The Ecology of Nettle Aphids - with Particular

THE ECOLOGY OF NETTLE APHIDS - WITH PARTICULAR

REFERENCE TO THEIR ROLE AS PREY FOR BENEFICIAL

NATURAL ENEMIES

ROBERT MICHAEL PERRIN B.Sc. (Leeds)

A thesis submitted for the degree of

Doctor of Philosophy of the University of London.

September 1974

Imperial College Field Station Silwood Park Sunninghill Near Ascot Berkshire - 2 -

ABSTRACT

The nettle aphid, Microlophium evansi, is one of the commonest and most abundant weed-infesting aphids in this country and frequently constitutes at least part of the diet of a great variety of natural enemies. The present work aimed firstly to provide an understanding of the population dynamics of this species on the perennial nettle, Urtica dioica and secondly to quantitatively evaluate the role of stinging nettles in relation to certain pest problems in the agro-ecosystem.

Populations of M. evansi were studied on three nettle

patches at Silwood Park. Numbers of the aphid and its natural enemies were estimated by destructive sampling of nettle stems each week from April to October. The size of infestations on several other patches was monitored by scoring randomly selected stems. Effects on aphid abundance of natural enemies, emigration of winged adults and food quality of nettles were investigated by construction of a simple computer simulation model.

The food value of M. evansi to the coccinellid beetles

Adalia bipunctata and Coccinella septempunctata was studied.

Laboratory and field experiments were designed to examine the feeding and egg-laying behaviour of adult Coccinellidae when offered a choice of stinging nettles and field beans.

Attempts were made to follow the movements of radioactively labelled coccinellid beetles between nettles and small plots of beans.

Plots of nettles were cut at various growth stages, primarily to investigate whether nettles might be manipulated to increase their value as a reservoir of beneficial natural 3

enemies. Finally, the overall contribution of U. dioica in relation to crop production is assessed in the light of present experiments and existing literature. - 4 -

"Nettles clearly are weeds, but, to a degree, weeds are a matter of definition."

MAY THEILGAARD WATTS, 1973 CONTENTS

Page No. ABSTRACT 2

SECTION I : General Introduction 12 Review of relevant concepts and knowledge. 13 (1)The diversity-stability concept 13 (2)The role of Diversity in relation to 18 problems of Pest incidence and control.

(3)The observation and analysis of change 25 in aphid populations.

(A)Special features of Aphids 25 (B)Population studies of Aphids 26 (C)The Population Dynamics of Aphids 29 (D)Special features of Microlophium 33 evansi

(4) The Ecological basis for control of 34 Aphid pests

(5) Urtica dioica and its aphid parasites 36 (A)Urtica dioica 36 (B)Aphis urticata 38 (C)Microlophium evansi Theob 39 (6) The role of the stinging nettle as a 39 host for Natural Enemies.

SECTION II : Laboratory studies on the response 41 of Microlophium evansi to temperature.

1. Introduction 41 2. Materials and Methods 44 (A)Rate of Development 44 (B)Mortality of Nymphs 45 (C)Longevity and Fecundity 45 3. Results 48 (A)Duration of nymphal instars 48 (B)Threshold temperature of development 50 - 6

(C)Fecundity 52 (D)Longevity 52 (E)Population growth statistics 52

4. Discussion 56

SECTION III : The population dynamics of 66 Microlophium evansi

1. Introduction 66 2. Materials and Methods 67

(A) Sites of study 67

(a)Sampled areas 67 (b)Scored areas 70

(B) Sampling procedure 71 (a)Aphids 71

(b)Natural enemies 73 (C) Scoring procedure 73

(D) Growth of nettles 74

(a)Fresh and dry weight 74

(b)Surface area 75 (E) Quality of nettles for growth and 76 reproduction of M. evansi

(a)Changes in quality as measured 76 by fecundity of M. evansi

(b)Inter-site variation in quality 78 as measured by growth rate of M. evansi nymphs

(c)Total nitrogen content of nettles 79

(F) Influence of natural enemies following 82 the decline in aphid numbers

(G) Trap catches 83

(H) Meteorological data 83

3. Results 84

(A) 1972 84 - 7 -

(a)Abundance of aphids at sampled sites 84

(b)Abundance of aphids at scored sites 95

(c)Abundance of natural enemies 95 (d)Pitfall and yellow trap catches 113

(e)Growth of nettles 113

(B) 1973 116 (a) Abundance of aphids at sampled sites 116

(b) Abundance of aphids at scored sites 119

(d) Pitfall and yellow trap catches 133 (e) Growth of nettles 133 (f) Nutritional quality of nettles 147

(i) Changes in fecundity of M. evansi 147 (ii) Differences in growth rate of 150 nymphs of M. evansi between sampled sites (iii)Differences in nitrogen content 153 of nettles between sampled sites (g) Influence of natural enemies following 156 the decline in aphid numbers

4. Simulation Model 161

(A) Introduction 161

(B) Structure of the model 163

(a)Rate of development 167

(b)Birth 167

(c)Production and emigration of alatae 168

(d)Insect parasitism and fungal infection 171

(e)Predation 171

(f)parameter updating 174

(a) Site 2 174

(h) Site 3 181 - 8 -

5. Discussion 186

(A)The simulation model 186

(B)Differences in sizes of aphid populations 191 between sites

(C)Year-to-year differences in sizes of aphid 194 populations

(D)The monophagous feeding habit of 198 Microlophium evansi (E)Nettles as a reservoir of natural enemies 200

SECTION IV : Laboratory studies on the food value of 203 Microlophium evansi for the coccinellid beetles Coccinella 7-punctata and Adalia 2-punctata

1. Introduction 203 2. Materials and methods 207

(A)Cultures of Coccinellidae 207

(B)Cultures of Aphids 208

(C)Experimental methods 208

(a)Rate of development 208 (b)Weight of food eaten 211

(c)Mortality 212

(d)Weight of pupae and adults 212

(e)Fecundity and longevity 212

(f)Fertility and size of eggs 212

3. Results 213

(A) Adalia bipunctata 213 (a)Rate of development at 20oC 213

(b)Weight of food eaten by larvae 213

(c)Mortality during development 213

(d)Weight of pupae and adults 215

(e)Fecundity and longevity 215

(f)Fertility and size of eggs 217

(B) Coccinella septempunctata 217 o, (a) Rate of development at 22 C 217 9

(b)Mortality during development 217

(c)Weight of pupae and adults 217

4. Discussion 222

SECTION V : Studies on populations of Coccinellidae 229 on stinging nettles

1. Introduction 229

(A) Influence of non-crop land on pests and their 229 natural enemies

(a)Reservoir of pests 229

(b)Reservoir of natural enemies 231

(c)Diversionary hosts for pests and 234 natural enemies

(B) Other influences of non-crop landon agriculture 235

(D) Aims of work described in this section 240

2. Materials and Methods 241

(A) Experiments using radiotracers on the habitat 241 preferences of Coccinellidae

(a) Laboratory tests 241

(i) Uptake of radioactivity by U. dioica 242 and M. evansi

(ii) Uptake of radioactivity by potted 243 plants

(iii) Biological half-life of P-32 in 243 adults of Coccinella 7-punctata

(b) Field tests 244 (B) Experiments using laboratory and field cages on 245 the habitat preferences of Coccinellidae

(a) Experiments in constant environment room 245 at 150C

(i) In petri-dishes 245

(ii) In small cages 248

(b) Experiments in the field 249 - 10 -

(1) Resting and feeding preferences 250 (ii) Egg-laying preferences 250 (C) Effects of cutting nettles on populations of 251 Coccinellidae (a)Experiments - year 1972 251 (b)Experiments - year 1973 251 3 Results 254 (A) Experiments using radiotracers 254 (a)Uptake of radioactivity by U. dioica and 254 M. evansi

(b)Uptake of radioactivity by potted plants 254 (c)Biological half-life of P-32 in 255 C. 7-punctata

(d)Labelling of adult Coccinellidae in the 258 field

(B) Experiments using laboratory and field cages 260- (a)Habitat preferences of Coccinellidae in 260 petri-dishes (b)Habitat preferences of Coccinellidae in 267 small cages

(1) C. septempunctata 267 (ii) A. bipunctata 268 (c)Habitat preferences of Coccinellidae in 268 field cages

(i) Preferences when resting or feeding 268 (ii) Preferences when egg-laying 269 (C) Effects of cutting nettles on populations of 272 Coccinellidae

(a)Experiments - year 1972 272 (b)Experiments - year 1973 275 4. Discussion 285 (A) Movement of adult Coccinellidae in the field 285 (a)Labelling of Coccinellidae with 285 radioactivity. (b)Labelling of Coccinellidae with paint 287 (B) Selection of plants by adult Coccinellidae 289

SECTION VI : General Discussion 294

1 The case for conservation of the stinging nettle 294

(A) Nettles as a source of alternative and alternate 294 prey for natural enemies

(B) Nutritional value of M. evansi 296

(D) Source of pests 296

(E) Aesthetic value 296 (F) Direct economic value 297 2. The feasibility of conserving nettles in farming areas 297

SUMMARY 299

ACKNOWLEDGEMENTS 302

BIBLIOGRAPHY 303

APPENDIX : Computer listing 320 - 12 -

SECTION I GENERAL INTRODUCTION

The work described in this thesis has two main objectives:- (A) Understanding of the population dynamics of a monophagous

aphid species that is limited to a wild host plant often

situated in habitats close to crops.

(B) Quantitative evaluation of the role of stinging nettles

in relation to certain pest problems in the agro-ecosystem.

In general terms, the work can be justified as a

contribution to the understanding of the role of diversity in

agro-ecosystems. Nettles (Urtica spp.) occur in a range of

habitats extending from arable land to relatively natural woodland and moorland. The annual species, Urtica urens is a weed of minor importance in crops, whereas the perennial

nettle, Urtica dioica is often found close to crops, especially in "waste areas", as well as in places more remote from

agricultural land, for example, in woodlands and in some moorland ecosystems. The perennial nettle is frequently found

in large patches, sometimes forming pure stands with character-

istics of crop monoculture. This is a notable example of lack

of diversity at the primary producer level in a wild ecosystem

and as such might provide data on the significance of diversity

in natural systems. Furthermore, our awareness of the

influence of diversity on crop pest incidence is inadequate,

particularly the role of diversity amongst wild plants in the

patchwork of crop and non-crop habitats characteristic of farmland in much of Britain and elsewhere in the world. There

is a crucial need for more profound understanding of such

situations in view of recent changes in agriculture e.g. destruction of hedgerows and the cultivation of previously - 13 -

uncropped areas of waste land. The consequence of these changes, at least in biological terms, is largely a matter for speculation, such is the paucity of existing knowledge.

The specific aim of the present work was to determine how diversity, as represented by nettles, might influence the incidence and control of certain crop pests, notably aphids.

This involved a background study of the population dynamics of the aphid, Microlophium evansi Theob. (Sections II and III) and of the natural enemies occurring on nettles, especially of the temporal pattern of arrival and emigration of predators in relation to the increase and decline in populations of their aphid prey on nettles. Other concurrent experimental studies aimed to determine whether the natural enemies which exploit

M. evansi might usefully contribute to the biological control of crop pests and whether their possible beneficial effects in this respect could be enhanced by manipulation of the nettle/ nettle aphid system (Section V).

The detailed description and discussion of the work that was undertaken is preceded by short reviews of relevant concepts and existing knowledge.

REVIEW OF RELEVANT CONCEPTS AND KNOWLEDGE

(1) The Diversity-Stability Concept

One of the central themes of population ecology is that increased stability of populations within communities results from increased diversity. Much confusion has arisen over the definition of diversity and stability, with the use of contrasting scales of time, e.g. millenium and season, of space, e.g. global and local, and of levels of biotic organisation, e.g. community and species population. Diversity in communities generally refers to the richness of species - 14 -

(Williamson, 1972) or to the number of alternative pathways or trophic links for energy flow in a food web (MacArthur, 1955). Stability in individual species populations within a community is commonly regarded as the persistence of a characteristic equilibrium level (van Emden and Williams, 1974), with the degree of stability determined by the amplitude of fluctuation around the equilibrium. Thus, highly stable populations show small fluctuations and less stable populations large fluctuations around their equilibria. The degree of stability in a population is measured in the normal range of environmental conditions for the community to which it belongs and should be distinguished from the degree of resilience (Clapham, 1973 ), which is the ability to return to the characteristic level of abundance following large external perturbations. Thus a community in which population stability is normally high may be very low in resilience and catastrophically destabilised following an unusually large disturbance e.g. the introduction of modern temperate-zone agricultural techniques in tropical rain forest.

The diversity-stability concept infers that there is a positive relationship between the richness of species populations in a community and the stability of .those populations. The more niches that exist in a community, the more interactions are possible and therefore the greater the ability to resist change in the abundance of individual species; hence, the stability will also be greater (Odum, 1953; C Elton,

1958). MacArthur (1955) used information theory to support this concept and demonstrated a proportional relationship between community stability and the log number of links in the food web. Several lines of circumstantial evidence were also forwarded by C Elton (1958) and these can be summarised as follows:- - 15 -

1. Mathematical models of simple systems (e.g. Lotka- Volterra) usually showed a lack of numerical

stability. 2. Population instability was common in Gause's laboratory

experiments on simple systems of Protozoa.

3. Small islands with relatively simple communities are often more vulnerable to the disturbing influence of invading species than are continents, which have more complex communities. 4. Outbreaks of pests are mostly associated with the

simplified agroecosystem or other systems disturbed

by man.

5. Population stability tends to be much higher in diverse communities such as tropical rain forest than in less diverse communities such as temperate forest.

6. Broad-spectrum insecticides have sometimes caused

pest outbreaks by eliminating beneficial natural enemies from the insect community on crop plants.

The evidence that stability is highest in very complex communities is related to the apparent coincidence between the polar-equatorial gradient of increasing diversity (Pianka, 1966) and a similar gradient of increasing population stability. The diversity of many communities in the tropical zone is largely explained by the striking consistency of many important abiotic environmental factors, especially favourable climate (Clapham,

1973). It seems likely that this consistency of favourable climate results in most of the energy in the community being available for speciation rather than for combating vagaries of climate - in Odumls (1971) terminology, anti-thermal maintenance costs in the tropics are low - and the large number of species can become highly specialised to their habitats. Populations - 16 -

in a stable abiotic environment are probably less subject to periods of stress as intense as those of less stable areas and thus there will be less environmentally-induced fluctuation in abundance, but when shifts in abiotic conditions, common to nearly all ecosystems, do take place, the sensitive interspecific reactions resulting from very high diversity then play an important role in buffering any movement towards instability in particular species populations. Thus, population stability in tropical rain forest may be largely due to consistently favourable climate, but any tendency for changes in abundance is further restricted by the complex'feedbacks within the trophic web. In contrast, where an inconstancy of favourable environmental factors exists, the energy of the community is largely expended in combating unfavourable conditions - i.e. anti-thermal maintenance costs are high - rather than in speciation and therefore communities are usually less diverse. Furthermore, in order to survive in areas where environmental stability is

low, populations must be adapted to a wide range of biotic and physical conditions e.g. there may be short-term changes in

the availability of suitable food, necessitating oligophagy or polyphagy in feeding habits and dormant states in many animals. In harsh environments, therefore, populations must be generalists rather than specialists. The relatively few populations of generalists in many communities results in a lack of sensitive interspecific links with which to resist climatically-induced tendency for change and this partly explains the low degree of population stability in many

temperate and arctic regions (Clapham, 1973). However, even where diversity in temperate-zone communities is high, stability may frequently remain low in the face of wide fluctuations in - 17 -

climate. Large amplitude fluctuations in scme aphid populations, for example, appear to result more from their rapid response to changes in temperature than from insufficient trophic links

between the aphids and their natural enemies. In summary, population instability in north temperate coniferous forests and arctic tundra may be caused mainly by the consistently unfavourable climate and this instability is sometimes

accentuated by the absence of sensitive feedbacks between

populations to buffer changes in abundance.

In addition to the species richness of a community, the

equitability of the populations in each niche is a further factor influencing their stability. In unfavourable climates, communities tend to develop one or two clearly dominant species,

the interactions between these species and others being relatively more important than other interactions (Clapham, 1973).

Conversely, in favourable climates dominance occurs less commonly and the interactions between populations are usually

more uniform. Where dominants exist and exhibit a disproportionate

effect on the whole system, variations in those populations may have repercussions throughout the community and therefore

stability of populations is less likely. In other cases, a

contributory factor to stability is simply the degree of

isolation of the community from other communities, preventing

the invasion of alien species which may disturb trophic

relationships (C Elton, 1958).

In conclusion, it is accepted that diversity and stability are often positively correlated and that Southwood and Way's

(1970) statement that "the greater the diversity, the greater

the chances of stability" is largely correct. However,

population stability in relatively simple communities e.g. on - 18 -

certain pacific islands, and instability in relatively complex ones e.g. coral reefs in the presence of Acanthaster planci, can arise according to the influence of several factors other than diversity, notably fluctuations in the abiotic environment and the nature of interactions within the trophic web. (2) The Role of Diversity in relation to problems of Pest Incidence and Control The aim of intensive agriculture to maximise productivity by the use of primary succession plants and artificial inputs of energy in the form of fertilisers, herbicides and pesticides creates a sub-climax ecosystem and largely precludes mimicking of the full diversity of natural ecosystems. The maintenance of arable areas in a simplified, immature condition was often held responsible for the apparent instability in most pest populations and there was a call for reversion to diversity in agricultural habitats (C Elton, 1958; Rudd, 1964), despite abundant evidence that certain kinds of diversity can often encourage pests (van Emden, 1961, 1965b; Lewis, 1965). It was thought that a return to greater diversity would aid the stabilisation of insect populations by re-introducing their natural checks and thus prevent economically important damage to crops (C Elton, 1958). In certain instances, diversity established by practices such as mixed farming and strip cropping appeared to result in fewer pest problems than with more traditional forms of cropping (Conway, 1971, 1973a).

Dempster and Coaker (1974) found that undersowing Brussels sprouts with clover reduced the numbers of Pieris rapae,

Brev.icoryne brassicae and Erioischia brassicae. van Emden

(1963) furthermore suggested that the relative absence of major pest epidemics on British farms was partly due to the large acreage of hedgerows and headlands, which create a - 19 -

mosaic pattern across the countryside, a feature gradually disappearing in regions of intensive arable farming (Southwood,

1972; Anon., 1973). However, in many cases it was erroneously assumed that epidemics of pests resulted from instability in their populations. It has become increasingly clear that most pest populations are stable in that they do possess equilibrium levels (van Emden and Williams, 1974) and that outbreaks result not only from violent instability, but either because populations are stable about an equilibrium level which is consistently above the economic threshold of damage (Figure la) or because they possess equilibria below the economic threshold but fluctuate above that threshold at frequent or infrequent intervals (Figure lb). Thus, three types of pest exist; those such as codling moth, Enarmonia pomonella, which are normally very stable, endemic pests; those such as black bean aphid,

Aphis fabae, which are much less stable and whose populations frequently rise above the economic threshold; and those whose fluctuations above the economic threshold are relatively rare e.g.spruce budworm, Choristoneura fumiferana.

Populations of highly stable pests frequently persist above the economic threshold because of their ability to blemish the crop product rather than just reduce yield or occasionally because they are efficient vectors of disease organisms within the crop even at low densities. The growth of the frozen food industry and supermarket packeting of fruit and vegetables in Britain has created a demand for a virtually unblemished crop (Southwood and Norton, 1973; Gair, 1974), thus lowering the threshold of economic damage and necessitating Figure 1 Fluctuations of Pest Populations in relation to

economic threshold of damage (E.T.)

(a)STABLE PESTS - always above E.T. e.g.

Codling moth, oriental fruit moth.

(b)LESS STABLE PESTS (i) Frequent fluctuations above E.T.

e.g._Black bean aphid, Wheat bulb

fly.

(ii)Infrequent fluctuations above E.T.

e.g. Spruce budworm, Red locust, Antler moth. - 21 -

FIGURE.!.

(0)

E.T. ------

(b)Ci)

E.T.

(b)Ci i)

E.T.

TIMI~ -:~ - 22 -

each year the reduction of some pest populations to extremely low densities during the vulnerable period of the crop i.e. control aims to destabilise the pest population. Alternatively, less stable pests whose equilibria are below the injury threshold need to be controlled only when large fluctuations above the threshold occur i.e. control aims to increase the stability of the pest population. The aim of pest control in general, and of manipulating diversity in particular, is thus to maintain numbers of pests below the economic threshold, whether this involves an increase or a decrease in stability. Pest control measures adopted by each grower are determined primarily by cost in terms of materials, manpower and the value of the crop, and possibly by the need to restrict contamination of both crop and non-crop environments. Most control schemes, particularly against low density pests, depend heavily on the application of chemical insecticides, but concern over the harmful side-effects of chemicals and the increase in insecticide resistance amongst some major pest species has stimulated the integration of chemical and alternative methods of pest control. Diversity can often contribute significantly to the successful integration of control methods, providing the right quality of diversity is established (Way, 1966a; Way, unpubl.). It has been shown in some cases that small changes in the diversity of non-crop habitats may be all that is required to prevent significant increases in pest abundance on nearby crops, usually by enhancing the activity of natural enemies or by preventing the successful completion of the pest's life-cycle. The manipulation of non-crop diversity in pest management schemes can serve three main purposes, as outlined below:- - 23 -

1. Increase in numbers of Natural Enemies on Crops

An increase in diversity is beneficial in terms of natural

enemy impact if it provides alternative but less preferred

habitats to supplement the diet of predators and parasites or

provides shelter and overwintering sites. Flowering plants, for example, along field edges or in orchards are a source of

nectar and pollen for Syrphidae and hymenopterous parasites (van Emden, 1965a; Remaudiere and Leclant, 1971). The planting

of blackcurrants around vineyards in California enhanced the

biological control of the grape leaf-hopper by providing an

important winter host for the parasite Anagrus epos (Doutt and

Nakata, 1965). Similarly, there is evidence that the continuous

sequence of breeding by Earias spp. on wild hosts as well as cotton in N. Tanzania enables sufficient control by natural

enemies to make the Earias spp. minor pests (Reed, 1970).

