table 25: Efficiency of sampling once with the D--vac suction sampler. Data referring to Table 21. Populations estimated by Kono's time- unit collecting method.
n 2 Accumulated catches Estimated o caught in the Nymphal afi-er the original instars third occasion population -First occasion
103 104 77.9 194 196 78.06 270 273 82.8 196 198 88.4 V 133 134 88.8
I to V 905 83.3
Table 27: Efficiency of sampling once with the D-vac section sampler. Data referring to Table 22. Populations estimated by ' n2 p Accumulated catches Estimated caught in the Nymphal after the original instars third occasion Population first occasion 41 49 63.0 46 71 49.3 87 114 57.9 197 206.4 69.3 110 112 68.7 166 174 74.1 V 164 173 71.7 I to 527 553 71.6 Table 28: Efficiency of sampling once with the D-vac suction sampler. Data referring to Table 23. Total Estimated original. Probability of capture catch Method population on a single sampling Reno 188.54 72.14% 182 Zippin 185.98 0.722 • 171 Table 29: Efficiency of sampling once with the D-vac suction sampler. Data referring to Table 24. Total Estimated original Probability of capture catch Method population on a single sampling Kono 132.5 76.2% 129 Zippin 130.76 0.762 Table 30: Efficiency of sampling once with the D-vac suction sampler. Data referring to Table 25. Populations estimated by Kono's time-unit collecting method. n2 Accumulated catches Estimated % caught after the original in the Stage third occasion population first occasion I+II+III 96 96 88.5 IV+V 115 117 78.6 I to V 211 212 83.5 Adults 106 110 85.4 based on data from removal trapping are not very close to the actual population values, unless large proportions of the population are captured. Zippin's (1956) equation: N - T , kR - Q,k (2) t N kN - Ex. • where T is the total catch (Eyi), k is the number of catching occasions, Exi is the sum of accumulated previous catches andQ=Ex.+Ey. was solved for N (the estimated total population) by a computer programme written by Mr. N. Small, Having found R, the probability of capture during a single sampling occasion (P) can be calculated as - p = probability of escaping capture during a single sampling occasion, then • 172 ,k = 1 - T/N = the probability of escaping capture during each of the k sampling occasions. Whence The probability of capture during a single sampling occasion () was estimated for each individual instar in tests made on oats of medium height. The lower numbers of instars I and II in tests on longer oats necessitated pooling of data of both instars to obtain the estimates of I7 and p. The assumption that both instars are caught in virtually equal proportions seems to be valid in this case since this is true for the shorter oats. On the other hand, the decision as to which instars were to be grouped for tests carried out on flattened oats was somewhat subjec- tive since there was no term of comparison. Tables 28, 29, 31 to 33 give the estimates of the original populations and of the probability of capture on a single occasion. Yono's and Zippin's method gave very close results both for the estimation of the original population and for the efficiency of the sampling method. The only discrepancy was in the combined estimations of instar I and II for long oats where a difference of 12.1% was obtained. When instar III was included both methods gave very close estimates. The sampling efficiencies adopted for deriving popu- lation estimates are given in Table 34. In 1973, sampling was carried out independently and in proportion to the area remaining erect (1.00 m to 1.20 m high oats) and to the collapsed area. The catches were therefore corrected in- dependently for the sampling efficiencies on long oats and on flattened. oats before adding up the population estimates • 173 Table 31: Efficiency of sampling once with the D-vac suction sampler. Data referring to Table 21. Original population (N) and probability of capture during a single sampling occasion (0) estimated by Zippin's procedure on maximum likelihood. Nymphal T instars (total catch) I 103 104.14 0.778 II 194 196.03 0.782 III 270 271.46 0.825 IV 196 196.35 0.879 V 133 133.21 0.884 I to V 896 900.4 0.830 Table 32: Efficiency of sampling once with the D--vac suction sampler. Data referring to Table 22. Original population 0'4) and probability of •capture during a single sampling occasion (D) estimated by Zippin's procedure on maximum likelihood. Nymphal T instars (total catch) 0 13 1-+II 87 89.4 0.700 I+II+III 197 202.7 0.696 III 110 113.3 0.692 IV 166 168.73 0.747 V 164 167.53 0.724 I to V 527 538.73 0.721 Table 33: Efficiency of sampling once with the D-vac suction sampler. Data referring to Table 25. Original population (N) and probability of capture during a single sampling occasion (p) estimated by Zippin's procedure on maximum likelihood. T Stage (total catch) 0 p I+II+III 96 96.11 0.894 IV+V 115 116.15 0.785 I to V 211 211.96 0.834 • Adults 106 106.27 0.863 174 Table 34: Sampling efficiencies used for deriving population estimates according to characteristics of the 0.65-0.75 m high 1.0-1.20 m high Flattened Stage oat plants oat plants oat plants I 78.0% - - I+IIA-III - 69.5% 88.5% III 82.5% - - IV 88.0% 74.0% - IV-+-V - - 78.5% V 88.5% 72.0% - I to V 83.0% 72.0% 83.5% Adults 72.0% 76.0% 85.5% for the oatfield as a whole. In 1974 each individual stage could be corrected for the sampling efficiency on short oats. The figures for long oats could not be applied to 1972 catches because the standard D-vac was used. Effectiveness of sweeping: metre equivalent sweeps 'Calibration' of sweep samples for a given species is possible to carry out once the density of a species per unit area is known. 0 Menhinick (1963) produced the following formula: (1) S T 11 = N xA. where in is the number of strokes necessary to capture the insects on one square metre, S is the number of strokes in first period of sweeping, N is the number of insects caught, T is the total number of insects in the total area and A is • 175 the total area in square metres. The formula can be also written as: (2) M = — x D Lv where D equals the density of the species per unit area 2 (m ) - The number of strokes needed to capture the insects on one square metre can be used to determine insect density from other sweep-samples with unknown area provided the sweeping method and related factors are the same or close to the original. The formula is then: (3) D = x M . Immediately following the weekly twenty Dietrick suction samples, a sweep net sample was taken according to the procedure described in C above. One hundred sweep strokes were taken along a longitudinal line in the oatfield. Metre equivalent sweeps were then calculated for M. sexnotatus adults over the whole sampling period for 1974. Account was also taken of nymphs caught, but sampling of nymphs by sweeping was regarded as unsatisfactory. Table 35 summarises the results and includes the height of oats which had an important effect on sweeping efficiency. Other factors such as the type of sampler, time of sampling, choice of dry days with no rain or high winds remained constant. A linear regression line was fitted to the points 4 of numbers caught in 100 sweep strokes (Y) and the density 176 Table 35: Variation in the number of sweep net strokes yielding the number of M. sexnotatus adults corresponding to the population of one square metre in the oatfield, 1974 (density per square metre estimated by Dietrick suction samples). Weeks one to nine, first generation females only; weeks 10 and 11, first and second generation females; week 12 onwards, second generation only. First generation males from week one to eight; second generation males from week 10 onwards. No. of strokes Average Numbers in yielding to height of Densitv/mz 100 sweeps numbers in 1 m2 1974 oatfield week (m) d ? 8 ? 8 1 0.30 1.9 3.9 12 6 15.8 65.0 2 0.40 16.9 8.5 27 14 62.6 60.7 3 0.50 13.9 37.1 21 18 66.2 206.1 4 0.58 8.5 23.2 13 21 65.4 110.5 5 0.66 3.1 24.7 11 22 28.2 112.3 6 0.75 7.6 26.2 4 181.1 0 190.0 145.5 7 0.75 5.1 16.4 0 6 0 273.3 8 0.75 4.6 16.2 0 4 0 405.0 9 0.75 0 10.8 0 2 0 540.0 10 0.75 6.1 10.0 0 4 0 250.0 11 0.75 27.8 1 5.5 16 10 173.7 155.0 12 0.75 82.5 39.3 43 40 192.0 98.2 13 0.75 81.0 69.4 49 46 165.3 150.9 .._,1 il 0.75 77.1 79.2 43 29 179.3 273.1 15 0.75 56.9 41.7 48 28 118.5 149.0 1 6 0.75 24.7 21.5 28 8 88.2 269.0 of the insects per square metre for each sampling date estimated by the D-vac. The linear regression equation for males is: (1) Y = 6.2842 0.5285x with r equal to 0.9037 and t equal to 7.6094 for 13 degrees of freedom. For females the equation is: s 177 (2) Y = 3.2675 + 0.5043x with r equal to 0.8137 and t equal to 5.2375 for 14 degrees of freedom. In both instances the null hypothesis was rejected at 0.001 level. Therefore, numbers of adults caught in 100 sweep net strokes reflect the real density of the population to be sampled. 4. Weekly Population Estimates The weeks are numbered serially for convenience for the three year study period, week zero being that preceding the invasion of the oatfield. The numbering of the weeks is given in Table 36. Table 36: The numbering of field work weeks Date on which week begins Week number 1972 1973 1974 0 2 June 3 June 27 May 1 12 June 10 June 3 June 2 23 June 18 June 11 June 3 28 June 25 June 18 June 4 6 July 2 July 24 June 5 12 July 10 July 1 July 6 19 July 18 July 8 July • 7 26 July 26 July 16 July 8 7 Atgust 31 July 22 July 9 15 August 8 August 29 July 10 21 August 14 August 5 August 11 30 August 22 August 14 August 12 5 September 29 August 20 August 13 14 September 5 September 27 August 14 20 September 11 September 6 September 15 26 September 20 September 1 3 September -I n 16 3 October 30 September -- u September 17 17 October - 28 September • 178 In 1972 regular suction sampling was started only on week 9. Previous catches (weeks 0 to 8) were carried out with a sweep net. The nymphal stages were therefore not represented in these catches taken in fully grown oats. Sau:l.ling with the D-vac was started after the peak of both instars I and II. The population curves for 1972 based on the D-vac samples also reflect the shortcomings of sampling on high oats with a standard D-vac collecting tube and net. No estimates were made from sampling data based on sweep net catches. In 1973 and 1974 weekly population estimates were based on D-vac suction samples and the figures were corrected for sampling efficiency as described in part 3D above. In 1973 after week 4 the oatfield was heterogeneous and separate estimates were made for the collapsed and the erect oats which afterwards were added to obtain the estimates for the total area. In 1973 and 1974 there was a slight overlapping in early August of female adults of the first and second generation. Old females could be distinguished from the young ones both by their 'wear and tear' and by examination • of the internal reproductive organs. In both years the first generation females lived two weeks longer than the males. No overlapping of males of first and second genera- tion occurred. First generation males and females invaded the oatfield simultaneously in 1973 and 1974, but females outnumbered males throughout the following weeks. In the second generation, the males appeared before the .females. Fig. 34A and B represent the fluctuations in • • .11 0 i- vi 7 ',. C IC- NOS. I NT HOUSANDS y: Wa 0 120- 40r. i,or 30- 6 so - 201- ,,, '.i EKO1 0 j \.(1 1-- ...... tc _JUN 10 -.- ..e i ' / 1 / t t - second (brokenline) generationM.sexnotatus Tiuctuation innuners offirst(solidline)and adults intheoatfield. / 10 A, - L 2 4 20 N I (1?) 3 ,14 N. 20 c'N. 3 \ 30 JUL10 1 4 N 1 ----- No- 30 jti!.10 5 6 -- 5 1 ' i 4, ------ 6 • 7 20 t 20 I -,,_ .-....• t) 910 i •• 7 - --': \ 30 Alit;10 ....., 3 % j 3 910 0 - .\1,t,10 , / , ------)...... -...1 (A) 19737(B)1974. / 4 / I t I I t / / I 11 1213 I P , 1 t 1 i 1 P- )4 I I i ---- I I I I r I I r 9 20 1 I 20 / ro 1 I r 11 i o r ,/ P/ t . _.,_ 14151617 '. /V 30 ,F1 / J) / 12 0 d' • 30 SEPT10 , . . 1 l o % 0, 0' % 1 t % . ■ 1 % , 13 . 10 b XO 1% . t X A% --\ t 1 ..- i . \ ■ \ I .0... 14 \ I N 4.3 I 20 . J , . 1•• .. N .... , 173 20 b, 300c1 . _ A , .• 30 OCT10 . 10 • 180 numbers of the first and second generation adults. The fall off in numbers of second generation adults will be dealt with in Section III. In 1973 and 1974, second generation first instar nymphs were already present on week 5 following invasion. Peak estimate of total nymphal population was on week 8 in 1973 and on week 10 in 1974, and on the third and fifth week after the initial hatching of the first nymphs. First instar nymphs were present from early July to late August both in 1973 and 1974 with peak numbers three weeks after their first appearance. The relation between the ovipesition period and the fluctuation in numbers of first instar nymphs is illustration in Fig. 35A and B. The weekly population estimates for 1972 to 1974 are set out in Tables 37 to 41 and represented graphically in Figs. 34A and B, and 36 to 38. • • a 181 Table 37: Weekly population estimates of second generation M. sexnotatus in the oatfield, 1972 (1,425 m2) Nymphal instars Week Total number I II III IV V nymphs 9 33,240 30,070 23,750 7,900 3,160 98,120 10 22,160 20,580 23,750 12,650 17,400 96,540 11 3,160 3,960 28,500 25,300 9,500 70,420 12 3,960 13,500 13,500 26,900 25,300 83,160 13 790 1,600 13,450 15,840 14 3,960 15,830 19,790 15 790 5,500 6,290 16 0 Week Adult Adult Total Total in number males females adults the generation 9 98,120 10 '1,580 3,160 '4;740 101,280 11 3,950 1,600 5,550 75,970 12 3,200 9,500 12,700 95,860 13 4,000 5,500 9,500 25,340 14 3,200 8,700 11,900 31,690 15 3,200 3,200 9,490 16 780 780 780 17 0 Table 38: Weekly population estimates of first generation adults M. sexnotatus invading the oatfield in 1973 (1,200 m2) Week Adult Adult Total number males females adults a 1 4,400 7,900 12,300 2 10,500 22,750 33,250 3 6,200 15,800 22,000 4 1,800 14,850 16,650 5 5,400 24,300 29,700 6 1,000 24,500 25,500 7 2,800 17,500 20,300 8 750 9,350 10,100 9 5,400 5,400 10 1,400 1,400 11 0 • • 182 Table 39: Weekly po-oulation estimates of second generation M. sexnotatus in the oatfield, 1973 (1,200 m2) Week Nymphal instars Total number I Il III IV V nymphs 5 72,000 5,000 900 77,900 6 52,400 41,700 21,600 115,700 7 84,600 43,200 43,500 9,600 180,900 8 100,800 111,900 60,600 62,950 21,300 357,550 9 24,100 61,600 97,400 67,500 61,900 312,500 10 34,900 28,200 54,500 86,100 95,900 299,620 11 6,000 9,700 8,300 23,050 65,500 112,550 12 720 3,840 9,700 10,160 29,200 53,620 13 720 720 5,000 10,900 16,620 14 1,670 7,600 9,270 15 0 Week Adult Adult Total Total in number males females adults the generation 5 77,900 6 115,700 7 180,900 8 750 750 358,300 9 6,200 1,500 7,700 320,200 10 28,500 14,150 42,650 342,270 11 26,900 32,040 58,940 171,490 12 30,900 37,950 68,850 122,470 13 15,640 28,850 44,490 61,110 14 7,910 23,000 30,910 40,180 15 4,700 7,000 11,700 11,700 16 2,350 3,100 5,450 5,450 Table 40: Weekly population estimates of first • generation adults M. sexnotatus invading the oatfield in 1974 (1,200 Week Adult Adult Total number males females adults 1 2,300 4,700 7,000 2 20,300 10,200 31,500 3 16,700 44,500 61,200 4 10,200 27,800 38,000 5 3,700 29,700 33,400 6 9,200 31,500 40,700 7 6,200 19,700 25,900 8 5,500 19,500 . 25,000 9 13,000 13,000 10 8,300 8,300 11 2,800 2,8002 12 0 • 183 Table 41: Weekly population estimates of second generation M. sexnotatus in the oatfield in 1974 (1,200 m2) Nymphal instars Week Total number I II III IV V nymphs 5 6,000 900 6,900 6 53,800 13,800 870 68,470 7 195,700 46,400 16,000 800 258,900 8 241,076 161,500 63,000 19,100 2,300 486,976 9 221,500 152,900 156,000 56,800 13,500 600,700 10 106,100 150,400 164,000 137,000 61,700 619,200 11 26,400 48,700 97,000 146,200 133,300 451,600 12 35,000 34,100 50,000 78,000 165,700 362,800 13 4,300 18,000 25,000 44,000 125,000 216,300 14 1,000 2,100 6,100 12,300 60,200 81,700 15 920 3,400 3,200 9,100 21,000 37,620 16 2,500 3,000 9,100 14,600 17 720 720 1,440 Week Adult Adult Total Total in number males females adults the generation 5 6,900 6 68,470 7 258,900 8 486,976 9 600,700 10 7,300 3,700 11,000 630,200 11 33,300 15,700 49,000 500,600 12 99,000 47,200 146,200 509,000 • 13 97,200 83,300 180,500 396,800 1 4 109,200 105,500 214,700 296,400 15 68,300 50,000 118,300 155,920 16 29,700 25,800 55,500 70,100 17 12,150 23,200 36,150 37,590 • • • • Fr./ \a-4 . AVERAGE NO. EGGS/DAY/ AVE RAG NO:E EGGS/DAY/y rC.1 „,) :;"-t. th — (A 0% 1 r r t 0 0-, ■•• 0 4- ri- Ci 1:(1 4- 0 C) (1) 0 U) I-1 co cl- s) 0 ■J (D I 73 ic 0 Co rt. P) 0 • C) H- / ';:; C) )i • ci) 0 .2.1' tJ i--h H- c't CD u) (:( (1) `FD4 0 (D H- . k.n no. 'SI 0 c), 0 `t1 ct- 0 -01 H- C) CD SI hi ii (D Hh H Qi H 0 -CD H. 0 pi al I-1 • kc: ri 0 • 0 CD 0 o r 0 t-h 1:2-1 CU („Q CD P.' 0 (I) ..... CM to Of Cr) tri 0 0 0 0 0 0 CM 0 Ui 0 SCINVS1101-11 .SON SCI N S1101-1.1. NI .SON • I • IV ds n a hous t wcat: 0 10 11 12 13 14 15 16 17 1 L - I {lug 20 20 sirpt 10 20 3 0 C Fig. 36. Fluctuations in the abundance of second generation M. sexnotatus nymphal stages and adults in the oatfield, 1972. • 1g, 37. Fluctuation inthe abundanceof second generation M.sexnotatus nymphal 0 z Lf) z THOU S AND S JUL F,tages and adultsinthe oatfield,1973. 10 a_.. • _1 20 30 AUG 10 20 30 SEPT .....1-_— 10 20 30 I OCT .---z 10 Is 1-' CO 0-1 250 ADULTS 200 150 DS OUSAN TH , 100 0 z 50 WEEK 4 5 6 7 8 9 10 11 12 13 14 15 16 17 1._ 1 ! 20 30 JUL 10 20 30 AUG 10 20 30 SEPT 10 20 30 OCT 10 ]-Tig. 32. Fluctuations in the abundance of second generation M. sexnotatus nymphal stages and adults in the oatfield, 1974. • 188 ▪ mbe Total Number of Eggs Laid A. Estimation of the daily fecundity (i) Material and methods Fecundity was measured directly by keeping first generation M. sexnotatus females caged in the field. The experiments were carried out in 1973 and 1974 from the time of invasion of the oatfield to the decline of adult population. The females to be caged were obtained from the regular weekly samples. The samples were sorted in the laboratory immediately afterwards. Females were taken at random and examined in a sample tube under the binocular microscope to discard any that could have been injured by the sampling procedures. Ten females were caged per week for six to seven days. Each week the whole batch of females was changed by a new one to maintain a representa- tive cross-section of the field population. The individual cages consisted of muslin sleeves. The sleeve was a cylinder of muslin around a framework of • cellulose acetate consisting of two distal rings connected two opposed longitudinal bars. The free parts of the sleeve projected 15 cm each side of the distal ring frame- work. The sleeve was 50 cm in total length, and the 22 x 8 cm cylindrical cage was of the same dimensions as the cages adopted for laboratory experiments on fecundity. The sleeve was desianed to allow for maximum ventilation and to provide conditions inside the cage as identical as possible to those of the open air. • 139 The sleeve was introduced over the oat plant or around. oat leaves and was closed near to the framework to avoid the possibility of the insect getting tangled in the folds of the fabric. The bottom part of the sleeve was fastened around the oat stem or the leaf blade by a pair of cellulose acetate bars glued together at one end and closed at the other by a bulldozer clip. The upper part of the sleeve was closed with a piece of string. The cages were numbered and the date and hour of the setting up and of the ending of an experiment was recorded. The sleeves were released from the plant by cutting the oat stems or leaves and were opened indivi- dually in the sorting cage in the laboratory. The numbered caged females were examined and dissected to assess the development of the ovaries and check for the presence of parasites. The numbered oat segments were examined under the microscope and oviposition was recorded. Records were discarded for those cages where females were found to have died by accident, for example, tangled amongst the folds of the cylinder. (ii) Results The average daily fecundity was calculated for each week and is given in Table 42 for both 1973 and 1974. The oviposition curve is represented in Fig. 39A and B for 1973 and 1974. In 1973 the daily averages were fairly consistent for each week and the largest variability was attained in week 1. Females from week zero were immature as determined • • LCD average no. eggs/day/? average no eggs/day/ r? rt)(•D C•J •-11 ,•••••,, 1-, ;Z:1, 1 H- CD . '1 .11 0 HC CD CD P) H; LCD P) 1:1 CD (1) ct 0 ri- ;71, ';',•." P) S!) I—' CD 1 CD H- !-h CD H CD 0 11 11; - C1 <1 0 CI- c'..) CD P.: 1-1 H. F1 it it 0 1-1 u) H I-h CD CD C) (") Cl Cl (X, . CD I 1<", H, • P.) O >4 cn H• Lo CD .1-1 u.") CD CD • WI -I CD cn P) 0 rt- H. f 0 r-rirri-TrrTrn Cl C • • C Cr; (-) 191 Table 42: Average daily fecundity of caged first generation M. sexnotatus females in the oatfield, 1973 and 1974 Fecundity x ± s.e. Week 1973 1974 0 Pre-oviposition Pre-oviposition 1 5.03±1.7 Pre-oviposition 2 4.21±0.27 4.38±0.42 3 6.33±0.23 3.65±0.43 4 5.41±0.53 2.76±0.49 5 3.45±0.48 2.47±0.55 6 2.17±0.30 2.42±0.37 7 Post-oviposition 3.3 ±0.56 8 Post-oviposition 1.6 ±0.50 9 Post-oviposition Post-oviposition by dissection of specimens in regular samples and from catches by the aerial suction sampler and on sticky traps. On week 7, out of 10 females caged only one oviposited and the oviposition consisted of five eggs over a week's period. The remaining females did not oviposit even when in gravid conditions as detected by dissections. A parallel check to these fecundity estimates was obtained in 1973 from the oviposition performances of caged • females kept for experiments on egg parasitism. The material and methods are described in 8B(i) below Females were caged for four consecutive days, from the 7 to 11 July 1973. A total of 40 females laid 670 eggs in four days with an average of 4.2 eggs per female per day. Reading off the graph of the oviposition period of caged females in 1973 in Fig.39A an average daily rate of 4.325 is 192 obtained for this four day period. The data were also analyzed separately for oviposition in the 11 cages that were flattened down with. the oats by the storm of 7 July and for those that remained upright amongst the erect oats. In the first case, there were 376 eggs in 11 cages and in the second 294 eggs in nine cages. Averages of 34.18±4.04 and 32.67±4.71 eggs per cage were obtained for this period for the 11 and the nine cages. A t-test was carried out to test the hypothesis of no difference between the ovi- position rates in the two sets of cages. The t value obtained was not significant and the null hypothesis was accepted. Therefore, there was no significant difference between the oviposition rate of females amongst the flat- tened and the erect oats assuming that the potential fecundity was the same for all the females obtained simul- taneously from field samples and that the cages did not induce differential mortality of the female in the four days considered. The serial numbers of weeks do not coincide in 1973 and 1974. The estimated oviposition period in 1973 corres- ponds to 10 June to 25 July, and in 1974 to 11 June to 28 July. Therefore, in both years, the oviposition period was • approximately of the same duration. In 1974 the daily averages were fairly consistent within each week but the oviposition rate in 1974 was consistently lower than in 1973. (iii) Discussion The high fecundity obtained in 1973 can not be • explained by effects of temperature when extrapolations are • 193 made from data obtained in constant temperature experiments. It was consistently higher than laboratory experiments would suggest. Messenger and Flitters (1959) have shown that, at least for incubation rates at variable temperatures, the mean developmental minimum, the optimum and the maximum are all shifted down by about 5°C from the values obtained under conditions of constant temperature. The average mean and maximum daily air temperature for each week recorded by the Stevenson screen is also illustrated in Fig. 39. The average mean temperature for the eight weeks was 15.6°C ranging from 14.0 to 17.0°C and the average maximum are 21.5°C ranging from 20.04 to 23,03°C. In both cases the average fluctuations over the eight weeks did not exceed 3.0°C. If these temperature readings are corrected to the temperatures at ground level up to 60 cm in height in the oatfield, then 0.88°C should be subtracted. Then the average mean temperature for the eight weeks becomes 14.7°c with a range of 13.03 to 16.07°C and the average maximum 20.58°C ranging from 19.14 to 22.13°C. The rate of egg laying calculated by the regression equation under constant temperature is given by the equation Y = 1.812 + 0.2402x. The oviposition rates under field conditions were consistently underestimated if use was made of the equation. Under constant temperature conditions and regular and continuous diet with a permanent nutrient status (oat seedlings) M. sexnotatus females did not exhibit a definite peak in the oviposition curve. The egg rate 41, 194 within each experimental temperature was fairly constant with a moderate decline in old age. Under field conditions in 1973 there was a progres- sive decline in the oviposition rate after the peak achieved on the third week. This decline cannot be explained in terms of field temperature as illustrated in Fig. 39. On the other hand, the nutrient status of the oat plant was such as that the maximum plant growth was achieved at week 3. It is suggestive that this decline in the oviposition rate after week 3 could result of an interaction between plant nutrient status and female age. It should be noted that in 1973 the oatfield had been treated with nitrogen pellets and the plants achieved great development of folliage. The 1973 oviposition curve (Fig. 39A) shows a kink on week 2. This decline in the oviposition rate might be due to heterogeneity in the age of the population, since there were three main fluxes of immigration to the oatfield. The first one was on week zero and resulted in the first lot -of settlers. The second and largest flux took place during week one and the third on week two. Migration flights in M. sexnotatus are by young, immature females. Therefore, the lowering in the average daily egg production could be caused by dilution of the settled population by new arrivals of immature females. In 1974 peak oviposition rate occurred early in the oviposition period. This was followed by a continuous and moderate decline except on week 7, when there was an increase in the rate. No 10 ausible explanation is suggested • • 195 for such an increase in the sixth week of the oviposition period, possibly this may be a spurious result, due to chance. The weekly average daily temperatures both for maximum and minimum were lower in 1974 and so was the ovi- position rate. The average mean temperatures for the nine weeks was 15.04°C ranging from 11.8 to 16.2°C and the average maximum was 19.8°C ranging from 17.4 to 22.1°C. If corrected by -0.88°C these values are 14.1 and 18.9°C. [•then mean temperatures are used, the oviposition rates under field conditions are consistently underestimated by the regression equation obtained from laboratory data. Better approximations are obtained using average maximum temperatures. In 1974 the oatfield was not treated with nitrogen. Plant growth, was considerably less luxuriant than in 1973 and the average height did not exceed 0.75 m whereas in 1973 it attained 1.10 to 1.20 m. However, the oviposition curve in 1974 does not seem to be related to the food plant growth. Mochida (1964) found that in Nilaparvata lugens a temporary exposure of the insect to a suitable high tempera- ture affected the oviposition remarkably for a long time after the exposure, both in terms of egg-formation rate and oviposition rate. Under field conditions he suggested that short exposures of females to some suitable high tempera- tures and other appropriate environmental factors induce an increase in the oviposition rate and in the total fecundity. Furthermore, total fecundity and the oviposition period were considerably affected by the condition of the host plant • during the nymphal stages. Similarly the nutrient value of 196 the host plant affected the adult insect, their oviposition rate being directly correlated with the nutrient status. Hetcalfe (1965) found a very significant relationship between the nitrogen status of the leaf and the fecundity of the delphacid sucrar-cane-fly Saccharosvdne saccharivora. This finding was also related to the heavy infestations by the delphacid of young cane and the subsequent decline of its population as the cane aged. Metcalfe (1970) reports that egg production of the delphacid Saccharosydne sacchari- vora was highest on young cane (seven weeks old) and declined on older plants (14-15 weeks old). Also, the plant nitrogen content was affected by the application of fertilizers and a linear correlation between egg production and leaf nitro- gen content suggested a causal relationship. House (1964) remarks that various agricultural chemicals have caused excess plant nitrogen and so benefitted the phytophagous insect pests. McNeill (1973) found a large variation in the numbers of eggs laid by the mirid Leptopterna dolabrata related to the nitrogen content of the food plant. Insects feeding on grasses with an inadequate level of nitrogen had lower. reproductive capacity. The differences encountered between field fecundity and laboratory experiments in M. sexnotatus could be related to several causes, perhaps mainly to the nutritive status of the food plant.