u • 1,,
THE INFLUENCE OF ADJACENT UNCULTIVATED LAND ON THE DEVELOPMENT OF INS 0:4" -PEST INFESTATION OF A CROP.
H.F. van Emden B.Sc.
A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
OF THE UNIVERSITY OF LONDON.
November, 1961. Imperial College of Science and Technology, Field Station, Silwood Park, Sunninghill, Ascot, Berkshire. 2.
ABSTRACT.
The literature on the relation of uncultivated land to crop pests
is reviewed with a distinction between biological and physical relations.
A description of preliminary experiments demonstrating types of
relationship suitable for quantitative work is followed by an account of
the techniques used in a two year study of Cabbage Aphid (Brevicoryne
brassicae L.) infestation of cultivated crucifers. The progress of this
infestation in both seasons is traced on a basis of the aphid's life
history, and the factors affecting the aphid populations are discussed.
Nutritional factors are considered of major importance.
The edgegrowch is assessed as a parasite and predator reservoir
and, in the crop, the edges and centre of the field are compared in respect
of edgegrowth effects. Physical effects of shelter on the growth of the
host plant and the adverse effects on the aphid's reproductive rate are
discussed; also the deposition of aphids from air currents. Heavy
predation at the open edges is considered due partly to the invasion of
predators from the edgegrowth and partly to heavier oviposition by
Syrphidae near flowers which provide food for the adult females.
Conclusions are given on the general role of uncultivated land in
a pest infestation of a crop, with particular reference to the pest species
studied and the strips of edgegrowth immediate to the crop.
3.
TABLE OF CONTENTS.
Abstract eoe .1.••• ... 2
Introduction ••• ...... ••• ...... 9 •
SECTION I. REVIEW OF THE LITERATURE. 12 I. EFFECTS'DUE-TO-TP/E130TARICAL -MAKE,UP_OF UNCULTIVATED LAND. 14
A) The importance of uncultivated land to crop pest species. 14
1) Injurious insects feeding on both the crop and plants on uncultivated land. 14
i) Injurious insects- feeding on crops and closely
related wild plants. 04, 0 16 ii) Injurious insects feeding on crops and unrelated
wild plants 90! 19 iii) Special causes of insect movement between un- cultivated- plants and the crop ... 21
a) Lack of food outside the crop 0 • . 21 b) The use of weed plants by injurious insects as an alternative source of food before the
crop becomes available - .4. ... 23 c) The use of weed plants by injurious insects as an alternative source of food after harvest or death of the crop plants . 24
d) The use of weed plants as food during winter. 26 e) Visits to the crop by injurious insects from breeding populations on wild plants. 26
iv) The economic importance, other than damage to the crop by feeding, attributable to polyphagy among injurious insects ... 27 a) The transmission of diseases from uncultivated plants to the crop 27 b) Implications on crop rotation 30 c) Beneficial effect of wild food plants. 31
4. Page
d) Effects on injurious insects of feeding on wild
food plants ... 000 000 ... 00. 31
2) The importance of flower-feeding to pest insects. 33
B) The importance of uncultivated land to beneficial insects. 33
1) The presence outside the crop of alternative hosts for
beneficial insects ...... 34
2) Effects on beneficial insects of attacking insects on
uncultivated land .00 066 .00. ... .00 38 3) Beneficial insects feeding phytophagously on uncultivated
land during the absence of their-prey. ... 0.0 40 4) The presence of flowers on uncultivated land 40
II. EFFECTS DUE TO PHYSICAL EFFECTS (PARTICULARLY SHELTER) OF
UNCULTIVATED LAND. ... 00. .00 ...... 44
A) Effects of edgegrowth on the deposition of insects from
air currents onto the crop ... .04p ...... 0.• 44
B) Uncultivated land as temporary shelter for insects
attacking the crop ...... , ••• a.. 46
C) Uncultivated land as shelter for hibernating insects. 49
D) Physical effects of the edgegrowth probably directly
affecting insects on the crop ...... OS, 54
E) Physical effects of the edgegrowth affecting insects on
the crop indirectly through the host plant 000 • • • 56
III. THE IMPORTANCE OF UNCULTIVATED LAND FOR POLLINATORS ... 56
SECTION II. PRELIMINARY EXPERIMENTS. O.. 0610 SOS 00. 59
1) General study of insect populations in Field and Hedgerow. 59
Sites ...... 59
Sampling methods ...... fee goo .$00 1100 Sill 60 Results ..• ...... ••• ••• ... 63
2) Effect of flowers on the activity of parasitic Hymenoptera. 67
5. Page
3) Experiments with specific infestations • • • 000 004 76 A) Macrosiphum avenae F. - (New Forest) ...... 76
B) Macrosiphum avenae F. - (Silwood Park) . 400 400 77 C) Dolerus haematodes Schr...... • • .•. 89
D) Brevicoryne brassicae L. • a • • • •• • ••• •• • 95
4) Discussion of preliminary experiments .. 004 VOO O00 99
SECTION III. MAIN PROJECT ...... • 0 0 • •• • •• ••• 102
Part I. Techniques. ... •• 0 ... ..• ••• • O• •• • 103
1) Experiment Site ... • . • ... •• • ...... 103
2) Meteorological records ... • •• • • • •• 0 ... ..• 108
3) Crop sampling • •• • • o • 0 0 00 a ••• If • • ... 110
4) Marked colonies ... •• • ...... ••• ••• •• • 112 5) Edgegrowth sampling ...... • . ... ••• ••• 114
6) Water traps ...... • .• • •• •• • •• • ••• 116 7) Plant growth . . ..• •.• ••. • . • • •• ... ••. 118 8) Parasite breeding ...... 119
Part II. Brevicoryne brassicae L. infestations in 1957 and 1958. 125
1) Life history ...... • GO ••• ... 0 •• 125
2) Initiation of the infestation ... • • • • .• ...... 127
3) Apterous generations . • a 0 • • • • • • • • • • • • 1.4. 128
4) Production of alatae ...... 060 *00 040 132 5) Winter eggs ... ••• •• .. • •• • • •• ... • •• 133 6) Progress of infestations ... •• • •• • ...... 136
S eason 1957 ...... ••• • • • • • • • • • • • • 136 Season 1958 • •• •• • ...... ••• ... 139 6. Page
Part III. Factors affecting the populations of Brevicoryne rassicae L. 00• •• 0 • • e •• Oe• eee 142
1) Review of literature • 0 • • • I •• •. • •. • •• • 142
2) Physical factors • • • •• • •• • •• • •• • •• • • 60 147
a) Temperature • • . •. • •• • •• • • 0• ... •. 147
b) Rainfall • • • •.. • •• 0 ••• •• • •• • ... 148 c) Factors affecting the production of alate nymphs 0. • 152
3) Leaf age • • • • • . •• • •• • 0 0 • • • • • Oa ••• 154
4) Hymenopterous parasites • • • • • • • • • •• • •• • •• • 156
a) Primary parasites. .0 0 • 0 • • • • • • • • • • • 156
Biology ..• 006 ... ••• 41•• ••• ... 156
Breeding .. 0,0 604 ... ••• .. . •• $ 0 • • 159
Parasitism in the field ..• ...... •. 160
Seasonal abundance •• • •• • •• . •• a • • • 160
b) Hyperparasites • •• • • • •• 0 • • • •• • • • • 161
Biology ... 000 0106 • • 0 • • • • O 6 •• • • 00 164
Breeding . ... .•. •• . •• • •• . •• • •• 6 164
Parasitism in the field •. • .. • . • . •• . 606 165
Seasonal abundance GO • O• 0 • • • ... • • • • 49 167
5) Predatory insects • • • •• • • • • • • . •• • . • • •• • 168
a) Hemiptera • • • •. • ... • • • • •• • •• • •• 169
i) Nabidae • • 0 O• • •• • •• • • •• ••• • e• 169
ii) Cimicidae • • f 0• • ••• •• • • • • • •• • •0 169
iii) Miridae O• • • • • •• • 0 • • ...... 169
b) Neuroptera • •• • • 0 •• • 60 • •• • •• • • •• 171
c) Diptera • 0 0 0•• • • • .. • ••• ... •• • 171
i) Cecidomyiidae .. • •• • •• • •. 0 ••• ... 171
ii) Syrphidae 000 ••0 000 000 0 0 11 0•0 172
d) Coleoptera - Coccinellidae • • • •• • •• • •• • 179
7.
Page
6) Entomophagous fungi • •• • •• • • • • • • ...... 181
7) Natural death of aphids ... 0•41 4, 4, 0 0, 00 ... 181
8) Summary of factors affecting B. brassicae populations 182
Part IV. The influence of edgegrowth on the abundance and distribution of Brevicoryne brassicae L. on the crop. 186
1) The experimental layout 4011 ...... 186
2) Physical effects of the edgegrowth 00* 414, 0 00. 0.0 187
a) Shelter from winds q-• ...... •• • ...... 187 b) The effect of shelter on the deposition of immigrating
elate aphids ... 992 ...... 189
c) Shade ... • •• ...... 192
d) Temperature ••• ...... 194
e) Relative humidity •• • • •. ... ••• ...... 194
f) Competition ... • • • ...... 194 g) Effects of the edgegrowth on the host plants of the aphid...... 194 h) Physical effects of the edgegrowth on the increase•of
B. brassicae ... ••• ...... ••• ••• 198
3) Effects of the presence of plant hosts of aphids in the
edgegrowth. • .4. ... • • • ...... ••• ... 200
a) The abundance of aphids on the edgegrowth crucifers . 200 b) Effects of alternative host plants on B. brassicae... 201 i) Rate of increase • • • • • • • • • • .. • • • 201 ii) Aphid size .. • •• • • • •• • ••• •• • 701 c) The edgegrowth as a parasite and predator reservoir 203
4) Effects of the presence of flowering plants in the edgegrowth. 207
a) Diaeretus ranee (Curt.) ... ••• ••• $00 0410 207
b) Aphidophagous Syrphidae ••• ••• ...... ••• 214
c) Coccinellidae ••• •• • • •• ••• ••• 217
8. Page
5) Analysis of levels of B. brassicae infestation in different
areas of the crop ... saps .. • • • • .• • 000 O011 221
Season 1957 ... •• • •• • •• • • •• • • ...... 221
Season 1958 ... • • • • .• •• • •• • ... saps 060 235
Part V. Discussion of main project.... 000 • . opo4 saps 246
SECTION IV. CONCLUSIONS. ... 000 080 0•0 060 263
Summary saps •• • •• • ••• • • • ••• •• • • •• ... 267
Acknowledgements • • • 000, saps saps saps saps ..• ••e 270 271 References ... ••• ... 00• ... 00• •00
Appendix I. Plant species present in the edgegrowth 00, *00 292
1) North-west Edgegrowth • •• ... 150 *SO ••• 292
2) Southwest Edgegrowth a•• ••• ••• ••• ••• 292
3) North-east Edgegrowth Se• 41•• ••• ••• 00.0 293
Appendix II. Specimen daily weather record sheet ... 6,0 295
Appendix III. Wind speed and direction recorder saps saps 296
Appendix IV. Table 80. Possible factors affecting the production
of slate nymphs of B. brassicae. 11 ,00 000 apse
Appendix V. Table 81. Measurements taken on B. brassicae from 304 sprout and mustard plants ... saps saps
Appendix VI. Key to instars of Brevicoryne brassicae L. saps 305 9.
INTRODUCTION
The proximity to a crop of uncultivated land in such forms as unmanaged pasture, woodland, headlands, roadside verges and hedgerows is a common feature in agriculture, particularly in Great Britain. Here both farms and fields are normally comparatively small and mixed farming is not uncommon. Little is known of the role with regard to insects of such uncultivated land in agriculture, apart from its obvious importance in providing nesting sites for insectivorous birds.
The Nature Conservancy became interested in supporting a programme of research on insect relationships between the crop and uncultivated land.
Hedgerows probably form an important reservoir of wild life, but this reservoir is currently being reduced in two ways, With the increasing emphasis on careful land utilisation, boundary hedges are being removed or replaced by post and strand fencing. Secondly, large mileages of hedgerow are being trimmed regularly by local authorities in the interestof good road vision for motorists. With these authorities, the replacement of cutting by the use of scorching or hormone weedkillers is rapidly gaining favour in view of the high cost of labour.
The aim of the present work was to evaluate the possible effects that such management of at present uncultivated land might have on insects attacking crops, and particularly to provide evidence of such effects in quantitative terms. In the time available it was clearly impossible to cover the whole problem; the literature has been reviewed in some detail as it was found possible to piece together a reasonably comprehensive picture in qualitative terms. 10.
It proved necessary to devote an entire year to a superficial preliminary survey of hedgerows and a variety of adjacent crops. In this way some types of insect dependence on crop and hedgerow could be determined as suitable for more detailed quantitative work. The remaining two summers were devoted to the main project, a study of Cabbage Aphid (Brevicoryne brassicae L.) infestation on cultivated crucifers. Here it was hoped to obtain numerical estimates of the part played by the whole complex of physical and biological characteristics of uncultivated land in a specific crop infestation by an insect pest.
The main project was unfortunately handicapped in both seasons.
In 1957, the Asian influenza epidemic halted work for an important period during the aphid peak in October, and gaps have had to remain in many of the tables and graphs. In 1958 the wet weather of early and mid summer caused aphid numbers to be so low that analyses of the results were only possible at a rather shallow level.
Throughout the text, the word "EDGEGROWTH" has been used to provide a covering term for all types of uncultivated land adjacent to edges of cultivated fields. Sampling has been referred to as random where the sample was determined largely by chance as by the throwing of a trowel.
The probability of p = .05 has been taken as the level of significance of differences and for fiducial limits. In certain instances, particularly when considering some 1958 data, tests have been used in spite of unsatisfactory figures to give a rough indication of the validity of any conclusions drawn.
Geological details have been taken from the 1920 edition of the 11. geological map for Windsor (Drift sheet 269) published by the Ordnance
Survey Office. Botanical names and authors are, where possible, reproduced from Clapham, Tutin and Warburg (1952). Similarly British insects are referred to by the nomenclature given in Kloet and Hincks (1945).
Exceptions include aphids which, as for foreign species, are quoted as given in any literature used, and Syrphids where Coe (1953) has been followed.
Colour descriptions have been standardised to the code given by Wilson
(1941). 12.
SECTION I.
REVIEW OF THE LITERATURE.
The literature of applied Entomology contains numerous references to the relation of uncultivated land to crop pests.
General aspects of the problem were investigated by Underhill and
Bodenstein (1946) in Virginia, U.S.A. A wildlife study of certain field crops was completed in an attempt to obtain information on insect injury and bird and rodent populations as related to edgegrowth. The assumption of the study was that intensive farming might reduce the numbers of birds and other wild life which normally consume insects, and that hedges also acted as windbreaks and reduced both soil erosion and moisture loss.
These advantages of edgegrowth might be outweighed by the land used, the labour required to maintain it, and the danger of supplying shelter for harmful insects. The authors failed to show differences in insect injury over the field and there was little evidence that the Lespedeza plants used as edgegrowth formed breeding grounds or hibernation sites for injurious insects.
Annand (1940) pointed out that interest in conservation ran counter to some common recommendations for insect control; these included the cleaning up of fence rows and corners, as well as the extensive elimination of weed growths and other places suitable for winter hibernation of injurious species. The recent agricultural practice in the U.S.A. of controlling soil erosion by the planting of trees and shrubs had the possible danger of forming uncontrolled reservoirs of pests, as well as bridging barriers to the further natural spread and extension of insect pests. 13.
Mel' nichenko (1937) discussed the importance of bolts of woodland as reservoirs for a variety of pests. On the other hand, he found that tl-ey also harboured insectivorous birds and various insect parasites and predators; also the alteration in the microclimate of adjacent fields increased the yield of crops and more than compensated for the loss due to pests. The yield of wheat was greater close to the trees, and there was a sharp drop in the yield of the distant plots, owing to severe drought.
At East Malling Research Station the effect of uncultivated land on infestations of the Red Spider Mite (Metatetranychus ulmi(Koch)) in apple orchards has been investigated over a fair number of years. Thus Collyer
(1953c) showed that biological control of the mite by predators was heavy in neglected orchards. Although in commercial orchards spray treatments could suddenly reduce both predator and mite populations, the high rate of increase of the mite compared with that of the predators soon produced mite populations exceeding those previous to spraying. Where constant spraying was practised, the ability of predators to re-infest the orchard from neighbouring unsprayed orchards, or hedges, ditches and woods, was of importance.
Numerous references point to particular aspects of the crop pest - uncultivated land problem. In the following pages these aspects have been classified, using sample instances from the literature, in an attempt to show the various facets of the effects uncultivated land may have on the attack of crops by insect pests. 14..
I. Effects due to the botanical make-up of uncultivated land.
A) The importance of uncultivated land to crop pest species.
1) Injurious insects feeding on both the crop and on plants on uncultivated
land.
The polyphagous habit of many crop pests is stressed in text books on agricultural entomology, and the literature in journals contains a great many instances of pests found feeding outside the crop.
A total of 429 such records, obtained from a search of the literature between approximately 1939 and 1960 is analysed in Table 1. The records are divided into the main groups of crops as well as the orders of pest insects.
The large number of records for cereals is not only a reflection of their importance, but also of the ubiquitous occurrence of related wild grasses. Particular emphasis in the table has been laid on the transmission of disease; in such instances heavy crop damage can be caused by comparatively few vector insects. Such records form approximately one-eighth of the total.
Parfent'ev (1937) found as many as 68 species of insect pests in protective strips of trees. Nagy, Reichert and Ubrizsy (1953) recorded that 203 plant species served as food plants for Hyphantria cunea Drury in
Central Europe, and emphasised the need to avoid planting primary food plants of this Arctiid in shelter belts. Kozhanchikov (1950), working on the Cabbage Moth, concluded that the widespread ability of insects to feed on plants on which they had not previously been recorded was important in crop cultivation, either when new crops were to be introduced or shelter belts to be established. 15. 15. TABLE 1. RECORDS IN THE LITERATURE (approximately 1939-1960) OF CROP PESTS FOUND FEEDING ON WILD PLANTS OUTSIDE THE CROP. Numbers in brackets are records of the transmission of disease to crops. 1
Orders of insects General Orthoptera Hemiptera. Thysanoptera! Lepidoptera Hymenoptera Diptera Coleoptera assessment' TOTAL ------1 I Insect groups of Acrididae Heteroptera Homoptera Noctuidae Tenthredinoidea Cecidomyiidae Chrysomelidae t particular importance I (Locusts) Miridae pssoidea Pyralidae Acalyptrata Curculionidae Pentatomidae Aphidoidea CROPS .ATTACKED 3 (3) Range of crops 3 7 (1) I 10 - 1 - 3 - 2 29 (4) Graminaceous and leguminous 3 3 3 1 6 2 fodder crops. 1 19 38 Cereals 1 8 15 10 (3) 2 20 3 19 17 95 (3) - Root crops - 3 1 14 (6) 1 7 - 3 15 (1) 43 (7)
Sugar cane - - - 2 (1) - 2 - - 1 5 (1)
Medicinal plants and herbs . - - 1 - - - - - 3 4 22 Tomato and potato - 1 3 (10) 1 2 - 2 9 40 (10) Soft fruits - - 1 4 7 (2) - - - - 7. 15 (2)
Orchard crops 1 kV 2 12 (1)
Tobacco 1
Fibre and latex crops 2 11 (2) Cruciferous vegetables 1 1 and salad crops 40 (4) Leguminous vegetables 1 2 (1)
Vines and hops
Tea and coffee
Miscellaneous crops
TOTAL 10 (1) 26 43 (1) 16.
Polyphagous insects normally have reasonably restricted food-plant preferences, often running parallel with botanical affinities, though sometimes habitat characteristics appear more important (Wiltshire, 1943). i). Injurious insects feeding on crops and closely related wild plants.
Hoffmann(1949) gave four examples of weevils which are pests of crops and feed on related weed hosts, thus demonstrating the importance of
wild plants in the propagation of pest insects, when these plants grow near the crops and there are botanical affinities. In the first example,
Hoffmannrecorded tracing an infestation of Ceuthorrynchus apicalis Gyll.
on celeriac to a dense clump of Heracleum sphondylium L. along the edge of
the field. The farmer had never associated these with the recurrent
attacks on the crop over a number of years. A similar second example given
was the failure of parsley over an area The pest causing the damage,
Ceuthorrhynchus terminatus (Hbst.) had its source in wild Chaerophyllum
temulum L. abundant on the paths and lanes of the area. Following these
examples of Umbelliferae, Hoffmands next example was the devastation of
carnation fields by Phytonomus arator (L.),where in spite of successful
nicotine treatment of the plants the insects always returned. The source
of infestation was located on Stellaria media (L.) flourishing in a
nearby patch of wasteland. Further records of plant hosts in the
Caryophyllaceae were cited. The final example given concerned an attack of
up to 80-85% of sugar beet plants by Lixus lunci Boh.. Chenopodium album L.
was undoubtedly the origin of the attack, showing the same symptoms and
found around the paths and headlands in the area. From these examples
Hoffmanaconcluded that, in combatting pests, adjacent land to the crops 17. should not be forgotten, so that the points of dissemination of pests could be removed by the destruction of dangerous weeds.
There are many instances in the literature of cereal pests also feeding on wild grasses. The frit fly (Oscinella frit L.) is an important pest of oats over much of Europe, but also completes the development of any generation in wild grasses (Jepson and Southwood, 1958). Similarly
Gough (1946) found larvae of the wheat bulb fly (Hylemya coarctata. Fall.) in species of Poa, Agrostis, Agropyron, Phleum and Lolium.
In examining grass alongside roads adjacent to infested wheat fields,
Post (1946) found the wheat stem sawfly (Cephus cinctus Nort.)generally
attacking Acropyraland Bromus. Lema melanop\a(L.) (Coleoptera, Chrysomelidae)
was recorded feeding on grasses as well as cereals by Urquijo (1940) and
Venturi (1942). The source of an infestation of seedling maize by the
Dynastid Euethola ruciceps Lac. in Arkansas was located by Baerg, Isely
and Sanderson (1938) in grasses, particularly Bermuda and Johnson grass.
Potato pests may also be found on other Solanaceae outside the crop.
The Colorado beetle (Leptinotarsa decemlineata Say) was found by Lange
(1942) to feed extensively on the leaves and shoots of bittersweet over
considerable areas in Germany. The beetle was first described in 1824 as
an obscure species feeding on solanaceous weeds on the eastern slopes of
the Rocky Mountains; it was not known as a pest of potatoes for a further
30 years, when pioneer settlers brought potatoes to these regions, starting
a spread of the pest across the Nen World at the average rate of 85 miles
a year (Metcalf and Flint, 1951). Similarly the vector of psyllid yellows
(Paratrioza cockerelli (Silk) of potatoes is almost confined to the Solanaceae
18.
(Wallis, 1955), though it is also known to breed on field bindweed and morning glory as well as the related sweet potato (Convolvulaceae). Wallis suggested an annual pre-season survey of adult Psyllid populations on non- economic food-plants to indicate the expected population on potatoes for the growing season.
The feeding by P. cockerelli on sweet potato and other Convolvulaceae recorded by Wallis is paralleled in other orders of insects. Watson (1944) ascribed an armyworm infestation of sweet potato by the Sphingid Herse cinqulatus F. to wild morning glory abundant in the surrounding fields and woods. Zorina (1939) stressed the value of destroying or treating with insecticide convolvulaceous weeds to control outbreaks of flea-beetles
(Londitarsus spp.) on the crop.
Two beetles attacking sugar beet are recorded as also feeding on wild Chenopodiaceae. Control measures against the Silphid Aclypea opaca (L.) recommended by Regnier (1944) included the destruction of wild Chenopodium and Atriplex. Ander (1941) found Cassida nebulosa L. and Cassida nobilis L. attacking beet also common on Chenopodium. Among dipterous pests, Yookoo
(1940) mentions qedomya hyoscyami (Panz.) as widely distributed, attacking beet and spinach, also the weed Cheno odium album L.
Some pest species of cotton are common to malvaceous weeds. Sauer
(1942) gave a list of mainly malvnceous wild food plants of the Mirid,
Horcios nobilellus (3erg), causing serious injury to cotton in Sao Paulo.
Cultural control measures included the early destruction of such wild food plants. The cotton stem weevil (Pempherulus affinis Fst•) was observed by
Krishna Ayyar (1940) feeding on 17 species of wild and cultivated plants, 19. which were mainly Malvaceae and Tiliaceae, in addition to cotton.
Triumfetta rhomboidea was the preferred food plant, and maintained a high weevil population independent of cotton; often the weevil preferred not to leave this plant even when cotton was available.
The destruction of wild crucifers has been recommended for the protection of susceptible cruciferous crops against flea-beetle (Phyllotreta spp.) attack. Such measures are advocated by Nole (1942) and Miles (1943), who particularly mentioned charlock and wild radish as natural food-plants for the beetles.
Illustrating the general nature of the problem of related wild plants near crops is a paper by Hallemans (1947) who reports the strawberry weevil
(Anthonomus rubi (Hbst.)), which also attacks raspberry and loganberry, being found on wild rosaceous plants. ii). Injurious insects feeding on crops and unrelated wild plants.
To judge from references in the literature, highly polyphagous insect pests are not so frequent as the more oligophagous species. Most of the highly polyphagous pest insects mentioned in the literature appear to occur within the orders of sucking insects.
Alternative food-plants of aphids appear to cover a wide range of plant families. Larsson (1941) listed the summer food plants of Aphis fabae
Scop., based ontobservations near Copenhagen, and included 79 new records.
By collecting on ditch banks and roadsides near where cotton had been grown during the previous season, Young and Garrison (1949) found Aphis gossypii Glov. on 21 species of plants of widely differing familes; most of these plants were quite normal and common hedgerow weeds. Shands, Bronson and Simpson 20.
(1942) found large numbers of aphids which were pests of potatoes (Myzus persicae Sulz. and Aphis rhamni q91) on the leaves of two cruciferous weeds,
Brassica rapa ssp., dampestris L. and Raphanus raphanistrum L. They concluded that these two weeds were potentially important sources of aphid infestation of potato.
The necessity for frequent applications of insecticide to check re- infestation of onions by Thrips tabaci Lind. was attributed by Sleesman (1943) to the re-appearance of the insects from protected parts of the plant, emergence of pupae in soil, or migration from the numerous alternative food plants.
Hixson (1941) recorded a wide host range for the Mirid Psallus striatus Reut.. The insect was known to feed on 138 species of plants, distributed between 28 families. The numbers of adults and nymphs on cotton were never so great as those on the preferred weeds, and yet the bug was considered one of the most serious cotton pests in the U.S.A.
Similarly Pickett, Neary and MacLeod (1944) listed 19 wild and cultivated food plants for a Mirid (Calocoris norveoicus Gmel.) infesting strawberries.
Rumex acetosella (Polygonaceae) appeared to be the preferred food plant.
The effect of such wild food plants on the crop was demonstrated by
Jacobson (1944) with a severe outbreak on wheat in Alberta in 1941 of a
Pentatomid (Chlorochroa umi Sal), which sucks kernels causing the grains to shrivel. Samples of threshed wheat were examined from up to 2 miles distant of the eastern end of the field. Here the field adjoined abandoned land overgrown with Salsola sp. (Chenopodiaceae), on which adults and nymphs of the bug had been abundant in early spring. A steady decrease in kernel 21. damage away from the Salsola was demonstrated over the 2 mile extent of the field; levels of percentage germination satisfactory for planting could only be obtained more than k mile away from the weed food plants.
The occurrence of Euacanthus interruptus (L.) on grassy roadside barks and hedgerows was noted by Massee (1943). The Jassid is frequently a serious pest of hops.
Among non-sucking pest insects, the weevil Otiorrhynchus liqustici L., the larvae of which attack lucerne and clover and many weeds, can be reared from egg to adult on plants covering several families - Gramineae,Leguminosae,
Rosacae, Umbelliferae, Polygonaceae and Compositae (Lincoln and Pack, 1941).
Kronenberg (1941) advised the destruction of stinging nectles near strawberry beds, as adult weevils (Phyllobius pomaceus Gyll.) feed on both plants. iii). Special causes of insect movement between uncultivated plants and the crop.
Besides the general extension of the area covered by an insect species during the crop season, many authors cite particular reasons why crops become invaded from weed plants, or why the crops are left in favour of uncultivated land. Kennedy and Booth (1951), working on Aphis fabae Scop., considered the phenomenon of food plant alternation as a particular instance of a shifting distribution pattern, probably concerned with the alternation of the seasons of active growth and senescence in the winter and summer food plants.
a) Lack of food outside the crop. The Sugar-beet Webworm (Loxosteqe sticticalis (L.)) occurs in great numbers on both waste and cultivated 22. ground. Pepper and Hastings (1941) reported that, if food became scarce, the larvae readily migrated to adjacent fields.
Scarcity of food outside the crop may be caused directly by the feeding of the insect. Kanervo (1947) recorded crop attack from weeds by
Plusia gamma (L.) (Lepidopteras Plusiidae) in Finland in the summer of 1946.
Most of the eggs had been laid oh weeds, and the larvae fed on these until there was no more food available before migrating to the crops.
The seasonal cycle of plant development and growth in uncultivated land is another factor which may produce a lack of food in uncultivated land.
Jacob (1944b) attributed an April attack on winter wheat by large numbers of weevils (Phyllobius pyri (L.)) to the scarcity of foliage in the hedgerows at the time; he suggested that the beetles had attacked wheat for lack of other food.
This record of crop attack before weed foliage appeared is balanced by Helson's (1942) work on the leafhopper Thamnotettix argentata Evans.
Field observations in Victoria (Australia), showed that the insect bred on capeweed and crowfoot and increased to considerable numbers before these weeds died at the end of November. Adults of the first generation which had reached a peak of abundance and were apparently carrying a virus, migrated to young tobacco plants for lack of other food. These adults returned to young weed seedlings to oviposit almost as soon as these appeared above the ground in summer.
Even when weed foliage is present, changes in the plant may render it unsuitable as a food-plant for the insect. Such an instance is recorded by 23.
Schmidt (1943), working in Germany. In mid-April the Silphids Aclypea
200 (L.) and A. undata (Mull.) first attacked young grasses and cereals,
but later migrated to chenopodiaceous weeds. When the tissues of these hardened, they attacked beet as well as sometimes rape, carrots and potato.
A similar example is given by Cherian and Kylasam (1938). Seedlings of
Eulesine sp. planted in strips on the edges of tobacco beds for erosion
control at Chirala (India) were heavily infested by larvae of Laphygma
exiqua (Hb.) (Lepidoptera: Caradrinidae). Tobacco beds surrounded by Eulesine tended to contain more larvae than those without it, owing to migration from
the borders, though the Eulesine sometimes contained 8 - 15 times as many
larvae as the tobacco beds. 14 days after germination the plants became
coarse and unpalatable to the larvae, which then tended to migrate to the
tobacco plants.
Lack of food outside the crop may be caused in an unnatural manner.
Steiner (1945) suggested that the use of weed-killers on uncultivated land
might force insects to migrate to crops in search of food. This has bearing
on the possible replacement of cutting road verges by weedkiller sprays.
b) The use of weed plants by incurious insects as an alternative source of food before the crop becomes available.
Simpson, Shands and Weyman (1945) claimed that certain weeds accounted
in large part for the aphids that flew to potatoes in July. Such weeds
were present when the aphids flew from their woody winter hosts; the insects
multiplied on the weeds if potato plants had not yet appeared above ground
and winged forms soon developed. As these matured they flew to other plants
which in July and August invariably included potatoes. From the standpoint 24. of large numbers of aphids produced, weeds growing in wasteland or other places where there was no competition from tall-growing crops were most important.
Among other Homoptera, the aleurodid Bemisia tabaci Genn. was recorded in N. India by Hem Singh Pruthi and Samuel (1942) as migrating to tobacco from wild food plants when the seedlings were planted in the fields.
As well as such Homoptera, the fleabeetles with other Chrysomelidae are regularly recorded in the literature as feeding on wild plants before the crop is available. Popov (1937) reported Chaetocnema sp. ovipositing heavily on Aqropyron repens (L.) before the summer crops began to sprout, causing crop infestations on land close to waste ground with the grass.
Similarly Glass (1940) found Epitrix parvula, F. on potato and most solanaceous weeds before tobacco seedlings appeared in spring. The beetles migrated to tobacco plants when these were set in the field. Adults of the striped cucumber beetle (Diabrotica melanocephala F.) were recorded on 61 plants in 20 families by Gould (1944). The overwintered beetles fed on many flowering plants, (chiefly Rosaceae)and on wild cucurbits in spring; but migrated to cultivated cucurbits, which are preferred, as soon as the seedlings appeared. In the same genus D. undecimpunctata Mannh. in Oregon was found by Rockwood and Chamberlin (1943) to feed on succulent weeds near their winter quarters and then to move to seedling clover, often in large bands.
c) The use of weed plants by injurious insects as an alternative source of food after harvest or death of the crop plants.
In some instances, the post-harvest spread to wild food plants is the same movement as the pre-crop feeding described just previously. Thus the 25.
fleabeetle Epitrix parvula F. (Glass, 1940) leaves the tobacco crop at
harvest and attacks other solanaceous plants, from which it migrates to
tobacco plants when these are set in the field. Gorham (1939) recorded
that Myzus persicae Sulz. in New Brunswick fed on weeds, particularly crucifers,
when potatoes were no longer available.
A movement to wild plants after harvest may be due to a succession of
generations, where, in the absence of the crop, the next generation must take
place elsewhere. Thus for the frit fly (Oscinella frit L.) Zhukovskii
(1937) recorded infestation of Acropyron repens (L.) in August and September
after the harvest of cereals. Jepson and Southwood (1958) gave densities
for the overwintering fly larvae in wild grasses.
Examples of insects whose adult feeding period is longer than the
growing period of the crop are probably more generally applicable to the
problem of alternative food plants outside the crop. Such a case is reported
by Wolfenbarger (1940). When the potato plants died, some fleabeetles
(Epitrix cucumeris Harr.) were able to hibernate, but others migrated to less
favoured plants in uncultivated areas, where they fed and bred. This provided
a source of re-infestation. A similar example was given by Duerden and Evans
(1954) of a Pentatomid Calidea dreqei Germ. attacking sorghum and sunflower
in Tanganyika. The species was highly polyphagous, and after harvest the bugs migrated to any plant still succulent.
Factors other than death or harvest of the plants may terminate the
availability of food in the crop for injurious insects. McLeod (1953) reported that, whereas cruciferous seed-crops became unsuitable for larval feeding by the cabbage seedpod weevil (Ceuthorhynchus assimilis (Payk.)) after July, wild species of Brassica remained green until October and became 26. food plants for the beetle. They were the source from which the cultivated plants became infected.
d) The use of weed plants as food during winter.
Although there is considerable overlap of this section with certain examples in the two preceding it, there are examples in the literature, particularly on aphids, of greater emphasis on an alternation of winter and summer host plants than would merit inclusion in the previous sections.
Thus Entomological Investigations in New South Wales (1941-42) showed that
Macrosiphum solanifolii Ashm. bred throughout the winter on a number of food plants, of which sow-thistle (Sonchus oleraceus L.) was the most important. Winged migrants of this aphid migrated to potato plots at about the same time as those of Myzus persicae Sulz. from peach trees.
Also in the SternorXyncha, the psyllid Paratrioza cockerelli (Sulc) was found to overwinter on wild Lycium by Wallis (1955).
e) Visits to the crop by injurious insects from breeding populations on wild plants.
Attacks on crops by insects may be incursions from breeding populations elsewhere. Leach and Mullin (1942) found the leafhopper
Macrosteles divisus Uhl. abundant on cereals and weeds as well as potato.
The leafhopper fed on potato in the field but apparently did not breed on it, and had migrated there from other plants. The insect transmitted aster- yellows virus.
Other references, mainly concerning Miridae, specifically refer to this particular relation between the crop and uncultivated land. Thus
Taylor (1947) found Lyqus spp., which bred on herbaceous plants, "visiting" 27. cotton in Uganda. Essig (1948) gave a similar picture for bugs of the same genus which were important pests of seedling alfalfa and other seed crops. Pickett, Neary and MacLeod (1944) found the mirid Calocoris norvegicus Gmel. ovipositing on Rumex acetosella. Although adults attacked strawberries, this attack was only for a period of two weeks when the fruit was ripe, after which the bugs immediately dispersed to other food plants.
iv). The economic importance, other than damage to the crop by feeding, attributable to polyphagy among injurious insects.
a) The transmission of diseases from uncultivated plants to the crop.
Essig (1948) cobtributed a general discussion of disease transmission by insects in relation to weed control. Because many weeds and grasses were reservoirs of virus diseases which could be transmitted to agricultural crops by insects, the destruction of such intermediate plant hosts was one very important method of dealing with these diseases. Essig thought it likely that virus diseases originated in wild hosts and had been transmitted to cultivated plants by insects. Fortunately only a few groups of insects, usually omnivores, were capable of carrying the virus to many host plants.
He listed the green peach aphid (Myzus persicae Sulz.) as capable of transmitting more virus diseases than any other known carrier aphids.
Examples were given of wild reservoirs and the vectors of virus diseases, particularly curly top virus of beet.
Sucking polyphagous insects are, of course, the most obvious vectors for virus disease between uncultivated land and the crop. A gazette on
Insect Pests (1948) reported Jassid transmission of rosette of tomatoes) a virus disease affecting a wide range of wild and cultivated plants. 28.
Destruction of weeds serving as reservoirs of the virus was recommended.
Another example of Jassid virus transmission is found in Linn (1940), who recorded common plantain (Plantaoo major L.) as the winter reservoir of yellows disease of lettuce and endives. This disease was the virus of eastern aster yellows, and was transmitted in New York by Macrosteles divisus Uhl. The eradication of weeds within 100 feet of prospective lettuce and endive beds resulted in appreciable control of the disease.
Among the allied Delphacids, Delphax striatella Fall., which transmitted virus to cereals in Siberia, was found by Sukhov and Sukhova (1940) to be two very attracted to/annual grasses, Echinochloa crus-oalli (L.) and Setaria viridis (L.). These grasses contained the virus, as did two perennial species Aciropyron repens (L.) and Zerna inermis (Leyss.). This is an example of the virus reservoir being botanically related to the crop.
Other records of uncultivated land serving as a disease reservoir are to be found in Hem Singh Pruthi and Samuel (1942) and van der Laan (1940) - an aleurodid Vector of leaf-curl between tobacco and numerous wild food plants; Severin (1939) - the leafhopper Eutettix tenellus Baker, transmitting curly top of beet from natural infections in 78 species of plants of 18 families; Leach and Clulo (1943), who considered that the pentatomid
Nezara hilare Say. became contaminated with yeast spot of beans, from attacking wild plants in spring before migrating to the lima bean crop.
Several authors cite examples of a virus reservoir botanically related to the crop, as in Sukhov and Sukhova (1940) above. Simons and
Coe (1958) showed that pseudo curly-top virus of tomato also occurred in nightshade (Solanum gracile) in Florida. The virus was transmitted by 29.
Membracids, and infection of the crop decreased progressively from the outermost rows towards the centre. de Meester-Manger Cats (1956) collected bittersweet (Solarium dulcamara L.) from 25 localities in Holland. All plants appeared to contain potato leaf-roll virus, though no symptoms were shown.
Myzus persicae Sulz, carried the virus from bitter-sweet to potato. The weed reservoir of western celery mosaic was located by Freitag and Severin (1945) as ringspot virus of poison hemlock (Conium maculatum L.). This plant was often heavily infested by Hydaphis sii Koch., which in experiments transmitted the virus to several cultivated Umbelliferae. Moskovetz (1941) reported Aphis aossypii Glov. breeding during the winter on weeds, particularly wild Mallow
(Malva), which was commonly infected with mosaic virus of cotton. Weeds which showed no evidence of infection might be symptomless carriers and a source of aphids infective to cotton in the spring.
Such an occurrence of wild plants infected with crop diseases, but themselves showing no symptoms, was also referred to by Poos (1955). The insect vectors in this case were not sucking insects, but fleabeetles carrying bacterial wilt of maize. Five species of symptomless grasses growing near infected maize on which Chaetocnema pulicaria Melsh. had fed were found to harbour the bacterium. Another fleabeetle (Phyllotreta sp.) was found by
Broadbent (1957) to transmit turnip yellow mosaic. Many cruciferous weeds were suceptible but, unlike the grasses mentioned above, they showed severe stunting.