2. Diversion of Pests from Crops during Vulnerable Periods

Wild hosts are sometimes beneficial in diverting important

pests away from the crop, providing they become attractive at

the right time. Stride (1970) suggested that plants such as

Cissus adenocaulis grown in hedgerows could be used to protect

cotton against large immigrations of Taylorilyous vosseleri.

In Zambia, another pest of cotton, Dysdercus superstitiosus, bred on one species of its wild host Hibiscus is diverted to

other Hibiscus species rather than to cotton in areas where these are attractive at the appropriate time (Way, unpubl.). The primary aim of strip-cropping alfalfa and cotton, a form of within crop diversity, is to retain Lygus hesperus on the

preferred host, alfalfa, rather than causing damaging migrations

to cotton when the alfalfa is harvested (Stern,1969). - 24 -

3. The Eradication of Pests It may sometimes be desirable to decrease diversity in order to create a more simplified environment lacking

essential host plants for completion of the pest's life-cycle.

Many polyphagous pests depend on wild hosts during the absence

of crops and the removal of these locally or regionally may

effectively eradicate the pest. This is exemplified by the

common practice of a "close" season for controlling pink

bollworm, Pectinophera gossy2iella on cotton and de Loach (1970)

suggested that cotton growing in alternate years only in the

U.S.A. would similarly control boll weevil. Successful control

of certain aphid pests might be achieved through eradication

of their winter hosts, providing these are well defined and limited in number, and likewise the removal of shelter provided

by hedgerow weeds would probably reduce the numbers of

immigrants to crops of a pest such as carrot fly, Psila rosae.

The role of diversity in relation to problems of pest

incidence and control thus varies greatly and the practical

aim should be to create the most beneficial type of diversity

in each crop/pest situation. Reversion to the degree of

diversity associated with most natural habitats is rarely

practical or necessary; pest - outbreaks are often avoided by

simple establishment or removal of one or few floral elements

in the agroecosystem. The overall value of existing diversity

is incompletely understood in most cases and since diversity,

once lost, is usually difficult to recreate (van Emden and

Williams, 1974), it is seemingly advisable to restrict any

further simplification of the landscape in arable areas until

we are more aware of the likely consequences of such action. - 25 -

Southwood's (1972) call for more work on the individual

components of diversity should be heeded, so that non-crop areas can be as skilfully managed as the crop itself.

(3) The Observation and Analysis of Change in Aphid Populations

(A) Special Features of Aphids

Aphids are plant parasites which imbibe the soluble

nutrients in phloem sap. They are characterised by parthenogenetic, viviparous reproduction in;the females of

some generations, which together with the telescoping of

generations i.e. the presence of embryonic daughters and grand-daughters within a parthenogenetic female, permits very

rapid multiplication on the host plant with up to nine

generations being possible during the favourable period of

the year in Britain (Dixon, 1973). Polymorphism is another notable feature of aphids; winged (alatae) and wingless

(apterae) forms occur as well as sexual and asexual phases of the life-cycle.

Aphids vary from monophagous species that are associated

with particular host plants, usually perennials e.g.

Drepanosiphum platanoides on Sycamore, through oligophagous

species which feed on a restricted range of annual or biennial

plants e.g. Brevicoryne brassicae on brassicas, to polyphagous

species like Aphis fabae and Myzus persicae which successively colonise a wide range of hosts during the summer season.

The ability to respond quickly to favourable conditions results in populations of aphids fluctuating within very wide

limits. Thus in temperate regions many species exhibit large

summer peaks followed by crashes to low density and temporary resurgances in early autumn, this being at least partly explained by their nutritional requirement for actively growing and senescing host plants. - 26 -

(R) Population Studies of Aphids

For practical reasons the majority of studies of aphid

populations have concentrated on pest species on single,

uniform crops (van Emden et al., 1969). Thus, Sharaf Eldin (1970) investigated the causes of change in aphid populations on potatoes. Rain and pathogenic fungi restricted increase in Macrosiphum euphorbiae and although exclusion tests showed that predators and parasites were influencing populations of

Myzus persicae, the poor host status of potato seemed to be the most important characteristic affecting population increase of this species.

Milne (1971) used exclusion techniques to evaluate the effects of predators on various aphids on broad beans. Birds

caused high mortality of Megoura viciae, whereas for Aphis fabae, invertebrate natural enemies were more important. In

both these species and in Acyrthosiphon pisum, intra-specific competition on the plants leading to a suppression of natality

and emigration of alatae was the main determinant of abundance. With many pest species, population levels on the host plants are partly dependent on the initial colonisation by alatae in

the spring. Dean (1971) found very sparse populations of several aphids, including Metapolophium dirhodum and Sitobion avenae, on cereals due to the infrequent arrival of alates

early in the season. Sharaf Eldin (1970) reported a high infestation of M. persicae on potatoes when alatae arrived early and in large numbers, this being partly governed by the proximity of overwintering host plants. Similarly, the size of mid-summer populations of A. fabae, on a regional basis, was considered by Way (1967) to depend on the timing and intensity of alate arrival on beans from the primary host, - 27 -

Euonymus europaeus and on factors such as weather and natural enemies that interact to affect their multiplication. A thorough study of Brevicoryne brassicae on kale in

Australia demonstrated that complex interactions between biotic and abiotic factors determine patterns of aphid abundance

(Hughes, 1963). Sensitive response to the decreasing quality of the host plant resulted in emigration of alates to fresh sources of food and a lowering of the reproductive rate which enhanced the hitherto insignificant impact of natural enemies. Way and Banks (1968) found the same effects operating on populations of A. fabae on spindle, E. europaeus. The peak and decline in numbers brought about by intra-specific competition was accelerated by syrphids, coccinellids and the parasite

Trioxys sp. It is notable that the numbers of overwintering eggs, as with the numbers of alates colonising annual crops, usually determined the size of populations on spindle early in the season.

Less attention has been paid to non-pest aphids on perennial hosts, even though this represents a commoner and more natural situation than economic species on artificially-created crop monocultures. The reproductive diapause of Drepanosiphum platanoides on the leaves of Sycamore during the summer was studied by Dixon (1963, 1966) who found that this was induced by the combined effects of population density and the poor nutritive status of the host. Natural enemies were not influential at this time because of the aphids extreme agility in avoiding capture. A notable feature of monophagous aphids is that their feeding may subsequently affect generations either much later in the same season or even in following seasons. For example, Dixon (1970a,h) concluded that for

D. platanoides , which typically shows three peaks of abundance, - 28 -

in spring, summer and autumn, abundant aphids in the spring affect the nitrogen metabolism of the leaves in a way that reduces aphid numbers in the autumn. This, in turn, results in fewer oviparous females developing, fewer overwintering eggs being laid, and a smaller population after egg hatch the following spring. Thus, while the basic pattern of fluctuation appears to be controlled by host plant quality, the size of infestation is additionally influenced by the history of aphid attack on the host. A similar process probably occurs in the Birch aphid, Euceraphis punctipennis, which also undergoes a mid-summer decline in reproductive activity and develops spring and autumn population peaks (Wratten, 1974).

The critical role of the host plant in population dynamics has been shown for other monophagous aphids. B.D. Smith (1957,

1966) studied Acyrthosiphon spartii on broom, Sarothamnus scoparius, in order to avoid the problems of host plant alternation and the lack of habitat continuity associated with annual plants.

The production and dispersal of alatae, possibly caused by a drop in food quality during pod formation, and predation by birds were the main factors checking population growth and causing its sharp decline. Frazer and Forbes (1968) discovered an unusual life history in Masonaphis maxima (Mason), which occurs only on the young leaves and terminal shoots of thimbleberry, Rubus parviflorus. Males and females were produced in late May and June and egg-laying had commenced by

July, exceptionally early in the temperate climate of Vancouver.

This sexual phase coincided with the cessation of growth by the host plant and in fact, aphids would not settle or feed on mature leaves or stems in field and glasshouse trials. The aphids were heavily preyed upon by syrphids, cantharids and - 29 -

cecidomyiids, but the authors suggested that the decline in numbers and early appearance of sexuales was due to a combined effect of host plant condition and sub-optimal temperatures.

Gilbert and Gutierrez (1973) found that after correcting for density-dependent effects, the fecundity of M. maxima individuals fell from 50 or more to less than 10 as the season progressed.

Similarly, changes in the plant were considered the most important factor in the population dynamics of the strawberry aphid, Chaetosiphon fragaefolii (Dicker, 1952). The fall-off in numbers coincided with a reduction in the number and quality of leaves during fruit production, whereas there was very limited and sporadic attack from natural enemies.

The foregoing examples of population studies of aphids have, with few exceptions, failed to provide adequate understanding of the population dynamics of the species concerned. As Hughes

(1963) points out, "highly accurate and laborious techniques have given good data from which only the vaguest conclusions can be drawn as to the relative importance of the many inter- acting factors involved in determining aphid numbers. " Three stages are involved in the understanding of the population dynamics of a species, namely:-

(1) Recording the kinds of numerical change that occur in the population.

(ii) Observational and experimental analysis of the

factors causing these changes.

(iii) Construction of a conceptual model to inter-relate

changes of abundance with their causes (Clark et al,

1967; van Emden et al, 1969). - 30 -

The incorporation of the causal relationships into a computer

model is also considered a worthwhile exercise (Hughes, 1973a).

The studies of Hughes (1963), Hughes and Gilbert (1968) on

B. brassicae and of Dixon (1963, 1966, 1970a,b) on D. platanoides are notable in that they have involved all these stages, although even in these cases, practical difficulties limited their

investigations to changes in populations on a small group of

plants. The abundance of many insects, particularly aphids

with strongly-developed powers of migration, is, however,

regulated in a patchwork of local populations on a regional

basis and the role of factors such as the distribution pattern of host plants has not been evaluated.

A brief outline of Hughes' approach to elucidating the

population dynamics of a local population of aphids is given, to illustrate the type of analysis needed. Firstly, the

recording of instar distribution in field samples and the

estimation of parasitism and fungal infection from sub-samples

kept alive in the laboratory is required. Using a physiological

time-scale of instar periods (Hughes, 1962), the integration of

relationships between the various causes of numerical change and observed abundance in the field is attempted through the

construction of time-specific life-tables (Hughes, 1972). This

initially involves projection of the numbers in each instar on

one sampling occasion to their potential numbers one instar

period later. Estimated losses due to parasitism, fungal

infection and emigration of alates are then subtracted to

provide the expected numbers in each instar, which are compared

with the observed numbers to which they correspond. A much

lower number of observed aphids indicates the existence of

"residual mortality" and represents maximum losses due to - 31 -

predation, which can be compared with predator activity as measured by numbers on the plants or suction trap catches. A higher number of actual than expected aphids suggests error

in the estimates of parasitism etc. or perhaps immigration if samples are small. Close agreement between the two sets of figures shows that all the major relationships have been included in the construction of the life-tables, examples of which are given in Table 1. Estimation of potential increase between

sampling periods by Hughes' method assumes a geometric series in the instar distribution i.e. a stable age distribution. This may not apply to most aphids in Britain, where populations often

grow and decline in about 8 to 10 weeks (Rendell, 1973).

van Emden and Way (1972) suggest that host plant influence

can also be studied through time-specific life-tables. The

potential population increase is based on actual reproductive rate in the field, so that the difference between this and the

maximum or unimpeded rate of increase determined under supposedly

optimal conditions in the laboratory is a rough measure of host

plant effects. The formation of life-tables for each sampling occasion can be a tedious method of analysing population change and a

more dynamic approach is sometimes possible by constructing a

computer simulation model. Simulation of B. brassicae

populations, based on Hughes' data, has further indicated that

the dynamics in this limited situation seems well elucidated

(Hughes and Gilbert, 1968; Gilbert and Hughes, 1971). Despite

the need to compromise with reality and to include non-causal

relationships, a reasonable match between computer output and

the natural population has been achieved. If the assumptions made during construction prove sound, modelling might also - 32 -

Table 1 Time-Specific Life-Tables for an Aphid Population (after Hughes, 1972)

I X

Y II I III II

III IV

IV Aap Aap Aal Aal

A B C D

A = Observed population and instar structure at sample period 1.

B = Potential population and instar structure one instar period later.

C = Expected population one instar period later.

D = Observed population one instar period later.

X = Known total losses during the instar period. Y = Residual losses - ascribed to predation. - 33 -

provide an opportunity to test various manipulations such as insecticide application or the altered impact of natural

enemies (Crawley, 1973). (D) Special Features of Microluhlum evansi

M. evansi is a non-crop aphid found exclusively on stinging

nettles. Although it has two host plant species, Urtica dioica

and Urtica urens, it is referred to in this thesis as a

monophagous aphid, because the two hosts are closely related and one of them, U. dioica, is by far the more abundant, both

temporally and spatially. Restriction to a perennial host imposes similar problems to those encountered by most other

monophagous species discussed earlier, in that there may be

periods when sufficient food supply for continued rapid increase.

of the population is unavailable. Wing formation and flight to other nettles, particularly to those less affected by previous

aphid attack, may contribute to survival during such periods, but this is less likely to be as rewarding as the switch of an

oligophagous or polyphagous species to an alternative host plant

in another genus or family which may be in a widely different

physiological condition compared to the original host.

It was observed in the present study that M. evansi can overwinter as parthenogenetic viviparae and this permits very rapid response to the return of favourable conditions in spring. Unlike the situation on annual crops, therefore, build-up of

populations begins with a small nucleus of apterae on the

plants and not with the invasion and settling of alatae.

Banks (1955) briefly referred to the pattern of abundance of M. evansi on perennial nettles at Rothamsted Experimental Station, Hertfordshire. Large populations were established by the end of April, with peaks early in June late June, the - 34 - aphids had virtually disappeared. There appear to be no other reports of population studies on this species.

(4) The Ecological Basis for Control of Aphid Pests

Well-documented evidence of the dangers of exclusive reliance on chemical insecticides in pest control e.g. pest resistance and deleterious side-effects, has provoked consideration and introduction of more ecologically-based control measures.

Sufficient ecological knowledge of certain pests exists to propose novel practices that are more harmonious with natural checks to pest increase such as predation, parasitism and plant resistance, and these practices can often be tested through classical experimental research·and quickly integrated with existing control measures (Way, 1973). What, then, is the role of extensive studies on the population dynamics of pest aphids, using methods such as those outlined earlier in this section?

While yielding potentially useful data on the relative importance and interactions of major components in the species! life-systems, in most cases they have not yet provided practical alternatives to over-dependence on chemical insecticides. Unfortunately, it has not yet been shown that ability to manage· pest populations and thereby ~educe their harmful influence on crop production depends on a thorough knowledge of the ~actors determining their abundance, as. asserted by Doutt and Smith (1971). Neverthe less, long-term population studies can be justified not only in providing insights to new control possibilities (Way, 1973) but also in enabling the construction of computer simulations that could determine the outcome to manipulations of the pest's environment. In a few instances, specific models have proved relevant to control practice in the field, notably where they permit dynamic feedback between field observations and the model e.g. Wilson et al (1972) for Heliothis on ·cotton and Macdonald et al (1968) for malaria. General insect population models also - 35 -

contribute to a growing theory of pest control, particularly since they can include the cost element and thus suggest optimal control strategies (Conway, 1973b). In this way, ecological studies may refine, if rarely instigate, integrated control methods.

Furthermore, understanding of the following aspects of population dynamics might usefully aid the improvement of control measures for pest aphids:-

(i) The numerical relationships between populations on winter and•summer host plants. This might enable the development of forecasting schemes such as those based on counts of overwintering eggs and active stages in spring on woody hosts, which predict, subject to climatic modification, the size of populations developing on annual hosts in the summer (Way and Cammell, 1973).

Shorter term warning schemes are possible if there is a predictable relationship between the number of migrants early in the season and the eventual size of infestation on the crop (Hull, 1968).

Forecasting is a vitally important feature of pest control in that it minimises the need for wasteful "insurance spraying" when the pest problem fails to materialise.

(ii) The relationship between the size and pattern of aphid infestation and crop damage in terms of yield and quality.

This establishes economic thresholds for various phases of crop growth (Stern, 1973) and permits the forecasting of damage rather than just aphid numbers.

(iii) The role of the host plant in population change. The development of plant resistance depends on knowledge of the influence of the plant on factors such as intra-specific competition and the efficiency of natural enemies (Pimentel, 1961; Way and

Murdie,1965; van Emden and Bashford, 1971 ; van Emden and Way, 1972). - 36 -

(iv) The impact of natural enemies on aphid populations. Exclusion techniques, for example, can be used to evaluate the effectiveness of natural enemies and the conditions under which they are economically significant as mortality factors, (Hughes, 1973b). Studies of the role of wild habitats as reservoirs of beneficial insects might enable the manipulation of natural enemy impact on crop aphids.

(v) Computer models of aphid populations. These, as previously mentioned, provide an overall insight into factors regulating aphid abundance, as well as aiding selection of biological control measures most likely to succeed in a given situation (Crawley, 1973; Hassell and May, 1973).

(5) Urtica dioica and its Aphid Parasites

(A) Urtica dioica

Two common species of Urtica occur in the British Isles; the small, annual nettle, Urtica urens L. which is particularly associated with light, cultivated soils, and the perennial nettle, Urtica dioica L., distinguishable by its larger size and tough, yellow rhizome system (Clapham, Tutin and Warburg,

1964).

U. dioica is an herbacious plant, ubiquitous and native in Britain and generally considered an obnoxious "weed" around buildings and agricultural land. It reaches a height of up to

150 centimetres, producing ovate, toothed leaves four to eight centimetres in length. It is usually dioecious, with green inflorescences produced on short lateral branches from June to

August. The cell sap of the stinging hairs contains sodium formate, acetylcholine and histamine, ejected in a syringe-like action on breakage of the silicified tip of the hair (Anon.,

Geigy Weed Tables, 1970). - 37 -

The perennial nettle is mostly confined to woodland in

"natural" habitats, preferring soils rich in nutrients with a high humus content. Elsewhere, nettle is an almost universal follower of man, establishing itself on disturbed ground, such as rubbish tips, compost heaps and ancient earthworks, where it often forms dense, almost pure stands. The biology and ecology of Urtica in Britain are described by Greig-Smith (1948) in the "Biological Flora of the British Isles".

Nettles have long been regarded as indicators of nitrogen conditions in the soil and their apparent association with areas plentifully supplied with this element is recorded by Watts (1973):

"In France, it seemed, we were nettled at the base of every wall, but in England we felt their pricks only where the wall was somewhat secluded - around a bend, or somewhat screened. Could the nettle distribution be indicating a difference in the relative sense of propriety with which Frenchmen and Englishmen relieve themselves?" Pigott and Taylor (1964), however, have demonstrated that phosphate is often the limiting factor in nettle growth and that on many woodland soils there is little or no response to nitrogen in the absence of phosphate. They suggest that the growth rate of U. dioica seedlings serves as a useful indicator of soil phosphate availability.

The exclusive status of nettle as a weed is of fairly recent origin, since prior to cotton importation, it was considered a valuable source of textile fibre and spun into cloth for sheets and tablecloths (Anon., 1973). It also enjoys some remarkable culinary uses, young tops being boiled as a vegetable or made into wine, while dried leaves can still be bought for making tea (Masefield et al., 1969). - 37 -

The perennial nettle is mostly confined to woodland in

Nettles have long been regarded as indicators of nitrogen conditions in the soil and their apparent association with areas plentifully supplied with this element is recorded by Watts (1973):

A great variety of insects is supported by U. dioica

(Richards, 1948). Twenty-seven species are more or less restricted to nettle and another nineteen oligophagous species are commonly associated with it. The life-cycles of some of the Hemiptera and Coleoptera on nettle have been studied by Davis

(1973a), but he did not treat the two species of aphid, Aphis urticata and Microlophium evansi.

(B) Aphis urticata

The author found A. urticata to be much less common on nettles than M. evansi. Infestation of the same nettle stem by the two species was observed during this study. A. urticata infests stems and upper leaves, often curling the latter. It exhibits a colour polymorphism similar to that recorded in

Aphis gossypi Glover (Wall, 1933), the apterous viviparae varying from bright yellow (a form previously described as

Pergandeida stanilandi Laing by Laing, 1923) to deep mottled green. This variation may be related to the nutritional status of the host plant (Hille Ris Lambers, pers. comm.), the oval to globular yellow form, 1 to 1.3 mm in length, being observed only on young, growing tips in early summer. A greater proportion of green forms, slightly larger in size and producing alatae, appeared later in the season, probably as a result of overcrowding or plant senescence. All the forms, including sexuales, are described in Theobald (1926).

Theobald (1926) lists the food plants as Urtica dioica,

U. urens and Rubus idaeus (probable confusion with Aphis idaei of Van der Goot) and states that unlike M. evansi, they are often attended by ants, including Formica rufa, Lasius fuliginosis and

Myrmica ruginodis. Attendance by Myrmica was observed at - 39 -

Silwood Park and at a site in Essex. Iperti (1965) records this aphid as being toxic to the Coccinellid Adalia bipunctata.

This aphid is much larger than A. urticata; apterous viviparae are usually about 3.5 mm in length. It is green in colour, sometimes with a marked reddish tinge and shows a contrast between the powdered larvae and the nude, slightly shiny adults. The morphology and biology have been documented by Hille Ris Lambers (1949), who distinguished two species in the genus, M. evansi and M. carnosum. Stroyan (1972) suggests these are, in fact, the same species and that evansi and its remaining synonyms should become synonyms of carnosum, which is the prior name (Buckton, 1876). However, the name evansi has been retained throughout this study.

Although oviparous females and alate males were described by Hille Ris Lambers (1949), they were not discovered in the field at Silwood Park, nor could they be induced by subjecting a population to short day-length at a constant temperature of

15°C. Apterous females continued to reproduce parthenogenetically on several nettle patches during the relatively mild winters of

1971 and 1972.

M. evansi has been recorded from Europe, Northern Asia,

North and South America. It infests both stems and leaves without causing the same degree of visible damage as A. urticata.