- Also fluctuations in the environmental temperature might have an effect on the oviposition rate. Nymphal nutrition does not seem to be an important factor since M. sexnotatus depends mainly on imaginal food intake fdr egg maturation. Another reason for the differences 0 s 197 fecundity could lie in the genetic constitution of the populations. B. Estimation of the number of ovipositing females and natality Female numbers were estimated by regular suction sampling. A random fraction of the catch was set up in cages for fecundity as described in A above and the re- maining females were diSsected to assess the development of the reproductive organs. Additional females from sweep net catches and also from the various traps were dissected weekly. This permitted the estimation of the reproductive state of the population. The total female population was estimated for the oatfield and the figures were plotted as a graph with female numbers on the ordinate and time on the abcyssa. The - points were connected by a line and the intermediate values per day were read off from the graph. The daily estimates were then multiplied by the estimated daily fecundity for each week. Natality was obtained by adding up the weekly esti- mates. The average number of eggs laid per day was obtained by dividing natality by total 'female days'. 6. Estimation of Incubation Rates and Nymphal Duration in the Field Developmental rates were based on field temperatures. Temperatures were read off from the daily chart from the Stevenson screen and were therefore corrected to those of the oatfield. Average daily temperatures were based on readings of maxima and minima for each two-hourly period for the Whole 193 day. The lowest temperatures were seldom less than either 11 or 11.8°C, that is, the theoretical developmental zero for M. sexnotatus eggs-and nymphal stages, respectively. Only temperatures above these values were considered as effective for development and were read off in either case. As a rule, temperatures exceeded these values and 12 pairs of readings were obtained daily and provided the average temperature for the day. The average temperature experienced by the eggs was calculated as follows: the average temperature for each week was multiplied by the natality in that week; the products for each week were added up and then divided by the total egg population for the season (total natality). For the temperature thus obtained the incubation period was calcu- lated from the logistic equation for eggs. The estimation of duration of each nymphal stage in the field was obtained in a similar manner. Duration of instar I and III to V were calculated from the logistic equation; for instar 11 from the regression equation. The incubation period for 1973 and 1974 was estimated as 23.94 and 25.44 days, respectively. The duration of each • nymphal stage was estimated as 7.22, 7.22, 7.43, 8.65 and 12.3 days for instars I to V in 1973 and 6.77, 7.06, 7.91, 10.29 and 16.91 days in 1974. 7. Integration of Population Estimates The series of absolute estimates. for M. sexnotatus from successive sampling days come from a population where 0 successive stages ovorlap. The analysis of this type of 199 data is difficult since the numbers of any one stage are being reduced by moulting and death and simultaneously are being increased by oviposition, hatching and moulting. The oviposition period in the field of first generation females extends for nearly two months so that recruitment is pro- longed and overlaps with mortality. The samples were taken at regular intervals and so as to take each stage in propor- tion to its abundance. As said elsewhere, the egg stage could not be sampled but estimates were made of recruitment into the egg stage. From these series of estimates resulting from the combined effects of recruitment and mortality it is neces- sary to calculate the total number of individuals of each stage for that generation. After this is obtained it can be assumed that the differences between successive stages represent mortality, dispersal, or both. The techniques so far developed for these integra- tions have in common the assumption that mortality is constant within a stage. These methods are applicable for different types of populations and range from simple to elaborate calculations. The crudest and simplest methods of integrations are those of Southwood and Jepson (1962). Both give underesti- mations of the populations. The methods give'estimates to the total population at the median age of a stage. Obviously this value will be very different from the total number of recruits entering a stage whenever mortality is both constant and heavy, unless mortality for some reason takes place at the end of the stage only. In the first procedure the values • 200 are obtained by plotting successive estimates on graph paper and then counting the area under the connecting line; the total is afterwards divided by the estimated duration of the stage. The other procedure consists in dividing the time when a stage is present in the field into periods of varying duration but equal to the estimated development time at the prevailing temperature. The average number of individuals of that stage found during each of these periods is calcu- lated and then summed. With the present data the weekly estimates were multiplied by the number of days in each week which gives values close enough to those to be obtained by graphical summation. The total number for each stage was calculated by adding up these estimates and then dividing by the estimated duration in the field. The figure thus obtained is an approximation of the number of individuals at the median age of a stage. The method based on the regression analysis proposed by Richards and Waloff (1954) is applicable to a stage with a well marked peak and an approximately steady mortality rate. That is, when the curve of numbers is regular and soon reaches an obvious maximum after which the numbers fall continuously away. The slope of the curve after the maximum can be measured and by producing the line back to the date when the stage first appeared in the field an estimate is obtained of the total number of individuals which entered that stage. The regression coefficient is the logarithm of the average fraction of the population surviving each day (daily survival) (Richards & Waloff, 1954; Dempster, 196; Richards, 1959; Southwood, 1966). As pointed out by Dempster 201 (1956) a long hatching period leads to a longer overlapping of the stages and therefore a greater effect of mortality in subsequent stages on estimates of population from a given stage. This method seems unsuitable for the analysis of M. sexnotatus populations since anomalous results were obtained when applied to the present data. The following methods are adequate for populations where the curves for each stage are irregular and a long recruitment takes place so that stages overlap widely. Richards and Waloff's second method (Richards, 1959; Richards, Waloff & Spradbery, 1960; Richards & Waloff, 1961; Southwood, 1966) to determine the number entering a stage can be adequately adopted to estimate mortality if the initial number of the population and the duration of the instars are known. If samples are taken at regular inter- vals each stage will be taken in proportion to its abundance. The total of the ith stage in all the samples is the accumu- lated total, N. This is equal to: a t = P. f S dt (1) 1 1 where! P. is the number recruited into the stage, a. 1 1 is the duration of the stage, Si is the fraction of the population which survives a unit time (the daily survival). Assuming that the mortality in the instar is constant or that it can be averaged then: (Sa - 1) (2) N = P i i loge Si The calculation for the value of S is simplified by substi- logeU a tutingthetermUforS. so that loge . = then: • 1 Ni a • '202 N. = aP U - or (3) logeU (4) aP. be U A table of (U-1)/log U was constructed for values of U from 0.001 to 0.999 by computer with, a programme written by Dr. G. Murdie. As the value of Nia.Pi is known a thecorrespondingvalueofUisreadoffwhichisS. , the survival for the whole stage. Since a is known, the daily survival S. is obtained by taking the antilog of log,,,,U/a. The corresponding percentage mortality is then 100 (1 - S7). In order to apply this formula it is necessary to have an estimate of N o (the. accumulated total of eggs in the field taken in all the samples) and of Po (the total number of eggs laid in the season obtained by the estimated oviposition rate in the field multiplied by the female population). Also, the duration of the egg stage (a0) under field conditions must be estimated. Therefore, knowing the survival in the egg stage it is possible to calculate the number entering the first nymphal instar. The process can be repeated to all instars since P, a and .N are known values in the equation. In the present population study, No could not be • estimated for the egg stage, only P o and ao. A modification of the original equation was carried out by Mr. Nicholas Small so that numbers entering a stage could be calculated in the absence of a value for the accumulated total, N. The method was applied assuming a common survival • rate, S, for two or more stages. • 203 a. 1 S N.1 (5) a. Ni+1 1 S a.14-1 where subscript i refers to the first stage and i+1 to the subsequent stage(s). The solution to survival i.e. S. was obtained by computer programme written by Mr. N. Small. The data necessary to the calculation were: (a) duration of the ith stage (ai); (b) duration of the subsequent stage(s); (c)theratioofNi tothesumofN.and N. After 1 to 1 the ith stage is obtained by Nlog S (6) P = e i 1 - Sa. The recruits entering the subsequent stage(s) can be calculated either by Richards and Waloff's formula (4) or by: a. (7) = (P.) (S 1+1 1 1). After the number of recruits to the first nymphal instar has been estimated, the egg survival can be calcu- lated by: 1/a0 (8) = (P./Po ) • ao Total egg survival S is then easily calculated as well as the corresponding percentage mortality 100(1 - Sao). As pointed out by Richards et al (1960) an accurate estimate of the initial P is necessary to apply this method_ Also, duration of the stage has a great bearing on the accuracy of the estimated mortality. Sampling errors and irregularity in mortality can lead to 'negative mortalities' • and to overcome this, nymphal stages are grouped and the • 204 total mortality in several instars is combined (Richards Waloff, 1061 Solomon, 1973). In the last instance, the estimation of the relative efficiency of the sampling method for each stage plays an important role since some stages may be sampled less efficiently than others and therefore underestimated, The tests carried out in 1973 to estimate the efficiency of D-vac sampling of the nymphal stages of M. sexnotatus in flattened oats and in long oats did not provide large enough numbers to permit estimates of efficiency for each instar separately. Therefore, a correction factor was adopted for numbers of instars I, II and III and another one for instars IV and V. Equation (5) was solved for instars I and II assuming an identical survival rate for both stages. Recruits to instar I were determined by equation (6). Egg survival was subsequently determined by equation (8). Recruits to instars II and III could be estimated either by equation (7) or by Richards and Waloff's equation (4). A negative mortality is obtained in instar III, since N > P-a if the calculation is started with estimates based on the common daily survival of instar I and II. This suggests that these stages are underestimated in relation to instar III, since its relatively high value could not be accounted for by migration. Equation (5) was then solved for several combinations of instars assuming a common daily survival: (a) I + II; • (b) I + (II + III); (c) III + IV; (d) III + (IV + V) and • 205 (e) IV V. The common daily survival rates obtained were: (a) 0.9709; (b) 0.9760; (c) 0.9743; (d) 0.9746 and (e) 0.9752. Comparison of the figures for survival indicates that instar III was overestimated in relation to instar I and II since (a) gave the lowest survival and when instar III was com- bined (b) gave the highest survival of the five combinations, whereas comparing (c), (d) and (e), survival of instar III was lower than that of IV and V. The results for (c), (d) and (e) are fairly consistent and give support to the assumption that instars III, IV and V are reasonably well weighted in relation to each other. The highest survival for these last three instars is given by instar V as can be seen by comparing (c) and (d). When instar V is included in the combination, a higher common daily survival is obtained. Recruits entering the third instar were then deter- mined by equation (6) after estimating a common daily survival for instar III, IV and V. Comparison was then made with the figure obtained for recruits entering the third instar based on the common daily survival of I and II and a difference of 10.04% of the former was found which was interpreted as the degree of underestimation of the first two instars. The 4 calculated figures for the instar I and II based on their common survival rate were then corrected by dividing by 0.8996. Recruits to instars IV and V were determined by equation (6) and to adult stage by equation (7). In 1974 the starting point for the estimates was based on the average daily survival for instar II and Recruits to instar I were obtained by equation (6). Recruits • to stages II and III were determined by formula (7). A • 206 'negative mortality' is obtained if equation (4) is applied to the third instar in order to estimate the survival within this instar Stages III, IV and V were then combined and equation (4) was applied to the data. An average daily survival was obtained for the three instars, and recruits to instar IV and V were determined by (7). In parallel, equation (5) was solved for several combinations of instars assuming a common daily survival as follows: (a) I + II; (b) I + (II + III); (c) I + (II + III +IV); (d) I + (II + III + IV + V); (e) II + III; (f) II + (III + IV); (g) II + (III + IV + V); (h) III + IV; (i)III + (IV + V); (j) IV + V. The daily survival rates obtained were: (a) 0.9526; (b) 0.9603; (c) 0.9608; (d) 0.9634; (e) 0.9755; (f) 0.9685; (g) 0.9693; (h) 0.9562; (i) 0.9648; (j)0.9761. Comparison of the figures for survival indicates that the highest survival rates are those of instar II and V, followed by IV, then III and lastly of I. Also, that survival of instar I is less than 0.9526, of instar II is greater than 0.9755, of instar III is less than 0.9562 and greater than 0.9526, of instar IV is less than 0.9685 and greater than 0.9603 and of instar V greater than 0.976. Equation (6) was then applied to each instar using the above estimated survivals. 0 Riritani and Nakasuji's (1967) method of estimation of the stage-specific survival rate and numbers recruited to an instar consists of an adaptation of MacDonaid's (1951) and Hokyo and Kiritani's (1967) formula to estimate daily survival rate of adult females censused at adequate regular intervals. The daily survival rate(s) is assumed as a constant throughout the life. The mortality in the ith instar is given by: 207 N. a. (1) = 1 S N t where N. is the accumulated total of the ith stage in all samples and N of the ith stage plus all subsequent stages, t and a_ is the developmental period of the ith stage. The survival rate of the ith stage is then: a. N. (2) S = 1 - N. If the total number of eggs deposited (Po) is known, the recruits into instar I (P*) can be estimated as: a P* = (3) Po) (S 0) As stated above, No could not be estimated for eggs. Furthermore, M. sexnotatus adults leave the oatfield soon after emergence. Hence, the inclusion of adult estimates in the above equations results in biased estimation of P and S. A formula adapted by Mr. N. Small where adults are removed from the equation is: N. a. (1 - S (4) Nt-a (1 - Sat-a) where t-a denotes the nymphal stages and excludes the adult stage (compare with the modification of Richards and Waloff's method). The solution to equation (4) for S was obtained by a computer programme written by Mr. N. Small. Equation (4) was applied to instars I to IV and S (daily survival rate) was estimated for each of the four stages. The number of recruits to the ith stage (P.) was obtained by the equation: a 208 P. = (5) a, S,(1 - - Si) where N. is the accumulated total of the ith stage in all the samples.• S. is the fraction of the ith population which survivesaunittime-(thedailysu anda.is the ai durationoftheithstage,sothatS.- is the survival of the whole ith stage. Equation (5) was applied to instars I to IV to estimate the recruits entering a stage. Numbers entering stage i+1 can be estimated by: a. (6) Pi+1 = (P ) (S 1) i i which is the same as numbers leaving the ith stage. Numbers entering and leaving the fifth stage, the latter correspon- ding to numbers entering the adult stage, are thus determined assuming a common daily survival rate for instars IV and V. Egg survival was determined by equation: (7) S = (P /P ) . o I o a Dempster (1961) proposed a method where it is not .necessary to know the duration of a stage and the initial number of - individuals but the rate of entry to the first stage must be known. It can only be applied when genera- tions are distinct. This method could not he used with the present data because it is necessary to have estimates of the number of eggs hatching between sampling occasions. Furthermore, this method requires a large number of accurate population samples taken at short intervals thus restricting its applicability. If the conditions above can be met with, this method provides the further advantage of giving estimates to the effects of factors affecting population changes like emigration or immigration. • 209 Other methods making allowances for the effect of migration are given by Waloff and Bakker (1963) to determine the total change in a population. Both methods proposed require that adult life may be divided into three phases consisting of a pre-flight period, the flight period and a post-flight period. The simplest one gives a graphical treatment to the data and the more elaborate one is based on Richards and Waloff (1954) regression method. Two regression equations are calculated. The first one on the estimates of the population at and after the peak to the end of the flight period. The regression coefficient represents here the combined result of mortality and flight. The second equation is calculated for the post-flight period hence the regression coefficient corresponds to mortality only. The difference between the two coefficients is expressed as a percentage of the larger and represents the decline in numbers of the initial population due to migra- tion. Since M. sexnotatus populations do not present a post-flight period the above methods are not applicable to the present data. A new method was recently proposed by Manly (1974) for populations with overlapping stages and allegedly with certain advantages over the earlier methods. The durations of stages, the numbers entering the first stage and the rate of entry to the first stage do not have to be known. Also, samples do not have to be taken at regular intervals of tine. The method gives estimates of the numbers entering each stage, the mean duration of a stage and the daily survival rate. According to Manly (1974) the method is similar to • that of Richards and Waloff (1954) but the time of entry to • 210 gages is assumed to follow a normal distribution. This method is supposed to provide reasonable estimates whenever the true entry distribution is unimodal. It has the dis- advantage of the complexity of the computation. An attempt was made to estimate numbers entering a stage by this method using the standard computer programme supplied by the author that does not make allowances for early migration by the adult stage. No reasonable estimates were obtained as compared to those given by other methods. The limit of. time prevented further attempts towards better estimates to the present set of data by altering the original programme. Due to insufficient sampling the data for 1972 is inadequate for the methods applicable to the two subsequent years. If an assumption is made that stages I and II were already sampled at peak numbers some estimation can be obtained using the crude method of Cheng and Le Roux (1966) where the number of recruits into a stage is calculated by taking the totals of each stage obtained at peak combined with subsequent stages. As pointed out by Solomon (1973) it is not always the sample on the occasion. of peak occur- rence of a stage that gives the greatest total of individuals in that stage. This figure can be obtained by calculating • the total of an instar and subsequent instars for every week in which the instar is present and selecting the highest total as an estimate of recruits entering a stage. This procedure was adopted for the 1972 data. The figures for the total area for recruits entering instars I to V were respectively 101307, 91810, 78350, 64900 and 37900. Unless immigration has taken place, this method will give under- 0 estimates since it disregards mortality between the time of • 211 the entry into the instar and the time of the sample (Solomon, 1973). The estimated numbers at median age of a stage for 1973 and 1974 obtained from Southwood and Jepsons's method are given in Table 43. Table 43: Numbers at median age of a stage estimated by Southwood and Jepson's method a Number at Year Stage Ni i median age 1973 I 2695621 7.22 373355 II 2179100 7.22 301814 III 2035207 7.43 273917 IV 1922900 8.65 222300 V 2105700 12.3 171195 1974 I 6302400 6.77 930930 II 4701600 7.06 665950 III 4380000 7.91 553730 IV 3804000 10.29 369680 V 4518000 16.91 267180 The estimates for recruits entering nymphal and adult stages for 1973 and 1974 are given in Table 44. The figures were obtained from the adaptations of the methods of Richards et al. and of Kiritani and Nakasuii. The procedure adopted for the 1973 estimates by the first method is described in the text above. For 1974 an average was taken for instar III and IV of the lower and upper ai bounds of estimates for P i, Si and S as given in Table 45. The values obtained from the adaptation of Richards et al. method were adopted in the construction of the budgets for 1973 and 1974 since it provides better estimates of the daily • survival than does the other method. • 212 lab In 44: Numbers of recruits entering a stage (nymphs and adults) estimated by: (A) adaptation of Richards et al. method; (B) adaptation of Kiritani and Nakasuji's method (see text for description). Recruits to egg stage estimated in the field. cti 0 H • -P w Year :15_; w Eggs I II II I IV V Adult 1973 (A) P i 3711293 460789 372388 300895 247918 184718 134665 S.1 0.9165 0.9709 0.9709 0.9746 0.9746 0.9746 c. 0.1241 0.8081 0.8081 0.8262 0.8007 0.729 (B) P. 3711293 410943 327589 304790 250498 201517 147892 Si 0.9122 0.9766 0.98 0.9746 0.9752 0.9752 S'7,' 0.1107 0.8432 0.8646 0.8263 0.8045 0.7339 1 974 (A) Pi 3748512 1092144 725952 662226 443004 325620 216192 S.1 0.9527 0.9526 0.9755 0.9544 0.9644 0.9761 Sa1 0.2913 0.72 0.8393 0.6914 0.6892 0.6639 (B)P.1 3748512 1073136 753680 647475 422758 329498 218765 Si 0.952 0.9634 0.9693 0.9648 0.9761 0.9761 Sa1 0.2863 0.777 0.8023 0.7531 0.7794 0.6639 2 Table 45: Numbers per m of recruits entering a stage (Pi) based on different estimates of daily survival • (Si), 1974. (See text for description). Common S for instars I and II and also for. instar III, IV and V S.. S.a P. Stage 1 I 0.9526 0.72 910.12 II 0.9526 0.7099 P* 655.27 III 0.9735 0.6314 P* 465.19 IV 0.9735 0.7587 P* 293.72 V 0.9735 0.6353 P* 222.86 • Adult - - P* 141.58 • 23.3 Table 45 (Continued) Independent estimates of S for each instar S. S. P. =(P.)