Frazier and Posnette (1957) studied the strawberry green-petal virus transmitted by two Cicadellids from various wild plants. Annual incidence of the disease was usually 2 - 5%, but heavier infections resulted when the crop was in the vicinity of high cicadellid populations on infected weeds, 30. especially if the insects had migrated from wild food-plants to the irrigated crop in time of drought.
b) Implications on crop rotation.
Attention has already been drawn to the importance of wild food plants to injurious insects during seasonal absence of the crop, frequently caused by harvest. The availability of wild food plants to a pest species has similar
implications for crop rotation (the annual absence of a crop) which is standard practice as a natural control measure against many pests and diseases. Thus
Kettlewell (1945) considered that oviposition and larval feeding of the
Caradrinid Oria musculosa (Hueb.) was usual on grasses at the edges of fields.
He regarded these grasses as the normal source of infestation of cereal fields, rather than the stubble and straw of the previous year's cereals when crop rotation was not practised. Similarly, Cockerham (1943) emphasised the
importance of wild food plants to the sweet potato weevil, Cylas formicarius
(Sum.), since it was evident that the beetles could breed abundantly on farms
where the sweet potato was suppressed as a control or eradicative measure.
The cotton weevil, Anthonomus qrandis Boh.,was found to survive on wild food
plants independently of cotton by Szumkowski (1954). Food plants of the
cotton stem weevil, Pempherulus affinis Faust, were recorded by Krishna Ayyar
(1940), who named Triumfetta rhomboidea as the preferred wild food plant.
Among the aphids, Jarvis (1945) found that infestations of the carrot root
aphid (Anuraphis tulipae Boy) could be carried over on dock when carrots
were absent.
The infestation of crops grown in isolation has bearings on crop rotation. Johnson (1957) showed that Taeniothrips niciricornis Schmitz
carried pod twist to french beans from the wild Phaseolus lathyroides. 31.
This weed flowered throughout the summer, and was able to provide a continuous source of infection, which would explain the occurrence of the disease in bean crops well separated from other bean-growing areas.
c) Beneficial effect of wild food plants.
Polyphagy among injurious insects is undoubtedly normally harmful to crops in the ways outlined above, yet records do exist of beneficial effects of wild host plants in protecting the crop by acting as a trap food•-supply for crop pests. Thus Mills (1942) quoted a report of the grasshopper
Melanoplus mexicanus Sauss. causing little damage to winter wheat owing to the abundance of green vegetation along field edges and on abandoned land during most of the summer.
Jones (1942) had in mind this trap effect of food plants outside the crop when he studied the order of colonisation of wild and cultivated summer food plants by Aphis fabae Scop. . He considered it unfortunate that the two preferred summer host plants should be crop plants, but pointed out the possibility of protecting seed sugar-beet from heavy initial infestations by planting the headlands with the preferred beans, and destroying the latter after they had become infested.
d) Effects on injurious insects of feeding on wild food-plants.
Different food plants may affect the size, fecundity, length of life cycle and susceptibility to disease of polyphagous insects. The latter three characteristics are clearly of importance in the case of crop pests.
How far effects on size may be important is suggested by the work
(referred to later, p.38) of Clausen (193114, Salt (1940), and J.M.Smith
(1957). Effects of wild food-plants on the size of injurious insects may 32.
afreta, the value of populations of ouch insects outside the crop as a
parasite reservoir.
Pepper and Hastings (1941) conducted laboratory experiments on the
effect of food plant on the length of larval life and the size of larvae of
the sugar-beet webworm (Loxosteqe sticticalis (L.)). They found that larvae
reared on Russian thistle (Salsola kali L.) were the largest and first to
pupate.
Wide variations in the fecundity of the Silphid Aclypea opaca (L.)
on different food plants were demonstrated by Kaiashnikov (1940). Compared
with females reared on the leaves of beet, those which fed on weeds laid
considerably more eggs. Adults which fed on chickweed (Stellaria media (L.))
laid twice as many eggs, and those which fed on Rosebay Willow-herb
(Chamaenerion anqustifolium (L.)) laid sixteen times as many. Scheiding
(1956) studied the numbers of eggs laid on crucifers by the weevil
Ceuthorhynchus pleurostiqma (Marsham). Compared with eight eggs on rape,
20-29 eggs were laid on cabbage, but the highest number (32 eggs) was laid
on charlock. The susceptibility of lepidopterous larvae attacking cabbage
to diseases when on wild and cultivated plants was investigated by Kawada
and Sekiya (1940), also by Vago and Cayrol (1955). The former authors showed
that Pieris rapae (L.) was less susceptible to a particular disease on
Cardamine hirsute L. than on field cabbage. Vago and Cayrol studied a virus
(Borrelina) attacking Plusia gamma (L.). As against a 60-100% kill of
larvae on cabbage, the virus killed less than 60% on Plantaqo and only 20%
on Sonchus. All larvae on Plantaqo transferred to cabbage died, but there
was no increase in mortality among those not transferred. The authors regarded
these results as an effect of a food plant activating the virus. 33.
2) The importance of flower-feeding to pest insects.
Adult feeding on flowers is common during the egg-maturation cycle in insects. Where this applies to injurious species and the crop is cleanly cultivated, flowers on adjacent uncultivated land may be an important source of food and thus act as a factor in the increase of an infestation.
Willer (1941) studied the emergence of overwintered adults of the
Rape Beetle (Meligethes aeneus (F.)) from their hibernation quarters. The gonads of the males were fully developed, but those of the females were not.
The females fed for some time on the nectar of spring flowers, and when the eggs were mature, the beetles migrated to rape fields to oviposit.
Flower feeding has similarly been recorded for some well known dipterous vegetable pests. Miles (1951) stated that adults of the Cabbage
Root Fly (Erioischia brassicae (Bch'e))visited flowers outside the crop for essential nectar feeding, and Petherbridge, Wright and Davies (1942) found adult Carrot Flies (Psila rosae (F.)) of both overwintered and first generations feeding on the flowers of wild chervil (AnOrtscus sylvestris
(L.)) and hemlock (faajlam maculatum L.)
B) The importance of uncultivated land to benefidial insects.
Uncultivated land may be important to beneficial insects in ways largely parallel to the importance of such land to injurious insects. Many of the references cited are of a qualitative or speculative nature, and there is need for quantitative work on this aspect of relationships between crops and uncultivated land. The need for a careful consideration of the practical effects of large-scale clearance of hedgerows and other uncultivated land is stressed by many authors, particularly by Elton (1958) in a chapter headed 34.
"The Reasons for Conservation". Both Vote (1946) and GyOrfi (1951),
drawing examples from forestry, emphasised the paucity of insect plagues in
unmanaged forests, explaining this in the terms of richness in beneficial
insects. An example with economic emphasis was given by Isaakides (1954),
who ascribed the current high rate of infestation of olives in Greece by the
Trypetid Dacus oleae (Gmel.) to the elimination of natural enemies caused by
excessive clean cultivation in olive groves, involving the destruction of all
undergrowth, as well as the use of modern contact insecticides.
1) The presence outside the crop of alternative hosts for beneficial insects,
The polyphagous habit of many beneficial insects was considered in
general terms by VOute (1946), who compared the regulation of the density of
insect populationsin virgin-forests and cultivated woods. With the removal
of undergrowth, the number of herbivorous insects would be diminished, and the
total would drop below the critical level of the polyphagous carnivores, which
would then become reduced in number and variety. Many polyphagous carnivores
needed a succession of prey to hold their own, and rich woods contained such
food all the year round. Gybrfi (1951) considered in more detail the relations
between parasitic Hymenoptera and the ground cover. Many of these parasites had subsidiary or alternative hosts to the pest insects attacked. These hosts
played an important part in bridging the gaps that occurred when the generations
of the parasite and the main host did not coincide and in maintaining the parasite when the principal host became scarce. To demonstrate the importance of the ground vegetation for alternative hosts, C-ybrfi listed the hosts of the main parasites of the Gypsy Moth (Lymantria dispar (L.)). His lists are condensed in Table 2. Gybrfi claimed that the most important pests of timber trees shared common enemies with larvae in the lower canopy and undergrowth. 35.
TABLE 2. Hosts of the main parasites of Lymantria dispar (L.) condensed from Gyorfi (1951).
Parasite No. of secondary No. of hosts occurring hosts on ground vegetation.
Ichneumon dispar Poda 7 0 Pimple instigator (F.) 21 10 Theronia atalantae (Poda) 16 10 Apanteles fulvipes (Hal.) 27 19 Apanteles liparidis (Bch6) 3 2 Apanteles portnetriae (Muesb.) 4 4 Apanteles melanoscelus Rbzb. 1 0
Total 79 45
The larvae below furnished the secondary and alternate hosts of the parasites, thus the mixing of undergrowth enriched the forest fauna and encouraged the increase of parasites much more than the mixing of tree species in the upper canopy. The economically neutral insects in the undergrowth formed a reservoir of hosts which was a valuable weapon in maintaining biological equilibrium. The explosive outbreak of pests was the result of the absence of hosts combined with favourable weather conditions.
General conclusions from woodland ground-cover studies have bearing on agricultural problems, when canopy and ground flora are represented by crop and adjacent uncultivated land. Five species of Trypetids collected by Schwitzgebel and Wilbur (1943) from a weed (Vernonia interior Small) on grassland in Kansas yielded 22 species of hymenopterous parasites, some of which were also parasites of crop pests.
Over winter, economically unimportant insects on uncultivated plants may be necessary alternate hosts for parasites of crop pests. Hardy (1938) studied the Diamond hack Moth (Plutella maculipennis (Curt.)) in England, 36. and came to the conclusion that the important parasites (Anclitia cerophaqa
(Gray.) and A. chrysosticta v. fenestralis (Holmgren)) both overwintered in other hosts. P. maculipennis does not hibernate as a larva. O.W.Richards
(unpublished) located the latter parasite overwintering inSwarnmerdamia sp.
(Yponomeutidae) on Hawthorn. Muggeridge (1939) considered Plutella as a serious pest of cruciferous crops in New Zealand, but as rather more scarce in Europe, where it was parasitised to a high degree. He suggested that this might he due to the parasites having alternate hosts. Bobb (1942) and
Friend (1942) found alternative winter hosts of parasites of the Oriental fruit moth (Enarmonia molesta (Busck.)) on common weeds in peach orchards.
Bobl? considered these alternative hosts important, as few larvae developed towards the end of the season in the peach twigs, where the level of parasitismwas much higher than in the fruits.
Many predatory insects are polyphagous and may have similar relations be parasites with prey on uncultivated land. The larvae of many species of Syrphids are general aphid predators. Speyer (1936) listed wild and cultivated plants on which larvae of Syrphus ribesii L. and S. balteatus
Deg. had been observed feeding on aphids. The prey of cbccinellid species were listed by Schilder and Schilder (1928), and comprised many groups of pest insects, particularly the aphids. Examples from these lists include
41 named species of prey for Adalia bipunctata (L.) and 32 species for
Coccinella septempunctata L. Banks (1955) recorded the accumulation and breeding of coccinellids on nettles infested with Microlophium evansi (Theo.); many of the ladybirds dispersing from nettle appeared on bean plots after the arrival of Aphis, fabae Scop. Lal and Singh (1945) studied the control of
Woolly Aphis, Eriosoma laniqerum Hsm., by Coccinella septempunctata L. on 37. apple. At the beginning of winter the coccinellids migrated to two wild grasses, Andropocion pertusus and A. assimilis. In November, the coccinellids fed on the aphids infesting these grasses, and the emerging adults hibernated there during the rest of the cold season. The coccinellids were scarce in orchards where these grasses were absent, and it was considered desirable to foster the predators by introducing the grasses into such areas. Knowlton (1943) observed the predatory bug Nabis alternatus
Parshley feeding on aphids of various species, including EPUS persicae Sulz. on potato, Macrosiphum onobrychis Buz. on lucerne, and M. avenae F. on grass.
In addition, a female was found feeding on a leafhopper nymph on birch foliage.
In addition to the value of uncultivated land for maintaining alternative hosts and prey; and for bridging gaps when generations of beneficial insects do not coincide with their prey on the crop or when the latter naturally become scarce, uncultivated land may preserve a reservoir of beneficial insects after the spraying of crops with insecticides. Such a reservoir may be maintained if either the parasites and predators are polyphagous or if the crop pest is itself polyphagous. Work on the re-invasicy of sprayed orchards by predators (Collyer, 1953c) has already been mentioned
(p. 13), Carlson, Lange and Sciaroni (1951) studied attacks on cruciferous seed crops in California by the weevil Ceuthorrhynchus assimilis (Payk.), as well as parasitisation of the pest by ectoparastic Pteromalids. These were absent from the experimental crops, though 27.2% parasitism was found in heavy infestations of the beetle on neighbouring wild yellow mustard.
They presumed that parasites on the crop had 'been repelled or killed by insecticide residues. 38.
A case of parasites being unable to utilise alternative hosts was given by Barnes (1952). Parasites of the gall midge Contarinia tritici (Kby.) on wheat did not complete their development without overwintering. The generation of C. tritici which occurred on couch grass in August-September was therefore likely to be free from attack, and would then constitute a dangerous site for population increase.
2) Effects on beneficial insects of attacking insects on uncultivated land.
It has been previously noted that uncultivated land may supply hosts for parasites of crop pests of different species, or/within the same species of different sizes and rates of development. Such differences may be reflected in the parasites emerging from them.
Salt (1940) showed how the size of individuals of the egg parasite
Trichogramma was largely controlled by the size of hosts in which they developed. The vigour, fecundity, longevity and rate of development of individuals was also affected by their hosts. Through their effect on size, the hosts influenced the behaviour of females selecting hosts for their progeny.
Clausen (1939) reviewed work suggesting that mostly female parasites emerged from large host individuals. J.M. Smith (1957) showed variations in mean size of the Californian Red Scale (Aonidiella aurantii (Mask.)) from 1.29 - 1.49 mm. on various cultivated plants and demonstrated that parasites associated with the larger scales were larger, more predominantly of the female sex, of greater longevity and more prolific than parasites bred on host plants which produced smaller scales.
The host plant may confer a kind of immunity to parasitisation on the 39. host insect. J.M.Smith (1957) showed that, while the Encyrtid parasite
Habroplepis rouxi Comp. suffered only 3.1% mortality of its immature stages when attacking the Californian Red Scale on grapefruit, when the scale was
Grownon sago palm 100% mortality of the immature parasites occurs. Smith found a similar effect on another Encyrtid, Comperiella bifasciata How.
Although work without direct bearing on cultivated plants has been recorded, it is possible that there is a similar contrasting effect on parasites with cultivated and uncultivated plants in the case of a polyphagous pest species or genus. Certainly, cases of different degrees of parasitisation of pests on cultivated and uncultivated plants can be found in the literature.
Thus, of the parasites of Armyworms (Laphygma spp.), Bianchi (1944) recovered most cocoons of Apanteles marcliventris Cress. on grasses and weeds; Meteorus laphygmae Vier. was similarly common on grasses. The Bethylid Perisierola sp. was found on both full grown maize and short weeds or grasses. Krishna Ayyar
(1940), studying the preferred wild food plant (Triumfetta rhomboidea) of the
Cotton Stem Weevil (Pempherulus affinis Faust.), found that parasites associated with T. rhomboidea were usually absent from cotton fields.
An observation by Richards (1940) on parasitisation of Pieris rapae (L.) by Apanteles rubecula Marshall may have a bearing on the importance of uncultivated land, where sheltered conditions prevail in the case of hedgerows and where the vegetation is usually more dense than in many crops. Richards found that percentage parasitim was influenced by the site of the food-plant, as well as by host density. It was twice as great on cabbages growing close together in sheltered situations as on those growing farther apart and in more exposed conditions. 40.
3) Beneficial insects feeding phytophaqously on uncultivated land during the absence of their prey.
Some insects, particularly the Mirid bugs, are known to take both
plant and animal food (Southwood and Leston, 1959). Thomas (1945) studied
the Mirid Enqytatus cieniculatus Reut. on tobacco. E. cieniculatus does not
harm the plants, but is a partial predator on another Mirid (Dicyphus
minimus Uhl) which occurs as a pest. E. cieniculatus did not hibernate,
and both nymphs and adults fed intermittently on native food-plants during
the winter.
4) The presence of flowers on uncultivated land.
The feeding of adult beneficial insects on the nectar and pollen of
flowers can make uncultivated land useful to adjacent crops which, in many
cases, are kept as free of weeds as possible and harvested before flowering.
Among predators, Syrphids are the subject of a considerable literature
which may partly apply to other predators as well as some parasites. Zimina
(1957) described the mouthparts and alimentary canal in the genus Syrphus
as adapted for the intake and digestion of both pollen and nectar. Previously
Schneider (1948) had remarked that Syrphids were among the most common flower
visitors, feeding on pollen and nectar. Experiments had repeatedly shown that the females hatched with small ovaries, and that reserves in the fat body did not usually suffice for development of the ovaries. Schneider considered that pollen feeding was essential to maturation, and that egg laying could not occur without it. Schneider-Orelli (1945) cited pollen feeding by Syrphids as a reason supporting the planting of flowering hedges and shrubs near fields in the interests of bee-keepers. The attractiveness of flowers to Syrphids was considered by Dixon (1959) to account for a peak of Syrphid eggs on broom 41. during the flowering period of the shrub, but before the peak in abundance of aphids. Parmenter (1952) reported attraction of Syrphids to yellow colours, including fresh yellow paint on a pavement. The dominant dipterous visitors to cow parsnip (Heracleum sphondylium L.) in a list compiled by
Grensted (1946b) were Syrphidae including four aphidophagous species. Allen
(1954) recorded four aphidophagous species visiting umbellifer blossom. The economic implications of pollen feeding were mentioned by Fluke (1929) describing work on the Pea aphid in North America. He considered that there was a relationship between the abundance of Syrphid larvae and the occurrence of flowering plants in the vicinity.
Other predators visiting flowers include Dolichopodidae, which Parmenter
(1942) listed from hogweed, sea-carrot, tormentil and lesser stitchworI, also adult Neuroptera, Dolichopodidae, Empididae and Rhagionidae on Umbellifers
(Allen, 1954).
The importance of flowers as a source of food to parasitic insects, as well as predators, was recognised by Schneider-Orelli (1945) in connection with promoting flowers for bees (as cited earlier in connection with flower visiting by Syrphidae). He claimed that Ichneumonids had to feed on nectar for egg maturation. In the realm of forest entomology, Voute (1946) stated that a lack of flowers was not only fatal for bees, but also for many species of Ichneumonids and digger wasps, and GyOrfi (1951) considered that it was important for the increase of parasites that ground cover and flowering meadows or clearings should be left in woodlands. Gy3orfi named two umbellifers, Daucus carota L. and Pastinaca sativa L. as preferred flowers.
Where these plants flowered in large numbers swarms of parasites congregated, especially Ichneumonids and Braconids. 42. Allen (1954) studied the insects feeding on plants of the family
Umbelliferae. The number of Hymenoptera visiting the blossoms of all species studied was very large indeed, especially of Parasitical of which so many were taken that individual identification was impossible. Allen mentioned the importance of nectar as food for many adult parasites; umbellifers appeared to be a popular feeding site.
The food preferences of some adult parasitic Hymenoptera were studied by Leius (1960). Itoplectis conquisitor (Say) and Ephialtes buolianae (Htg.) fed generally on umbelliferous flowers, but also accepted flowers of other families, as well as body fluids of their hosts. Oroilus obscurator (Nees) fed on umbelliferous flowers only, with a strong preference for wild parsnip.
Leius concluded that information on the adult feeding habits was essential when parasites were being introduced for biological control purposes. If the favourite food, such as umbelliferous flowers, were absent from the release area the parasites would have less chance to become established.
This point was illustrated by a series of papers by Wolcott (1941a & b, 1942) on the establishment of a Sphegid parasite (Larra americana Sauss.) against the mob-cricket Scapteriscus vicinus Scud. in Porto Rico. Wolcott drew attention to the dependence of the survival of insect parasites, not only on the presence of their hosts, but also to factors favourable to the non- parasitic stages. Failures in establishing the parasite were apparently due to the scarcity of Borreria verticillata and Hyptis atrorubens which supplied nectar to the wasps. Destruction of these weeds by cultivation, shortly after a release had been made, apparently eliminated one factor essential to their survival. In 1939, liberations of the parasite succeeded only at one point, and no spread was recorded till 1941, when adults were seen feeding on 43. flowers some miles from the points of liberation. Wolcott (1938, 1942) gave a second economic example of the importance of flowers to adult parasites.
Adults of the Scoliids Tiphia spp. which parasitised white grubs (Lachnosterna sp.) commonly fed at the flowers of wild umbellifers. In Porto Rico the parasites were scarce though their hosts were common, probably because the frequented umbellifers were uncommon. In Haiti, where the weeds are wide- spread, Tiphia spp. were much more numerous. Friend (1946) reported that
Tiphia popilliavora Rohw., which attacked the Japenese beetle (Popillia japonica Nem.), had failed to become numerous prior to 1946. This was attributed to the cutting of the flower stalks of wild carrot when the flowers began to open; the adults were thus deprived of their cnly known source of food.
The establishment of flowers to attract parasites was recommended in a bulletin (1943) on maize stem borers (Sesamia vuteria Stoll and Crambus malacellus Dup.) in Mauritius. It was suggested that clumps of Cordia interrupta, on the flowers of which the adult parasites fed, should be left at intervals of 200-300 ft. near maize fields.
The behavioural aspects of flower feeding by adult hymenopterous parasites were considered by Thorpe and Caudle (1938) in work on Ephialtes ruficollis (Gray.) a parasite of the Pine Shoot Moth (Evetria buoliana (Schiff.)
These workers correlated the attractiveness or repellence of pine oil to the parasites with the degree of development of the ovaries. Parasites were only attracted by the oil when they were three to four weeks old. During the first week after emergence the ovaries were minute, but towards the end of the three to four week period they were well developed and the insects 44. were ready to lay their eggs. The period when the insects were repelled by the odour of pine was that during which the host caterpillars were not
yet available and during which the parasites probably left the pine trees to feed on the flowers of ulmbelliferous and other plants. After the third
or fourth week of life, the parasites presumably returned to the pine shoots,
where the host was in a stage suitable for attack.
Parasitic Diptera are similarly recorded feeding at flowers. Tachinids
supplied 17 out of 27 species of Diptera found on the flowers of Golden Rod
(Solidaoo viraaurea L.) by Grensted (1946a). Beard (1940) recorded the
Tachinid Trichopoda pennipes F., the controlling parasite of the Coreid
Anasa tristis DeG. in Connecticut, feeding on the nectar of flowers. Santoro de Crouzel and Salavin (1943) found adult Neorhynchocephalus (Nemestrinidae),
which parasitised grasshoppers, frequenting flowers.
The flower-feeding habit appears to apply generally to important
groups of beneficial insects. Besides the mention of such insects being recorded on flowers, the importance of the habit has been indicated by several authors as quoted above, particularly with reference to experiments
in biological control.
II. Effects due to physical effects (particularly shelter) of uncultivated land.
A) Effects of edoecirowth on the deposition of insects from air-currents onto the crop.
The deposition of insects held in air-currents has been studied particularly with regard to aphids, where the literature is very extensive.
Although many other economically important groups of insects, including 45,
Thysanoptera and many small root flies and stem-boring diptera, are
similarly carried by air-currents, there has been little experimental
work on the deposition of these insects comparable to the attention given
to aphids.
Taylor and Johnson (1954) suggested that the shelter of tall hedgerows may induce settling,on the cropl of migrating aphids. Bean
fields were often heavily attacked by Aphis fabae Scop. along the edges,
especially on sides facing the wind. Johnson (1950) considered that wind
direction gave a plausible explanation of the pattern of infestation by
Aphis fabae round the edge of a bean field. However, other factors, such
as local eddies and subsequent movement of alatae, were also important
and blurred the pattern. Klostermeyer (1953) studied the distribution
of elate Myzus persicae Sulz. in a potato field by means of yellow water
traps. More alatae were trapped at the edges than at the middle of the
field. Both the distribution of the apterous population and the frequency
of infection of plants by leaf-roll was closely associated with the elate
distribution. Comparable work by Schreier & Russ (1954) on virus of
beet-yellows and the infestation of beet by Aphis fabae and Myzus persicae
showed that plants infested by A. fabae and those showing yellowssymptoms
were predominantly and fairly evenly distributed along the edges of fields.
Such plants were slightly more numerous along the west edge, which was
the one exposed to the prevailing wind. Infestation by M. persicae was
also concentrated along the west edge. A similar pattern of disease
incidence in a crop was reported by Broadbent, Tinsley, Buddin and Roberts
(1951) who studied the spread of lettuce mosaic and its vectors Macrosiphum 46. euphorbiae (Thomas) and Myzus persicae Sulz. There were more diseased plants near hedges, woods, trees, or buildings than in the open field.
Such barriers halted the flying aphids. The greater the area of the crop, the smaller the proportion of the crop would be formed by the infested edge rows, thereby reducing the economic loss. The percentage infection of the crop often increased near hedges and trees even on the side of the field farthest from the source of infection. Earlier, Broadbent (1946) had reported that sheltered potato fields were the least infested by Myzus persicae, but such fields were situated in deep valleys and therefore sheltered from the air-currents carrying the aphids. This was followed by Fidler (1949), who suggested that potato fields which were very sheltered
(particularly from east winds) might receive fewer aphid migrants than exposed ones if air-currents carried the aphids across above them.
Nevertheless, maximum populations of potato aphids were found in such fields.
B) Uncultivated land as temporary shelter for insects attacking the crop.
The assemblage of adults in the shelter of dense vegetation around the field, with visits to the crop for oviposition in suitable weather conditions, appears to be a feature of several important pests among the acalyptrate Diptera. This suggests the spraying of marginal vegetation or the eradication of shelter as possible control measures.
Nishida and Bess (1950) noticed that 90% of field adult populations of the Melon fly (Dacus cucurbitae Coq.) consisted of gravid females, which apparently entered the fields to oviposit rather than to feed. Nearly all of them left the fields before dusk and spent the night on neighbouring weed vegetation. Control by spraying the hedgerows at night was suggested. 47.
The less regular movement of the cabbage root fly,Erioischia brassicae (Bdhe), between hedgerow and crop was reported by Miles (1951). Feeding and egg- laying occurred in warm, sunny weather, whereas in cold or wet weather the
flies sheltered in the soil or in thick herbage. If immobilised they died of starvation, and in this way the onset of cold weather checked the development of an attack.
Reference to the importance of edgegrowth shelter in attacks by the carrot fly (Psila rosae (F.)) is a recurrent theme in papers on the bionomics of this pest. Almost any type of shelter is utilised by the fly; thus Petherbridge, Wright and Ashby (1945), also Williams (1954) found the adults sheltering on tall maize plants growing near carrot crops. Baker,
Ketteringham, Bray and White (1942) swept flies from hawthorn hedges of from 2' - 10' in height. The flies were scarce in the lower hedges, but large numbers were recovered from the higher ones. Most flies congregated on the sheltered side of a hedge when the wind was blowing through it, even when the wind speed was low; when a gale was blowing the flies assembled on the lower surfaces of weeds on the sheltered side of the hedge.
Petherbridge,Wright and Davies (1942) similarly found adults on broad-leaved plants and low bushes under conditions of high winds. Although Wilson
(1945) thought that there was a daily movement from hedge to crop, he considered that weather factors played an important part. During warn and still weather the flies remained on the crop. Baker et al. (1942) found adults in the hedges till 4 p.m. They did not move in cool, windy weather, but in warm, still weather at least half left during the evening by a general dispersal rather than a migration to carrots and returned at nightfall. 48.
Williams (1954) observed that adults were hard to find on carrot fields, even when infestation was high, but many flies could be found sheltering outside the crop after sunset and numerous flies remained there in the early morning. He concluded that females probably visit the crop for oviposition a few at a time. Petherbridge and Wright (1943) considered that the flies remained sheltering from 10 a.m. - 5 p.m. if the weather was hot and sunny, though on dull, warm days they could be found in the fields throughout the day. A marked increase in the number of adults in a celery field was noticed by Watkins and Miner (1943) as the wind velocity and brightness of sunlight decreased. The flies preferred the darkest shade available. The effects of the movement of flies on crop attack were mentioned by most of the authors cited. Oviposition was concentrated near sources of shelter (Petherbridge et al.,1942), especially in sheltered positions where the hedges were thick or trees occurred (Baker et ale, 1942).
Such differences in oviposition were reflected by a heavier infestation near shelter (Petherbridge and Wright, 1943; Petherbridge et al„1945).
Wilson (1945) studied a 92' x 10' plot of carrots bordered on two sides by 8' high hawthorn hedges, and concluded that infestation was heaviest near hedges which provided shelter from the prevailing wind. Wright and
Ashby (1946) made a detailed study of the variation in infestation resulting from the distribution of ovipositing adults, determined by the shelter available. By plotting the percentage attack at various distances along transects into the crop from the two adjacent headlands, they were able to build up a three-dimensional figure which clearly showed a decrease in infestation towards the centre of the field and a particularly high attack near the corner where the effect of the two headlands overlapped. 49.
The chemical control of the flies while sheltering was suggested by Baker et al. (1942). The infestation of the crop by first generation larvae could be reduced by spraying hedges during the morning or late evening round fields in which carrots were grown in previous years.
Petherbridge and Wright (1943) recommended a bait spray applied six times between 22nd May and 12th June to dykes surrounding early carrot fields.
Such sprays had failed where the weedy condition of the crop provided alternative shelter or not all the headlands had been sprayed. The spray should be applied to both sides of hedges with egnipment fitted with coarse nozzles to give good penetration.
Cultural measures, based on the importance of siielter for adult carrot flies, have been recommended by several authors, particularly the cleaning up of hedges and the destruction of unnecessary dense vegetation
(Petherbridge and Wright, 1943; Petherbridge et al., 1945; Wright and Ashby,
1946). Wright and Ashby claimed that where this had been done spraying could be discontinued. On the basis of their headland-crop transects,
Wright and Ashby considered that fields should be as large as possible to minimise the high edge infestation; large open fields with little shelter had previously been recommended by Watkins and Miner (1943) and Petherbridge et al. (1945).
C) Uncultivated land as shelter for hibernating insects.
Tischler (1950) made a general study of the hibernation relationships of agricultural pests, in respect of crop rotation, mixed crops, weed control, the timing of sowing, the mowing of verges, and the possible importance of hibernation ecology in chemical control. Tischler's lists 50. of the hibernation habits of pests in a variety of crops are summarised in Table 3. Tischler concluded that pests, particularly bugs and beetles, which overwintered as adults, nearly all left the crop to hibernate in the shelter of uncultivated land, and that such insects were of particular importance in beet, crucifer, and legume fields, where they included the most important pests.
Among the beetles hibernating in the shelter of uncultivated land, fleabeetles are of particular importance. Wolfenbarger (1940) in New York
State studied the distribution of injury to potatoes by Epitrix cucumeris
Harr. in a 10-12 acre field bordered on one side by trees and underbrush and by cultivated land on the other three sides. The highest percentage
(18%) of injured tubers occurred near the uncultivated area, and attack decreased as far as 100' - 200' into the field, where it became fairly constant. Further investigations showed that the adult beetles hibernated to the extent of 188,000 per acre in various types of uncultivated land near a potato field, and 22,000 per acre within the field. Anderson and
Walker (1940) studied E. cucumeris in E. Virginia and similarly found the adults overwintering in soil or under debris in protected places, especially on the edges of woods. Fleabeetles of another genus (Chaetocnema tibialis
Iii.) attacking beet were located by Watzl (1950) hibernating at 250 per sq. metre in the surface soil of a lawn 100 metres away from fields, at
150 per sq. metre in the surface litter of a conifer wood 200 metres from fields, and at 40-50 per sq. metre in the surface soil of cultivated fields.
Chaetocnema pulicaria Melsh., carrying bacterial wilt of maize, has also been found overwintering in the top inches of soil, mainly under wild grasses (Poos, 1955). Moreton (1945) investigated the migration of several TABLE 3. The Hibernation habits of crop pests (Summarised from Tischl?r, 1950).
Number of pests cited in-classification group (including pests listed under several groups). Classification of method of hibernation. Cereals Total (Modified after Tischler) and grasses Root crops Crucifers Legumes non-orchard crops Orchards Hibernation on or within the crop plant or on 12 1 4 5 22 27 remains of:the crop plant (e.g. stubble).
Species hibernating in 14 8 13 10 45 14 soil of crop field.
Species seeking Feeding distant and on 2 3 0 1 6 0 different wild biotopes for plants hibernation (based on Sheltering Tischler's in debris 3 7 8 5 23 2 observations etc. on the borders of woods, hedges, verges etc.) 52. species of Phyllotreta attacking Brassica crops. At the beginning of the season, beetles were most numerous in an edge of the field that was adjacent to a wood, in which they had probably overwintered. Infestation
spread most rapidly along the sheltered headlands.
Phaedon brassicae Baly, the cruciferous leaf-beetle, is another
Chrysomelid recorded sheltering for hibernation. Cheo and Ling (1943)
found adults sheltering in loose soil, under crop debris, among weeds, and in other protected situations. The beetles sought such shelter during
winter, for aestivation in high temperatures, or when lack of food rendered conditions unfavourable.
The Mexican bean beetle (Epilachna varivestis Muls.) (Coccinellidae)
was found by Elmore (1949) overwintering in shelter near the margin of
fields in which they had been feeding at harvest. Numbers decreased rapidly as the distance from the fields increased; none was found more than half a mile away. The beetles were found in moist places under debris at the foot of various trees. Adult Epilachna have also been located over-
wintering under leaves, grass and weeds, and along the banks of ditches
(Leaflet issued by the Department of Agriculture, United Provinces, ? 1943;
Tanner, 1943).
Several weevils similarly seek shelter for hibernation. In the
literature on cotton pests, Fenton and Chester (1942) considered that
abundant woodland provided protection during winter for Anthonomus orandis
Boh. Bondy and Rainwater (1942) found 70 of weevils on the crop within
50' of the edge of woods bordering the cotton fields and practically none
was found more than 150' from the edge. A similar pattern of A. orandis 53. infestation of cotton near wooded areas, fence-rows and other shelter was reported by Reinhard (1943). Within the same weevil genus, A. signatus
Say., the Strawberry Weevil, was found to be particularly injurious to strawberries grown near woods, hedgerows or other cover supplying suitable hibernation quarters for the adults (Christ and Driggers, 1949). Obrtel
(1957), working on a species of Apion attacking clover, found that the sexually immature adults hibernated in forest borders, bushes or other shelter.
A further instance of the importance of hibernation shelter to many beetles was provided by Blaszyk and Madel (1950). Overwintering adults of the Silphid Aclypea opaca (L.), which is a pest of suglr beet, were found beneath thick clumps of long grass between trees bordering a road and an aujacent orchard. Dry soil was preferred, and the presence of thickly matted grass was favourable to the pest.
Bugs were the second insect group quoted by Tischler (1950) as being particularly dependent on hibernation as adults on uncultivated land. de Alwis (1941) recorded that adults of the Pentatomid Scotinophora lurida
Burm. from rice remained inactive from harvest until the new crop became available in the next season. They sheltered both in the soil of the rice- fields and in the neighbouring patches of high land and jungle. The
Overwintering of the "Wheat bug", Eurydaster austriacus seabrai China, was studied in Spain by del Canizo (1941). The adults overwintered beneath the ground litter of oak forests, or just below the surface of the soil of woods or thickets. Acanthocoris sordidus Thnb., a Coreid damaging sweet-potato, potato, tomato and egg-plant, also overwintered outside the crop in the adult stage, among fallen leaves and dry grasses (Yamazaki, 1940). 54. Similar overwintering habits under rubbish and weeds for the Lygaeid,
Nysius vinitor Bergr., causing damage to tomatoes in hot dry summers, were recorded by Pescott (1940).
Apart from the beetles and Heteroptera, there STA few references to
insect pests in other groups sheltering in soil, litter, etc. on uncultivated land. In the Homoptera a Delphacid transmitting virus to cereals was reported sheltering outdide the crop by Sukhov and Petlyuk (1940). The nymphs of the Delphacid, Delphax striatella Fall., overwintered, chiefly
among grasses, on uncultivated strips at the edge of meadows until activity was resumed in mid-April. Sukhov and Petlyuk considered that the destruction of these hibernating nymphs, concentrated on narrow strips of ground over- grown with weeds,was the most effective control measure, and added that an insecticide which scorched the plants would not be disadvantageous.
The use of uncultivated land for oviposition by the Caradrinid,
Hydraecia xanthenes (Germ.); which attacks globe artichokes, was described by Thermes, Delmas and Cessac (1954). The larvae entered the fields from overwintering eggs laid round the edges of the fields in cracks in the bark of trees, and on stakes, fences and windbreaks.
D) Physical effects of the edgegrowth probably directly affecting insects on the crop.
Tall vegetation and particularly the presence of trees round a field will alter the microclimate in that part of the field adjacent to such edge- growth. It is likely that in many cases a differential in favourability for any insect species on the crop will arise between such edges and the centre of the field. 55.
Thus Knowlton (1948), studying a heavy infestation on alfalfa of the leafhopper Empoasca filamenta DeLong., swept an average of 17 leafhoppers per sweep along a margin of the field well shaded by large cottonwood trees; beyond the shade of these trees, the leafhoppers averaged 1.2 per sweep and alfalfa foliage injury was not conspicuous. Knowlton concluded that the shade afforded by the trees created an especially favourable environment for the leafhopper, resulting in an unusual concentration. Wolfenbarger
(1940) reported.that shade had the converse effect on the fleabeetle Epitrix cucumeris Harr. in potato fields. There were both fewer injured tubers and fewer adult fleabeetles in conditions of dense shade. Similarly van der Helm (1942) found that infestations of leeks by the Onion Maggot, hyjemya antique (Meig.), and the Tineid, Acrolepia assectella (Zell.), could be reduced by sowing in sheltered positions. Attack by these pests was less on the north side of hedges than on the south.
Comparable effects have been observed on beneficial insects. Arthur
(1945a), releasing the Braconid, Aphidius oranarius Marshall against a variety of aphids infesting wheat, noted that the centre of the field stimulated more flight of the adult parasites, since it was hotter and less sheltered than the remainder and stayed warm for a longer period during the day. Advantages of shade to beneficial insects were recorded by Taylor
(1910). Neglect of coffee plantations had resulted in the scarcity of
Pentatomid pests of the genus Antestia, as the predators and parasites had been favoured by the shade, absence of wind, humidity and abundant soil moisture resulting from neglect, as well as by the presence of weed under- growth sheltering alternate hosts. 56.
E) Physical effect of the edgegrowth affecting insects on the crop indirectly through the host plant.
Several authors attribute the effect of physical factors on insects to an effect working through the host plant. One of the possible factors causing the heaviest damage by the carrot fly (Psila rosae (F.)) to occur in parts of the fields sheltered by hedges was considered by Wilson (1945) to be water competition from the hedgerow weakening the plants and rendering them more susceptible or more responsive to fly attack.
A second type of effect working through the host plant was quoted by
Royal (1955), who pointed out that the close planting of groundnuts was
known to reduce infection with rosette disease carried by Aphis craccivora
Koch. Densely planted strips round a field had been shown to arrest the
spread of the aphid. Real established that the reason lay in the reduced
exposure to the sun received by the crowded or shaded plants. This resulted
in reduced phytosynthesis, in turn reducing the fecundity of the aphid and
the production of elates.
An interesting example of a plant effect of a type which might arise
in connection with edgegrowth and concerning a beneficial insect was given
by Vote (1958). An Encyrtid parasite (Ageniaspis sp.) of the leaf-mining
Gracillariid, Phyllocnistis citrella Staint. could not find its host if the
leaf cuticb:became too thick due to lack of shade. Insufficient shade
resulted in a severe outbreak of the miner.
III. The importance of uncultivated land for pollinators.
Although this section does not fall strictly within the province of
the literature review on the relation of uncultivated land to crop pests, 57.
its inclusion is perhaps justified on two grounds. Firstly, an insect
relationship between crops and uncultivated land may be considered to exist
with regard to pollinators and, secondly, the use of insecticides or weed-
killers as control measures in connection with aspects of crop-edgegrowth
relationships previously discussed may have a profound effect on the
generally beneficial flower-visiting insects. The dangers of killing bees
by the use of sprays have periodically received mention in the national press.
In "The Observer", Butler (1956) stressed the importance of roadside verges
as nesting sites and feeding grounds for wild bees, especially where large
areas of land were devoted to crops requiring pollination. He was sorry
to see that in some districts many of the roadside flowering plants were
being destroyed by the use of s•el'ective weedkillers. Hints to farmers given
by the Ministry of Agriculture and quoted by Trow (1961) in the "Daily Express"
included :- "Avoid letting sprays and dusts drift onto hedgerow flowers or
neighbouring fields where bees are foraging".