Its defence mechanisms to natural enemy attack have been studied by Dixon (1958) and some aspects of its predation by coccinellids in the spring by Banks (1955).

(6) The Role of the Stinging Nettle as a Host for Natural Enemies

U. dioica supports a great variety of natural enemies, including hymenopterous parasites, predators and entomophagous - 40 -

fungi. Some of these are largely confined to nettle but the majority frequently occur on other plants, both wild and cultivated, during the year. Predator groups previously recorded on nettles include Coccinellidae (Banks, 1955), Anthocoridae

(Anderson, 1961) and other predatory Heteroptera (Southwood and

Scudder, 1956; Davis, 1973a).

The natural enemies which feed or might feed on Microlophium evansi can be broadly classified as follows:-

(i) Non-specific e.g. adult Cantharidae and Carabidae, larval and adult Staphylinidae and Dermaptera.

(ii) Mostly Aphid-feeding

e.g. Coccinellidae, Anthocoridae, Cecidomyiidae, and parasitic Hymenoptera. Some of these probably also feed on Psyllidae on nettles.

(iii) Mostly or exclusively Nettle-aphid feeding

e.g. Calocoris sexguttatus (Heteroptera: Miridae). - 41 -

SECTION II

LABORATORY STUDIES ON THE RESPONSE OF Microlophium evansi TO TEMPERATURE

1. INTRODUCTION Temperature is a major climatic factor determining the abundance of poikilothermic organisms such as aphids in any one year. All insect populations possess an optimal temperature range for growth, above and below which the innate capacity for increase, determined by the rate of development, fecundity and the length of the reproductive period, is lowered (Barlow,

1962a,b; Legay and de Reggi, 1964). However, as indicated in section III, it is difficult during field studies to distinguish the relative importance of the complex biotic and-abiotic factors which influence populations of aphids; responses to climatic conditions interact with the effects of aphid density, the condition of the host plant and the action of natural enemies.

Laboratory experiments may help to overcome this problem, if potential rates of increase of aphid populations can be calculated at various temperatures and related to development in the field, where prevailing climatic conditions have been measured (Adams and Drew, 1964a,b).

Temperature affects aphid populations directly by influencing rate of development and adult reproduction, or indirectly by affecting the suitability of the plant host

(Milne, 1971). Murdie (1969) found that high temperatures were detrimental to the green variety of Acyrthosiphon ryisum, populations maintained at a constant temperature of 25°C dying out after three generations. Individuals were progressively smaller in successive generations, with a corresponding decrease in fecundity. Sharaf Eldin (1970) roared five successive - 42 -

generations of Myzu.s persicae at 27.50C and noticed a steady decrease in size and fecundity in each generation. Lees (1959) suggested "heat injury" to be the cause of these effects and Murdie (1965) showed that individuals took longer to recover from the stress of heat than from overcrowding when transferred to a more equitable environment.

The responses of many species of aphid to temperature have been investigated (e.g. Graham, 1959; Lamb, 1961; Barlow, 1962a,b;

Messenger, 1964; Miller, 1966; Sharaf Eldin, 1970; Milne, 1971).

Some authors consider the. daily range of temperature to be important and thus compared the results obtained from fluctuating and constant temperatures. Alternating temperatures are more biologically realistic, but there is some dispute as to whether they increase (Siddiqui et al, 1973) or decrease (Bonnemaison,

1951) developmental rates. Siddiqui et al found that intrinsic rates of increase of A. pisum were slightly higher at fluctuating temperatures compared to constant temperatures approximating to the mean of the fluctuations. This was due to faster development and earlier attainment of maximum fecundity. Milne (1971) recorded a shorter nymphal period for several aphids when exposed to fluctuating temperatures in the field, than when they, were reared at constant temperatures in the laboratory.

The pattern emerging from laboratory studies is of a temperature threshold above which the rate of most physiological processes is directly related to temperature (Hughes, 1972). This threshold, based on the assumption that the relationship between rate of development and temperature is linear, has been estimated for several aphid species (Bodenheimer and Swirski, 1957; Hughes,

1963; Milner, 1966). Bodenheimer and Swirski considered the rate of development of many species to be proportional to - 43 -

environmental temperature in excess of the threshold value.

They expressed the relationship as: D(T- ) = K, where D =

development time, T = environmental temperature, c = threshold

of development and K = thermal constant. Several authors (Hughes,

1963; Milner, 1966; Sharaf Eldin, 1970) determined developmental

thresholds by extrapolations of the regression line of rate of

development on temperature. The disadvantage of such a procedure is that the development velocity/temperature curve departs from

linearity towards the upper and lower extremes of the temperature

range (Andrewartha and Birch, 1954) and sometimes even at moderate

temperatures (Lamb, 1961).

However, temperature-summation has been widely used in

aphid studies. Thus, Hughes (1963) devised a physiological

time—scale for. B. brassicae in the field by the integration of

the daily temperature in excess of the development threshold. It was found that the immature instars of B. brassicae and

M. persicae were of equal duration on such a scale, so that an

'instar period, could be employed as "a useful and biologically

meaningful unit" (Hughes, 1972). It would not apply to Aphis fabae under field conditions in Britain (Milne, 1971), but the

assumption that each instar lasts for a certain number of day-degrees above the temperature threshold is of value in

simulation modelling (Crawley, 1973). The construction of

time-specific life-tables can be simplified where such an

assumption proves valid (Hughes, 1972).

This section describes the effects of five constant

temperatures on populations of Microlophium evansi, in terms of development and mortality of nymphs and longevity and fecundity of adults. - 44 -

2. MATERIALS AND METHODS

(A) Rate of Development The duration of the four nymphal instars of M. evansi was determined in rooms at constant temperatures of 6, 12, 15, 20 and 25°C. The nymphal development of Aphis urticata was studied at the four higher temperatures. Some observations on both species were also made at 27 and 30°C. There was artificial illumination for 16 hours per day and ambient relative humidity in the rooms ranged from 45 to 75%.

Several rearing procedures were tried at the start of this work. The Hughes and Woolcock (1965) method of using leaf-discs floated on cultilre solution proved unsuitable for M. evansi, because of its restlessness. Sharaf Eldin's (1970) modification of inserting the tubes of culture solution in a water-filled box merely provided the aphids with an opportunity to drown, which they frequently did! The use of clip-on leaf cages (Honeyborne,

1969; Adams and van Emden, 1972) did not facilitate rapid inspection of nymphs and was hampered by the frequent need to replace the plants at the higher temperatures. Excised leaves of Urtica dioica, taken from the field, proved most satisfactory.

This technique has been used extensively in aphid research (e.g.

MacGillivray and Anderson, 1957; Maltais and Auclair, 1962) and is acceptable where uniformity of substrate is the main requirement

(Adams and van Emden, 1972). The mature, excised leaves were maintained in the apparatus described by Blackman (1965,1967b) for rearing the larvae of Coccinellidae. This provided twelve individual rearing cells, each one consisting of a 1" diameter by 1/2" high perspex cylinder resting on a leaf laid with its undersurface uppermost on wet filter paper (Figure 2). The lid of each cell consisted of fine nylon mesh stretched across - 45 -

the perspex cylinder. The leaves were usually renewed once a week at temperatures up to 15°C and twice a week at higher temperatures. Individuals used in these experiments were taken from cultures maintained at the appropriate temperature on potted stinging nettles for at least one month beforehand. The plants were replaced and re-infested as often as necessary at each temperature. The cultures were started from single parthenogenetic, viviparous specimens in each room.

At the beginning of the experiments, third or fourth instar nymphs from the stock cultures were placed in each rearing cell and allowed to reach maturity. The first few offspring deposited were rejected as potentially atypical

(Murdie, 1969) and then one nymph was subsequently retained in each cell. Observations were made at about three hourly intervals, except between 23.00 hr. and 9.00 hr., but only twice daily at 6°C. Exuviae were recorded and removed at each inspection.

(B)Mortality of Nymphs

Mortality in the nymphal stages was estimated by enclosing three or four nymphs in each cell. About fifty nymphs were reared at each temperature. The numbers reaching maturity were recorded and the percentage mortality estimated.

(C)Longevity and Fecundity

The performance of adult apterae was studied at the five constant temperatures, using the Blackman rearing apparatus.

The adults in these experiments were not those whose development had been recorded earlier, but were ones that had been isolated on excised leaves from birth to maturity. The recording and removal of nymphs was performed daily and regular inspections continued until the adults had died. From the data obtained, Figure 2

Exploded diagram of the apparatus for rearing nymphs and measuring fecundity of Microlophium evansi.

- 47 - FIGURE 2.

NYLON MESH REARING CELL .

LEAF

0 0 0 PERSPEX SLAB

SLOTTING PAPER

WATER RESERVOIR - 48 -

life-tables were constructed and various population growth statistics computed.

3. RESULTS (A) Duration of Nymphal Instars

At least 12 individuals were reared at each temperature.

Any which developed into alatae were replaced and the experiment repeated. No attempt was made to measure the duration of the alate nymphal periods.

The mean duration of the nymphal instars of M. evansi at each of the constant temperatures is given in Table 2. Data for A. urticata are included for comparison. The first three instars of M. evansi were of approximately equal duration, the third instar being slightly the longest, except at 12°C and at the two highest temperatures. The fourth instar was always longer than earlier instars, ranging from an average of 74% longer at 6°C to 32% at 15°C.

There was an inverse relationship between developmental period and temperature up to 25°C. At 27°C the aphid took fractionally longer to reach adulthood than at 25°C. This was mainly due to a lengthening of the first instar at the higher temperature, suggesting that premature birth might have occurred under heat stress and, in fact, more than half the nymphs were o o born dead. A rise in temperature from 6 to 12 resulted in a shortening of the total nymphal period by 12.2 days (43%). A reduction of 4.6 days (28%) occurred with the 3° rise from 12 to o o 15 . Between 20 and 27 , the average developmental period remained virtually the same, approximating to 9.0 days. It proved impossible to maintain cultures of M. evansi or its plant host at 30°C. Newly-born nymphs could not be reared past the third instar at this extreme temperature, when transferred from the 25° room. - 49 -

Table 2 Data for the Development of Nymphs of M. evansi and A. urticata

LENGTH OF INSTAR TEMP (DAYS) TOTAL RANGE 5 DEVELOPMENT C SPECIES ± S.E. PER DAY I II III IV M.e. 6.0 5.3 6.8 10.5 28.6+0.34 27.0-30.5 3.5 6 A.u. - - - - M.e. 4.2 3.4 3.5 5.3 16.410.15 16.0-17.5 6.1 12 A.u. 4.3 4.4 4.6 5.4 18.7±0.32 17.0-20.0 5.3 M.e. 2.6 2.5 3.1 3.6 11.81:0.23- 10.0-12.9 8.5 15 A.u. 3.2 2.9 3.0 3.3 12.4±0.39 10.7-14.8 8.1 M.e. 2.1 2.0 2.3 2.9 9.3±0.14 8.0- 9.8 10.8 20 A.u. 2.1 2.0 1.9 2.4 8.4+0.17 8.0- 9.7 11.9 M.e. 2.2 1.9 2.1 2.7 8.9±0.19 8.0-10.0 11.2 25 A.u. 1.4 1.5 1.5 1.9 6.3±0.31 5.0- 8.1 15.9 M.e. 2.3 1.8 1.8 3.1 9.04'0.21 8.4-10.9 11.1 27 A.u. 1.4 1.3 1.2 1.9 5.81'0.16 5.0- 6.7 17.2 - 50 -

Correlation coefficients calculated for developmental period v. temperature, including and excluding 27°, were 0.85 and 0.91 respectively. Both were significant at P 0.05. Figure 3 shows the relation between temperature and the duration and rate of development at the various constant temperatures. The development period showed a curvilinear relation while the rate of development followed a typical sigmoid curve.

At 12° and 15°, development times for A. urticata (yellow, apterous form) were longer than for M. evansi, but at the three higher temperatures they were shorter. The negative correlation between development period and temperature held even up to 27°

(r = 0.945), although there was a reduction of only half a day for the rise from 25 to 27°C. Although the first three instars of A. urticata were of approximately equal duration at each temperature, the first instar was invariably the longest of the three. The final nymphal instar was longer than any preceding it, but not significantly so at 15°. As with M. evansi, full development of this species did not occur at 30°.

(B) Threshold Temperature of Development

The method of Hughes (1963) was used to calculate the threshold of development for each instar. The rates of nymphal development of M. evansi and A. urticata were plotted against temperature and a regression line fitted by the method of least squares. The threshold temperature was read from the graph and checked by substituting y = 0 in the regression equation. The values obtained are given in Table 3. The mean temperature thresholds of development were -3.4° and 5.2° M. evansi and

A. urticata respectively.

Thermal constants were calculated using the formula: K =

D(t k), where k = threshold of development, t = ambient Figure 3

Relationship between temperature and duration and rate of nymphal development for M. evansi. X X RATE OF DEVELOPMENT FIGURE .3. e * DURATION OF DEVELOPMENT

30 -12

25 -10

S) AY

D 20 -8 ( D AY O D / T PERI 15 -6 T MEN EN OP PM EL V

ELO 10 -4 DE V DE %

5 -2

6 12 15 20 25 27 TEMPERATURE °C. - 52 -

temperature and D == time of development in hours or days (Bodenheimer and Swirski, 1957). Relative instar periods,

based on the first instar as standard, were also determined

(Table 3).

(C)Fecundity The average number of births per female of M. evansi

reached a peak at 12°C, falling sharply between 15 and 20°, and. between 20 and 25°. Average offspring per female per day, however, peaked at 20°, due to reproductive life at this temperature being about half that at 12 or 15°. The length of

the reproductive period was similar in the range 6 to 15° (Table 4). Differences in fecundity were tested by Student's 't'. A positive correlation existed between the weight of adult apterae

during the pre-oviposition period and average fecundity for each

temperature (Table 5).

At 15° and 20°, a comparison of fecundities on excised

leaves and in clip-on cages on potted plants was made (Table 6).

The method of rearing did not significantly affect the mean number of offspring deposited, although the difference in

fecundity was greater at the higher temperature. This is probably related to the performance of U. dioica at the two

temperatures (see Discussion).

(D)Longevity Mortalities during nymphal development of M. evansi on

excised leaves were 2, 2, 1, 17 and 43 -70 at 6, 12, 15, 20 and o C respectively. Longevity decreased with increasing

temperature (Table 4).

(B) Population Growth Statistics

Life-tables were constructed at each temperature from the

data on age-specific longevity (lx) and fecundity (nix) - 53 -

Table 3 Threshold Temperatures of Development and Thermal Constants for M. evansi and A. urticata

REGRESSION THRESHOLDo *THERMAL INSTAR SPECIES INSTAR EQUATION TEMP. C CONSTANT PERIOD M.evansi I y= 7.490+1.726x -4.3 54.5 1.0 II y= 8.680+1.891x -4.6 48.8 0.9 III y= 5.873+1.761x -3.3 57.2 1.0 IV y= 1.856+1.518x -1.2 78.6 1.4 Mean=-3.4 Total=239.1 A.urticata I y=-22.961+3.687x 6.2 29.9 1.0 II y=-16.623+3.339x 5.0 29.4 1.0 III y=-19.241+3.490x 5.5 29.6 1.0 IV y=-10.088+2.548x 4.0 35.5 1.2 Mean=5.2 Total=124.4

*Thermal Constant = Day-Degrees above mean development threshold. 'EMP. 'MEAN NO. OF ADULT LIFE REPRODUCTIVE OFFSPRING/9/DAY PRE-LARVIPOSITION PRE-DEATH oC OFFSPRING+8.E. (DAYS) TOTAL LIFE LIFE PERIOD PERIOD

6 36.3+4.2 48.8 77.4 36.1 1.01 9.45 3.32 12 86.71-5.8 40.0 56.4 33.9 2.46 2.92 3.17 15 83.3+8.0 31.6 43.4 28,2 2.95 2.54 2.68 20 46.9+4.1 17.7 27.0 15.6 3.01 0.95 1.12 25 20.8+1.4 14.0 22.9 12.2 1.70 0.78 0.98

*Difference in mean no. of offspring at 12 and 15° not significant (P:1-:-.=-0.05). All other differences significant (P.‹..Z0.01). - 55 -

Table 5

Relationship between Adult Weight and Fecundity of M. evansi

TEMP. AVERAGE PRE-OVIPOSITION AVERAGE (°C) WEIGHT (MG.) ± S.E. FECUNDITY

6 2.09±0.14 36.3 12 2.86+0.15 86.7 15 3.09±0.23 83.3 20 1.37+0.08 46.9 25 0.901'0.04 20.8 r = 0.91, P---::: 0.05

Table 6 Comparison of Fecundity of M. evansi on excised and on intact leaves

TEMP. AVERAGE FECUNDITY ON:- (2- S.E.) (°C) Excised Leaves* Intact Leaves*

15 74.8+2.8 77.1±4.0 20 42.0±2.4 35.6+2.1

*Differences not significant (P2:-----0.05) - 56 -

(Southwood, 1966). The life-tables are presented graphically in Figure 4. Various statistics were determined for each table:- (a)Gross Reproductive Rate, G.R.R. (.2 mx)

(b)Net Reproductive Rate, Ro (= lx.mx)

(c)Intrinsic Rate of Increase, rm, calculated from the 7 formula 2 e7-rxlx.mx = e (Birch, 1948), by the graphical method of Southwood (1966).

(d)Finite Rate of Increase, )\ , the number of times the

population increases per week (= erm).

(e)The Generation Time, T, the mean time (in weeks) elapsing between births in two consecutive generations

(= loge Ro/rm) (Table 7).

The gross reproductive rate was highest at 15°, decreasing with any further rise in temperature. The net reproductive rate ranged from only 8.17 at 25° to 77.51 at 12°, due to differences in survival during the reproductive period.

The intrinsic rate of increase increased with a rise in temperature up to 20°, due to reproductive rate being greatest at this temperature, but decreased at 25° despite the earlier attainment of maturity. A similar trend occurred with the finite rate of increase.

The generation time, T, is largely dependent on the rate of development and the length of the pre-reproductive period. Thus, values of T decreased with increasing temperature between 6° and 25°.

4. DISCUSSION

Through its effect on rate of development, fecundity and longevity, temperature profoundly affects the innate capacity for increase of Microlophium eva.nsi. The general relationship Figure 4

Age-specific life-tables for M. evansi at various constant temperatures.

6° 12°

TY I D UN

AO it 12 10 EC F - 30 200 zs°

15° - 25 W ,A i PORT // , O ' —20

F. / 1

P / 1 / 1 -15 al i 1 / 1 I 1 I 1 - to / 1 / \ I \ \ \ N.. /I \ r

2 4e 7 6 9 i 2 3 A 5 G a TIME SINCE BIRTH (WEEKS) - 59 -

Table 7

Population Growth Statistics for Microlo-phium evansi

TEMP. (°C) Ro rM T

6 36.28 26.90 0.45 1.57 7.32 12 86.68 77.51 1.02 2.77 4.27 15 89.24 75.40 1.07 2.92 4.04 20 46.50 39.39 1.43 4.18 2.57 25 21.68 8.17 0.99 2.69 2.12 Table 8 Data for Development and Reproduction of Various Aphids at Constant Temperatures

Threshold Total Nymphal Period % Development/Day Mean Fecundity Temp. of (Days) Source of Data Species Development o o C 12° 15° 20 25° 12 15 20 25 12 15 20 25

M.evansi (on -3.4 16.4 11.8 9.3 8.9 6.1 8.5 10.8 11.2 86.7 83.3 46.9 20.8 Personal nettle) M.maxima Gilbert & 1 (on - 20.3 13.8 10.8 - 4.9 thimble 3.3 7.2 9.3 - - - - Gutierrez, berry) 1973 1 * M.persicae (on 4.3 19.9 11.8 7.6 5.8 5.0 8.5 13.2 17.2 19.5 36.7 46.6 53.6 Sharaf Eldin potato) 1970 A.pisum (on broad 4.5 - 11.4 7.2 5.7 - 8.8 13.9 17.5 - 91.5 96.1 85.7 Milne 1971 bean) M.viciae (on broad 5.6 - 12.1 8.4 6.2 - 8.3 11.9 16.1 - 115.4 101.1 98.8 Milne 1971 bean) A.fabae (on broad 5.3 19.4 11.3 7.4 5.3 5.2 8.8 13.5 18.9 - 50.8 67.9 72.1 Milne 1971 bean) Rendell 1973

*Data for M. persicae at 10° not 12° - 61 -

between rate of development and temperature agrees with that found for several pest aphids (Table 8). For all these species, the development period was noticeably shortened by a rise in o temperature up to 20 , but any subsequent increase had little overall effect. There was an increase in the duration of the first instar of M. evansi between 25 and 27°, suggesting that premature birth might have occurred. Mortality was greater at the two highest temperatures, being 43% and 60% respectively, than at temperatures below 25°.

The threshold of development was approximately -3.4°C for M. evansi, lower than that found for many other aphids (Table 8).

This threshold is a tentative one based on extrapolation of the linear relationship between rate of development and the temperatures used in this study. Correlation coefficients given in Table 9 suggest that extrapolation is less justified for

M. evansi than for A. urticata, but nevertheless it can be concluded that the former species seems tolerant of quite low temperatures and this could explain both the parthenogenetic overwintering and early population growth observed at Silwood. Park and elsewhere.

Table 9

Correlation Coefficients for Temperature v. Rate of Development of M. evansi and A. urticata

INS TAR SPECIES I II III IV

M. evansi 0.927 0.976 0.988 0.974

A. urticata 0.993 0.999 0.997 0.992 - 62 -

The optimum temperature for fecundity of M. evansi was 12°,

again much lower than that reported for other species (Table 8). It agrees most closely with the 15° optimum found for Megoura

viciae (Milne, 1971), maximum fecundity being 33% more in this aphid than in M. evansi. Fecundity was comparatively low at

constant temperatures of 20 and 25°, with an average of only

46.9 and 20.8 offspring per female respectively. Compare this,

for example, with the corresponding values of 101.1 and 98.8 for M. viciae or 96.1 and 85.7 for A. pisum (Milne, 1971).