(s.a ) Stage 1 i Stage 1+1 ' i 1 I 0.9526 0.72 910.12 II 655.27 II 0.9755 0.8393 604.96 III 507.78 III 0.9562 0.7015 548.05 IV 384.46 III 0.9526 0.6812 555.66 IV 378.53 IV 0.9685 0.7194 361.58 V 260.12 IV 0.9603 0.659 376.76 V 248.28 V 0.9761 0.6639 271,35 Adult 180.16 Adult - - P7'180.16 8. The Population Budgets The primary advantage of a budget is to present a simple summary of population change within one generation. It gives figures for the age distribution and mortality that can be readily analysed. Budgets were constructed for M. sexnotatus second generation populations in the oatfield in 1973 and 1974 with the following headings: x = age interval lx = number living at beginning of stage in x column dx = number dying within age interval in x column 100 qx = percentage mortality Sx - survival within x. The figures for the lx column are the estimates of the numbers at the beginning of each stage calculated by the adaptation of Richards et al. method (Table 44). The budget for 1973 is given in Table 46. a • 214 Table 46: Population budget for M. sexnotatus, 1973 lx dx 100qx Sx Egg 3711293 3250504 87.6 S 0.124 Instar I 460789 o 88401 19.2 S 0.808 Instar II 372338 I 71493 19.2 S 0.808 Instar III 300295 II 52977 17.6 S 0.824 Instar IV 24791.8 III 63200 25.5 S 0.745 Instar V 184718 IV 50053 27.1 S 0.729 Instars I to V 1566708 v 1432043 91.4 0.086 Adult 134665 S I to V Generation totals 3576628 96.4 S 0.036 g For 1974, three parallel budgets were constructed according to the different values of P1 estimated by the different procedures as given in Table 45. The lx column in Table 47 is derived from the figures given in Table 44 for numbers of recruits entering a stage. In Table 48, from instar II onwards these figures were obtained as di a = (P)(S iP i+1 1) where Pi and S are respectively the numbers entering the preceding stage and the survival of the preceding stage calculated from the independent esti- mates of daily survival (S i) for each instar. Here again an average was taken for instar III and IV of the lower and upper bounds of estimated for Pt+1. In Table 49, from instar II onwards the values from the lx column were again obtainedasPt+l aboveandP.and S were calculated from 1 i a common S for instars I and II and also for instars III, IV and V as given in Table 45. • 215 Table 47: Population budget for M. sexnotatus, 1q74 (see text) x lx dx 100qx Sx Egg 3748512 70.9 S 0.291 2656368 o Instar I 1092144 3 66192 33.5 ST 0.665 Instar II 725952 0.912 63726 8.8 S_11 Instar III 662226 219222 33.1 STII 0.669 Instar IV 443004 117384 26.5 SIV 0'735 Instar V 325620 S 0.664 109428 33.6 y Instars I to V 3248946 S 3032754 93.3 I to V 0.067 Adult 216192 Generation totals 3532320 94.2 S 0.058 g Table 48: Population budget for M. sexnotatus, 1974 • (see text) x lx dx 100qx Sx Egg 3748512 70.9 S 0.291 2656368 o Instar I 1092144 305820 28.0 ST 0.72 Instar II 786324 176988 22.5 SII 0.775 Instar III 609336 0.751 151542 24.9 SIII Instar IV 457794 S 0.666 152754 33.4 IV Instar V 305040 29.1 S 0.709 88848 y Instars I to V 3250638 3034446 93.3 SI to V 0.066 Adult 216192 Generation totals 3532320 94.2 S 0.058 g • • 216 49: Population budgets for M. sexnotatus, 1974 (see text) x lx dx 100qx Sx Egg 3743512 2656368 70.9 S 0.291 Instar I 1092144 o 305820 28.0 SI 0.72 instar II 786324 228096 29.0 S11 0.71 Instar III 558228 S 0.631 205764 36.9 III Instar IV 35246 85032 24.1 S 0.759 instar V 267432 IV S 0.635 97536 36.5 y Instars I to V 3056592 2886696 94.4 S 0.055 Adult 169896 I to V Generation totals 3578616 95.5 S 0.045 g 9. The Analysis of Budget Data The calculation of a meaningful population trend from generation to generation cannot be carried out for most pest insects of arable crops. Both the Morris's key factor analysis and the Varley and Gradwell's method to test the role of each factor in the population trend require con- tinuous populations for which a series of estimates have been made in successive generations. The investigation of • the roles of different factors in single-generation budgets can be carried out to determine the relative importance of natality as compared with mortality in determining the size of the final population. This bears a practical approach in the integrated pest control programmes for pests of arable crops to compare the role in population fluctuation due to mortality by parasites and predators in relation to the variation in the numbers of eggs laid (total natality) resulting from the numbers of females invading the crop and S 217 their survival and fertility (Southwood, 1966). The budgets can be analysed either by a graphical method or by a regres- sion analysis and only data on total natality and number of the resulting population (here emerging adults) are necessary. The present data were insufficient for this analysis since a series of generations are necessary to use either method. Comparison of mortality factors within a generation can be made by several means. Apparent mortality is the measured mortality representing the numbers dying within a stage as a percentage of the numbers entering the stage. It corresponds to column 100qx in the budget. Real mortal- ity is calculated as the percentage dying within a stage in relation to the initial numbers i.e. number of recruits, in that generation. The values are additive and permit comparison of the role of population factors within the same generation. Indispensable (or irreplaceable) mortality is that part of the generation mortality that would not occur should the mortality factor in question be removed from the life system, after allowance is made for the action of subsequent mortality factors. Here an assumption is made that these factors will cause the same percentage mortality a independent of the change in density (Southwood, 1966). The results for these calculated mortalities for 1973 and 1974 are given in Tables 50 to 53. The total real mortality for second generation M. sexnotatus in the oatfield was as high as 96.4% in 1973 and between 94.25 and 95.5% in 1974 according to the different estimates of P. and S.. • 218 Table 50: Comparison of mortality factors in 1973 (data based on budget given in Table 46) % % % Apparent Real Indispensable Stage mortality mortality mortality Egg 87.6 87.6 25.6 I 19.2 2.38 0.86 II 19.2 1.93 0.86 ITT 1 7.6 1.42 0.77 IV 25.5 1.7 1.24 -,1 27.1 1.35 1.35 Total: 96.4 Table 51: Comparison of mortality factors in 1974 - (data based on budget given in Table 47) . % % % Apparent Real Indispensable Stage mortality mortality mortality Egg 70.9 70.9 14.03 I 33.5 9.77 2.9 II 8.8 1.7 0.56 III 33.1 5.85 2.85 IV 26.5 3.13 2.18 V 33.6 2.9 2.92 Total: 94.25 Table 52: Comparison of mortality factors in 1974 (data based on budget given in Table 48) a % % % Apparent Real Indispensable Stage mortality mortality mortality Egg 70.9 70.9 14.02 I 28.0 8.16 2.4 II 22.5 4.72 1.67 III 24.9 4.04 1.9 TV 33.4 4.07 2.9 V 29.1 2.37 2.37 • Total: 94.26 219 Table 53: Comparison of mortality factors in 1974 (data based on budget given in Table 49) o % % Apparent Real Indispensable Stage mortality mortality mortality Egg 70.9 70.9 11.01 I 28.0 8.16 1.76 II 29.0 6.08 1.85 III 36.9 5.49 2.64 IV 24.1 2.27 1.43 V 36.5 2.6 2.6 Total: 95.5 The greatest mortality occurred in the egg stage in both years, being 87.0% in 1973 and 70.9% in 1974. Mor- talities in the nymphal stages were much lower in both years. The heaviest nymphal mortality occurred in the first stage both in 1973 and 1974. Total natality in both years was remarkably similar. In 1973 there were lower numbers of invading females but the oviposition rates were higher than in 1974 when larger numbers of females invaded the oatfield. Egg survival was higher in 1974 but the larger numbers entering the nymphal • stages suffered heavier mortality than in 1973. The numbers of resulting second generation adults in both years were not much different. No details are available for losses in the egg stage apart from the field experiment on parasitism carried out in 1973 where a high percentage parasitism was detected following peak natality in the field. • • '7 0 The main cause of nymphal mortality was probably predation by Arthropods Parasitism was consistently low throughout the season in both years. The initial nymphal population in the field in 1974 was 1=e than twice as large as in 1973. Corresponding with this higher nymphal density in 197A was a much heavier mortality than in 1973. 10. Parasitism in the Egg Stage In natural grasses Two experiments were set up in the summer of 1972. The first one was designed to assess parasitism of eggs oviposited by first generation females of M. sexnotatus and the second one in eggs of second generation females. Both basically consisted of the planting of an egg population in the field and its subsequent re-examination to obtain the egg parasites since the very low density of the natural population of :Ti. sexnotatus prevented direct observation by sampling leaf blades containing Macrosteles eggs. Three females and two males were isolated in a cage described in section 1, part 4D. Four oat seedlings were planted in each pot and females were allowed to oviposit for a week. All females were in the beginning of the oviposition period and together with the males were obtained from the 20°C constant temperature room cultures of M. sexnotatus. Oviposition took place at-25°C to accelerate the rate of egg maturation and oviposition. After a week the insects and the cages' top cylinder were removed and the pots were taken to the experimental plot. The pots were placed directly on the soil amongst the tufts of 0 221 grasses and the position was recorded in order to recover the pots after a period in the field. Three pots were allocated per each sub-area; two amongst the longer grasses and one amongst the shorter, growing grasses. Therefore, a total of 24 pots was exposed to parasites. On the eighth day the pots were brought back to the laboratory. The cellulose acetate cylinder was replaced in each pot. The cages were then covered by black paper cylinders to prevent light reaching the cage except for a 2.5 cm distal hole coincident with the cages distal one through which a collecting tube was connected to the interior of the cage. This was a 7.5 x 2.5 cm inverted glass tube containing a tightly fitting 3 cm long inverted • cone made of thin cellulose acetate. The cone was open at both ends to act as a funnel and imprison insects attracted by this only source of light. The cages were covered by the black hoods and the collecting tubes were set up every other day for a 24 hour period. After that the collecting tubes were brought into the laboratory to be examined under a binocular microscope. In the first experiment eggs were exposed from 28 June to 5 July, corresponding to the main period of ovi- position of first generation females. In the second experi- ment eggs were exposed from 17 to 24 August, corresponding to the beginning of the oviposition period of second generation females. In the first experiment, out of the 24 pots exposed, five were attacked by egg parasites (20.8%) while 19 were not. • 222 Of these five, four. had been among the longer grasses and one amongst the short grasses. M. sexnotatus nymphs were obtained in variable numbers from all of the 24 cages. Altogether, 14 specimens of egg parasites were obtained. The first specimen was available after 16 days at the 20°C constant temperature room and the last one after 32 days. The specimens were identified as Anagrus near subfuscus FOrster and Gonatocerus possibly paludis (Debauche) (Mymaridae) and Oligosjt.a engelhardti Kryger (Tricho- grammatidae). The last one is already reported in the literature as a parasite of the eggs of Cicadula sexnotata Fall. in Sweden (Thompson, 1943-58). Also as egg parasites of Cicadula sexnotata Fall. in Sweden are recorded Anagrus atomus L. and Gonatocerus radiculatus Ahlberg (Thompson, 1943-58). the second experiment, six of the 24 pots were attacked by egg parasites, that is, 25% of the total. All pots with parasitised eggs had been amongst the long grasses. Thirty specimens of egg parasites were bred out. The first specimen was obtained after 15 days in the 20°C constant temperature room and the last one after 32 days. Twenty of the specimens were Anagrus near subfuscus FOrster and emerged between 8 and 15 September 1972. The remaining specimens were the trichogrammatid Oligosita engelhardti Kryger and the mymarid Gonatocerus litoralis (Haliday) emerging respectively between 15 and 25 and between 18 and 20 September. • • 223 B. In the oatfield (i) Materials and methods An egg population of M. sexnotatus was 'planted' in the oatfield to assess qualitatively and quantitatively egg parasitism. Again, the low density of eggs per foliage prevented direct assessment by egg sampling. Females set up in this experiment were obtained from samples from the oatfield. on 6 July 1973. Altogether, 40 females were set up in twenty cages in the field, each one containing two females. The cages consisted of 25 cm long and 10 cm in diameter cellulose-acetate cylinders. The cylinder was provided with two 7 x 7 cm windows covered with terylene placed opposite to each other at different heights. Both ends of the cylinder were prolonged in 20 cm long terylene sleeves. Two such cages were set up in each sub-area, the location within each sub-area being defined by random numbers as described for sampling, in part 3B. • The sleeve was introduced over the oat plant and set up in such a way as to include two consecutive leaves. The two leaves were so arranged as to fit properly inside the length of the rigid cylinder. The area enclosed in the sleeve was approximately 30 cm long. The average leaf length was 34.78±0.7 cm based on the measurement of 36 leaves. The sleeves were fastened firmly around the culm with a piece of string, as close as possible to the cylinder gr to avoid any-loose fold of the fabric where the females 224 could get fondled. The average distance of the bottom leaf from the soil was 40.55±2.42 cm. The cages were set up in the field on 7 July and were removed on 11 July. The eggs were exposed to egg parasites from 11 to 17 July, This period was immediately after peak oviposition in the field as assessed by caged females (part 5A). On the day of the setting up of this experiment a storm flattened down 60% of the area of the oatfield as described in part 1E. Eleven of the 20 cages were then flattened down and were almost or totally covered by adjacent collapsed oats. The other nine cages remained erect. On 11 July the cages were located and examined. All had remained firmly attached to their original positions. The points of contact of the sleeve with the culm were identified by a cotton thread around the culm and the sleeves were then removed. On 17 July the plants were again located and the segment of the oat plant where the sleeve had stood was cut with a pair of scissors and placed in a numbered polythene bag. The plants were immediately examined in the laboratory under the binocular microscope. The number of eggs was recorded for each plant and their position, either in the leaf blade or leaf sheath. The segments of the oat plant containing the eggs were then cut off and set up as des- cribed in Section I, part 4E(i). A pair of 8.7 cm Petri dish lids were adopted as the container and were placed one against the other so that the borders met neatly and the • 225 height was 2.7 cm. Fragments of plant material from each field cage were set up in individual numbered boxes. Small bits of adhesive tape were used to hold firmly together the two lids. The 20 numbered boxes were then transferred to the 20°C constant temperature room with 16 hours light per day. The boxes were examined at 24 hour intervals. The emerged first instar M. sexnotatus nymphs and egg parasites were removed from the boxes in a sorting cage described in part 3B. The numbers were recorded daily for each cage and the egg parasites were transferred to numbered tubes with 70% alcohol for future identification. (ii) Results A total of 670 eggs were oviposited by the caged M. sexnotatus females. Of this total 69 eggs were oviposited in the leaf sheath (10.3%) adjacent to the ligula and 601 eggs were laid in the leaf blade (89.7%). The experiment was analysed separately for eggs in cages where oats had been flattened down and for those that remained erect. The data is summarized in Table 54. X- tests • were carried out to test the null hypothesis of homogeneity between parasitism in the areas where oats were flattened 2 down and where they remained erect. A X test-was carried out for the hypothesis of homogeneity in both areas of numbers of egg parasites hatched, of M. sexnotatus eggs 2 hatched, and of unhatched non-parasitized eggs. A X of 17.05 for 2 degrees of freedom was obtained, and the null hypothesis was rejected at P < 0.001. Another X- test was carried out this time for the hypothesis of homogeneity in • 226 Table 54: Parasitism in M. sexnotatus first generation population of eggs in the oatfield, 1973 Number of cages 11 in 9 in flattened erect Total oats oats 20 Total number of eggs 376 294 670 Total number of egg parasites 142 157 299 Total number of sexnotatus nymphs 205 124 329 Total number of unhatched non-parasitized 29 13 42 eggs % egg parasitism 37.8 52.9 44.63 % of M. sexnotatus nymphs 54.5 41.6 49.1 % unhatched non- parasitized eggs 7.7 4.4 6.27 numbers of parasitized eggs and non-parasitized eggs in both areas. A X2 of 16.32 for one degree of freedom was obtained and the null hypothesis was rejected at P < 0.001. On 27 July the twenty boxes were examined by trans- mitted light under the binocular microscope and 238 eggs were easily detected as parasitized since the parasite could be seen through the transparent chorion. Therefore, after • 10 days of incubation under laboratory conditions it was possible to detect 79.6% of the total parasitism. Only one parasite occurred per egg. Both from direct observation and from counts of numbers of emerging egg parasites, M. sexnotatus nymphs and subsequent check up of unhatched eggs, the number of parasites per egg remained constantly one. Egg parasitism per cage ranged from zero to 100% in • the flattened oats and from 5,5% to 89.0% in tl-le erect oats. • 227 The average percentage parasitism per cage for flattened oats and erect oats was respectively 31.13 and 48.6. In three of the 11 cages amongst the flattened oats there was no parasitism at all and in one of the cages 100% of the 52 eggs were parasitized. Fig. 40 illustrates the hatching curve of egg parasites in the laboratory. Eggs were exposed to the attack of parasites for six consecutive days in the field. The peak hatching in the laboratory took place on the 15th day of incubation at 20°C. The parasitism figures are summarized in Table 54. A higher percentage parasitism was obtained from eggs in erect oats (52.9%) as compared to eggs in flattened oats (37.8%). A linear regression analysis was carried out to test the hypothesis of independence between density of hosts and percentage parasitism. The data was transformed to logarithms. The x values were the log number of eggs per cage and _the y values were transformed to k values where k = log(N/S), N is the number of hosts exposed and S is the number of hosts not parasitized. The analysis was carried out separately for cages in the erect oats and flattened oats. In the first case a correlation coefficient of 0.662 was obtained and t-value for two degrees of freedom was 2.335 where t > 0.10 < 0.05. The null hypothesis was rejected at the 0.10 level for the eggs in the erect oats cages and accepted for the eggs in cages amongst flattened oats. • • • Fig, 40.Thehatchingcurveof4. sexnotatuseggparasites Q 61 hatc hing / day 60 60 r- 70 50 30 20 10 0 a 23 2527 1 iuly _.---0 I -0- represents mainly Anara,rusnearsubfuscusForster; incubated at20CCconstanttemperature (solidline broken linemainly othereggparasites). 8 I -- I 10 I i 12 29 311 I days ofincubationat20°C I 14 I I august 16 I 3 I 18 I 5 I 20 I 7 I 22 I 9 1 24 22 I 11 1______I 26 M 229 The Spearman rank correlation coefficient for the nonparametric measure of correlation was also calculated. The hypothesis tested was that these two variables (host density and percentage parasitism) are not associated in the population and that the observed value of rs differs from zero only by chance. The analysis was carried out k separately for eggs in erect and in flattened oats. In the first case the Spearman rank correlation coefficient (rs) was 0.82 > P 0.01 0.783 and for n = 9 and the null hypothesis was rejected at the 0.01 probability level for a one-tailed test. Thus, in the population of eggs from which the sample was drawn, egg numbers and percentage parasitism are associated. The null hypothesis was accepted for eggs in flattened oats with rs = 0.44 for n = 11. The main species of parasite was the mymarid Anagrus near subfusCus F8rster corresponding to approximately 85% of the total number of parasites hatched. Other species were the mymarids Gonatocerus litoralis (Haliday) (9), G. paludis (Debauche) (1), Gonatocerus sp. (1) and Anagrus sp. (1), the trichogrammatid Oligosita possibly engelhardti Kryger (1) and the aphelinid Centrodora livens (Walker) (1). • The egg parasites both in this and in the preceding experiments were identified by Dr. M.W.R. de V. Graham. (iii) Discussion According to personal communication of Dr. I. Walker, the species here named Anagrus near subfuscus FOrster is also found in Silwood Park parasitizing the eggs of the delphacids Laodelphax elegantulus (Boheman) and Dicranotropis 1 • 230 hamata (Beheman). She was able to breed them for twelve consecutive generations on eggs of the latter. Tf truly bred'out from an egg of M. sexnotatus the aphelinid specimen Cenfrndora livens (Walker) is a new record for leafhopper egg parasites. Other species of the same genus have been reported to parasitise eggs of locustids and cicads (Clausen, 1962). A trend towards a density dependent relation between host density and percentage parasitism of eggs in the normal, erect oat plants is indicated by the linear regression analy- sis and the Spearman's coefficient of correlation. When k-values are plotted against host densities the scatter of the points indicate that the relationship varies to some extent. A much larger variation is found where eggs were amongst the collapsed oats and no statistical evidence was found of a density dependent relationship. There are records in the literature of non-random search by mymarids e.g. Whalley (1969) for Anaarusincarnatus and A. atomus; May (1971) for A. incarnatus; Checke (1974) for Alaptus fusculus. It is not known whether the mymarids parasiting M. sexnotatus eggs will also display a non-random a behaviour in the search of hosts eggs. When parasites search in a non-random fashion they respond to the spatial distribution of the hosts and in some instances tend to remain for a longer time in areas where a prey had already been found (Varley, Gradwell & Hassell, 1973). As a consequence, a higher percentage mortality is found in areas with a denser prey population (Hassell, 1966). The • 231 density dependent response to the host distribution therefore results from the tendency of the parasites to search more intensively where more prey is found. From the data available it seems likely that the parasite population was rather evenly distributed where • oats remained erect since eggs from the nine randomly distributed cages were all attacked to a higher or lesser extent. On the other hand, the egg parasite population seems to have been much less evenly distributed amongst the collapsed oats. It is possible that eggs amongst the collapsed mat of oats were less accessible to search and attack by the parasites. A rather patchy distributZ.011 of parasites is suggested by the fact that in three of the eleven cages all eggs remained sound while in one cage • 100% of the 52 eggs were parasitized. In the erect oats the situation is closer to the artificial environment of the laboratory arenas where a given number of parasites is confined with a given number of hosts. 11. Parasitism in the Nymphal and Adult Stages A. Introduction • The nymphal and adult stages of auchenorrhynchous Homoptera are commonly subjected to endoparasitism by the Pipunculidae (Diptera), the Drvinidae (Hymenoptera) and the Strepsiptera. Waloff (1974b) has recently studied the parasitoids of nymphs and adults of leafhoppers of acidic grasslands in Silwood Park, She reports a frequent range of 10 to 25% parasitism of the cicadellids by the Pipunculidae and no more • 232 than 209, by the Drvinidae. Host specificity of the most common parasitoids in general extends to family or sub- family level. Parasitism by Strepsiptera was found to be restricted to the Delphacidae, the exception amongst the cicadellids being Ulopa reticulata which has a specific strepsipteran parasitoid. Also, most of the leafhopper species were multiparasitised by the pipunculids and the dryinids. In the present study only minor incidence of para- sitism by Dryinidae and PipuncUlidae was found in the M. sexnotatus populations in the oatfield. Waloff (1974b) gives the combined percentage parasitism of three species of Macrosteles (M. sexnotatus, M. laevis and M. viridigriseus) in natural grasses for eight continuous generations. The percent parasitism ranged from zero to 38.7 and the higher percentages occurred either in the first or second generation. According to Waloff (1974b) the general trend in cicadellids and delphacids is to have greater parasitism by the Dryinidae in the first generation •and by the Pipunculidae in the second. Density dependent relationships are suggested, since dryinids cause greater mortality when prey is in low numbers and pipunculids when prey populations are at their maximum. Macrosteles specimens were dissected immediately after being killed in 70% alcohol to facilitate the detec- tion of young first instar pipunculid larvae, since they are not detectable from external examination oft he host. The bigger, mature first instar larvae and the second instar 0 larvae can be easily detected by the expansion of the hosts • 233 abdomen. Female host specimens are more difficult to sort from non parasitized ones since the distention of the abdomen also occurs in gravid females. Individuals parasitized by the Strepsiptera and Dryinidae are readily detected by external inspection since the parasites protrude from the host. The set up for obtaining parasites was the same as that of Waloff (1974b) with the same results: it is satis- factory for both dryinids and pipunculids that emerge in the same season out of the first generation hosts but high mortality is obtained from the diapausing parasites. B. Parasitism by Pipunculidae Altogether 907 specimens of M. sexnotatus were dissected in 1973 to detect pipunculid parasitism. Of this total, 35 specimens i.e. 3.86% were parasitiZed. Lower parasitism was found in the first generation invading adults available in field samples from week zero to eight of 1973. The overall parasitism detected for this period was 1.78%. Higher figures were obtained for the second generation. In weeks nine to 12 the percentages•cfparasitism were 5.5, 2.7, 2.9 and 1.8. - Higher values were obtained during. weeks 13 to 15 where virtually all specimens available in the field were already adult. Parasitism increased to 10, 8 and 17.6. In 1973, seven specimens of second generation M. sexnotatus suspected of harbouring pipunculid parasites were set up as described in Section I, part 4E(ii). In all specimens the larvae eventually left the host abdomen and pupated. The rearing tubes were transferred to an outdoor insectary where the pupae diapaused. The pupae were • 234 occasionally .sprinkled with water during the second fort- night of May 1974. Of the seven pupae, only two hatched. The first approximately on 31 May 1974. The host was a male M. sexnotatus collected in the outfield on 29 August 1973. The second parasite emerged between 12 and 18 June 1 974. The host was a female M. sexnotatus collected in the oatfield .on 5 September 1973. Both specimens were identi- fied by Mr. F. Benton as Eudorylas fuscipes Zetterstedt, the first was a male and the second a female. In 1974, 1 256 specimens were dissected for pipunculid parasitism. Of this total, 21 specimens were parasitized, corresponding to 1.67% for the whole season. As in 1973, the lower percentage parasitism was in the invading first genera- tion adults, where it was only 0.64%. The higher percentages were again obtained late in the season when the majority of the second generation specimens were already adult. The values were 2.7, 2.2, 4.9, 1.7 and 5.0% respectively for weeks 13 to 17. No attempts were made in 1974 to breed out para- sites since overwintering takes place in the pupal stage and adults emerge only in the next spring. According to Coe (1966) E. fuscipes has been bred out in Germany both from Macrosteles variatus FalI6n and M. laevis Ribaut. Its present host record is therefore new. Waloff (1974b) reports the presence of E. fuscipes in Silwnod Park both from aerial suction traps 1.2 m above ground and from sweep samples from May to August. No host species have been identified. According to Mr. F. Benton (personal communication), • Pipunculus cam;Destris was bred out from a first generation • 235 M. sexnotatus female collected in Gunnes's Bridge, Silwood Park on 27 June 1972. Coe (1966) records P. campestris parasitizing M. laevis in Germany. According to Waloff • (1974b) P. campestris is the most common species within its genus in aerial suction traps 1.2 m above ground, from May to October, in Silwood Park. The low percentage parasitism of M. sexnotatus in the oatfield as compared with the high percentages for cicadellids determined by Waloff (1974b) may perhaps be explained in terms of habitat. The study area adopted by Waloff (1974b) harboured a complex of 42 persistent breeding colonies of cicadellids and delphacids thus providing a spread of potential hosts throughout the year. This included leafhoppers overwintering as nymphs or adults thus providing reservoirs fOr pipunculids that diapause as larvae. Further- more, it was a continuous habitat. On the other hand only a few species of leafhoppers bred in the oatfield and the majority of them only occasionally and in negligible numbers. The host range was thus much curtailed. The second aspect is the transitory nature of the oatfield. It is invaded every year and cultivation practices can deplete the popu- lation of puparia in the soil (Way et al, 1969; Southwood and Way, 1970). Van Emdem (1964-65a & b) stresses the importance of uncultivated land in relation to. crop insects as reservoirs both of the crop pest and of its natural enemies. He states that in Great Britain agricultural plots are never large enough for even the centre to be entirely free of the effects of invading insects of cultivated land. Both Eudorylas fuscipes and P. campestris were reported in • aerial trap catches (Waloff, 1974b) indicating that dispersal • 236 does take plac. Raatikainen (1967) reports negligible pipunculid populations in several catfields in western Finland and low percentage parasitism of Javeselia pellucida. The high percentage parasitism of second generation adults in,the oatfield in 1973 is probably an overestimation of the actual if it is assumed that parasitized individuals do not migrate in equal proportions to non-parasitized ones. Migratory flight was delayed in 1974 so that high numbers of adults accumulated in the field and those parasitized represented smaller proportions than in 1973. No parasitized specimens were detected either in the aerial suction