Linsley and MacSwain (1947) considered that seed production of lucerne
in California was especially dependent on the pollinating activity of wild
bees, and recommended their encouragement by allowing uncultivated strips to
remain near fields. Both these authors and Knowles (1943) considered it
advisable, however, to eliminate more attractive weed flowers (e.g. Sonchus
arvensis L. and Chamaenerion spicatum) at the same time as the flowering of
lucerne.
The dangers of poisoning bees by applying insecticides to roadside and
other wild flowers, either as part of an intentional control measure on uncultivated land or as drift from a crop spray, also apply to other
beneficial insects visiting flowers. Even non-lethal doses may have effects 58. on such insects. Flanders (1943) found that hymenopterous parasites which had received small doses of sulphur insecticide lost and never recovered their ability to recognise scale insects as hosts.
59.
SECTION II.
PRELIMINARY EXPERIMENTS.
1) GENERAL STUDY OF INSECT POPULATIONS IN FIELD AND HEDGEROW.
Sites.
3 sites were chosen for a general study of insect populations in field and hedgerow. The plant species present were recorded in the notation given by Sankey (1958)*. Site (a) lies in the middle of a large area of London
Clay overlain to the south by the Bagshot Beds in which sites (b) and (c) are situated.
(a) Foliejohn Park. 150 yards (map ref. SU 903752) of the verge and hawthorn hedge along the south side of "Drift Road" (near Fifield, Berks.).
The vegetation was cut back regularly; consequently there were few flowers and no tall vegetation except for the hedge itself. The site was chosen as an example of a managed hedgerow. The plant species present were:-
d Crataequs monoclym Jacq., d Holcus lanatus L., a Festuca rubra
f Anthriscus sylvestris (L.) Bernh., f Aqrimonia eupatoria L., f Aqrimonia
eupatoria L., f Lotus corniculatus L., o Rubus sp. (subgEmus Rubus)., o Rumex acetosa L., o Trifolium repens L., o Urtica dioica L., r Quercus sp. - small shoots in the hedgerow, r Chamaenerion anqustifolium (L.) Scop.,
✓ Cirsium vulqare (Savi) Ten., r Taraxacum officinale egg., r Geranium pretense L.
(b) Silwood Park Farm. 150 yards (map ref. SU 939692) of the grassy
bank on the south side of the sunken lane to Silwood Pork Farm from the road
* d = dominant, a = abundant, f = frequent, o = occasional, r = rare,
vr = very rare. 60. linking A 329 with B 383 through Cheapside. The vegetation was left uncut throughout the year and consisted of small trees, shrubs and numerous flowers
(especially tall and vigorous Umbelliferae). The site was chosen as an example of a wild hedgerow. The plant species present were :- d Salix caprea L., d Holcus lanatus L., a Festuca rubra L., f Angelica sylvestris L., f Cirsium vulgare (Savi) Ten.,f Rubus sp. (subgenus Rubus), f Trifolium pratenae L., f Plantago lanceolate L., f Lotus corniculatus L., f Crepis capillaris (L.) Wallr., o Quercus sp. - small scrubby plants, about
3' high, o Trifolium repens L., o Holcus mollis L., o Rumex acetosa L., r Rumex crispus L., r Achillea millefolium L., r Campanula rotundifolia L.
(c) Silwood Park Farm. The pasture field (map ref. SU 938692) border- ed by Site (b). The plant species present were :- d Holcus lanatus L., d Lolium perenne L., a Poa pratensis L., a Rumex crispus Lof Trifolium repens L., f Ranunculus repens L., o Trifolium pretense L., o Polvoonum persicaria L., r Cirsium palustre (L.) Scop., r Chenopodium album L.
Sampling methods.
1) Sweeping. Standard collections were made at each site at intervals over a period of 9 months (September 1955 - May 1956). Each collection consisted of 4 3-minute sweeps with a net of the pattern recommended by Smart
(1940). After each sweep the end of the net bag was shut into a jar containing a cotton-wool pad soaked with ethyl acetate, and the dead insects were sorted into collecting tubes. Each collection took approximately 30 minutes and was made, whenever possible, between 1400 - 1500 hrs. G.M.T. under conditions of sunshine and light wind. 61. In the laboratory the collected insects werd identified as far as necessary to place them in one of three biological groups :-
(i) Carnivorous - Insects which are at least for one stage of their life cycle predators or parasites of insects and other small arthropods.
(ii) Herbivorous * Insects feeding on living plants, but excluding the herbivorous stages of group (i).
(iii) Scavengers and insects parasitic on vertebrates.
Sweeping was considered the only available method for this study over a period of several months, although its limitations for quantitative work are very serious (cf. Beirne, 1955).
a) Only free-living insects are collected - miners, borers and internal parasites escape capture. It is, however, the free-living stages
which are probably most important in field-edgegrowth relationships.
b) Even for free-living insects the efficiency of the method varies
with their activity and their position on the vegetation at different times
and under different weather conditions.
c) The efficiency of the method also varies with the nature of the
vegetations rather different sweeping movements are required for sampling hedgerows and for sampling pasture.
To minimise these limitations of sweeping, large numbers of insects
were collected and only two simple numerical expressions of the data have
* - This distinction proved difficult in several groups - e.g. Miridae
were included in group (ii) although they take a considerable amount of
animal food (cf. Collyer 1952, 1953a, 1953b;Clausen 1940). 62.
been used for comparing the collections at the three sites (Fig. 2, p.66 ):-
i) The total number of insects collected by 12 minutes sweeping.
ii) The percentage of carnivorous insects in each collection (not
calculated when the total catch was less than 50 insects).
2) Emergence tins. The numbers of insects in the sweeping collections remained low from the end of November to mid-April, and a technique was
developed to investigate the overwintering populations more fully. Site (b)
- the hedgerow at Silwood Park Farm - was divided into 100 equal squares and
sampled at each of 17 points (determined by random numbers). Each sample
consisted of a deep 1' sq. turf together with the vegetation. Similar
samples were taken along two lines 50' and 100' out into the pasture field
(Site c) and parallel with the hedgerow. The turfs were cut in mid-April*
and kept in emergence tins in an outside insectory until mid-July.
These tins (Fig.IA) were based on a technique used by Wilbur and
Fritz (1939) but were designed to accommodate taller vegetation. A glass
funnel was sellotaped over a 2" diam. hole punched in the lid of a full-sized
biscuit tin. The method of fixing the funnel (Fig.1B) left gaps which were
too small to allow insects to escape yet prevented condensation inside the
funnel. Insects moving upwards towards the light were trapped in an inverted
collecting tube over the stem of the funnel.
* - The turfs were cut as late as possible before the insects became active
to lessen the time for which the vegetation was exposed to conditions inside
the tins,
63.
Besides numerous Collembola*, only 1 Ichneumonid and 1 Curculionid were found in an examination of all 51 turfs at the close of the experiment.
The insects trapped were counted and identified as in the sweeping collections (see p. 61). The results are presented in Fig. lc, compared with averages for September from the sweeping data.
Results. (Figs. 2 and lc, Table 4).
SEPIEMBBR . APRIL Average Per 12 thin. Emergence from 17 sweep. 1 ft. s . turfs. Site io.insects % carp. no.insects carn.I Cut hedgerow (a) 108.6 35.9 - - Wild hedgerow (b) 204.2 45.3 394.0 43.2 Pasture (overall) (c) 385.0 12.9 - - " 50' from hedge (c) - - 607.0 31.6 " 100' from hedge (c). 628.0 24.9
TABLE 4. Comparlson of data for late summer and winter.
Data from the sweeping collections suggest that, although the actual number of insects collected per 12 minute sweep was rather higher in the field samples (c) than in the hedgerow samples, the latter supported a higher percentage of carnivorous insects. The wild hedgerow (b) supported a larger number of insects and a higher percentage of carnivores than the managed hedgerow (a).
Collembola, which had presumably continued reproducing throughout the experiment, were not included in the results. 64.
Vote (1946) and Gy8rfi (1951) have pointed out the importance to predators and parasites of a mixed ground flora in forests; it is possible that the contrasts referred to above between sites a, b and c are expressions of the greater variety of hedgerow plants (both in number of species and in growth form) and the abundance of flowers in the wild hedgerow.
The same general trends are shown by the overwintering populations
(Fig. lc). A high percentage of carnivores was found to emerge from the hedgerow samples, decreasing towards the centre of the field. Fleabeetles
(Coleopteran Chrysomelidae) were found only in the hedgerow samples;
Moreton (1945) and Watzl (1950) record them overwintering at the edge of woodlands and migrating to crops in periods of sunny weather with light wind during April-May. b5.
B EMERGENCE TIN
trop Sellotope
A cork
funnel
10 cms
- 400 —40 tal U) CC z • 0
2 — 0 SEPTE BER APRIL I SITES b L
50 100' out out C. COMPARISON OF NUMBERS OF INSECTS SWEPT AND EMERGING FROM A FIELD AND THE ADJACENT HEDGEROW FIG. 1.
6b.
FIG. 2. COMPARISON OF NUMBERS OF INSECTS SWEPT FROM A
. •...... - •-• r •• .- . - . -
.1
- Site c 3X • 1,-
)" 0
10C,' 20
1YI -T I III T 1 1 r r— T 0
EP 400- KEY (f) Site NO. OF INSECT %CARNIVOROUS SWE
b
N. 300- 9 z 11. cc 12 MI E 40 8 ER
100- TS P EC S I 0 ••T 0 F IN O
O. 400 N Site a 300-
2C0- 40
100- -20
0,111111Er. 7 1 0 1 M I 1956 1955 A gust S 0 I J 1 F A May MONTHS 67.
2) EFFECT OF FLOWERS ON THE ACTIVITY OF PARASITIC HYMENOPTERA.
Schneider-Orelli (1945) has pointed out that parasitic Ichneumonids must feed on nectar to produce their full complement of eggs, and GyOrfi
(1943-44) records parasites settling on blossoms and feeding on nectar.
In a later paper (1951) Gy8rfi mentions two Umbelliferae (Daucus carota L. and Pastinaca sativa L.) as being most preferred, adding that where these plants flower in large numbers swarms of parasites congregate - especially
Ichneumonids and Braconids. The literature (see p. 40) contair additional records of parasites visiting flowers, and in view of the results of the general population study (p. 63) it was decided to investigate the effect of flowers quantitatively.
In 1956 a series of experiments was set up using white water traps
(see p.116 ), from which Hymenoptera in predominantly parasitic families were collected every few days. The results are analysed in Table 8, p.?5 ,
Numbers caught in such traps are regarded as a function of the number of insects in the area and their activity, for the traps only sample the aerial population and are likely to be selective in their attractiveness to parasites.
That white traps are attractive to parasitic Hymenoptera was indicated by a crude colour trial (in August 1956) in which six colours were tested in one arrangement only:-
White I Blue I Black Green Red Yellow
The top area of a water trap was partioned equally into six coloured portions, with the water in the dish continuous through large gaps in the partitions to stabilise temperature. The trap was set up at a height of 68.
2' an a grass verge at Silwood Park Farm.
The number of parasitic Hymenoptera trapped in the six portions of the trap is shown in Table 5. Black was assumed to be non-attractive for purposes of analysis.
TABLE 5. Single water trap colour trial.
Col. ref. Expected no. Colour Wilson 1941 No. tra.. d assuming attraction p' Significance White 11 Control >.99 Attractive
Enamel 48/1 2 11 <4,001 Non-attractive Blue Black - 2 11 4:.001 Scheeles 8 60/1 10 11 p.98 Attractive Green
Red 11 ›.99 Attractive 19 11 i (Scarlet) t Canary 2 11 11 ).99 Attractive Yellow
* Estimated by a simpleX test.
The results indicate that White, Green, Red and Yellow are more
attractive to parasitic Hymenoptera than Blue or Black.
Expt. A. (Site N.E. corner of Silwood Park, the verge beside
the tractor path - map ref. SU 947691). An area of grass (almost pure
Holcus lanatus L.) and one with small Umbelliferae (Anthriscus sylvestris
(L.) Bernh.) were chosen on the verge. Two water traps (1'6" apart) in
each area were set level with the top of the vegetation (which was
approximately 1'6" high in both areas). The experiment was divided
into two periods :-
(i) 14-20 June - Umbelliferae flowering.
(ii) 21-29 June - After flowering of Umbelliferae. The distinction 69, was made visually at a distance from the whole patch of flowers, and it was found that the patch was considered as "after flowering" when approximately 60% of the florets had lost all their petals.
Expt. B. (Site - Silwood Park Farm, a patch of Umbelliferae and nettles at the east side of the lane near the farm buildings - map ref.
SU 941692). One water trap was placed among the Umbelliferae (Angelica sylvestris L.) and.two control traps were placed in patches of neetles
(Urtica dioica L.) of the same height az the Umbelliferae (approximately 4'), one on each side of the flowers. The tfaps were set level with the top of the vegetation.
Expt. C. (Site - N.E. corner of Silwood Park, a three acre field of spring wheat (var. Atle) and the flowering verge along the east side of the field - map ref. SU 941692). The main plant species flowering on the verge were:- Rumex acetosa L., Taraxacum officinale egg., Cirsium vulgare (Savi)
Ten., Ranunculus repens L., Trifolium repens L.
Paired water traps 6' apart were fixed at two heights (regulated at crop level and 6') on slotted angle-iron posts both on the verge (sq. 59, Fig. 4, p.80) and at the centre of the crop (68 yards distant - sq. 44, Fig. 4).
The data were divided into three sections for analysis :-
During the flowering of the verge plants, 21 June - 17 August,
(i) traps at crop level
(ii) traps at 6'
(iii) crop level traps after flowering of the verge plants
(assessed visually) to harvest of the crop, 18 August - 17 September. 70.
Expt. D. (Site - Manor Farm, Foliejohn Park. A 1 * acre cabbage
field and hawthorn hedgerow on the west side - map ref. SU 897747).
Although the experiment was carried out late in the year, the following species of plants were still flowering in the hedgerow:-
Chenopodium album Lm Polygonum persicaria L., Achillea millefolium L.,
Senecio jacobaea L.
One pair of water traps was set level with the vegetation in the hedgerow
and one pair was set at crop level 45 yards out into the field.
,Expt. E. (Site - N.E. corner of Silwood Park, a 1 acre grass slope
(mixed Holcus lanatus L. and Holcus mollis L.) behind the lodge cottage,
Pairs of water traps 1'6" apart in a N.E, - S.W. direction were set up 1'
above ground level at three sites (all more than 50' from the edge of the
meadow and more than 200' apart:-
I. Near the N.E. corner of the area,
II. Near the S.W. corner of the area,
III. Near the centre of the area.
From 26 May to 1 June the three grass areas were compared for uniformity
(i); from 9 June the traps at site III were surrounded by a ring of 30
potted Umbelliferae (Anthriscus sylvestris L.)
Results of water trap experiments. (Table 8, p.75).
The results indicate that the activity of parasitic Hymenoptera is
significantly increased by the presence of flowers; most of the experiments
were conducted in vegetation of uniform height to eliminate effects caused
by the disturbance of air-currents.
Within areas of uncultivated land traps among Umbelliferae caught 71. significantly more parasites than traps in vegetation without flowers.
After the flowers had fallen there was no significant difference between catches.
A significantly higher number of parasites was trapped near the
flowering edgegrowth at the side of fields of wheat and cabbage than over
the crops themselves. This difference was not noticeable at 6' above the vegetation level and was less noticeable (though still significant)
after plants on the verge had finished flowering. These results suggest that differences are not solely due to air-current effects.
There was no significant difference in the number of parasites caught
in control and experimental areas on a grass slope until potted Umbelliferae
were introduced in the experimental area only. Numbers trapped in this area were then significantly higher than those trapped in the control areas.
Expt. F. The effect on the activity of parasitic Hymenoptera of
edgegrowth at the corner of a field was investigated in an additional
experiment (Fig. 3A). 3 water traps (Cl, B2, A3) were spaced along a diagonal line outwards from the corner of a pasture field at Silwood Park
Farm (site c, p. 60). Trap Al was placed near one hedgerow but at the same total distance from both hedgerows as trap B2. All traps were at a height of 1' above ground level; parasitic Hymenoptera were collected from
4 - 22 May 1956.
The number of parasites per trap (y) was plotted against the total distance from both hedgerows (x) and a linear regression line
(y = 214.38 - 1.0117 x) was fitted (Fig. 3B). The regression was tested by an analysis of variance, and was found to be significant (p = <.01). 72.
Numbers trapped (Table 6) decreased with distance from the
hedgerows, and were highest at the corner of the field.
TABLE 6. Activity of parasitic Hymenoptera at various distances from two hedgerows. , 1 Trap Distance from Distance from Total distance No, parasites N hedgerow (yds.) E. hedgerow (yds.) (yds.) trapped
Cl 3 3 6 205
Al 3 50 53 166
B2 25 28 53 162
A3 50 53 103 107
Wright and Ashby (1946) describe a similar effect with hedgerows
which provided shelter for gravid W of Psila rosae F.. Fly infestation
was highest at the field corner, where the effect of two hedgerows overlap.
Expt. G. If adult female parasites are attracted to flowers and
feeding affects ovarial development, one would expect to find a high
proportion of young females among parasites trapped near flowers. An
Ichneumonid (Mesochorus sp.) * was chosen to test this theory, and a few
dissections showed that immature and mature ovaries could easily be
distinguished. The immature ovaries dissected out easily, and the largest
egg was approximately 0.150 mm. in length. In other females, large eggs
(approximately 0.615 mm.in length) were clearly visible as soon as the
abdomen was opened; such ovaries were considered as mature for the purpose
of the experiment.
' The parasites used in the experiment came from the water trap catches of
Expt. C. (p.69) for the period 21 June - 21 July 1956.
73. A. ARRANGEMENT OF WATER TRAPS IN PASTURE FIELD ,
SCALE 1 10 yds. 1
A3 • A10
B2 •
C1 •
II
18
b=1.0117 160 0 w a. :}t 14 I--x Lou' r-,-; 120 Q a.4
Lci5 DO
9 ••• o o ' 20 40 6-10 80 160 yards TOTAL DISTANCE FROM BOTH HEDGE ROWS B. REGRESSION OF NO. OF PARASITIC HYMENOPTERA TRAPPED ON TOTAL DISTANCE FROM BOTH HEDGEROWS FIG. 3. 74.
25 females collected from traps on the flowering verge beside the wheatfield were dissected and the ovaries examined. A further 25 females from traps at the centre of the crop were also dissected - as a control, for it seemed possible that the catch of white water traps might be selective for immature females. The results (Table 7) show that a significantly higher proportion of females with immature ovaries was trapped on the flowering verge than at the field centre.
TABLE 7. Ovarial condition of Mesochorus sp. 99 trapped near flowers and at the centre of a wheatfield.
No. 9? with % 99 with Item Number dissected immature immature ovaries ovaries
Situation of tra Flowering verge 23 92.0 25 Centre of crop 7 28.0 25
Total 30 60.0 50
Standard error of difference 19.2 P 4.001 1 75.
TABLE 8. Analysis of series of parasite activity experiments.
EIPIL. .purationi CONTROL Exp. EXPERIMENT P Si o, of expt. Site 1 No. Site No. A i 14/6 to Grass (aggregate of 2 28 28 Flowering Umbelliferae 48 <,:4,001 20/6 traps). (aqgreqateof 2 tra.s . ii 21/6 to Grass (aggregate of 2 . 62 62 Umbelliferae, after flower- 48 "›.05 29/6 traps). inq (aggregate of 2 traps). ,1 :001 +++ B 22/8 toNettles (2 traps). 106 109.5 l' Flowering Umbelliferae 1 147 <.- 27/9 . 113 (1 trap) I C 1 21/6 to Wheat, crop level (aggr.of 267 267 , Flowering verge, crop level 372 Ar„.001 +++ 17/8 2 traps). aggr. of 2 traps). ii 21/6 to Wheat, 6' (aggr. of 2 148 148 Flowering verge, 6'- (aggr. 126 3..05 - 17/8 traps). of 2 traps). iii 18/8 to Wheat, crop level (aggr. 115 115 0 Verge after flowering 150 4f.01 ++ 17/9 of 2 traps). (anqr, of 2 traps}. s.›.001 D 4/10 to Cabbage field aggr. of 33 33 Hedgerow (aggr. of 2 traps). 100 G.001 +++ 14/11 2 traps). i 26/5 to Grass areas (aggr. of I 42 37.5 Grass area III 47 77.01 -- 1/6 2 traps). IT 33 (aggr. of 2 traps). ii 9/6 to Grass areas (aggr. of 11.26 124 Grass area III with Umbell- 185 <7.001 +14 23/6 2 traps) . IL122 iferae (agar. of 2 traps).
Key. No. = No. of parasitic Hymenoptera trapped.
Exp. = Calculated expected value for experiment on a nul hypothesis.
Sig. = Significance at the 5% level.
P was estimated by a simpleg test. 76.
3) EXPERIMENTS WITH SPECIFIC INFESTATIONS.
The results, of the experiments on edgegrowth described earlier suggested that there was some justification for proceeding with a series of experiments on crop infestations. In 1956 and 1957, populations of phytophagous insects and their predators and parasites were studied on a variety of crops (including pasture) to demonstrate any effects of the proximity of uncultivated land.
Expt. A. (Site - Meadow at New Park Farm, New Forest. - map ref.
SU 293047). 7th July, 1956.
An area of Dactylis glomerate L. was chosen in the west corner of the field. The corner was bordered by deciduous woodland and grassy banks with abundant flowering Rubus (subgenus Rubus) sp.. At each of 16 stations in the field 10 flower heads of D. glomerate, taken in a circle within arm's reach, were sampled for Macrosiphum (Sitobium) avenae F. and aphid predators and parasites. The number of aphid colonies on the 10 flower heads for each station is given in Table 9a.
A linear regression (y = 30.54 - 12.8566 t 1.276x) of the number of colonies per station (y) on log. combined distance (yards) from both edges (x) of the field was tested by an analysis of variance and found to be significant (p = <%001), indicating that the infestation in the field decreased with the combined distance from both edges. Fidler (1949),
Broadbent et al. (1951) and Taylor and Johnson (1954) all refer to high infestations near the shelter of trees and tall hedges (see p, 45).
The stations really close to the shelter ( 8 stations on tne grassy 77.
banks and possibly also the corner station - with 13 colonies) did not
show a maximum infestation, in contrast with a previous experiment on
parasitic Hymenoptera (p. 71) and the work of Wright and Ashby (1946) on
Psila rosae F.. This may be expected, for the elate aphids initiating the
infestation were deposited from air-currents against the barrier formed by
the woods (Broadbent et al., 1951) whereas the distribution of parasitic
Hymenoptera was probably due to movements at or near vegetation level.
Psila rosae F, infestation is the result of adult female flies leaving the
shelter of the edgegrowth during the day for oviposition (Williams, 1954).
The number of aphid predators and aphid mummies found in each sample
of 10 flower heads is recorded in Table 9b and c. Numbers were rather low,
and a simple analysis (Table 10) was attempted by pooling the predator and mummy totals for each of three blocks of stations :-
Block H. Stations on the grassy banks.
I. Stations within 15 yards of either edge.
II. Stations more than 15 yards from the edges of the field.
There appeared to be a significant decrease in the number of predators and aphid mummies away from the grassy banks, although these yielded fewer aphids than the meadow.
Expt. B. (Site - N.E. corner of Silwood Park, a three acre field of spring wheat (var. Atle) and the adjacent uncultivated land - map ref.
SU 941692).
1) 18th July 1956. A 100-square grid was superimposed on the field area (Fig. 4) and squares were sampled in three areas:- 78. TABLE 9. Numbers of aphids and aphid predators at the corner of a Dactylis qlomerata L. meadow. Wood 9 6 8 4 Bank (5) S .W. wind . 5 (5) 13 16 7 5 (15) 4 6 Wood 9 17 (15) 15 e 10 8 3 2 5 16 6 11 3 Bank a. Number of Macrosiphum avenae F. colonies (Distances in yards between stations are given in brackets).
2 3 2 0 0 1 5 2 0 2 1 0 0 0 II 5 1 5 1 1 0 2 0 0 5 0 :1 0 0 H I 1 II b. Number of aphid predators. 1 0 1 1 1 0 3 1 1 0 0 1 0 0 0 I 0 0 0 i 0 3 0 0 r 0 0 0 II c, Number of parasitised adult aphids (aphid mummies).
TABLE 10. Distribution of predators of Macrosiphum avenge F.
Block No. predators No. stations. Expected no. Significance and mummies. predators and of mummies. difference (p)
H 25 8 16 < .05
I 18 7 14 <.01 II 5 9 18 C' .01) 79.
1) 20 squares on the NNW side of the field, adjacent to a dense deciduous copse.
2) 20 squares along the other three sides of the field, adjacent to rough grassland.
3) 15 squares at the centre of the crop.
In each square sampled, 20 ears were examined for Macrosiphum
(Sitobium) avenae F. and predators and parasites of the aphid. The data are summarised and analysed in Table 11. No significant difference in the number of aphids was observed between areas 2 and 3, but significantly fewer aphids were found adjacent to the copse (Area 1). A possible explanation for this contrast with earlier results (p.76) may be the position occupied by the shelter. For May, June and July 1956 the prevailing wind at the wheatfield site was parallel to the line of shelter.
In an earlier experiment (see Table 9a, p.78) shelter parallel to the prevailing wind had less effect on aphid distribution that shelter at right angles.
At the edges of the wheatfield adjacent to rough grassland with abundant flowering herbaceous plants (see p. 69) the numbers of parasites and predators were significantly greater than at the centre of the crop.
No significant differences were found in the percentage parasitism of the aphids in the three areas. FIG.4. WHEATFIELD (1956) - SAMPLING PATTERN.
= Aphid sampling
0 = Sawfly larva sampling
APPROX. SCALE E 20 yds. TABLii 11. Analysis of Macrosiphum avanae F. samples on wheatfield.
Item No. sampling o/ sampled Area stations No. Exp. s p Sig. Aphids 2 20 1788 2 1790.3 3 1342.7 )0' .99 3 15 1345 3 1199.1 < .001
1 20 1453 1 1598.9 +++ Parasites 2 20 82 2 52.6 + +++ and 3 . 39.4 - .‹.001 Predators ,., 3 15 ':10 3 14.1 „...20 1 20 23 1 13.9 + ___
No. adults and I mummies P(s) Parasitised 2 267 67 25.1 2 3.S >. .20 ••••••••• adults 3 + (aphid 3 284 85 29.9 3 mummies) 3.9 1 222 51 23.0 1 Area = see p. 79. d = Nature of difference (+ or -). Exp. = Calculated expected value (on a nul hypothesis). No. = Number observed in experiment. P was estimated by a simple X test. p(s) = p estimated from the standard error (Arkin and Colton, 1929, Table 32a). so/6 = standard error of difference, Sig. = Significance at the 5% level. 82. 2) 25th July 1956. As Macrosiphum avonae F. is an aphid polyphagous on Gramineae (B5rner, 1952), both the wheatfield and the rough grassland along its SE edge were searched for the aphid and aphid mummies.
Wheat, Holcus lanatus L. and Dactylis glomerate L. were sampled by examining up to 50 inflorescences in areas where three estimates of stem density per
1' sq. ft. quadrat were within 10% of their mean.
Aphid infestation was assessed by an arbitrary classification of stems
(modified after Banks, 1954);-
0 Zero (no aphids seen)
V = Very light (from one aphid to a few not as a distinct colony),
L = Light (one distinct colony).
M = Medium (more than one colony, the colonies separate and distinct).
H = Heavy (aphids present in large numbers, massed on the inflorescence ).
In doubtful cases the higher of two possible categories was recorded. Aphid mummies were recorded with the primary parasite (Aphidius spp. or Praon spp.)
identified by the appearance of the mummy (Dunn, 1949).
The data are given in Table 12.
Both Macrosiphum avenae F. and Aphidius cocoons were found on all
three species of Gramineae sampled, but no Praon coccoons occurred in the
samples of wild grasses. An attempt was made to assess whether the distribution
of Aphidius mummies indicated a preference for wheat by the parasite. Such
a preference would reflect on the importance of grassland as a parasite reservoir.
If the frequency of mummies in wheat and grasses were similar, a
simple relationship might be expected between stem density irrespective of 83. plant species and the number of Aphidius cocoons per sq. ft. A linear regression (Fig. 5), y (no. of cocoons) ='0.749,+ 0.2444 (t0.019 x (stem density over the range of 2-49 stems per sq. ft. was tested by an analysis of variance and found to be significant (p =4.001). This range of stem densities was chosen as, on the wheat field, the number of cocoons per . sq. ft. was considerably reduced at densities of 58 and 71 stems per sq. ft.
The regression suggested either that the parasites showed no preference for wheat or that the aphids were markedly more abundant on the grasses.
A second regression calculation confirmed the former suggestion.
An "infestation index" was calculated from the stem classification as
follows:-
Infestation index = (No. of "V" stems) + (2x No. of "L" stems)
+ (4x No. of "M" stems) +(8x No. of "H" stems).
By dividing this index by the number of parasitised aphids found, a
value indicating the relative degree of parasitisation at each stem density
was obtained ("Infestation index" per mummy, Table 12). The logarithm
of this value (y) gave a linear regression against stem density, again
irrespective of plant species (y = 0.146 + 0.0159 (t 0.0098)x). The regression was again significant (p = (.05).
As in the two above regressions both the grassland and wheatfield
data fitted a common line, it was concluded that there was no evidence of
a preference fox wheat by searching Aphidius females. The results indicated
the possibility that the wild grasses adjacent to the field might form a
useful reservoir of overwintering Aphidius cocoons in spite of the compara-
tively low density of inflorescences. This parasite reservoir was estimated
quantitatively in experiment 3. 84.
FIG. 5. REGRESSION OF Aphidius COCOONS PER SQ. FT. ON STEM DENSITY
x Dactylis glomerata L.
o Holcus lanatus L.
• Wheat
b = +0.2444
•
o o •
10 20 30 40 50 60 70 STEM DENSITY PER SQ. FT. TABLE'12. Number of Macrosiphum avenae F. on Wheat, Holcus lanatus L. and Dactylis glomerate L. - 25th July,1956.
Stem density No.stems No.stems with aphid Aphid munmies with primary parasites:- Host Plant per sq. ft. sampled infestation of:- Infestation Aphidius - per sq.ft. Infestation index Praoli 0 V L M H index per mummy*
Wheat 71 50 28 9 11 2 0 39 1 1.4 39 0 58 50 22 17 9 2 0 43 2 2.4 21.5 4 49 50 27 20 3 0 0 26 15 14.7 1.7 2 39 50 23 20 7 0 0 34 11 8.6 3.1 0 16 50 28 9 11 2 0 39 19 6.7 2.1 2 i Holcus 40 50 8 17 16 7 2 93 10 8.0 9.3 0 lanatus L. 15 50 28 10 8 4 0 38 17 4.5 2.2 0 6 50 28 10 6 5 1 50 15 1.5 3.3 0
Dactylis 2 20 15 5 0 0 0 5 3 0.3 1.7 0 glomerate L.
* See text. 86.
3) On 25th July 1956 the stem density on the wheatfield was estimated by counting the number of stems in 16 1 sq. ft. quadrats chosen at random, The mean stem density was calculated as 48.38 - 7,06 stems per sq. ft. The area of the wheatfield was calculated as 118746 sq. ft.
With these results it was possible to estimate the number of
Aphidius cocoons on the wheatfield using the results of expt. I (Table 11, p. 81) corrected from Table 12. for the proportion of Praon cocoons
(Aphidius accounted for 86.0 32.3% of the total number of cocoons). This estimate was 990,28203 ± 61% (a mean of 4.011 - 0.69 cocoons per 20 stems) which agreed well with an estimate of 962,584 - 55% cocoons calculated from the individual stem densities of the 16 quadrats and the regression line for Aphidius cocoons from an earlier experiment (Fig. 5, p. 84).
It was found difficult to attempt a similar estimate of the number of Aphidius cocoons on the wild grasses because of the overlapping and patchy distribution of the various grass species. A technique was evolved for estimating the number of parasites emerging in spring from sample areas in the grassland. Most of the old inflorescences had died down by the end of the year and no parasite cocoons were found in an examination of 200 inflorescences in February 1957. However, parasites emerged from samples of ground litter collected with a vacuum sampler (see p.87) and kept at
23°C and 68% R.H. in a constant temperature room. It seemed likely that these parasites had emerged from cocoons which had fallen off the old grass inflorescences during the winter.
The rough grassland adjacent to the SE edge of the wheatfield was divided into 50 squares of a 20 ft. grid, In each square samples were 87. 1 taken at 3 points determined by using random numbers * on a 2 ft. grid of the square. Samples were taken with a vacuum sampler (equipment and technique as described by Johnson, Southwood and Entwistle, 1955); the three samples from each square were combined and kept in an emergence tin (p. 62) at 23°C and 68% R.H.
Table 13 shows the number of parasites emerging from 3 1 sq. ft. samples on a diagrammatic grid of the area sampled.
TABLE 13. Number of parasites emerging from wild grassland. ( ) = Charips and Asaphes (hyperparasites). ) = Aphidius (primary parasite).
0 Area of grassland - 32,400 sq. ft. Mean emergence per 3 sq. ft. - Aphid parasites - 1.6 -40.47 2 Aphidius - 0.76 - 0.41.
2 3
2 1 2 0 1
3 2 1 3 1 1 1 3
0 5 1 0 1 ' 0 1 2
1 1 1
Wheatfield The emergence of aphid parasites in spring from the wild grassland
(an area of 32,400 sq. ft.) was calculated as at least*2 17,280 ±29%, of which 8,208 - 54% would be Aphidius adults.
In comparing the parasite productivity of the wheatfield and wild
1* See note d), p. 88. 2* The values are not corrected for sampling efficiency, which is unlikely to be below 75% - see table 1, Johnson et al., 1955. 88. grasses, several considerations require mentions-
a) The productivity of the wheatfield was measured while the crop was still standing, and many cocoons must be destroyed and removed by harvest and ploughing in of the stubble before spring.
b) Table 14 shows the results of breeding out (at 23°C and 68% R.H.) parasite cocoons from wheat and wild grasses. Although the number of
cocoons used was very low it seems likely that a high proportion of Aphidius
(about 70%) are hyperparasitised by Lyqocerus.
Lyqocerus adults did not emerge from the vacuum samples of wild
grasses. Dunn (1949) has suggested that this genus overwinters in an
immature stage in cocoons of other hosts or may survive as adults. Cocoons
which would produce Lyqocerus as well as non-emergences* are included in
the wheatfield calculation.
c) Fraon did not appear to attack aphids on the wild grasses, and
cocoons of this genus are not included in the wheatfield totals.
d) Emergencesfrom the vacuum samples of wild grasses may include
parasites of aphid species other than Macrosiphum avenae F., though
significant numbers of other species were only noticed on Rumex acetosa L.
In using random numbers to determine sampling positions points within 3 ft.
of R. acetosa plants were not used.
* The non-emergence rate of Table 14 may have little relation to the rate in natural conditions. 89.
TABLE 14. Hyperparasitism of Aphidius cocoons from Macrosiphum avenae F.
Item Wheat Wild grasses
No. cocoons bred 28 26 No. emergences 22 23 % non-emergences 21.4 11.5
No. Aphidius 1 1 % of emergences 4.5 4.3
No. Lyclocerus 16 15 % of emergences 72.7 65.2
No. Charips and Asaphes 5 7 % of emergences 22.7 30.4
% hyperparasites 95.4 95.6
Expt. C. (Site - as for an earlier experiment p. 77). 13th - 27th
June, 1956.
A survey was made of sawfly larvae in the wheatfield. The most abundant species, Dolerus haematodes Schr., was also found on wild grasses, apparently feeding on Holcus lanatus L. Lorenz and Kraus (1957) record this species as polyphagous on Gramineae. As it was not found possible to distinguish the different instars, three size categories were used to describe the stage of growth.
1) 18th June, 1956. Series of size 3 larvae collected from wheat and Holcus lanatus L. were used in a feeding trial. The leaves of 3 plant species were offered: (A) Wheat and (B) H. lanatus as apparently acceptable, and (C) Trifolium repens L. as a presumed non-acceptable species.
The aim of the trial was to establish whether the uncultivated grass was markedly less acceptable to D. haematodes than the crop. This might effect the importance of the grass as a sawfly reservoir in the absence of the crop. 90.
The trial was based on a type described by Richards and Waloff (1954,
p.11 ). Approximately equal areas of each leaf* were put in 3 x 1 inch
specimen tubes. A larva was added to each tube and the tube closed with
cotton wool. The foods were made up in 2 tubes for every possible
combination:- ABC, AB, AC, BC, A, B, C. This gave 24 tubes in each series,
with each plant offered four times. After 24 hours the tubes were examined
and the foods offered were scored 0-3 according to a visual estimate of
the quantity eaten :-
0 = No signs of feeding.
1 = Feeding just recognisable a small excision of the leaf edge.
2 = A definite strip of leaf tissue eaten.
3 = Large amounts eaten - the leaf attacked in several places.
The scores are given in Table 15. The two species of Gramincae
appeared to be almost equally acceptable and preferred to Trifolium, though
in each series the species from which the larvae had been collected showed
a slightly higher score.
Dolerus haematodes Schr. has been recorded as a pest of cereals in
Britain (Roebuck, 1922). The results of the feeding trial suggest that
wild grasses may be important in maintaining populations of the solidly under conditions of crop rotation.
When wheat and H. lanatus were offered together, the Holcus leaf was distinguished by a small mark with a ball-point pen. Care was taken to cut leaf areas with straight edges to facilitate the recognition of feeding marks. 91. TABLE 15. Feeding trial with Dolerus haematodes Schr. larvae. A - Wheat. B - Holcus lanatus L. C - Trifolium repens L.
Larvae from Wheat Larvae from Holcus
(1) Food scored (1 ) A :B C A B C (2) Food A 0+2 0+0 0+0 A 2+3 1+1 0+0 offered in (2) B 3+0 2+1 0+0 B 2+0 2+2 1+1 combination with (1) C 2+1 2+2 1+0 C 0+0 3+3 0+0 ABC 2+2 1+2 0+0 ABC 3+0 0+2 0+0
(3) Total score 12 10 1 (3) 10 14 2
2) 27th June, 1956. The wheat crop was sampled for larvae of
Dolerus haematodes Schr. on the same grid used for aphid sampling in another experiment (Fig. 4, p.80). 50 sampling points were determined using random numbers and at each point two samples were taken, making a total of 100 samples. Each sample consisted of a semi-circle swept in 20 traverses of a sweep net. Each 2 samples swept a circle 7 ft. in diameter; each sample therefore covered 19.2 sq. ft. of the crop (total area of crop
= 118746 sq. ft.). Sweeping was considered the only feasible method of sampling the crop though it was realised that larvae might escape by dropping to the ground. 27 similar samples were taken at random on the adjacent wild grassland (area = 32,400 sq.ft.) on the SE side of the wheat-
field. A warm fine day was chosen for sampling; 50 samples taken a week earlier on a dull day contained a significantly lower number of larvae per sample. The data are given in Table 16. TABLE 16. Estimate of the number of Dolerus haematodes Schr. 13rvae on the wheatfield and on wild grasses.
Wheatfield Wheatfield Wild grasses Item (dull day) (fine day)
Area (sq. ft. ) 118746 32400 Number of samples 50 100 27
No. larvae captured 21 65 18 ± Mean no. of larvae per sample 0.42 ± 0.19 0.65 ± 0,20 0.67 0.41
Significance of difference of means at the 5% level (p = <.001) --- (p =1P.80) + + Estimated total population 4011.2 - 31% 1122.5 - 61% 93. No significant difference in the density of larvae was found
between the field and the wild grasses. The field samples were separated
into larvae from the edge and larvae from the centre (the dividing lines used were nos. 2 and 8 on Fig. 4). The mean density of larvae was estimated
as .67 per sample at the edges and .60 per sample at the centre of the crop,
but numbers were so low that the fiducial limits of these estimates were
greater than ± 100%.
Larvae of size 2 collected from the edges and centre of the wheat-
field and from the wild grasses were examined for parasites. The results
are given in Table 17. There was no significant difference between the
three areas when all types of parasites were counted, and it was noticed in dissection that only 2 out of 6 types observed in the field samples were
found in larvae collected from the wild grasses. It was outside the scope
of this preliminary experiment to attempt an identification of the immature
parasites or even to determine whether some types were just different stages
of the same parasite species. Attempts to breed adult parasites from larvae
failed.