The fecundity of M. evansi at each temperature correlated

strongly with average adult weight during the pre-oviposition

period, which ranged from 3.09 mg. at 15° to only 0.90 mg. at o 25 , more than a three-fold difference.

It appears that 25° may represent a state of real stress

to M. evansi and that it is more suited to temperatures from 12 to 15°. This conclusion is reached despite finite and in-

trinsic rates of increase being greatest at 20°, since it is important to consider the growth of its host plant at the various

temperatures. Excised leaves, used in the present study, were

replaced as often as necessary in each room. This was twice a o week at 20 and above, but usually once a week below this

temperature (excised leaves remained in 'good' condition for o over three weeks at 6 ). Potted plants flourished and often flowered at 12 and 15° and tolerated large infestations of aphids

and psyllids, whereas at 20 and 25°, plants grew poorly, rarely flowered and often collapsed under moderate aphid infestation.

It is, in fact, recommended from the author's experience that

nettle aphid cultures are most easily maintained at temperatures

between 10 and 15°C. Replacement of excised leaves ensured a

satisfactory substrate for the experimental aphids throughout - 63 - development and reproduction at all temperatures. The study of a single generation by this method thus indicated that population increase rate was highest at 200. The greater vigour of whole plants at lower temperatures, however, would probably result in a higher rate of aphid increase over several generations in the field. The suggestion that temperatures around 15° are optimal for aphid reproduction and plant growth is supported by the comparison of numbers of offspring produced on excised leaves and in clip-on cages on potted plants (Table 6).

The conclusion emerging from the results of these experiments is that M. evansi is adapted to lower ambient temperatures than most aphid species that have been studied. This aphid increases in abundance in the field during April, and large populations occur on some nettle patches before the end of May (see next section). Population peaks occur in June, followed by a decline during July in most years. Average weekly temperatures from

April to July, together with maxima and minima, are shown in

Table 10 for 1972 and 1973 and it is notable that mean daily temperatures for this period rarely exceeded 15° in either year and minimum temperatures were probably never at a level that completely inhibited the development of nymphs or the reproduction of adults. In 1972 day-time temperatures rarely reached a level detrimental to M. evansi until the time of population crash in July. In 1973 temperature was much higher during June than in 1972 and this probably reduced the reproductive rate of the aphid. This was also an extremely dry period (0.5 mm of rain in the first half of June) which may have had adverse effects on the quality of the host plant, Urtica dioica.

Experiments described in this section thus show that while field conditions during April and May are usually within the - 64 - optimal range for rapid population increase of M. evansi, heat stress in June and July may contribute to the eventual decline in abundance. Other factors affecting populations of this aphid are described in the following section.

- 65 -

Table 10 Mean Daily Temperatures at Silwood Park from April to July 1972 and 1973

1972 1973 Week Beginning Mean Mean Dail Mean Mean Mean Dapy Mean Min. Temp. C Max. Min. Temp. C Max.

April 12 4.6 8.7 13.3 3.2 6.5 10.3 19 3.6 7.5 11.6 2.3 9.3 15.0 26 3.4 8.6 14.0 5.0 9.0 13.3 May 3 6.4 10.8 15.7 6.4 10.1 13.7 10 4.7 8.8 13.4 6.3 12.6 17.4 17 5.6 11.0 16.5 9.0 14.7 19.9 24 8.3 11.4 15.0 8.3 12.5 17.1 31 6.5 11.3 15.9 8.7 15.4 20.7 June 7 6.4 10.4 15.5 7.5 16.2 22.6 14 7.2 12.2 16.9 10.3 15.7 20.9 21 7.8 13.2 16.7 11.5 16.9 22.6 28 9.0 13.5 17.8 11.8 16.7 22.6 July 5 10.0 14.4 18.5 12.6 16.4 20.7 12 10.4 17.6 24.8 10.9 14.2 18.0 19 13.9 17.2 22.1 9.9 14.7 20.0 26 10.2 15.0 21.3 12.2 16.7 21.6 - 66 -

SECTION III

THE POPULATION DYNAMICS OF Microlophium evansi

1. INTRODUCTION Microlophium evansi is one of the commonest and most abundant species of aphids which colonise weeds during the_ summer and on some patches of Urtica dioica, it reaches numbers more frequently associated with epidemics of pest species on crop monocultures. As with many aphid species, there is not only a year-to-year variation in abundance of M. evansi in any one area, but also a considerable variation between nettle patches during any one year. Regional influences, such as climate, and more localised factors such as impact of natural enemies, micro-climate, host plant condition and topography require investigation to reveal the causes of variation. This has not hitherto been done in British conditions or elsewhere in the world, despite the aphid's widespread availability and its suggested importance as an alternative prey for certain coccinellids (Banks, 1955).

Aphids must often adapt to violently changing conditions, whereby host plants mature and senesce in a cyclical nature.

Some species, such as Aphis fabae and Myzus persicae, have seemingly achieved this by exploiting a sequence of temporary host plants (Way and Cammell, 1971) while others, such as

Drepanosiphum platanoides and Masonaphis maxima are monophagous

and possess other adaptations, e.g. early production of sexuales, for surviving periods when the host is in an unfavourable condition. Major advances in the population dynamics of aphids

have largely been made with species of the former type on single,

uniform crops i.e. polyphagous or oligophagous pests (van Emden et al., 1969). Comparable work on monophagous species is - 67 -

therefore needed to add to that done by Dixon (1963, 1966,

1970a) on D. platanoides. The M. evansi/U. dioica system reported in the present section is noteworthy in that it provides

an example of a sap-feeding insect noted for its rapid powers of

increase exploiting a dense natural or semi-natural stand of

relatively uniform non-crop plants throughout the year. Further- more, understanding of the population dynamics of this aphid, particularly its temporal pattern of abundance and the range of

natural enemies it supports, is an essential background to the

attempted evaluation of nettles as a reservoir of beneficial

natural enemies and thus of their relevance to biological control

of crop pests.

This section describes the detailed sampling of M. evansi

on three sites at Silwood Park. Aphid numbers were recorded,

together with corresponding changes in the numbers of natural

enemies, the condition of the host plant and prevailing climatic conditions; certain data were analysed by incorporation in a

computer simulation model. Infestations of M. evansi on other nettle patches were scored "in situ".

2. MATERIALS AND METHODS (A) Sites of Study

(a) Sampled Areas

Three nettle patches were chosen at Silwood Park for a

programme of regular destructive sampling for aphids and their

natural enemies. These patches differed in the amount of shade

received from adjacent vegetation (Figure 5). Plot 1, 54 square

metres in area, was completely exposed, i.e. no shade, and was

frequently waterlogged following heavy rain. The stems on this

plot were stunted, suffering intense competition from cleavers

and several species of grass, and produced smaller leaves than stems on sites 2 and 3. Plot 2, 66 square metres, was shaded Figure 5

Diagrams of the sampled sites at Silwood Park, showing adjacent vegetation and arrangement of sub-plots.

— 69 —

FIGURe 3.

SITE .1. F 0 o-r pAT

1 to

AREA OF 2 9 GRASS, THISTLES

8 DoCK. GRASS 3 G2 ASS

4 7

5 to

N C ARABLE FIELD (MuSTARD /paTATO

I4AZEL ELM 0 TREE LIME 0 TREE OAK TREE

AD 4 5 3 2 1 AREA OF RO NETTLES et

E YOUNG SPINDLE BuSH ES

AT 7 8 9 10 PRIV FOOT PATH N

MANURE IIEAP

SITE .3. ir.;■, °AK TREE FOOT PATH 0 ON< TREES 0 GRASS 6 7 8 9 10

O SYCANIOV 0 7. r: SALLOW A — 4 1 TREE Trst.e-

GRASS L - 70 -

along its southward- and westward-facing edges by lime, elm and hazel trees, but exposed along the other two edges. The stems on Site 2 formed a pure stand and were the most vigorous in appearance. Plot 3, 60 square metres, was heavily shaded

beneath a canopy of sycamore, oak and sallow trees. The stems

on this site also formed a pure stand but their distribution was patchy.

Each plot was divided into 10 equal sub-plots (Figure 5)

and the density of nettle stems on each sub-plot estimated in

April and August using a one foot quadrat. Average densities on the plots for the two years of sampling are given in Table 11. Table 11

Density of Nettle Stems on Sampled Sites 1972 and 1973

STEMS/SQ. METRE (Mean of April and August values) YEAR Site 1 Site 2 Site 3

1972 116 120 78

1973 72 134 63

(h) Scored Areas

In addition to the detailed sampling of the three plots described above, simple scoring of infestations of M. evansi on eleven other plots at or near Silwood Park was done from May

to July. This covered the time of peak numbers and the subsequent decline. These eleven sites were selected for their differences in size and situation and were classified as exposed, semi-shaded and shaded types to correspond with the three main sampled sites. - 71 -

(B) Sampling Procedure

(a) Aphids

The destructive sampling procedure consisted initially of

cutting ten stems per sub-plot each week, making a total of 100

stems from each site. Stems were selected by removing at random

six along a central line and one from each corner of the sub- plots. After careful cutting with scissors, stems were placed

individually in polythene bags for removal to the laboratory. M. evansi is a sensitive aphid and a proportion of individuals dropped from the stem during sampling, this being minimised by

holding the stem firmly between the aphids and the point of

cutting and rapidly placing it into the polythene bag. In 1972 aphid numbers were estimated separately on each stem, but in

1973 the stems from one sub-plot were bagged together and

numbers per sub-plot recorded. It was impossible not to disturb

the nettle stand during sampling; any stems brushed against were not sampled on that day.

It was soon found that sampling 300 stems per week was time-consuming, so early in May, a comparison was made of aphid numbers at Site 2 based on a 50 stem and a 100 stem sample. As no significant difference was obtained between the two samples on any of the sub-plots, 5 stems per sub-plot were removed for the remainder of the study. This represented at each sampling 0.8% of the stems at Site 1, 0.6% at Site 2 and 1.1% at Site 3.

Numbers of aphids were usually determined on the same day as sampling, the stems being stored at 4°C until just before counting to prevent significant increase in numbers or changes in instar structure in the warm laboratory (Heathcote, 1972).

In 1972 an attempt was made to count all the aphids on each stem - 72 - in a large white plastic tray, the sides of which were coated in "Fluon". Each stem was thoroughly searched and shaken to avoid missing too many concealed aphids. Total counts proved impossible during peak infestation period on Sites 2 and 3, so numbers on heavily-infested stems were determined by counting those on one upper, one middle and one lower leaf plus three

Inches of stem and multiplying the counts by the number of leaves in each category and the length of.infested stem respectively. Complete counts were rarely attempted in 1973. The five stems from each sub-plot were bulked and aphids removed into the plastic tray by vigorous shaking and where necessary, with a camel-hair brush. When numbers were low i.e. at the beginning and end of the sampling period, all the aphids were counted, but usually they were transferred to airtight glass tubes for storage in 905 alcohol. Aphids were later estimated by transferring them to a perspex dish measuring 15 cm x 15 cm, which contained alcohol and was divided into four equal parts. Aphids in either one quarter or one half of the tray were counted, according to total numbers present. Counts of aphids in one quarter of the tray were repeated after swilling the contents around the dish, and an average of the two figures was taken.

In both years, instar distribution was determined each year in a simple and rapid manner; relative size of individuals and any major distinguishing features were the only criteria used.

The shiny appearance of the adult body separated it from the apterous fourth instar. Second and third ins-tars were indistinguishable and were consequently grouped. The distinction between 1st and 2nd/3rd instars and between 2nd/3rd and 4th instar was based only on relative size and although size - 73 -

variation due to temperature and over-crowding in aphids is well documented (Murdie, 1965, 1969), periodic checks under high-powered microscope showed the method to be reliable enough with practice. Alate nymphs were easily recognised by their conspicuous wing pads. In the first year, instar distribution of whole samples was determined. The following year, instar of all individuals in sub-samples (i.e. those counted in the perspex dish) was recorded.

(b) Natural Enemies

To estimate degree of parasitism and fungal infection, weekly sub-samples of 200 aphids, or whole samples where there were less than 200, were kept on fresh cuttings of U. dioica

in small perspex containers at 20°C for about 7 days. Any parasite mummies or aphids infected by fungus appearing during

the 7 days were counted. Mummies and fungus-infected aphids were also counted on the sampled plants. Specimens of adult parasites and of the fungi were sent to the British Museum

(Natural History) and Rothamsted Experimental Station respectively for identification.

Adult Coccinellidae were recorded on stems in the field, -

but all other predators were recorded from the samples taken to

the laboratory. Specimens of each species encountered were

preserved in 90% alcohol and identified later. Syrphid and coccinellid eggs were counted, but not those of predatory

Heteroptera. Syrphid and coccinellid larvae were occasionally reared through to adult on a diet of M. evansi to confirm their identity.

Development of aphid populations on the eleven nettle plots

was monitored from May to July by scoring infestations on fifty randomly-selected stems "in situ" each week. Categories used - 74 -

were as follows:-

0 = No infestation I = 1 to 10 aphids, mean 5

II = 10 to 50 aphids, mean 30

III = 50 to 200 aphids, mean 120

IV = over 200 aphids, mean 300 This procedure enabled maximum, minimum and mean estimates of aphid numbers to be made. Early in May, several plots were scored and stems then removed to the laboratory for checking by means of detailed counts. These counts confirmed that estimates based on destructive sampling were always between maximum and minimum estimates based on scoring and furthermore that the mean estimate was usually reliable. On two occasions, infestations at the three sampled sites were scored prior to destructive sampling and the population estimates compared (Table 12). This further confirmed the adequacy of the scoring method, which had the added advantages of rapidity and lack of disturbance of the site. Table 12

MAY 24TH MAY 31ST

Scoring (Aphids/ Sampling Scoring 100 stems) (Aphids/ Sampling 100 stems) Min. Mean Max. Min. Mean Max.

Site 160 600 980 657 380 1150 1900 1066 1

Site 2900 6100 9900 7184 9000 15800 25750 18084

Site 400 1300 2050 1985 700 2100 3750 2355 3

(D) Growth of Nettles

(a) Fresh and Dry Weight - 75 -

In 1972 both fresh and dry weight of stems taken from each sub-plot were recorded. In 1973 dry weight of samples from each main plot was considered sufficient information. To obtain the dry weight, stems were placed in large paper bags and maintained in an oven at 75°C until weight was constant.

(b) Surface Area Surface area of nettle stems and leaves was calculated in

1973, so that aphid densities could be recorded. Initially, 20 stems were randomly sampled from each site. The area of each leaf was estimated by placing it on light- and ammonia-sensitive paper (Grade A90/6 supplied by Admel Division of Addressograph-

Multigraph Ltd., Dacre Works, Brooklands Road, Weybridge, Surrey). Leaves were carefully arranged on the paper, which was then exposed to a 100 watt bulb for 5 minutes. After removing the leaves, the paper was placed in contact with fumes of ammonia, leaving dark blue images of the leaves against a lighter blue background, which were cut out, desiccated for 24 hours and then weighed accurately on a beam balance. Surface area of leaves was easily calculated from the weight of standard areas of the paper. Aphid densities were calculated in terms of the area of leaf undersurfaces, since M. evansi is virtually confined on leaves to the undersides.

Preliminary tests on the volume of water displaced in a

250 ml measuring cylinder suggested that a nettle stem could be treated as a cylinder with the diameter measured approximately half-way along its length. Surface area of sampled stems was consequently measured as 2TErh, the curved surface area of a cylinder.

This method of determining surface area proved to be time-consuming, with 60 stems sampled on each occasion. The - 76 -

relationship between surface area and dry weight was therefore investigated and the correlation found to be significant

(P < 0.01) for both leaves and stems (r = 0.988 and 0.978 respectively). Thus, dry weight data was used for the remainder

of the season to calculate surface area. Dry weight and "Admel paper" methods were compared at various growth stages during the

season, and the difference between the two estimates was never

more than 14% (Table 13).

(E) Quality of Nettles for Growth and Reproduction of M. evansi

(a) Changes in Quality as measured by Fecundity of M. evansi

To further understanding of the influence of the host plant on changes in abundance of M. evansi, the aphid's

reproductive rate was used to bioassay the plant at various

stages of its growth i.e. to determine the temporal pattern of

plant quality in terms of aphid reproduction. Once a month

from May to August 1973 batches of ten first instar nymphs of

M. evansi were reared at a constant temperature of 15°C to the fourth instar and then transferred to various nettle patches

where they were caged individually on leaves at the second node

of otherwise uninfested stems. Offspring produced during the

first 12 days' larviposition were recorded as a measure of the

suitability of the nettles for reproduction. Mean numbers of offspring during the 12 day period each month were not directly

comparable due to fluctuations in temperature throughout the

season, so the actual numbers of offspring were compared with

estimated potential numbers, based on data from the constant

environment rooms, which would have been deposited at the

prevailing temperatures for each experiment. - 77 -

Table 13 Estimates of Surface Area of U. dioica (Stems and Leaves)

Estimate from Accurate Determination 5 Error Date Dry wt./S. Area using Admel Paper of Correlation (sq.cm.) (sq.cm.) Estimate Apr 18

Site 1 65.7 72.1 8.9 Site 2 123.5 141.7 12.8 Site 3 93.7 109.3 14.3

May 2

Site 1 124.5 112.8 10.4 Site 2 250.9 279.2 10.1 Site 3 184.7 165.1 11.9

Jul 12

Site 1 330.9 372.0 11.0 Site 2 656.4 711.1 7.7 Site 3 490.7 536.0 8.5 78 -

(b) Inter-site variation in Quality as measured by Growth Rate of M. evansi nymphs

During May 1974 an attempt was made to investigate differences in host plant quality between the three sampled sites. Mean relative growth rate of M. evansi was used as a measure of suitability of U. dioica, since it can give more rapid and less variable results than the use of fecundity data (van Emden, 1969). Use of relative growth rate as a measure of aphid growth enables comparison of the performance of aphids of some range in initial size, even in conditions of fluctuating temperature etc., provided relative growth rate remains constant during nymphal development of each aphid (van Emden, 1969).

This was checked in the laboratory by individually rearing six

M. evansi from birth and weighing them at intervals of a few days up to pre-reproductive adult stage. Mean relative growth rate over a five-day period (days 3 to 8),given by the formula:-

loge final weight - loge initial weight

No. of days over which weight difference is measured

(Fisher, 1921; Radford, 1967) was calculated back and forwards from the start of the period to the days of initial weight and pre-reproductive adult weight.

This gave a good fit of actual weight increase of the aphids until just before the final moult, when the calculated rate overestimated observed rate (Figure 6). Thus, relative growth rate of nymphs of M. evansi can be assumed constant for most of the developmental period.

Field experiments were done initially by individually caging first instar nymphs, previously weighed on an electro- microbalance, on the second node of 12 randomly selected nettle stems at each site during early May. After four days, each aphid was reweighed and mean relative growth rate calculated. - 79 -

To determine whether there might be a direct influence of shade, which differed at the three sites, on aphid growth, the experiment was repeated by removing stems from each site and measuring aphid growth rate in the 15°C constant environment room.

To complement bioassay of the suitability of nettles for growth and reproduction of M. evansi, total nitrogen in nettle shoots was measured. This aimed to discover whether possible differences in aphid "performance" between sampled sites and changes in aphid numbers during the season coincided with differences in host plant nitrogen content, known to be relatively high in U. dioica (Pigott and Taylor, 1964). Further- more, it was hoped to investigate effects of the aphid on content and distribution of nitrogen between stem and leaves of various ages.

Twenty aerial shoots were taken from the three sites and from one of the scored sites (Site E - Dog Kennels) during the third week of each month from April to September, 1973, always between 14.00 and 16.00 hours. Shoots from each plot were bulked and then separated into young growing tips, mature leaves, senescent leaves (i.e. with visible signs of yellowing) and stems. The plant material was oven-dried at 75°C, ground in an electrically-driven mill and nitrogen determined by the slightly-modified micro-kjeldahl technique of Varley (1966).

A known weight of ground material, approximating to 60 mg was digested in 2 ml nitrogen-free sulphuric acid in the presence of a selenium catalyst (two digests were prepared for each plant category). The resultant solution was diluted to exactly

100 ml and two aliquots taken for analysis in a Technicon Mark Autoanalyser. Figure 6

Fit of the mean relative growth rate calculated over a short period to the actual weight increase of M. evansi nymphs up to pre-reproductive adult weight.

The value of the fitted mean relative growth rate is shown at the top of each graph. - 81 - FIGURE .6.

0-314 0.296

3000

200

C D

0.502 0- 26i

11:3000- •

Z2000 U. O •

I I i I I I t 1 II II

4 0-282. 0.2.98

2000

1 4 G to 122. 2 4 G 8 10 12. DAYS S MICE: B RTI-I - 82 -

Investigation of the effects of M. evansi on distribution of nitrogen required samples of infested and uninfested shoots frow.the same patch and to this end randomly-taken shoots on the scored site adjacent to the Dog Kennels were divided into heavily-infested and uninfested batches, excluding any with low infestations. On other plots, populations of aphids were either too small or too evenly distributed to make such comparisons. Additionally, a large area of nettles was roped off into eight plots for a replicated study of growth and total nitrogen content of nettles under aphid attack. Four plots were given two applications of menazon granules, in April and May, to ensure freedom from infestation, but unfortunately natural infestation was low and attempts to artificially infest appropriate plots proved unsuccessful; the experiment was thus abandoned.

It was felt that total nitrogen might be a suitable indicator of aphid effects on the host plant, although it was appreciated that reciprocal effects of the plant on growth and fecundity of individual aphids and thus on population development might not be indicated by this factor; alteration in amino acid composition of phloem sap or formation in the leaves bf feeding inhibitors might be more likely causes of fluctuations in numbers of phloem-feeding insects than total nitrogen. However, with limited time available in a full sampling programme, it was decided that estimates of total nitrogen were all that could be attempted as a yardstick of both nutritional value and any aphid-induced effects on the host plant.

(F) Influence of Natural Enemies following the Decline in Aphid Numbers

The possible influence of natural enemies in maintaining aphids at a low level during August was investigated using perspex and muslin sleeves. Populations of about ten to twenty - 83 -

M. evansi were caged on nettles either in fully-closed or in

partly-open sleeves which allowed free access of predators.