traps, sticky traps or water traps. Waloff (1973) reports that parasitized wachen in Silwood Park aerial suction traps and in laboratory tests flight in cicadellids was altogether deterred when the parasitic larvae- attained maturity. C. Parasitism by Dryinidae (i) In field populations In 1973 a total of 2,715 M. sexnotatus specimens consisting of 834 adults and 1,881 nymphs were examined for • dryinid parasitism. The overall parasitism in the oatfield was as low as 0.37%. No parasitized individuals were detected in the June and July samples corresponding to first generation invading adults and also to young larval instars of the resulting second generation. Parasitized individuals were obtained from weeks 9 to 14, the percentage parasitism figures being 0.24, 1.14, 0.46, 0.59, 1.26 and 1.66 respectively. A total of 10 parasitized M. sexnotatus specimens were obtained in the oatfield in 1q73. Four of 237 the specimens were fourth instar nymphs, five were fifth instar nymphs and one was a female adult. The specimens were set up in the 20°C constant temperature room in indi- vidual tubes as described in Section I, part 4E(ii). When the larvae emerged the tubes were transferred to an outdoor insectary. In the spring of 1974 the cocoons were occa- sionally sprinkled with water and four of the 10 specimens hatched. Two males hatched approximately on 18 June. The hosts were M. sexnotatus nymphs collected in the oatfield in August 1973, a fifth instar on 14 and a fourth on 29. Two females hatched on approximately 24 June 1974. The hosts were M. sexnotatus fifth instar nymphs collected in the oatfield on 19 August and 5 September 1973. All the four specimens were identified by Professor O.W. Richards as Chelogynus ephippiger (Dalman) var. collaris (Dalman). In 1974, 5679 M. sexnotatus specimens were examined for dryinid parasitism. Of this total, 1,653 were adults and 4,026 nymphs of all instars. In only 20 was a dryinid sac detected, so that the overall percentage parasitism for 1974 is 0.35. Of these 20 specimens, three were female adults; the remaining were either fourth (7) or fifth (10) instar nymphs. In all leafhoppers the dryinid sac protruded between the second and third thoraccic segments, either on the left or right hand side. On one instance, a fifth instar nymph had a dryinid sac and abdominal dissection showed a newly hatched second instar piapunculid larva. As in 1973, the lowest percentage parasitism was obtained in• June and July. With the exception •of a parasitized first generation female in week 3 and another in week 8 no other • 238 parasitized specimens were found. From “eeks 11 to 16 the percentaTes of parasitism were 0.47, 0.32, 0.84, 1.1, 2.3 and 2.5. No attempt was made in 1974 to breed out dryinids from the parasitized hosts. There are no records in the available literature for dryinid parasitism in M. sexnotatus or other European Macrosteles species apart from Waloff (1974a & b) , who obtained a male specimen of Anteon lucidum (Haliday in Curtis) on 1 September 1971, out of a fifth instar Macro- steles nymph collected on 20 July 1971, in Silwood Park. The present parasite-host record is therefore new. Miller and de Lyzer (1960) report Epigonatopus plesius (Fenton) as a parasite of the nearctic species M. fascifrons. This is a wingless form and oviposits in the hosts' abdomen where the sac develops. Two parasites per host were reported but the usual number was one per host. Only 2% of 19,000 specimens examined by Miller and de Lyzer (loc. cit.) were parasitized and some individuals of the early migrating populations were captured with ruptured abdomen as a result of the endoparasitic larval growth. a Gonatopus ombrodes is reported by Ainslie (1920) parasitising M. fascifrons. (ii) Infection tests In 1972 a small patch of grass near to the site known as 'Four Acre Field' in Silwood Park was sampled for Macrosteles species. Between 23 and 26 June 1972, 26 slpeci-• mens were collected with a hand net. Eight of the specimens 239 had a drvinid sac. The hosts, all of them fifth instar nymphs, were regarded as M. viridigriseus (Edwards) since the majority of the specimens collected at that site belonged to this species. The eight parasitized specimens were set up on 28 June. Five of the drvinid larvae emerged between 1 and 5 July, one on the first and two on the fifth. One of the larvae failed to leave the sac entirely and died. Of the remaining seven, only three pupated successfully. Two males emerged on 31 July and one female a few hours later. At adult emergence the wasps produced a circular hole in the cocoon. Soon after the emergence the adults were fed on a mixture of honey and water in equal proportions. On 13 July, 7 fifth instar specimens were collected with a hand net in the same site. Five of them had dryinid sacs. They were set up in the usual manner and three died a few days later before the dryinid larva left the sac. The remaining two spun cocoons three days later but failed to emerge as adults. An infection experiment was carried out using the dryinids emerged on 31 July. The dryinid female was induced to oviposit in M. sexnotatus nymphs by confining them together in the labora- tory. Forty-five M. sexnotatus nymphs from parasite-free laboratory cultures were introduced in a culture cage described in Section I, part 4A. A pair of dryinids was simultaneously caged less than two hours after the female emergence. A few oat seedlings were planted in the pot for the leafhoppers to feed on and the sugar solution was provided for the dryinids. The first attack by the dryinid female took place approximately 10 minutes after the S confinement with the nymphs. The female took a close hold • 240 of the nymph and curled its abdomen 45 degrees to oviposit. The second attack took place approximately 10 minutes later this time unsuccessfully since the nymph escaped before being grasped. A third attack took place some 30 minutes later. The female grasped the nymph and oviposition lasted for approximately one minute followed by the release of the nymph which was grasped again almost immediately. The nymph afterwards remained immobile for about two minutes. After, the nymph moved with difficulty, the legs were semi- rigid and were shaken repeatedly. The female remained all the time on the top of the cylinder cage walking constantly in such a way as to make several loops. The male remained on the upper third of the cylinder's lateral walls. Soon after the female's third attack the male approached and seized her from the back adopting a 'male above' position. Mating lasted for five minutes and 10 seconds. During mating the male continuously vibrated the antennae in a quick beat. During the first two minutes and again during the last minute or so the male vibrated the wings rhythmi- cally. The wings were beaten twice and then remained still for a lapse of time approximately four times as long as that spent in beating, followed again by the two beats and so on. After mating the female attempted to attack a fifth instar nymph in its vicinity on the top cover of the cylinder. When touched by the wasp the nymph reacted by jumping some 15 cm downwards landing on the lateral walls of the cylinder. The nymph was closely followed by the flying female that grasped it the moment it landed. The wasp inserted its ovipositor in the nymph's body for 15 seconds, withdrew it afterwards and approximately two • 2 41 seconds later introduced it again this time for about one minute. The nymph was then released and was immobile for four minutes and 15 seconds moving afterwards as described above. A new attack took place some 10 seconds later. No further observations were made. The cage was open three days later, on the third of August 1972. Neither the male nor the female were found in the cage. Of the original lot of nymphs, seven had visible dryinid sacs six days later, therefore nine days since they were parasitized. All the specimens were fifth instar nymphs and were set up in the usual way. Some of the specimens had the dryinid sac on the left side of the thorax, others on the right. One of the nymphs died before the larva emerged from the sac. The remaining six emerged either on 13 of 14 August, at 20°C constant temperature, approximately two weeks after para- sitization. All the six larvae spun up and only one failed • to hatch as adult. The remaining five hatched 24 to 28 days later. A male specimen hatched on 7 September, 24 days after forming cocoon, two males on 10 and a male and a female on 11. All adults were kept in individual tubes and were fed with honey solution at emergence. A new 'infection' experi- ment was carried out using the pair emerged on 11 September. • Forty-eight hours after emergence the female and male were confined to a 7.5 x 2.5 cm glass tube to induce mating which took place. within five minutes. The wasps were then trans- ferred to a cage containing 22 fourth instar M. sexnotatus nymphs from laboratory parasite-free cultures. The arena this time was reduced to a plastic box 4 cm high and 10.5 cm in diameter. The bottom of the arena was covered with moistened filter paper and the usual honey solution was 242 provided for the wasps as well as a few oat seedlings for the nymphs. Within 10 minutes of confinement two successful attacks had taken place. Soon afterwards the male attempted to mate but•was rejected by the female. No further observa- tions were carried out. The box was opened and examined five days later and both wasps were not found. The nymphs were transferred to a cage with fresh oats and seven days later the dryinid sacs were already visible in five of them. The parasitized specimens were set up in the usual way and one died before the larva could leave the sac. The first two larvae emerged four days later, the other two within the next two days. Of the four specimens three failed to form cocoons and died and the remaining one pupated but failed to emerge as an adult. All the specimens were identified as Chelogynus ephippiger (Dalman) var. collaris (Dalman) by Professor O.W. Richards. The observations during the tests were carried out with a naked eye. The characteristic predatory behaviour of dryinid females was not observed. This might have been excluded by feeding the female at emergence with honey solution. The avoidance reaction by the nymphs is mentioned by Waloff (1974a) in nymphs of several species of Cicadel- lidae.. Waloff (loc. cit.) reports that Gonatopus sepsoides searches for its hosts apparently at random and the attack reaction is activated by visual stimulus. This also seems to be the case for C. elphipipier (Dalman) var. collaris (Dalman). e 0 243 12. Predation in the Nymphal and Adult Stages A. Predator feeding trials Feeding trials were carried out with a number of potential predators from the oatfield to detect those that would feed on M. sexnotatus nymphal stages and adults and also in an attempt to determine maximum daily rate of consumption of prey. The specimens to be tested were obtained both from regular suction sampling and also from sweep net samples. They were collected in the oatfield during July, August and early September of 1973. Only the commonly occurring potential predators were tested and this included spiders and nabids, opilionids being far less frequently obtained from field samples. The cage adopted for these tests is described in Section I, part 4F. The tests were carried out at 20°C and 16 light hours/day. The specimens to be tested were starved for 72 hours before any prey was introduced into the arena. The cages were checked every 24 hours for predation and the number of prey was maintained constant. Four prey a day dk were offered to each predator and those preyed upon were replaced daily by new ones so that the number of prey per day remained constant throughout the test. Only nymphal stages III to V and occasionally some adults were offered as prey since the presence of the younger stages is diffi- cult to detect as well as the cause of death. The distinc- tion between prey which had been killed by predation and those which had died of other causes was particularly easy • to make when the predator inflicted severe damage to the 244 prey. In doubtful cases the dead prey was examined under the binocular microscope. Death by other causes rather than predr, i_on was not frequently expected since the set up excluding the predators proved to be satisfactory and high survival was achieved both for nymphs and adults of M. sexnotatus. Each predator was tested for a period of 10 to 14 days. Table 55 summarizes the data on predators and respec- tive mean daily rates for the consumption of M. sexnotatus nymphs and adults. These are the values obtained with no alternative prey and to a definite and constant searching area at a constant temperature of 20°C. The average amount of prey consumed per day was termed 'predatory value' by Fewkes (1960). No attempt was made to have an estimate of the population numbers of spiders and nabids in the oatfield due to the time involved in the identification of specimens. Furthermore, the D-vac sampler is rather selec- tive for large specimens like nabids and larger spiders and therefore a reliable estimate of the predator population would require a diversification of the sampling programme. The more effective predators amongst the species tested are the two nabid species, with an average of more than one M. sexnotatus eaten per day. All the remaining predators. averaged one prey eaten every two days. The small Linyphiidae are apparently nearly as effective as larger spiders. No estimation of the weight of food eaten was attempted. This could have been carried out by feeding • predators on weighed Macrosteles and re-weighing the prey after predation (Fewkes, 1960; Turnbull, 1962). 245 Table 55: The mean daily consumption rate of arthropod predators supplied with M. sexnotatus nymphs and adults in laboratory feeding trials Mean daily Number consumption Species of predator tested (x 1 s.e.) Hemiptera: Heteroptera Nabidae Nabis rugosus (L) (6) 4 0.8510.15 Nabis rugosus (L) (Q) 3 1.2310.46 Nabis rugosus (L) (IV instar) 2 1.3 Nabis rugosus (L) (total) 9 1.1 10.16 Nabis ferus (L) () 1 0.6 Nabis ferus (L) (Q) 5 1.64±0.29 Nabis ferus (L) (IV & V instars) 5 1.1 ±0.16 Nabis ferus (L) (total) 11 1.2810.17- Araneae Clubionidae Ciubiona sp. 12 0.6 -10.08 Thomisidae Misumeninae Xysticus cristatus (Clerk) 5 0.6 ±0.1 Lycosidae Pardosa sp. (immature stages) 5 0.7210.06- Pisauridae Pisaura mirabilis (Clerk) 22 0.5710.06- Theridiidae Enoplognatha ovata (Clerk) 0.14±0.03 Theridion bimaculatum (L) 0.43±0.07 Tetragnathidae Tetragnatha pinicola L. Koch. 1 0.2 Pachygnatha degeeri Sundevall 3 0.4 ±0.1 Pachygnatha clercki Sundevall 8 0.64±0.08 Meta segmentata (Clerk) 0.7 Araneidae Araneus sp. 2 0.2 ±0.09 Linyphiidae Erigoninae Erigone atra (Blackwall) 7 0.46±0.1 Linyphiinae Bathyphantes approximatus (0.P. Cambridge) 1 0.3 B. gracilis (Blackwall) 5 0.422_0.05 Lepthyphantes nebulosus (Sundevall) 1 0.1 L. tenuis (Blackwall) 13 0.52±0.1 Linyphia hortensis Sundevall 0.4 Linyphia sp. (immature stage) 2 0.5 '-0.4 ♦ 246 Detection of predation by the precipitin test (i) Introduction A serological technique was used to detect predation of M. sexnotatus. It is based on the principle that anti- body reacts specifically with the antigen used to prepare that antibody. This reaction can be detected in many ways including the precipitin test used in this work. It follows that an antiserum prepared against an extract of the orey organism can be used to detect prey material present in the gut of a predator. The ideal antiserum would have a high titre and would be specific to a single prey species, or, alternatively, the prey species should be phylogenetically isolated in its habitat (Dempster, 1960). This arises from the fact that related species have antigens in common and thus the anti- serum may cross-react with extracts of related species. Higher specificity can be obtained by the absorption pro- cedure but it can reduce the sensitivity of the antiserum (Weitz, 1952). In the present work, tests were carried out on V p-redators collected in the oatfield in the summer of 1973. In that season, M. sexnotatus was by far the most abundant cicadellid. Within the Auchenorrhyncha the only group also present in any significant number were delphacid species. Since phylogenetic isolation occurs in this study area it will provide a better estimate of true predation by arthro- pods on Macrostcles. 247 The precipitin test was used to detect predation only on a Qualitative basis since it cannot be assumed that only one prey would be taken at each feed and the degree of digestion is unknown (Dempster, 1960). Also, because absolute estimates of predator numbers in the field were not available. (ii) Preparation of the antigen Antigen was prepared from laboratory cultures of M. sexnotatus. The insects were starved for approximately 24 hours to remove oats from their guts. All five nymphal stages were used as well as adult specimens since it is known that some antisera prepared using a single instar does not necessarily react with all other instars (Loughton et al. 1963; Boreham, personal communication). The insects were sorted in the laboratory, anesthetised with carbon dioxide and immediately transferred to a deep-freeze at -20°C. This material was freeze-dried and stored in a desiccator until sufficient material was available to carry out the extraction of proteins. A total of 8.07 g of fresh weight of insect was collected from which a dry weight of 2.45 g was obtained. The insects were crushed in a pestle and mortar. Distilled • water was added to the powder and the mixture was trans- ferred to a Buckner flask. The material was then stirred mechanically for 20 h at 4°C and later centrifugated at 1500 g for 5 min. The supernatant was removed and a second extraction of the residual was carried out. The extracts were pooled and freeze dried. The solid material thus obtained was dissolved in 10 ml of distilled water and this constituted the antigen used throughout this study, 242 (iii) The preparation of the antiserum The method adopted permits the preparation of high titred antisera using small amounts of antigen (Boreham & Gill, An emulsion of 0.5 ml of Freunds complete adjuvant and an equivalent of Macrosteles antigen extract was pre- pared.pared_ . Aliquots were injected in four lymph nodes of an adult New Zealand white rabbit, weighing 2.5-3 kg. The emulsion was injected into each of the axillary and popli- teal lymph-nodes. The injection was repeated at intervals of seven days until four injections had been given. Twelve days after the last injection a 10 ml sample of blood was taken from the marginal ear vein of the rabbit. The serum, containing antibodies, was separated initially by placing the blood sample tube in a water bath at 37°C for 45 min. The serum was then transferred to centrifuge tube and centrifugated at 1500 g for 5 min. The serum was then filtered through a Hemmings filter and was then ready to be tested for specificity and titre to Macrosteles antigen. • (iv) Titre of antiserum The titre of the antiserum is defined as the greatest dilution of antigen which shows a positive reaction against the antigen. The titre of the anti-Macrosteles serum was deter- mined by preparing serial dilutions of the antigen. Twelve doubling dilutions were prepared ranging from 1/10 to 1/20840. 249 The dilutions were tested using a multiple automatic dispenser (Weitz, 1957). A sample of 0.05 ml of the twelve dilutions of the antigen was drawn up into each tube of the dispenser. This was followed by an equal volume of the anti- Macrosteles serum so that the antigen and antiserum formed two distinct layers. When the antiserum reacts with the antigen a white ring of precipitate forms at the interface between the two liquids. The tubes were allowed to stand at room temperature for 2 h. Positive reactions were detected up to the maximum dilution. Thus the titre of the antiserum was at least 1/20840. Since the titre of the antiserum was satisfactory the rabbit was bled by cardiac puncture and about 120 ml of blood obtained. Serum was prepared as described above and stored at -20°C until required. (v) Specificity of antiserum The specificity of the anti-Macrosteles serum was tested against a wide range of insect extracts and also against the food plant. Most of the extracts to be diluted were already available in stock in the Immunology Laboratory so that only delphacid and oats extract had to be prepared. The delphacid extract was prepared from adult and nymphs collected in high density areas in Silwood Park. The oats extract was prepared from leaves collected in the oat- field. The material was diluted to equivalent proportions to that of the Macrosteles extract. The procedure to prepare the extract was the same as described above for Macrosteles. 250 The results of the tests are given in Table 56. Table 56: The specificity of the anti-Macrosteles serum to a range of.antigens Antigen from Titre Lepidoptera (Pieris brassicae) 1/10 Coleoptera (Tenebrio sp.) 0 Diptera (Drosophila melanogaster) 0 Diptera (Anopheles stephensi) 0 Orthoptera (Locusta migratoria) Aphididae (Metopolophium festuca Sitobium avenae) 0 Aphididae (Aphis fabae) 0 Aphididae (Microlophium evansi) 0 Cicadellidae (Cicadella viridis) 0 Cicadellidae (Graphocephala coccinea) 1/80 Cicadellidae (Macrosteles sexnotatus) 1/20840 Delphacidae (Several species) 1/1280 Oats variety Condor 0 Positive reactions were obtained for the Auchenor- rhyncha extracts and also a very weak reaction (titre 1/10) was obtained for Pieris sp. In general, the cross-reaction of the antiserum with the extract is proportional to the phylogenetic. relationship. In the present case however, a higher titre 0 was obtained for the delphacid than to the cicadellid extract of Cicadella and Craphocephala.. These cicadellid extracts had been stored in the laboratory for at least two years and some break down may have occurred accounting for this result. Since only delphacids were nearly as abundant as Macrosteles in the oatfield in 1973 the absorption procedure 251 was carried out to eliminate cross-reactions with delphacid material. The procedure consists of adding antigen extract of the related species to the antiserum and precipitating out the antibodies directed against it. Provided that care is taken not to add excess of the cross reacting antigen the unwanted antibodies can be precipitated out without reducing the titre to the homologous antigen significantly. To prevent the anti-Macrosteles serum from reacting to delphacid extract 0.05 ml of delphacid material was added to 5 ml of antiserum thus precipitatin the antibodies to the delphacid. antigen. This was allowed to stand for 48 h at 4°C. After absorption had been carried out, the absorbed anti-Macrosteles serum was tested against the range of dilu- tions of both delphacid and cicadellid. There was a negligible reaction at 1/20 for the delphacid extract. The titre for Macrosteles was reduced by one dilution to 1/10240. (vi) Tests on predators from the field Arthropods to be tested as potentially feeding on Macrosteles were collected in the oatfield during August and early September 1973 coinciding with peak numbers of instars III, IV and V of M. sexnOtatus and of adults. The potential predators were sorted and killed with chloroform within one hour of sampling in the field. They were stored individually in gelatin capsules. Each capsule contained two cr three grains of silica-gel topped by a plug of non-absorbent cotton-wool. The capsules were numbered and S kept in a tightly closed tin at -20°C for taxonomical * 252 identification and precipitin test. After identification the arthropods to be tested were crushed in individual tubes in 0.2 ml of .saline using a glass rod. The extracts obtained were cloudy due to the presence of fat. A clear solution of the predator extract is essential in order to carry out tests using the multiple dispenser and the capillary ring test since a positive result is detected as a white ring at the interface of the two flujds. An attempt was made to clarify the extract by centrifuging. This procedure improved the condition of the extracts but they were still cloudy and unsuitable for testing. As an alternative to the capillary ring test, pre- cipitin tests were carried out in agar gel by the method of Ouchterlonv (Ouchterlony & Nilsson, 1973). The principle of this method is that precipitation in agar occurs at the point where optimal proportions of antigen and antibody occur after diffusion through the agar. The precipitate appears as a sharp white line. An even laver of hot agar was poured over 7.6 x 2.5 cm microscope slides. After cooling, the slides were transferred to a moist chamber for 4 to 5 h. Wells were then punched in the agar film by means of a pipette. Three sets of six equidistant holes were punched in each slide each set consisting of a central well and five wells in a circle around it. The absorbed anti-Nacresteles serum was placed into • the central well. The numbered predator extracts were 253 individually pipetted into the outer wells. Each slide contained a control of Macrosteles extract. In some of the slides the delphacid extract was also tested against the absorbed antiserum. After 2-4 h each well was refilled where diffusion had occurred. The slides were transferred to a moist chamber and examined after 48 hours for lines of precipitate. To improve the detection of a weak positive reaction the slides were washed in saline for 24 h to remove proteins other than those already precipitated. The slides were afterwards immersed in a protein stain (0.2% naphthelene black) for three minutes. Excess stain was removed by immersing the slides in 2.0% acetic acid in methyl alcohol. A total of 229 tests were carried out on possible predators collected in the oatfieid. A list of species tested is given in Table 57 together with the number tested per species and number of positive results obtained. No reactions were seen with the delphacid extracts and positive results were obtained for all the Macrosteles extracts. The overall percentage positive results was 5.2 with 12 positive results for 229 specimens tested. ,t- should be remembered that there is no way of telling whether the predators had any food in their guts and so many of the negative results could reflect unfed specimens. The precipitin tests confirmed the findings of the laboratory feeding trials where the two rabid species were determined as the most voracious predators on M. sexnotatus • • 254 Table 57: List of arthropods from the oatfield, 1973, that were precipitin tested. to detect feeding on M. sexnotatus. Number of Number positive Species of predator tested results Hemiptera: Heteroptera Nabidae Nabis rugosus (L.) 23 6 N. ferus (L.) 8 1 N. sp. 2 - Araneae Gnaphos idae Zelotes latteillei (Simon) 4 Clubionidae Clubiona terrestris Westring 2 C. reclusa 0.P-Cambridge 5 C. compta C.L. Koch. 1 C. subtilis L. Koch 1 Cheiracanthium erraticum (Walckenaer) 1 1 Agroeca proxima (0P.-Cambridge) 4 1 Zora spinimana (Sundevall) 16 Thomisidae Xysticus cristatus (Clerck) 11 X. kochi Thorell 1 Philodromus aureolus (Clerck) 4 Thanatus striatus C.L. Koch 3 • Tibellus oblonqus (Walckenaer) 3 1 Salticidae Heliophanus flavipes C.L. Koch 1 Euophrvs frontalis (Walckenaer) 1 Lvcosidae Pardosa nigriceps Thorell 1 P. luqubris (Walckenaer) 4 4`. P. amentata (Clerck) 10 Alopecosa pulverulenta (Clerk) 1 Trochosa ruricola (Degeer) 7 Plrata hvgrophilus Thorell 1 Pisauridae Pisaura mirahilis (Clerck) 2 Mimetidae Ero carrhridgei Kulczynski 1 • 255 Table 57 (Continued) Number of Number positive Species of predator tested results Theridiidae Crustulina guttata (Wider) 1 Theridion bimaculatum (L.) 11 2 Enoplognatha ovata (Clerk) 20 1 Robertus lividus (Blackwall) 2 Tetragnathidae Tetragnatha extensa (L.) 6 Meta. segmentata (Clerbk) 2 Pachygnatha clercki Sundevall 6 1 P. degeeri Sundevall 6 Araneidae Araneus quadratus Clerck 2 1 Hypsosinga pygmaea (Sundevall) 2 Mangora acalypha (Walckenaer) 3 Linyphiidae Erigoninae Walckenaera antica (Wider) 1 Gonatium rubens (Blackwall) , 3 1 Maso sundevalli (Westring) 1 Erigone atra (Blackwall) 22 Linyphiinae Microneta viaria (Blackwall) 1 Centromerus sylvaticus (Blackwall) 1 Bathyphantes parvulus (Westring) 1 B. gracilis (Blackwall) 6 B. approximatus (0.P.