The most frequent parasite type (type 4) was found in larvae from
both wheat and grassland. Percentage parasitism by this type was significantly higher (p =(.05) at the edges of the field than at the centre. The larvae
from wild grasses also showed a higher percentage parasitism by type 4 than
the centre of the crop, though the difference was just less than
significant (p =.7.05).
It was noted that among the parasite types not found in larvae from
wild grasses were oval eggs (2 types) laid externally on the sides of the thorax near the head of the larva. TABLE 17. Parasitisation of Dolerus haematodes Schr. larvae on the .eheatfield and on wild grasses.
Wheat Wheat Wild grasses Item (Edges) (Centre) Number larvae examined 62 43 59
No. parasitised 20 8 10 % parasitised 32.2 18.6 16.9
Standard error of difference 8.4% 7.6% P 7,.10 7.80 Significance at the 5% level ---
No. parasitised by type 4 9 1 7 % parasitised by type 4 14.5 2.3 11.9
Standard error of difference 5.8% 5.3% P 4'45 -., .05 Significance at the 5% level + 95. Expt. D. (Site - Manor Farm, Foliejohn Park. A acre cabbage field - map ref. SU 897747). 8th October - 14th November 1956.
Series of 15 colonies of the Cabbage Aphid (Brevicoryne brassicae L.) were chosen at random from within 15 yards of the edges and from the centre of the field. For each colony the plant was identified by a numbered stake, and the leaf carrying the colony was marked by a transparent plastic "clothes- peg" clip. The colonies were visited weekly and the following records taken for each colony :-
a) Number of aphids in the colony.
b) Aphid mummies. These were collected and the adult parasites bred out (see Techniques, p. 119).
c) Number of aphids attacked by entomophagous fungi,
d) Clear signs of predatory activity.
From these records it was possible to follow the progress of the marked colonies over a period of several weeks. The limitations of the technique are discussed under "Techniques", p. 113. No significant difference between the series was found in the percentage loss through factors classified as "Various". The cabbages in this experiment were planted in August and had been colonised by the summer elate generations from other fields. No alate nymphs were observed during inspection of the colony.
The results are given in Table 18. A significantly higher percentage of aphids was lost at the edgesof the field than at the centre. This appeared due to the action of parasites and predators, though the difference in the percentage taken by parasites was not significant. The significantly greater mortality through fungus at the centre of the crop is not explained 96. but was again recorded at a different site the following year (p.228).
Data from the breeding out of aphid mummies are given in Table
19. Adult parasites emerged from about 70% of the mummies taken in for breeding. Over the whole field area, there appeared to be about 50% hyperparasitisation of Aphids. . No differencesbetween the field edges and centre were statistically significant because of the low total numbers
of parasites, but there was a slight indication that Aphididdae were more heavily attacked by Charips at the centre of the crop,
TABLE 18. Marked colonies of Brevicoryne brassicae L. . 8th Octobel - 14th November 1956.
j: p was estimated from the standard error. a% = standard error of difference. Sig. = Significance at the 5% level. Within 15 yds. Centre of Item of field edges. field p Sio.1 Total number of aphid individuals involved in the experiment 276 180
Loss of aphids by: - Various No. 51 46 % 18.5 25.6 ! 3.9 ).05 ! - 1 Predators No. 179 94 1 64.9 52.2 i 4.7 <.01 1 -44 1 Parasites* No. 39 17 , % 14.1 c.4 3.1 ).10 1 ...... Fungus No. 6 13 1 I 2.2 7.2 i 1.9 Key: p was estimated from the standard error. s% = standard error of difference. Edge of Cente of Item Total field field s% Aphid mummies collected for breeding 59 39 j 20* Parasites emerging No. 41 I 27 14 69.5 ; 69.2 70.0 1 12.7 ).90 Aphidiidae (Diaeretus?) No. 21 16 5 % 51.2 1 59.3 35.7 16.4 ").10 Asaphes No. 6 i 4 2 i 14.6 i 14.8 14.3 1 11.6 ).90 i Charips No. 14 7 7 % 34.2 i25.9 50.0 ! 15.6 ).10 % hyperparasitisation of Aphidiidae 48,8 I 40.7 64.3 16.4 1 )..10 * Mummies were collected in the course of inspecting the marked colonies. The additional three mummies from the field centre were collected on the day the marked colonies were set up. 99. 4) DISCUSSION OF PRELIMINARY EXPERIMENTS, The preliminary experiments described in this section were conducted in a variety of situations with a variety of insect groups and insect species, It has been possible to demonstrate three types of relation between uncultivated land and crop infestation which are likely to be of general application:- a) Uncultivated land as an insect reservoir. Uncultivated land (especially if not regularly cut) appears to support a high percentage of carnivorous insects and may serve as a general reservoir of predators and parasites. Specific examples of the importance of this reservoir can be demonstrated where the carnixforous insects or their pest hosts are polyphagoucs and the crop is absent from the area for a year or more (crop rotation) or just for a season (harvest). The feature of parasites overwintering on uncultivated land adjacent to fields is probably of special importance, as in some cases (e.g. Aphid parasites) they are likely to remain behind in parasitised hosts when the host insect overwinters elsewhere, Uncultivated land serves equally as a reservoir and overwintering site for injurious species (e.g. sawflies such as Dolerus, fleabeetles). Many similar examples are recorded in the literature. b) The im ortance of flowers in the edgeerowth to parasites and predators. In the preliminary experiments this aspect has been stadied on parasites. The distribution of predators on crop areas and work by Schneider-Orelli (1945) and Zoebelein (1956) on adult feeding indicate that flowers may also be of importance to many predators. The attractiveness of flowers to parasites has been demonstrated in 100. a number of situations. The dissection of female parasites suggests that, in some cases, adult feeding affects ovarial development and egg maturation as well as extending the life of the adult. It is suggested that the presence of flowers on uncultivated land adjacent to a crop may be important in the biology of the parasites and predators of pest insects on the field. c) The effect of edoeorowth on the distribution on the crop of injurious species and their predators and parasites. Apart from acting as a natural insect reservoir, the edgegrowth may affect the distribution of insects on the crop - partly as a result of movement to and from the edgegrowth and partly due to the physical resistance of tall vegetation to air-currents. The number and activityof predators and parasites appears frequently to be greater at the edges of a crop near the flowering verge than at the centre. This feature has been observed even where the uncultivated vegeta- tion is of similar height to the crop and therefore unlikely to disturb air-currents to the same extent as a sheltering hedge or strip of woodland. It is suggested that this distribution of parasites and predators is connected with the feeding requirements of the adults and with the presence 'of flowers in the edgegrowth. Therd are numerous records in the literature (see p.44 ) of the effects of shelter at the sides of fields on insects held in air-currents. Edge concentrations of Aphids associated with tall vegetation acting as a barrier to air-currents have been observed in the course of the preliminary experiments. 101. The review of the literature and the whole series of preliminary experiments broadly suggest that the relation between uncultivated land and the insect fauna of a crop may be divided into two components:- a) Biological. - The botanical components of the uncultivated land (with the insect species they support) and the place of the various plants in the life-history and biology of the insects found on the adjacent crop. b) Physical. - The shelter and shade caused by the height and density of the edgegrowth and the effect of these factors on insects on the crop and those moved by air-currents. 102. SECTION III. MAIN PROJECT Infestation by the Cabbage Aphid (Brevicoryne brassicae L.) of cultivated and edgegrowth crucifers. 103. PART I. TECHNIQUES 1) EXPERIMENTAL SITE (at Silwood Park) map ref. SU 941684. The Field Station is situated on Bagshot Beds in a fairly heavily wooded district near Windsor Great Park, Berkshire. The field. A 14 acre field was chosen at Ashurst Lodge, between the laboratory buildings and the A329 road (Virginia Water to Reading), and situated at one corner of the Sunninghill crossroads. The field sloped gently from N.W. to S,E. The edgegrowth. To the NAI, the field was bordered by rough grassland with clumps of thistles. A solitary elm tree stood at this edge of the field. The grassland was 100 ft. wide, and was separated from the minor road to Ashurst Lodge by a dense deciduous copse. A 70 ft. wide strip of rough grassland and lawn separated the N.E. edge of the field from the buildings and orchard. The tractor path running down the centre of this strip was lined with a few tall oak trees. To the S.E. the field abutted on fallow land. The S.W. side of the field was separated from the main road by a narrow but tall strip of conifers and sycamore with some tall rhododendron bushes. Plant species present in the edgegrowth are listed in Appendix 1, p.292 The crop. Although the whole field was planted with crucifers, only the top 14 acres were set out for experiments (Figs. 6, P.106 and 7, P.107). 104. In both seasons (1957 and 1958) the field was planted with the following mixture of crucifer transplants:- Brussel3sprouts - about 6000 plants. Autumn cauliflower - about 1500 plants January King cabbage - about 1500 plants. Christmas Drummhead cabbage Only the sprout plants were used in experiments. One row of cauliflowers or cabbages was planted for every four rows of sprouts. Rows were planted 2'6" apart with a distance of 2' between plants. In 1957 the field was planted by hand, with rows running N.E S.W. In 1958 a mechanical planter was used, and this necessitated changing the direction of the rows to N.W - S.E., parallel to the slope of the field. 50 plants were examined during planting in 1958; no aphids or aphid predators were found. The field was divided into a 30 ft. grid by numbered posts along the verges; the numbered blocks (Figs. 6 and 7) show the areas for which data were compared:- 1 and 2 = Field edges adjacent to rough grassland. 3 = Field edge adjacent to shelter and shade. X = Field corner. 0 (1 and 2) = Centre of crop. Mustard plots. 10' square plots at the N. corner of the field were sown with mustard, swede and turnip to provide both alternative hosts for the aphids and a mass of flowers to augment a patch of Anthriscus sylvestris L in the shade of the oak tree on the N.E. side of the experimental part of the field. The plots were sown by hand. The density of plants, calculated 105. by counting plants within 4 1' sq. quadrats in each plot, was 31.92 - 31.50 per sq. ft., with fiducial limits of the mean of - 3.72. 18 plots were used in 1957 and 15 in 1958. Table 20 (p. 108) shows dates in the development of crop and edge- growth relevant to the experimental work. KEY TO FIGS 6 AND 7. a-m = Water traps X,1,2,3,0,01,02, = Crop sampling blocks met. = Sites of meteorological readings Bulb of thermograph w = Anemometer of wind recorder Edgegrowth sampling 1958 1-30 = Pair of marked colonies FIG.6. FIELD PLAN, 19.;7 00' MET. ,,-MUSTARD ■Paul kg c d ■ 1 2 3 4 Scale - ) II 30ft. b ... . gll . COPP 10 9 8 a I I rYi I 2 LL. 11 12 13 14 15 cz:L (-) Sq. 63 FLOWERS m 20 19 18 17 16 3 1• T .._ 4 g, 21 22 23 1 24 25 I 1 30 29 28 27 26 (1 0DEN .... . )0 ._ T cc 0 1 2 3 4 5 6 7 8 9 FIG. 7. FIELD PLAN ,1958 MUSTARD & a N 40)-- x iiiiiii4 Tikii TURNIP IF _ !WIMP • 0 id • Scale - • /C 1 • e 30ft. 1 89 • 2 • 2 ILI ,d b —§—d-"--P h .--FLOWERS 3 v i e k f 1 0 4 g' at Sq. 75 5 i; 4 1 2 3 4 5 6 fr 7 8 9 108. TABLE 20, Field Calendar. ...... Season 197 v'44Vvia J./VV Item White mustard sown in alternate plots 4/4/57 Mustard sown in White mustard and remaining plots 25/4/57 7/5/58 Green stone Turnip sown in empty plots Umbelliferae in 20/4/57 15/5/58 Umbelliferae in flower to 25/5/57 to 13/6/58 flower First mustard in 3/6/57 12/5/58 Swedes in flower flower to 20/6/57 to 10/6/58 Crop planted . 12-14/6/57 10-13/7/58 Crop planted Second mustard in 18/6/57 1/7/58 Mustard in flower to 17/7/57 to 25/7/58 flower Swedes sown in 10/8/57 alternate plots Field cut and . late Feb. ploughed in 1958 2) METEOROLOGICAL RECORDS. Daily records of a number of weather factors were kept in the form shown in Appendix II, p. 295. Fig. 8 shows the mean daily temperature (4 hourly means) and total daily rainfall data. Temperature. A continuous record of temperature at crop level was provided by a mercury-in-steel thermometer bulb housed in a radiation shield in square 80 (Fig. 6, t). The shield was kept at crop level by adjustment on a slotted angle-iron stake. The bulb recorded on the weekly chart of a Negretti and Zambra thermograph in the main recording site adjacent to square 80, '957 20 t! tE ~U 0 (} ~ t+ 2 a ~ \ ::0 :ra 1958 0- -3 ? 1 '-" 1----....20 1----....10 FIG. 8. RAINFALL & MEAN TEMPERATURE DATA - 1957 & 1958 110. Shielded maximum and minimum thermometers at a height of 1 1 3" were set up at the main recording site and at a subsidiary recording site (adjacent to square 03) in the shelter of the coppice on the S.W. side of the field. Wind speed and direction. A continuous record on daily charts was provided by a wind speed and direction recorder (described in Appendix III, p.296 ). General conditions. A daily assessment of the prevailing conditions for the past 24 hours was made at 1800 hrs. G.M.T. The following data was available from the records of the Silwood Park main meteorological sites- Total rainfall (mm.) - from Dines tilting siphon recorder on the front lawn, checked by M.O. gauge. Hours rainfall - from Dines recorder on front lawn. 3) CROP SAMPLING. Sprout plants were sampled at approximately four day intervals. On each plant sampled the heart and young leaves, one middle leaf and one lower leaf were examined (cf. George, 1957) and the following records kept for each leaf age on each plant :- 1) For Brevicoryne brassiaae L. and "other aphids". no. alatae no. apterous adults no. eggs no. nymphs presence of elate numphs no. aphid mummies (parasites not emerged) - these were removed and kept for breeding out. 2) No. adult parasites (identified to genus). 3) No. Syrphid eggs, larvae (recorded in 3 size categories) or pupae. 4) No. Coccinellid eggs, larvae (recorded in 3 size categories) or pupae. 5) No. and stage of other aphid predators. Season 1957. Sampling was commenced on 25 June 1957 and ended on 9 January 1958. Five sprout plants, chosen at random, were sampled in each of the 30 unnumbered squares on the field plan (Fig.6 ). On each occasion 150 plants were sampled. Season 1958. Sampling was commenced on 17 July 1958 and continued until 5 December 1958. Four sampling blocks of 250 plants (see Fig. 7) were marked out - consisting of either 10 rows of 25 plants or 25 rows of 10 plants. 30 plants were sampled * in each area, giving a total of 120 plants sampled. On alternate dates the centre block was sampled twice, giving a total of 150 plants sampled - the same total and similar distribution of samples as in 1957. For the remaining sampling dates the numbers of the centre block were doubled for comparisons with the previous season. On 1 September 1958 leaves examined in the first sample of the centre block were marked with a ball-point pen, and the second sample showed that only two hearts, one middle and two lower leaves had been sampled for a second time. Sampling error. The sampling error was estimated over the whole season directly from the replicated sampling of the central area in 1958 and using sY Working back and forward along the rows from a randomly chosen start, every fifth plant was examined until 30 sprout plants had been sampled. 112. groups of alternate squares in the central area as replicates in 1957. Table 21 shows the sampling errors for 30 plants sampled expressed as a percentage x t (p=1.05).x 100). Mean TABLE 21. Error of crop sampling. Season "other aphids" Brevicoryne brassicae L. 1957 ▪- 26.7% 1958 ▪- 24.6% 4) MARKED COLONIES. Season 1957. In each numbered square on the field plan (Fig. 6) two leaves with aphids were marked on separate plants with a celluloid tag slipped over the petiole. The plants were identified with stakes. At approximately four day intervals the colonies were examined and the following records kept- 1) Age of leaf (medium, old or senescent). 2) No. of alatae. 3) No. of apterous adults. 4) No. of nymphs (recorded in two size categories). 5) Presence of nymphs with wing pads. 6) No. of aphid mummies whose parasites had not emerged (classified as sessile Aphidius or tented Praon types - cf. Dunn, 1949). 7) No. of adult parasites visiting the colony. 8) No. of Syrphid eggs, larvae (recorded in three size categories) or pupae 9) No, of Coccinellid eggs, larvae (recorded in three size categories) or pupae. 113. 10) No. and stage of other aphid predators. 11) Any attack by entomophagous fungi. 12) Signs of predatory activity - staining of the leaf and sucked skins. 13) Signs of elate dispersal (skins cast by nymphs moulting to alatae). From these records it was possible to follow the progress of the marked colonies during the season, though the technique has certain limitations, 1) Aphids which both were added to the colony by reproduction and disappeared between inspections would not be recorded in many cases. The conclusions drawn from the data would only be affected if this error were not proportionately constant between compared areas. The error was reduced by separating the aphids into adults and two size categories of nymphs. 2) It was found impossible to apply the technique to the heart leaves of the plant, as the inevitable disturbance was found to dislodge a proportion of the aphids. This may have introduced errors in absolute values at least up to the end of August 1957, during which period most aphids were on the young leaves. Comparisons between areas, however, were thought to be valid in spite of this limitation. 3) Losses of aphids when the cause could not be clearly identified were recorded under "Various". A large proportion of these losses is probably due to climatic factors and senescence of the adults. Table 22 shows that there was no significant difference in the losses in this category between two areas of open field. This indicates that this technique for separating the factors causing losses to colonies is satisfactory. 114. 4) There is a danger of confusing aphids sucked by predators with skins cast in ecdysis. It was found possible to identify sucked aphids with the aid of a x10 hand lens; also a general indication was obtained from the appearance of the colony. Large numbers of cast skins combined with loss of aphids from a colony are common during the production of elates. 5) When no more aphids remained on a marked leaf, the first infested leaf found on plants nearby was marked and given the same number. It was hoped that by choosing leaves at random the error of having a mixture of ages in the colonies of each series would remain constant. Season 1958. The numbers of B. brassicae remained so low that a sufficient number of colonies for marking was not available for the period of predator activity. TABLE 22. Losses in marked colonies caused by the factors classified as "Various" -Season 1957. Number % No. of aphids Area lost loss in experiment 1 + 2 (Open edges) 586 14.53 4034 01 + 02 (Field centre) 564 16.13 3496 TOTAL 1150 15.27 7530 Standard error of difference 0.831 p = >.05 5) EDGEGROWTH SAMPLING. The plots of mustard, swede and turnip planted along the S.E. and S.W. margins at the N. corner of the field were sampled in both seasons, and records kept as for crop sampling (p. 110). 115. Season 1957- Mustard sampling (30 May_ - 31 July 1957). At approximately four day intervals, five plants were chosen at random in each plot and the flowers, leaves and stem examined (a total of 90 plants were sampled). Swede sampling (25 November 1957 - 10 January 1958). At weekly intervals, five leaves were chosen and examined (a total of 90 leaves sampled). Season 1958. Mustard and Turnip sampling (3 July - 17 September 1958). At weekly intervals, two samples were taken in each of the four plots marked with on the field plan, Fig. 7. The samples were composed as follows:- 3 July - 25 July 10 Mustard flower heads 10 Mustard leaves 10 Turnip leaves 10 Aug. - 17 Sept. (The Mustard leaves and flowers had mainly fallen). 5 Mustard flower or seed heads 10 Turnip leaves Samples were plucked and transferred to 2 gallon jars containing 2 pints of water and 50 c.c. Stergene. The jars were returned to the laboratory and shaken vigorously. The contents were poured into a large polythene funnel sealed with a length of rubber tubing and a spring clip and the liquid was allowed to stand for 15 minutes. Plant material was then washed and discarded. The spring clip was opened and the liquid strained through muslin stretched over a smaller funnel. The muslin was transferred to a petri dish with alcohol, and the insects were picked off under a binocular microscope. No insects were found in an examination of the plant material after washing. Table 23 shows the errors of sampling expressed as a percentage (S.E. x t (p=0.05) x 100),. Mean 116. TABLE 23. Error of Edgegrowth Sampling. B. brassicae Item "Other aphids" (Period of occurrence only) SEASON 1957 Mustard sampling _11. 33.67% - 86.72% Swede sampling 30.75% very low numbers SEASON 1958 Mustard and Turnip very low sampling numbers ▪- 30.13% Samples frequently consisted of many zero counts and a few counts of large colonies. The sampling errors involved are thus considerable, but the data do give satisfactory qualitative results. 6) WATER TRAPS. The traps used were metal trays measuring 24 x 19 x 6 cms. painted black on the outside and with the trap colour inside. Besides white, the following colours (Wilson, 1941) were used:- Chrome Yellow (light) col. ref. 60 5 58 Paris Green col. ref. 1 The volume of each trap was approximately 2 litres. The traps were re- filled every other day with one litre of a mixture of 12 gallons water, 15 c.c; Stergene and 15 c.c. 40% formalir. The detergent caused most insects to sink with the majority of the elate aphids left floating with outspread wings; the formalin prevented algal and fungal growth. The traps were examined daily at 10.00 G.M.T. Aphids and their predators and parasites were collected and re- turned to the laboratory for sorting and counting under a binocular microscope, Stakes fitted with platforms were used to raise the top of the traps to 1' 6" above soil level. Season 1957. White traps were arranged as shown on the field plan (Fig.6, 22 April - 27 May 4 traps, a,b,g and h. 18 June - 28 November 12 traps, a-m, 117. Season 1958. A comparison was made between catches of yellow and green .or.: white traps. From 21 May to 26 June yellow traps were set up at positions a and m (Fig. 7) with white traps at a distance of five feet. The white traps were then repainted green and replaced from 27 June to 11 July. The number of aphids and parasitic Hymenoptera trapped is given in Table 24. There were no significant differences in the numbers of parasites, but yellow traps caught more aphids than white or green ones. TABLE 24. Comparison of yellow, white and green water traps, 1958. Trap No. p (eAtimated Significance-Tat 5% Item coloui‘ trapped by test) level) Aphids Yellow 329 '.001 ÷++ White 72 Yellow 104 <.001 ++4- Green 36 Parasitic Yellow 152 .20 Hymenoptera White 133 Yellow 59 .20 NO OM OM Green 45 Yellow traps were chosen for use in 1958. They were arranged as shown on the field plan (Fig. 7, a-m). 2 May - 11 July 5 traps, a, b, i, e, f. 12 July - 3 December 12 traps, a-m. It was realised that the trap catches in the two seasons were not comparable. The two colours would trap different fractions of arriving and leaving alatae. The consistency of the catches. The consistency of single water trap catches expressed as a percentage (standard error x t (p=0.05) x 100) is mean 118. given in Table 25. Traps a - b and c - d in 1957 and traps a - m in 1958 were considered as replicates. In using data from water traps, the catches of all 12 traps were combined for each day or the total catches of single traps over a period of several days were compared. TABLE 25. % Error of single water trap catches. Colour of No. of Item trap replicates. Error % All aphids White 25 67.26 (alatae) Yellow 15 104.02 Brevicoryne White 45 171.90 (alatae) Yellow 15 72.00 Aphid parasites White 25 - 174.40 20 and hyperparasites Yellow 233.31 7) PLANT GROWTH. As the sampling of the crop included the heart and young leaves, one medium leaf and one old leaf it was considered necessary to keep a record of the number of leaves in each category throughout the season. In 1957 six sprout plants were chosen and identified with numbered stakes. Two plants were selected in each of the three squares, 60, 72 and 75. In 1958 20 plants were chosen - five round sampling area 1, ten round sampling area 0 and five round area 3 (Fig. 7). The plants were inspected at approximately four day intervals in 1957 and weekly intervals in 1958. At each inspection the following routine was adopted:- a) The number of young, medium and old leaves was recorded. As all young leaves and the heart were included in sampling the crop the number of young leaves was recorded as 1. 119. b) Leaves which had reached medium age since the last inspection were marked with a coloured staple - the colour being different for each inspection. c) Fallen leaves were examined, and the colour of the staple recorded,, Expressed as a percentage of the mean, the fiducial limits of the mean leaf number of six marked plants over the season were - 17.09% for medium leaves and - 12.80% for old leaves. The average number of leaves of 20 plants chosen at random from the whole field area on three occasions in 1957 was well within the fiducial limits of the mean leaf number of the six marked plants (Table 26). A graph of the average number of leaves of the three age divisions per plant is given in Fig. 9. TABLE 26. Comparison of mean leaf numbers of 6 marked plants and 20 plants chosen at random, 1957. Plants chosen Date Leaf Age Marked plants at random + 23/7 medium 8.7 -. 1.48 9.6 old 3.7 - 0.47 4.0 + 16.0 - 2.73 16.8 30/8 medium + old 6.0 - 0.77 5.7 25/11 medium 21.7 +- 3.71 20.6 old 14.7 - 1.88 13.0 8) PARASITE BREEDING. Parasitised aphids were collected for breeding in 2" x 71" collecting tubes. The open end of the tube was placed over the aphid mummy and a leaf disc punched out by pressure on the cork on the other surface of the 120. 30- 1957 20- • 10- (f) _J YOUNG 0 0 I T lir E r trir 30 - OLD 0 1959 MEDIUM 20- 10- YOUNG I 1, I I 1 I „ july IA I I S I " I -'/C2. 9 GROWTH OF SPROUT PLANTS,1957 13 1958 121. leaf. The top of the cork was labelled with the reference number of the plant and leaf age. In the laboratory the leaf disc was pushed halfway along the tube and a celluloid strip with a reference number inserted. The tubes were re- stoppered with absorbent cotton wool and placed into a breeding rack (Fig.10)e The tubes (tu) stood inverted in a honeycomb mesh (h) on a perforated zinc cradle (z). This cradle fitted into a metal tray (tr) of the pattern used as water traps, and rested on a sheet of moistened foam plastic (F). The following records were entered against the tube reference number (r) a) Source of specimen and date of collection. b) Primary parasite (identified from the appearance of the mummy, cf. Dunn, 1949). c) Species of parasite emerging. d) Date of emergence of parasite. Each breeding rack could accommodate about 150 tubes. The racks o were kept in a constant temperature room at 23 C and 68% R.H. ; they were inspected daily for emerged parasites. Emerged parasites were clearly visible through the glass tops of the tubes when the zinc cradle was lifted from the tray. Parasites emerged from 1 - 16 days after collection. The percentage emergence achieved by this technique is shown in Table 27 and is significantly higher than the percentage emergence from corked tubes kept under the same conditions. 122. KEY TO FIGURE 10. c = cotton wool stopper f ... foam plastic sheet h = honeycomb mesh m = aphid mummy on leaf disc r = reference number tr = tray to = 2 x e collecting tube w = water level z = perforated zinc cradle APPROX. SCALE 5 cms. • 111111 MI ...... • • • •'IVMMWZZZMZCM:F.I.F.r.rZJWZJ'.l.IYMJrranCVZZ.rr.r..'vYrrIWW'zr.r ,rd-.r.r.e.cr;rdw.ar.rz frit e FIG. 10. PARASITE BREEDING RACK 124. TABLE 27. Percentage emergence of parasites bred in breeding racks, 1957 and 1958. Number % Number of mummies Item emerging i emerging bred All mummies 411 88.39 465 Praon mummies 10 66.67 15 Diaeretus mummies 401 89.11 450 Diaeretus mummies in corked tubes 41 69.49 59 Total 442 86.84 509 Standard error of difference 4.6 P <.001 125. PART II. Brevicoryne brassicae L. INFESTATIONS IN 1957 AND 1958. This section describes the progress of the infestations of Brevicoryne brassicae L. studied in both seasons. Aphid populations during any one season are extremely variable in respect of numbers, parts of the plant colonised, distribution of apterous and elate generations and emi- and immigration. The effects of uncultivated land were expected to differ in nature and degree under different population conditions. A study of the nature of the aphid infestation, without reference to uncultivated land but with particular reference to those variables most affecting aphids, was considered an essential initial step in the investigation. This study provided a framework of stages in the infestation to which effects of uncultivated land could be assigned. 1) LIFE HISTORY. (Fig. 11). Brevicoryne brassicae is an aphid with a fairly simple life-cycle. It is not a migratory aphid; the whole annual cycle can take place on one plant species. The aphid has been monographed by Markkula (1953) in Finland and the life cycle has been described in England by Petherbridge and Mellor (1936) and in America by Herrick (1911) and Herrick and Hungate (1911). The winter may be passed as viviparae or in the egg stage. Empson (1952) states that in North and East England the winter is passed mainly in the egg stage but as viviparae in the South and West, although here eggs may be laid in suitable years. In Finland (Markkula, 1953) eggs are the only overwintering stage. Where the host plant remains standing over winter (e.g. crucifers grown for fodder) the eggs hatch in April or May (Smith, 1951) giving rise to apterous viviparae. These produce several 126. SPROUT FIELD AT OTHER CRUCIFERAE Si LWOODRIRK o • J apterous egg F Crop ploughed in M A M J I slate A J A >-- S 0 AS IN LEFT HAND COLUMN N D FIG. 11. LIFE HISTORY OF Brevicoryne brassicae L. 127. crenerations of apterae, and winged forms may be produced as early as late May (Petherbridge et al., 1936). The production of nymphs by apterae and alatae continues throughout the summer and autumn; as a rule apterae predominate. Towards the end of the season sexual forms are produced, the males winged and the females wingless. After mating, eggs are laid by the females. Markkula (1953) gives data for fecundity and speed of development of B. brassicae. 2) INITIATION OF THE INFESTATION. The growth of B. brassicae populations* in the present experiment is shown in Fig. 14, p.134 (1957) and Fig. 16,p. 138 (1958). The crucifers were grown as vegetables - planted in summer and the harvested plants ploughed in at the end of February. No aphids or eggs were found in an examination of the dead leaves and stumps in February 1958 or in samples of the weeds and planted crucifers in the edgegrowth during winter. The build-up of an aphid infestation on the freshly planted crop was therefore dependent on the arrival of summer alatae from other sites. Such alatae appeared in June and July 1957 and in July and August 1958 (Figs. 14 and 16). When the crop was sampled during these periods in both seasons it was found that a significantly higher proportion of alatae was found in the heart leaves than on medium and old leaves (Table 28). From both crop sampling and water trap records it appears that the number of alatae arriving in 1958 was considerably less than in 1957. * Calculated from the crop sampling and plant growth records. 128. In the field, alates were observed to alight mainly on the outer medium leaves. It seems probable that there is a movement to the hearts of elate which have descended onto the plants from an air-stream. Markkula (1953) regards the young actively growing leaves of the heart area as providing the best food for the cabbage aphid. TABLE 28. Nos. of alatae on samples of 150 leaves of different ages during flight peaks in 1957 and 1958. —,-- Age of 1957 1958 leaf 25 June to 20 July 14 July to 20 August % No. Expt. No % No. Expt. No.* Young 70.6 180 85 93.1 27 9.7 Medium 20.8 53 85 3.5 1 9.7 Old 8.6 22 85 3.5 1 9.7 P = 4,...001 4(.001 * Expected values are based on a hypothesis of random distribution, regarding the heart area, medium and old leaves as approximately equal in landing area for alatae. 3) APTEROUS GENERATIONS. A succession of apterous generations followed the arrival of the alatae. Markkula (1953) has shown in Finland that by taking the firstborn of the first generation and rearing the firstborn nymphs of each ucceeding generation there are nine generations in the year. With the last born nymphs there are only five generations. In the field, therefore, generations overlap each other considerably, especially as the alatae founding the infestation arrive over a period of several weeks. B.D. Smith (1957) graphed the percentage of adult Acyrthosiphon spartii (Koch) per sample 129. to distinguish generations, but in the present experiment such a graph did not show generation peaks. From the records of the marked colonies in 1957 the approximate length of the nymphal stage could be determined in 95 instances, although the routine of inspection every four days prevented the accurate timing of birth and moult to adult. The results are shown in Fig. 12. Markkula (1953) gives the mean period between moult to adult and the birth of the first nymph as 1.5 days. In the present experiment many adults were observed to reproduce on the day following the moult to adult. From Fig. 12 it is possible to deduce a maximum of 11 generations from the planting of the crop to the end of the year in 1957. The distribution of B. brassicae on young, medium and old leaves is shown in Fig. 13, where the number of aphids found on 150 leaves of each age is plotted as a percentage of the total observed on each sampling occasion. The graphs for 1957 demonstrate an initial concentration of aphids in the heart leaves where the arriving elates settled, followed by peaks on the medium and old leaves in September and October. The plant growth records of 1957 (Table 29) show that the heart leaves of July became of medium age from 9 August to 2 October and old from 17 September to 7 November. These periods fit the peaks of aphid distribution on the various leaf ages. In 1958 the young leaves remained the most heavily infested through- out the season. The spring and summer were wet and aphid numbers were much lower than in 1957. In both seasons, the percentage of B. brassicae on young leaves rose with heavy rain (Fig. 13). It seems likely that the heart leaves provide protection from heavy rain (see p. 148). 130. 34 32 birth moult to adult 30 28 26 _ z -0 a 24 4,O 022 020 L :E 18 c sw 16 No. of o• 14 instances recorded 0 ow- z• 12 10 ... FIG.12. LENGTH OF NYMPHAL STAGE OF APTEROUS Brevicoryne brassicae L. 131. SEASON 1957 I t II III I I I II II I II I MI 0C 0 O O ••• O U J July .c U o SEASON 1958. f o ves Leaf age:- lea :young 50 medium. 1 old on ->5 rrm. rain in 24 hrs. J July A S 0 FIG.13. PERCENTAGE B.brassicae ON DIFFERENT LEAF AGES 132. 4) PRODUCTION OF ALATAE. Alate nymphs were first observed on the crop* on 3rd September 1957 and 18th September 1958. During the period of alate production, apterae predominated in both seasons.- alate nymphs were found in only 5.1% of colonies in 1957 (maximum - 13.4% on 25 October)and 5.8% of colonies in 1958 (maximum - 15.4% on 28 November). TABLE 29. Development of July heart leaves, Season 1957. Number of Date of Date labelled Date of change Date of leaf as medium age to old age senescence leaves separating category category from heart 23 July 9 Aug. 30 Aug. 17 Sep. 1 9 Aug. 9 Sep. 27 Sep. 1 9 Aug. 9 Sep. 21 Oct. 2 9 Aug. 17 Sep. 21 Oct. 1 17 Aug. 4 Sep. 20 Sep. 1 30 July 9 Aug. 9 Sep. 2 Oct. 1 9 Aug. 17 Sep. 7 Oct. 1 17 Aug. 4 Sep. 20 Sep. 1 17 Aug. 17 Sep. 7 Oct. 1 17 Aug. 17 Sep. 21 Oct. 2 22 Aug. 9 Sep. 24 Sep. 1 22 Aug. 17 Sep. 7 Oct. 1 3 Aug. 22 Aug. 17 Sep. 27 Oct. 1 22 Aug. 20 Sep. 7 Oct. 1 22 Aug. 20 Sep. 21 Oct. 2 22 Aug. 24 Sep. 21 Oct. 1 22 Aug. 24 Sep. 30 Oct. 1 26 Aug. 20 Sep. 21 Oct. 1 26 Aug. 27 Sep. 21 Oct. 1 26 Aug. 2 Oct. 7 Nov. 1 30 Aug. 20 Sep. 21 Oct. 1 Table 30 shows the distribution of colonies with elate nymphs on * Water traps were catching elates from 27 August 1957 (Fig. 14). 133. young, medium and old leaves. In 1957 the production of alate nymphs was greatest on the older leaves; in 1958 numbers were too low on the older leaves to compare with the young leaves. 5) WINTER EGGS. Eggs were found from 14th November in 1957, though many viviparae also overwintered. No eggs Were found up to the end of sampling (5 December) in 1958. A comparison of daily minimum temperatures at crop level for October showed that in 1957 the average minimum temperature was 3.6°C with several light frosts, and 5.40C with no frosts in 1958. Most eggs were found on the underside of the lower* leaves while sampling, and occasional eggs were observed on the stems of the plants and in the crevices formed by overlapping leaves on the sprouts themselves. The Amid elongated eggs are almost black in colour, though they are a light green when first laid. TABLE 30. The proportion of colonies with elate nymphs on young, medium and old leaves during the period of elate production, 1957 and 1958. 1957 (3 Sep. - 12 Dec.) 1958 (18 Sep. - 8 Dec.) Leaf age Number No. of Expected Number No. of Expected colonies colon- no. with colonies colon- no. with with ies alate with ies elate elate nymphs on elate nymphs on nymphs nul hypo- nymphs nul hypo- thesis thesis. Young 11 652 30.4 9 181 9.8 Medium 45 846 39.4 2 28 1.5 Old 52 819 38.2 2 32 1.7 p. 4..001 Numbers too low for valid analysis. 4 * From 14-28 November no eggs were found in the heart or young leaves, one egg on medium leaves and 45 eggs on old leaves. t I x1000 more than 5mm rain in 24hrs. III II crop planted first alate nymphs first eggs 25- - 3°No of predators III -207nelgoe -10 •-0 INI population in 1958 to the same scale I IR V May V 0 FIG.14. NO. OF Brevicor ne brassicae L. ON THE CROP AND TRAPPED - 1957 135. 1957 1958 x1000 x 100 15- 15- 10— 10- a a. 0to on hids ap Na 0 J July July 'I A FIG.15. COMPARISON OF NOS. OF Brevicoryne brassicae L. AND OTHER APHIDS ON THE CROP AND TRAPPED UP TO 31st AUGUST — 1957 and 1958. 136. 6) PROGRESS OF INFESTATIONS. SEASON 1957. (Fig. 14). Alatae of B. brassicae and other aphids, particularly Myzus persicae Sulz., alighted in the area in June and July. The proportion of B. brassicae in the water trap catches (Fig. 15) was generally less than 1/10 of the total catch of elate aphids. The build-up of the other species of aphids in the early part of the season (Fig. 15) was of a markedly different character from that of B. brassicae. Alates of other aphid species arrived in large numbers. There was a rapid rise in numbers on the crop to a peak, the numbers dropping again quickly as predators increased and rainfall was heavy. Alates of B. brassicae were much fewer in number and the initial peak was correspond- ingly smaller. Numbers were kept low during the summer by the wet conditions and predators, and the peak was not reached till late October (Fig. 14). In the early part of the season B. brassicae appeared to suffer less from the adverse conditions that, the other species of aphids. It is suggested that this is explained by the different parts of the plant colonised by the species. It has already been shown that B. brassicae was most abundant in the heart leaves (Fig. 13) early in the season and Table 36 013.150 suggests that heart leaves appear to offer more protection from rain than the outer leaves. The other species of aphids, however, were most abundant on the outer leaves, particularly those classified as "old" (Table 31). Such a difference between B. brassicae and M. persicae has been recorded by Palmer (1956) on swedes, wheret. persicae was twice as numerous on the four oldest leaves as on the four youngest (5-10 times more abundant on young leaves, however, when assessed as number per gram. of green leaf) and B. brassicae was most 137. abundant on young leaves (30-40 times when assessed as number per gram.of green leaf). Taylor (1957) states that M. persicae colonises the lower leaves first, spreading upwards as leaves enter the senescent stage. TABLE 31. Distribution of aphids other than B. brassicae on young, medium and old leaves, 25 June - 31 August 1957. Expected no. on nul Leaf age No. aphids on % of 150 leaves total hypothesis (Mean) Young 298 9.0 1107.7 Medium 1071 32.2 1107.7 4.001 Old 1954 58.8 1107.7 Total 3823 During the same period, more predators were observed on the old leaves (Table 32), and it seems likely that B. brassicae in the heart leaves would suffer less predation than the other species of aphids on the outer leaves. TABLE 32. Distribution of aphid predators on young, medium and old leaves, 25 June - 31 August 1957. Leaf age No. of predators Expected no. on nul p on 150 leaves hypothesis (Mean). Young 59 71.3 Medium 63 71.3 <.02 Old 92 71.3 Total 214 Alate nymphs' of B. brassicae were observed on the crop from 3 September, when the population had built up well above the low level of most of the summer, to the end of sampling on 9 January 1958. During this period elate nymphs were seen in only 4.7% of the colonies, and appeared most frequently 5 traps_i nly x1000 41111111 .41,J,i, 1 ,,,,, , , r. more than 5mm. rain in 24hrs. x 100 I pC crop planted first slate nymphs O -30 No. of -20 predators cn 450 leaves -10 a 0 O I I I II II I 1 1 1 1 III I June J A S 0 FIG. 16. NO OF Brevicoryne brassicae L. ON THE CROP AND TRAPPED — 1958 139. in colonies on the outer leaves (Table 30, p.133). From Fig.12, p. 130 it appears that the first alate nymphs were produced in the seventh generation. The peak in numbers of B. brassicae occurred at the end of October well after the most favourable temperature conditions for development. The peak was followed by a rapid decline in numbers with the onset of winter. Eggs were observed from the 14th November, though many viviparae were still alive when sampling was discontinued. SEASON 1958. (Fig. 16). Alatae of B. brassicae alighted in the area in late June, July and August. Many other alatae (of M. persicae and other aphid species) also appeared on the crop and the difference in timing of peaks noted in 1957 was again observable. B. brassicae elates constituted only 1/-10 of the total number of elate aphids trapped, and the early peak of the other species and the late peak of B, brassicae were repeated (Fig. 15). The summer of 1958 was particularly wet and numbers remained very low throughout the season (see Fig. 14 for comparison of the two seasons). B. brassicae was again most numerous in the heart leaves (Fig. 13, p. 131) and never established successfully on medium and old leaves as the species had in 1957; other aphids were again most abundant on the old leaves (Table 33). As in 1957, predators were most abundant on the old leaves (Table 34) in the early part of the season. Alate nymphs of B. brassicae were observed on the crop from 140. 