Differences in development between protected and unprotected populations during one week were recorded.

(G)Trap Catches To examine the incidence of alatae and the arrival of adult predators, a yellow water trap was set up on each sampled plot. This was a plastic container 7 inches in diameter and

2 inches deep, painted green on the outside andon the top

1/2 inch of the inside; the remainder of the inside being yellow.

A small piece was removed from the edge of the container and the gap covered with muslin to serve as an overflow in the event of

heavy rain. The trap was fixed to a wooden pole and the height changed to keep it level with the tops of the nettle stems. A solution made from 20 ml of stergene-formalin in 4.5 litres of water was used in the containers, which were topped up and cleaned when necessary.

Pitfall traps, made of 4" plastic beakers painted black inside, were placed at 2 widely-spaced points on each site and filled with the same solution as the yellow traps. Predators were removed from these weekly.

(H)Meteorological Data

A continuous record of temperature at plant height was

provided on each site by a mercury-in-steel thermometer bulb

housed in a radiation shield, temperature being recorded on the weekly chart of a Negretti and Zambra thermograph. Rainfall Data was obtained from the Meteorological Site at Silwood Park. - 84 -

3. RESULTS (A) 1972 (a) Abundance of Aphids at Sampled Sites

An average of less than 1 aphid per stem was present at the beginning of the sampling programme in April, with slightly more aphids on Site 2 than on the other sites. Apterous viviparae had been found throughout the previous winter on all sites. Numbers rose rapidly in April despite the relatively low temperature; by early May there were large variations between the three sites (Figure 7). By May 10th, numbers on the semi- shaded plot, Site 2, were fifteen times more than on the exposed plot, Site 1, and over four times more than on Site 3, the heavily shaded plot. This order was maintained until late July, when numbers decreased to similarly low levels on the three areas (Table 14).

Peak populations coincided on Sites 1 and 3 at the end of June, but on Site 2 it occurred two weeks earlier. After the peak, decline in numbers was very sharp, particularly on Site 2 where some stems had each supported more than 1000 aphids for most of June. Populations were sparse during August and after temporary resurgance in September, virtually disappeared.

In addition to variation between main sites, there were significant differences in abundance on the sub-plots (Table 15).

For example, on the semi-shaded site, sub-plot 7 had a peak population of 708 aphids/stem on June 16th, while on sub-plot 1 a maximum of only 158/stem was reached on June 30th. On this site the five unshaded sub-plots (6-10) supported between two and three times as many aphids as the five shaded sub-plots.

On Site 3, numbers were distributed evenly over the total area; Figure 7

Populations of M. evansi at sampled sites 1972

- 87 -

on Site 1, aphids were largely confined to sub-plots 3 to

Instar distribution showed that during population increase, first instar was dominant, accounting for about 50% of total numbers (Figure 8). During the period of maximum abundance, 70 to 80% of the population on each site was composed of instars I, II and III. As numbers crashed the proportion of older instars increased, with up to 50% adults recorded late in the season. Except for the first few weeks, no attempt was made to distinguish between second and third apterous nymphs during sampling. When Hughes' (1962) test for stable age distribution was appliedto early samples, numbers in the first three instars were infrequently found to conform to a geometric series. Hughes' method for analysing aphid populations strictly applies to persistent populations showing a steady:..increase or stable numbers, whereas populations of M. evansi increase and decline rapidly over an eight to ten week period.

Alate forms first appeared early in May (Figure 9). The proportion of alate nymphs and adults in the total population remained fairly low, reaching a peak of 25% on Site 2 a fort- night after overall peak in aphid numbers. Secondary peaks of alatae occurred on Sites 1 and 3 in September, but. this was due to the presence of a few immigrant adults in what were very small populations. A more accurate estimation of alate production is obtained by determining proportion of 4th instar nymphs bearing wing pads (Figure 10). This provides an indication of potential emigrants being recruited to the adult population between samples. On Site 2, more than half the 4th instar sampled during the six-week period from the end of May to early July were alate and over 40;x, during the same period Table 14

Numbers of M. evansi/100 stems at Sampled Sites 1972

Maximum Sub-Plot Minimum Sub-Plot Total Aphids/100 Stems Total/10 Stems Date Total/10 Stems Week Site 1 Site 2 Site 3 Site 1 Site 2 Site 3 Site 1 Site 2 Site 3

1 Apr 12 33 67 53 7 11 16 0 1 0 2 19 14 72 40 5 17 8 0 2 1 3 26 48 184 43 11 38 9 0 11 1 4 May 3 67 433 108 20 129 16 0 9 6 5 10 104 1607 358 43 338 146 0 28 9 6 17 201 3677 465 86 675 96 1 66 8 7 24 657 ' 7184 1985 211 1350 603 9 266 42 8 31 1066 18084 2355 360 3672 346 6 648 110 9 Jun 7 1052 22848 3988 289 3780 530 14 354 148 10 14 2624 35458 4708 752 7080 826 24 912 116 11 21 2264 30550 5736 690 7533 1364 16 1040 268 12 28 4486 28985 7632 950 5315 1213 34 1064 183 13 Jul 5 3596 19350 7372 1369 3337 1362 10 235 230 14 12 2467 6725 3361 920 1365 620 20 170 95 15 19 1320 2753 1297 345 630 260 0 47 50 16 26 486 689 457 70 117 73 10 37 24 17 Aug 2 55 135 120 20 29 34 0 7 5 18 9 9 23 30 3 7 10 0 0 0 20 23 0 3 7 0 3 4 0 0 0 22 Sep 6 23 14 30 20 10 13 0 0 0 24 20 10 7 33 4 4 10 0 0 0 26 Oct 4 3 0 0 3 0 0 0 0 0 - 89 -

Table 15 Peak Numbers of M. evansi/stem on Sub-plots at Sampled Sites 1972

Sub-Plot Site 1 Site 2 Site 3 Site Peak/stem=45 Site Peak=.355 Site Peak=76

1 4 158 35 2 26 220 121 3 91 201 136 4 70 217 136 5 39 293 84 6 92 450 112 7 137 708 75 8 50 594 106 9 27 471 81 10 8 753 83 Figure 8

Instar distribution of populations of M. evansi at sampled sites 1972.

- 91 -

la 4- In op. FIGURE .8. IM at. I9 op- sITE.i. ra at. ADULT ap. 60- ADULT al.

4o-

SrVE.2.

GO-

(I) a 40-

2o-

U- 0

SITE .3.

Aprt L MAY SEPT. 90Th - 92 -

on Site 3. On Site 1, where total numbers were lowest of the

three sites, samples always contained more apterous than alate

4th instars, reaching a peak of 4859 with wing pads on June 7th. Alate nymphs continued to be formed until the end of July. M. evansi was found exclusively on the undersurface of

upper leaves and on the top few centimetres of stems in early samples, but as numbers increased they spread down the stems

and on to mature leaves. On heavily infested shoots in June, the upper half of the stem was densely packed with aphids,

although a small degree of spacing, usually about 3 or 4 mm,

was maintained on the leaves. Yellowing, senescent leaves and

the lower woody parts of stems seemed to be the least preferred feeding sites.

The potential number of M. evansi at various intervals was

calculated on a computer from the instar distribution on the

preceding sampling date and the known maximum reproductive rate

at the prevailing temperatures (Figure 11). Large discrepancy between observed and potential numbers during the first week of the study was due to initial fall in numbers in the field which

may reflect sampling error while populations were very small.

Subsequently, increase during May was generally 75% or more of the estimated potential. As peaks were approached, difference

between observed and potential numbers widened and after the

peaks, populations dropped to between 1 and 5 per cent of their theoretically maximum level (Table 16).

On Site 2, population increase appeared to be unimpeded

during May; the count on May 17th actually exceeded calculated

potential, possibly because in the computer calculations all

aphids were assumed to he middle-aged in their respective instars.

Age structure of adult populations would have differed from this, Figure 9

Proportion of alatae (nymphs and adults) in populations of

M. evansi at sampled sites 1972.

Figure 10

Proportion of alatae in 4th instar at sampled sites 1972.

- 94 -

FIGURE .9. z 0 26 11 a_ 0 20 I' FQ- 0 \ is I \ t.r) I I < to \\ 0 II

5 I/ A

APR'S MAY JUNE I JULY AUG. 1 SEPT. I 0 CT.

FIGURE .10. / 3 R 1 TA S 1 IK t4 S 1 TE A AL

APRIL MAY TUNE uuLy - 95 -

but nevertheless the inference is that increase was virtually

unrestrained at this time. On Site 2, where there was a very

large infestation, potential population development for the whole season was also calculated from the instar distribution for the first sampling date (Table 17). In early May observed

numbers were 95% of the potential, decreasing to 50% by the end -

of May and to only 95; at the peak in mid-June.

(b)Abundance of Aphids at Scored Sites

Although degree of infestation on the scored sites varied

enormously, temporal pattern of abundance was consistent and followed the same trend as on the main sampled plots. Numbers rose rapidly in May, peaked during June and quickly declined

during July. Peak populations, ranging from 1 aphid/stem to

419/stem, are given in Table 18.

Infestations on same of the nettle clumps beneath

Euonymus (Plot A) reached phenomenal levels relative to other

patches monitored. Even by mid-May, numbers in this area were five times greater than at Site 2. They peaked earliest

(June 7th) and were still averaging 150 aphids/stem in mid-July.

At the other extreme, Plots I, J and K at Silwood Bottom were

scarcely infested and very few aphids were found after the middle of July. It is perhaps notable that Plots A and B, with

the largest populations of M. evansi, were situated within

30 metres of Site 2, while Plots I, J and K, with the smallest

populations, were situated within 50 metres of Site 1.

(c)Abundance of Natural Enemies

Total numbers of active predators associated with M. evansi. on nettles are shown in Figure 12. The ratio of predators to aphids was relatively high during April, but decreased rapidly in May as the exponential rise in aphid numbers progressed Figure 11

Potential and observed populations of M. evansi at sampled sites 1972. Cf) Lu 0 A PI41D S / i00 ST EMS FIGURE .11. APRIL. SITE .3.

MAY - 97

a UNE

J ULY

AU G. Table 16 *Potential Increase at Various Times of Populations of M. evansi at Sampled Sites 1972

APHIDS/100•. STEMS Site 2 Site 3 Date Site 1 0 as % 0 0 as % P 0 as Observed. Potential of P of P 0 of P April 19 14 215 7 72 223 32 40 235 17 May 17 201 273 74 3677 3618 102 465 826 56 June 14 2624 3032 87 35458 44052 80 4708 7726 61 July 12 2467 8553 29 6725 57810 12 3361 18568 18 Aug 9 9 215 4 23 1585 1 30 600 5 (*Each potential figure based on instar distribution of sample one week earlier.)

Table 17 *Potential Increase (April to June) of Population of M. evansi at Site 2 1972 oo

Date Aphids/100 Stems Observed as % Observed Population Potential Population of Potential April 19 72 223 32 May 3 433 457 95 May 17 3677 7693 48 May 31 18084 36295 50 - June 14 35458 386491 9 June 28 28985 2611528 1 • (*Potential figures based on instar distribution of first sample.) - 99 -

Table 18 Peak Numbers of M. evansi/100 Stems at Scored Sites 1972

Plot Degree of Shading Peak Pop./100 Stems

A Semi-shade (S.S.) 41900 B S.S. 38650 C S.S. 16700 D S.S. 12800 E Exposed (E.) 12750 F Heavy Shade (H.S.) 12500 G H.S. 650 H H.S. 350 I S.S. 200 J E. 150 K S.S. 100 Figure 12

Total numbers of predators (active stages) at sampled •sites 1972. APRIL

MAY

SUNS.

JULY

AUGUST

SEPT.

OCT. H O 'Zi. 9. 1t151J - 102 -

(Figure 13). At Site 2 there was only one active predator per

1000 aphids at the beginning of June and about one per 100 aphids a month later.

Anthocoridae, mainly Anthocoris nemorum and occasionally

A. nemoralis, particularly on the exposed site, were the earliest specific predators. to appear and also the most abundant and persistent (Figures 14 and 15). Coccinellidae arrived at

Sites 1 and 2 early in May but were rarely observed at the heavily shaded Site 3 and not at all until July. A few coccinellid eggs and larvae were recorded, mainly on the exposed site. Two peaks in numbers of adults were observed; first in mid-June before re-dispersion to other habitats and a new generation early in August following a peak in numbers of pupae about two weeks earlier. The commonest species in

1972 was Adalia bipunctata, and then Coccinella septempunctata

(Figure 14). Other less frequently recorded species were

Propylea 14-punctata, Adalia decempunctata and Coccinella 11- punctata.

Apart from unidentified spiders, whose significance as predators of M. evansi was not assessed, all predatory groups other than those discussed above appeared in extremely low numbers and are unlikely to have made significant impact on populations of nettle aphids. Larvae of Syiphidae, most commonly Platycheirus albimanus and more rarely. Metasyrphus luniger, were highest in number at Site 2, as were larvae of the Cecidomyiid, Aphidoletes sp. Syrphidae first appeared as larvae on May 24th and Cecidomyi1dae on June 21st, but never on more than three or four stems in a fifty stems sample.

Heteroptera other than Anthocoridae were mostly herbivorous

(notably Lygocoris pabulinus and Liocoris tripustulatus) but: Figure 13

Numbers of predators (active stages)/100 aphids at sampled sites 1972. PREDATORS/ 100 APt-%I DS 13 - 13 1 15- 12 15 FIGURE .13. 9- 3 2- APR1 I- SITE S1TE..2.

.1. - 104 32 27.5 6G-7303 n 11 4 Ica 91.9 [1 rl11 r l

133 124fier3 52.1 can

11 [1 11 1.03 [1 Figure 14

Populations of Anthocoridae and Coccinellidae at sampled , sites 1972.

Figure 15

Relative abundance of predator groups at sampled sites 1972. - 108 - FIGURE .i5.

I. coccimzuADAE 2. SVRPHIoAs SI T AtiTHOcORiDA.E 4. OTHER 47::1EROPTERA

40- S. CECIOOINNYIIDAS 6. CH RV'S° MAE. 7. PERMAPTERA 8. CANTHARI DAS 30- 9. SPIDERS

20-

10 -

SITE .2.

5 9-

SITE. 3. SO

40-

30-

20-

to-

. S 9 - 109 -

included the carnivorous mirid Heterotoma merioptera and, at

Site 3 only, Calocoris sexguttatus. Forficula occurred at

Site 2 and two species of Cantharidae, Cantharus nigricans and Rhagonycha sp were occasionally taken at Sites 2 and 3.

Recording of predator numbers on a sub-plot basis enabled

their spatial distribution in relation to variations in density of aphids to be determined. Accumulated totals of aphids and

active predators on the sub-plots, shown in Table 19, were

positively correlated (r = 0.81, 0.91, 0.50 for Sites 1, 2 and

3 respectively). The correlation was highly significant

(P -c: 0.01) at Sites 1 and 2 where large differences in

infestations of aphids between sub-plots existed. Thus predators

appeared to be aggregating in areas of high prey density. However, consistent ratio of aphids to predators was not

necessarily maintained. At Site 2, for example, there was a

four-fold infestation of M. evansi on sub-plot 7 compared to

sub-plot 1, yet little more than a two-fold difference in

corresponding numbers of active predators.

Parasitism, determined from samples of aphids kept at 20°C until mummification, remained at a low level at all sites until

the second week of June, when it increased rapidly (Figure 16). Percentage parasitism was greatest during July, reaching about 8%, but by early August less than 2% of sampled aphids were

•becoming mummified. Over 90% of primary parasites identified

were Aphidius ervi, the remainder being Enhedrus lacertosus.

The hyper-parasite LygOcerus emerged from mummies, but degree

of hyper-parasitism in samples of aphids.was not measured.

Infection by fungus was similarly low and did not appear until

late in June. Maximum rates of infection were recorded in

mid-July, perhaps partly due to heavy rainfall on July 7th. and 8th, (Figure 16). Highest percenlicje infection in samples - 110

Table 19 Accumulated Totals of Aphids and Active Predators on each sub-plot at Sampled Sites 1972

Sub-Plot Accum. Aphid Total Accum. Active Predator (per stem) Total (per stem) Site 1 Site 2 Site 3 Site 1 Site 2 Site 3

1 27 741 202 4 6 5 2 90 975 437 3 7 7 3 298 903 514 5 7 8 4 299 1151 454 5 5 8 5 270 1325 437 8 7 6 6 396 2237 325 14 12 5 7 354 2989 385 10 13 7 8 191 2772 426 4 10 4 9 84 2135 412 3 10 5 10 37 2616 405 3 11 5

GRAND 2046 17844 TOTALS 3997 59 88 60 Figure 16

Rates of parasitism and fungal infection in populations of M. evansi at sampled sites 1972. FIGURE AG. - 112 -

SIM 10 PARA:31115M FUNGAL iNFScrICM

SED TI S1TE.2. SI A 10- PAR TED OR FEC IN N O LATI PU PO OF % 6IT 101

AU: U - 113 -

maintained at 20°C occurred at Site 1 (85), which was the dampest of the three nettle patches studied. Species of fungus identified were Entomophthora aphidis and E. planchoniana.

(d) Pitfall and Yellow Trap Catches

Carabidae, Staphylinidae, spiders and Phalangidae were

taken in pitfall traps, most frequently during August after the

decline in populations of aphids. Catches were dominated by two adult carabid beetles (with up to 20 per trap each week in

August); one of these beetles was Carabus nemoralis.

Syrphidae and Cantharidae were the commonest predatory

species in the yellow traps. Small numbers of Heteroptera,

Dermaptera and Coccinellidae were also collected. Alatae of

•M. evansi were first observed in yellow traps in early May, about the same time as they appeared on stems at the sampled sites. Alatae of Myzus persicae and the Aphis fabae complex

were frequently taken during June and July, in much greater numbers than those of M. evansi.

(e) Growth of Nettles

Growth curves based on dry weight of sampled stems are

given in Figure 17. Weight and height of stems at the three

sites ranked in the same order as their aphid infestations.

Under intense competition from grasses and cleavers, nettles

at Site 1 developed poorly and dry weight at maturity was

significantly less (P-‹ 0.05) than for the other two sites.

Growth curves were typically sigmoid in shape, levelling off

towards the end of July just as flowering reached its peak.

Due to greater leaf-fall during August, dry weight of samples

at Site 2 was eventually lower than that of samples at Site 3.

With growth of stems there was a corresponding increase in

surface area. Changes in average surface area during the season • Figure 17

Growth curves of nettles at sampled sites 1972. APRIL- 1

MAY

slim .2. JUNE

X X TULY x

AUGUST 0

SEPT.

OCT.

Z.V Bth1913 - 116 -

are given in Table 20. These are based on the dry weight/area correlation determined in 1973.

Table 20

Average Surface Area of Single Stems of U. dioica (sq. cm.)

SITE DATE 1 2 3

April 12th 49.8 88.8 83.1

July 12th 287.1 482.7 418.7

Oct. 4th 191.9 313.1 335.1

Rapid growth of stems during April and May resulted in low densities of aphids being maintained and at Sites 1 and 3 there were only about 20 aphids per 100 square cm. of plant even at

peak population times (Figure 18). It was only at Site 2, where a very large infestation developed, that density rose dramatically to a peak of 69/100 sq. cm.

(B) 1973

(a) Abundance of Aphids at Sampled Sites

Infestations were generally much lower than in 1972, but

the pattern- of abundance was strikingly similar (Figure 19).

From resident populations of less than two aphids/stem rapid

increase occurred during May and as in the previous year,

differences between sites were soon apparent. The same order

of infestation was retained, with Site 2 most: heavily infested

and Site 1 least heavily (Table 21). Peak populations We le

attained by mid-June at Sites 2 and 3, and in early June at

Site 1; these maxima were about one third, one quarter and

one half those of 1972 for Sites 1, 2 and 3 respectively.

Variations in abundance of M. evansi in the sub--plot s

were less conspicuous and did not always foiloo! the pattern Figure 18

Density of Microlophium evansi at sampled sites 1972.

70 G o APRIL I

___.._...... ,'''' MA'( ../."..

...---

• •

,'.

.. A ..'

, ...... ---. ..---•

/ /

// JUNE -----

.---/--*- ---"\\ \\ / 1"

-- - A \ \ \\ JULY

\ N.

N•

..".••■ N

N. ....,,,...... AUGUST

'81'313C 1O1d - 119 -

of distribution observed in the preceding year (Table 22). At

Site 2, however, largest infestations remained on the unshaded side and at Site 3 aphids were again relatively evenly distributed. At Site 1, sub-plots 9 and 10, which had very few aphids in 1972, supported the largest populations in 1973. Conversely, sub-plot 7, with a very large population in 1972, was least colonised in 1973 (see accumulated totals of aphids in Tables 19 and 25).

Instar distribution followed similar trends; with a preponderance of first instars until the end of June and significant increase in the proportion of adults at the end of the season as reproductive rate fell (Figure 20). Alatae appeared in early May and were commonest at Site 1; several peaks occurred during the season (Figure 21). In keeping with lower total

populations, fewer alatae were produced and none were formed

after mid-August. The proportion of winged 4th instar nymphs was also lower and fell to zero by the beginning of July (Figure 22).

Potential numbers of aphids each week were calculated as before (Figure 23). Unexplained drop in observed numbers

between the first two sampling dates resulted in a large initial difference between observed and potential figures, but during May populations were realising at least 80 per cent of their calculated potential level. Ratios of observed/potential had fallen dramatically by the time of maximum abundance and in early August numbers of aphids were 4-6% of their estimated

potential (Table 23).