-Cambridge) 2 1 Lepthyphantes tenuis (Blackwall) 3 Linyphia hortensis Sundevall 3 L. (Neriene) clathrata Sundevall 2 L. sp. I • Of the specimens of Nabis ferus tested 12.59% gave positive results and as much as 26.1% positive results were obtained for N. rucTosus. Nabidae were also found as the most effec- tive predators of Cicadellidae by Tay (1972) and Solomon (1973). 0 256 SECTION III STUDIES ON DISPERSAL 1. Introduction Some of the best known examples of long range dis- placemonts of insects come from Auchenorrhynchous insects of the family Cicadellidae. These include studies on long distance displacements in North America by Macrosteles fascifrons (Sad) (Wallis, 1962; Drake & Chapman, 1965; Chiykowski & Chapman, 1965; Nichiporick, 1965) , the beet leafhopper Circulifer tenellus (Baker) (Dorst & Davies, 1937; Cook, 1967), and the potato leafhopper Empoasca fabae (Harris) (Huff, 1963; Pienkowski & Medler, 1964; Meddler et al, 1966). Short the median range displacements often over only a few kilometres usually follow the direction of the local wind. On the other hand, long range displacements of insects generally take place on specific occasions where temperature conditions favourable for a sustained flight coincide with winds blowing consistently in one main direc- tion. Large numbers of new migrants may accumulate at the source probably as a result of weather unsuitable for flight (Chiykowski & Chapman, 1965). At the same time, minor • migrations less easy to detect may occur whenever weather conditions are suitable. As pointed out by Southwood (1971) most arable crops start E.,s 'ecological vacuums' where the whole area is open to colonization. The major initial population process is then the number of immigrants into this 'vacuum'. Migration is regarded Southwood (1971) as often the most important 257 process for pests of annual crops since its magnitude will determine whether or not there will be a pest outbreak. One of the basic factors to affect the size of an invasion of an annual crop is the number of potential invaders (Southwood & Way, 1970). The identification of the origin of the invaders, whether from a nearby source (short range displacement) or from a distant source (long range displacement) involves a large amount of cooperative work over wide areas to assess overwintering sites or alternative host plants. Therefore, in the present work no attempts were made to identify the origin of the invaders since in this country a polyphagous species such as.. sexnotatus benefits from the extensive areas available covered by various grasses. Also due to time restrictions the measure- ment of the migratory potential of individuals by laboratory experiments with tethered flight was not carried out. The invasion of leafhoppers into fields of cereals has been looked into by authors such as Razviaskina (1960), Miyashita et al. (1964) , Raatikainen (1967, 1972), Chiykowski & Chapman (1965) , Jurisoo (1964) , Kanervo et al. (1957) and Rose (1972a & b). According to Dingle (1972) the available data on a variety of insect migrants suggest that an holistic view should be adopted in the studies of insect migrations. This ideally would include studies on the insect's physiology, behaviour, life history and also as a population. As defined by Southwood (1962) the level of migratory movement of a species is a combination of its frequency S within a given-sized population and its duration. A 0 250 cuantitative measure of this level might be a comparison of the proportions of different species flying to that of their actual populations on the ground or vegetation from which the sampled air had come (Southwood, 1962). Waloff (1973) recorded 115 species of Auchenorrhyncha in aerial traps in Silwnod Park. About half of each annual catch was composed of M. sexnotatus • M. laevis and Ja-c7esella pellucida. M. sernotatus comprised 27% and 26% of the total catch for 1970 and 1971 respectively,. while M. laevis comprised 8% and 9%. Both species, together with J. pellucida were not included amongst the most abundant local species in the acidic grasses. 2. Methods of Sampling Dispersing Insects A. Aerial suction trap This sort of trap has the advantage of working with equal efficiency independent of weather conditions. The only environmental variable to be taken into account is wind speed. Thus suction traps provide more consistent catches than impaction traps which are directly affected by wind (Lewis, 1959b). In 1973 and 1974 a suction trap of the standard 13 inch propeller enclosed cone type (Johnson & Taylor, 1955) • was sited adjacent to the oatfield. This trap samples air at a height of 1.2 m from the ground and has a Woods 230-50 volt, 50 c/s, 1-phase, 400 W, 46 cm propeller fan mounted at the base of a galvanized iron cylindrical duct 51 cm in diameter and 91 cm high. The collecting jar is screwed to the tip of a suspended 43 cm deep 32-mesh copper gauze cone. According to Taylor (1955) this sort of trap samples 4615 m3 of air per hour, 110760 m3 per day. 259 The position of the trap in relation to the oatfield in 1973 and 1974 is shown in Fig. 32. The trap was placed in an exposed situation over short vegetation. The nearest trees were approximately 30 metres distant from the trap. The data on dispersal of M. sexnotatus was supple- mented by three suction traps permanently sited on the area named Silwood Bottom. The traps are usually referred to in Silwood Park as trap II, IV and the Rothamstead trap and are suspended above ground level respectively at 1.2, 9.1 and 12.2 m. Traps II and IV are of the same type as the one running adjacent to the oatfield referred to for convenience at the oatfield suction trap. The Rothamstead trap has a box base 3.05 x 0.76 x 0.76 m containing a centrifugal fan and the collecting jar screwed to the bottom of a gauze cone. The cone is connected to a 9.15 m long and 25.4 cm in diameter conducting tube. The total height of the trap 3 is 12.2 m and the volume of air sampled is 2832 m per hour. The position of these traps in relation to the oatfield is shown in Fig. 33. Traps II and IV are located approximately 270 m south of the oatfield and the Rothamstead trap 330 The oatfield trap was operated from 20 May to 2 October in 1973 and from 20 May to 29 September in 1974. The collecting jar was changed every day of the week .between 9:00 and 9:15. The contents were examined under a binocular microscope as soon as brought in from the field. All Macrosteles species were sorted, sexed and recorded. Every male specimen was dissected for identification. The females, which cannot be distinguished at species level were recorded a as species in proportion to the males for each occasion. 260 Every female was dissected to assess the state of develop- ment of the reproductive organs. In the course of dissec- tion, internal and external parasites were looked for and recorded. The three supplementary traps were emptied at the same hour and stored in 70% alcohol for sorting and identi- fication of the Auchenorrhyncha in the winter. The data on Macrosteles spp. catches from these traps were kindly supplied by Dr. N. Waloff. The three traps were also operated for the period May to October in 1973. Data from Trap II• and the Rothamstead trap could not be obtained for 1974 within the period of this work. In 1974, trap IV was operated from 11 July onwards. B. Sticky traps The positions of the sticky traps in relation to the oatfield in 1973 are shown in Fig. 32A. The sticky traps were made after Broadbent's et al (1948) design of a cylindrical trap that samples at random the passing air. The catches of small insects on such traps can be converted to aerial densities if wind speed is known (Taylor, 1962). The cylinders were made of cellulose acetate 30 cm long and 12.5 cm in diameter. The cylinder was closed at both ends except for a central perforation to let the supporting pole through. The height of the cylinder was easily set by a few thumb tacks pressed onto the pole with the base of the cylinder resting on their heads. This type of cylinder besides being light and easy to transport stays 261 firmly attached to the pole so that the original position of the catches in relation to the environment can be inferred. The adhesive used was the 'Plant protection banding grease' supplied by J. Gibbs Ltd., Middlesex. The adhesive was applied to a grease proof paper of the same dimensions as the cylinder and was diluted in turpentine to an adequate consistency to be brushed evenly over the whole surface. The papers were changed weekly throughout the season. The traps were checked every day of the week and the leafhoppers were picked up with a forceps and brought into the laboratory in specimen tubes each corresponding to a trap. The catches were then examined. Ten sticky traps were set up around the oatfield -in 1972. They were in use from 5 September to 3 November. Only movements of M. sexnotatus second generation adults were available for sampling. Five of the traps were at the height of 1.80 m and five at 1.50m. In 1973 sticky traps were set up as early as 24 May to assess movement of migrating insects into the oatfield. They were in use throughout the season until 2 October. Sixteen traps were set up around the oatfield. Nine traps sampled between 0.90 and 1.20 m above ground level and four between 1.20 and 1.50 m above the ground. Three sticky traps were fixed at different heights on a single pole to compare catches at different heights. Each sticky trap cylinder was 10 cm distant from the next. The heights sampled were 0.70-1.0, 1.10-1,40 and 1.50 to 1.80 m. This set of sticky traps and the nine traps 1.20 m high were set up two metres 262 away from the oatfieldborder. The remaining four traps were. set up four metres away from the oatfield border. The pairs of +- raps situated at the four transects running radially NW, NE, SW and SE away from the corners of the field were intended to sample in relation to wind direction. This design was after Southwood, Jepson, and Van Emden's (1961) for water traps catching Oscinella frit in an oatfield. The height of the traps was chosen to catch alighting insects over the growing period of the oat plants. On 1 August all traps except the three standing on the same pole were raised so as to sample between 1.40 and 1.70 m above ground. A higher position was adopted to sample in accor- dance to the height of the crop which by then had already achieved full growth. The aim was to sample the movements of the second generation adults leaving the oatfield and to avoid catching individuals engaged in trivial flights at crop level. A pair of traps was set up in 1973 inside the oat- field so as to trap leafhoppers moving at crop level. The position of the traps inside subarea 9 is shown in Fig. 32A. The traps were respectively 4 and 6 m away from the margin of the oatfield. The height of the traps was continually adjusted to the growth of the oat plants so that the base of the cylinder rested at the level of the top of vegetation. The low catches in the sticky traps around the oat- field both in 1972 and 1,473 were unrewarding in relation to the time spent on their maintenance. Due to the low numbers caught no conclusions could be drawn either from the traps arranged in the four transects or from that sampling at 263 different heights. Satisfactory results were obtained from the pair of traps sampling at crop level inside the oatfield. C. Water traps A water trap of the type designed by Coon and Rinicks (1962) was set up by the oatfield in 1973. It consisted of a metal tray each side of 52 cm subdivided into four compartments. of eaual size by two 45 x 24 cm upright baffles at right angles to each other. The trap rested on a 50 cm high frame and both baffles and trays were yellow. The side of each compartment was 24 cm and was filled with 75% water, 20% formaldehyde solution and 5% stergene. The trap was run from 21 May to 2 October. Daily examination was made from 14 June onwards and the specimens trapped were picked out with a forceps for further examina- tion in the laboratory. From 21 May to 13 June catches were examined weekly. The site of the trap is shown in Fig. 32A. It was set up one metre away from the border of the oatfield on the contact line between the Lolium area and a narrow strip of dense weeds and sparse short oats. Quadrants named one and two faced the oatfield and quadrants three and 264 four faced. the Lolium. D. uiar netting and suction sampling Movements of . sexnotatus into and out of the oatfield were also assessed by the weekly estimates of population numbers by suction sampling. These movements wereTellecred in fluctuations in the size of the estimated populations. Sweep .netting was specially adopted to deter- mine the onset of the invasion and for surveying the area when infestations were light. Though only rela- tive estimates can be obtained by this method sweep netting permits sampling over a much larger part of the habitat than D-vac suction sampling. During this initial period the low height of the oat plants gave a better chance of sampling insects over most of the vertical axis. 3. The Invasion of the Oatfield A. Macrosteles spp. as invaders Some of the species of the genus Macrosteles exhibit considerable powers of dispersal and there are several reports in the literature of their readiness to colonize 0 265 field crops. The long range migrations of M. fascifrons over North America are at the extreme of the rangf,, of the dispersal powers of the genus. There are no reports of equivalent flights by any of the other fourteen nearctic species or by the thirty European species of Macrosteles. Heterogeneity of migratory activity within genera is known in other groups as well, for instance in species of the Heteropteran genera Eurygaster and Aelia where some of the species are entirely non-migratory and others are strongly migratory (Brown, 1965). On a smaller scale, Macrosteles species in Europe have been reported invading cultivated crops in a number of coun- tries. Some of the species have been noted as vectors of plant diseases mainly mycoplasmas. In the British Isles there are no records of serious damages to crops by Macrosteles species. In Scandinavia and the U.S.S.R. they have been reported to cause damage to cultivated crops. Lindsteh et al. (1970) refer to heavy infestations of M. laevis in Sweden in 1948 where densities of several thou- sands per square metre were reported in invaded spring sown cereals. Bollow (1950) reports a heavy infestation of cultivated crops in Bavaria by M. laevis where density as high 2 as 100,000 specimens per m was estimated by counting dead specimens after chemical control. He refers to M. laevis as an inhabitant of meadow grasses which eventually invade cultivated crops such as wheat, oats, barley, potatoes, alfalfa and clover and says that most of the damage attri- buted to that date to M. sexnotatus in Europe was in fact caused by M. laevis and M. cristatus. 266 Prior (1965) reports an evenly distributed large population of Macrosteles spp., mainly M. sexnotatus in Tierts., in a white clover seed crop undersown with oats. Also present but in smaller numbers were M. viridigriseus and M. laevis. Rice fields have been known to be invaded by sexnotatus in Italy but the distribution of both nymphs and adults is restricted- to the periphery of the plots (Olmi, 1968), Raatikainen (1971) reports on the leafhopper communities in oatfields. in Finland, Sweden and Norway. The same community occurs throughout the Finish cereal zone and also in equivalent climatic zones in the rest of the Scan- dinavia. M. cristatus was the second most abundant and common leafhopper just after Javesella pellucida. In years when it was the most abundant species, H. laevis was also present. M. ossiaAalssoni was only found occasionally. According to Raatikainen and Vasarainen (1971) M. cristatus and M. laevis are abundant on oats, barley and in wheat fields in Finland. Also, M. cristatus was the most abundant leafhopper on crops of oats, barley, wheat and winter rye in a study area in Finland. Raatikainen (1972) reported that M. cristatus and M. laevis invade oatfields in Finland by migratory flights. Further, Raatikainen and Vasarainen (1973) report that M. sexnotatus is occasionally found in cultivated crops in Finland, particularly in cereals. M. cristatus is referred to as frequent in leys from which it migrates especially to - cereal crops. M. laevis is reported as very frequent in leys and migrates to a lesser extent to crops where it is outnumbered by M. cristatus. According to Jiirisoo (1964) N. cristatus in Sweden is not restricted to any specific crop in particular. Together with M. laevis and M. alpinus it is S present in cereal crops where M. laevis is the most abundant • 267 of the three followed by M, cristatus. Razviaskina (1960) reports that in the U.S.S.R. M. cristatus is the only species of the genus that invades cultivated crops in large numbers. . laevis is also said to invade cereal crops while plant hosts of M. quadripunctulatus are restricted to the Compositae. The economic importance of these invasions is related I to the transmission of plant diseases. In North America M. fascifrons is the main. vector of the aster yellows disease which severely affects vegetable crops, barley and flax. The incidence of the disease could be reduced by controlling the vector but high reinfestation takes place within a few days of spraying neutralizing the efficiency of the chemical control (Meade & Peterson, 1964). The relation of M. fascifrons to aster yellows was found by -Kunkel in 1926 and to oat blue dwarf virus by •Banttari and Moore in 1962, Recently, Lindsten et al. (1970) recorded two new virus diseases similar to oat blue dwarf and aster yellows transmitted by M. laevis in Finland. Maramorosch et al. (1968) demonstrated by electron microscopy and chemotherapy that the so-called yellows-type diseases of plants classified as virus diseases on the basis of their, • transmission by leafhoppers and the type of syndrome are caused by mycoplasma-like organisms. M. laevis, M. cristatus and M. ossiannilssoni have been shown to be vectors of virus and mycoplasma in Finland (Raatikainen, 1970). Although abundant in cereal crops in Finland, M. cristatus is of minor economic importance since very few individuals carry aster yellows in that country (Raatikainen & Vasarainen, 1971). According to Raatikainen (1971 ) the earlier the leafhopper vectors reach an oatfield the greater will be the chances that the transmitted viruses or mvcoplasmas will reduce the yield of the crop. In the U.S.S.R. only i. cristatus and M. quadripunctulatus have economic importance as vectors of diseases to cultivated crops (Razviaskina, 1960). In England M. viridigriseus has been shown to be a vector of the phyllody disease caused by the green petal 'virus' of the aster yellows group (Frazier & Posnette, 1 956). The same disease is transmitted in Holland by M. cristatus (Evenhuis, 1958). and in •America by M. fascifrons (Chiykowski, 1962). Prior (1965) attempted without success to carry out virus transmission tests for the phyllody disease with M. sexnotatus collected in the field. So far, there are no definite records in literature of A. sexnotatus transmitting virus or mycoplasma diseases. B. The mode of colonization (i) Introduction Studies on this subject have generally been carried out where a regular sampling programme yields large numbers of insects such as thrips (Lewis, 1959a) and the frit fly (Southwood, Jepson & Van Emdem, 1961). As pointed out by Johnson (1969): "The end-point of a migration flight comes when the condition of the insect, largely determined by phase, age, sexual state, and duration of flight, is such that after a host or habitat is located it is accepted. Flight thereafter tends to become associ- ated with particular activities such as feeding or egg-laying • within a more circumscribed locality, though the different S types of flights are not sharply demarcated". S 269 In the present study the low densities at the process of infestation of the crop together with the size 2 of the plot (1200 m ) were limiting factors in the assess- ment of the mode of colonization. Areas as large as 24000 m9 2 and 49000 m were used by Raatikainen (1972) for the study of invasion of leafhoppers into oatfields, and farm fields of alfalfa by Kieckhefer and Meddler (1966) for the same purpose. (ii) Local immigrants In 1974 the mode of colonization was investigated to assess whether invasion was of a local origin over a distance covered by jumps and short flights or from other sources and by flight only. A local aerial suction trap provided data on flight . in the oatfield area and sweep net surveys provided data on the onset and size of the infestation. The immediate surroundings of the oatfield in 1974 are illustrated in Fig. 32B and described in Section II, IB. In the summer of 1973 second generation Macrosteles specimens were collected both from the Lolium patch (area III) and also from the low grasses east of the oatfield (area I) though in much lower numbers. If diapausing eggs were laid in these grasses these areas could contribute the invaders to the oatfield in the next spring. The suwey of the oatfield was carried out on three occasions with four day intervals between the sampling dates. Six sets of 100 sweep strokes were taken along parallel longitudinal lines in an east- west direction at definite • 270 distancr-s from the south border of the oatfield adjoining area III. The first set ("-5") was taken 5 m from the south 'border on the Lolium/nolcus area (area III) along a line parallel to the oatfield. The second set ("0") was taken along the south boundary of the oatfield. The third set ("5") was taken 5 in inside the oatfield, the fourth ("10") 10' in inside, the fifth ("20") 20 m and the sixth ("30")30m. Thus, the first four sets were taken at 5 m intervals and the last two at 10 m intervals. Sets "0", "5" and "10" were taken along subareas 1 to 5 on single longitudinal lines. Set "20" was taken on area VIII in two longitudinal sets of 50 strokes and set "30" in the same manner on area IX. Samples were taken on 27 and 31 May and on 4 June. The weather was dry, sunny, winds negligible on the three occasions and averacre temperature and relative humidity • being respectively 20°, 18', 21°C and 90, 88, 90%. The average height of the grasses on area III ("-5") was 20 cm on the three occasions; on sites "0" - t "20" of the oatfield between 30 and 32 cm and on site "30", 45 to 53 cm. The last sampling date before these surveys was on 20 May when no Nacrosteles specimens were found in the oat- field. The oatfield suction trap was in operation since 20 May and no flight was recorded until 31 May. Table 58 summarizes the data on catches on the three sampling occasions and on the state of the reproductive organs of the females caught. On the first occasion more nymphs than adults were caught in the Lolium/Polcus area. In the oatfield no nymphs S • 27] Table 58: Numbers of M. sexnotatus per 100 sweeps caught at different distances from the border of the oatfield adjoining a Lolium/Holcus area. Also, state of maturation of the reproductive organs of females caught (I=immature; D = developing oocytes; M = mature, oocvtes with chorion). Sampling sites Fifth Sampling (m away from instar Adults Ovaries date border of oatfield) nymphs Male Female I D :1 27 May -5 9 2 1 1 - 0 - 2 3 - g 5 - 4 4 2 1 1 10 - 3 2 1 1 20 30 Totals 9 15 10 31 May -5 1 1 2 2 - - 0 - 11 4 1 3 - 5 - 10 10 1 5 4 10 - 9 3 1 - 2 20 - 10 2 - 2 - 30 - 10 3 - 2 1 Totals 1 51 24 4 June -5 1 4 1 - - 1 0 - 21 15 1 9 5 5 - 16 18 2 7 9 10 - 43 9 1 7 1 20 - 22 7 4 1 2 30 - 40 10 4 2 4 Totals 1 146 60 were found and adults were caught in sites "0" to "20" only. In site "20" only males were obtained. The sex ratio was 1:1.4 females to males. One third of the females were sexually immature, more than 50% were in the process of maturation and only one specimen was already mature. The only female caught in the Lolium/Holcus area was totally immature. The total number caught in this occasion was still too low for any statistical analysis to be carried out on the distribution pattern along the sampling sites. a • 272 On the second sampling occasion both sexes were obtained in all s es_ sampled. Numbers caught were 3.5 times as large as on the previous occasion. Only one nymph was obtained in the Lolium/Holcus area and the two females caught were immature. The sex ratio in the oat- field favoured males 2.3:1. The ratio of female to male was 1:2.7, 1:1, 1:5 and 1:3.3 for sites "0" to "30". Flit* five percent of the females had maturing oocytes, 32% were sexually mature and 14% were totally immature. A 'A9- zest" was carried out for the hypothesis of equal availa- bility of insects for sampling along the five sites in the 2 oatfield. A x of 3.14 for 4 degrees of freedom was obtained and the null hypothesis accepted. On the last sampling occasion numbers caught in the oatfield were 2.6 times as large as on the previous occasion four days before. Numbers in the Lolium/Holcus area remained as low as before. The sex ratio in the oat- field was 1:2.3 females to males. The ratio female to male was 1:14, 1:0.9, 1:4.7, 1:3 and 1:4 for sites "0" to "30". 2 A X test was again Carried out and the null hypothesis 2 was rejected at 0.05 level for a X of 10.36 for 4 degrees of freedom. Flight at the outfield area was detected for the first time by the oatfield suction trap on 31 May. On this occasion two males and one female were caught followed by a catch of four males and two females on 3 June and five males and four females on 4 June. The sudden rise in numbers from 27 to 31 May when catches more than trebled within four days, coincided with 273 the first catches of M. sexnotatus by the oatfield suction trap. Catches increased by more than 2.5 times within the. next four days coinciding with a high catch of N. sexnotatus in the oatfield suction trap. Numbers of insects were not significantly different along the five sectors of sampling on 31 May but departed from randomness on 4 June with higher numbers 10 and 30 metres away from the south border of the oatfield. The results suggest that colonization took place from two sources. Firstly, an initial local population moved into the oatfield from the adjacent areas by low and short flights and jumps. This lot was composed of young adults. No movement was recorded by the oatfield suction trap either because flight was not high enough or because numbers moving were too low to have a chance to be detected. The sex ratio of 1:1.14 females to males also suggests that the invading population was at the beginning of the imaginal moult period. Secondly, adults from other sources invaded. the oatfield by migratory flight. Flying specimens were intercepted by the oatfield suction trap and the catches coincided with increases in the oatfield population that could not be accounted for by the low density local popula- tion only. Sex ratio was then 2.3:1 males to females and 70% of the females sampled were immature following both influxes of invasion by flight. Local movements by highly mobile and migratory species of Macrosteles have been reported in literature. Though of short range the displacement is post-teneral and the insects reach a new breeding ground. 