18 September to the end of sampling. The first elate nymphs were recorded soon after the first large rise in aphid numbers (Fig. 16). Alate nymphs were seen in only 5.4% of the colonies during the period of their occurrence. There was no significant difference between young, medium and old leaves in the number of colonies with elate nymphs (Table 30, p. 133). TABLE 33. Distribution of aphids other than B. brassicae on young, medium and old leaves, Juiy - 31 August 1958. Leaf age No. aphids on % of Expected no. on nul p 150 leaves total hypothesis (Mean) Young 194 12.4 520.3 Medium 393 25.2 520.3 Ar.001 Old 974 62.4 520.3 Total 1561 TABLE 34. Distribution of aphid predators on young, medium and old leaves, 17 July - 31 August 1958. Leaf age No. of predators Expected no. on nul on 150 leaves hypothesis (Mean) Young 8 33.0 Medium 18 33.0 4c.001 Old 73 33.0 Total 99 The peak in numbers of B. brassicae occurred at the end of October, on the same sampling date as the 1957 peak, and again well after the more favourable temperature conditions for development of the summer. The maximum population was much smaller (Table 35) than in 1957 (about 1/20) and the peak was of short duration, falling off rapidly with the onset of winter. No eggs were observed. The two seasons are compared in Table 35. TABLE 35. Comparison of B. brassicae infestations at Silwood Park in 1957 and 1958. Item 1957 • 1958 Crop planted 14 June 13 July Peak of infestation of crop by alatae 18 June - 20 July 14 July - 20 August 1 No. alatae trapped in twelve water traps during above period * 139 81 First elate nymphs observed 3 September 16 September Peak•in numbers 31 October 31 October No. of aphids on 150 plants at peak 33,279 1,760 First winter eggs observed 14 November - * Not strictly comparable, as the colour and arrangement of traps was different in the two seasons. 142. PART III. FACTORS AFFECTING THE POPULATIONS OF Brevicoryne brassicae L. In order to keep the section on the effects of uncultivated land on crop infestation discrete, it was decided to keep separate the observations on the nature and overall importance of the various factors affecting aphid numbers as well as data on the biology and the abundance on crop and edge- growth of the parasites and predators of aphids. The later section on the effects of uncultivated land could be reserved for an analysis of the types of edgegrowth effects and their importance and incidence in the season. The importance of these effects was considered as being largely an inter- action of the factors described in this section with the changes in the nature of the aphid population described previously. Thus reference to the data on the progress of the aphid infestations and the factors affecting them has been made extensively in the later section on edgegrowth effects. 1 REVIEW OF LITERATURE. There is a widespread literature on factors affecting aphid populations,. Mainly this consists of the correlation of prevailing weather conditions or the appearance of predators with changes in aphid numbers observed in the field, or lists of observed aphid predators and parasites with notes on their life histories. In addition, some workers have attempted a quantitative estimation of the effect of factors (particularly environmental) under controlled conditions. Campbell (1926), working on the Pea Aphid (Acyrthosiphon pisum (Harris); in California, listed observed predators but considered adverse weather conditions more important in checking infestations. Heavy rain dislodged and killed many aphids, especially those on exposed terminal clusters of the 143. plants. Chittenden and White (1926), in a bulletin on the control of Melon aphid (Aphis oossypii Glover) stated that warm, dry periods favour increase of the aphid. In addition, they recorded parasites and predators as effective, particularly Coccinellids, which were present at the outset of the aphid attack. Fluke (1929) listed 79 predators and parasites attacking the pea aphid in North America and gave data for the numbers of aphids destroyed under experimental conditions. Wadrkey (1931) gave the life histories of some predators of Toxoptera graminum (Rond.) in the U.S.A, with data for their aphid consumption. Although rainfall and high summer temperatures were important factors in destroying aphids, predators (mainly Coccinellidae) were of value if they were abundant early in the season. A list of the predators and parasites of Hyalopterus arundinis (F.) in Switzerland was given by Dill (1937). Knowlton, Smith and Harmston (1938) reported the rapid re-infestation of sprayed fields by the pea aphid in the U.S.A. in the absence of predators. A list of predators was given with life history data and laboratory determined feeding rates. Dicker (1940) considered that Coccinellid larvae were largely responsible for the decline of populations of aphids on Rubus in August, though he did not measure this in the field. Studying Tiyzus persicae Sulz. on potatoes, Staniland (1943) found that 144. areas with low population index figures were generally much more humid and more exposed to winds than were areas from which high counts were obtained. The failure of M. persicae and Macrosiphum solanifolii Ashm. to build up as rapidly as expected was considered by Jacobs (1944a) to be the result of a cool wet summer. Broadbent (1946) found that infestations of potato aphid (particularly M. persicae) were least in sheltered fields. No evidence was obtained to show that altitude and aspect (_,F the field influenced the intensity of the infestation. Fidler (1949), also working on potato aphids) found large populations of aphids in fields which were sheltered, particularly from east winds. Dunn (1949) listed parasites and predators of potato aphids and included some life history data and laboratory feeding rates. Swarms of Coccinella septemp,Inctata L. caused a total check in early August, but predators and parasites which appeared from about mid-July seldom checked the growth of aphid populations until the number of aphids was stable or declining. In work on the pea aphid, Dunn and Wright (1955) considered physical factors as most important, particularly rain, but also hot summer weather when the host plants were becoming less palatable. There was a wide variation in the percentage production of alatae in Spring and thus emigration as a pe;-ulation reducing factor was variable. Broadbent, Tinsley, Buddin and Roberts (1951) found that predators (especially Syrphids) and parasites successfully checked M. persicae populations on lettuce, though the heat near the soil surface on sunny days may also have been detrimental. 145. The biology and population fluctuations of the Strawberry aphid (Pentatrichopus fraoaefolii (Cock.)) were studied by Dicker (1952). There was a sudden decline in May or June each year which could not be ascribed to climatic factors, elate dispersal or predators and parasites. Dicker advanced a theory which related the population decline primarily to the physiological state of the strawberry plant, which he believed became unattractive at the time of fruit development. This caused the adult aphids to move and generally depressed the reproductive rate of the population. R6a1 (1955) reported similar host plant effects on Aphis craccivora Koch attacking groundnut plants. When the plants were crowded photosynthesis was affected and this in turn reduced the production of alatae and the fecundity of the aphids. Szalay-Ma4S6 (1957) attributed a sudden decline of Aphis fabae Scop. on beet to weather factors rather than to natural enemies, which were scarce. Leaf age as a factor affecting aphid populations is discussed else- where (p. 154). Previous work on Brevicoryne brassicae L. has followed similar lines. In the U.S.A. Herrick (1911) recorded natural enemies. He considered that in most seasons they were effective in holding the pest in check. Petherbridge and Mellor (1936) considered rain important, though they reported that more -ain in 1935 ',lad less effect on the aphid than in 1934. Predators and parasites were fairly abundant, but were not sufficient to prevent serious damage. Petherbridge and Wright (1938) found that increase of the aphid was 146. favoured by warm, dry conditions, whereas cool wet weather and especially heavy rainstorms reduced the numbers. Hoverflies and gall midge larvae were among the more important predators. Smith (1951) stated that B. brassicae was liable to become a serious pest in a drought when the plants were not in a vigorous state. He listed the predators and parasites: the latter could kill 80-90% of the aphids towards the end of a season. Empson (1952) compiled a survey of cabbage aphid populations on Brussel sprouts from 1946-1951, and reported that his October "severity index" was greatest when the previous August was warm and dry. Markkula (1953) monographed the cabbage aphid in Finland, and included an investigation of factors affecting the reproduction and development of the aphid. Besides demonstrating the extreme importance of temperature, his experiments with controlled watering of plants suggested that rainfq11 may affect the reproduction of aphids adversely through the water relation- ships of the host plant. George (1957) recorded parasites and predators. He assessed the percentage parasitism as ranging between 0% and 7.2%. B.D. Smith (1957), working on broom aphids (particularly Acyrilosiphon spartii (Koch.)), attempted to account for observed fluctations of aphid numbers in the field with data from laboratory experiments on the various factors affecting them. He could not find any lobs of aphids after rain, and reached the conclusion that predation by birds was the most important single factor. He calculated the number of adult aphids which birds would 147. have had to destroy to account for the residual mortality unaccounted for by the factors he had been able to measure in the laboratory. In the present work the technique of marking colonies (see p.112) has been used to measure the effect of the various mortality factors in the field directly. 2) PHYSICAL FACTORS. a) Temperature. The speed of development at different temperatures has been measured for many species of aphids, including Toxoptera qraminum Rand. (Headlee, 1914), Myzus persicae Sulz. (Weed, 1927), Myzus houqhtonensis Troop. (De Long and Mathewson, 1925), Acyripsiphon spartii (Koch), (B.D.Smith, 1957), and Brevicoryne brassicae L. (Markkula, 1953). The records of the marked colonies (Fig. 12) clearly showed changes in the length of the nymphal stage during the Beason. When the length of the nymphal stage was plotted against the mean temperature of the period (Fig. 17), a decrease in duration with e rise in temperature was observed. Using the equation of Blunck (1923)* to estimate the effect of temperature on the development of B. brassicael Markkula (1953) calculated t(T - 1.7) = 187°C as the sum of effective degrees. The hyperbola drawn from this equation has been included in Fig. 17. * Blunck (1923) uses a hyperbola to express the effect of temperature on the development of an insect:- t (T - c) = a constant (the sum of effective degrees) where t = the time required for development I = the mean temperature °C c = the calculated constant (the "threshold of development") 148. The results of the marked colonies appear to fit a sigmoid curve (Andrewartha and Birch, 1954, pp. 148, 149) with limits of 5 and 36 days more closely than the hyperbola suggested by Markkula. Considering rate of development as a response to temperature the data can be plotted in probits (Fig. 18) and a probit regression line fitted by eye. The effective linearity of the points was tested and found to be good (p = <1.001). Markkula's experimental data were plotted and tested against the results of the present work as a variance ratio, which indicated no significant difference between the two sets of results (p = > .10). b) Rainfall. Rainfall is considetea as an extremely important factor in reducing aphidpopulations (Campbe11,1926; Wadley, 1931; Petherbridge and Mellor, 1936; Petherbridge and Wright, 1938; Dunn and Wright, 1955), though4Smith (1957) concluded that a few A. spartii might possibly have been dislodged by a combination of wind and rain in the field, but that these would most likely regain the broom bushes quickly. He did, however, stress the difference in leaf size between broom and row crops. In order to assess the effect of rain on numbers of B. brassicae in the field, the marked colonies were counted before and after a heavy shower of rain between 10 a.m. G.M.T. and 1.0 p.m. on 19th July, 1957, when 5.3 mm of rain was recorded. At 1.0 p.m. 83 (69.7%) of the 119 aphids counted before the rain were still to be found on the marked leaves, a reduction in numbers of 30.3%. The aphids counted were all on leaves outside the heart, whereas at% that time a much higher proportion of aphids could be found in the heart leaves (Fig. 13, p. 131), An examination of the crop sampling records for the period (17 - 21 July, 1957) including 19th July • present work o from Morkkula (1953) • • • 12 f • t(T- 1.7) .167•C o 8° 12° 16° 20° 22°C Mean Temperature Mean Temperature FIG. 1ZRELATIONSHIP BETWEEN MEAN TEMPERATURE FIG.18. REPRESENTATION OF THE LINEAR RELATIONSHIP BETWEEN PROBIT OF TIME OF DEVELOPMENT .sh AND THE RATE OF DEVELOPMENT OF APTEROUS AND MEAN lEMPERATLRE. Brevicoryne brassicae L. NYMPHS. 150. showed a significantly greater reduction in the number of aphids on the medium leaves than on the young and heart leaves (Table 36). TABLE 36. The fall in numbers of B. brassicae on young and medium leaves during a period of heavy rainfall; 17 - 21 July, 1957. a) Rainfall data Date Total Rainfall (mm) Hours Rainfall 18th July 1.2 0.3 19th 6.2 2.5 20th 13.6 6.6 Total 21.0 9.4 b) Aphid data Leaf age No. aphids on 150 leaves Decrease % Decrease 17 July 21 July Heart and young 285 146 139 48.77 Medium 41 11 30 73.17 Total 326 157 169 51.84 Standard error of difference 8.35 p it.o1 An indirect effect of rainfall is suggested by Markkula's (1953) work on the effect of controlled watering of the host plant on the reproduction of B. brassicae. He found that many more nymphs were produced on plants given little water than on those watered normally. Data from the marked colonies were used to tabulate this effect (Table 37) over a period (1 July - 15 August 1957) of the highest daily mean temperatures available in the field which' were coveredloy .marked colony records. 151. Data for the production of nymphs since the last inspection were available for each data of inspection of the marked colonies, though it has been admitted that nymphs which both were added to the colony and disappeared between inspections were often not recorded (p. 113). The mixture in ages of adults made it impossible to determine accurately the relevant amount of rain received by the plant, but, as daily means of periods approaching a fortnight were concerned, it was considered justifiable to use the nymphal period of adults first reproducing on the day before inspection for calculations. This period could be determined from Fig. 12, p. 130. TABLE 27. The effect of rainfall on the production of nymphs by B. brassicae ,amoimemors• Date of Dates used Average Mean daily Reciprocal No. new inspection for nymphal daily mean rainfall (mm) of mean nymphs pery duration of T 0C during during daily rain- 100 plants period. period fall (mm) aphids. 12 July 1 - 11 July 18.1 2.91 0.344 16.2 •16 July 4 - 15 July 16.6 2.43 0.412 13.6 19 July 6 - 18 July 15.5 2.50 0.400 22.7 .23 July 7 - 22 July 15.0 3.29 0.304 10.4 25 July 12 - 24 July 15.4 2.49 0.402 22.2 30 July 17 - 29 July 16.0 2.75 0.364 12.1 3 Aug. 22 July - 2 Aug. 17.3 0.56 1.786 34.2 8 Aug. 27 July - 7 Aug. 17.8 0.24 4.167 137.4 13 Aug. 1 - 12 Aug. 17.9 3.96 0.253 7.3 The number of new nymphs per 100 aphids since a previous inspection (y) was plotted against the reciprocal of mean daily rainfall during the calculated nymphal period of the adults (x) and a linear regression line (y = 2.107 + 30.515 -1 10.986x) was fitted. The regression was tested by an analysis of variance, and found to be significant (p =,.001). This tends to support Markkula's work, suggesting that rainfall may have an 152, important effect on the size of aphid populations by acting through the host plant, as well as causing the direct loss of aphids in heavy rain. c) Factors affecting the production of elate nymphs. Factors affecting the production of alate nymphs are included in this section as the emigration of alatae can bring about a reduction (or diminish the rate of increase) in the size of a given population of aphids. A number of workers have stressed different factors as affecting the production of elate nymphs. Shull (1928, 1930) found an increase in the proportion of elate nymphs produced by Macrosiphum solanifolii (Ashm.) with decreasing light intensity, in contrast to Kenten (1955) on Acyrtosiphon pisum (Harris) and B.D.Smith (1957) on A. spartii (Koch) who both considered that elate nymphs were produced at low temperatures in a proportion increasing with length of photoperiod. Reinhard (1927) and Bonnemaison (1951) connected elate production with overcrowding. Rivnay (1937) connected decline in photoperiod length with water relations of the plant, and concluded that elate production in Toxoptera aurantii Boy. was mainly a consequence of lack of water in the plant and a lower atmospheric humidity. Markkula (1953), working with B. brassicae, also found that the production of elate nymphs increased when plants suffered lack of water. Evans (1938) considered that a decrease in nitrogen (especially protein) content of the host plant increased elate production in B. brassicae. Table 80 (Appendix IV) gives data for possible factors affecting the production of alate nymphs for various occasions when the proportion of elate nymphs had been recorded during crop sampling. The procedure used to determine the relevant periods was that already given on p. 151. In the table, photoperiod is given in hours from sunrise to sunset, water relations 153. of the plant are represented by mean daily rainfall, and overcrowding by the number of aphids found in the samples. Mean temperature and photoperiod decreased steadily in each season. The first elate nymphs in 1958 were produced under conditions of higher temperature but shorter photoperiod than in 1957, though there was no significant difference (p .5) between the proportion of colonies with elate nymphs in the two seasons (5.1% in 1957 and 5.8% in 1958). The fluctuating factors were rainfall and the number of aphids found. From the literature it was concluded that the number of aphids per sample and the reciprocal of mean rainfall might prove suitable variables for a multiple regression from which to predict values for the proportion of colonies with elate nymphs. The equation * calculated from the 1957 data was tested by an analysis of variance but the fit was found not to be significant (p = Individual factors did not appear to exercise a quantitative control on the production of elate nymphs. It seems more likely that it is the condition of the food which the host plant provides (governed by a combination rinf environmental factors with aphid density) which determines the production of alatae. The number of aphids lost to the population on the crop by dispersal in 1957 are shown in Fig. 24, p. 183. Although only a small proportion of .111•••••••••••ftweli. * y(angular transformation of % = 5.9079 +(.3252 9.2320)X1 ( 1 ) colonies with elate nymphs) Ewan daily rainfall +(.003782 t 0.0154)X2 (No. of aphids in sample) 154. colonies was found to have elate nymphs when sampling, alatae were dispersing over a considerable span of the season, mainly from the larger colonies. The dispersal of alatae was largely responsible for the decline of the populations after the peak in late October. LEAF AGE, Markkula (1953) demonstrated peaks of increase by B. brassicae on young and old leaves, though the peak on young leaves was considerably higher than on old leaves. Similarly, Kennedy and Booth (1951) found that Aphis, fabae Scop, reproduced faster on young and early senescent leaves than on mature leaves. Markkula correlates his results with water content and the translocation of nitrogen in the plant. There is much evidence from the reactions of aphids (Kennedy, 1958) that they suffer nutritional deficiencies in the absence of young growing or senescing organs on the plant. Mittler' (1958) has demonstrated that the seasonal fluctuations in amino acid and total nitrogen in the composition of stylet sap of Tuberolachnus salionus (Gmelin) on Salix are identical with the sieve-tube sap of an aphid-free plant. He points out that these fluctuations are in accordance with plant physiological concepts of the mobilisation and conservation of nitrogenous matter by a plant during the growth and senescence of its foliage. Lindemann (1948) showed similar results with Cryptom'zus ribis (L.) on Ribes and demonstrated a depression in the birth and growth rates of the aphid from when the growth of the leaf slows until senescence sets in. Bonner (1950) summarised the literature on the nitrogen metabolism of leaves. Protein hydrolysis accompanies the excision or starvation of leaves as well as conditions of high water stress, factors which to a large extent 155. are to be found operating in ageing leaves. Amides and amino acids have been shown to accumulate with protein hydrolysis. Kennedy (1956) put forward the hypothesis that the results of Kennedy and Booth (1951) could be explained by the especial richness of soluble organic nitrogen compounds at or near regions of the plant, where growth and hence protein sythesis, or senescence and hence protein breakdown occur. The percentage increase of aphids on 150 young, medium and old leaves Was compared for the period 26-30 September, 1957 (Table 38a). The least significant difference (6.02%) was calculated from the young and old leaves, where least difference occurred between both the percentages and the numbers on which they were based. All differences in percentage increase between the leaf ages were significant (p = <.001). The increase of aphids on young leaves appeared greater than on.mature leaves, and the increase on ageing leaves was even greater than on young leaves. During the period studied, the distribution of predators found in crop sampling was 3 on young leaves, 6 on medium leaves, and 16 on old leaves. Reduction in numbers by predators during the period would appear to increase the significance of the high increase on old leaves. Results of a similar comparison of percentage increase on young, medium and old leaves for 21-26 September, 1957 (Table 38b) confirmed these results. The distribution of predators for this period was nil on young leaves, 9 on medium leaves, and 16 on old leaves. 156. TABLE 38. Comparison of percentage increase of B. brassicae on 150 young, medium and old leaves. Leaf age No. aphids at 'No. aphids at Increase during Least beginning of lend of period period increase significant period difference I a) 26-30 Septembe:', 1957. Young 457 703 246 53.8 Medium 801 1021 220 27.5 6.02% Old 488 858 370 75.8 b) 21-26 September, 19574 Young 371 457 86 23.2 70••••••••.a Medium 703 801 98 13.9 7.56% Old 305 488 183 60.0 4) HYMENOPTEROUS PARASITES. Different workers have attached widely varying degrees of importance to the hymenopterous parasites as factors affecting aphid populations (p.142 onwards). Some figures for percentage parasitism are given in Table 39. Hymenopterous parasites were bred in considerable numbers from B. brassicae mummies, and the species found are listed below (Table 40), together with the records of B. brassicae parasites in Britain from Barnes (1931), Ripper (1944)and George (1957). a) Primary parasites. Biology. Vevai (1942) recorded the life history and results of fecundity studies for Aphidius matricariae Hal.; Ullyett (1938) gave similar TABLE 39. Some figures for % Parasitism in Aphids. 157. Author Date Aphid species % parasitism Method of calculation 13 1:),Smith 1957 Acyripsiphon s.partii (Koch.) 0.26 to 0.66 % mummies of total at aphid peaks aphids recorded at each sampling date. Dunn 1949 Myzus persicae Sulz. 1944 1945 13.1 8.9 as above Macrosiphum solanifolii Ashm. 15.7 9.4 as above Aulacorthum solani Kalt. 15.4 as above Dunn & 1955 Acyrlipsiphon pisum (Harris) 80 - 90 in Wright May, 1949. normal 6 - 30 not stated George 1957 Brevicoryne brassicae L. 0.6 - 11.3 Observing aphid colonies,collectee in the field on each .ampling date and kept in an insectary. Dicker 1952 Pentatrichophus fragaefolii virtually observation (Cock.) absent Arthur 1945b Macrosiphum granarium Kirby Peaks:- not stated Aphis avenae F. Aphidius Myzus festucae Theo. granarium 68% Aphidius avenae 67% TABLE 40. Records of B. brassicae parasites in Britain. Species bred in the present experiments Species not seen but recorded by other authors Primary parasites Aphidiidae Diaeretus rspae (Curt) Aphidius pol oni Marsh Barnes (1931) Barnes 31) Ripper (1944) George (1957) Praon volucre (Hal.) bred from B. brassicae in 1957 only, though Praon mummies of other aphids were seen on the crop in 1958. Hyperparas ites Cynipidae-Charipinae Ch. lonqi cornis (Hart) CharipS dol ichoc spereucsies(Cam.) Barnes (1931) George (1957) - ChariPs, Asa phes vulciaris Walk. pter°malidae-SPhegi::::::::p6piesGeorge (1957) Barnes (1931) CeraPhr°ntidae-Mega apilinae Lygocerus species Geor e '1957' 158. data for Aphidius species in South Africa as well as assessing their importance. Sekhar (1957) described the mating and oviposition behaviour of Aphidius testaceipes (Cressen) and Praon aquti Smith. Beirne (1942a, 1942b) described the developmental stages of Aphidiinae including Praon volucre (Hal.) From these papers, h e fo.lowiiig general account of primary parasites can be comp±led. The adult females have been observed to geed on the aphid secretions of honeydew and wax, and can mate within one or two hours of emergence. Males pair frequently but females only once. The parasite lays an egg on the-aphid frequently on the abdomen, and usually avoids hosts which have already been parasitised. Most eggs are laid on the second and third days of the ovi- position period, but Sekhar reports the third to fifth days for Praon. 300 - 400 eggs may be laid by one female. Sometimes a host may be parasitised several times (Vevai reports up to ten), but only one larva survives. Vevai (Aphidius) and Beirne (Praon) record 5 instars, but Ullyett (Aphidius) only records 3 in South Africa. Aphids parasitised in the third of five instars or earlier do not reproduce,and those parasitised later may reproduce until the parasite larva reaches the third instal.; therefore the number of offspring is always much reduced. The offspring of mated females include both males and females,but the offspring of unmated females are all male. In the field, aphid mummies of Aphidius or Diaeretus and Praon are easily distinguishable. The Praon larvae mature% inside a web between the mummy and the leaf surface, whereas Aphidius and Diaeretus mature within the mummy, which is fastened down in contact with the leaf. The parasites over- winter as mature larvae within the host mummies. Adults were seen to feed on 159. the honeydew secreted by aphids, and at 10.45 B.S.T. on 26th June 1958 an adult P. volucre was observed feeding on flowers of Pastinaca sativa L. in the edgegrowth. Breeding. Under breeding conditions (p.119) Diaexetus rapae adults emerged in up to 17 days, depending upon the age of the cocoon when collected for breeding. (The mean of 168 cocoons bred was 6.33 3.7 days). Only 8 P. volucre were bred. Adults emerged in up to 12 days (a mean of 8.0 - 2.306). There was no significant difference between the two primary parasites in time taken to emerge under the breeding conditions (p = > .4). Table 41 shows the emergences of mummies bred in 1957 and 1958. TABLE 41. Emergences from mummies of Diaeretus rapae and Praon volucre, f957 and 1958. 1957 & 149D8 1957 195 Item D. rapae % P. volucre • U. .1.122.9.1 N Mummies taken in 1 for breeding ' 363 10 55 Mummies emerged 297 81.8 6 60 38 69.1 Primary parasite emerged 212 58.4 2 20 16 29.1 Asaphes. vulgaris emerged 14 3.9 2 20 1 ¶ 1.8 Charips dolichocerus emerged 71 19.6 2 1 20 21 38.2 Mummies failing to emerge 66 18.2 4 40 17 30.9 Apparently empty, though no emergence 38 10.5 0 not examined hole or crack ) Dead larva 16 4.4 3 30 Dead pupa 10 2.8 1 10 Dead adult 1 Diaeretus 0.6 0 0 11 1 Asaphes 160. Parasitism in the field. Percentage parasitism in the field could not be assessed owing to the labour involved in dissecting or rearing in an insectary large numbers of aphids of all ages, for each sampling occasion. The measure of parasitism used was the ratio of unemerged mummified apterous adults to the total of apterous adults. This method is liable to considerable inaccuracies, which are discussed subsequently (p. 258). In 1957 only 15 out of 410 (3.7%) mummies were parasitised by P.volucre, D. rapae parasitising the remainder (96.3%). All mummies found in 1958 were parasitised by D. rapae. Taking the crop sampling data Df the whole season in each case, it was estimated that in 1957 8.6% of all adult B. brassicae were mummified by D. rapae and 0.3% by P. volucre. In 1958 D. rapae mummified 14.9% of adults. The marked colonies showed that the loss of aphids by other agencies reduced the mortality due to parasitism of all stages of aphids (including alatae) to 0.99% in 1957. Seasonal abundance. The seasonal abundance of D. rapae on the crop, in water trap catches, and as percentage adults mummified is shown in Figs. 19 and 20 for 1957 and 1958. The numbeniof P. volucre were too low for similar observations to be made. D. Tepee adults were trapped from early May to early November in 1957, and from late July to early November in 1958, In 1957 numbers trapped rose to a sharp peak at the end of June as hosts appeared on the crop, and this was followed by the peak in parasitism of the host. Low numbers of the parasite 161. were found on the crop and trapped throughout the season, and parasitism by D. rapae fell in August and remained low as hyperparasites emerged from a large percentage of the mummies (p.165). The initial attack on the aphid appeared to cover several weeks and generation peaks were not very apparent. The mummifying of eleven adults and the subsequent emergence of parasites was followed in the field with marked colonies in July and August. The average time taken to emerge was 12.4 ▪- 2.8 days. The average nymphal period of the aphids observed was calculated (see p.151) as 11.4 •- 0.92 days. This suggests a time of 23.8 days under similar temperature conditions from egg to adult parasite. This is likely to be maximum figure as it assumes both parasitism of the aphid in the first instar and the emergence of the primary parasite - it was not known whether or not the mummies observed had been hyperparasitised. Vevai (1942), Beirne (1942b), and B.D. Smith (1957) found generation times of approximately one month for species of Aphidius, Praon and Ephedrus. Accepting the present data the 1957 graph suggests four generations in the year with peaks of mummies in mid-July, early August, mid-September and late October - after this there was an accumulation of unemerged mummies as aphid populations declined towards winter. In 1958 there was again an initial peak in parasitism. Hyperparasitism was again high, but numbers of adult aphids were much lower than in 1957 and even a few mummies in a sample gave a high parasitism index. There appeared to be three generations with perhaps a partial fourth in early November. 6) HYPERPARASITES. Species of Charips and Asaphes are well known as secondary parasites FIG.19. SEASONAL ABUNDANCE OF Diaeretus rapae (Curt.), 1957, -12 ,.. No. adults in 12 water -8 traps -4 ‘.-ii-TT II 1111 I I« 0 No. adults on -4 90 mustard plants in edgegrowth r- -12 - No. adults on 450 leaves -8 of crop -4 0 F1G.20. SEASONAL ABUNDANCE OF Dia retus ramie (Curt.), 1958 -6 No. adults -4 in 12 water traps -2 I Hi 1 1111 1 1 1 11 0 0 50- d ie f i m m mu 30- ae ic ss 8 bru B. - No. adults on 450 leaves lt _4 10- of crop du a °I. n i JI 164. of aphids (Barnes, 1931: Dunn, 1949: George, 1957: B.D.Smith, 1957). Biology. Barnes (1931) recorded Charips as an internal parasite of Aphidius, and Asaphes as an ectoparasite on both Aphidius and Charips. Asaphes can therefore become either harmful or beneficial, depending upon whether or not the primary parasite has already been attacked by Charips. Dunn (1949) following Haviland (1921, 1922) recorded the life histories of species of Charips and Asaphes. Eggs of Charips are laid in first to fourth instar Aphidius larvae while the aphid hosts are still alive. Only one egg is laid: when more occur (up to six have been found) they are the result of other attacks. The growth of the Aphidius larva is arrested immediately it has spun its cocoon and fastereithe aphid down. Charips overwinters as a mature larva inside the cocoon of the host. Asaphes vulgaris is a polyphagous species. The hosts are attacked after they have spun the cocoons and cemented the aphids down. Eggs are again laid singly a female may deposit 30-40 eggs. The winter is probably passed within the host cocoon. Breeding. Charips dolichocerus and Asaphes vulgaris were each found to attack both D. rapae and P. volucre. Table 43 gives a summary of the emergence of hyperparasites from aphid mummies bred in 1957 and 1958*. Under breeding conditions, adult C. dolichocerus emerged in up to 16 days after collection. The mean (of 36 D. rapae mummies) was 10.4 days - 1.21. A. vulgaris adults emerged in up to 15 days: only 6 adults were bred from D. rapae mummies (a mean of 14.33 days). As P. vulucre was not abundant, * The figures for A. vulgaris may be rather lower than the corresponding figures in the field. Mummies removed for breeding might have been attacked later by this species, which oviposits after the death of the aphid host. 165. few hyperparasites were bred from this host (Table 41). There was no evidence that there was any preference of either species of hyperparasite for any one species of primary parasite (p = Parasitism in the field. A high level of hyperparasitism was observed in both years. In 1957 the overq11 attack on D. rapae by C. dolichocerus was estimated as 23.0%,but levels of 10096 occurred in the samples taken in early August, and levels of above 60% were observed regularly till mid-September. A. vulgaris was bred from D. rapae comparativel- infrequently (an overall percentage attack of 4.9%). In 1958 C. dolichocerus attacked 55.3% of D. rapae mummies: again 100% hyperparasitis was observed in some samples. Only one A. vulgaris adult was reared from 38 D. rapae mummies (2.6%). Assuming that hyperparasitised and healthy primary parasites were attacked equally by A. vulgaris, it was estimated that 5.3% C. dolichocerus were attacked by A. vulgaris in 1957 and 2.6% in 1958. The relationship between B. brassicae and its parasites and hyper- parasites in the two years is illustrated by Table 42. TABLE 42. Relationship of B. brassicae and its parasites and hyperparasites in 1957 and 1958. 1957 1958 B. brassicae No. adults on 450 leaves No. adults on 450 leaves host ,,(4580) (586) 8.6% Primary 0.3%r /\ 14.9% IC\ Pracin volucre Diaeretus rapae Diaeretus rapae parasites C4: 28.6% .-----i 123.095 '''' -..,s 1 4.996 ><28.6% 2.6% / 55.3% Hyper- Asaphes vulgaris Charips dolichocerus Asaphes' Charips parasites vulgaris dolichocerus \"-- 5.3%-' /'5 2.6% 166. F1G. 21. SEAMNAL ABUNDANCE OF .HYPERPARA SITES. 1957 No. 1958 hyperparasi tes in 12 water traps (per 5 days) No. hyperparasite s on 450 leaves of crop %attack of Ilmipac by hyperparasTtes 11- le adult B.brassicae mummified by D.rapae 140: No. adult B. brassicae 1 Oa on 450 leaves of crop 60- 20- J JnJIA SOND 167. Seasonal abundance. The seasonal abundance of the hyperparasites on trapp the crop, in water catches, and as % attack on D. rapae is shown in Fig. 21 for 1957 and 1958. The numbersof both A. vuloaris and C. dolichocerus have been combined to form total numbers of hyperparasites. Numbers of A. vuloaris were too low both on the crop and emerging from D. rapae mummies, to give satisfactory separate counts. The species was, however, quite abundant in water traps - it is known to be a polyphagous species and may have been more abundant emerging from other species of aphids in the area. Adult hyperparasites were trapped and observed on the crop from mid- July to early November in both seasons. Numbers in 1958 were rather low, but mainly followed the pattern of 1957. The peaks in trap catches, numbers on the crop, and attack on D. rapae were a little later in each case than the corresponding peaks cif the primary parasite (Table 43). TABLE 43. Comparison of initial peaks of B. brassicae with D. rapae and its hyperparasites. a) 1957 B. brassicae D. rapae Hyperparasites Water traps 24th June 26th June 5th July Adults on crop 5th July 8th July 25th July In aphid mummies 17th July 31st July b) 1958 Water traps 11th July 27th July 5th August Adults on crop 31st July 8th August 8th August In aphid mummies 13th August 1st September Dunn (1949) gives 30 days as the time taken by Charipids to complete a generation during summer. This agrees with Fig. 21, and four generations in 1957 are suggested by the graphs, with peaks of aphid mummies in early August, mid-September, late October and a generation in the overwintering 168. primary parasite larvae. In 1958 numbers of both adult aphids and hyper- parasitised mummies were low. Each mummified aphid in a sample caused a considerable increase in the calculated percentage attack of D. yapae by hyperparasites. Three generations are probably represented by the four peaks - occurring in mummies in mid August, late September, and late October to early November. This last peak may include the overwintering generation. 5) PREDATORY INSECTS. Predation has been considered by many workers to be one of the main factors affecting aphid populations, (p.142 onwards). Syrphidae, Cecidomyiidae and Coccinellidae have commonly been listed as the most important of the predatory insects. B. D. Smith (1957) determined laboratory feeding rates for all the possible aphid predators he encountered while sampling. Attention has already been drawn (pp.139 and 140) to the observation that in both seasons the peak in numbers of B. brassicae on the crop occurred well after the more favourable temperature conditions for development. Figs. 14 and 15, p. 134 and p.135 , illustrate the occurrence of the aphid peak after the general decline in numbers of predators on the crop, and Fig. 24, p.183 (graphed from marked colonies data) shows that predation was the main single factor reducing aphid numbers in 1957 up to the peak in late October. Taking the season 1957 as a whole, predators were the most noticeable single factor causing the loss of aphids, accounting for 44.9% of all aphids included in the marked colonies observations. A variety of possible aphid predators were trapped over the crop in both seasons, and a more limited range of predators was observed feeding 169. on B. brassicae during sampling of the crop or edgegrowth crucifers and during examination of the marked colonies. The abundance of the main predators on the crop and on edgegrowth crucifers is shown in Fig. 22. A) HEMIPTERA. i) Nabidae. Nabids were observed infrequently and were never sampled •feeding on B. brassicae. Nabis ruclosus (L.), Nabis ferus (L.) and Nabis flavomaroinatus Scholtz were the species occasionally trapped and encountered on the edgegrowth crucifers in June, July and August. The only record of a Nabid on the crop was a single N. ferus in late June 1957. ii) Cimicidae (Anthocoridae). Little importance has generally been attached to the Anthocoridae, though Dicker (1952) found Anthocoris nemorum L. among the most numerous predatory insects on strawberries. Anthocoridae were frequently observed feeding on B. brassicae on the crop and in edgegrowth samples from mid-July to early October, and were trapped up to early November. Not all adults and nymphs were identified, but all specimens collected for checking were identified as Anthocoris nemorum (14.)•Anthocorids were most numerous on the crop in late July and Auguet,and before the peak of Syrphid- larvae towards the end of August they appeared to be the most effective predators. Nearly all the individuals observed were encountered feeding in B. brassicae colonies. iii) Miridae. Mirids were trapped in large numbers from mid-May to early November 1958, but were only rarely observed on the crop or on edgegrowth crucifers; no individuals were found with aphid colonies. In 1957 only occasional Mirids were trapped in late June and July and none was found during sampling. The most abundant species trapped were Harpocera 170. 1958 A nthocoridae C kCP I En GL.GRC11'T H 2 Neuroptera CROP EDGEG ROW TH 4 2 Cecidomyiidae " to ..GROWTH Syrphidae Larvae CROP EDGEGROWTH Adults Coccinell idae CROP J EDG EGROWTH Size of samples:- CROP-450 leaves EDGEGROWTH —90 plants Jn ' . 2 S I O I N I D 1:1 JI A ' ' Ni ID FIG 22 . NUMBERS OF PREDATORS IN CROP AND EDGEGROWTH. 171. thoracica (Fallen) presumably from oak, Psallus varians (Herrich-Schaeffer), Plagiognathus chrysanthemi (Wolff), Orthotylus adenocarpi (Perris) presumably from broom, and Notostira elongate (Geoffrey). Both broom and enti oak were fairly abundant near the margins of the field. It was not known how far these Mirids could be regarded as occasional aphid predators. B) NEUROPTERA, Many Neuroptera arc well-known predators of aphids, Fluke (1929) and Dunn (1954) found them abundant and of importance in reducing Pea Aphid populations. In the present experiments stalked Chrysopa eggs were found both on the crop and on the edgegrowth crucifers in early July; a few larvae were included in samples early in the season before the build-up of Sryphid populations and may have been of some importance in reducing aphid numbers at the outset of the infestation. Neuroptera larvae were practically always found feeding in aphid colonies. Adult Chrysopa phyllochroma Wes. were trapped regularly in both seasons, from late August to early October in 1957, and late July to early August, and early October in 1958. Chrysopa vittata Wes., C. carnea Steph., C. albolineata Kill., and Hemerobius lutescens F. were also recorded from the traps. C) DIPTERA. i) Cecidomyiidae. Clausen (1940) considered predacious Cecidomyiid larvae as largely restricted by their limited powers of locomotion to hosts of a gregarious habit. Barnes (1929) reviewed the literature on host preferences and quoted some examples of the economic importance of aphidophagous Cecidomyiid larvae. B.D. Smith (1957) found that the number 172. of aphids consumed per day by Cecidomyiid larvae was only half that eaten by Syrphid larvae. In the present experiments, Cecidomyiid larvae were only observed while sampling in 1957. Numbers were low, and larvae were found for only a short period at the end of August. There was also an isolated record in early October. These larvae were in an early instar, and were all found feeding on B. brassicae on the crop. As numbers of Cecidomyiid larvae were very low and Syrphid larvae were numerous at the same time, it was considered that Cecidomylids were of little importance in the reduction of numbers of B. brassicae. ii) Syrphidae. Syrphid larvae are widely recorded as important predators of aphids. George (1957) gives 230 - 600 as the number of B. brassicae individuals eaten by the larval stages of an individual under laboratory conditions. Dunn (1949), in work on potato aphids, found that Syrphids only occurred in large numbers after aphid infestation was already well advanced. In the present experiments, there seemed little doubt that Syrphid larvae were important in reducing aphid numbers. Larvae were often observed feeding in colonies with signs of considerable reductions in numbers of aphids through their activity. They were by far the most abundant predators, and must have been responsible for a large fraction of the aphids destroyed by predation in August and September, 1957. (Fig. 24, p. 183 ). In both seasons, rain and probably predation by Coccinellids had kept aphid numbers low at the beginning of the season. (Figs. 14 and 15), and the later predators (especially Syrphids) proved effective until aphids reached the older leaves. 