(b) Abundance of Aphids at Scored Sites

There was considerable variation in aphid numbers from site to site. With the exception of the Doc" Kennels Site (E), nettles heavily infested in 1972 supported far fewer M. evansi Figure 19

Populations of M. evansi at sampled sites 1973. API-I losf 100 STEMS (LOG ECALE) U 0 io,:aitt.'1 TEW.ri, Ett:111E'Z'?C•

-0 r

4 C

C r

C C -4

to in -c)

0 P zg • z p vraEKLY RAI FALL rek rn

'6I' I I - Table 21

Numbers of M. evansi/100 stems at Sampled Sites 1973

Maximum sub-Plot Minimum sub-Plot Week Date Total Aphids/100 Stems Total/10 Stems Total/10 Stems Site 1 Site 2 Site 3 Site 1 Site 2 Site 3 Site 1 Site 2 Site 3

1 Apr 17 46 66 196 16 12 36 0 2 6 2 24 38 58 152 10 12 34 0 0 2 3 May 1 38 88 102 10 16 20 0 0 0 4 8 134 106 156 46 20 .58 2 0 4 5 15 164 372 446 28 56 78 6 20 14 6 22 392 1048 1358 144 168 218 8 42 74 7 29 1258 1340 1664 300 214 238 68 86 106 8 Jun 5 1448 2166 2964 370 338 468 52 132 134 9 12 594 5150 4382 146 1010 938 20 264 192 10 19 580 7914 3310 106 1386 462 20 498 216 11 26 292 5058 2064 96 1092 404 0 234 106 12 Jul 4 74 1058 368 26 230 82 0 50 6 13 10 48 174 72 14 40 18 0 4 0 15 24 12 38 14 6 16 6 0 0 0 17 Aug 7 2 16 14 2 4 6 0 0 0 19 21 0 6 4 0 4 4 0 0 0 21 Sep 4 0 30 10 0 10 4 0 0 0 23 18 24 22 8 12 8 6 0 0 0 25 Oct 2 12 18 2 6 10 2 0 0 0 - 123 -

Table 22 Peak Numbers of M. evansi/stem on Sub-plots at Sampled Sites 1973

Sub-Plot . Site 1 Site 2 Site 3 Site Peak/Stem=15 Site Peak=79 Site Peak=44

1 10 51 94 2 19 55 46 3 9 81 40 4 14 50 45 5 9 70 41 6 14 139 63 7 9 87 46 8 10 102 46 9 23 106 26 10 37 50 22 Figure 20

Instar distribution of populations of M. evansi at sampled sites 1973.

- 126 -

in 1973 (Table 24). Conversely, many of the patches relatively free from colonisation in the first year developed higher populations in the second year. Peaks at Sites G and H increased nine and twenty-five times respectively. Site A was colonised by less than 1/20 of. the numbers in 1972, Site B supported only

1/3 of the 1972 numbers and Site C about 1/16. (c) Abundance of Natural Enemies

Numbers of active predators each week at the three main sites are shown in Figure 24. Ratios of predators to aphids were always higher than in 1972, particularly during June when aphid numbers were at their maximum (Figure 25). During July predators, mainly Anthocoridae, may have been retained by populations of Psyllidae, since there were seemingly insufficient aphids to provide even a maintenance diet.

Coccinellidae and Anthocoridae were the earliest to arrive and the commonest specific predators (Figures 26 and 27). Site

3 was again an unfavoured habitat for adult coccinellids and despite frequent occurrence at Sites 1 and 2, very few eggs were deposited on nettles. C. 7-punctata was the most abundant species, with A. 2-punctata appearing less frequently'than in the previous year (Figure 26). A. nemorum and A. nemoralis were the most persistent predators at all three sites (Figure 26). Larvae of Syrphidae,

Cecidomyiidae and Chrysopidae occurred spasmodically. The predatory bugs Heterotoma merioptera and Calocoris sexguttatus appeared in greater numbers than previously and an additional herbivorous species, Lygocoris lucorum, was identified. Spiders were very abundant and at Site 1 formed a greater percentage of the total numbers of predators than Anthocoridae (Figure 27).

Accumulated totals of aphids and active predators on the sub-plots were not significantly correlated (P.2>- 0.05) although Figure 21

proportion of alatae (nymphs and adults) in populations of

M. evansi at sampled sites 1973.

Figure 22

Proportion of alatae in 4th instar at sampled sites 1973. - 128 -

FIGURE .25.. ON I AT L POPU TAL O IN T S TE A % AL

APR1 MAY I SUNS LY AUG.

FIGURE .22.

I I ,,/ J. , V I ".-,/ ‘ 4 / 1 1 APRIL' TY:AY I 3" Li t'; SLILY AU G. ea PT. I OCT. Figuxe 23

Potential and observed populations of M. evansi at sampled sites 1973.

- 130 - FIGURE .23. to 511E .1. OBSERVED T POTENTIAL

1000

1. / /

too /

10 ) E AL (LOG SC S EM T S 0 10

IDS / APH

SITE.3.

APRIL INS NY JUNE Table 23 Potential Increase at Various Times of Populations of M. evansi at Sampled Sites 1973

APHIDS/100 STEMS

Site 1 Site 2 Site 3 Date O as % Observed Potential O P 0 as 5 0 P 0 as 5 of P of P of P

April 24 38 202 19 58 309 19 152 879 17 May 22 392 477 82 1048 1096 96 1358 1462 93 June 19 580 2773 21 7914 21146 37 3310 15646 21 July 10 48 500 10 174 1812 10 72 1122 6 Aug 7 2 50 4 16 274 6 14 254 6 - 132 -

Table 24

Peak Populations of M. evansi/100 stems at Scored Sites 1973

Plot Degree of Shading Peak Pop./100 stems

A Semi-shade (S.S.) 2,000 B S.S. 13,500 C S.S. 1,100 D S.S. 550 E Exposed (E.) 16,500 F Heavy Shade(H.S.) 1,800

G H.S. 6,100 H H.S. 9,200 1 S.S. 850 J E. 350 K S.S. 1,350 - 133 -

there was a tendency for more natural enemies to occur on the less shaded side of Site 2 where aphids were generally more numerous. The lesser degree of aggregation by predators reflected the more even distribution of aphids between sub- plots in 1973 (Table 25). Percentage parasitism followed a similar pattern to that of 1972, reaching a maximum of 7-10% about two weeks after the aphid peaks (Figure 28). Despite differing availability of prey, percentage parasitism was similar in both years. As before, Aphidius ervi was the major primary parasite.

A small percentage of fungus-infected aphids, probably insignificant in the population dynamics of M. evansi, developed during June (Figure 28). The same species of Entomophthora were present as in 1972. (d)Pitfall and Yellow Trap Catches

Pitfall traps captured a wide range of general predators, mainly carabidae and Staphylinidae. Syrphidae, including

P. albimanus, M. luniger and Syrphus balteatus, formed the bulk of the yellow trap catches. Syrphid adults were present when sampling commenced in April and were most numerous during

June. A few Cantharidae, Coccinellidae and predatory Heteroptera were also caught in the yellow traps. Fewer predators (P< 0.05) appeared in the trap at the heavily shaded site than elsewhere.

Alatae of. M. evansi were first captured in the traps at the end of May, two weeks after they were first recorded in field samples.

(e)Growth of Nettles

Growth curves based on dry weight of stems followed sigmoid shape until the end of June, when there was a resuroance of vegetative growth (Figure 29). By the end of July, stems at Figure 24

Total numbers of predators (active stages) at sampled sites 1973.

3 1: 10 1 1 200 SITE .2. V31: 17

V 1 (1) I / I I ul 1-- / • / (1) / \/ 3 ITS fr...... _.-1 SVTE.3. 0 o N-I I --_, / / tn ,

, r----_,, / , 0 , / 4., 1 / 0 --./ us z CC \\ z CL ),,

AUGUST SEPT. APRIL 1 MAY JUNE JU LY Figure 25

Numbers of predators (active stages)/100 aphids at sampled sites 1973. PREDATORS / 1.00 APHIDS FIGURE.25. 12- 13 - is - 3- G 9- n APRIL SITE.i. SITE .3. SITE .2. r - - 137 MAY n

n r I rf LI:NIS

3389 575Ifoo n q5 45a 11 nn rl 32! 700 Figure 26

Populations of Anthocoridae and Coccinellidae at sampled sites 1973. S VT 100 74x re.-ts so IA

CI 10

LO A 2M9 C.5 0 B . '98 tn dc-

- A C.-7J LTG COCC 1NEt.Lt DAE 0 --- PI) PAS co A . bi puilaaira 0 11-

Ln a

0 4 5 z

- AOU LTG (10 C. septemponcial-a --- Pu PAE 12 12

4 r. 4 ilk AA\ A SUN L MAY APSZI MAY ,ACAS JULY Au G. SEPT. APO MAN JUNE 7 U LY AUG. SEPT. E 7 ULy AUG. SE Pt Figure 27

Relative abundance of predator groups at sampled sites 1973. - 141 - FIGURE .27.

SITE .1.. I. COCc1 NSLLI DAIS Z. SYRPHI OAF:. SO - 3. ANTHOCORIDAS. 4. OTH7.-2R HSTEROP-IS.RA 5. c.s.ct Do rviv °Ala. 40- G. C9P(S0PIDAri 7. 10:7:-.PINIAP1 SRA B. C.AlsITH ARIONE. 9. :RIDERS BO-

20-

I 2 3 4 5 6 7 8 9 SITE .2. 50— ri 9.5

RS 40-4 TO

30- PREDA L 20- OTA T

F 10- O %

a 5 7 S SITS .3. So

7 9 - 142 -

Table 25

Accumulated Totals of Aphids and Active Predators on each Sub-plot at Sampled Sites 1973

Accum. Aphid Total Accum. Active Predator Sub-Plot (per stem) Total (per stem) Site 1 Site 2 Site 3 Site 1 Site 2 Site

1 37 191 236 5 10 8 2 53 160 195 6 5 9 3 37 195 177 5 10 9 4 42 155 218 6 10 9 5 32 219 168 i6 10 6 6 43 428 201 4 14 12 7 31 279 159 4 17 7 8 41 338 148 6 13 ' 6 9 89 321 110 5 13 8 10 112 180 100 6 14 5 i Grand p Totals 517 2466 1712 i 53 116 79 Figure 28

Rates of parasitism and fungal infection in populations of M. evansi at sampled sites 1973.

- 144 -

Ft GORE .28.

PARASITISM - - - FUNGAL INFNCTIora

1 SED I SIT RA R PA ED O T C NFE I N ATIO PUL PO OF %

AUGU5T Figure 29

Growth curves of nettles at sampled sites 1973. MAY NE U LY AUGUST r - SEPTEMBeR I OCT.

63- rioki - 147 -

Sites 1 and 2 were about 655 heavier on average than they had been at the corresponding time in 1972, and 205 heavier at Site 3. According to K. Gales (pers. comm.) this could have been caused by extremely dry conditions during the normal period of flower initiation in U. dioica. This was probably in the first half of June when conditions were very dry, and the hypothesis was supported by the absent or sparse flowers, particularly at Sites 1 and 2. During July and August stems produced excessive side-shoots, which resulted in continued increase .in dry weight and surface area and which may have been a response to the lack of flowers and fruits earlier in the season. Average surface areas are given in Table 26.

Table 26 Average Surface Area of Single Stems of U. dioica (sq. cm.)

SITE DATE 1 2 3 April 17th 65.7 123.5 93.7

July 2nd 330.9 656.4 490.7

Oct. 2nd 316.4 658.2 407.4

There was less than 1 aphid per 500 sq. cm. until the beginning of June and maximum densities were only 5, 13 and 10 aphids per

100 sq. cm. at Sites 1, 2 and 3 respectively (Figure 30). Thus prolonged vegetative growth of stems led to much greater surface areas and correspondingly much lower aphid. densities.

(f) Nutritional Quality of Nettles

(i) Changes in Fecundity of M. evansi

Mean number of offspring produced during the first 12 days of larviposition was used once a month as a measure of the quality of nettles for aphid reproduction on various. sits at Figure 30

Density of Microlophium evansi at sampled sites 1973. 13 0H -1 il n ' 'OE - 150 -

Silwood Park. Results for each month were compared with estimated

potential numbers of offspring determined for the same 12-day period at a constant temperature in the laboratory approximating to the mean temperature in the field (Table 27).

Actual numbers of offspring were presumably a reflection of (a) host plant condition and (b) climate. Thus, a mean of 36 offspring/lst 12 days' larviposition was achieved in June when

temperatures were mostly optimal as determined in laboratory experiments and when nettle stems were still actively growing.

The relatively few offspring in May was probably due to low temperatures, while the small mean number in August was partly due to poor condition of the host plant. By comparing results with the potential for prevailing temperatures during each experiment, it was shown that in May, rate of larviposition

(86% of the potential) was little affected by food quality of

the host plant. Rate of larviposition was gradually reduced,

compared to the corresponding potential, from May to August,

the number of offspring in August being only 30% of the estimated

potential for a mean temperature of 18.4°C.

(ii) Differences in Growth Rate of Nymphs of M. evansi between Sampled Sites

Mean relative growth rates of nymphs of M. evansi were

determined in clip-on cages at the three sampled sites in May

1974. Results, based on 12 replicates at each site, are given

in Table 28a. Analysis of variance showed significant differences

< 0.05) between sites. Mean relative growth rate at Site 2

was approximately 3.1/2 times greater than at Site 1 and twice

as great as at Site 3. It is notable that this order of

difference corresponded to differences in both size of aphid - populations and size of stems in previous years at the three

sites. - 151 -

Table 27

Suitability of Stinging Nettles for Reproduction of M. evansi

Mean Mean No. of offspring Date on which Daily during First 12 days o x 100 Experiment began Temp. of larviposition p C *Potential Observed

May 2 9.0 22 19 86

June 6 16.0 43 36 84

July 10 15.8 43 1 21 49 Aug. 7 18.4 40 i 12 30 I

*Based on laboratory data at constant temperature approximating

to mean in the field. - 152 -

Table 28 (a) Mean relative growth rates of M. evansi nymphs at sampled sites 1974 (/4.1g//ug/day)

Rep. Site 1 Site 2 Site 3 1 0.020 0.138 0.025 2 0.012 0.206 0.046 3 0.028 0.033 0.022 4 0.037 0.062 0.093 5 0.004 0.102 0.050 6 0.025 0.063 0.022 7 0.067 0.069 0.013 8 0.018 0.034 0.030 9 0.035 0.149 0.085 10 0.034 0.055 0.068 11 0.027 0.113 0.031 12 0.023 0.096 0.035 Mean±S.E. 0.0281-.005 0.0934-.014 0.043-1-.008

(b) Mean relative growth rates of M. evansi nymphs on nettle stems from sampled sites - measured in 150C constant environment room (pg//ug/day)

Rep. Site 1 Site 2 Site 3 1 0.281 0.291 0.313 2 0.265 0.259 0.332 3 0.344 0.345 0.275 4 0.353 0.388 0.355 5 0.388 0.371 0.344 6 0.294 0.298 0.326 7 0.341 0.379 0.360 8 0.343 0.316 0.286 Mean 0.326 0.331 0.324 153 -

When the experiment was repeated by placing nymphs on cut stems from each site in a 15°C constant environment room, there was no significant difference (P 0.05) between relative growth rates (Table 28b). Values obtained were almost identical at the three sites, suggesting that differences in suitability of nettles in the field for aphid development may have been due to non-plant factors e.g. degree of shading. However, it is perhaps more likely that physiological changes were induced by cutting stems and removing them to a constant environment, thus producing stems of similar food quality for growth of M. evansi.

Until further experiments are done it can only be concluded that the three sites chosen for sampling appeared to differ in suitability for growth of nymphs of M. evansi.

(iii) Differences in Nitrogen Content of Nettles between Sampled Sites

Total amounts of nitrogen in whole stems, shown in Figure

31, gradually increased, reaching a peak in the June sample at

Sites 2 and 3 and in the August sample at Site 1. Thereafter, there was a slight drop and levels in September were about the same as those in May. Differences between sites in total amounts of nitrogen were merely a reflection of differences in size of stems and not of differences in concentration of this element. Thus amounts of nitrogen per milligram of stems and leaves were very similar at the three sampled sites and at

, scored site E and do not appear to explain differences in growth rate of aphids described earlier (Table 23). Concentration of nitrogen declined steadily in growing tips and stems but remained virtually unchanged in mature and old leaves. At Site

E, where nettle shoots were separated into those relatively uninfested and those heavily infested, aphids did not appear to affect distribution of nitrogen within shoots (Figure 32). Figure 31

Total nitrogen content of nettle stems at sampled sites 1973.

- 155 - FIGURE .31.

SITE. 1. 90-

GO-

30-

132 SITE .2. I

SN • ROG T NI 30 AL OT T

SITE.3. 120

APRIL uLS AUG. SEPT. - 156 -

(g) Influence of Natural Enemies following the Decline in Aphid Numbers

Development of small populations of M. evansi in 'closed'

and 'open' muslin sleeves on nettle stems in the field indicated that the activity of predators, notably Anthocoridae, may have

contributed to the trough in numbers of nettle aphids during August. Populations in 'open sleeves' declined during the seven day experimental period and predators were active in these

sleeves at the end of the experiment (Table 30). In 'closed

sleeves', aphid populations increased.

Numbers of M. evansi were scored on fifty randomly-selected stems at each of the sites sampled in the previous two years

and also on the previously scored sites 13, E and K. This was done mainly to record size of infestations for a third year

and thus to see whether regular patterns were emerging. Stems were scored on May 8th, June 4th, and June 17th; results are given in Table 31. Table 31

Populations of M. evansi at Various Sites 1974

Nos. of SITE M. evansi/100 stems MAY 8TH JUNE 4TH JUNE 17TH

1 130 1150 2100

2 570 9650A 14800*

3 360 4100 6700

B 520 8900 18150

E 900 13800 19600

K 2 15 50

Based on sub-plots 1 and 10 only - others accidentally cut.

Table 29 Mean concentration of Nitrogen (/ug/mg of dried material) in various parts of Urtica dioica - April to September 1973

MONTH PART SITE E SITE E SAMPLED INFESTED STEMS UNINFESTED STEMS SITE 1 SITE 2 SITE 3

Growing Tip(GT) 58.7 56.8 55.4 57.2 55.1 Mature Leaves(ML) 40.7 37.6 31.7 41.0 38.9 APRIL Old Leaves(OL) - - - - - , Stem(St) 34.3 32.3 34.5 40.2 37.2 1 H GT 53.8 49.7 49.3 57.6 57.9 tr, ML 36.2 35.2 27.3 39.2 41.0 --.1 MAY OL 22.7 17.7 16.9 17.5 19.4 1 St 19.0 27.7 17.7 19.8 25.8 GT 44.7 53.9 48.6 49.6 44.9 ML 45.1 4.2.6 37.5 46.3 43.7 JUNE OL 24.6 24.9 17.2 28.3 30.0 St 19.9 17.5 14.2 17.6 18.9 GT 51.2 54.2 44.8 43.0 50.9 ML 34.0 39.5 32.8 31.2 35.7 JULY OL 16.2 20.7 15.2 20.1 26.7 St 15.2 13.3 12.9 14.3 13.6 GT 52.9 49.2 49.8 41.8 47.5 ML 40.7 33.2 36.1 29.6 29.7 AUG OL 24.3 18.7 19.1 17.7 22.3 St 12.4 11.4 11.4 11.7 11.1

GT 41.0 39.7 40.0 36.9 35.4 SEPT ML 36.6 31.0 28.1 31.3 27.6 OL 22.4 16.8 17.4 19.3 22.9 St 10.6 11.2 11.5 9.0 13.4 Figure 32

Concentration of nitrogen in various parts of nettle stems

at scored site E.

- 159 - FIGURE .32. 1. YOUNG LEAVES & TIP OF STEM 2. MATURE LEAVES Go 3. OLD LEAVES (A) INFESTED PLANTS 4. STEM

1 -1 0

C. I 2 3 4 1

APRIL MAY JUNE JULY AUG. SEPT. W Go t.9 0 (ES) NON- IN5ESTED PLANTS Ct

So U- 0

APRIL MAY :rums JULY AUG. SEPT. - 160 -

Table 30 Development of Small Populations of M. evansi in the presence and absence of Predators

Net Instar Structure On:- Predators Type Change in cage of August 14 August 21 in at end of Sleeve I II/III IV Ad. I II/III IV Ad. Pop. expt.

Open 15 4 2 0 0 2 3 3 -13 3 Antho- corid nymphs + 1 Coccin- ellid adult

Open 10 0 0 2 0 2 4 0 -6 4 Antho- corid nymphs

Closed 15 8 0 1 6 7 5 8 +2

Closed 7 0 0 1 4 3 4 1 +4 - 161 -

As in previous years at the sampled sites, the highest infestation appeared at Site 2 and the lowest at Site 1. Numbers of aphids/100 stems increased rapidly during May and at Sites 1 and 3 infestations were very similar in mid-June to those recorded in 1972. The population at Site 2 in mid-June was about half that of 1972. Thus a pattern of relatively large and small infestations of nettles in alternate years appeared to be emerging at the sampled sites.

At 'scored site' B numbers of M. evansi in mid-June were almost 1.1/2 times greater than at the corresponding time in

1973 and about half the numbers at the time of peak population in 1972. Infestations at sites E and K were similar to the previous two years i.e. very large at Site E and very small at Site K.

4. SIMULATION MODEL (A) Introduction

A computer model was constructed to simulate the development of populations of M. evansi on two of the stinging nettle patches sampled, Sites 2 and 3 (Appendix), using published information and laboratory and field data gathered during the present study.

The model aims to provide insight into possible causes of numerical change in the observed populations and thus to indicate whether or not some substantial relationships have been overlooked. Because stable age distributions occurred infrequently in the sampled populations, it was not possible to calculate the

potential increase rate in the field directly and then to analyse the data by Hughest (1963, 1972) time-specific life-table approach. However, since the data obtained was easily incorporated into a simple simulation model, a more rapid analysis was permitted than that by static life-table methods. - 162 -

There were two basic problems in the development of the model. Firstly, one of the most important aspects of biological simulation modelling is the definition of the problem to be investigated. Specific questions should be asked and data then collected to enable construction of the model that might best provide the answers. Since the idea of modelling was acquired relatively late, this was not the sequence of events realised in the present study and some necessary data were not collected. Nevertheless, intuition and the use of published information

On this and other aphid species partly circumvented the problem.