274 As pointed out by Johnson (1974) the distances flown by the migrant insect depend on the ecological needs and successive breedincT grounds may be separated by thousands of kilometres or by a few metres. Whether an insect is called a migrant depends rather arbitrarily on the distance between such places and the adaptive characteristics of behaviour that enable the more or less lengthy flight to be made before the maturation of the ovaries. Pazviaskina (1960) reports that M. cristatus in the U.S.S.R. invade crops by short, interrupted flights of 1.5-2.0 m at the height of 50-60 cm from neighbouring grasses. Miller and de Lyzer (1960) report that local populations in Southwestern Ontario of the highly mobile species M. fascifrons overwinter in winter cereals in the egg stage, disperse to vegetables, ornamentals and other vegetation in early summer and return to autumn sown winter cereals in the autumn. Also Hervey and Schroeder (1947) report local movements between adjoining crops by M. fascifrons. In Western New York migration by M. fascifrons is of no significance and invasions are due to local populations. A gradient in numbers both of the spring and autumn genera- tions was found in the invaded crops with higher numbers closer to the source of invasion. According to Juriseo (1964) young adult Nacrosteles spp. in Sweden invade oat crops by crossing over from surrounding habitats to the fresh shoots; in awheaf field leafhopper invasion was . detected along an extensive border facing natural habitats. Also, that when great infestations of cereal crops take place the invaded crop is usually adjoining first year leys • consisting of clover, timothy grass and weeds. Raatikainen • 275 and Vasarainen (1971) attribute the greater abundance of M. cristatus in oats in relation to wheat to the fact that oats in Finland are grown in outlying fields. According to Raatikainen (1972) nymphs of Macrosteles spp. move into an oatfield from legs bordering it but not for more than a few metres. Both in 1972 and 1973 local surveys indicated that there were no adjacent populations to account for any detectable local migration. (iii) Immigrants from other sources Both in 1973 and 1974 flight was intercepted by the oatfield suction trap for the. first time on 31 May. The main flight period extended from 31 May to 24 June in 1973 and from 31 May to 17 June in 1974. The build up in the oatfield of the young immigrant population commenced in the very beginning of June and reached a clear peak in both years a few days before 20 June. A noticeable decline in numbers occurred in 1973 and 1974 in the two weeks following peak. This was followed by a new rise in numbers in the week after, a second peak 0 being reached by July 10. Thereafter, numbers declined steadily in both years so that by 20 August adults were no longer available in the field. It is interesting that the pattern of numbers was the same in both years, including the timing of the changes. The only difference was that the second peak in 1973 was proportionately higher. These figures were obtained from catches in the • oatfield suction trap, and are related to higher numbers in • 276 the field which resulted from influxes of immigrants. Similarly, the lower catches by the suction trap after the end of the migratory period are reflected in the steep fall in numers of the field populations since mortality was no longer obscured by the new arrivals. The first decline in numbers immediately after the first copulation peak is interpreted as resulting from a period of trivial movements between neighbouring areas after the end of migration. Lower numbers were then caught by the suction trFtp. The data on suction trap catches is summarized in Table 59 and 60 for 1973 and 1974. The daily catch figures are grouped under the sampling weeks. The number of speci- mens collected at different time intervals (weeks) are expressed as fractions or percentages of the total collection made during the flight season. Table 59: Numbers of first generation M. sexnotatus captured in the oatfield suction trap, 1973. Daily catch figures grouped under sampling weeks. Week ?-11 4°- 6'6 ? + 6' 9634 9683 %? + 6' 0 5 13 18 23.8 50.0 33.3 1 9 4 13 42.8 15.4 27.6 2 5 5 10 23.8 19.2 21.3 3 1 2-) 3 4.8 7.7 6.4 4 0 2 2 0 7.7 4.2 5 1 0 1 4.8 0 2.1 Totals 21 26 47 • 277 Table 60: Numbers of first generation M. sexnotatus captured in the oatfield suction trap, 1974. Daily catch figures grouped under sampling weeks , Week .6rn -r 0 0 1 2 3 3.7 4.8 4.4 1i 5 9 14 18.5 21.4 20.3 2 11 19 30 40.7 45.2 43.5 3 3 3 6 11.1 7.1 8.7 4 4 7 11 14.8 16.7 15.9 5 3 1 4 11.1 2.4 5.8 6 0 1 1 0 2.4 1.4 Totals 27 42 69 The relation between fluctuation in numbers of the immigrant population in the oatfield and percentage flight as determined by the oatfield suction trap is represented graphically in Fig. 41A and B for 1973 and 1974. The estimated weekly populations are plotted on the day of the weekly D-vac sampling; the percentage flight of a given week obtained from the accumulated daily catches of the given week is plotted on the last day of the week-. Additional data on flight was obtained in 1973 from the sixteen sticky traps placed around the oatfield though the very low catches over a long trapping period are a limiting factor in the interpretation of the data. Peak percentage flight was obtained one week later than that estimated by the suction trap. The data relevant to sticky traps is given in Table 61 where catches are grouped under sampling weeks and percentages of the total seasonal catch are included per week. -140 - 30 20 - -20 10 C) L• r 1 0 W EEK 0 1 2 • 4 5 6 7 8 9 10 11 1 JUN 10 41.) 301[1. 10 20 30 AUG 110 20 44 B nn rn • 1. 20 - !. ON 01 ;1 •,/\ „ NVS CI 101-- , S qi i 6' o. -10 N ; I I_ WEEKu 1 2 4 5 6 7 9. 10 11 i2 131 jt.A1 0 LU 7] io 20 30 ALGIO 20 30 Numbers of first generation M. sexnotatus captured by the cat field suction Ira:. reT)resented as wea(Jy, percentages of the tollaL. 1:1)2r caught(broken line) and fluet'Jati.ons in numbers of first generation ad•alt immigrants in the o;s,.tfield (solid line). (A) 1973, (B) 1974. • 279 Table 61: Numbers of first generation M. sexnotatus captured in the sticky traps placed around the oatfield, 1973. Daily catch figures grouped under sampling weeks Week 86 + 6 541- 6889 58? + 0 3 1 4 23.1 11.1 18.2 1 7 4 11 53.8 44.4 50.0 2 2 1 3 15.4 11.1 13.6 3 0 3 3 0 33.3 13.6 4 1 0 1 7.7 0 4. Totals 13 9 22 In 1973 the higher catches of first generation M. sexnotatus in traps II and IV were obtained in weeks 0 and 1. The data from trap IV confirming the estimated peak flight by the oatfield suction trap is of particular significance since trap IV samples aig at 9.10 m above ground. No Macrosteles were obtained in the Rothamstead trap throughout the sampling period in 1973. As said, data from the high suction traps was not available for the first generation in 1974. In the long range displacements of Macrosteles fascifrons over North America, the invasion influxes of the insect into Wisconsin are not correlated with the local weather conditions but are a function of temperature, air currents and general growing conditions in southern source areas (Drake & Chapman, 1965). Chyikowski and Chapman (1965) postulate that the long range displacement of M. fascifrons. from southern United States to Wisconsin takes place passively in warm southern air currents and settling to the ground occurs when cold winds blow from the north; east or west. The invasions into Manitoba, Canada, take place S 7s under these same conditions (1NriCriCk, 1965). Meddler (1962) suggested that M. fascifrons migrates in an essen- tially passive manner over long distances, except for taking off and landing. Lawson et al. (1951) reported that the beet-leafho-oper Circulifer tenellus in flight move at the same speed as the dominant wind. Johnson (1974) points out that there is no evidence that migratory insects are blown passively without wing beat to extreme distances and that it is doubtful if long distances can be covered in case the insect does not maintain itself airborne by active flight. Raatikainen (1967) reports that maximum population figures for Javesella pellucida in oatfields occur after the main migration period, numbers declining thereafter. Also, that J. pellucida migrates within a few days of adult emergence but migration may take longer to start if the weather conditions after emergence are unfavourable for flight. Males apparently migrate slightly earlier than females and predominated in invaded oatfields during the beginning of the invasion period then declined sooner than the female population. Also, the proportion of migrating males captured in interception traps was higher than that obtained from sampling the invaded crops. Raatikainen and Vasarainen (1973) say that in Finland M. cristatus migrates from levs to cereal crops apparently within one week of adult emergence. According to Rose (1973c) there are higher proportions of females among the long distance fliers of Cicadulina sip. compared to males dispersing downwind. Also, young. cereals in Rhodesia are predomin- antly settled by females. • 221 Secondary movements of the invading populations have been reported in literature. Meade and Peterson (1964) found several peaks in the M. fascifrons adult populations in oats; these sudden changes in numbers were attributed to movements into and out of the crop after invasion. The graphs given by Jurisoo (1964) for the fluctuations in abundance of adult populations of M. laevis, M. cristatus and M. alpinus in oatfields also show irregu- larities and secondary peaks. Lawson et al. (1951) detected secondary movements of the beet-leafhopper Circulifer tenellus and suggested that outgoing movements might be obscured by incoming ones. The sex ratio of the invading population has been used as an indicator of the distance from the source of the migrant population in the long range displacements of leaf- hoppers (Lawson et al. 1951). This was also found in Macrosteles fascifrons where the distance of the insects from the source of migration is indicated by the sex ratio: theoretically, the original population leaving a breeding area would have a sex ratio of 1:1 and the numbers of females increasing progressively with distance from the source (Chiykowski & Chapman, 1965; Wallis, 1962; Miller & de Lyzer, 1960). 4. The Trivial Flight of Males Behaviourally, Kennedy (1961) characterized migration as:"...persistent, straightened-out movement that is accom- panied by and dependent upon the maintenance of an internal inhibition to those 'vegetative' reflexes that will, even- tually, arrest movement." Here veaetative activities are • 282 considerc:d to be fecding, mating and oviposition. Thus a migratory flicf,At could he distinguished from a purely appetitive flight by a concomitant suppression of feeding and reproductive behaviour (Caldwell & Rankin, 1974). Studies of the relation between migration and reproduction in insects have shown that in general repro- duction begins after the migratory flight. This relation based on the fact that migrating females are usually sexually immature has been referred to as "oogenesis-flight syndrome" by Johnson (1969). More recently, Dingle (1974) proposed to refer to this relation as "reproductive-flight syndrome" to include male activity as well. Southwood (1962) distinguishes between migratory flight and trivial movements. The first takes an animal away from its population territory or habitat while the second is restricted to the animal's habitat and leads only to a limited increase in the mean distance between indivi- duals. Also, the duration of the trivial movement is vari- able and may be terminated at almost any time on perceiving a vegetative stimulus such as mate, food or shelter. Southwood (1962) names "vagrants" the individuals engaged in a trivial movement that are carried out passively by air currents. Flight within the habitat was studied in 1973 by means of sticky traps and water traps. As said, a pair of sticky traps permanently adjusted to the crop height were run inside the oatfield in 1973. 40, Also, data from a water trap lower than the crop level and adjacent to the oatfield was available. 233 The peak of male invasion took place on week 0 in 1973 and that of the female one week later. Intense flight activity within the habitat as determined by the pair of sticky-traps inside the oatfield extended from week one to three with a peak on week two. Water trap catches provided peak numbers on week 1 with a decline in numbers in the two subsequent weeks. The data from the sticky traps and water traps are summarized in Tables 62 and 63. The daily catch figures are grouped under the sampling weeks and the numbers collected within each time interval (weeks) are expressed as fractions or percentages of the total collection over the trapping period. Table 62: Numbers of M. sexnotatus captured in two sticky traps placed inside the oatfield, 1973. Daily catch figures grouped under sampling weeks. Week 9 + 6 %?? %86 . 96? + 6 0 0 3 3 - 4.8 4.5 1 2 17 19 67 28.6 30.3 2 1 28 29 33 44.4 44.0 3 0 13 13 - 20.6 19.7 4 0 1 1 - 11.6 1.5 Totals 3 62 65 Table 63: Numbers of M. sexnotatus captured in 1973 in one water trap with baffles placed on the boundary of the oatfield to a Lolium perenne area. Daily catch figures grouped under sampling weeks Weeks +90 + Scr ? + S 95W- % Se 96 ? + 0 0 1 1 - 2.8 2.6 1 0 21 21 - 60.0 55.3 2 0 7 7 - 20.0 18.4 3 3 6 9 100 17.2 23.7 Totals - 3 35 38 IP 284 hales were in overwhelming majority in all weekly catches from both types of traps with the total catch been g of 95.4% males in the sticky traps and 92% in the water traps. The movements recorded could be associated with search of mating partner rather than search of food, Moreover, the excess of males captured occurred over a rather definite period after invasion. Mating in M. sexnotatus takes place early in the adult life and the invading population consisted of sexually immature indi- viduals. The relation between catches of trivial flight ("flitting") and that of "true flight" (migratory) for males is represented graphically in Fig. 42. The data from the oatfield suction trap, sticky traps and water traps related to field population numbers suggest a flight activity performed by young males over a definite period after invasion coinciding with peak numbers of females in the field and with the beginning of the oviposition period. 5. The State of Maturation of Females Ovaries as an Indication of Migratory Activity Ecologically, migration is essentially a flight by sexually immature females to sites where the next generation can develop. Flights classified as migratory take place soon after imaginal ecdysis and continue during part of the pre- oviposition period after which they often end (Johnson, 1969, 1974). Females of migrant species have been shown to be fairly constantly pre-reproductive fliers. The strategy of • 60 50 40 30 -NP o 20 10 0 J une 10 20 30 july 10 20 Co Wr-ekly percentages flight of first generation senotatlas males, 1973 oatfield suction trap catches; (o---o) catches from sticky traps inside the cofficid; Co o)water trap catches. e 286 the "oogenesis flig-nt-syndrome" consists of minimizing the development of the rer,roductive system when the flight system is maximized (Dingle, 1972). The timing of reproduction is obviously an important factor for the success of migrant insects as colonizers of new habitats. To be a successful colonizer the migrant must be able to reproduce and leave descendants. The "reproductive value" of an individual has been shown to reach a maximum just as reproduction begins hence the significance of pre-reproductive migrations (Dingle, 1972, 1974). Two moments of the migratory flight activity of M. sexnotatus can be followed in the oatfield. Firstly the end product of the migration in the first generation of invaders and settlers. Secondly, the exodus from the oat- field in a migration to overwintering sites by the second generation. In both generations, the state of maturation of the female ovaries was estimated by regular dissection of every female caught either in traps or by D-vac sampling and sweep net sampling. The state of the reproductive organs was classified as: immature, ovarioles thin and delicate, previous to the deposition of yolk (I); oocytes developing and already with some deposition of yolk (D); mature, indicated by the fully sized cocytes with chorion (M). The first two stages are named 'nen-gravid' and the third stage is named 'gravid'. 287 In 1973, 141 first generation females were dissected and 202 of the second generation. In 1974, 357 and 397 females were dissected of the first and the second generation. Tables 64 and 65 summarize the data for the state of maturation of the ovaries of first generation females in 1973 and 1974. Tables 66 and 67 give these data for the second generation of 1973 and 1974. As said, female Macrosteles cannot be identified to species level and were taken in proportion to males. Here, however, all Macrosteles females were dissected and included in the tables. Table 64: Comparison of the state of maturation of the ovaries of first generation M. sexnotatus, 1973. (a). specimens captured by the oatfield suction trap and in sticky traps 1 to 16; (b) specimens taken in D-vac suction samples and in sweep net samples; (I) immature ovaries; (D) with developing oocytes. Numbers % non-gravid Week Dissected % I % D % gravid 0 a 7 57.1 42.9 b - - - - 1 a 19 57.9 31.6 10.5 b - - - - 2 a 8 12.5 12.5 75.0 b 13 100.0 3 a 1 100.0 b 6 16.7 83.3 111 4 a 1 100.0 b 7 100.0 5 a 1 100.0 b 41 100.0 6 a b 11 100.0 7 a b 8 100.0 8 a - b 10 - - 100.0 9 a b 8 100.0 • Totals 141 * 288 Table 65: Coparison of the state of maturation of the ovaries of first generation M. sexnotatus, 1974. (a) specimens captured by the otZtfield suction trap; (b) specimens 'aken in D-vac suction samples and in sweep net samples; (I) immature ovaries; (D) with developing cocytes Numbers non-gravid Week dissected % I o D I gravid 0 a 1 100.0 b 31 19.2 55.0 25.8 a 6 16.7 66.6 16.7 h 69 17.4 45.0 37.6 2 a 15 - 86.7 13.3 b 12 16.7 - 83.3 3 a 3 33.3 33.3 33.3 b 47 4.4 10.6 85.0 4 a 6 50.0 50.0 b 24 8.3 91.7 5 a 5 40.0 60.0 h 42 100.0 a 5 100.0 b 37 100.0 7 a 1 100.0 b 19 100.0 8 a 1 - - 100.0 b 19 - - 100.0 9 a - - - - b 14 - 100.0 Totals 357 Table 66: Comparison of the state of maturation of the ovaries of second generation M. sexnotatus, 1973. (a) specimens captured by the oatfield suction trap; (b) specimens taken in D-vac suction samples and in sweep net samples; (I) immature ovaries; (D) with developing oocytes Numbers % non-gravid Week dissected % s % D % gravid 10 a 22.2 77.8 b 8 100.0 11 a 8 25.0 62.5 12.5 b 45 77.8 15.5 6.7 12 a 18 72.2 27.8 b 35 54.3 34.3 11.4 13 a 6 16.7 50.0 33.3 b 21 28.6 33.3 38.1 14 a 10 70.0 30.0 b 24 45.8 20.8 33.4 15 a 8 12.5 37.5 b 7 1 4.3 28.6 57.1 • 16 a b 3 100.0 Totals 202 289 Table 67: Comparison of the state of maturation of the ovaries of second generation M. sexnotatus, 1974. (a) specimens captured by the oatfield suction trap; (b) specimens.taken in D-vac suction samples and in sweep net samples; (I) immature ovaries; (D) with developing oocytes Numbers % non-gravid Week dissected % I % D % gravid 10 a 1 100.0 - b 4 100.0 - 11 a 3 - 100.0 b 17 76.4 11.8 11.8 12 a 4 50.0 50.0 b 47 89.4 4.2 6.4 13 a 7 14.3 57.1 28.6 b 87 85.0 4.6 10.4 14 a 30 26.7 40.0 33.3 b 87 61.0 12.6 26.4 15 a 32 31.2 28.1 40.7 b 49 67.3 14.3 18.4 16 a 1 100.0 b 25 48.0 12.0 40.0 Total 397 The aerial suction trap catches though rather low in numbers for both years for first generation females are fairly consistent in that females of the main, initial influxes are overwhelmingly immature. Those on later dates, in the 'tail. of the flight period contain a progressively higher percentage of mature females. A similar picture 0 was obtained from the oatfield suction trap catches for the second generation in both years. At the beginning of the flight period females are overwhelmingly immature and the percentage of mature individuals progressively rises with time though never exceeding one third of the total except at the very tail of the 'exodus'. In both years, after the main invasion influxes • the percentages of mature females in the oatfield rose to 290 10075. This level was retained for the rest of the first generation pn In the secor,,,, generation, in both years, females in field samPles were predominantly immature throughout the sampjing weeks, predominantly non-gravid female population in the oatfield paralleled by similar females in the oatfield suction trap indicate a phase of intense flight activity by the young adults of the second generation. Temperature influences migration via reproductive maturation: migration being largely a pre-reproductive activity, the lengthening of the pre-reproductive period would favour a prolongation in migration in autumn (Dingle, 1968, 1972). Lawson et al. (1951) report that disseminating female beet-leafhoppers are predominantly non-gravid but gravid females were also captured in aerial traps. Also, non-gravid females covered longer distances than the gravid ones. According to Rose (1972b) flight in Cicadulina spp. is apparently limited to the immediate pre-reproductive stage • or to inter-oviposition periods. Also, about 90% of the dispersing Cicadulina spp. captured in aerial suction traps were immature while in settled breeding populations there was a predominance of mature females (Rose, 1973b). According to Raatikainen and Vasarainen (1973) M. cristatus in Finland apparently starts migrating less than a week after adult emergence. Raatikainen (1967) reports that the females of Javesella pellucida are immature when invading cereal crops in Finland. 291. 6. The Emigration from the Crop by Second Generation Adults Flights that are classified as migratory due to their simultaneity and persistence generally begin soon after imaginal ecdysis and are adapted to ensure change of habitat or a temporary abandon of a habitat destined to become unsuitable (Johnson, 1969, 1974). In females, these flights occur during part of the pre-oviposition period after which they often end. Following the adult moult there is a period of maturation prior to migration when the cuticle hardens and internal changes such as growth of fat body and reproductive organs take place. Depending on the insect species, this period will vary from a few hours to several days. When it is over there is a 'choice point' where individuals either remain in the habitat and begin reproducing or migrate before doing so. This 'choice point' can be described by the normal curve where a variable proportion of any given population will be migrants ranging from zero to virtually 100% depen- ding on species, habitat and previous individual history (Dingle 1974). According to Southwood (1962) the major evolutionary significance of migration lies in that it allows insects to keep pace with temporary habitats. Thus the less permanent a habitat the greater the probability that species utilizing it will engage in migratory flights. Evidence is building up that the exodus flight that takes migrants away from the habitat in which they emerged as adults is a specifically adapted flight (Johnson, 1974). 292 IL is generally accepted that take-off of migrants is characterized by the )dispersivc-movement behaviour pattern' in which the insect ascends to the tips of vegetation launching itself into the air, flying upwards to be engulfed by air currents, or alternatively, fly upwards until a more or less specific height is attained after which they orient themselves. This sort of flight is clearly an adaptive exodus (Southwood, 1962; Johnson, 1969, 1974). In those species where the insect flys upwards to be caught by air currents the migratory species has acquired a flight behaviour pattern to get free from the restrictions of its boundary layer for flight in order to ensure wind-assisted migration (Taylor, 1974). Take-off on a migratory flight requires the appropriate conditions of temperature, sunshine and winds° that populations can build up on the ground until conditions are favourable and a mass exodus may occur (Johnson, 1969). In 1973, flight activity as determined by the oatfield suction trap recommenced on week 7. This flight period extended to week 16 and reached a peak in week 12. In parallel, second generation adults were available in D-vac samples from week 8 to 16, the - last sampling date for the season, where numbers were virtually reaching zero. The numbers of Second generation M. sexnotatus captured in the oatfield suction trap in 1973 are given in Table 68. The daily catches are grouped under sampling weeks and flight within each week as a percentage of the whole flight period is also included. 29_3 Table 68: Numbers of second generation M. sexnotatus captured in the oatfield suction trap, 1973. Daily catch figures grouped under sampling weeks. Week 7 2 0 2 3.2 - 2.4 8 1 1 2 1.6 5.0 2.4 9 0 11 11 - 55.0 13,4 10 9 7 16 14.5 35.0 19.5 11 8 0 8 12.9 - 9.8 12 18 0 18 29.0 - 22.0 13 7 0 7 11.3 - 8.5 14 10 0 10 16.1 - 12.2 15 6 1 7 9.7 5.0 8.5 16 1 0 1 1.6 - 1.2 Totals 62 20 82 The relation between percentage weekly flight and fluctuations in numbers of adult and total nymphal popula- tions in the oatfield is represented graphically in Fig. 43. It is assumed here that the specimens captured in the oatfield suction trap originate from the oatfield itself. This assumption is based on the fact that the oatfield repre- sents the main source of M. sexnotatus in the vicinity of the trap. In 1973 catches from traps II and IV that were absent from weeks 3 to 8 and weeks 4 to 8 respectively, recommenced in both traps in week 9. Catches in trap II extended to week 14 and in trap IV to week 15. In both traps catches were consistently low with no clear peak. In 1974 the flight period, as determined by the oatfield suction trap, extended from week 8 to 16. A clear 7300 1250 -4200 25 —100 20 5 0 z 15 0 T GH 50 oz FLI Y0 ( 10 1 / 25 5 0— — / 0 i__—._-10 0 wEEK7 8 10 11 12 13 14 15 16 17 1 1 I 1 I J 30 30 AUG 10 20 30 siPTIO 20 OC T Fig, 43. Numbers of second generation N. sexnotatus captured in the oatfield suction trap, 1973 (broken line) and fluctuations in numbers of adults (solid line) and of total nymphal population (double solid line) in the oatfield. •2 9 5 peak occurred in week 15 accounting for almost 40% of the total flight for the season. Second generation adults were available in field samples from week 10 to 17. The last sampling data was 29 September when numbers were approaching zro. The numbers of second generation M. sexnotatus captured in the oatfield suction trap in 1974 are given in Table 69 with daily figures grouped under sampling weeks and weekly percentage flights. Percentage weekly flights and fluctuations in the abundance of adult and total nymphal populations is repre- sented graphically in Fig. 44. Table 69: Numbers of second generation M. sexnotatus captured in the oatfield suction trap, 1974. Daily catch figures grouped under sampling weeks Week 88 ? + S % %+0 + --A6 8 1 0 1 1.6 - 1.2 9 0 1 1 - 4.3 1.2 10 1 1 2 1 .6 4.3 2.3 11 3 1 4 4.8 4.3 4.6 12 3 1 4 4.8 4.3 4.6 13 7 4 11 11.0 17.4 12.8 14 22 4 26 34.9 17.4 30.2 15 25 9 34 39.7 39.2 39.5 16 1 2 3 1.6 8.7 3.5 Totals 63 23 86 As said, data from trap II and the Rothamstead trap were not available for 1974. Trap IV was in operation from 11 July to 1 October (weeks 6 to 17) in 1974. M. sexnotatus catches recommenced on week 14 with a definite peak in week 15 and a sharp decline in week 16. S 296 6.50 550 450 350 250 0 40 J200 0 CU 0 30 150 cA ,e to 20 100 O 10 5 0 wit 0 WEEK8 2 0 30 A Li G 10 20 30 SE PT 10 20 30 o c T 10 Fig. 44. Numbers of second generation M. seznotatus captured in the oatfield suction trap, 1974 (broken line) and fluctuations in numbers of adults (solid line) and of total nymphal population (double solid line) in the oatfield. 