173. The seasonal abundance of Syrphids is shown in Fig. 23. Eggs were first observed shortly after the planting of the crop in both seasons when adults were trapped regularly. More eggs were laid per leaf on medium leaves than on young and old loaves (Table 45, part 2). The peak of ovi- position followed after something over a month. Eggs were frequently laid in groups of two and three, and up to nine eggs were recorded in a single group (Table 44). TABLE 44. Distribution of Syrphid eggs into groups, 25 July 31 August, 1957, No. laid together Frequency 1 136 2 138 3 85 4 53 5 30 6 13 7 4 8 5 9 2 Total No. eggs 1,193 The most frequent size group found in sampling varied from 1 - 3 on different sampling dates, but bore no apparent connection with either the number of aphids present or the total number of Syrphid eggs found. Various workers (e.g. Bhatia, 1939; B.D.Smith, 1957; Dixon, 1959) have stressed the oviposition of Syrphids near aphid colonies. Schneider (1948) also makes this point, but adds that eggs were occasionally laid without reference to the proximity of aphids. Such eggs, also eggs laid if aphids were not available, were unfertilized even when sperm could be found in the receptacula seminis. 174. 1 o5C1 li ILI TI III :1111 :15 L Aphidophagous adults in 12 water traps Eggs se CROP E ) GEGROWTH size 2 larvae size 1 larvae CROP EDGEGROWTH size 3 larvae CROP • e EDGEGROWTH 2 pupae CROP 0 EDGEGROWTH Size of samples:— CROP —450 leaves EDGEGROWTH— 90 leaves lJnIJI IA IS 101 N 1 DIJ' J1'1' '4I1 1. 0 1 N ID FIG.23. NO. OF SYRPHID STAGES IN CROP AND EDGEGROWTH. TABLE 45. Comparison of the numbers of B. brassicac, Syrphid eggs and Syrphid larvae on different &oaf ages during three periods in 1957. (2) 1 (1) (3) 1 143-- I Expected li Expected i Expected Expected no. based on i no. on 01o. no. on .;No. no. based on ''Leaf No. _B. i nul tiSyrphid nul tSyrphi I no. of no. B. 'Age brassicae hypothesis p Ileggs hypothesis p larvae eggs laid p brassicae 25 June - 31 July Young 1743 879.3 j 28 107 !I 25 3.7 27.7 f Medium 500 879.3 :7.0011 150 107 .004 5 19.6 !---.0011i 8.0 h:- .05 Old 395 879.3 143 107 d 12 18.7 6.3 5 August - 19 August 372.3 i 13 214 2 1.1 31.1 Young 631 I 1 I t Medium 301 372.3 <.001 480 214 C. ' 31 41.1 -.02 14.8 k.001 Old 185 372.3 149 214 22 12.8 9.1 23 August - 21 September 22 i 127 27 7.0 I 30.1 Young 2688 3631.3 t I I i 217 i 127 ..:".001 34 1 69.5 11<.001 56.7 ! (' ..001 Medium 5058 3631.3 C .0011 1 01d 3148 3631.3 ! 142 127 t 61 45.5 i 35.3 1 176. In the present experiment, no association between oviposition and the presence of aphids* could be shown. Data for two sampling occasions in August were chosen, one occasion when aphids were low in numbers and one when aphids were considerably more abundant (Table 46). No association of eggs and aphids could be demonstrated for either occasion. Combining both occasions by summing for 2.D.F. obtained from -nat. log. probability gave p = 3,20. Similarly, there was no evidence of an association between the numbers of eggs and the number of aphids on young, medium and old leaves (Table 45, part 2). TABLE 46. Data for test of association of Syrphid eggs with the presence of aphids on 3 leaves of 150 plants. + = greater than expected value on nul hypothesis = less 11 5th August, 1957 - 27th August, 1957 - 232 aphids per 450 leave 1521 aphids per 450 leaves Syrphid eggs Syrphid eggs Present Absent Present Absent + Present 23 21 Present 22 48 Aphids ,Aphids Absent 69 - 37 Absent 30 50 - No association No association p = .10 P = •5 First instar larvae were frequently observed with their empty egg shell:, on leaves without aphids, and Table 47 shows a large decrease in numbers, probably due to starvation, between egg and the second growth stage recognised * Entirely satisfactory data for the presence of aphids were not available. It was thought that to use presence or absence of aphids and eggs on single leaves might be too restrictive, and the data chosen were presence or absence on the total of the three leaves sampled per plant. 177. in sampling. That starvation may have been a large factor is indicated by the big difftrence in numbers between larvae of the second and third sizes in 1958, when aphids were scarce, and the small difference in 1957, when aphids were much more abundant; in both cases the data are taken from the beginning of September onwards. TABLE 47. Numbers of Syrphids in the various growth categories found in 1957 and 1958 per 450 leaves. 1957 1958 No. in 1 No. in stage % decrease stage % decrease Egg 1569 920 First size 91.1 92.5 category 140 67 Second size ou,v 56.7 category 56 29 Third size 7.1 44.8 category 52 16 The most abundant species found as larvae on the crop were Sphaerophoria scripta L. and Svrphus balteatus Deg. . Not all larvae found in sampling were identified, but Platychirus manicatus Meig. and Svrphus luniger Meig. were also numerous. Of 8 pupae bred in 1957, five were parasitised by Diplazonini* and two S. balteatus and one S. scripta emerged. In 1958 only two pupae were bred; one failed to emerge and the other was parasitised. The variety of species of aphidophagous Syrphids trapped during the two seasons is shown in Table 48. * Diplazon tarsatorius (Panz.), Promethes sulcator (Gray.), and P. pulchellus (Hlgr.) were identified after Beirne (1941). 178. TABLE 48. Species of a.phidophagous Syrphids caught in water traps; 1957 and 1958. Species 1957 1958 Platychirus manicatus Meig. Mid and late July. Late July. P. scutatus Meig. Mid July to late September. P. albimanus F. Early October. Late May. P. sticacus Meig. Mid July. Mid September. Melanostoma scalare F. Late August. Sphaerophoria scripta L. Mid July. Mid August. Early September. Late August. Late September and early October. Early November. S. menthastri L. Early and mid August. Scaeva selenitica Meig. Early July to early Early and mid August. August. Syrphus torvus Ost.-Sack. Late August. S. ribesii L. Mid and late August. Mid to late August. Late September. S. vitripennis Meig. Early July. S. luniger Meig. Late June to late July. Early June. Late August to mid Mid and late August. September. Early and mid November. Early and mid November. S. balteatus Deg. Mid July, Early and late August. Early August. Early and late September. Late September and Early October. October. All November. 179. D) COLEOPTERA - Coccinellidae. Coccinellid adults and larvae have probably received the greatest attention of all aphid predators. Schilder and Schilder (1928) summarised the literature on the feeding habits of Coccinellidae, and aphids figure largely as prey. The work of Clausen (1916), Speyer (1935), and B.D.Smith (1957) show figures for aphid consumption varying from 9.5 to 24 per day. Coccinellids have often been considered of great importance in reducing aphid numbers (e.g. Fluke, 1929; Wadley, 1931; Dicker, 1940; Dunn, 1949), especially when they occur early in the build-up of the aphid infestation. Dunn (1952) analysed the effect of temperature on the Pea Aphid - Ladybird relationship and found an important effect of temperature on both the rate of multiplication of the aphid and on the activity of the Coccinellid. He concluded that high temperatures were the most favourable for the effective- ness of predation by Coccinellids. Banks (1955, 1956a and b, 1957) found a high mortality of first instar larvae due to their undirected searching behaviour. At low aphid population levels, Coccinellid eggs were laid without reference to the presence of aphids, which were frequently most numerous on the young leaves at the top of the plants. In the present experiments, Coccinellids were among the earliest predators on the crop, and had already been observed feeding on aphids (including B. brassicae) on the edgegrowth crucifers. They were thus likely to be of some importance under the high temperature conditions of July and in the comparative absence of Syrphid larvae. There was no significant difference between the numbers of Coccinellids found on young, medium and old leaves in 1957 (Table 49): the numbers in 1958 were too low for a 180. comparison to be made. At the time of occurrence of Coccinellids in 1957, B. brassicae was most abundant on the young leaves, and other species of aphids on the old leaves (Fig. 13, p.131 and Table 31, p. 137). By mid July numbers of aphids (particularly those on the older leaves) had fallen to a low level, and mortality of Coccinellid larvae through starvation was considered likely. During late June and July 1957, when Coccinellids were the most abundant predators, the loss of aphids by predation (Fig. 24, p. 183) did not appear to be more than 5-15%. TABLE 49. Distribution of Coccinellidae (adults and larvae) on young, medium and old leaves, 25 June - 31 July, 1957. No.of Coccinellids Expected no. on nul Leaf age s on 150 leaves. hypothesis (Mean) p Young 14 11.3 Medium 5 11.3 > .10 Old 15 11.3 Total 34 The most abundant species on the crop was Adalia decempunctata (L.), recorded in late June and July, 1957 and early August, 1958. A. de6empunctata was trapped in late July and August, 1957 and July, 1958. Adalia bipunctata (L, was trapped in July and mid September, 1957, and in mid June and the beginning of July, 1958. Although not recorded from the crop, this species was found attacking B. brassicae on the edgegrowth crucifers in early June, 1958. Coccinella septempunctata L. was the only species regularly recorded from both crop and edgegrowth crucifers, where it was found in late June and July in both years. The species was trapped in July, early August and early September, 1957, and in mid June, late July, mid August and early September, 1958. Propylea quatuordecimpunctata (L.) was recorded in 1958 181. only, when the species was frequently found in traps from late May to mid June, mid and late July, and mid August. Larvae were found in fair numbers on the edgegrowth crucifers from mid July to early August. 6) ENTOMOPHAGOUS FUNGI. Steinhaus (1949) recorded fungi of the genera Entomophthora, Empusa and Cladosporium killing aphids. Fluke (1929) found Entomophthora aphidius (Hoff.) appearing whereever heavy epidemics of the Pea Aphid occurred. The fungus developed rapidly during periods of high humidity with warm nights. Dunn (1949) found two types of fungi attacking potato aphids; in some cases fungal attack was more important than parasitism. MacLeod (1955) reported Empusa aphidis Hoff. attacking the pea aphid and causing up to 41.4% mortality. B. D. Smith (1957) found broom aphids only very occasionally attacked by fungi. In 1957, aphids killed by fungi and sprouting mycelial threads were observed from late September onwards. The proportion of aphids killed in this way was usually very small, though almost 10% were found attacked by fungi in late December. Over the season as a whole, fungi were estimated as accounting for 3.1% of B. brassicae. In 1958 a similar attack was not observed, despite the wet season. Large colonies were, however, eradicated by fungus attacking the host plant, and spreading over the heart leaves which were occasionally crowded with aphids in late October. 7) NATURAL DEATH OF APHIDS. The large proportion of aphids accounted for by the factors already described left little importance to be attached to the natural death of 182. aphids, which may live up to 80 days in the laboratory (Markkula, 1953). Such deaths would probably have been included under "various" factors in the loss analysis. 8) SUMMARY OF FACTORS AFFECTING B. brassicae POPULATIONS. Fig. 24 summarises the loss analysis which could be made by the "marked colonies" technique in 1957. A consideration of factors causing losses of B. brassicae is unlikely by itself to account for fluctuations of aphid numbers observed in the field. The variation of the reproductive and developmental rate of B. brassicae with climatic conditions and the food quality of the host plant is extremely important in determining how effective the control of the strictly "population reducing" factors can be in any one season. Such factors were probably the main determinants of the size of the B. brassicae populations studied. Such interaction could not be investigated in this two years' study, though the investigations on leaf age effects (p.154 ), the markedly higher population level in the warmer and dryer 1957, and the analysis by Empson (1952) of the "October severity index" (Table 50a) were probably expressions of the importance of the reproductive rate of the aphid. An "October severity index" for 1957 and 1958 was calculated from the crop sampling records for 31st October and included in Table 5010. Both indices were relatively low and in both cases followed relatively cold and wet months of August. The difference between the years is slight in terms of economic damage (the severity index is a geometric scale) and was not expressed by Empson's weather criteria. 183. FIG. 24. B. brassicae LOSS ANALYSIS, 1957 — ON 150 MEDIUM & 150 OLD LEAVES. No. of aphids A. Numbers lost 153D- -. 779 z V \A ta T. 13 O ••• • eeeeee III Various 1111111 Predators KEY TO FACTORS M Dispersal Parasites Fq Fungus 9 A B. Percentage lost TABLE 50. Data for the "October Severity Index"of B. brassicae. Severity index = 1j slight attack + 2 x % moderate attack + 3 x % severe attack (25 plants sampled) 3 a) From Empson (1952). Year October Mean air T F in August Rain days in August severity index. (Deviations from average) (Deviations from averag21_ 1947 40 + 5.2 - 12 1949 35 + 2.3 - 6 1948 25 - 0.8 + 1 1950 20 + 0.3 + 3 1946 10 - 1.7 + The present experiments Total Deviations from averages (1921-19511 rainfall cmm) 1957 20 -2.8 + 7 69.5 1958 12 -2.8 + 6 83.7 185. The factors classified as "various", which probably largely included direct climatic effects, appeared fairly constant in effect throughout the season, but appeared more important at the end of the year as winter conditions began, and in 1957 were associated with the beginning of the season in July when heavy rainfall was recorded. At the beginning of the season Coccinellid adults and larvae were of some importance in limiting the early infestation; the appearance of a variety of predators (particularly Syrphid larvae) accounted for heavy losses of aphids in August and early September, after which the predators declined, probably partly through starvation. The aphids reached their peak in late October, at which time elate progeny were being produced in the larger colonies. The dispersal of alatae and the onset of winter conditions appeared to have been the main factors reducing aphid numbers on the crop at the end of the season. Little importance could be attached to parasitism, attack by fungi or the natural death of aphids. Table 51 summarises the estimated losses of aphids due to the various factors over the whole season in 1957. The importance of the losses caused by any factor is dependent on its particular incidence in the season, and this has been summarised in Fig. 24a. Any calculation of % survival from the table is not valid, as it includes a whole season of succeeding generations of aphids. TABLE 51. Loss analysis for season 1957. (From 9309 individual aphids used in the marked colonies experiment). Factor % Loss. Various 14.86 Parasites 0.99 Predators 44.86 Dispersal 29.40 Fungi 3.05 Total 93.16 186. PART IV, THE INFLUENCE OF EDGEGROWTH ON THE ABUNDANCE AND DISTRIBUTION OF .asyipararaloastLe_L. ON THE CROP. It should be emphasised that in the present study particular attention has been paid to the effects of the "edgegrowth", i.e. the cultivated land immediate to the edges of the crop. It was here that physical effects of uncultivated land would have been likely to be of maximum importance. That "biological" effects on the crop of uncultivated land over a considerably wider area are important was clear from the review of the literature. Such widely extending effects would have affected the whole area of the field. The basis of the analysis of edgegrowth effects described here was a contrast between changes in the aphid population at the centre of the crop and at edges of differing characters. The adjacent edgegrowth was therefore primarily considered, especially bearing in mind the relatively small size of the field available. No doubt several of the effects described were not entirely dependent on the nature of the immediate strips of edgegrowth studied. 1) THE EXPERIMENTAL LAYOUT.(Figs. 6 p. 106 and 7 p. 107). The results of the preliminary work of the first year (p.99) suggested that three factors in the edgegrowth might show an effect on pest populations on a crop in a two years study. These factors were the height and density of the edgegrowth, the occurrence of alternative hosts for the pest, and the presence of flowering herbaceous plants. • The site of the study (p.103) was chosen accordingly to include both an edge bordered by trees and edges adjacent to grassland with added alternative crucifers and flowering plants. One side of the crop was continued by fallow land. 187. The crop field was divided into squares which could be grouped into areas to demonstrate edgegrowth effects (Table 52). TABLE 52. Grouping of crop squares for demonstrating edgegrowth effects. 1957 1958 Area Area Interest of area Code Squares included Code squares included Low flowering 1 20,30,40,50,60,70 1 50,60 edgegrowth 21, 31,41,51,61,71 Low edgegrowth with few flowers 2 82,92,83,93,84,94' 2 91,92 after June 85,95 Corner with flow- X 80,90,81,91 ering edgegrowth _ • •. * • • Trees 3 00,10,01,11,02,12, 3 03;04 ; 03113,04,14,05,15 01 22,32,42,52,62,72, 23,33,43,53,63,73 Centre 0/ 44,45 02 24,34,44,54,64,74 25,35,45,55,65,75 2) PHYSICAL EFFECTS OF THE EDGEGROWTH. a) Shelter from winds. A comprehensive study of the aerodynamics of shelter and its effects on microclimate has been made by Jensen (1954). He showed that the shelter effect (defined as a reduction in wind velocity) is dependent on the hole percentage of the screen, and that natural screens (i.e.hedgerows) can be considered as perforated screens. The shelter effect of trees without foliage is reduced by about 60%. Maximum shelter when there is a gap between ground and screen is at 4 - 8 times the height of the screen behind it. The shelter effect to the windward ranges from between 7% and 11% of the total effect of the screen. The line of trees on the S.W. edge of the field was equivalent to N cr (i) CO U) KEY 0 N z NE E Leeward shelter SE belt (50%) of winds indicated SW FIG. 25. WIND SHEL BELTS ON CROP AREA W NW a‘Arligirciond fteciter 0 Single trees F 189. Jensen's "medium" category (corresponding to a screen with hole areas of 50-65%). The overall height of the trees was 45 feet. Table 53 shows the likely % shelter at various distances from the trees, calculated from Jensen's graphs. Fig. 25 shows the likely extent of shelter (up to an arbitrary limit of 50%) on the experimental field. TABLE 53. Shelter effect at various distances into the crop from trees on the S.W. edge of the experimental field. Distance in feet from trees to which shelter extends % Shelter Leeward Windward S.E. S. S.W. W. N.W. N. N.E. E. 85 21 78 93 51 - - - - 75 52 153 183 i 96 - - - - 65 66 231 270 150 - - - - 50 108 __ 384 450 244 0 0 0 0 40 126 303 6 21 27 15 30 180 _405 , 12 45 54 , 30 20 234 21 75 , 90 71 10 315 42 141 168 93 0 66 231 270 150 b) The effect of shelter on the deposition of immigrating elate aphids The water trap catches of elate B. brassicae during the flight peaks in the two seasons are shown in Fig. 26. Accompanying the diagrams are wind roses for 0600 - 2100 hrs. G.M.T. for the respective periods, and "aphid roses' showing the numbers of alatae trapped on all days when the same wind prevailed. Wind directions have been corrected to the nearest 450 From the two roses it was possible to calculate an "aphid index" for each wind by calculating the number of aphids carried in 10 hours*. The "aphid index" for each wind in the two seasons is tabulated (Table 54). No. of aphids on days with prevailing wind in directiond * "Aphid index" ' 'X 10 No. of hours on which wind 'd' was recorded for any wind direction (d) 190. A. °/ohrs.wind - 0600 -21 CC rr v.T. Average speed 2.6inph ,71 2.9 2 c No. alatae per 9C 32 Al 1B leaves on crop 27 CT ID I 2 07 18 E G 39 smi ti 3004,_ Wag M 27 52.2 38.5 49.8 L K l""i"„ ."•:! 21st June -11th July, ig F n 10 20 B. No. alatae trapped on days with 20 a- eva d ing wind in direction shown _ 1M ric I I E 21 2. 4 llth -31st July,1958 No. alatae per 90 leaves on crop to 8th August FIG.26. NOS. OF ALATE B.brassicae TRAPPED AT THE BEGINNING OF THE SEASON I N WATER TRAPS A— M . 191. TABLE 54. "Aphid indices" of winds during B. brassicae flight peaks, 1957 and 1958. Season Item Wind direction N ' N.E. E. S.E. S. S.W. W. N.W. No. of hours 5 38 18 31 7 32 0 40 No. B. brassicae 28.5 13 7.5 48 2 9 0 4 1957 t trapped "Aphid index" 57 3.4 4.2 5.9 2.9 2.8 0 1 No. of hours 27 58 7 19 0 51— 9 46 No. B. brassicae 1 20 0 2 0 11 0 15 1958 trapped "Aphid index" 0.4 3.4 0 1.1 i 0 2.2 0 3.3 The high"aphid index" in 1957 of the North wind, which had the lowest mean wind speed (2 m.p.h.), is at once apparent. It is possible that some of the many alatae produced on the edgegrowth crucifers in late June were carried onto the crop or at low levels over it and contributed to the high index. In 1958, far fewer alatae were produced on the edgegrowth crucifers (p.201), and no similar high index for the North wind was observed. The numbers of alatae on the crop suggested that aphids had landed in the windward shelter of the belt of trees on the S.W. edge of the field from northerly and easterly winds, but this was not reflected in the trap catches on this edge. The trap was situated underneath the edge of the canopy and may have been too near the trees. The shelter belt on the windward side of an obstruction is extremely narrow (Table 53) and the area of aphid deposition would have been similarly restricted. In 1957, heavy catches were taken in traps F and H, which were sited near the single large tree on the N.E. side of the field. These traps were in the windward shelter area for the S.E. winds, which carried the majority of the immigrating aphids in 1957. 192. In 1958, wine were low in power and very variable. The majority of the alatae were brought by N.E., N.W., and S.W. winds. The largest numbers of alatae were recorded on the N.W. side of the field in traps G and C and crop sampling area 1. This area was not sheltered from S.W. winds, but neither was it at a windward area of shelter for any of the aphid- carrying winds; with such low wind speed5 windward shelter effects would probably be less important. The importance of the disturbance of air-current. by barriers in determining the areas of the crop where aphids are deposited has been shown by several workers, including Broadbent et al. (1951) and Johnson (1950). c) Shade. The shade cast on the crop by the trees on the S.W. side of the field in the present experiments may have affected the growth of the sprout plants in combination with other physical characteristics of crop edges adjacent to tall edgegrowth. A series of spot readings taken in bright sunshine at 1300 hrs. G.M.T. on 1st August, 1957 (Table 55) showed a definite area of shade adjacent to the trees. Under the conditions on that occasion, the shade belt was about 30 feet wide. The light reflected from a sheet of white cardboard at crop level was measured with a Weston Master exposure meter held at 3'6" above the crop. The readings (in candles per square foot) were converted to foot candles* (nearest whole number) for the table. One measurement was made in each square of the experimental grid. TABLE 55. Light in foot candles reflected from white cardboard on the experimental field. 1.8.1957. 3266 7536 7536 8164 8164 i 8792 8792 8164 8792 6280 3266 7536 8792 9169 7536 8792 9169 8792 8792 7536 3768 6908 6908 7913 7536 7536 8415 8164 7536 8164 E 3391 6280 6280 7536 7536 7536 7536 7536 6908 6908 S 2512 6908 6908 7536 7536 8164 7536 7536 7536 -024 Tree 2198 6280 7536 7536 7536 7536 7536 7536 6908 4710 * 4g12.56) ft. candles = 1 candle per sq. ft. RELATIVE HUMIDITY 80- 11 th SEPTEMBER 1957 12 th SEPTEMBER 1957 1 I ,13th SEPTEMBER. 1957 .. 4°1 • • T 1 I 1 I I r I I I I -•••••- "k--- 6 7 8 9 10 11 12 13 1415 16 1718 1920 21 222324 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 1819 2021 222324 1 2 3 4 5 6 7 8 9 10 11 1213 14 15 1617 18 M 7 Illi_•••• •••ii••••••III • • • • I I l I I I I t I I I I I I 1 I 1, •• A T°C 20 TEMPERATURE OVERALL CONDITIONS CLOUDY-BRIGHT CLOUDY T BRIGHT PERIODS ;E PE RtODS AND AND SHOWERS WITH SHOWERS I RAIN FIG. 27 SAMPLE RECORDS OF RELATIVE HUMIDITY AND TEMPERATURE OBTAINED AT AN OPEN ( )AND THE SHELTERED EDGE(-----) OF THE CROP. 194. d) Temperature. Records of the thermometers at the main recording site and in the shelter of the trees on the S.W. side of the field showed consistent differences throughout both seasons. The open edge of the field showed a greater range of temperature fluctuation than the sheltered site, with maximum temperatures 4 - 10°C higher and minimum temperatures at night lower by 0 - 7°C. The diurnal pattern of temperature at the two recording sites is illustrated (Fig. 27) by a sample of the records obtained in September 1957 when thermohydrographs were put out at both situations. e) Relative humidity. The diurnal pattern at the two sites is illustrated in Fig. 27. With a rapidly rising temperature during the morning at the open site, the % relative humidity fell steeply. At the shaded site the temperature rose more slowly and there was a corresponding gradual fall in the % relative humidity, which, for most of the day, tended to remain higher at this site than at the open edge of the field. f) Competition. The roots of the trees and shrubs on the S.W. side of the field were certainly drawing some of their water from soil within the field, and would be in competition with the crop plants adjacent to the edge. g) Effects of the edoeqrowth on the host plants of the aphid. In 1958, measurements (p.118) were made on the growth of sprout plants in the centre of the field,near crop sampling Area 1 (open edge), 195. and near crop sampling Area 3 (edge sheltered by trees). Fig. 28 compares the increase in leaf number during the season as an average of 5 plants for the open and sheltered edges. The leaves were counted from the time they were included in the medium age category. Table 56 gives the data for the individual plants of all three areas as up to 5th November, 1958. TABLE 56. Plant growth records up to 5th November 1958, comparing the number of leaves produced and the rate of leaf fall at crop sampling areas 0, 1 and 3. Area 0 Area 1 Area 3 (Centre of field) (open edge) (Edge sheltered by trees) No.of leaves % fallen No. of leaves % fallen No. of leaves % fallen produced produced produced 54 34.1 50 39.2 37 36.3 55 41.3 49 36.1 34 34.7 50 38.1 54 33.0 34 34.7 54 28.2 44 37.1 43 40.3 47 31.0 55 33.7 42 30.8 50 29.3 58 32.8 58 36.0 60 38.3 37 33.0 Mean 52.3 34.2 50.4 35..8 38 35.7t Data for Area 0 and Area 3 were tested against Area 1 by the analysis of variance, with percentage leaf fall transformed to angles. There was no significant difference between Areas1 and 0 in either number of leaves per plant (p = ).20) or in the percentage leaf fall (p = -7,-.20). Area 1, however, showed a significantly larger number of leaves per plant than Area 3 (p = <.01) as indicated by the steady greater rate of increase in Fig.28 . The figure also suggests that the plants in Area 1 consistently 196. 30- EDGE 3 20- Z 10' a. cr 0 a_ al 10- 4 iiiii s111 1 1111 KEY 0 medium leaves on the old plant fallen leaves 30- EDGE 1 Z 20_ z .- 2 10- 10- Iji111141 1 1 (23, 1 i N S FIG.28. COMPARISON OF PLANT GROWTH IN OPEN AND SHELTERED AREAS OF THE CROP, 1958 197. had more leaves than those in Area 3; i.e. that the rate of leaf fall was certainly not greater than in the sheltered area. In the analysis of variance, no significant difference between rate of leaf fall in the two areas was found (p = N.20). It was concluded that the physical conditions experienced by crop plants adjacent to tall edgegrowth may influence the pattern of growth of the plant. This in turn may affect the development rate and fecundity of phytophagous pests through the quality of the food available to them. It is generally considered that aphid populations are greatest in warm, dry years. The moisture content of host plant leaves has specifically been mentioned in the literature as a factor affecting the speed of build-up of aphid populations and the production of the elate forms. (Markkula, 1953; Rivnay, 1937). Data for this factor as an indirect effect of rainfall have already been given. It was decided to compare the moisture content of leaves from the sheltered area (crop sampling Area 3) with the remainder of the field. On several occasions in 1958 a batch of medium aged leaves was collected at random from each situation. From each batch 2 sets of 50 leaf circles (13 mm in diameter) for replication were punched with a No. 8 cork borer. Each set of 50 circles was immediately weighed and transferred o to an oven at 104 O. The circles were weighed hourly until there was no further change in weight. The loss in weight was taken as representing moisture content of the original circles and % moisture content could then be tabulated (Table 57). The angular transformations of these percentages were subjected to an analysis of variance. There was a significant difference between dates of sampling (p =<.01) but the difference between sites was also significant - 198. TABLE 57. Comparison of % moisture content of medium aged sprout leaves in Area 3 and the remainder of the field, 1958. Origin of Date leaves 22/7 31/7 11/8 27/8 Area 3 86.27 86.30 88.73 87.53 86.60 86.89 88.12 87.15 Centre and open 86.36 85.17 87.13 85.55 edges of field 86.18 86.03 86.91 85.67 to a higher level (p = -4(.001). The plants in Area 3 had a higher moisture content than those elsewhere on the field. Plants in shelter, however, normally have less sclerenchymatous tissue, and an accurate assessment of the weight of fully turgid leaf discs would be necessary to express differences in water deficit unambiguously. h) Physical effects of the edgegrowth on the increase of B. brassicae. In view of the observed effects of the edgegrowth on moisture content and number of old leaves of the host plant, the reproductive rate of B. brassicae on plants adjacent to the trees was compared with the rate on plants at the centre of the field. Marked colony records for a period of fairly steady temperature from 13th August to 20th September 1957 were used. The reproductive rate of colonies which had suffered no losses between inspections was expressed as number of young born per adult per day. 26 such examples were available from Area 3 and 31 from Area 0. At the sheltered edge (Area 3) the reproductive rate was calculated as 1.43 ± .72 young per adult per day. The corresponding rate at the centre of the field was 2.36 ± 1.78. The two sets of data were tested by a t-test, and the difference was found to be significant (p = 17.001). 199. <-1957 —F-1958--> KeY. B, brassicae RI Other aphids 011 Syrphidae MOther predators FLOWERING OF MUSTARD Edgeqrowth crucifers (Nos. on 90 plants) Crop (Nos. on 450 leaves) FIG.29. SUCCESSION OF APHIDS AND PREDATORS IN CROP AND EilGEGROWTH 200. 3) EFFECTS OF THE PRESENCE OF PLANT HOSTS OF APHIDS IN THE EDGEGROWTH. The plots of crucifers sown in the edgegrowth (p.l01) provided alternative hosts for B, brassicae. Other species of aphids, particularly Myzus persicae Sulz., also colonised the crucifers as well as a variety of plants naturally occurring in the uncultivated margins. Such aphids were of interest in the present investigation as furnishing food material for a reservoir of general aphid predators. a) The abundance of aphids on the edgegrowth crucifers (Fig. 29). In both years, the edgegrowth crucifers provided hosts for immigrating elate aphids earlier than the planting of the crop. The edgegrowth plants were already well established by the time that slate aphids first appeared in water traps. B. brassicae was a late species to arrive, and in both seasons appeared in traps and on the edgegrowth crucifers shortly before the planting of the crop. In 1958, water trap records were continuous and showed that the crop was absent during the first part of the B. brassicae elate peak (Fig. 16, p.138). The different constitution of edgegrowth crucifers in the two seasons made comparisons difficult. Numbers of all aphid species in 1958 were markedly lower than in 1957. Noticeable in both seasons was the initially low infestation of B. brassicae followed by a rapid increase shortly after the peak in the flowering of the mustard plants. From a small infestation of the leaves massed aphids appeared on the flowering stems within 7-40 days, Markkula (1953) similarly observed a rapid rate of increase of B. brassicae on flowering stems. He demonstrated with a variety of cruciferous host plant species that the rate of increase of the aphid on 201. leaves was on the average 74% of that on the flowering stems. In 1957, the massed colonies observed on the mustard plots produced large numbers of alatae. In 1958, fewer alatae were observed as the numbers of aphids on a stem were much smaller (11-63 as against 79-222 in 1957) and the number of mustard flower stems in the mixed crucifer plots sampled that year was reduced by at least two-thirds. In coming to flower (in contrast to the crop) the mustard plants in the edgegrowth were important as hosts of B. brassicae to a degree not envisaged when the initially low population of aphids was sampled. In both seasons this initial population was established before the crop was planted and therefore available to arriving alatae. b) Effects of alternative host plants on B. brassicae. i) Rate of increase. A difference in the rate of increase of B. brassicae on various host plants might affect the importance of crucifers in the edgegrowth. The rate of increase of B, brassicae on the leaves of a variety of crucifers has been studied by Markkula (1953) and Farrell (1958). Markkula found that more nymphs were produced per winged adult in a given time on white mustard than on sprouts, but Farrel was unable to demonstrate this difference with apterous adults, though he found evidence of reduced fecundity on swedes. In the present field experiment, numbers on the leaves of the edge- growth crucifers were so low that no rate of increase could be assessed. There was certainly no evidence of a high rate of increase. 202. The rate of increase on mustard and sprout plants was compared in a greenhouse experiment in July 1957. Three five week old plants of both host species were potted and placed in a ring of ventilated celluloid cages under a battery of 60 w. lamps including a central Sie-Ray lamp. The lamps supplied a 16 hour day. A central cage contained a sprout plant with screened maximum and minimum thermometers. There was considerable variation in temperature during the experiment; maximum readings ranged o o o o from 25.8 C to 28.3 C and minimum readings from 17.8 C to 19.2 C. Apterous nymphs reared in stock cultures on the respective hqst plants were transferred to a medium aged leaf on each experimental plant. Nymphs were transferred just before the moult to adult to ensure that temperature fluctuations would be reasonably uniform. For eight days after the moult to adult (which occurred in all cases within 48 hours) the daily number of progeny were counted. The mean number of young per day per adult was 2.00 - 2.16 on sprouts and 2.33 - 3.76 on mustard. These means were tested by a t-test, and the difference was found not to be significant ( p = > .90). No difference in rate of increase of B. brassicae on mustard and sprouts could be demonstrated either in the field or under controlled conditions. ii) Aphid size. Salt (1940)observed differences in vigour, fecundity, longevity and behaviour of female Trichoqramma associated with the size of the host in which development had taken place. Parasites bred from large hosts tended to reject small hosts for oviposition. It is possible that any size difference in B. brassicae on mustard and sprouts might affect the effectiveness on the crop of parasites reared from hosts on mustard in the edgegrowth. 203. The greenhouse experiment referred to above was continued for 12 days, when the aphids were washed off, separated into instars* and the following measurements taken with a micrometer eyepiece:- a) Length of body from most forward point of head between antennal insertions to tip of cauda. b) Length of hind tibia. c) Width of head including eyes. The results are shown in Table 81, Appendix V. The body lengths on sprouts and mustard for each instar were compared by a t-test. The direction in which the means differed was not consistent. The figures for mustard bred aphids were larger in the first three instars, but sprout bred aphids appeared larger in the last two instars. As there was only one instar (second) where the figures showed a significant difference, it was concluded that no size difference between aphids on the two host plants had been demonstrated. c) The edgegrowth as a parasite and predator reservoir. The edgegrowth was considered as a possible parasite and predator reservoir in the early summer before the crop was planted, and over winter, when the crop would be harvested and the resting stages of parasites and predators removed. Aphids other than B. brassicae were considered of importance in the edgegrowth reservoir, as many aphid predators have a wide host range, and at least Praon volucre and Asaphes vuloaris among respectively the parasites and hyperparasites of B. brassicae are polyphagous (Dunn, 1949). * A key for the separation of instars of B. brassicae is given in Appendix VI, p.305 . 204. Only low numbers of aphid mummies were encountered on the edgegrowth crucifers and all failed to emerge on transfer to the breeding racks. No mummies of B. brassicae occurred in edgegrowth samples, even where massed colonies of the aphid occurred on flower heads of mustard. Before the crop was planted in 1957, 14 parasitised adult aphids were found in sampling the edgegrowth mustard plants. These included a maximum of 5 on 3rd June; this would represent a total population of between 17,820 and 14,100 parasite larvae in the edgegrowth. No Praon mummies were seen, and it seemed probable that of the parasite complex affecting B. brassicae only the hyperparasite A. vulgaris could emerge from aphids on the edgegrowth mustard. As the hyperparasite Charips was abundant on the crop and attacked a large proportion of Diaeretus mummies (see p. 165 a reservoir of A. vulgaris would probably be beneficial (Barnes, 1931). Only three mummies (again not caused by Praon) were found over- wintering on Swedes in November 1957. In the early summer of 1958, no aphid mummies were found in sampling the edgegrowth crucifers. A very crude sample of some of the common non- cruciferous weeds in the edgegrowth was taken on 8th July (Table 58). Such weeds were abundant and the total number of mummies of a variety of aphid species was certainly considerable, though again no Praon cocoons were observed. TABLE 58. Crude sampling of non-cruciferous weeds for aphid parasites and predators (8th July, 1958). Plant Description No. of aphid No. of No. of of sample mummies Syrphidae Coccinellidae Holcus lanatus L. 10 flower heads 1 1 larva Dactylis glomerate L.10 flower heads 1 1 egg 1 larva 10 flower heads 5 plants 35 Rumex acetosa L. 10 plants 31 1 adult Cimium arvense (L.) 10 plants 1 larva 205. The edgegrowth was probably of greater importance as a predator than as a parasite reservoir (Fig. 29, p.199 ), though predators were not found on the vegetation over winter. In both seasons, however, predators were quite numerous in the edgegrowth in early summer, particularly Syrphidae in 1957 and Coccinellidae in 1958, The species and abundance of predators in the edgegrowth have been given in Fig. 22, p. 170 and pp.168 to 181 Table 58 also includes predators found on non-cruciferous weeds in July 1958. The Syrphid larvae on edgegrowth crucifers did not reach the pupal stage before the crop was planted, and in view of the large numbers of Syrphid eggs laid on the crop, it seemed unlikely that any contribution from the edgegrowth would have a significant effect on aphids on the crop. With larvae and adults of Coccinellidae (particularly Coccinella septempunctata) there may possibly have been some movement from the declining edgegrowth crucifers to the crop at the beginning of the season. The absence of young stages of Coccinellidae in crop sampls, taken at the beginning of the season is contrasted with the stages of Syrphidae in Table 59 and suggests that the initial population of Coccinellidae was due to the movement of active stages into the crop. The degree to which flowering of plants in the edgegrowth was important to the adult stages of aphid predators and perhaps also parasites is considered in the following pages. As flowering of the edgegrowth plants largely coinciddd with the planting of the crop in both seasons, it was not possible to divorce the effects on the crop of a predator and parasite reservoir from effects caused by flowering of the edgegrowth plants. Observations on the distribution of parasite and predator attack attributable to a combination of these effects are included subsequently (p.221 onwards). TABLE 59. Comparison of the growth stages of Coccinellidae* and Syrphidae found in samples of crop and edgegrowth crucifers 1957 and 1958. (* Only Coccinellid species found on both crop and edgegrowth are included in this table). Season Period Location Predator Growth stage (Larvae described by size category, p.111) Larvae egg First Second Third Pupa Adult size size size category category category 1957 17/6 - 5/7 Edgegrowth Coccinellidae - 2 1 - - 7 Syrphidae 33 1 1 1 25/6 - 8/7 Crop Coccinellidae - 1 2 3 17 Syrphidae 31 9 6 - - - 1958 8/7 - 1/8 Edgegrowth Coccinellidae - 2 1 1 1 Syrphidae 2 - - - 17/7 - 5/8 Crop . Coccinellidae - - - 2 Syrphidae 63 5 1 1 - 207. 4) EFFECTS OF THE PRESENCE OF FLOWERING PLANTS IN THE EDGEGROWTH. The importance of flowers as sources of adult food to parasites and predators has been summarised in the literature review (p. 40 onwards). Adults of most aphid parasites and predators will also feed on honeydew (Zoebelin, 1956). In the present experiments the aphid used, B. brassicae, was not a prolific producer of honeydew, which was only observed as free droplets in the largest colonies late in the season. As a result adult parasites and predators which were commonly trapped in white water traps and seen visiting flowers were never observed feeding on honeydew during crop sampling. Fig. 29, p.199 shows a regular pattern of peaks in the number of predators on the edgegrowth crucifers occurring before the aphid peaks and coinciding with flowering, which tends to confirm that in the present experiments flowers were a more important source of adult food for predators than aphid honeydew. Of the recorded parasites and predators of B. brassicae, Diaeretus rapae among the parasites and Syrphidae and Coccinellidae among the predators were considered of sufficient importance for a study to be made of their distribution in and around the crop. Numbers of aphidophagous Syrphidae have been bulked as it was found impossible to distinguish eggs and young larvae of different species. Numbers of Coccinellidae have been bulked to give sufficient numbers. a) Diaeretus rapae (Curt.), In May 1957, Diaeretus rapae was trapped before the crop was planted and before the edgegrowth crucifers came into flower. Highest numbers of 208. the parasite were caught on the side of the field where white Umbelliferae were in flower than on the other open side (Fig. 30a). The least significant difference for single trap totals over the period given was computed from the residual mean square of an analysis of variance of single trap catches per day. Whereas trap B had a significantly low catch when compared with traps G and H near the flowers, the catch in trap A was sufficiently high to render the experiment inconclusive. Later in the season, the D. rapae peak trap catches (Fig. 30b) occurred when the edge- growth mustard was in flower. The highest catches were again obtained in the traps among flowers (B and G), and were significantly greater than the catches in traps at the fringes of the flowering strip (A and H), and the trap at the edge sheltered by trees (I). Catches in the traps in the centre of the field (L,K,M) were not significantly smaller than in the traps among the flowers. The peak trap numbers of D. rapae in 1958 were very low (Fig. 30c), although they were again obtained at the time when edgegrowth mustard was in flower. Numbers were too low to show up a significant pattern, though all the adults trapped were taken in traps at the field edges. In all three periods of D. rapae trapping examined, daily catches in single traps were very low. The error variance formed a large fraction of the total variability and it was not possible to confirm that higher catches of the adult parasites were obtained near flowers. The first peak of parasitisation followed the peak flight of adults recorded by trapping. The proportion of adult B. brassicae mummified by D. rapae in areas of the crop adjacent to flowers and elsewhere was compared 209. 