Secondly, after only two years' field work ecological research is often barely beyond preliminary stages whereby regular patterns and associations are only tentatively. revealed. This model of M. evansi populations therefore lacks the prolonged background studies which are inherent in mathematical models such as those of Macdonald, (1961), Morris (1963), Varley and

Gradwell (1968), and Dixon (1970b).

Accepting these limitations the model aims to answer two questions:-

(i)How complete is the understanding of the population dynamics of M. evansi on the sampled patches of

U. dioica?

(ii) What is the relative contribution of each of the major variables in the aphidls life-system to the regulation of its abundance?

In effect an attempt was mane to mimic the temporal pattern of aphid numbers recorded during samplino. Understanding of the population dynamics was measured by goodness of fit of the model output to the observed population trend. Errors -were introduced by the use of laboratory and published data and by - 163 -

certain approximations of environmental values e.g. mean daily

temperature. However, an explanation of population pattern e.g. causes of peak and crash, was sought rather than exact numerical fit.

A standard or control model which includes all the variables under consideration was built and run on the computer (Run 1,

Table 32). Figure 33 indicates all the factors incorporated into the model and the interactions between them. Table 33 identifies the origin of data used to quantify the variables

in the model. Field data used was that recorded at Sites 2 and 3 in 1972 and output from the standard run represents an attempt to mimic aphid populations at those sites. The major variables

(i.e. predation, insect parasitism and fungal infection, emigration of alates, effects of host plant quality) were then

each omitted in turn, in order to evaluate their possible

importance in the system (Runs 2 to 5, Table 32). A significant change in the population curve resulting from removal of a single factor was taken to indicate the importance of that factor in determining aphid abundance at the level at which it operated in 1972. Finally, all the major variables mentioned above were deleted from the model (Run 7) to obtain the potential pattern of population increase at recorded field temperatures. This set a theoretical ceiling to the numerical abundance of

the aphid and served as a yardstick against which the total impact of restraints to aphid increase could be judged. (B) Structure of the Model

The model considered the aphids living on 100 nettle stems as a population unit, with the Fortran symbol APTA representing the total number of aphids, which were assumed to be evenly distributed over the available stems. The population was - 164 -

Table 32 Computer Runs of the Simulation Model

Run No. Conditions 1 All restraints applied (Control) 2 No predation 3 No parasitism or fungus infection

4 No emigration of alatae

5 No effects of plant quality 6 No functional response of predators

7 No restraints applied

8 Run from 2nd week with all restraints

Table 33

Origins of Data used in the Model

Factor Source of Data

1 Production of alatae Field

2 Emigration of alatae Intuitive

3 Initial instar structure Field

4 Parasitism and,fungal Field infection

5 Species and Nos. of Field predators

6 Voracity of predators Laboratory and published data

7 Development rate of aphid Laboratory

8 Reproductive rate of aphid Laboratory

9 Mean daily temperature Field

10 Quality of host plant Field and intuitive Figure 33

Flow chart for simulation model of populations of M. evansi. - 166 - FIGURE .33.

NUMBER OF PREDATION PREDATORS

EMIGRATION BY ALATAE

APHIDPREDATOR PROPORTI ON RATIO BECOMING ALA1AE

APHID nas-rAR STRUCTURE MATURATIoNcf 1-10sT PLANT

INSECT PARASITISM FEEDING By APH IDS cr, NUMBER OF APTEROUS API-ez

FUNGAL FOOD INFECTION QUALITY

RATE OF J DEVELOPMENT MEAN TEMPERATURE A

V REPRODUCTIVE NYMPHS RATE OF API-I IDS BORN

N U M BEA AGE OF OF ADULTS ADULT APR IDS - 167 -

divided into five instars and the number of apterous aphids of any particular age in days was given by APTERAE (I), where I took any value between one and the maximum value of 50 days to which an individual of M. evansi was assumed to live (Section

II). The processes of birth, death and emigration which acted to increase or decrease aphid numbers are examined in detail below.

(a) Rate of Development

The model ran with real time units (days) and simulated the effects of temperature on development rate by assuming that each instar lasted for a fixed number of day-degrees aboveT.the development threshold (Hughes, 1962; Hughes and Gilbert, 1968); this threshold was calculated for M. evansi in Section II.

DAYDEG (I) was defined as the total number of day-degrees experienced by an aphid aged I days since birth. The mean. daily temperatures recorded in the field in 1972 were used in the model. When the aphids in a particular instar experienced the required number of day-degrees, DEGTHR (K) (i.e. when

DAYDEG (K) for that instar), they passed into the next instar, and so on until they became adult. (h) Birth

Each run started with the instar structure as recorded in the first field sample. Aphids in this sample were assumed to have the median age of their respective instar. The reproductive rate, governed by adult age and air temperature, was based on the findings of laboratory experiments at constant temperatures between 6 and 25°C (Section IT). PECMAX (I) was the maximum rate of larviposition at each day's mean temperature for a female aged I days. The actual birth rate was reduced below this level by the condition of the host plant and the - 168 -

density of aphids present. In all runs, except where plant quality factors were omitted, the maturation of the plant was

assumed to result in a gradual loss of food quality for the aphids (Figure 34) and hence a drop in fecundity according to

data obtained from fecundity experiments in the field (Table 27).

This was generalised data based on replicates of a number of nettle patches and may not have applied directly to Sites 2 and

3. An intuitive factor, the loss in food quality under the stress

of aphid attack was also included and this could have been important at Site 2 where the infestation was very large. Thus, food quality for aphid reproduction was assumed to decrease in

relation to the size of infestation experienced by the plants i.e. aphid-days calculated as the summed daily product of aphid numbers and time which then governed the reproductive rate of the

adult aphids on a particular day (Figure 35). The effect of

aphid density on reproduction was not thoroughly investigated

during this study, but it was noted that the mean numbers of offspring deposited by five individuals enclosed together in

clip-on cages did not differ significantly from the mean

numbers when aphids were caged singly 0.05)- Microlophium evansi does not form tight aggregates on host stems, but since

it is known that high densities reduce the reproductive rate of

several aphids e.g. Acyrthosioh2LEium (Murdie, 1965) and Drepanosiphumylatanoides (Dixon, 1966) , a mild density-dependent factor was incorporated into the model (Figure 36).

Polymorphism greatly complicates the modelling of aphid

populations (Crawley, 1973; Hurihes, 1973a). Production of alatae at various stages of population development is most

commonly thought to be related to aphid density and the condition Figure 34

Natural changes in food quality of nettle stems for reproduction of M. evansi.

Figure 35

Changes in food quality of nettle stems induced by aphid feeding.

- 170 - FIGURE .34. TY LI A U D Q FOO

X. A M OF ON TI R PROPO

20 40 GO Rio 100 120

TI ME (DAYS)

FIGURE .35. Y T ALI U Q D O FO AX. M OF 0.4 ON TI

POR 0.21 O PR

2 A. id' 3x105 4,4 - 171 -

of the host plant (Lees, 1966, 1967; Sutherland, 1969). Although

in most species the proportion of fourth instar nymphs bearing wing pads continues to increase even after the number of aphids

per plant decreases, for M. evansi it proved adequate to relate the appearance of alates to current density (Figures 37 and 38)

since in the field there was often a site-specific positive correlation between the size of the population per 100 stems

and the rate of alate production. It was assumed that all

alates emigrated from the plot, although this was not verified.

The few adult alatae of M. evansi taken in yellow water. traps indicated little immigration of alatae and such recruitment was not included in the model.

(d)Insect Parasitism and Fungal Infection

Mortality due to insect parasites and fungal pathogens probably depends on several variables in the field, such as

climatic conditions, aphid density and abundance of adult

parasites. Insufficient was known of the relationship between

these variables and the intensity of parasite and fungus attack

to be modelled in detail. The recorded levels of parasitism

and fungus infection at Sites 2 and 3 were included as rigid

data sets and were assumed to act equally on all age-classes of aphid present.

(e)Predation

The accurate simulation of rates of predation presented

another difficulty since it was numbers of predators rather

than their influence which was recorded in the field. For the

purposes of the model numbers of active predator stages each

week were multiplied by their voracity, as determined in the

laboratory (Section IV; Anderson, 1961; Blackman, 1965;

Hodek, 1973), to obtain a maximum daily aphid loss factor. Figure 36

Effect of density on rate of reproduction of M. evansi.

Figure 37

Production of alatae at Site 2 in relation to aphid density.

Figure 38

production of alatae at Site 3 in relation to aphid density. — 173 —

FIGURE .3G. N O I CT U D PRO RE OF E RAT

100 200 300 400 APHIDS/ STEM FIGURE .37. E 0. LAT A G N I OM BEC

so° 200 Zoo APHIDS/ STEM FIGURE .59. z _ 0

• J."- 7 1- 3- • --- tc) !.!n .60 40 5 GO A :4 / - 174 -

Only the most abundant predator groups, the Coccinellidae, the

Anthocoridae and the spiders were included and it was assumed that prey were removed from each age-class according to their proportions in the total population. This maximal kill was reduced according to the availability of aphid prey on the plants (Popov, 1960). The relationship between aphid numbers and the feeding efficiency of predators included in the model is shown in Figure 39. The inclusion of a functional response by predators to aphid density was probably realistic in that low densities of M. evansi on nettles may have caused a switch by general predators to other prey e.g. Psyllidae. The effect of removing the functional response from the model, i.e. maximum food intake by predators for the whole season, was investigated in Run 6 (Table 32).

(f) Parameter Updating

The foregoing processes were simulated once every day.

Before beginning the next day's calculations, the aphids and the parameters associated with them were "aged" or updated by one day. The vectors were looped through their subscripts from the maximum age to the minimum, advancing each by one day such that aphids of maximum age were culled in the process. Any outputs required at the end of each day were printed at this stage e.g. total number of aphids, instar structure, etc.

(a) Site 2

The control run, in which all the variables were included, gave a reasonable it of the population trend at Site 2

(Figure 40b). Except for the first two weeks the model agreed closely with the observed population up to the end of May, when the model curve flattened sharply to reach a peak of 10,000 Figure 39

Functional responses of predators included in the model.

(i) Feeding efficiency of predators in relation to availability of aphids per predator.

(ii) Feeding efficiency of predators in relation to aphid density. PROPORTION OF MAXIMUM FOOD INTAKE ACHIEVED 0 FIGURE .39. it APHIDS AVAILABLEJPREDATOR - 176 APHIDS / STEM 10

20 25 - 177 -

aphids/100 stems. This was considerably less than the real

peak of 35,000 aphids, although the peaks of both the model

and field population coincided in mid-June. The fall-off in

numbers was similar in model and field population until the end of July, when the model curve again flattened and ended

with a slightly higher population than in the field.

Thus there are discrepancies between model output and the real situation at three stages:-

(i) In the first few weeks when the simulated population increased too quickly compared to the real population,

which barely increased between the first two sampling dates.

This could have been due partly to an incorrect assumption of the age-structure of the adults in the initial sample or to an over-estimate of developmental rate at the relatively low

temperatures during April. Alternatively, the model may have under-estimated the impact of natural enemies early in the

season. A model run begun at week 2 (with the instar structure

as recorded in the field at that time) gave a curve which closely fitted the real trend until early May, when it increased less

rapidly than in the field and resulted in a peak population

slightly less than that of the control run (Figure 40c).

(ii) In late May, when the real population continued to rise steeply in comparison to the model. The most likely explanation seems to be premature simulation of loss of plant quality, which when excluded (Figure 40g), mimicked the observed population very closely until just before

the peak in aphid numbers. A more remote possibility is that

the predation rate applied in the model was too severe at high

prey numbers, although predation up to the time of peak aphid numbers appeared relatively unimportant in the simulated Figure 40

Output of simulation model for Site 2 - 1972. b.

ACTUAL DEVELOPMENT OF POPULATION f MODEL - RUN 1 AT Stral.

C. 4

Cf) G. 1.11 Win. 0 0 -r-t

L€1 0 MODEL FROM SECOND WEEK- RUN 0 NO PREDATION - RUN 2

a 4-1

NO INSEcT PARASITISM OR FUNGAL NO EMIGRATION OF ALATAE RuN 4 INFECTION -RUN .21 9- 4

3 NO RESTRAINTS TO INCREASE -RUN 7

NO FUNCTIONAL RESPONSE

NO EFFF..CTS HOST PLANT cUALITY PREDAToRS- RUN 6 - RUN S - 180 -

population. However, field experiments indicated that predation may be important at certain times e.g. in maintaining populations at low densities in July and August (see Table 30).

(iii) The last two weeks of the simulation (late

July/early August) when the observed population crashed to little more than one aphid/stem. The deterioration in plant quality may have been simulated inaccurately late in the season or data on predation and parasitism in small populations may have been unreliable.

Nevertheless, the relative contributions of the various factors used in the model seem to be clear. Predation had least overall impact (Figure 40d) and its effect was to accelerate decline of aphid numbers following the June peak; unless realistic functional responses by predators were included the aphid population crashed to extinction within three days

(Figure 40h). The effects of parasitism and fungus attack, mainly the former, were detectable from early in the season. parasitism, which appeared to be the only major restraint to increase until the latter half of May, was more significant in determining aphid abundance than predation (Figure 40e).

Production and emigration of alatae had greater effects than natural enemies, which was most evident from June onwards

(Figure 40f). Without emigration, numbers remained at a high

level until the beginning of July. In the field this might

have led to a compensatory increase in predator and parasite

action. However it is clear from the sii ulation. that removal of a large proportion of the reproductive stock via emigration of alatae had more impact during June and July than hymenopterous

parasites which at no time were re.:;pen,Jble for more than 10--,; mortality of the total aphid population. - 181 -

The simulated population escapes control almost completely if aphid reproductive rate remains maximal for the prevailing temperatures throughout the season (Figure 40g). Thus, without the drastic drop in reproductive rate caused by deterioration in food quality of the host plant, the impact of natural enemies might have been insufficient to cause a decline in aphid abundance. The total effects of major "mortality factors" was guaged from the computer run with all restraints except temperature removed (Figure 40h). In these circumstances, the initial population in April would have reached an average of 1000 aphids/stem by the end of May, instead of the observed 180/stem.

Thus, potential for increase of the aphids was considerably reduced by the combined effects of natural enemies, emigration

of alatae and changes in quality of the plant.

(b) Site 3

A similar pattern to Site 2 resulted from simulation of the Site 3 population. The control model closely resembled the observed population, but with the peak occurring two weeks

earlier. The model over-estimated aphid numbers due partly to its failure to mimic the drop in numbers in the field between.:.

sampling dates 1 and 2 (Figure 41b). As with the Site 2 model, the simulated curve gave a closer fit when started at week 2

(Figure 41c) showing that the discrepancy between model and real

populations was largely a legacy of poor simulation in the first

week. The shape of the population curve was, however, well

simulated. The decrease in numbers followed a realistic pattern

until the end of July, when the simulated curve flattened out

instead of following a steep decline as in the field. In-

accuracies in data on changes of host plant quality for aphid - 182 -

reproduction may account for this.

The outcome of removing individual restraints was usually

similar to that discussed for the Site 2 model. A noteworthy

difference from Site 2 was the greater importance of parasite and fungus attack relative to emigration of alatae in affecting

aphid numbers (Figure 41e,f). Parasitism and emigration of

alatae appeared to be more important than at Site 2; this would

be expected on the basis that percentage parasitism and percentage

of alatae in the sampled populations were similar at the two

sites, whereas aphid density was much lower at Site 3. Predation

seemed unimportant, its effect being to cause a slightly more rapid decline in aphid numbers than would otherwise have occurred

(Figure 41d).

Reduction in reproductive rate of M. evansi due to the

effects of food quality changes in nettle stems was indicated

as a vital limiting factor to population growth (Figure 41g).

When the model ran with the production of offspring unchecked

by loss of food quality, "mortality" due to natural enemies and alate emigration had little effect on the exponential rise in

the simulated population. This supports the integrated control concept that small amounts of host resistance could sometimes

tip the balance towards success in pest control by natural

enemies (van Emden and Wearing, 1965)- This is also evident

from the International Biological Programme field project on

Myzus persicao infesting potatoes, where poor host status of

potato was largely responsible for reducins population increase

in the field (van Emden and Way, 1972; van Emden, 1973).

M. persicae and Brevicoryne brass:icae on Brussels sprouts

(van Emden and Bashford, 1971) and Rhopalosiphum maidi on

barley (El-Ibrashy et al., 1972) were found. like evansi on Figure 41

Output of simulation model for Site 3 - 1972. - 0. 4 FIGURE .41.

4-,

AC.:TUAL DEVELOPMENT OF POPULATION MODEL - RUN 1 AT SIT. .3.

MODEL FROM SECot,10 WEEK- RUM S NO PREDATION - RUN 2

NO INSECT PARASITISM OR FUNGAL. I NO EMIGRATION OF ALATAE- RUN 4 INI-ECTION-RUN3- 9•

NO RESTRAINTS TO INCREASE - RUN

NO FUNCTIONAL RESPONSE r.-W PREDATORS - RUN G NO EFFECTS OF 140ST PLANT QUALITY -RUN S - 185 -

stinging nettle in the present study, to reproduce maximally on young plants and to become less fecund as the plants aged. Thus, sensitive responses of many aphids to changes in host plant quality seem to be important in determining their abundance in the field and the model would benefit from the incorporation of more information on changes in the suitability of nettles for the development and reproduction of M. evansi.

An intriguing feature of the simulations for Sites 2 and 3 in 1972 is that they produced a similar numerical as well as a similar temporal pattern. Thus, although the latter agreed with the real situation, the variation in observed numbers, five-fold at the peaks between the two sites, was not mimicked by the models. The similarity in peak numbers of aphids in the simulations (control runs) result from underestimation of abundance at Site 2 and overestimation at Site 3. It was stated earlier that the underestimation at Site 2 may have been due to premature simulation of changes in plant quality, whereas the overestimation of abundance in the Site 3 simulation may have arisen by assuming that the quality of the host plants and therefore the reproductive rate of the aphids was the same at the two nettle patches, at least until different rates of deterioration in plant quality due to aphid attack began to take effect. Results from experiments in 1974 indicated that mean relative growth rate of M. evansi was twice as fast on

Site 2 as on Site 3 (Table 28a). Most adults produced on Site 3 would therefore have been smaller than those on Site 2 and correspondingly less fecund (Murdie, 1965). The model was thus modified by assuming that aphids at Site 3 deposited only half as many nymphs per day as those at Site 2, although this had not been checked in the field. The outcome (Figure 42) was to - 186 -

produce a peak population about one-third of that in the control run and about one-half of that observed in the field. By assuming the reproductive rate to be 255 less than on Site 2, very close agreement between observed and simulated peak populations was obtained (Figure 42). This suggests that differences in the size of populations on the two nettle patches may be explained by mean relative growth rate and reproductive rate of the aphids being lower at Site 3, the heavily-shaded patch.

5. DISCUSSION

(A) The Simulation Model

Successful simulation of the population trends at sampled

Sites 2 and 3 suggests that reasonable understanding of the population dynamics of M. evansi in these limited situations

has been achieved. In structure the model most closely resembles the simulations of Hughes and Gilbert (1968) and

Crawley (1973), although it was designed to answer different questions. The advantage of such models lies in the ease with

which they can be improved as more field data become available.

Developing this simple model has clearly exposed the need for further research, particularly on the aphid--plant relationship

(Gilbert and Gutierrez, 1973) and on the effects of aphid density on reproduction and survival.

The conclusions to be drawn from the model are that the

major factors affecting the birth- and death-rates of the

populations varied in their intensity through the season and

that without restraints to the aphid Ys capacity for increase,

particularly loss of food quality and emigration of alatae,

the numbers of natural enemies recorded in 1972 were not

sufficient to explain the downward trend in abundance durind Figure 42

Output of simulation model for Site 3 after modifications to rate of reproduction of the aphids. - 188 - F 1 GURSI 42.

/"------,,, \ 4- /

( --, \ . 7- \ \\ .. \ , \ \ .. \ \)c . •

3 MS E ST 100 DS / PH I

m A i

+ G n LO

ACTUAL DEVELOPMENT OF POPULATION AT SITE-3.

MODEL FOR SITE 3. (RUN I)

MODEL WITH APHID RATE OF REPRODUCTION 50% LESS THAN ON SITE.2. MODEL Wal4 APHID RATE OF REPRODUCTION 2.S"/fl LESS THAN ON SITE.2.

APRIL I MAY I JUNE I TULY - 189 -

July. These findings are summarised in Table 34 where the relative effects of major factors influencing the aphid populations are classified as small, medium or large. The effects are mostly cumulative, so that small predator impact at the beginning of the season largely explains its continued small impact during June. Similarly, the removal of alate emigration from the model had little effect before the end of May, since by then it was still not an important factor in the field population. By the end of July, however, the continuous effect of simulating complete retention of adults on the nettle patch produced a population which, unlike the real situation, declined very little from its maximum level (Figures 40, 41,f).

During the first half of the season the only slight restraint to aphid increase came from hymenopterous parasites, which seemed capable of inflicting 6-8% mortality irrespective of whether host density on nettles was one/stem or 100/stem. The rapid crash during July to very low numbers appeared to be largely caused by deterioration in food quality for aphid reproduction and to a lesser extent by the emigration of alate adults. Output from the model thus supports the suggestion that careful selection of crops at least partially resistant to pest aphids, i.e. reducing their rate of population increase, may be of great benefit in schemes of integrated control, which often'involve use of indigenous or introduced natural enemies.