297 Inspection of Figs. 43 and 44 shows that numbers of adults in the oatfield decline sharply after peak paralleling the descent in numbers of the nymphal stages and indicating that adults leave the habitat soon after being recruited into the stage. Simultaneously, during the main period of adult emergence higher percentages of flyers in a given week correspond to lower numbers in the field population in the ensuing week. Conversely large numbers build up in the field when the percentage of flights is low. This situation is well illustrated in 1973 and 1974. In 1973, the build up of the field population of adults in week 11 resulting in a peak of adult numbers in week 12 (sampled on- 29 August) corresponded to one of the lowest percentage flights of the main flight period in week 11 (22-28 August). This was followed by peak flight throughout week 12 (29 August-4 September) corresponding with 22% of the total flight of the season. Accordingly, the estimated field population sampled on 5 September (week 13) was reduced by almost 50% of the estimated population of the previous week. In 1974, due to adverse weather conditions, flight was consistently • low up to week 13 inclusive. In parallel, high numbers of adults built up in the field reaching a clear peak in week 14. Both in 1973 and 1974 peak adult numbers were achieved in the fourth week after the appearance of the first adults in field samples. The number of recruits entering the adult stage was 40% greater in 1974 than in 1973, but the peak achieved in 1974 due to the low migratory activity over the main period of recruitment was three times as high as • that in 1973. Conditions for flight were favourable in 298 week 14 and 30.2% of the total flight for the season took place between 6-12 September. Accordingly, the estimated weekly population of week 15, sampled on 13 September was reduced by nearly 50% of the estimated population in the previous week. Over week 15 (13-19 September) 39.5% of the total seasonal flight took place; the adult field numbers sampled on 20 September for week 16 were reduced to less than 50% of the previous week. In both years male captures by the suction trap were approximately only 1/3 of that of the females. The data are consistent for both years for the second generation and also is for the first generation where male catches were higher than female catches. The same trend is obtained in trap IV where more femels were obtained in the flight season of the second generation both in 1973 and 1974. No inter- pretation is proposed here for this discrepancy in sex-ratio in the trap catches since as determined by D-vac sampling males were nearly as abundant as females in the oatfield throughout the second generation and there is as yet no information on a differential behaviour in the take-off by the two sexes. M. sexnotatus is neither as large as Eurygaster and Aelia (Brown, 1965) nor as abundant as the frit fly (Southwood et al., 1961) to enable direct obser- vations on take-off behaviour under natural field conditions. Raatikainen (1967) reports that take-off in Javesella_ pellucida consists of an upward flight for a few centimetres to a few metres where the insect is carried up by air currents. At the time of descent to host plants the height of flight slowly decreases and the insect often makes horizontal turns in the flight direction previous to landing on the leaves of 299 plants in a cereal field. Ballard and Evans (1928) describe a differential behaviour for both sexes of the hemipteran DNy, ercus sidae where the male take-off usually consists of an upward flight to a much greater height than that of the female that is a low straight flight. It has been argued whether migrants rely directly on ultimate environmental stimuli such as the drying up of the food plant to initiate a migratory flight. As pointed cut by Johnson (1969) migration ensures that insects leave, either temporarily or permanently, habitats that by a natural course of events become unsuit- able for breeding. The principal causal factors, from the evolutionary aspect, seem to be the adverse changes of climate and diet. Factors such as particular deficiencies in the food or a changed photoperiod may act as 'token stimuli' that trigger the ontogenetic processes that will produce migrant individuals that soon after adult emergence will migrate from sites that apparently are still suitable for breeding. M. sexnotatus first generation adults leave natural grasses to migrate to the young oat crops and second genera- tion adults leave the withering oatfield to settle in natural grasses once again. The exodus from the oatfield by the young second generation adults coincides with the general poor state of the crop. An experiment was carried out under laboratory conditions to dupplement information on movements out of witherina oat plants. was made on 2 and 5 October 1972 in the out of door insectary. young specimens a few days after adult emergence 300 from 20°C laboratory cultures were used. The host plants tested were oats. Half of the plants used were 8 cm high fresh oats seedlings grown in pots and the others were 30 cm high drying oats. Though withering, the oats could poten- tially maintain an adult and nymphal population as deter- mined by a control culture. The food plant was placed inside a 33 cm long and 13 cm in diameter cellulose acetate inverted cylinder. The 10 cylinders were placed side by side on a bench, alternating those with fresh with those with withering oats. The insects were distributed at random in sets of 10 per cage. The cylinders were kept closed for 15 min to allow time for the insects to settle down on the food plant. The experiment was repeated twice and altogether 200 specimens were tested. On the first occasion, the numbers remaining in the •cages were checked after 2, 24 and 48 hours after the setting up. On the second occasion, the cages were checked after 2 and 24 hours of the setting up. 2 A X test was carried out to test the hypothesis that departures from the fresh and from the withering oats were homogeneous. The contingency tables for both sets of experiments are given in Table 70. A X2 of 23,62 for three degrees of freedom was obtained for the first experiment and the null hypothesis was rejected at P < 0.001. The null hypothesis was agains rejected in the second experiment with a X2 of 53.23 for two degrees of freedom. The results indicate that departures from withering oats are signifi- cantly higher than those from fresh oats. According to Miller and De Lyzer (1960) Macrostel.es fascifrons adults disperse to vegetables, ornamentals and 301 Table 70: Contingency table relating the numbers of sexnotatus adults departing from fresh and \Ti_thering oats in an out of door insectary, October 1972 Departures after Numbers State of the set up remaining host plant 0-2 h 2-24 h 24-48 h after 48 h Totals Fresh 22 5 2 21 50 Withering 37 10 2 1 • 50 Totals 59 15 4 22 100. X2 = 23.62 3 d.f. > P 0.001 Departures after Numbers State of the set up remaining host plant 0-2 h 2-24 h after 24 h Totals Fresh 11 4 35 50 Withering 21 28 1 50 Totals 32 32 36 100 2 X = 53.23 2 d.f. > P 0.001 other vegetation in early summer when the cereal crop begins to ripen; when vegetable crops have been harvested they move again to autumn-seeded winter grains where diapausing eggs are laid. Westdal et al. (1961) report that M. fascifrons move from ripening plants such as rye in spring to new crops such as flax and oats. Chiykowski and Chapman (1965) report • that M. fascifrons leave grain crops shortly after the crop has headed. Hervey and Schroeder (1947) suggest that the maturity of winter barley would hasten the migration of M. fascifrons adults originated from overwintered eggs. According to Raatikainen (1967) migration in Javesella pellucida is governed by internal causes and the ready availability of suitable host plants is unable to prevent it. Lawson et al. (1951) report that the major migratory 302 flights of the beet-leafhopper Circulifer tenellus take place in autumn when large areas of host plants are mature and dying and once more in spring when the winter annuals dry up. Rose (1972a) says that the main flight season of Cicadulina spp. in Rhodesia is composed of individuals leaving low density populations in widespread drying host grasses. 7. Flight and Weather Conditions The number of insects in the air is influenced partly by the size of the population in the source area and also by the proportion of that population in flight. The proportion in flight depends on many factors, especially on weather. Flights vary in function, duration and fre- quency according to the insect species but also with the age and physiology of the individuals. Insect flight may be modified by weather. In migrating insects the numbers in the air are related to the proximity of the source from which the primary exodus is made. Under these circumstances aerial numbers are also controlled by rates of moulting and teneral development. Also, numbers of insects flying near a trap differ according to whether they are accumulating in a locality or whether they are departing (Johnson, 1969). In an attempt to relate flight in M. sexnotatus with weather conditions, a multiple linear regression analysis was carried out on the daily catches by the oatfield suction trap and the independent weather variables. These comprised mean daily temperature, maximum daily temperature, mean daily percentage relative humidity, average wind speed in knots, hours of sunshine and total rainfall in mm. 303 The analysis was carried out initially with numbers caught in relation to the daily means or daily totals of the above variables. No significant results were obtained when trying to relate daily numbers caught to the values representing the whole of the 24 hour period. Waloff (1973). found significant results at 1% level on the relationship between maximum daily temperatures on the daily catches of M. sexnotatus from several suction traps in Silwood Park. Lewis and Taylor (1965) have demonstrated that flight in insects is periodic and with rare exceptions all species fly during only part of the 24 hours. Also, that time of flight is determined largely by physiological res- ponses to cyclic factors in the environment. They demon- strated that a periodicity curve for M. sexnotatus shows a long period of limited flight activity followed by a sudden increase. This curve was interpreted as perhaps coming from a mixed population of insects of different flight capability, habitats and perhaps age, or even a mixture of migratory and non-migratory individuals. Like many leafhoppers, M. sexnotatus shows a crepuscular peak flight., This was preceded by a steady low activity starting at dawn. Peak of flight was determined as between 17.00-19.00 h GMT. Thus, numbers caught were analysed by the multiple linear regression in relation to weather conditions at these hours of the day. Values of temperature and wind speed were calculated for 17.00, 18.00 and 19.00 h for each day. The analysis was carried out separately for the flight season of first and second generation since the 304 former consists mainly of specimens congregating to the oatfield while the latter represents an exodus flight. 277ma1yses were carried out separately for 1973 and 1974. For the first generation, 1973, the combined effect of temperature and wind speed on flight gave significant results between 5% and 10%. Wind speed had no significance "per se" and in the partial analysis temperature gave signi- ficant results between 2.596 and 5.0%. For 1974, multiple analysis gave significant results with a value of F higher than 0.1%. The same significance level was obtained in the partial analysis for temperature; again, the result of the partial analysis for wind speed was not significant. 4 For the second generation, 1973, the result of the multiple analysis was significant between 0.5 and 0.1%. The same significance level was obtained in the partial analysis for temperature. Once again, wind speed was not significant. In'1974, the result of the multiple analysis was significant between 2.5 and 5%. The partial analysis for temperature gave results not significant statistically while that for wind speed had an F value between 2.5 and 5%. These results suggest that the exceptionally high wind coinciding with the peak emergence period and teneral period of the second generation in 1974 obscured the relation of flight to temperature in that season. The result of this situation is illustrated in Fig. 44 where percentage weekly flights are related to population numbers in the field and the 4 curve for flight is clearly delayed in relation to field numbers. 30: GENEP,AL DISCUSSION AND SUIE.IARY Essentially this is a study of a highly mobile migrant secies Macrosteles sexnotatus (Fall4n) which invades an unoccupied area (the crop) at the beginning of each season. This thesis is divided into three sections that concern the biology, population studies and dispersal sexnotatus. Detailed discussion are given within each section but the main results are summarised and only briefly discussed below. Section I. Biology of Nacrosteles sexnotatus 1. The natural host plants in the habitat where M. sexno- tatus is found are listed. M. sexnotatus is a poly- phagous species and under laboratory conditions it was bred out successfully for consecutive generations on oats variety Condor. It was the dominant Auchenor- rhyncha species in an oatfield during the three-year study period. 2. Three species of Macrosteles were found breeding in Silwood Park; M. sexnotatus, M. viridigriseus (Edwards) and M. laevis (Ribaut). 3. Continuous breeding under laboratory conditions was I maintained and also egg, nymphal and adult parasites were bred out. 4. The five nymphal instars were described and illustrated as well as the rudiments of genitalia from instars III to V of both sexes. The most diagnostic features for the species are the number and distribution of abdominal setae (;athirithamby, 1971). The cnvmphal stages are b es Oagnoej by the sape and relative sizes of the 306 meso- and metathorax and from third instar onwards by the genitalia as well. The rate of growth of the nymphal stages was related to duration of instars and was well described by a straight line but for the third instar a poorer fit was obtained. 5. The relevant taxonomical literature was reviewed and stressed that in identifying Macrosteles spp. use should made both of the aedeagus and second abdominal apodemes. 6. A short-winged form obtained in low numbers in labora- tory was described and the wings of both forms illus- trated. The degree of reduction of the wings was variable and involved suppression of parts. Fecundity and fertility of such females seemed to be normal. 7. The internal reproductive organs of both sexes were illustrated and described. Minimum time for first mating under laboratory conditions was tested. At 20°C, females can copulate with either young or older males as early as on the second or third day after emergence. 8. Rates of egg development were studied under constant temperature conditions of 15, 20, 25 and 30°C. The relationship between developmental rates and tempera- • ture was well described by the logistic equation. The 'optimum' both for speed of development and synchroni- zation of hatching was found to lie between 25 and 30°C. 9. No effect of acclimatization 'per se' was found on development rates of eggs incubated either in the same or in a different temperature for oviposition. 10. The theoretical 'developmental zero' for eggs was found to be at 10.99°C. 307 11. The effect of humidity on development rates was -7tdied. No hatching was seen in relative humidities • bel.ow 10096. Even then, proportions hatching were lower than whr,n. oggs were in direct contact with moist surfaces. This suggested that eggs either absorb a certain amount of water by imbibition or that a saturated atmosphere is necessary to prevent desiccation. 12. Diapause was induced in the eggs by transferring nymphs from continuous cultures at 16 light hours/day to a short-day regime of 12 light hours/day. The resulting females produced diapausing eggs. Diapause in M. sexnotatus is facultative and the reaction to the environmental conditions is of the 'long-day' type (Danilevskv, 1961). 13. 'MorphogenPsis' and hatching of diapausing egg was obtained by subjecting such eggs to a period at low temperatures followed by incubation at 25°C. Dia- pausing eggs were shown to be less successful in their incubation rates, hatchability and synchronization of eclosion as compared with non-diapausing eggs incubated at comparable conditions. 14. The rate of development of the nymphal stages was studied under conditions of constant temperature of 15, 20, 25 and 30°C. The relationship between rates and temperature was well described by the logistic equation for all instars except the second where the regression line was adequately fitted. The greatest developmental speed was at 30°C in all the five instars. • 302 15. The theoretical 'developmental zero' was determined for the five nymphal stages and it was found that it increased systematically from the first to the fourth instar thus being paralleled by the seasonal increase of temperature in spring. 16. Fecundity was studied under constant temperature condi- tions of 15, 20,•25 and 30°C. The duration of the pre-oviposition period was found to be inversely affected by temperature. Fecundity was affected by temperature both in terms of total egg number and daily rate of oviposition. The highest fecundity and egg rate was attained at 30°C. The post-oviposition period increased at lower temperatures and sometimes was altogether suppressed at higher temperatures. 17. The weight of a female was shown to fluctuate with - maturation and rate of oviposition. In ageing females especially in temperatures where the metabolic rates are particularly depressed or accelerated there was a progressive increase in weight since the rate of egg laying was lower than that of egg production. 18. Very old females were found to deposit eggs outside the host plant. 19. Under laboratory conditions, females ovipositing in oat seedlings prefer the leaf sheath to the leaf blade. 20. In natural conditions, females lay eggs as frequently in the leaf sheath and ligula as in the leaf blade. In older oats, with more rigid leaf sheaths, they oviposit especially in the leaf blade. Oviposition took place at all heights of the oat plant. 309 21. Lonc;evity was followed under conditionsconstant temperature of 13, 20, 25 and 30°C and was found to be inversely related to temperature. i:.aximum longevity observed was 195 days for a male at 15°C. Section II. Population Studies 1. During the three years study period M. sexnotatus occurred in very low densities in natural grasses in Silwood Park: 2. Population studies were carried out in an oatfield spring sown each year. In 1973 and 1974 the oatfield was 1200 m2 and was subdivided into 10 subareas of equal size. The surroundings of the oatfield were described. 3. Temperature in the oatfield at different heights and situations was determined and was found to be consis- tently lower than the air temperature measured in the Stevenson screen. 4. Sweep net sampling was adopted only to determine the beginning of the invasion into the oatfield by first generation M. sexnotatus. Metre equivalent sweeps were calculated and number caught by 100 sweep strokes was found to reflect the real density of the population to be sampled. 5. D-vac sampling was adopted to sample the population and to derive absolute estimates for the study area. An adaptation of the suction head was made to sample on fully grown oats. 6. The efficiency of suction-sampling the nymphal instars and adults on medium cats, high oats and collapsed oats was determined by the removal sampling technique. The 310 data was analysed by Kono's 'time-unit' method and by Zippin's procedure based on maximum likelihood. D-vac sampling on oats of medium height was more efficient - than either sampling on high or collapsed oats. 7. Weekly population estimates based on D-vac samples were made for the invading population of adults and for all the nymphal stages and adults of the second generation in 1973 and 1974. For 1972 estimates were available for the second generation only and did not cover the whole generation time. 8. The daily fecundity of first generation females under field conditions in 1973 and 1974 was estimated from caged females in the oatfield. The high field fecun- dity encountered cannot be explained by effects of temperature when extrapolations are made from labora- tory data under conditions of constant temperature; the main cause was suggested to be the nutrient status of the food plant. 9. Natality in the field was estimated by relating fecundity rates to female numbers in the field. The figures for natality were similar in 1973 and 1974 though arrived at by different processes: in 1973 fewer females laid more eggs. 10. Incubation rates and nymphal duration in the field were estimated by the logistic equation and regression equation for the average temperatures in the field estimated for each immature stage. 11. Population estimates were integrated by different methods. That of 1972 due to insufficient sampling was only analysed by the crude method of Cheng and Le Roux (1966). Those of 1973 and 1974 were analysed 311 a modification of Richards et al. (1960) method permitting the analysis in the absence of a value for the accumulated total of samples of the egg stage (M0). Kiritani and Nakasuji's (1967) method was also adapted so as not to be affected by the early migration of the second generation adults. 12. Population budgets were given for 1973 and 1974. Numbers living at the beginning of a stage were those estimated by the adaptation of Richard's et al. method. 13. The budget data was analysed in terms of comparison of mortality factors within a generation since only discontinuous generations occur in the oatfield as is usual for most pests of arable crops (Southwood, 1966). The greatest mortality occurred in the egg stage; mortalities in the nymphal stages were much lower * than that in the egg stage in both years. Egg sur- vival was higher in 1974 but the larger numbers entering the nymphal stages suffered heavier mortality than in 1973. 14. Experiments on egg parasitism were carried out in natural grasses (1972) and in the oatfield (1973). In both cases the main egg parasite was the mymarid Anagrus near subfuscus F6rster. Other egg parasites occurred in smaller proportions: the mymarids Gonato- cerus possibly paludis (Debauche), G. litoralis (Haliday), the trichogrammatid Oligosita engelhardti Kryger and the aphelinid Centrodora livens (Walker). 15. Parasitism of the nymphal stages and adults by the Pipunculidae in the oatfield was low in 1973 (3.86%) and in 1 974 (1.76%). Eudorylas fuscipes Zetterstedt was bred out from M. sexnotatus and is a new host record. 312 16. Parasitism of nymphs and adults in the oatfield by the Dryinidae was low in 1973 (0.37%) and in 1974 (0.35%). Chelogynus ephippiger (Dalman) var. collaris (Dalman) was bred out from M. sexnotatus and M. viridigriseus (Edwards). These are new parasite-host records. Female dryinids of this species were successfully induced- to oviposit in M. sexnotatus nymphs under laboratory conditions. The female attack behaviour was briefly described. 17. Predator feeding trials were carried out using both Nabidae and Araneae collected in the oatfield in 1973. The most effective predators among the species tested were Nabis rugosus (L.) and N. ferus (L.). 18. A serological technique was used to detect predation of M. sexnotatus. An anti-Macrosteles serum with a titre of 1/20840 was prepared. It was 'absorbed' to avoid cross-reacting to Delphacidae extract and the titre of the absorbed serum for Macrosteles was reduced to 1/10240. The precipitin tests were carried out in agar gel by the method of Ouchterlony. The overall percentage positive results for predation was 5.2 and the two nabid species were confirmed as the most . voracious oredators on M. sexnotatus. Section III. Studies on Dispersal 1. Dispersing insects were sampled by aerial suction traps, sticky and water traps. . Movements of M. sexnotatus into and out of the catfield were inferred by the fluctua- tions in numbers of the estimated populations. IP 313 2. The relevant literature of MacRosteles spp. as invaders of crops was reviewed. The genus shows heterogeneity in terms of migratory activity. 3. The mode of colonization of the oatfield in 1974 was studied. Invasion took place in two stages: firstly, by a small local population thainoved into the field jumps and low flights; secondly, by fluxes of flying invaders from other sources as determined by local surveys, fluctuations in numbers in the oatfield, suction and sticky trap catches. 4. The period of trivial flight of males of the invading population was determined in 1973 by means of sticky traps and water traps at crop level. It took place over a definite period after invasion coinciding with peak numbers of females in the field and the beginning of the oviposition period. 5. The state of maturation of the female ovaries was used as an indication of migratory activity in 1973 and 1974. Both invasion and exodus flights were made by post- teneral females. Following the main flight invasion period the settled female population in the oatfield was sexually mature. Second generation females abandon the oatfield soon after emergence. 6. The pattern of emigration from the crop by second generation adults was followed in 1973 and 1974. The young adult population leave the oatfield continuously unless adverse weather conditions prevent flight so that large numbers of adults may temporarily build up in the oatfield. Under laboratory conditions it was shown that M. sexnotatus adults leave withering oats in significantly higher numbers than the fresh oats. 314 7. The relation of flight and weather conditions was examined by multiple regression analysis. Significant results were obtained when daily catches in the suction trap were related to weather conditions taking place at the hours of peak flight for M. sexnotatus as determined by Lewis and Taylor (1965). Temperature was found to have a greatet effect than wind speed. This relation was obscured if wind speed was consis- tently above the threshold for flight. ArYAOWLED01:!11ENTS 1 am grateful to Professor T.R.E. Southwood for granting facilities to carry out this research in Silwood Park. In particular, I wish to express my gratitude to my supervisor, Dr. N. Waloff, for her encouragement, interest and guidance throughout this work and the preparation of the thesis. I wish to express my acknowledgements to Mr. Nicholas Small for unfailing interest, discussions and advice on the statistical analysis. I am grateful to Dr. P.F.L. Boreham who provided advice and facilities of the Immunology Laboratory at Silwood Park for the serological experiments and for criticism of the manuscript on this theme. I would also like to express my thanks to the following individuals and institutions: Professor O.W. Richards, F.R.S., for the identification of the Dryinidae. Dr. M.W.R. de V. Graham, of the Hope Department of Zoology (Entomology) for the identification of the egg parasites. Dr. I. Walker for examination of the Mymaridae. Dr. F. Wanless, of the British Museum (Natural History) for the identification of the spiders used in the laboratory feeding trials. Mr. David Robertson, for the identification of the spiders tested in the serological experiments Mr. F. Benton, for the identification of the Pipunculidae. Miss S. McCarthy and Mr. P. Thompson for the sortinq out and identification of the Macrosteles species from the Silwood Botton suction traps. Mr. R. Dransfield, for the joint use of the water trap. Mr. E. Thornhill for the unfailing cooperation in the maintenance of the D-vac machine. Mr. A. Broodbank and especially Mr. N. Vanuzzi for helping in the sampling programme by carrying the D-vac suction sampler. This work was carried out while I was a recipient of a British Council Scholarship, and the generosity of this Institution is gratefully acknowledged. Finally, I am indebted to the Universidade Federal do Rio Grande do Sul and to the Government of the Reptiblica Federativa do Brasil for granting me three years of study leave. +11 * 317 REFERENCES ATNqLIE, (1°20). Notes on Conatopus ombrodes, a Pal-ac,ite of Jassids. Fnt. News, 31, 169-173. ANDREWARTHA, E.G. (1952). Diapause in relation to the ecology of insects. Biol. Rev., 27, 50-107. ANDREIRTHA, E.G. & BIRCH, L.C. (1954). The distribution and abundance of animals. The University of Chicago Press, Chicago and London. ANDRZEJEWSKA, L. (1962). Macrosteles laevis Rib. as an unsettlement index of natural meadow associations of Homoptera. Bull. Acad. pol. Sci. Cl. II, Ser. Sci. Biol., 10, 221-6. ANDRZEJFWSKA, L. (1965) Stratification and its dynamics in meadow communities of Auchenorrhyncha (Homoptera) (31). Ekol. pol. Ser. A, 13, 685-715. BALLARD, E. & EVANS, M.G. (1928). Dysdercus sidae, Montr., in Queensland. Bull. ent. Res, 18, 405-32. BANKS, C.J. & BROWN, E.S. (1962). A comparison of methods of estimating population density of adult sunn pest Eurygaster integriceps Put.(Hemiptera, Scutelleridae) in wheat fields. Ent. exp. & apple , 5 (1962) , 255-60. BANTTARI, E.E. & MOORE, M.B. (1962). Virus cause of blue dwarf of oats and its transmission to barley and flax. Phytopathology, 52, 897-902. BEAMENT, J.W.L. (1961). The water relations of insect cuticle. Biol. Rev., 36, 281-320. BEIRNE, B.P. (1952). The Nearctic species of MacRosteles (Homoptera: Cicadellidae). Can. Ent., 84(7), 208-32. BEIRNE, B.P. (1956). Leafhoppers (Homoptera: Cicadellidae) of Canada and Alaska. Can. Ent., 88(2), 1-180. BOLLOW, H. (1950). Uber die zwergzikade and ihr Wiederauftreten in Bayern. Pflanzenschutz, 2, 102-4. BOREHAII, P.F.L. & GILL, G.S. (1973). Serological identifi- cation of reptile feeds of Glossina. Acta trop., 30, 356-6c. BROADBENT, L., DONCASTER, J.P., HULL, R. & WATSON, M.A. (1948). Equipment used- for trapping and identifying alate aphides. Proc. R. ent. Soc. Lond. A 23, 57-8. BROWN, E.S. (1965). Notes on the migration and direction of flight of Eurygaster and Lelia species (Hemiptera, Pentatomoidea) and their possible bearing on invasions of cereal crops. J. Anim. Ecol., 34, 93-107. 