1957 1958 Al is KEY x - Position of flowers 10 x Diaeretus rapae (Curt.) 5 —Scale L.S.D. -Least significant H difference a) 1st-25th May A- M - Water traps L.S.D. = 7.26 Alx Bxxxxxxxx x Alxxx a.Mxxxx CT D x —x 101 x x II 1 x M Diaeretus ralag (Curt.) DB lj K L H L K b) 21st June- 11th JUly c)24th July -21st August L.S.D. = 4.54 L.S.D.= 2.60 Aax1B xxxxxxxxxx Ajxxx IMxxxx CI ID Aphidophagous M Syrphidae •IN R r H -rt d) 24th June -30th July e 1st July - 31st August L.S.D. = 6.61 L.S.D. = 4.09 Aix ABxxxxxxxxx A A xxx _Mxxxx CI —D x . X E G — X II X x Coccinellidae X 1 x I M D E AK LAK uri Wow f) 28th June -30th July g) 1st July - 31st August L.S.D. = 4.25 L.S.D. = >1 FIG.30. DISTRIBUTION OF PARASITES 8 PREDATORS IN WATER TRAPS TABLE 60. Comparison of attack on B. brassicae by D. rapae in different areas of the crop during the first peak of attack. Area Situation No. of adults No. of % Expected no. of p • and mummies of mummies mummies mummies on nul B. brassicae hypothesis (based on total % mummies) Season 1957 (July 8th - August 23rd Adjacent to 2 1 flowers 100 15 15.00 12.98 Sheltered, few flowers near 3 edge of field 175 26 r 14.85 I 22.72 Adjacent to 1 flowers 143 20 13.99 18.56 >.8 Adjacent to X flowers 33 4 12.12 4.28 0 Centre of crop 381 43 11.29 49.45 Total 832 108 12.98 - Season 1958 TJuly 30th - August 27th). Sheltered, few flowers near 3 edge of field 6 2 33.33 1.24 Adjacent to 1 flowers 26 8 30.76 5.35 O Centre of crop 8 0 0.00 1.65 ›.5 2 Adjacent to flowers 28 4 14.29 5.77 Total 68 14 20.59 - 211. by 9(2 test (Table 60). No significant differences between areas occurred in either season and it was concluded that the distribution of aphid mummies on the crop had no relation to the presence of flowers in the edgegrowth. In May and June 1957, two white and two green water traps were set up in grassy areas of the edgegrowth at the unsheltered sides of the crop field. Female aphid parasites trapped were dissected and the reproductive organs examined to see whether eggs had already been laid. If the visiting of flowers for adult feeding played an important role in the egg maturation and oviposition cycle of these parasites, it might have been possible to separate females searching for hosts on the vegetation and females searching for flowers by the use of coloured traps. The female reproductive organs of the Aphidiidae have been figured and described for Aphidius by Henneguy (1904). The female reproductive organs of Diaeretus rapae are very similar (Fig. 31). The two ovaries are unusual in that, although follicles are differentiated, the rather sac- like ovaries are not divided into ovarioles. The number of egg rudiments was impossible to assess,and the technique published by Lewis (1958) for the recognition of nulliparous and parous Simulium damnosum Theo. and Anopheles qambiae Giles was attempted, but no corpus luteumor cord on the oocytes could be distinguished. Very few D. rapae were trapped, and another species of Aphidiidae in the genus Aphidius trapped in larger numbers was used for the experiment. The following measurements were made on each half of the reproductive organs, using a micrometer eyepiece :- 212. KEY TO FIG. 31. acc ' Accessory gland e = Egg g = Germarium lag = Length of accessory gland le = Length of egg ig = Length of germarium lo = Length of ovary o = Ovary ov = Oviduct sp = Spermatheca t9 = 9th tergum 213. I 1 mm. _____i g FIG. 31. Diaeretus fripae. (Curt.) FEMALE REPRODUCTIVE SYSTEM 214. a) Length of ovary (10, Fig. 31) b) The ratio of the length of the germarium to the total length of the ovary 0.2., Fig. 31) lg c) The length of the largest egg. d) The ratio of the greatest width to the length (lag, Fig. 31) of the accessory gland. The mean values for each measurement from green and white traps (Table 61) were compared by a t-test, and no difference proved significant. There was therefore no evidence of any attraction to white of the nulliparous parasites. The effect of flowers on adult parasites had been tested by analysing the distribution of adults as recorded by traps, by examining the possibility of ovarial characters being associated with a possible colour response contrasting flowers with the habitat of the host, and by studying the distribution of parasite attack on the crop. No evidence was found for supposing that adult feeding on flowers by aphid parasites was of any importance in the present experiments with reference to parasite attack on B. brassicae on the crop. b) Aphidophaqous Syrphidae. In both years, peak numbers of Syrphids were trapped when the edgegrowth was in flower. In 1957, there was no consistent difference (Fig.30d) between the numbers trapped among flowers and in different areas of the crop. In 1958, very few Syrphids were trapped on the crop, and numbers in traps A and B among flowers had a significantly large catch. It was noticeable that the majority (53 out of 77) of Syrphids trapped were TABLE 61. Measurements on the female reproductive organs of Aphidius sp. caught in green and white water traps in 1957. In green traps In white traps Measurement No. measured Mean Fiducial No. measured Mean !Fiducial , limits -. Ilits Ovary length 21 0.451 mm ±0.217 mm 14 0.489 mml ±0.186 ma 4.3 i Length of ovary Length of germarium 21 2.18 ±1.74 14 2.05 .32±0 ›.9 Length of + 0.069 mm ±0.026 mm 14 0.076 mm -0.043 mu largest ogg 20 .....10, 4 Length of access►r land + ± 16.00 -7.30 Greatest width ' 19 15.32 .609 9 .. _, 216. males. In view of the evidebce in the literature (p. 40) concerning the importance of flower feeding to female Syrphids, the high proportion of males in the water trap catches may explain the rather even distribution of adult Syrphids obtained by trapping. A number of females was dissected, and pollen grains found in the alimentary canal showed that flower feeding had taken place. The numbers of eggs laid in different areas of the crop, as recorded in the crop sampling records, were totalled for each season. The maximum egg numbers (Fig. 23, p. 174) occurred after the adult peak in traps when the edgegrowth was in flower. As only one peak of egg-laying was recorded, it was considered justifiable to include in the total the small proportion of eggs laid later in the season. A single count of Syrphid eggs was made on 5th August 1958, when one young, one medium, and one old leaf were sampled on 40 plants in each of three areas of the crop. A„.2 The results (Table 62) were compared by a )„ , test. Areas of the crop adjacent to open flowering edgegrowth consistently had a significantly high catch (p = (.001) when compared with the sheltered edge and the centre of the crop. The field had no edges without weed flowers, except for the sheltered edge, which showed a significantly lower number of eggs than both the open edges and the centre of the crop in both season comparisons (p = 4001). In order to check that the apparent effect of flowers was not purely an edge effect, the egg data from the open edges were divided into two blocks for the Syrphid peak oviposition period in 1957. (.17th July - 31st August:- 1) Edges of the crop adjacent to flowering mustard - squares 50, 217. 61, 70 of Edge 1, squares 81 and 90 of Area X, and square 92 of Area 2 (Fig. 32A). ii) Parts of the same edges of the crop outside the strip of flowering mustard - square 10 of Area 3, squares 30 and 21 of Area 1, and squares 83, 95 and 94 of Area 2 (Fig. 32B). The number of eggs (Table 63) found in those parts of the open crop edges adjacent to the strips of flowering mustard was significantly higher (p.=•x'.001) than the parts of the same edges outside the flowering strip. The latter areas had an egg total which hardly differed (p. = 2>'.7) from the figure at the centre of the crop. It was concluded that flowers in the edgegrowth caused a marked increase in Syrphid oviposition at the adjacent edges of the crop, whereas the non-flowering belt of tall edgegrowth resulted in few eggs being laid on the adjacent crop edge. c) Coccinellidae. Coccinellidae were the first aphid predators to be found in numbers on the crop. As mentioned earlier, (p.205), there appeared to be no oviposition on the crop, and the late immature stages and adults must have arrived from outside the crop. Coccinellidae were most numerous in the edgegrowth at the time of flowering of the mustard (Fig. 22, p.170), but, as aphid numbers were highest at the same time, it was impossible to separate any effect of flowers from the effects of prey availability. Only the remains of aphids were found in the alimentary canal of dissected Coccinellid females; no pollen grains to indicate flower feeding were observed. The invasion of 218. X XXXXXXXXXXXX x o x i B x 2 X 3 C 4 B O i 2' 3 4 5 ' 6 7 1 8 9 X - Flowers A - Open edges adjacent to flowering mustard B- Other parts of open edges C - Centre of crop FIG. 32. DIVISION OF CROP FOR COMPARISON OF SYRPHID OVIPOSITION T — Trees X— Flowers — Larvae — Pupae — Adults i i 1 FIG. 33. DISTRIBUTION OF ON CROP AREA, T COCCINELLIDAE 25th. June to 31st. August 1957 TABLE 62. Comparison of numbers of Syrphid eggs laid in different areas of the crop, 1957 and 1958. Period Area Situation Size of No. of i Expected no. eggs I Nature of p sample Syrphid eggs on null hypothesis! difference Season 1957 1,X Open edges 180 leaves 739 627.6 + <:001 2 adjacent to flowers 1 01 Centre of 180 leaves_ 572 1 627.6 - 02 crop .G001 3 Sheltered, few 90 leaves 258 1 313.8 - - flowers near i edge of field Season 1958 1,2 Open edges 180 leaves 529 i 352.0 3 + 1 <.001 adjacent to t flowers 1 0 Centre of 180 leaves 240 I 352.0 - crop or 90 leaves x 2 <.001 3 Sheltered, few 90 leaves 111 176.0 - - flowers near edge of field 5th August 1958 1 Open edge 120 leaves 38 i 25.7 i + adjacent to c:.02 flowers i. 1 3 Sheltered, few 120 leaves 20 25.7 - • flowers near ›.10 edge of field 0 Centre of 120 leaves 19 25.7 J - - crop TABLE 63. Comparison of numbers of Syrphid eggs laid near flowering and non-flowering edgegrowth and at the centre of the crop, 17th July - 31st Auiust, 1957. Squares on Situation Size of No. of Expected no. eggs P field plan sample Syrphid 3qqs on nul hypothesis 50, 61, 70 Edges adjacent 90 leaves 367 297.5 < .001 81, 90, 92 to flowering mustard (Fig. 32A) 10, 21, 30 Parts of same 90 leaves 228 297.5 83, 85, 94 edges of crop (Fig.32B) 232.3 )a Areas 01, Centre of crop 180 leaves 469 464.6 02 (Fig. 32C) 221. the crop by the active stages of Coccinellidae appeared to take place very rapidly and no differences in distribution on various areas of the crop ascribable to the presence of flowers in the edgegrowth could be observed, either in trapping (Fig. 30, f and g) or in crop sampling (Fig. 33). 5) ANALYSIS OF LEVELS OF B. brassicne INFESTATION ON THE DIFFERENT AREAS OF THE CROP Season 1957. The availability of both crop sampling and marked colonies records enabled a fairly detailed analysis to be made. On the basis of Fig. 24, p. 183, the season was divided into 3 periods (Table 64). The size of the aphid population in the various areas calculated from the crop sampling records is shown in Fig. 34, while the progress of an initial 100 aphids per crop area calculated from the marked colonies records is illustrated in Fig. 35. Differences in the proportions -,2 of aphids were tested by a X, test, considering the centre of the crop as a control area. TABLE 64. Division of season 1957 into periods for analysis. Period Dates 1 Apparent main factors causing aphid loss , (cf. Fig. 24, p. 183) T I 8th July - 9th August' Various, Predators (Coccinellidae) II 10th August - 7th October Predators (Syrphidae) III 24th October - January Dispersal Period I (8th July - 9th August 1957). The initial arrival of alatae in late June and early July appeared related to the distribution of shelter around the field (pp. 189-192 ). This resulted in the numbers of aphids at the crop edges being slightly,though consistently, higher than at the centre 222. NO. aphids on No. aphids on 90 leaves 90 leaves No. aphids on 90 waves No aphids on 90 leaves 7lnr EDGE 3 50- 40. 3(0- 1 PERIODS FIG. 34 COMPARISON OF BrevIcoryne brcss,coe . P'7 4." .S IN DIFFERENT AREAS OF THE CROP 1G5 Least significant difference 16 8°1. 223. of the crop. The heavy rains of mid-July (Fig. 8, p.109) were probably responsible for the high loss caused by "various" factors, and, together with Coccinellid predators, accounted for the decline of the arrival peak in all areas. As the weather improved at the end of July the numbers began to build up again though the first Syrphid larvae appeared at the same time. The losses of aphids during the whole period are tabulated (Table 65) from the marked colonies records. "Various" factors (Mainly rainfall ?). There was no significant difference between the open edges (1, 2 and X) and the centre of the crop (0), but the sheltered edge (3) had a significantly high loss (p = .01). This may have been due to the heavier"rainfall" of wet trees stirred by the wind or the larger drops running off the foliage. It was noticed that the soil in this area of the crop was often heavily pitted by water drops after rain. TABLE 65. Losses of B. brassicae in different areas of the crop - Period 1, 195/. Factors causing aphid loss No. aphids Croparea duringeriod Various Predators Parasites No.lost yj loss No.lost % loss No. lost AL loss 359 113 31.5 5 1.4 18 5.0 1 268 100 37.3 43 16.0 23 8.6 2 + X 178 46 25.8 46 25.8 10 5.6 3 139 62 44.6 39 28.1 14 10.1 Predators. Predation at all edges was heavier than at the centre (p = .001,. Among the edges, Edge 1 had a significantly lower loss than the other open and the sheltered edge (p = .<.01). The difference between Edges 3 and 2 + 1 and the centre was in agreement with the numbers of active predators (mainly 1 2 3 F1G. 35. PROGRESS IN DIFFERENT AREAS OF THE CROP OF A HYPOTHETICAL COLONY OF 100 a brossicae ON 8th JULY.1957 225. Coccinellidae) recorded in these areas while crop sampling; a similar comparison with Edge 1 and the centre gave no significant difference (Table 66). Parasites. There was no significant difference in the numbers of aphids lost through parasitisation in the different areas (p = > .2). The lack of any demonstrable effect of edgegrowth flowers on parasitism on the crop has already been noted (p.214). TABLE 66. Number of active predators in different areas of the crop - Period I 1957. A No predators Expected no on nul hypothesis it 3 (6 squares) 17 5 4. .001 2 + X (6 squares) 17 1 (6 squares) 8 5 1,43 0 (12 squares) 10 Control Overall effects - Period I. A comparison of marked colony records (Fig. 35) for this period shows the more rapid decline in numbers near the sheltered edge (Area 3) than in the other areas of the crop. The losses on Edge 1 appear to have been of the same order as at the centre of the crop but higher numbers of aphids remained due to the broken pattern of the decline. Up to 25th July very few, and sometimes no predators were observed during the crop sampling routine in Area 1. At this time, Coccinellidae appeared to be the main predators, active stages of which, from the small quantity of data available (Fig. 33, p.218), seem to have been more infrequent in Area 1 than elsewhere. This early lack of predators may account for the broken decline of the graph. 226. Period II (10th August - 7th October, 1957). Throughout this period, predators appear to have been the principal factor causing aphid loss (Fig. 24A). Syrphid larvae (Fig. 23, p.174 ) were at their most numerous, though aphid numbers still tended to increase with the more favourable climatic conditions until late August, when the general level of temperature fell (Fig. 8, p.109 ). Towards the end of September, the number of predators on the crop fell and the aphids built up towards their October peak. The level of the aphid population in the edge areas of the crop, particularly the corner of the open edges(Area X), never reached the high peaks found at the centre of the crop in early September and early October (Fig. 34). The losses of aphids during this period are tabulated (Table 67). "Various" factors. There was a significantly low loss in this category at the sheltered edge (p = -c.001) and at Edge 1 of the open edges(p = The other open edge (2), however, showed no significant difference when compared with either Edge 1 (p = >-.30) or with the centre of the crop (p = 7.›.05). These conflicting results were not analysed further, as losses attributable to the "various" factors were below 10% in all areas. Predators. Predators, particularly Syrphid larvae, caused heavy losses of aphids during this period. There was no significant difference between the two open edges (p = )' .20), but losses were considerably heavier than at the centre if the crop (p = <7.001). The sheltered edge had low')r losses, of the same order as the centre of the crop (p = > .70). These differences were considered to be the result of the more abundant oviposition by Syrphids near flowers along the open edges (p.217 ). A very low percentage of Syrphid larvae reached the 2nd size category (Table 47, p. 177) and the TABLE 67. Losses of B. brassicae in different areas of the crop - Period II, 1957. Factors causing aphid loss INo. aphids Crop Arealduring period Various Predators Parasites Fungus Dispersal No. lost % loss No. lost % loss No. lost % loss No. lost,% loss Nolost % loss 1 0 2497 202 i 8.1 1199 48.0 3 0.1 68 ' 2.7 507 20.3 j 1 1222 73 6.0 789 64.6 1 i 0.1 0 1 0.0 162 13.3 2 + X 1885 129 6.8 1281 70.0 4 1 0.2 18 1.0 206 10.9 F 3 1453 67 4.6 689 47.4 2 i 0.1 0 0.0 3.9 228. bulk of predator loss must be ascribed to young larvae. This is illustrated by the decline in predation losses on the whole field area (Fig. 24B, p. 183 ) in mid-September, when, although second-and third size larvae were still as numerous as before, the number of first size larvae had fallen off markedly (Fig. 23, p.174 ). A greater number of Syrphid eggs and young larvae were found at the edges of the crop, rather than at the centre or sheltered edge during this period (Fig. 36). Parasites. After a high percentage emergence of hyperparasites from the due first peak of parasitised aphids (Fig. 21, p.166 ), the loss/to parasitisation was extremely and equally low in all areas of the crop. Fungus. Attack of aphid colonies by fungus was noted to be sporadic in appearance. The loss of very few aphids was causeCiby this factor, and the significantly high loss at the centre (p = <.001) of 2.7% was due to the inclusion among the marked colonies of one large colony which subsequently suffered fungal attack. Dispersal. A significantly higher loss of aphids at the centre of the crop than at the edges was recorded (p = 4:.001), though all edges differed significantly from each other (p = <.02 for edges 1 and 2+X, p = <.001 for edges 2+X and 3). The density of aphids, whether acting directly or through the host plant, is probably an important factor in the production of alate nymphs. From the marked colonies records(Fig. 24) dispersal began to be of importance from mid-September onwards. The angular transformation of percentage dispersal (y) in Period II (Table 67) was plotted against the log. number of aphids per 90 leaves in each area (x) in early September (from the crop sampling records for 9th September, Table 68). 229. Nos. on 180 leaves KEY Eggs •Size 1 larvae ■Size 2 larvae Size 3 larvae ■Pupae Nos. on 180 leaves Nos. on 90 leaves FIG. 36. NUMBERS OF SYRPHIDAE IN DIFFERENT AREAS OF THE CROP, PERIOD 2 -- 1957 230. A linear regression could be fitted by the equation y = -47.85 + 31.243 (± 15.631)x for the range covered by the data. The regression was tested by an analysis of variance, and was found to be significant (p = The percentage dispersal per area could be predicted in terms of aphid density. TABLE 68. Number of B. brassicae in various areas of the field crop sampling recor.s for 9th September, 1957. Area No. aphids per 90 leaves 0 250 1 153 2 + X 156 3 91 Overall effects - Period II. A comparison of the marked colony records for this period (Fig. 35) shows the lower aphid numbers in the edge areas than at the centre of the crop, particularly the steady decline in numbers caused by predators from the end of August at the open edges. The corner area (Fig. 34) maintained the lowest numbers of aphids, this area falling within that part of the field where most Syrphid eggs had been laid (Table 63, p220 ). Of the two open edges, Edge 1 started the period with a higher aphid level on the marked colony assessment due to fewer early predators (Table 66, p.225 ). The initial increase in aphids, up to the end of August, was of the same order of magnitude at both edges. There was, however, a contrast between the sudden decline at Edge 1 and the gradual decline at Edge 2 +X in Fig. 35. The marked colony records for these two areas showed very different losses between the 3rd and 10th September (Table 69). 231. TABLE 69. Comparison of main factors causing aphid loss in Areas 1 and 2 X, 3rd - 10th September, 1957. 3rd - 6th Sept. 6th - 10th Sept t/::)- loss caused by Area Predation I Dispersal Predation Dispersal 1 57.0 0.0 17.9 61.0 2 + X 13.9 3.5 18.0 8.3 Probability of difference p =<-.001 p =<.001 p =>.5 p ..--<.0011 arising by chance * See note to p(s), Table 11, p.81. The steep decline in Area 1 was undoubtedly caused by the high losses from predation and dispersal between 3rd and 10th September. Between 1st and 6th September, crop sampling records contained 17 active predators for Area 1 and 7 for Area 2 X and, from 6th - 13th September, 8 for Area 1 and 7 for area 2 + X: although such records are insufficient, they do give the same trend as the pattern of predationin Table 69. During Period II, the sheltered edge (3) formed an interesting contrast with both the centre and with the other edges. Although aphids in the sheltered part of the field appeared to suffer considerably smaller losses than any other area (Table 67), numbers increased only slowly (Fig. 34), and the populations reached were comparable with the open edges rather than with the centre of the crop. This was attributed to the slower rate of reproduction observed at the sheltered edge compared with other areas of the crop (p. 198). Period III (24th October, 1957 - January, 1958). This period commenced at the tail end of the peak in aphid numbers and continued for the decline of 232. this peak towards winter. Dispersal appeared to be the main factor causing aphid loss, although losses falling in the "various" category were consider- ably increased over those of the previous period. The numbers of predators (fig. 23, p. 174) had considerably decreased. The mean temperature dropped rapidly; rainfall was not heavy (Fig. 8, p.109). The number of aphids in all parts of the crop declined fairly steadily, and the final population levels reached in January were of much the same order in tall parts of the crop. The losses of aphids during this period are tabulated (Table 70). "Various" factors. Losses attributable to these factors were lighter than in Period II and were fairly even over the areas compared. Only the higher losses at the sheltered edge (3) differed significantly from losses at the centre of the crop (p = 44%001). The losses in Area 3 were noticeably higher than in the other areas only during the rainy period (Fig. 8) from the 4th - 11th December, and some effect of the trees on the direct depletion of aphid numbers by rainfall (cf. p.223) may have been operative. Predators. The overall losses by predators (Fig. 24A) were considerably lower during Period III than in the previous period, and were negligible after November 20th. The centre of the crop again had significantly lower losses than the open edges (p = <.001), which did not differ significantly from each other (p = .05). The sheltered edge showed very little depredation by predators, with a loss significantly lower than the centre of the crop (p = predator distribution during Period II, though numbers of predators after mid-October were too small to enable a comparison between areas to be attempted. TABLE 70. Losses,of B. brassicae in different areas of the crop - Period III, 1957. Factors causing aphid:loss. Crop area No. aphids during period—Various Predators -Parasites--. Fungus Dispersal No...lost % loss No. lost % loss No. lost % loss No. lost% loss No. lost % loss 0 1269 256 "20.2 - 129 10.2 ' 9 0.7 1 93 7.3 715 i 56.3 1 527 116 22.0 136 258 . 0 0.0 8 1.5 259 I 49.2 , 2 + X 541 .124 . 22.9 160 29.6 4 0.7 69 • 12.8 142 i 32.9 359 .103 . 26.9 13 3.6 1 0.3 27 7.5 178 439.6-- 3 • 234. Parasites. Attack by parasites was again extremely low, with no significant differences between areas of the crop (p = 7:7.10). Fungus. Fungus attack was more important during this period than in Period II. Edge 2 + X of the open edges had a significantly high loss attributable to this factor (p = <.001), whereas the other open edge showed a significantly low loss (p =.<:.001) when compared with the centre of the crop and the sheltered edge. 61 out of 69 aphids attacked by fungus in Edge 2 + X were included in a single record of fungus attack on a large colony of 218 aphids, whereas the 8 lost in Edge 1 were in a single colony of 24 aphids. With such large proportions of the losses caused by fungus based on single colonies, it was felt that no valid conclusions as to the distribution of fungal attack could be drawn. Dispersal. The pattern of dispersal was hard to interpret, probably owing to the number of elate generations which had elapsed, and the fluctuations in aphid numbers in the various, areas since the middle of Period II, when dispersal first became important. The centre of the crop retained the character of higher dispersal loss than the edges (p .001): but, of the edges, although Edge I had a higher loss than the sheltered edge (p = x.001), Edge 2 had significantly lower losses (p = .02). This could not be explained. Overall effects - Period III. A comparison of marked colony recorls for this period (Fig. 35) shows the decline in all areas mainly caused by the departure of elate aphids and partly by the factors classified as various, which probably included the death of aphids with the onset of winter conditions. The rapid decline at Edge 1 by early November and the steepness of the decline at the centre of the crop reflected the high dispersal loss 235. in these areas. Season 1958. Because of the very low numbers of aphids in 1958, the setting up of marked colonies proved impracticable, and the detailed analysis of aphid losses attempted in 1957 could not be repeated. The size of the aphid populations in the various areas of the crop calculated from the crop sampling records as shown in Fig. 37, and on the pattern of the graphs the season was divided into 4 periods (Table 71). In attempting to analyse the differences in aphid populations between areas, data from the crop sampling records on the numbers of predators, the proportion of aphids mummified by parasite attack, and the proportion of colonies showing elate nymphs have been used, though no information relevant to aphid losses in the "various" category was available. (2 Data from the various areas of the crop were compared by a A" test, considering the centre of the crop as a control area. TABLE 71. Division of season 1958 into periods for analysis. Period Dates Apparent main factors causing aphid loss (cf. p.236) 1 I 17th July-18th August Predators, Rainfall II 13th August-9th September Predators, Rainfall III 10th September-lst November Parasites IV , 2nd November-5th December Climate, Dispersal Period I (17th July - 12th August, 1958 ). The distribution of the initial arrival of alatae in mid- and late July (p. 192) resulted in a higher * The areas compared are shown in Fig. 7, p. 107. 236. % APHIDS ON DIFFERENT LEAF AGES - EDGE I APHIDS ON DIFFERENT LEAF AGES - EDGE 3 No.aphids on 90 leaves 20r No. aphids on 90 leaves 180. HD. EDGE 3 150. 813. 140- 60. 120-1 40- EDGE 1 113D- Centre 20- 80, 60. 40. Centre APHIDS ON DIFFERENT LEAF AGES - EDGE 20. 0 1 2 f 3 4 PERIODS No. aphids an 90 leaves % APHIDS CN DIFFERENT LEAF AGES - CENTRE 120- EDGE 2 40-1 Centre 20- FIG. 37 COMPARISON OF Brevicorre brass/cm L. POPULATIONS IN DIFFERENT AREAS OF THE CROP, 1958. Least significant difference .11-0 '1. 1 2 3 4 PERIODS 237. infestation in Area 1 than in the other areas. The regular rainfall (Fig. 8, p.109 ) throughout the period and depletion by predators reduced the arrival peak in all areas to low levels by the end of the period. "Variousufactors. An increase in the losses of aphids in this., category at the sheltered edge was noticed under conditions of heavy rainfall in 1957. Rainfall during the period was frequent and it is possib1? that Edge 3 suffered more heavily than the other areas of the crop. Predators (Table 72). Anthocoridae were fairly numerous at -else beginning of the period and Syrphid larvae began to appear in samples at the July (Fig. 23, p. 174). TABLE 72. Number of active predators in different areas of the crop Period I, 1958. Area Size of sample per No. predators, sampling occasion 0 180 leaves 3 1 90 leaves 10 2 90 leaves 4 3 90 leaves 10 Edges 1 and 3 had significantly high numbers of predators (p =.4-1.01). Samples at the sheltered edge consistently contained nymphs and adults of Anthocoridae which were very numerous feeding on aphids on the leaves of the overhanging trees and may have fallen from these onto the crop beneath. The predators in Edge 1 were also mainly Anthocoridae, but these were absent at both the other open edge and the centre of the crop. Parasites (Table 73). Numbers of adults on the crop were very low and estimations of their percentage mummification were too unreliable to show any differences between areas (p = 238. TABLE 73. Mummification of adult aphids by parasites, Period I, 1958. Area No. adults and No. mummies % adults parasitised mummies 0 6 0 0 1 17 6 35.3 2 16 2 12.5 3 3 0 0 Overall effects, Period I. A comparison of the area graphs for the crop (Fig. 37) shows the low population level at the centre of the crop falling away to almost zero at the end of the period. Losses caused by rainfall were possibly heavier at the sheltered edge and this combined with the higher number of predators kept the numbers at Edge 3 very low indeed. Of the open edges, Edge 1 received the higher initial infestation, but this was reduced rapidly by rainfall and predators. The higher population at Edge 2 than at the centre of the crop could not be explained in terms of factors causing loss, but may possibly have been the result of a difference in initial infestation not nshown in the water trap catches. Period II. (13th August - 9th September, 1958). Rainfall during this period was again heavy and frequent, and this, combined with the rise in the numbers of Syrphid larvae on the crop (Fig. 23, p4 174), kept numbers of aphids low in all areas. "Various" factors. Losses at the sheltered edge may again have been heavier than in the other areas (P.M). Predators (Table 74). Owing to the low numbers of aphids, mortality of Syrphid larvae by starvation was probably heavy (p. 176). The differences in oviposition noted (p.217) did not show in higher numbers of larvae at the open edges in contrast to the previous season, though the significantly 239. low numbers of predators at the sheltered edges was in agreement with the recorded egg numbers. TABLE 74. Number of active predators in different areas of the crop - Period II, 1958. Area Size of sample per No, predators Significance of sampling occasion (Mainly Syrphid larvae) difference between area and centre p = 0 180 leaves 27 MO 1 90 leaves 21 ›..05 2 90 leaves 19 >.10 3 90 leaves 5 .‹....05 Parasites. (Table 75 ). As in the previous period, numbers of adults were very low, and no differences in percentage mummification by parasites between areas were observed (p =7..30). TABLE 75. Mummification of adult aphids by parasites - Period II, 1958. Area No. adults No. mummies adults parasitised and mummies 0 4 0 0 1 10 2 20.0 2 16 2 12.5 3 6 2 33.3 Overall effects,Period II. A comparison of the area graphs for the crop (Fig. 37) shows the low aphid numbers in all areas, particularly after the heavy rain of 20th - 25th August. The sheltered edge showed extremely few aphids under heavy rainfall conditions in spite of fewer predators than were present on the remainder of the crop. Numbers at the open edges appeared to fluctuate considerably more than in the other areas, but at such low aphid levels this was entirely due to the periodic inclusion in the samples of 240. single larger colonies which had probably survived from the higher levels of initial infestation. Period III. (10th September - 1st November). During this period, numbers in all areas increased to a late October peak. This general increase after the consistent low levels during the first two periods of the season could be due to three factors, to some extent connected (a) Days with rain became slightly less frequent (Fig. 8, p.109 ). (b) Small numbers of aphids began to appear consistently on older leaves (Fig. 13, p.131 ) thus probably speeding aphid increase (p.155). (c) By 25th September predators had virtually disappeared from the crop. "Various" factors. (See Periods I and II). Predators. (Table 76). As in Period II, no differences in the number of predators showed between the open edges of the crop, which had been bordered by flowers earlier in the season, and the centre. The sheltered edge also showed similar numbers of predators to the other areas (p = ;>.20). TABLE 76. Number of active predators in different areas of the crop Period III, 1958. Area Size of sample per No. of predators sampling occasion (Mainly Syrphid larvae) 0 180 leaves 11 1 90 leaves :7 2 90 leaves 10 3 90 leaves 8 Parasites (Table 77). Parasitisation appeared to be the main factor 241. effecting differences in,aphid loss between areas of the crop in this period. The proportion of adults which became mummified at Edge 1 and the sheltered edge was significantly M*Tile* than at the centre of the crop or the other open edge. It is suggested that this difference may have been due to heavy losses of the unmummified larvae caused by predators in the earlier generation of parasites in Period L, though one might expect the searching of emerging parasites from this earlier generation among a low population of aphids to have ironed out the differences between areas. Dispersal. Alate nymphs began to appear occasionally in samples from 18th September in the larger colonies of aphids. It seems unlikely that dispersal reduced the numbers of aphids remaining on the crop significantly, and a crude comparison of areas (Table 78) based on the proportion of colonies with elate nymphs showed no difference between areas (p = > .50). Overall effectst Period III. A comparison of the area graphs for the crop (Fig. 37) shows differences in the degree of the autumn build-up of aphids which occurred in all areas. All areas started the period with similarly low numbers of aphids and parasitisation appears to have been the only major factor of aphid loss which could cause later differences. A comparison of the leaf age distributions of the centre and open edges (Fig. 37) suggests that there would be no basis for assuming that a differential reproductive rate between these areas was operative. Regarding the build-up of aphids as a logarithmic rather than as an arithmetic progression, it was possible to predict fitting values for the aphid peak at both the open edges from the aphid peak at the centre of the crop, by comparing the proportions of adults which escaped parasitism in TABLE 77. Mummification of adult aphids by parasites - Period III, 1958. * Expected no. = Antilog. (fraction adults surviving in area x log. No. aphids at peak ) fraction adults surviving at centre at centre of crop Area No. adults + iNo. mummies' % adults Significance ' No. aphids Expected' Significance mummies ' parasitised of cliff. ! per 90 leaves no.* ! of diff. between area ; at peak 1 between area and centre 1 ; and centre p = I p = 0 73 21 28.8 - 78 78.0 - 1 102 I 3 2.9 <.001 231 204.9 77.90 2 82 16 19.5 > .10 115 137.0 >.05 3 75 5 6.7 <.001 93 162.5 <.001 TABLE 78. The proportion of colonies with elate nymphs in different areas of the crop, Period 111, 1958. Area No. of colonies No. colonies with % colonies with elate nymphs elate nymphs 0 81 2 2.5 1 38 3 7.9 2 41 0 0 3 20 2 10.0 244. each area (Table 77). The value similarly obtained for the sheltered edge was significantly discrepant, indicating that aphid build-up had been slower than a common level in the other areas. The possibility that rainfall caused heavier losses at the sheltered edge has been mentioned (p. 223), but probably very important was a slower increase of aphids at this edge, as had been recorded in 1957 (p. 231). Period IV. (2nd November - 5th December, 1958). This period included the decline of the late October peak in aphid numbers with the onset of winter conditions. A large proportion of aphid loss was probably due to weather conditions, particularly the drop in numbers in all areas in early November could be associated with the rainfall (Fig. 8). Predators were very few (Table 79), and numbers of adults and parasite mummies were too low to show any reliable differences between areas (p =';'.20). The percentages of colonies with elate nymphs differed widely, but were based on too low numbers to be reliable (p =77-.30). A linear regression of the number of aphids at the end of sampling (y) on the number of aphids at the late October peak (x) in each area could be fitted by the equation y = 26.98 + 3612 (1-.2655) x over the range covered by the data. The regression was tested by an analysis of variance, and was found to be significant (p =.4‘ .05). This result confirmed the crude analysis of Table 79 that there were no major differences in aphid loss between areas during this period. TABLE 79., Analysis of several factors accounting for aphid losses in different areas of the crop - Period IV, 1958. PREDATORS PARASITES DISPERSAL ...., Size of sample No. of No. colonies % colonies per sampling predators No. adults 1% adults ' No. with elate with alate Area occasion observed and mummies iNo. mummies parasitised colonies nymphs nymphs 0 180 leaves 0 23 I 1 1 4.4 32 8 25.0 I-- 1 90 leaves 0 96 15 15.6 22 6 27.3 2 90 leaves 1 28 8 j 28.6 26 1 3.9 3 2 8 0 0.0 9 1 1.1 90 leaves i 246. to 249. PART V. DISCUSSION OF MAIN PROJECT. The preliminary experiments had shown a variety of insect relations between uncultivated land and several crops, but such experiments remained isolated examples without reference either to the progress of a pest infestation or to any combination of edgegrowth effects on such progress. Two years' work was therefore devoted to the study of a particular pest infestation of a crop. It was felt that the disadvantages of choosing a single example were offset by the more detailed evaluation of edgegrowth effects at all stages of the infestation, and that such an approach, combined with the preliminary experiments, would be the best start for an analysis of insect relations between uncultivated land and the crop, and for the develop- ment of suitable techniques for such a study. The basis of the study was to analyse contrasts in the fluctuations of aphid numbers at the centre and edges of the crop in terms of the changing nature of the aphid population during the season and the effects of uncultivated land on the factors affecting aphids. Owing to the small size of the field, particular attention was paid to the strips of edgegrowth adjacent to the field edges. The importance to be attached to the isolation of fields in large areas of uncultivated land could not be investigated satisfactorily with the plan of work of the main project. The choice of the Cabbage Aphid (Brevicoryne brassicae L.) as the pest species for the study was governed by several considerations. Aphids could be expected to colonise freshly planted crops in fair numbers in most years, and a considerable volume of literature on their bionomics and population fluctuations would be available for the interpretation of the rather specialised data for a study on edgegrowth relations. Also work 250. on aphids had already given some useful data in the preliminary experiments. The types of parasites and predators of aphids were already familiar, and the preliminary experiments showed that signs of predation in aphid colonies could be both recognised and distinguished from other factors affecting aphid populations. Similarly, parasitised adults were easily recognisable, and a technique for breeding parasites from mummies had been developed. It was, however, recognised that aphids showed several features disadvantageous to this particular study. The changing age distributions in the population and the overlapping of generations as well as the production of both alatae and apterae would be complicating factors in population work. A further important consideration was that the production of honeydew would affect the importance of flowers as sources of adult food for predators and parasites. B. brassicae had already been used in the preliminary work, and was known from Markkula (1953) to have a range of wild and cultivated host plants. The parasites and predators had been identified to generic level. The aphid is covered with a wax exudate which would tend to make it distinguish- able from many other aphid species on its range of host plants, and the individuals of a colony appeared to be sufficiently sessile to enable the same colony to be revisited and changes to be assessed. The area of the crop (It acres) was probably rather small for studying edgegrowth effects. In 1957 the whole field was sampled proportionally, but this gave too small sampling units to assess gradations from the edges to the centre; intermediate regions had to be bulked with the centre of the crop. In 1958 the plan was changed so that discrete areas were sampled more intensively. Though the second year's plan was thought to be more 251. likely to show up edgegrowth effects, it was recognised that total population estimates would be less accurate than in 1957. The technique used for sampling the crops proved satisfactory and had the advantages of not disturbing the aphids as well as being independent of weather conditions, so that sampling intervals could be controlled. With higher numbers of aphids such sampling proved laborious without removing leaves for washing (as in George, 1957) and a single sampling often required a full day to complete. The marking and inspection of colonies appeared to be a new technique for assessing aphid population changes, and had several advantages over the application of laboratory data to the fluctuations of numbers in the field (3.D.Smith, 1957). The results were indepexdent of data on the searching behaviour of predators, as well as the interaction of factors such as the effect of ambient weather conditions on the behaviour of predators and the predation of parasites within the host. Difficulties in interpretation caused by the fluctuating age distribution of aphid populations were eliminated to the extent by which the colonies marked reflected the age distribution of the entire crop population. The main limitations of the method were the difficulty of checking accurately the number of nymphs which both . were added and again dis- appeared between inspections and the unsuitability of the technique for the heart region of the plant. These limitations are discussed elsewhere (p.113). The technique appeared to give consistent results which agreed with the main crop sampling. It was possible to calculate approximate generation times and relative reproductive rates from the data, and the 252. method enabled the factors affecting aphid fluctuations to be linked quantitatively with the pattern of infestation in different areas of the crop. Unfortunately numbers of aphids in 1958 were so low that the marking and inspecting of colonies proved impracticable: the comparison of areas in this season could only be based on indirect data of aphid losses, in terms of the distribution on the crop of other insects and aphid colonies with elate nymphs. Water traps proved to be unsatisfactory. Although catches in coloured water traps are biassed by attractiveness to insects and it is uncertain how far this varies under different environmental and climatic conditions, it was hoped to be able to obtain qualitative information in terms of relative catches. Water traps had the virtue of cheapness, enabling a fair number to be distributed in and around the crop. The consistency of catches in white traps was poor in 1957, and, in an attempt to increase the daily aphid catch, yellow traps were used in 1958. Numbers of alatae were so low in 1958 that replication was equally poor and the colour difference in the traps made comparisons between seasons dubious. To use water trap data it was necessary to resort to considerable bulking of catches to reduce errors of replication. Considerable difficulty was experienced in establishing crucifers in the edgegrowth in "semi-weed" conditions. The mustard plants died back soon after the crop was planted, and it was necessary to fill in with crucifers of different speecies in the edgegrowth at intervals. The main purpose of establishing edgegrowth crucifers was to evaluate their importance as a parasite reservoir over winter, but with the failure of the plants and the absence of B. brassicae in the edgegrowth in autumn, no importance could be attached to the factor. The importance of the edgegrowth as a parasite 253. reservoir would be influenced by any effect of the type demonstrated by Salt (1940) on developing parasites caused by differences in aphid size on different host plants. Measurements on all instars of B. brassicae bred on sprouts and mustard dailed to demonstrate such a difference. Of the B. brassicae parasite-hyperparasite complex, only the hyper- parasite Asaphes vulqaris would be likely to emerge from other species of aphids in the edgegrowth and this hyperparasite though probably acting beneficially (p.204 ) was not numerous in either year. In both seasons, B. brassicae appeared on mustard in the edgegrowth before the crop was planted, although numbers on the leaves remained low. The higher rate of increase of the aphid on the leaves of mustard than on sprouts recorded by Markkula (1953) could not be demonstrated. In both years a sudden rapid increase of B. brassicae on mustard was observed with flowering, and it was apparent that weed hosts had become a far more important source of infestation of the crop than had been envisaged from the earlier low ambient level of aphid numbers. Wind analyses indicated that alatae from the edgegrowth mustard arrived on the crop under conditions of low wind force. Aphid predators were present in the edgegrowth before the crop was planted in both seasons, and this early establishment of predators may have been of some importance. An analysis of the stages of development of Coccinellidae found on the crop showed that invasion of larvae and adults had occurred from outside the crop. Species of aphids other than B. brassicae in the edgegrowth were probably of equal importance to B. brassicae as a reservoir of polyphagous predators. The variability of individual physical factors was measured over the field area to indicate the order of magnitude of the effect of edgegrowth on 254. factors which might affect aphids through water and nutrient relations of the host plant. The quality of food obtained by B. brassicae appeared to be the major factor affecting the aphid populations. The sharp rise in aphid numbers on mustard associated with flowering was comparable with the apparent importance of leaf age on the crop. Once the older leaves were colonised the reproductive rate appeared to increase and numbers of aphids rose steeply in spite of the activity of numerous predators and the apparently less favour- able temperature conditions as expressed by mean generation time. During the period 3rd July - 13th August 1957 before the aphid graph began to rise steeply, it could be calculated from the marked colony records that predators were destroying approximately 20% of the aphids, but from 30th August - 10th September 1957 the graph was rising steeply, although predators were destroy- ing nearly 30% of the aphids. Rainfall undoubtedly had some indirect effect through the host plant on the speed of the aphid's reproduction, comparable with the leaf age effects described. In the time available it was not possible to analyse plant nitrogen relationships, but the results point strongly to the close dependence of aphid numbers on the state of the host plant, a conclusion drawn by Kennedy and Stroyan (1959) in a review of literature on the biology of aphids. The differencesin plant growth, especially in the number of old leaves and leaf moisture content between the sheltered edge and elsewhere on the crop, were considered to be caused by a combination of physical effects of the edgegrowth and resulted in a lower reproductive rate of B. brassicae at the sheltered edge. This aspect of the work would repay enlargement and further investigation 255. The association between warm, dry years and large B. brassicae infestations is well known (Empson, 1952). Although neither summer in the present experiments was particularly favourable for the aphid, B. brassicae populations in the extremely wet summer of 1958 showed a marked contrast with those in the previous summer, with peak numbers reduced by a factor of twenty. In both years, heavy rainfall accounted for several noticeable checks in aphid build-up. Apart from the important indirect effect of rain- fall on the speed of the aphid's reproduction through the host plant, evidence was obtained which suggested that significant numbers of aphids could be washed off in heavy rain, though no estimate of the number surviving and regaining plants could be obtained. The infestation of the late planted crop was dependent on the arrival of summer alatae from other sites. This took place in late June and July, and was identified by trapping. Wind speed and direction during the arrival period were analysed in respect of any effects of shelter by edge- growth on the settling of elate aphids. From Jens'n's (1954) data it was possible to map out shelter effects over the field area, which showed that the field was likely to prove too small and enclosed to yield a clear elate alighting pattern. The calculation for each wind of an approximate "aphid index" from hourly data for 0600 - 2100 hrs. G.M.T. was considered justifiable_ in view of the diurnal take-off and settling cycle established for Aphis, fabae Scop. by Johnson et al. (1957). The arrival of alatae appeared prolonged and at a low numerical level, and this, together with the sheltered nature of the field, made the interpretation of water trap catches only speculative. Indications were obtained of higher numbers at the edges of the crop, particularly with reference to the position of trees. 256. Alatae appeared to alight on mature leaves and there was evidence of a movement of alatae from the outer leaves (where the aphids appeared to alight) to the heart area, where the nitrogen relationships of the plant suggest that food of optimum value to the aphids would be found at that time. The heart area also seemed to offer more protection against the direct effects of rainfall than the outer leaves. In both seasons, the size of the initial infestation was checked by heavy rainfall and also by early predators such as Anthocoridae and Coccinellidae, particularly after the numbers of other species of aphids more abundant on the older leaves had been reduced. There was evidence that higher direct losses from rainfall occurred at the sheltered edge than elsewhere on the crop, possibly due to heavier "rainfall" running off foliage above the crop, especially when wet trees were stirred in the wind. Of the early predators, Coccinellids invaded from the edgegrowth (p.253). Adult females dissected had only aphids in their gut; there was no trace of pollen feeding. In 1958 Anthocoridae were numerous on the crop at the sheltered edge, and here they were abundant also on overhanging Sycamore trees. It seemed probable that Anthocoridae on the crop were derived from the reservoir on the edgegrowth trees. Syrphidae were undoubtedly the most numerous predators and destroyed large numbers of aphids in August and September. Under conditions of regular rainfall and until large numbers of B. brassicae reach the older leaves, predators appeared capable of controlling aphid numbers. Although the condition of the host plant and its regions colonised by aphids appeared 257. to be the main factor influencing the numbers of B. brassicae, predation was undoubtedly the main factor causing aphid loss up to the autumn peak. Early in the season both Coccinellidae and Syrphid larvae (Table 45 pt.3, p. 175) were more numerous on the outer leaves than in the heart area and on young leaves, and it seems possible that importance may be attached to predators, not only through the number of aphids consumed, but also through any tendencies of searching behaviour and oviposition to prevent the Vim parts of the plant most favourable for aphid increase from becoming infested. The association of Syrphid eggs with aphid colonies stressed by several workers (e.g. Bhatia, 1939; Schneider, 1948: B.D.Smith, 1957; Dixon, 1959) could not be shown; a large proportion of the Syrphid eggs were laid on uninfested leaves and plantg the crop sampling data indicated a high mortality among hatching larvae. Flowers in the edgegrowth proved to be an important source of adult food for Syrphidae. The pattern of oviposition of Syrphid eggs confirmed rather uncertain results of trap catches which indicated that Syrphids were more abundant near flowers, and dissection of adult females showed that pollen feeding had taken place. Considerably greater numbers of eggs were laid at the open edges than at the centre of the crop, and further analysis showed that higher numbers of eggs were laid near flowers than elsewhere along the open edges, where no edge effect could be demonstrated. In 1957, differences in losses through predation in different areas of the crop could be linked with the observed predator distribution. The importance of Syrphidae was particularly noticeable, and the pattern of predation during the period when these predators were abundant could be explained in terms of the egg distribution previously established in connection with flowers in the edgegrowth. 258. In 1958, predators were few and most Syrphid eggs hatched during a period when aphid' Were scarce in the crop. There was evidence that numbers of active predators were considerably reduced by starvation and adverse conditions; the pattern of infestation in different areas of the crop in this year could not be related to the presence of flowers in the edgegrowth, although differences in Syrphid oviposition between the areas had again been demonstrated. The difficulties of determining a satisfactory parameter of percentage parasitism were considerable, largely because of the continually changing age distribution of the aphid population with overlapping generations. Another complicating factor was the destruction of parasites within immature aphids by predators and other factors causing aphid mortality. The emergence of hyperparasites did not confuse the pattern of primary parasitism as it was possible to identify the primary parasite of an aphid mummy without reference to the adult which emerged. The parasitism index chosen was the ratio of mummified apterous adults to the total of apterous adults. This measure obviated the dissection of aphids in age proportions representative of the crop population or rearing in an insectary large numbers of aphids for each sampling occasion. The measure was subject to two main sources of inaccuracy. Although Ullyett (1938) stated that aphid predators destroyed parasitised and healthy aphids indifferently, it was felt that this was unlikely to be true in the adult stage where the parasitised fraction had become mummified. Such mummies tended to remain on the crop longer than equivalent adults, especially under adverse conditions such as occurred at the end of a season (p. 161, and George, 1957). In general, it was felt that unemerged mummies 259. would be likely to remain on the crop for more sampling occasions than unparasitised adults. The second source of inaccuracy was the difficulty of assessing the parasitism of alatae in view of the departure of unpara- sitised individuals from the crop leaving those parasitised in the form of mummies. There is also evidence (Vevai, 1942 - Aphidius matricariae Hal. parasitising Myzus persicae Sulz.) that alatae are not readily parasitised; indeed very few mummified B. brassicae alatae were observed in either season. For this reason the parasitism index was calculated on apterous adults only. The marked colonies provided a check on the accuracy of the parasitism index, giving a percentage parasitism for apterous adults unbiassed by any difference in the persistence of mummies and unparasitised adults on the crop. The overall figure for 1957 of 8.99% agrees remarkably closely with the 8.95% for Diaeretus rapee and Praon volucre from the crop sampling records. The importance of depredation of parasites within unmummified hosts by factors causing aphid mortality could be illustrated from the marked colony records. The overall 8.99% adult mummification in 1957 represented an actual 0.99% mortality from parasitism based on all stages of aphids. This latter figure of necessity included elate generations. Although trap catches of D. rapae tended to be higher near flowers, the distribution of adult parasites and aphid mummies could not be correlated with the presence of flowers in the edgegrowth. From such data, as well as ovarial measurements, it was concluded that no importance could be attached to flowers as a factor affecting parasite attack on B. brassicae on the crop. Parasitism proved to account for only a small proportion of aphid mortality in 1957, when hyperparasites destroyed a very large fraction of the first generation of D. rapae. 260. In 1958, the rate of parasitism was again not high,. but appeared to have been the most important factor causing differences in aphid numbers in the unsheltered areas of the crop. Differences in the size of the final aphid peak could crudely be accounted for by the loss of adults by parasitism. Such differences between areas in the proportion of parasitised adults could only be ascribed to the heavy predation of unmummified aphids early in the season particularly by Anthocoridae, but without other evidence than the distribution of predators the explanation remained inconclusive and speculative. In 1957, predators appeared to check any rise in aphid numbers until large numbers of B. brassicae reached the outer leaves and the speed of reproduction increased (p. 254), resulting in the late October peak. In both years an adverse indirect effect of physical factors on the aphids' reproductive rate could be demonstrated at the sheltered edge of the crop after allowing for measured aphid losses. As parasitised adult aphids were evenly distributed over the crop area, the differences in peak infestation between the open edges and the centre of the crop appeared to be related to predator activity, which in turn was affected by the presence of flowers in the edgegrowth. In 1958 only a small proportion of the aphid population reached the older leaves and a steep build-up in numbers was not marked as in 1957 until after most predators had disappeared from the crop. Soon after the first major rise in aphid population level in both seasons, alate nymphs appeared in samples, though the proportion of colonies with elate nymphs at any one sampling occasion remained low in both years. Although the dispersal of alatae was not in itself a mortality factor it was considered justifiable to include dispersal among factors causing 261. the loss of aphids from the experimental field. Certainly in 1957 some colonies with alate nymphs were very large but were greatly reduced by alate take-off. The marked colonies showed that dispersal was a significant factor of aphid loss from the field. With the techniques employed the accuracy of any comparison of marked colony records with crop sampling during this period suffered by the unknown increase in the number of colonies in the field caused by the local alighting of elates. The result of wind analyses suggested that such local dispersal of elate aphids could occur in conditions of low wind force. Attempts to correlate alatae production with ambient conditions were unsuccessful. In 1957, differences in losses by dispersal between the edges and the centre were shown to be related to the size of the aphid population in these areas. The size of the population in each area had been largely determined by predator activity and host plant conditions affected by edge- growth factors. Losses by dispersal could therefore be considered as being indirectly affected by the edgegrowth. Differences in dispersal losses tended to minimise differences in peak infestation between areas. In both years, dispersal and the onset of less favourable climatic conditions caused a rapid decline of the October peak; eggs were observed occasionally in 1957. The natural death of aphids would be included among the factors classified as "various". Other factors accounted for such a large proportion of aphid losses that the inability to assess the natural death of aphids was considered unlikely to lead to significant errors. 262. Although the most important factor affecting aphid numbers appeared to be the quality of the food obtained by B. brassicae, predators checked the rise of aphid numbers until the older leaves became colonised. In 1957, evidence was obtained that the distribution of Syrphidae, the most important predators, was affected by the presence of flowers in the edgegrowth and that this resulted in a lower peak infestation at the open edges than at the centre of the crop. Dispersal tended to minimise this difference. Those results which could be obtained in the adverse weather conditions in 1958 failed to demonstrate effects of the edgegrowth on the low infestation of the crop apart from the indirect physical effects of shelter. 263. SECTION IV. CONCLUSIONS. A study of the literature showed a wide range of possible effects which the presence of adjacent uncultivated land may have on the development of insect infestation of a crop. The botanical components of the flora on such uncultivated land may provide alternative food and breeding sites for injurious insects, sometimes also forming a reservoir of crop diseases. Similarly, alternative and alternate hosts may be available for predatory and parasitic insects, many of which also visit uncultivated land for flower feeding. Physical effects of uncultivated land are also important in influencing the deposition of insects from air currents and providing shelter diurnally or for hibernation. The proximity of tall vegetation may affect the physical characteristics at the field edges, which may affect insects on the crop either directly or indirectly through the host plant. Before the crop is planted, a possible predator and parasite reservoir, which may extend over a wide area, is provided by uncultivated land. Such a reservoir may exist if either alternative host plants of the pest are present or the beneficial insects are polyphagous, as is the case with many aphid predators. Where alternative host plants of the pest are present, or the pests seek shelter among debris for overwintering, uncultivated land may also serve as a pest reservoir. Even with an initially low pest population, this reservoir may suddenly assume importance; Brevicoryne brassicae increased very rapidly on the edgegrowth crucifers once they came into flower. In determining the initial infestation of the crop, the immediate edgegrowth may be an important factor not only by suppling individuals of 264. the pest species but also by interfering with air currents carrying the pest from other areas and causing the deposition of insects onto the crop. With insects which are carried passively in air currents, the initial infestation can be related partly to general shelter conditions of the field and the position of trees. Insects on the crop are affected by two physical characteristics of the proximity of tall edgegrowth. Numbers of small and susceptible insects may be depleted near the edgegrowth, where more intense "rainfall" is caused by the large drops of water running off the leaves of overhanging trees. Secondly, the different microclimate near such edgegrowth affects the growth pattern of the host plant, and this in turn may affect the insect feeding there. In the present work, the reproductive rate of B. brassicae was markedly reduced along the sheltered edge of the crop. The distribution of beneficial insects on the crop may be affected by uncultivated land in two ways. Predators may be more numerous at the edges of the field as a result of their invasion into the crop from the edgegrowth plants. Secondly, many predator females, prior to egg maturation, feed on the pollen and nectar of flowers, which are usually present outside rather than within the crop. Oviposition by these predators (as with Syrphids in the present experiments) may be much heavier near flowers than elsewhere on the crop. Similarly, many parasites feed on flowers before their eggs mature. Evidence for this was obtained in the preliminary experiment, but such behaviour by aphid parasites could not be demonstrated. At the edges of the crop, beneficial insects are commonly more abundant with heavier pest losses through their feeding than at the centre of the field. 265. The presence of flowers and alternative prey is probably of importance to parasites and predators over a far wider area than was studied in the main project. It is common in Great Britain for fields to be small and to form islands within large areas of uncultivated land. Many beneficial insects are sufficiently mobile to render uncultivated land at same distance from crops of importance in restricting the numbers of pests over the whole field area. With regard to the specific infestation of the Cabbage Aphid (Brevicoryne brassicae) on crucifers which formed the main study, the most important single factor affecting aphid numbers appears to be the effect on the pest's reproductive rate of the quality of food obtained from the host plant, whether wild or cultivated. It is suggested that cruciferous weeds in the edgegrowth should be eliminated or otherwise prevented from flowering. If flowering occurs, large populations of the aphid may be produced from initially low numbers, and alatae may be carried onto the adjacent crop under conditions of low wind speed. It seems likely that the presence of other species of aphidsin the edgegrowth will act beneficially early in the season before the crop is planted in encouraging aphid predators which may then invade the crop. In the shelter of tall hedgerows and trees plant growth is modified and the reproductive rate of the aphid is depressed; in addition, direct "heavy" rainfall further reduces the aphid population increase. On balance, however, in view of the poor plants produced in the shelter of such edge- growth and the limited extent of the beneficial effect, it seems likely that tall edgegrowth would be disadvantageous, especially as it forms a barrier to air currents from which aphids are deposited onto the crop. 266. Predators can he encouraged by the presence near fields of non- cruciferous flowers, which supply adult food to the beneficial insects. Particularly Syrphidae, which are valuable predators, oviposit near such flowers. The abundance of these predators on the outer leaves of the crop plants is valuable in keeping the reproductive rate of the aphid in check. The initial alatae move to the heart leaves, and the infestation spreads outwards from this point. Once the outer leaves have become heavily colonised by B. brassicae, predators appear unable to check the population increase of the aphid. 267. SUMMARY. 1. The literature on the relation of uncultivated land to crop pests is reviewed, subdivided on a framework of the importance of the botanical components of the flora and physical effects. 2. A series of preliminary experiments is described, which were aimed at identifying aspects of the problem suitable for quantitative work. Examples are given of the importance of uncultivated land as an insect reservoir, the importance of flowers in the edgegrowth to parasites, and the effect of edgegrowth on the distribution on the crop of injurious species and their predators and parasites. 3. The main project chosen for study in 1957 and 1958 was the influence of adjacent edgegrowth on the development of an infestation of the Cabbage Aphid, Brevicoryne brassicae L. on cultivated crucifers. 4. The techniques used in this study are described, including a technique for assessing aphid population changes in detail by the marking and regular inspection of individual colonies in the field. 5. The progress of the infestation followed a similar course in both years, though populations were very much lower in 1958 than in 1957. Alatae, borne on air currents,arrived from June to August and moved to the heart leaves of the crop plants; the infestation moved outwards from this point during the season. At the end of September the aphids reached the older leaves in some numbers, and, although conditions had passed the optimum temperature,the rate of increase rose sharply, presumably due to favourable nutritional conditions on the old leaves. The peak in aphid numbers was reached in late October, and declined with the onset of winter conditions. Only a small percentage of the 268. colonies produced alatae, and very few winter eggs were observed. 6. The factors affecting populations of B. brassicae are discussed. The rate of reproduction and development of the aphid, which varied with climatic conditions and the food quality of the host plant, was probably the most important factor affecting aphid numbers. Predators caused heavy losses of aphids in August and early September, and appeared able to check the rise of aphid numbers until the latter had colonised the older leaves. The dispersal of alatae appeared to be an important factor causing the decrease in numbers of aphids on the field after this point; attempts to correlate alatae production with ambient conditions were unsuccessful. Little importance could be attached to parasitism, attack by fungi and the natural death of aphids. 7. The centre of the field, two open edges and an edge of the field sheltered by trees were compared to demonstrate the operation of edgegrowth effects. 8. Deposition 6f aphids appeared related both to shelter and the edge- growth crucifers, where initial low populations increased rapidly with flowering of the host plants. 9. The sheltered area of the crop was more shaded than the open edges of the crop, and the relative humidity was higher. Plants at the open edges produced more leaves and these leaves had a lower moisture content than at the sheltered edge. Aphids at this edge showed a lower reproductive rate than at the centre of the field. There was evidence of higher direct losses from rainfall at the sheltered edge. 10. Of the early predators in the crop it seemed likely that Coccinellidae and Anthocoridae had invaded from the edgegrowth. Syrphidae were 269. probably the most important predators, and their eggs were more numerous near flowers than at the centre of the crop or elsewhere at the open edges. Dissection of females showed that pollen feeding had taken place. Losses of aphids caused by predation could be linked with the observed predator distribution. No association of Syrphid eggs with aphid colonies could be shown. Both Coccinellidae and Syrphidae larvae were more numerous on the outer leaves of the crop plant, and would tend to restrict the progress of the infestation from the heart area. 11. The distribution► of adult parasites and aphid mummies could not be correlated with the presence of flowers in the edgegrowth. 12. Differences between the areas•in the proportion of colonies producing elate nymphs were hard to interpret, but largely reflected the size and number of the colonies present. Reduction of the number of aphids present in the areas by alatae take-off tended to reduce the difference caused by heavier predation at the open edges. 13. It was concluded that with regard to B. brassicae infestations, crucifers in the edgegrowth should be eliminated, though other flowering herbaceous plants would prove valuable. It was likely that, on balance, tall trees or similar edgegrowth would be disadvantageous. 270. ACKNOWLEDGEMENTS. I should like to thank Professor O.W.Richards, F,R.S., for accepting this research programme in his department, for undertaking its supervision, and for all his help. I am also indebted to the Nature Conservancy for the Studentship award which made this research possible. Dr. R.E.Blackith, my father - the late Dr.F.I. van Emden, Dr. T. Lewis, Mr. J.W.Siddorn and Dr. T.R.E.Southwood have given me valuable advice in discussions for which I am most grateful. I should also like to thank Mr. R.D.Eady and Mr. G.J. 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Summary of the Scientific Research Work of the Institute of Plant Protection for the Year 1936. Part I. Pests and Diseases of Cereals and Shelterbelts: 56-58. Leningrad. Lenin. Acad. agric. Sci. (In Rev. appl. Ent. (A), 26: 422). ZIMINA, L.V., 1957. Structure and Function of the Intestine of ayphus (Diptera Syrphidae). (IN RUSSIAN . Zool. Zh., 36: 1039-1043. (In Dixon, 1;57). ZORINA, L.M., 1939. Some Data on the Biology of the Convolvulus Flea-Beetles. DIN RUSSIA. Bull, Inst. Zool. appl., Leninor., 7: 34-43. In Rev. appl. Ent. (A), 29: 479). ZOEBELEIN, G., 1946. Der Honigtau als Nahrung der Insekten. Tell I. Z. angew. Ent., 38: 369-416. Tell II. Z. &mew. Ent., 19: 129-167. 292, APPENDIX I. PLANT SPECIES (EXCLUDING GRASSES) PRESENT IN THE EDGEGROWTH MAIN PROJECT. 1) NORTH-WEST EDGEGROWTH. Ranunculus sp. (rep ens L. ?) Sinapis alba L. Capsella bursa-pastoris (L.) Stellaria sp. Sperqularia rubra (L.) Chenopodium sp. Acer sp. (Seedling) Trifolium sp. Lotus corniculatus L. Epilobium sp. Chamaenerion angustifolium (L.) Polygonum sp. P. persicaria L. Rumex sp. R. acetosella agg. Faqus sylvatica L. Quercus sp. (Seedling) Convolvulus arvensis L. Solanum niqrum L. S. tuberosum L. (One plant only) Veronica sp. Senecio sp. Achillea millefolium L. Cirsium sp. Sonchus arvensis L. Hieracium sp. Taraxacum officinale agg. 2) SOUTH-WEST EDGEGROWTH. Cupressus macrocarpa L. Taxus baccata L. Ranunculus sp. Capsella bursa-pastoris L. Viola sp. Stellaria sp. Sperqularia rubra (L.) 293. Chenopodium sp. Tilia sp. Acer sp. Aesculus hippocastanum L. Ilex aquifolium L. Trifolium sp. Sorbus aucuparia L. Hedera helix L. Polygonum sp. P. persicaria L. Rumex spp. Urtica dioica L. Faqus sp. Castanea sativa Mill. Quercus sp. Rhododendron ponticum L. Veronica sp. Senecio sp. Achillea millefolium L. Lapsana communis L. Sonchus arvensis L. Endymion nonscriptus (L.) 3) NORTH-EAST EDGEGROWTH. Ranunculus sp. Sinapis alba L. Hypericum sp. Stellaria sp. Sperqularia rubra (L.) Chenopodium sp. Genista sp. Trifolium sp. Lotus corniculatus L. Epilobium parviflorum Schreb. Angelica sylvestris L. Anethum qraveolens L. Heracleum sphondylium L. Polyqonum sp. P. persicaria L. Rumex spp. R. acetosella agg. 294. Urtica dioica L. Quercus sp. (1 Seedling, 1 large tree) Fraxinus excelsior L. (Sapling) Solanum ninrum L. Veronica sp. Senecio sp. Gnaphalium sylvaticum L. Achillea millefolium L. Matricaria sp. Centaurea sp. Sonchus sp. Hieracium sp. 295. APPENDIX II. Specimen daily weather record sheet. WEATHER RECORDS Day... Saturday Date... 22nd June 1957.. HOURS G.M.T.. T°F 1 WIND SPEED WIND DIRECTION I 0600-0700 61 2 NW ! GENERAL CONDITIONS 0700 68 2 NW 0800 64 2.5 NW 0900 75 3.5 NW 1000 77 3.5 NW - Cool, cloudy, 1100 72 4 NW A few showers 1200 66 3.5 NW 1300 66 4.5 NW 1400 64 5 NW 1500 61 4 NW 1600 59 4.5 ENE Main Sta. Sub. T°F 1700 55 3,5 NNE 80 / 1 Max. 1800 54 3 NNE 36 , Min. 1900 52 2.5 NNE 82 1% R.H. 2000 51 2 r NNE 3.0; Hrs.sunshine 2100 50 2 NNE 3071 Total radiatiol Mean of 0.81 Hrs.rainfall 62.1 3 NNW 16 hr. day 0.91mm.tot.rainfalll 2200 50 2 NNE WORK RECORD 2300 48 2 NNE 2400 45 2 NNE Water traps 0100 43 2 NNE 0200 41 2 NNE 0300 36 2 NNE 0400 1 43 2 NNE 0500-0600 47 2.5 NNE Mean of 24 hours 56.1 2.5 NNE I 296. APPENDIX III. WIND SPEED AND DIRECTION RECORDER. A portable apparatus for obtaining continuous records of wind speed and direction was constructed from Meccano and scrapped materials at a cost of approximately (1956). The instrument has the advantage of being independent of mains electricity supply. The apparatus (Fig. 38) consisted of 2 boxes - the cup anemometer and the recording unit - each with a plug for a connecting cable. The anemometer. 4 light plastic egg cups served as anemometer cups, held out from the central spindle (b,AS) by 14 cm. long arms of stiff wire. The arms were attached to the central spindle 10 cm. above the top of a small box housing a mercury switch. Details of the switch are shown in Fig. 38,b. The pointed end of the central spindle (AS) rotated in a cup bearing* and was supported by a steel washer where it passed through the roof of the box. A cross-spindle (SS) carrying a 25-tooth gear wheel (25G) was turned by a worm gear (WG) attached to the central spindle. The cross- spindle was cut into two parts and rejoined with a sleeve (S1) of glass tubing held by sealing wax to form an insulator between the halves. The two cup bearings could then be used as terminals (C,D) for the switch contacts° The switch (MS) consisted of a small plastic collecting tube in which a drop of mercury could make contact across the ends of 2 platinum wires (PW) soldered to the halves of the cross-spindle. This switch therefore closed and opened once for each complete revolution of the cross-spindle (i.e. for every 25 revolutions of the anemometer cups). * Cup bearings - these were made by drilling the ends of bolts with progressively smaller drills, this producing a cone to act as a bearing for the pointed ends of Meccano rods. 297. KEY TO FIG. 38. A = terminal to electromagnet AS anemometer spindle B terminal to electromagnet terminal to mercury switch Ca = carriage on worm Cl = clutch terminal to mercury switch Dr.l = alarm clock main drive Dr.8 8-day clock movement DD = wind direction drum DP = wind direction pen 25G = 25-tooth gear wheel LG right angle gear PW pl#inum wire S1 glass-tubing and sealing wax sleeve SD = wind speed drum Sp wind speed pen SS mercury switch spindle = worm WG worm gear 298. FIG. 38. THE WIND SPEED AND DIRECTION RECORDER a. THE RECORDING UNIT APPROX. SCALE 2 cms 1 53333)3)333 Dr 1 Dr8 A B SS —,-.--' 6v 100 mfd f 1500n 7 C D APPROX. SCALE c. CIRCUIT BETWEEN 1 2 cms. I TERMINALS AB—CD b. THE ANEMOMETER FIG. 39. SPECIMEN OF WIND RECORDS Direction Speed 1140 hrs. 21.10.58 1000 ■ V -1 T 1 1 -T 1137 hrs. 20.1Q581 1200 20-21 OCTOBER 1958 300. In conditions of low wind speed the switch often remained closed for a considerable period and an electrical circuit (Fig. 38c) to eliminate prolonged discharge of the battery was devised by Mr. J.W.Siddorn. A condenser charged through a resistance by a 6 volt battery was discharged by the closing of the switch to give a momentary impulse to the recording unit. The circuit was housed below the spindles in the anemometer box. The recording unit. This was based on a similar unit designed by Mr. J.W.Siddorn for a colleague (Lewis and Siddorn, 1959). Wind speed. The wind speed chart was attached to an empty 1 lb cocoa tin (SD) equipped with a central spindle and rotated through holes in angle brackets. The spindle carried a spring-loaded clutch, of which the sliding section was fixed to two toothed chain wheels driven by clockwork. The power was provided by the main drive of an alarm clock (Dr. 1) (i.e. the escapement and small gears had been removed). The second chain wheel was coupled to an 8-day clock movement (Dr.8) restricting the unwinding of the alarm clock spring so that the chart drum (SD) revolved once an hour. A worm (w) thread supported in cup bearings* alongside the chart drum was rotated by chain drive from the central spindle of the drum. A carriage (Ca) threaded on the worm with a guide running between two rails was thus carried along the thread by the rotation of the spindle and the attached hinged pen (SP) traced a continuous spiral on the chart. The pen was fixed to the arm of an electromagnet with leads to the anemometer. A discharge from the condenser when the mercury switch closed was recorded as a flick of the pen on the spiral trace (Fig. 39), and the number of flicks per hour could be calibrated in terms of wind speed (see p.110). * See footnote to p. 296. FIG. 40. CALIBRATION OF ANEMOMETER WIND SPEED MEAN M.PH, 9-• 8 7 ER ET 6- OM 5 ANEM 4 ED T RA IB L CA 2 I { I 1 I I i 10 SO 50 70 90 1 130 1(:) CONTACTS/ HR 2 3 4 5 6 7 8 9 WIND SPEED MEAN M.PH. UNCAL1BRATED ANEMOMETER 302. Wind direction. A length of diem. electrical conduit was used as the direction drum (DD). This tube was held in position on a divisible central spindle by two bevel gears, one of them sliding and spring-loaded. The spindle could be locked with the drive from a right- angle gear (LG) by opposing bevel gears. The drum was rotated through the right-angle gear by a wind-vane above the recording box. A second hinged pen (DP) fixed to the worm carriage (Ca) produced the wind direction trace (Fig. 39). Operation. Except for a fixed corner supporting the axis of the wind vane the lid of the recording box could be slid open to change the charts. The pens were moved away from the drums and the carriage wound back down the worm by disengaging the clutch and turning the wind speed drum manually. The chart on this drum was removed after slitting down with a razor blade and a fresh direction drum was slipped onto the divisible spindle. Charts were fixed to both drums by glueing along a narrow overlap of chart paper. The clocks were rewound and the pens were set onto the drums again (a short reference line was marked on the direction drum with the wind vane held in the North position). Calibration. A calibrated Shepherd cup anemometer and recording unit were set up alongside the uncalibrated instrument with the 2 sets of cups at the same height. The records for three days were compared (Fig. 40) in terms of mean wind speed per hour. Although a steady wind of 2 m.p.h. was insufficient to turn the egg-cup anemometer the instrument rotated for a longer time after gusts than the Shepherd cups and in terms of mean wind speed per hour proved accurate to within 2 m.p.h. down to 2 m.p.h. APPENDIX IV. 303. TABLE 80. POSSIBLE FACTORS AFFECTING THE PRODUCTION OF ALATE NYMPHS IN B. brassicae. Date of No. colonies Total % colonies Dates used for nymphal Mean daily Mean daily Mean daily No. aphids on sampling with alate number with alate duration of adults temperature photoperiod rainfall 450 leaves nymphs colonies nymphs °co (hrs.) (mm.) 1957 19/8 0 86 0 6/8 - 18/8 15.7 14.91 4.5 526 23/8 0 101 0 10/8 22/8 15.1 14.68 3.2 1112 27/8 0 112 0 13/8 26/8 14.2 14.46 2.3 1446 31/8 0 116 0 17/8 30/8 14.0 14.21 0.7 1626 •/9 8 119 6.7 20/8 3/9 13.6 13.99 0.8 1845 9/9 1 107 0.9 25/8 8/9 13.4 13.67 1.0 896 13/9 4 104 3.9 29/8 12/9 13.5 13.41 1.6 1201 17/9 3 126 2.4 31/8 - 16/9 12.8 13.21 1.7 1389 21/9 3 128 2.3 5/9 - 20/9 13.G 12.91 1.5 1379 26/9 6 150 4.0 9/9 25/9 12.8 12.61 3.7 1746 30/9 6 184 3.3 12/9 29/9 12.3 12.38 2.8 2582 4/10 3 165 1.8 13/9 - 3/10 11.7 12.22 2.4 2173 25/10 17 127 13.4 30/9 24/10 10.3 10.96 1.7 2337 31/10 11 198 5.6 9/10 - 30/10 11.5 10.47 1.8 2608 6/11 6 161 3.7 13/10 5/11 10.8 10.16 4.5 1568 14/11 15 185 8.1 16/10 13/11 9.4 9.75 4.0 1856 21/11 17 181 9.4 23/10 - 20/11 8.4 9.40 2.6 1453 28/11 7 181 3.9 29/10 - 27/11 6.8 9.04 2.5 1164 12/12 8 121 6.6 10/11 11/12 4.8 8.41 0.9 638 27/12 0 86 0 25/11 26/12 4.3 7.94 1.6 310 1958 7376 0 10 0 8/8 19/8 16.5 14.82 1.6 56 27/8 0 13 0 14/8 - 26/8 15.0 14.43 3.3 48 1/9 0 8 0 19/8 31/8 14.7 14.12 3.8 25 9/9 0 8 0 29/8 - 8/9 16.9 13.54 2.6 9 13/9 0 7 0 1/9 12/9 16.5 13.31 2.0 29 18/9 1 26 3.9 6/9 - 17/9 16.0 12.98 1.5 181 23/9 1 10 10.0 10/9 22/9 14.9 12.68 2.0 108 30/9 0 14 0 15/9 - 29/9 13.6 12.28 4.3 109 - 6/10 1 15 6.7 19/9 5/10 12.9 11.95 4.2 199 15/10 1 15 6.7 27/9 - 14/10 12.0 11.39 4.0 241 22/10 0 35 0 30/9 21/10 11.5 11.06 2.4 316 31/10 2 45 4.4 7/10 - 30/10 10.7 10.54 1.2 624 7/11 1 15 6.7 12/10 - 6/11 9.9 10.16 2.6 141 13/11 2 24 8.3 17/10 - 12/11 9.0 9.81 2.3 189 28/11 4 26 15.4 30/10 - 28/11 6.7 8.98 2.1 194 5/1 0 16 6 4/11 - 4/12 5.9 8.70 0.8 74 APPENDIX V. 304. TABLE 81. MEASUREMENTS TAKEN ON B. brassicae FROM SPROUT AND MUSTARD PLANTS. a) Body lengths. Food Aphid No. of Max. Mean (mm) Min. plant instar individuals (mm) with fiducial limits p (mm) of distribution Sprouts I 28 0.888 0.737 -+ 0.180 > .3, 0.518 Mustard I 5 0.851 0.777 - 0.188 0.629 . + Sprouts II 29 1.037 0.942 - 0.177 .02 0.814 Mustard II 32 1.259 0.993 - 0.201 0.814 Sprouts III 14 1.333 1.164 - 0.216 , .03 1.037 Mustard III 33 1.481 1.204 i 0.289 1.037 + Sprouts IV 6 1.814 1.524,+524.,- 0.294rw05, 1.407 Mustard IV 13 1.666 1.379 T 0.335 ".-- 1.111 Sprouts T 0.208 1.518 Adult 9 1.851 1.732 ›.3 Mustard Adult 16 1.962 1.673 ± 0.363 1.296 b) Length of hind tibia. Sprouts I 28 0.255 0.222 0.180 Mustard I 5 0.240 0.222 0.195 Sprouts Mustard II 32 0.330 0.307 0.270 Sprouts III 14 0.450 0.422 0.390 Mustard III 33 0.450 0.412 0.390 Sprouts IV 6 0.600 0.591 0.540 Mustard IV 13 0.600 ,0.572 0.525 Sprouts Adult 8 0.900 0.855 0.825 Mustard Adult 16 0.870 0.838 0.810 c) Head width, Sprouts I 28 0.270 0.243 0.210 Mustard I 5 0.270 0.237 0.195 Sprouts II 29 0.330 0.288 0.240 Mustard II 32 0.345 0.283 0.270 Sprouts III 14 0.345 0.323 0.285 Mustard III 33 0.390 0.337 0.300 Sprouts IV 6 0.375 0.354 0.345 Mustard IV 13 0.405 0.377 0.315 Sprouts Adult 9 0.420 0.390 0.360 Mustard Adult 16 0.335 0.404 0.360 305. APPENDIX VI. Key to the instars of Brevicoryne brassicae L. 1. Antennae composed of 5 segments only 2 Antennae composed of more than 5 segments 3 2. Antenna with 3rd segment as long or shorter than flagellum Instar I .1140 ••• Antenna with 3rd segment markedly longer than flagellum Instar II 3. Typical adult antenna, long thin flagellum, 3rd segment markedly narrower than 2nd segment. Flagellum and preflagellum less than twice as long as the remainder of antenna Instar V rem ••10 /lb Shortened antenna, 3rd segment almost as wide as 2nd segment. Flagellum and preflagellum twice or more as long as the remainder of antenna ...... 4 4. Thickened antenna, not tapering after 3rd segment.Instar III- Short "adult" type, flagellum twice as long as preflagellum Instar IV Measurements of instars are given in Appendix V.