318 BROWNING, T.O. (1952). The influence of temperature on the rate of development of insects, with special reference to the eggs of Gryllulus commodus. Aus. J. sci. Res., B.5, 96-111. CALDWELL, R.L. & RANKIN, M.A. (1974). Separation of migratory from feeding and reproductive behaviour in Oncopeltus fasciatus. J. comp. Physiol., 88, 383-94. CALLAHAN, R.A., HOLLBROOK, F. & SHAW, R.F. (1966). A comparison of sweeping and vacuum collecting certain insects affecting forage crops. J. econ. Ent., 59, 478-9. CARPENTER, J.R. & FORD, J. (1936). The use of sweep net samples in an ecological survey.. J. soc. Brit. Ent., 1, 155-61. CLARIDGE, M.F. & REYNOLDS, W.J. (1972). Host plant speci- ficity, oviposition behaviour and egg parasitism in some woodland leafhopper of the genus Oncopsis (Hemiptera Homoptera: Cicadellidae). Trans. R. ent. Soc. Lond., 124, 149-66. CLARKE, J.M. & MAYNARD-SMITH, J. (1961). Independence of temperature of the rate of aging in Drosophila subobscura. Nature, Lond., 190, 1027-8. CHEKE, R. (1974). Experiments on the effect of host spatial distribution on the numerical response of parasitoids. J. Anim. Ecol., 43, 107-13. CHENG, H.H. & LEROUX, E.J. (1966). Preliminary life tables and notes on mortality factors of the birch leaf miner Fenusa pusilla (Lepeletier) on blue birch, Betula caerulea grandis Blanchard, in Quebec. Ann. ent. Soc. Queb., 11, 81-104. CHYIKOWSKI, L.N. (1962). Clover phyllody virus in Canada and its transmission. Can. J. Bot., 40, 397-404. CHYIKOWSKI, L.N. & CHAPMAN, R.K. (1965). Migration of the six spotted leafhopper, Macrosteles fascifrons (Stal). Part 2. Migration of the six spotted leafhopper in Central North America. Res. Bull. agric. Exp. Stn. Univ. Wis._ (261), 21-45. CLAUSEN, C.P. (1940). Entomophagous insects. Hafner Publishing Company, New York. COE, R.L. (1966). Diptera: Pipunculidae. Handbk. 'dent. Br. Insects, X, 1-83. COON, B.F. & RINICXS, H.B. (1962). Cereal aphid capture in yellow baffle trays. J. econ. Ent., 55, 407-8. COOK, W.C. (1967). Life history, host plants and migrations of the beet leafhopper in the Western United States. Tech. Bull. U.S. Dept. Agric. No. 1365, 122pp. 319 COUPE, T.R. & SCHULZ, J.T. (1 968) . The influence of controlled environments and grass hosts on the life Cycle of Endria inimica. Ann. ent. Soc. Am., 61, 74-7. DANILEVSKY, A.S. (1961), Photoperiodism and seasonal development of insects.- English translation published by Oliver & Boyd, London (1965). DANILEVSKY, A.S. GORYSHIN, N.I. & TYSHCHENKO, V.P. (1970). Biological rhythms in terrestrial arthropods. A. Rev. Ent., 15, 201-44. DAVIDSON, J. (1944). On the relationship between temperature and rate of development of insects at constant tempera- ture. J. Anim. Ecol., 13, 26-38. DE LONG, D.M. (1932). Some problems encountered in the estimation of insect populations by the sweeping method. Ann. ent. Soc. Am., 25, 13-7. DE LONG, D.M. (1970). An Alaskan leafhopper that lives normally beneath icy tidal submergence. Ohio J. Sci., 70, 111-4. DE LONG, D.M. (1971). The bionomics of leafhoppers. A. Rev. Ent., 16, 179-210. DEMPSTER, J.P. (1956). The estimation of the number of individuals entering each stage during the development of one generation of an insect population. J. Anim. Ecol., 25, 1-5. DEMPSTER, J.P. (1960). A quantitative study of the predators of- the eggs and larvae of the broom beetle, Phytodecta olivacea Forster, using the precipitin test. J. Anim. Ecol., 29, 149-67. DEMPSTER, J.P. (1961). The analysis of data obtained by regular sampling of an insect population. J. Anim. Ecol., 30, 429-32. DIETRICK, E.J. (1961). An improved backpack motor for suction sampling of insect populations. J. econ. Ent., 54, 394-5. 4 DIETRICK, E.J., SCHLINGER, E.I. & VAN DEN BOSCH, R. (1959). A new method for sampling arthropods using a suction collecting machine and modified Berlese funnel separator. J. coon. Ent., 52, 1085-91. DIETRICK, E.J., SCHLINGER, E.I. & GARBER, M.J. (1960). Vacuum cleaner principle applied in sampling insect populations in alfalfa fields by new machine methods. Calif. Agric., 14(1), 9-11. DINGLE, H. (1968). The influence of environment and heredity on flight activity in the milkweed bug Oncopeltus., J. exp. Biol., 48, 175-84. DINGLE, H. (1972). Migration strategies of insects. Science, N.Y., 175, 1327-35. DINGLE, H. (1974). The experimental analysis of migration and life-history strategies in insects. Experimental analysis of insect behaviour (ed. by L. Barton Browne), pp. 329-43, Springer, Verlag Berlin, Heidelberg, New York. DORST, H.E. & DAVIS, E.W. (1937). Tracing long distance movements of beet leafhopper in the desert. J. econ. Ent., 30, 948-54. DRAKE, D.C. & CHAPMAN, R.K. (1965). Migration of the Six- spotted Leafhopper Macrosteles fascifrons (Still) . Part I. Evidence for long distance migration of the Six- spotted Leafhopper into Wisconsin. Res. Bull. agric. Exp. Stn. Univ. Wis., No. 261, 3-20. EGUAGIE, W.E. (1972). Effects of temperature and humidity on the development and hatching of eggs of the thistle lacebug, Tingis ampliata (Heteroptera, Tingidae). Ent. exp. & appl., 15, 183-9. ELTON, C.S. & MILLER, R.S. (1954). The ecological survey of animal communities: with a practical system of classifying habitats by structural characters. J. Ecol. 42, 460-95. VAN EMDEM, H.F. (1964-1965a). The importance of adjacent :uncultivated land in relation to crop pest insects. Proc. XII Int. Congr. Ent. London, p.577. VAN EMDEN, H.F. (1964-1965b). The role of uncultivated land in the biology of crop pests and benefitial insects. Scient. Hort., 17, 121-36. EMSLEY, M.G. (1957). A coarse method of estimating mirid populations in the field. Emp. Cott. Grow. Rev., 34, 191-5. EVENHUIS, H.H. (1958). Investigations on a leafhopper- borne clover virus. Proc. III Conf. Potato virus diseases (Lisse-Wageningen, 1957), pp.251-4. FENTON, F.A. & HOWELL, D.E. (1957). A comparison of five methods of sampling alfalfa fields for arthropod populations. Ann. ent. Soc. Am., 50, 606=11. FEWKES, D.W. (1960). The food requirements by weight of some British Nabidae (Heteroptera). Ent. exp. & appl., 3, 231-7. FEWKES, D.W. (1961). Diel vertical movements in some grassland Nabidae (Heteroptera). Entomologist's mon. Mag., 97, 128-30. FRAZIER, N.W. & POSNETTE, A.F. (1956). Leafhopper trans- mission of a clover virus causing green petal disease in strawberry. Nature, Lond., 177, 1040-1. 321 GUSTIN, R.D. & STONER, W.N. (1968). Biology Cf Dc,Htecenhalus sonorus, Ann. ent. Soc. Am. 61, 77-32. HAT:VEY, W.R. (1962). Metabolic aspects of insect diapause. A. .7nt., 7, 57-80. HASSELL, L.P. (1966). Evaluation of parasite and predator responses. J. Anim. Ecol., 35, 65-75. HEIKIUHEIMO, O. & RAATIKAINEM, M. , (1962). Comparison of suction and netting methods in population investiga- tions concerning the fauna of grass leys and cereal fields, particularly in those concerning the leaf- hopper Calligvpona pellucida (F). Valt. Maatalousk. Julk. Helsingfors, 191, 5-31. HAGEL, G.T. & LANDIS, D.J. (1967). Biology of the aster leafhopper, Macrosteles fascifrons, in eastern Washington and some overwintering sources of aster yellows. Ann. ent. Soc. Am., 60, 591-5. HAMILTON, A.G. (1950). Further studies on the relation of humidity and temperature to the development of two species of African locusts Locusta migratoria miqratorioides, R. and F., and Schistocerca gregaria Forsk. Trans. R. ent. Soc. Lond,, 101, 2-56. HERVEY, G.E.R. & SCHROEDER, W.T. (1947). The life cycle of Macrosteles divisus Uhl. in relation to carrot yellows in western New York. J. econ. Ent,, 40, 505-8. HINTON, H.E. (1954). The initiation, maintenance and rupture of diapause: a new theory. Entomologist, 86, 279-91. HINTON, H.E. (1957). Some aspects of diapause. Sci. Prog., Lond., 45, 307-20. HOKYO, N. & KIRITANI, K. (1967). A method for estimating natural survival rate and mean fecundity of an adult insect population by dissecting the female reproduc- tive organs. Res. Popul. Ecol., 9, 130-42. HOLLINGSWORTH, N.J. (1968). Environmental temperature and life span in poikilotherms. Nature, Lond., 218, 869-70. HOUSE, H.L. (1964). Contributions of nutritional research to Entomology. Proc. N. Carol. Brch. ent. Soc. Am., 19, 13-20. HOWE, R.W. (1967). Temperature effects on embryonic development of insects. A. Rev. Ent., 12, 15-42. HUFF, R.I. (1963). Relation between leafhopper influxes and synoptic weather conditions. J. appl. Meteor. 2 39-43. • 322 JOHANSSON, A.S. (1958). Relation of nutrition to endocrine- reproductive functions in the milkweed bug Oncopeitus fasciatus (Dallas) (Heteroptera: Lygaeidae). Nytt. Hag. Zool., 7, 1-132. JOHANSSON, A.S. (1964). Feeding and nutrition in repro- ductive processes in insects. Insect Reproduction (Ed. by K.C. Highnam), pp.43-55, Royal Entomological Society, London. JOHNSON, C.G. (1969). Migration and dispersal of insects by flight. Methuen, London. JOHNSON, C.G. (1974). Insect migration: aspects of its physiology. The Physiology of Insecta (ed. by M. Rockstein), vol. III, pp.280-334. Academic Press, New York and London. JOHNSON, C.G. & TAYLOR, L.R. (1955). The development of large suction traps for airborne insects. Ann. appl. Biol., 43, 51-61. JURISOO, V. (1964). Agro-ecological studies on Leafhoppers (Auchenorrhyncha, Homoptera) and Bugs (Heteroptera) at Ekensgard Farm in the Province of Halsingsland, Sweden. Stat. Vaxtskyddsanst. Medd., 13, 101, 1-147. KANERVO, V., IiEIKINHEIMO, O., RAATIKAINEN, M. & TINNILA, A. The leafhopper Delphacodes pellucida (F.) (Hom., Auchenorrhyncha) as the cause and distributor of the damage to oats in Finland. Valt. Maatalouskoet. Julk., 160, 1-56. KATHIRITHAMBY, J. (1971). Taxonomy, development and morphology of the immature stages of Cicadellidae (Homoptera). Ph.D. Thesis, University of London. KATHIRITHAMBY, J. (1974). Development of the external male and female genitalia in the immature stages of Cicadellidae (Homoptera). J. Ent. (A), 48(2), 193-7. KAYUMBO, H. (1963). The water relations of eggs of some Hemiptera-Heteroptera. M.Sc. Thesis, University of London. KENNEDY, J.S. (1961). A turning point in the study of insect migration. Nature, Lond., 189, 785-91. KIECKHEFER, R.W. & MEDLER, J.T. (1966). Aggregations of the Potato Leafhopper in alfalfa fields in Wisconsin. Ann. ent. Soc. Am., 50, 180-2. KIRITANI, K. & KAWAHARA, S. (1963). Effect of adult diet on the longevity, fecundity, oviposition period and phototaxis in the black carpet beetle Attagenus megatoma (F.). Jap. J. Ecol., 13(1), 21-8. KIRITANI, K. & KIMURA, K. (1965). The effect of population density during nymphal and adult stages on the fecundity and other reproductive performances. Jap. J. _Fool., 13, 233-6. 323 KTRITANT, K. & NAKASUJI, F. (1967). Estimation of the stage-specific survival rate in the insect population with overlaping ,- L-Iages. Res. Popul. Ecol., 9, 143-52. KISIYOTO, R. (1956). The effect of crowding during the larval period on the determination of wing form of an adult planthopper. Nature, Lend., 178, 641-2. KONTKANEN, P. (1950). Quantitative and seasonal studies on the leafhopper fauna of the field stratum on open areas in North Karelia. Suomal. Clain-la kasvit. Seur. van Julk., 13, 1-91. KONTKANEN, P. (1954). Studies on insect populations. I. The number of generations of some leafhopper species in Finland and Germany. Suomal. elain-ja kasvit. Seur. van Tiedon P6vtak., 8(2), 150-6. KUNKEL, L.O. (1926). Studies on aster yellows. Am. J. Bot., 13, 646-705. LANGER, R.H.M. (1972). How grasses grow. The Institute of Biology's Studies in Biology No. 34. LAWSON, F.R., CHAMBERLAIN, J.C.,& YORK, G.T. (1951). Dissemination of the Beet Leafhopper in California. Tech. Bull. U.S. Dep. Agric., No. 1030, 59pp. LEES, A.D. (1955). The physiology of diapause in Arthropods. University Press, Cambridge. LEES, A.D. (1956). The physiology and biochemistry of diapause. A. Rev. Ent., 1, 1-16. LE QUESNE, W.J. (1968a). Macrosteles ossiannilssoni (Hem., Cicadellidae), a new species previously confused with M. sexnotatus (Fallen). Entomologist's mon. Hag., 103, 190-2. LE QUESNE, W.J. (1968b). Macrosteles ossiannilssoni Le •Quesne (Hem., Cicadellidae) - a correction. Entomologist's mon. Mag., 104, 3. LE QUESNE, N.J.. (1969). Hemiptera: Cicadomorpha, Deltocephalinae. Handbk. Ident. Br. Insects, II, 2(b), 65-148. LEWIS, T. (1959a).. The annual cycle of Limothrips cerealium Haliday (Thysanoptera) and its distribution in a wheat field. Ent. exp. & appl., 2, 187-203. LEWIS, T. (1959b). A comparison of water traps, cylindrical sticky traps and suction traps for sampling thysanoptcran populations at different levels. Ent. exp. & appl. 2, 204-15. LEWIS, T. & TAYLOR, L.R. (1965). Diurnal periodicity of flight by insects. Trans. R. ent. Soc. Lond., 116, 4 393-479. k 224 LINDSTEIN, K., VACKE, J. & GERHARDSON, B. (1970). A preliminary report of three cereal virus diseases new to Sweden spread by Macrosteles and Psarcgtettix Leafhoppers. St. Vaxtskvddsanst. Meddr., 14, 281-297. LINNAVOURI, R. (1952). Studies on the ecology and pherology of the leafhoppers (Homoptera) of Raisio (SW Finland). Suomal. elain-ja kasvit. Seur. van Julk., 14, 5, 1-32. LOUGHTON, B.C., DERRY, C. & WEST, A.S. (1963). Spiders and the Spruce budworm. Dynamics of epidemic Spruce budworm populations. (ed. by R.F. Morris). Mem. ent. Soc. Can., 31, 249-88. MacDONALD, G. (1957). The epidemiology and control of malaria. Oxford Univ. Press. MANLY, B.F.J. (1974). Estimation of stage-specific survival rates and other parameters for insect populations developing through several stages. Oecologia (Berl.). 15, 277-85. NANSINGH, A. (1971). Physiological classification of dormancies in insects. Can. Ent., 103, 983-1009. MARAMOROSCH, K., SHIKATA, E. & GRANADOS, R.R. (1968). Structures resembling mycoplasma in diseased plants and in insect vectors. Trans. N. Y. Acad. Sci., 30, 841-55. MAY, Y.Y. (1971). The biology and population ecology of Stenocranus minutes (Fabricius) (Delphacidae-Homoptera). Ph.D. Thesis, University of London. McNEILL, S. (1973). The dynamics of a population of Leptopterna dolabrata (Heteroptera: Miridae) in relation to its food resources. J. Anim. Ecol., 42, 495-507. MEADE, A.B. & PETERSON, A.G. (1964). Origin of populations of the six-spotted leafhopper, Macrosteles fascifrons in Anoka County, Minnesota. J. econ. Ent., 57, 885-8. MEDLER, J.T. (1962). Long-range displacement of Homoptera in the Central United States. Proc. XI Int. Congr. Ent. (Vienna, 1960), 3, 30-5. MEDLER, J.T., PIENKOWSKI, R.L. & KIECH, R.W. (1966). Biological notes on Empoasca fabae in Wisconsin. Ann. ent. Soc. Am., 59(1), 178-80. MENHINICK, E.F. (1963). Estimation of insect population density in herbaceous vegetation with emphasis on removal sweeping. Ecology, 44(3), 617-21. MESSENGER, P.S. & FLITTERS, N.E. (1958). Effect of constant temperature environments on the egg stage of three species of Hawaiian fruit flies. Ann. ent. Soc. Am., 51, 109-19. 32 5 MESSENGER, P.S. & FLITTERS, N.E. (1959). Effect of variahle temperature environments on egg development of three species of fruit flies. Ann. ent. Soc. Am. 52, !91-204.- METCALFE, J.R, (1965). Nitrogen status of sugar-cane leaves and the fecundity of a hemipterous post. Nature, 207, 21 9-20. METCALFE, J.R. (1970). Studies on the effect of the nutrient status of sugar-cane on the fecundity of Saccharosydne sacoharivora (Westw.) (Hom., Delphacidae). Bull. ent. Res., 60, 309-25. MILLER, L.A. & DE LYZER, A.J. (1969). A progress report on studies of biology and ecology of the six-spotted leafhopper Macrosteles fascifrons (Stal) in South- western Ontario. Proc. ent. Soc. Ont., 90, 7-13. MIYASHITA, K., ITO, Y., YASUO, S., YAMAGUCHI, A. & ISHII, M. (1964). Studies on the dispersal of plant- and leafhoppers II. Dispersals of Delphacodes striatella Fallen, Nephotettix cincticeps Uhler, and Deltocephalis dorsalis Notschulsky in the nursery and paddy fields. Jap. J. Ecol., 14, 233-41. MOCHIDA, 0. (1964). On oviposition in the brown plant- hopper Nilaparvata lugens (Stal). (Horn., Auchenor- rhyncha). I. Oviposition and environmental factors with special reference to temperature and riceplant. Bull, Kyuchu agric. Exp. Stn., 10 (3,4), 257-85. MOCHIDA, O. (1970). A red-eyed form of the brown plant- hopper Nilaparvata lugens (Stal). (Horn., Auchenor- rhyncha). Bull. Kyuchu agric. Exp. Stn., 15(2), 141-273. MOCHIDA, 0. (1973). The characters of the two wing-forms of Javesella pellucida (F) (Homcptera: Delphacidae), with special reference to reproduction. Trans. R. ent. Soc. Lond., 125(2), 177-225. MORAN, P.A.P. (1951). A mathematical theory of animal trapping. Biometrika, 38, 307-11. MORRIS, M.G. (1971). Differences between the invertebrate faunas of grazed and ungrazed chalk grasslands. IV. Abundance and diversity of Homoptera Auchenorrhyncha. J. appl. Ecol., 8, 37-52. MORRIS, M.G. (1973). The effects of seasonal grazing on the Heteroptera and Auchenorrhyncha (Hemiptera) of chalk grassland. J. appl. Ecol., 10, 761-80. MULLER, H.J. (1956). The taxonomic value of the male genitalia in leafhoppers in the light of new studies on the seasonal forms of Eusoelis (Brune). Proc. 10th int. Conar. Ent., 1, 357-62. 4 bh 326 NICHIPORICK, W.- (1965). The aerial migration of the Six-spotted Leafhopper and the spread of the virus disease Aster Yellows. Int. J. Bioclim. Biomet. 219-27. NORRIS, M.J. (1964). Environmental control of sexual maturation in insects. Insect Reproduction (ed. by K.C. Highnam), Royal Entomological Society, London. OLMI, M. (1968). Cicaline della Risaia da Vicenda Vercellese (Homoptera Auchenorrhyncha). Annali Fac. Sci. agr. Univ. Torino, 4, 247-60. OSBORN, H. (1916). Life histories of leafhoppers of Maine. Bull. Me. agric. Exp. Stn., 248, 53-80. OSSIANNILSSON, F. (1949). Insect Drummers. A study on the morphology and function of the sound-producing organ of Swedish Homoptera Auchenorrhyncha. Opusc. ent., suppl. X, 1-146. OSSIANNILSSON, F. (1951). On the shape of the apodemes of the second abdominal sternum as a specific character in the genus Macrosteles Fieb. (Hom., Auchenorrhyncha). Opusc. ent., 16, 109-11. OUCHTERLONY, 0. & NILSSON, L.A. (1973). Immunodiffusion and immunoelectrophoresis. Handbook of Experimental Immunology (ed. by D.M. Weir), 2nd ed., Chapter 19, 19.1-19.39. Blackwell Scientific Publications, Oxford. PALMITER, D.H., COXETER, W.J. & ADAM, J.A. (1960). Seasonal history and rearing of Scaphytopius acutus (Say). Ann. ent. Soc. Am., 53, 843-6. PIENKOWSKI, R.L. & MEDLER, J.T. (1964). Synoptic weather conditions associated with long-range movement of the Potato Leafhopper, Empoasca fabae, into Wisconsin. Ann. ent. Soc. Am., 57, 588-91. PRADHAN, L. (1945). Insect population studies. II. Rate of insect development under variable temperature It of the field. Proc. natn. Inst. Sci. India, 11, 74-80. PRADHAN, L. (1946). Insect population studies. III. Idea of biograph and biometer (Instruments for estimating developmental periods and numbers of generation of insects in nature). Proc. natn. Inst. Sci. India, 12, 301-14. PRIOR, R.N.B. (1965). Taxonomy and some aspects of the biology of the principal leafhopper vectors of clover phyllody disease. M.Sc. Thesis, University of London. PRIOR, R.N.B. (1967). Occurrence of Macrosteles cristatus (Ribaut) (Homoptera, Auchenorrhyncha) in Britain with notes on related species. Entomologist's mon. Mag., 102; 286-8. 327 RAATIKAINEN, T- (1967). Bionomics, enemies and population dynamics of Javesella pellucida (F.) (Hom., Delphacidae). Annls. agric. Fenn. , 16 (suppl, 2) , 149pp. RAATIKATNEN, iii. (1970), Virus diseases of cereals and leafhoppers. (In Finnish with English summary). Luonnon Tutk., 75, 65-74. RAATTKATNEN, H. (19 71). Seasonal aspects of leafhopper ( om., Auchenorrhyncha) fauna in oats. Annls. agric. Fenn., 10, 1-8. RAATIYAINEN, M. (1972). Dispersal of leafhoppers and their enemies to oatfields. Annls. agric. Fenn., 2, 146-53. RAATIKAINEN, M. & TINNILA, A. (1959). The feeding and oviposition plants of Callygypona pellucida (F.) (Hom. Auchenorrhyncha). and the resistance of different oats varieties to the damage. Van. aatalouskoet. Julk., 178, 101-9. RAATIKAINEN, M. & VASARAINEN, A. (1971). Comparison of . leafhopper faunae in cereals (Hem. Hom. Cicadellidae). Annls. agric. Fenn., 10, 119-124. RAATIKAINEN, M. & VASARAINEN, A. (1973). Early- and high- summer flight periods of leafhoppers. Annls. agric. Fenn. 12, 77-94. RAZVIAZKINA, G.M. (1960). Bioecology of cicads belonging to genus Macrosteles and their epiphytological impor- tance. (In Russian with English summary). Zool. Zh. , 39, 1855-65 RIBAUT, H. (1952). Homopt.eres Auch6norhynques. II. Jassidae. Faune Fr., 57, 474pp. RICHARDS, 0.W. (1949). The relation between measurements of the successive instars of insects. Proc. R. ent. Soc. Lond., A, .24, 8-10. RICHARDS, O.W. (1959). The study of natural populations of insects. Proc. R. ent. Soc. Land., C 23, 75-9. RICHARDS, 0.W. & WALOFF, N. (1954). Studies on the biology and population dynamics of British grasshoppers. Anti-Locust Bull., 17, 182pp. RICHARDS, O.W. & WALOFF, N. (1961). A study of a natural Population of Phytodecta olivacea (Forster) (Coleoptera: Chrysomeloidea). Phil. Trans. B. 244, 205-57. •RICHARDS, 0.W., WALOFF, N. & SPRADBERY, J.P. (1960). The measurement of mortality in an insect population in which recruitment and mortality widely overlap. Oikos, 306-10. ROCKSTEIN, M. & MIQUEL, J. (1973). Aging in insects. .11 The Physiology of Insecta. (ed. by M. Rockstein), Vol. 1, pp.371-478. Academic Press, New York and London. 328 ROMNEY, V.E. (1945). The effect of physical factors upon catch of the beet leafhopper (Eutettix tenellus) (Bak.) by a cylinder and two sweep-net methods. Ecology, 26, 135-47. ROSE, D.J.W. .(1971). The biology and dispersal of Cicadulina species (Hemiptera, Cicadellidae). Ph.D. Thesis, University of London. ROSE, D.J.W. (1972a). Times and sizes of dispersal flights by Cicadulina species (Homoptera: Cicadellidae), vectors of maize streak disease. J. Anim. Ecol., 41, 495-506. ROSE, D.J.W. (1972b). Dispersal and quality in populations of Cicadulina species (Cicadellidae). J. Anim. Ecol., 41, 589-609. ROSE, D.J.W. (1973a). Laboratory observations on the biology of Cicadulina spp. (Hom., Cicadellidae), with particular reference to the effects of temperature. Bull. ent. Res., 62, 471-6. ROSE, D.J.W. (1973b). Field studies in Rhodesia on Cicadulina species (Hem., Cicadellidae), vectors of maize streak disease. Bull. ent. Res., 62, 477-95. ROSE, D.J.W. (1973c). Distances flown by Cicadulina species (Hom., Cicadellidae) in relation to distribution of maize streak disease in Rhodesia. Bull. ent. Res., 62, 497-505. SACHER, G.A. (1967). The complementarity of entropy terms for the temperature-dependance of development and aging. Ann. N.Y. Acad. Sci., 138, 680-712. SAINI, R.S. (1967). Low-temperature tolerance of the aster leafhopper. J. econ. Ent., 60, 620-1. SEVERIN, H.H.P. (1940). Potato naturally infected with California aster yellows. Phytopathology, 30, 1049-51. SOLOMON, M.E. (1951). Control of humidity with potassium hydroxide, sulphuric acid and other solutions. Bull. ent. Res., 42(3), 543-54. SOLOMON, M.G. (1973). Ecological studies of grassland leafhoppers with special reference to Psammotettix confinis (Dahlbom)(Cicadellidae, Hemiptera). Ph.D. Thesis, University of London. SOUTHWOOD, T.R.E. (1956). The structure of the eggs of the terrestrial Heteroptera and its relationship to the classification of the group. Trans. R. ent. Soc. Lond., 108, 163-221. SOUTHWOOD, T.R.E. (1962). Migration of terrestrial arthropods in relation to habitat. Biol. Rev., 171-214. 329 SOUTHWOOD, R.E. (1966). Ecological methods with par- ticular: reference to the study of insect populations. 7iethuen, London. SOUTH:JOOD, T,R.E. (1971). The role and measurement of migration in the population system of an insect pest. Trop. Sci., 13, 275-8. SOUTI17100D, T.R.E., JEPSON, W.F. & VAN EnDEN, H.F. (1961). Studies on the behaviour of Oscinella frit L. (Di-atera) adults of the panicle generation. Ent. 3x-p. & aPpl., 4, 196-210. SOUTHWOOD, T.R.E. & JEPSON, W.F. (1962). Studies on the populations of Oscinella frit L. (Dipt.: Chloropidae) in the oatcrop. J. Anim. Ecol., 31, 481-95. SOUTHWOOD, T.R.E. & WAY, M.J. (1970). Ecological background to pest management. Concepts of pest management (ed. R.L. Rabb and F.E. Guthrie) pp.6-28, N.C. State University, Raleigh. STONER, W.N. & GUSTIN, R.D. (1967). Biology of Graminella nigrifons a vector of corn (maize) stunt virus. Ann. ent. Soc. Am., 60, 496-505. SURTEES, G. (1964). Observations on some effects of temperature and isolation on fecundity of female weevils Sitophilus granaries (L) (Coleoptera, Curculionidae). Ent. exp. & appl., 7, 249-52. TAKAI, A., ITO, Y., NAKAMURA, K. & MIYASHITA, K. (1965). Estimation of population density on the green rice leafhopper Nephotettix cincticeps Uhler by the mark- recapture and sweeping methods. Jap. J. appl. Ent. Zool./ 9. 5-12. TAY, E.B. (1972). Population ecology of Cicadella viridis (L) and bionomics of Graphocephala coccinea (Forster) (Homoptera: Cicadellidae). Ph.D. Thesis, University of London. TAYLOR, L.R. (1955). The standardization of air flow in insect suction traps. Ann. appl. Biol., 43, 390-408. TAYLOR, L.R. (1962). The efficiency of cylindrical sticky insect traps and suspended nets. Ann. appl. Biol., 50, 681-5. TAYLOR, L.R. (1974). Insect migration, flight periodicity and the boundary layer. J. Anim. Ecol., 43, 225-38. THOMPSON, W.R, (1943-1958). A Catalogue of the parasites and predators of insect pests. Commonwealth Inst. Biol. Control, Ottawa, Ont., Canada. TURNBULL, A.L. (1962). Quantitative studies of the food of Linyphia tniangularis Clerk (Araneae: Linyphiidae). Can. Ent., 94, 1233-49. VARLEY, G.C., GRADWELL, G.R, & HASSELL, M.P. (1973). Insect population ecology an analytical approach. Blackwell Scientific Publications, Oxford. WALLIS, R.L. (1962). Spring migration of the six-spotted leafhopper in the western great plains. J. econ. Ent., 55, 871-74 WALOFF, N. (1973). Dispersal by flight of leafhoppers (Auchenorrhyncha: Homoptera). J. appl. Ecol., 10, 705-30. WALOFF, N. (1974a). Biology and behaviour of some species of Dryinidae (Hymenoptera). J. ent. (A) 49, 97-109. WALOFF, N. (1974b). The parasitoids of nymphs and adults of leafhoppers (Auchenorrhyncha, Homoptera) of acidic grassland. Trans. R. ent. Soc. Lond., in press. WALOFF, N. & BAKKER, K. (1963). The flight activity of Miridae (fleteropter_•a) living on broom, Sarothamnus scoparius (L.) Wimn. J. Anim. Ecol., 32, 461-80. WALOFF, N. & SOLOMON, M.G. (1973). Leafhoppers (Auchenor- rhyncha: Homoptera) of acidic grassland. J. appl. Ecol., 10, 189-212. WATT, K.E.F. (1968). Ecology and resource management. McGraw-Hill, New York. x WAY, M.J., MURDIE, G. & GALLEY, D.J. (1969). Experiments on integration of chemical and biological control of aphids on brussels sprouts. Ann. appl. Biol., 63, 459-75. WEITZ, B. (1952). The antigenicity of sera of man and animals in relation to the preparation of specific precipitating antisera. J. Hyg., 50, 275-94. WEITZ, B. (1957). An automatic dispenser for multiple serological titrations. J. clin. Path., 10, 200-7. WESTDAL, P.H., BARRETT, C.F. & RICHARDSON, H.P. (1961). The six-spotted leafhopper Macrosteles fascifrons (Stal) and aster yellows in Manitoba. Can. J. Pl. Sci., 41, 320-31. WHALLEY, P.E.S. (1969). The mymarid (Hym.) egg-parasites of Tettigella viridis L. (Hem. Cicadellidae) and embryoparasitism. Entomologist's mon Mag., 105, 239-44. WHITTAKER, J.B. (1969). Quantitative and habitat studies on the froghoppers and leafhoppers (Homoptera, Auchenorrhyncha) of Wytham Woods, Berkshire. Entomologist's mon. Mag., 105, 27-37. WIGGLESWORTH, V.B. (1972). The principles of insect Physiology. 7th ed., 1972, Ilapman & Hall, London. 331 1)7 WILD, J. (1962), Photoperiodism in insects and mites. A. Eev, Tnt., 7, 1-26. WTNSTON,P.W.L DATES, B.H. (1960). Saturated solutions for the control of hurAdity in biological research. Tology, ZIPPIN, C. (1956).. An evaluation of the removal method of estimating animal populations. Biometrics, 12, - 163-89. ZIPPIN, C. (1958). The removal method of population estimation. J. Wildl. Mgmt., 22, 82-90.