Minimising quality of food supply for pests with notorious tendencies for explosive increase in abundance may aid the early establishment of favourable natural enemy/pest ratios on agricultural crops (Tamaki and Weeks, 1968b). - 190 -

Table 34

Relative Impact of various restraints to the increase of M. evansi populations

Restraint Relative Impact

1. Predation Small

2. Parasitism Medium

3. Fungus infection Very small

4. Emigration of alatae Medium

5. Changes in food quality Large - 191 -

(B) Differences in Sizes of Aphid Populations Between Sites

Despite the evidence that initial infestations in spring were similar (Tables 14 and 21), large differences developed later in the sizes of aphid populations on the three sites which were sampled and also on the scored sites. There was, however, a striking pattern to the differences on the sampled sites, namely that in both years the semi-shaded nettle patch,

Site 2, had the highest peak infestation and the exposed patch,

Site 1, the least (the same trend appeared in the third year - see Table 31). One or several factors could have accounted for the differences, including:-

(a)Temperature

(b)Rainfall

(c)Shade

(d)Natural enemies

(e)Emigration of alatae

(f)Effects of aphid density on reproduction

(g)Nutritional quality of the host plants

Although at the heavily shaded patch (Site 3) maximum temperature on warm days was reached later in the day and temperature fell more slowly in the evening, mean daily temperatures at plant height were very similar on all the sites. The aphids on the exposed patch were less protected from rainfall but they would have been less subject to the effect of large drops of rain than aphids under tree shade.

The effect of rain was probably insignificant for the following reasons:-

(i) Aphids dislodged from nettles in a dense stand were unlikely to experience difficulty in relocating suitable host plants. - 192 -

(ii) Aphids were commonest on the most unshaded sub-plots at Site 2.

(iii) No significant changes in aphid numbers during the rise and early peak in abundance occurred following heavy rain.

Therefore temperature and rainfall effects were seemingly unimportant. The small population at Site 1 suggested that complete absence of shade may be detrimental to M. evansi, but data obtained from the scored sites indicated that aphid abundance was not correlated with the degree of cover their host plants received from adjacent vegetation.

Influence of natural enemies, emigration of alatae and density of aphids are unlikely causes of differences in infestation between sites, since differences appeared early in the season when the impact of these factors was still small.

This assumes that numbers of unidentified ground predators did not differ on the three sites. The relative insignificance of

natural enemies and alate emigration in determining abundance during April and May was also indicated by the simulation model discussed earlier. Aphid densities were mostly too low to

induce a reduction in reproductive rate, except at Site 2 where

aphids were anyway most abundant.

It is notable that the differences in aphid abundance corresponded with differences in nettle growth in the three

patches, as measured by dry weights and surface areas. This was not simply because smaller plants were capable of supporting fewer aphids; overcrowded conditions did not exist on most stems at Sites 1 and 3 in either year and the five-fold difference in peak infestations in 1972 between Sites 2 and .3 corresponded to less than a two-fold diffe,- ence in size of stems. Similarly in 1973 there were about i we times as manv - 193 -

aphids and only twice the mean size of nettle stem at Site 2 as at Site 1. Differences in total nitrogen per stem followed the same pattern as differences in stem size for 1973, since concentration of nitrogen per unit weight of stem was very similar on all sites. Thus, with most stem nitrogen per aphid occurring at the least infested site, it is unlikely that shortage of nitrogen was a limiting factor to population increase. However, amounts of total nitrogen were not necessarily an indication of the relative availability of this element at each site and certainly not a measure of the quality of nitrogen, e.g. composition of the phloem sap and the concentration of individual amino acids and carbohydrates, which is the important factor in terms of the nutritional value of the plant (van Emden, 1973). Experiments on the mean relative growth rate of M. evansi nymphs indicated that Site 2 was more suitable for aphid growth than Sites 1 and 3 and that Site 3 was more suitable than Site 1. Similar effects may have occurred in relation to aphid fecundity, suggesting that the quality of the nettle stems differed between the sampled sites and this may explain the observed differences in the size of M. evansi populations. Although not apparently important in themselves, the size and visually-assessed vigour of nettle stems might serve as indicators of food quality for aphids, since at Site 2 and the scored sites A and B, for example, stems grew prolifically and supported an abundance of aphids, whereas at Site 1 and the scored sites I and J, stems were stunted and supported much smaller numbers of aphids.

Differences in the amount of flowering or in the proportion of male and female stems between sites were not investigated, and these factors may also have affected food quality of host stems and thus the rate of increase of aphid populations. - 194 -

As previously stated, nettle patches appear to develop

aphid infestations of a characteristic magnitude of abundance,

this seemingly being correlated with surface area and dry

weight of stems and the mean relative growth rate of the aphids.

Thus, large stems on some patches seem capable of supporting

500 or more aphids, while smaller stems on other patches

appear to suppOrt about 50 aphids or less at the peak of infestation. The classification of an infestation as large

or small may therefore be regarded as site-specific; Tsmalll

infestations on taller growing patches may represent greater

numbers of aphids/stem than Ilarget infestations on less tall

patches. Using this site-specific classification of

infestations there appeared to be an alternation between

larger and smaller populations in the two years i.e. sites

with heavy infestations in 1972, with few exceptions, developed

much smaller infestations in 1973; while lightly-infested sites

were generally more infested in the second year. This is

illustrated in Figure 43. Development of aphid populations

at certain sites during May and June 1974 further suggested

that biennial alternation between relatively large and small

populations is a common pattern on many nettle patches.

A pattern of both increase and decrease at different sites

between the two years of this study suggests the operation of

a local factor or factors rather than of a r eional influence

like climate. Abundance of natural enemies, infection hy

fungi and rates of parasitism were similar in both years for

each of the three sampled sites, as also were initial numbers

of M. evansi on the stems in April. Tt is possible that, as

with variation between sit_s, variation at the same site Figure 43

Size of peak populations/100 stems of M. evansi in successive years on various nettle patches.

>—__ indicates change in peak populations from 1972 to 1973.

Figures in brackets are numbers of aphids/100 stems on June 17th

1974, which may have been prior to the time of peak population.

Classification of populations as large or small is site-specific

e.g. 4,500 and 35,000 are large populations for Sites 1 and 2

respectively; 200 and 13,500 are small populations for Sites I

and B respectively. - 196 - FIGURE 43.

41900

38650

35458

-(ts15o)

(14804 HtGI-IpEA Pop. 1,5500

92oo-

7914

(Sioo)

LOW PEAK Pop.

__.4436 4382

(210:4 2000 1448 f1350 850

350 200 100 (so) 1 SITE...J. 51Te..2. 5ITG.3. A 8 I K 1-1 SCORED SITES - 197 -

between years was due to the effect of changes in nutritional quality of nettles, which either fluctuated in a natural biennial pattern, perhaps due to a preponderance of vegetative growth and seed production in alternate years which could have altered concentration and/or composition of the phloem sap, or was influenced by degree of aphid attack in the previous year, which might have reduced accumulation of food reserves in the rhizome system. This is analagous to aphid-induced plant effects which appear to regulate numbers of the sycamore aphid, Drepanosiphum platanoides (Dixon, 1970b). A large population of this species in spring affects the nitrogen metabolism of the leaves and usually results in a relatively small population peak the following autumn. This in turn reduces the number of overwintering eggs laid on the tree and thus contributes to a relative scarcity of aphids after egg hatch in spring.

Dixon (1971) also reported that while aphid infestation on lime trees does not reduce leaf area or shoot growth, it seriously inhibits root growth. In addition, the leaves contain less chlorophyll and senesce earlier than the leaves of non-infested trees. If trees are uninfested the following year, they compensate by producing leaves with more chlorophyll and therefore capable of fixing more energy, but nevertheless, these trees may well be less nutritious for aphids in the year after heavy aphid attack. Both natural and artificial removal of the vegetative parts of plants can have subsequent effects on asexual and sexual reproduction by plants, e.g. work on

Rumex crispus by Cavers (1971), and it could similarly affect the nutritional quality of the plants for later populations of insects. If damage to Urtica dioica by the feeding of large numbers of M. evansi results in a less nutritionally - 198 -

suitable plant the following year, it would account for relatively large and relatively small infestations in alternate years.

Notable exceptions to the trend occurred at scored sites

E and J. The nettle stems at site E, which were consistently

colonised by large numbers of M. evansi (1972-1974) were the

tallest at Silwood Park and samples taken in 1973 had a greater

dry weight than samples from Site 2. This patch formed a

narrow strip alongside the open fence of a dog kennels and received much fertiliser in the form of urea and dung leachings

which might have overwhelmed the natural or aphid-induced cycle

in nutritional quality which it is suggested would otherwise

occur. The persistently small population at site J is less

easily explained and there were patches elsewhere which remained

either uninfested or poorly infested during both years. One of

these could not even be artificially colonised with M. evansi

on cut stems brought from other sites, despite several attempts,

suggesting that certain clones of nettle may be relatively

resistant to attack. If this proved correct, these nettle

patches would be worth studying with the object of determining

the causes of 'resistance,.

(D) The Monophagous feeding habit of Microlophium evansi

Nettles are actively growing from late March to mid-June

in most years and from then until the slight flush of growth

at the base of the old stems begins in September, M. evansi

must survive while its host plant, whether U. diolca or

U. urens, appears to be unsuitable for rapid reproduction and

yet when natural enemies, particularly Anthocoridae, are still

abundant. Many host alternating aphids are able to exploit a

more-or-less continuons supply of nutritionally favourable - 199 -

foliage which is either growing or senescing and they have the

added advantage of escaping the threat of over-predation and

parasitism by moving to other plants and multiplying rapidly before they are relocated by natural enemies. Such aphids are

normally very sensitive to stimuli influencing alate production e.g. a drop in food quality, this factor often conditioning the effects of aggregation. Several monophagous aphids

seemingly have different mechanisms for survival when the host plant is nutritionally unsuitable. Emigration of winged forms

would usually result in the colonisation of other plants that

are in an equally unsuitable physiological state, though natural enemies might be scarcer.

Some monophagous species continue to reproduce at a low rate e.g. Eucallipterus tiliae or enter reproductive diapause

during the summer e.g. Drepanosiphum platanoides. Escape from natural enemies during this vulnerable period is often achieved

through a spaced-out gregariousness amongst individuals which enables the rapid transmission of warning messages through the

colony. Strong avoidance reactions have also been developed,

such as kicking and leaping, which would not usually be possible in dense aggregates. Other monophagous aphids respond to the

onset of unfavourable host plant conditions by production of

sexuales and early oviposition e.g. Dysaphis devecta on apple,

Brachycaudus rociadae on larkspur and Masonophis maxima on

thimbleberry (Frazer and Forbes, 1968). Several Periphyllus spp.

on maple respond by aestivating as first instar sexuparae, known as dimorphs (Essig and Abernathy,1952).

Microlophiuni evansi appears to possess a combination of

the survival mechanisms characteristic of monophagous and polyphagous aphid species. It continues to reproduce at a low - 200 -

level on nettles during mid-sLwmer and minimises loss from many natural enemies through its agility in kicking and in falling from the plant. However, because of its exploitation by such a wide range of predator groups, production of alatae during

May and June may be another escape mechanism which results in a spread of numbers over a very large area, thus increasing the chances that some, perhaps very small populations will survive until the autumn regrowth of the plants. Widespread occurrence of dense stands of U. dioica certainly provides an opportunity for successful co19nisation of very many hosts and thereby could insure against the risk of local extinction while food supply is inadequate.

Whatever the significance of its possible adaptations to monophagy, M. evansi persistently survives the har~h environmental conditions to which populations are frequently exposed during mid-summer and winter. Its ability to develop and reproduce at low temperatures results in the early appearance of large populations, which often reach exceptionally high peaks for a weed-infesting aphid. This is probably due mostly to the ag~regation of its host plant in dense stand~, which permits rapid spread to fresh food supplies by non-migratory alates and apterous crawlers. Aphis urticata similarly colonises dense stands of nettles yet populations of this species rarely appear to reach the same size as those of !VI. evansi. A.urticata may be even more sensitive to the condition of the host plant than M. evansi and it appears to colonise only the growing tips.of stems. Alate production has not been investigated in this species and if this is relatively uncommon, spread of the population within a patch of nettles would probably occur much less quickly.

(E) Nettles as a reservoir of Natural Enemi~s Among the many different natural enemies which prey upon - 201 -

M. evansi, the following mostly aphid-specific groups were identified:-

GROUP PRINCIPAL SPECIES

1. Coccinellidae Coccinella 7-punctata; Adalia 2-punctata

2. Syrphidae Platycheirus albimanus; Metasyrphus luniger

3. Anthocoridae Anthocoris nemorum; Anthocoris nemoralis

4. Miridae Heterotoma merioptera 5. Cecidomyiidae Aphidoletes sp. 6. Chrysopidae Chrysopa carnea

7. Braconidae Aphidius ervi; Ephedrus lacertosus

8. Entomophthorales Entomophthora aphidis; E. planchoniana

The most important groups numerically were the Anthocoridae, the Coccinellidae and the Braconidae, all of which commonly appear on crops during the summer. The total numbers of natural

enemies on individual nettle patches in each year corresponded with the size of the aphid infestations i.e. most on Site 2, least on Site 1. However, the ratio of total numbers of

predators to aphids was generally much less for Site 2 than for Sites 1 and 3 (Tables 19 and 25). Thus, it does not appear

that numbers of natural enemies on nettle patches were determined

only by the abundance of aphids. Some predators may feed

largely on various species of Psyllidae and furthermore, the occurrence of certain groups of predators was seemingly

influenced by the degree of shading from nearby trees, although

there was insufficient replication to analyse in detail the

influence of shade on natural enemies.. A total of only 4 adult • and 28 larval Coccinellidae were recorded at Site 3, the

heavily-shaded patch, during two years, sampling; it seems likely, therefore, that Coccinellidae are not attracted to - 202 -

nettle patches in deep shade. Cover from adjacent vegetation may also affect some species of predatory Heteroptera on nettles; Calocoris sexguttatus was found only in the heavily- shaded plot, and Anthocoris nemoralis was sampled much more frequently in the exposed plot than elsewhere. - 203 -

SECTION IV

LABORATORY STUDIES ON THE FOOD VALUE OF Microlophium evansi FOR THE COCCINELLID BEETLES Coccinella 7-punctata AND Adalia 2-punctata

1. INTRODUCTION

In assessing the contribution of a wild plant system as a possible source of natural enemies that can act against pest insects on crops, it is essential to establish its ecological relationship with predators and parasites in terms of the following:-

(i)The species of natural enemy that are attracted - see Section III.

(ii)The attractiveness of the wild plant to natural enemies relative to crop plants e.g. colour, texture, micro- climate. - see Section V.

(iii)The acceptability of the alternative host/prey supported relative to hosts/prey on crops - see Section V and this section.

(iv)The temporal and spatial pattern of host/prey abundance on the plant - see Section III.

(v)The food value of the host/prey for natural enemies - see this section.

(vi)The distribution and abundance of the wild plant - see Discussion (Section V).

All the above factors play a part in determining the role of non-crop habitats in pest control. The temporal pattern of prey abundance on wild plants is particularly important, since sources of food away from the crop for natural enemies should preferably be available when pest aphids are scarce or absent i.e. weeds should provide alternate rather than alternative prey/hosts. However, it cannot be assumed that an alternate supply of food outside the crop is beneficial until the - 204 -

suitability of the hosts or prey for rapid development and

reproduction of natural enemies has been carefully examined.

Mobile natural enemies are initially attracted to a

particular habitat (Way, 1966b) but then there may be a

considerable degree of selection of hosts/prey within their

habitat. Casual observation may reveal the range of host

plants visited and prey attacked, but experimental techniques

of the type employed by food ecologists are needed to determine

the food value of the hosts/prey for survival and reproduction..

The food ecology of aphidophagous insects has been an important

branch of biological control work for many years (see reviews

by B C Smith, 1966 and by Hodek, 1966, 1973). The purpose of

work in this field is primarily to identify the prey specificity

of potentially useful natural enemies and the quality of the

different components in their diet. Hodek (1966) points out

that apparent acceptability has often been mistaken for real

suitability of prey. Controlled experiments to distinguish

between complete and purely maintenance diets therefore help

to develop methods of increasing the effectiveness of predators

and parasites as mortality agents (B C Smith 1966).

The suitability of different aphids as food for several

groups of predators has been studied (e.g. Laska, 1959 for

Syrphidae; Anderson, 1962 for Anthocoridae; Blackman, 1967b for Coccinellidae; Mondal, 1972 for Chrysopidae). Hodek (1973) classifies unsuitable prey into those which are rejected,

those accepted but inadequate for normal growth and reproduction

and those which are toxic; all others are termed "essential foods". Examples of the different categories are as follows:-

) Rejection or Prey

Hyalopterus pruni is rejected by laJ:vao of Adalia decempunctata after they have pierced the body wall but - 205 -

subsequent encounters with this aphid involve rejection on contact (Dixon, 1958). Some predators are repelled by waxy surfaces on aphids such as Brevicoryne brassicae (George, 1957).

In some cases, predators reject prey feeding on certain host plants which they normally accept on other hosts (Hodek, 1973).

Thus, Rodolia cardinalis does not prey on its normal host,

Icerya purchasi, if the latter feeds on Spartium or Genista and this may be due to the intake by Icerya of the alkaloid isparteinf and the yellow pigment Tgenisteinl which are unpalatable for the coccinellid.

(ii) Acceptance of Inadequate Prey Hodek (1956) found a comparatively low intake of Aphis sambuci by Coccinalla 7-punctata, with larvae failing to complete development and adults dying prematurely. Larvae of

Adalia bipunctata were able to complete their development on

A. sambuci but at a slower rate than on A. fabae (Hodek, 1957).

Smith (1965) reared Hippodamia tredecimpunctata to the adult stage on A. fabae, but the beetles were smaller and their development time longer than when fed Acyrthosiphon pisum or

Rhopalosiphum maidis. El Hariri (1966) recorded that

A. 2-punctata laid twice as many eggs when fed on A. pisum as when fed on A. fabae. Remains of Panonychus ulmi were found in the gut of A. 2-punctata and of three Coccinella spp.

(Putman, 1964), although these coccinellid beetles could not develop on this prey (Robinson, 1951; Putman, 1957).

(iii) Toxic Effects of Prey

Acute toxic effects have been observed following the ingestion of certain aphids by predators (Blackman, 1965, 1967b).

Blackman found Megoura viciae lethal to all active stages of

A. 2-nunctata, although it was suitable for C. 7-punctata. - 206 -

Aphis nerii from Nerium oleander was poisonous to most

coccinellids, although Adonia variegata developed normally

on this prey (Iperti, 1966).

Even suitable aphids vary in their value to natural enemies

as a source of food; behavioural and nutritional factors

interact to affect the degree of exploitation of a particular

species. Thus several aphids which have proved excellent

sources of food for development and reproduction of natural

enemies in the .laboratory are relatively 'unavailable' in the

field because of their self-induced low density e.g. Myzus

persicae (Way and Cammell, 1971). Furthermore, mechanical

defences of aphids, such as the smearing of an attacker's

mouthparts with cornicle secretions, may be harmful to predators

(Edwards, 1966; Heathcote, 1969). The escape responses of

Microlophium evansi to the larvae of Coccinellidae involve kicking, secreting of wax and dropping from the plant. Some

aphids often turn downwards on a stem or petiole, facing the

predators as they crawl up the plant, and so are well placed

to react (Dixon, 1958). Colonies of B. brassicae and some

lachnids perform rhythmic movements when alarmed. This is

elaborated into a "song and dance routine" by the citrus aphid,

Toxoptera aurantii (Williams, 1922). The stridulating

mechanism on the tibia and abdomen is similar to that of

grasshoppers. Attendance by ants is another factor reducing

the accessibility of certain "essential aphids" to natural

enemies (Banks, 1962; Way, 1963; Duckett, 1974).

The provision of alternative or alternate Coed for the

survival and reproduction of natural enemies may be essential for the success of biological control projects (Doutt and

Nakata, 1965; Smith, 1966), but the above evidence indica1es - 207 -

that sufficient hosts/prey outside the crop are not necessarily guaranteed by an abundance of aphid populations on a wide range of wild plants. Thus, the decision to preserve or encourage non-crop habitats as a supplementary source of food should be influenced partly by the quality of prey those habitats support.

In this section, the food value of M. evansi for two important species of coccinellid, Adalia 2-punctata and Coccinella

7-punctata, is investigated. Comparisons are made with the food value of Acyrthosiphon pisum and Aphis fabae. Other factors affecting the role of stinging nettles within agro-ecosystems are considered later (See Section V).

2. MATERIALS AND METHODS

(A) Cultures of Coccinellidae Cultures of Adalia bipunctata and Coccinella septempunctata were started from adults collected in the field during April and May and maintained in a 16 hour photoperiod at a constant temperature of 20°C. All work on the former species was done in these conditions; studies on C. 7-punctata were performed at 22°C, due to an unavoidable change in conditions in the controlled environment room between experiments on the two species.

The rearing technique was as described by Blackman (1965,

1967a). Adults of both sexes were kept in perspex boxes measuring 7.5 cm x 13-75 cm x 5.5 cm high ventilated by two

111 diameter holes in the lid covered with bolting silk (Figure

44). A piece of corrugated cardboard on the floor and one side of the box provided an absorbent substrate and also a suitable surface for oviposition. Food was provided daily in the form of cuttings from aphid-infested plants (usually Acyrthosiphon pisum on broad beans), with the cut-ends inserted in a small - 208 -

perspex container holding damp cotton wool (Figure 44). Large

supplies of two-spot ladybird eggs were readily obtained by this method. Egg production by C. 7-punctata was.always less but oviposition was often stimulated by transferring adults to

another cage. Cannibalism of eggs is common in the latter

species, so newly-laid eggs were quickly removed. (B)Cultures of Aphids

Three species of aphid - Microlophium evansi Theob.,

Acyrthosiphon pisum (Harris) and Aphis fabae Scop., were

reared on potted plants in a 16 hour photoperiod at a constant

temperature of 20°C. The plants, U. diolca for M. evansi and Vicia faba var. Dwarf sutton for A. pisum and A. fabae were

maintained in 2v x 21 x 21 wooden-framed cages covered in fine-mesh nylon gauze. Plants were watered and replaced weekly.

(C)Experimental Methods

For Adalia 2-punctata the following factors were measured

as indicators of the "food value" of the three aphid diets:-

(a)Rate of development of larvae

(b)Weight of food eaten by larvae

(c)Mortality during development

(d)Weight of pupae and adults

(e)Fecundity and longevity

(f)Fertility and size of eggs

For C. 7-punctata only factors a, c and d were recorded, on diets of M. evansi and A. fabae.

(a) Rate of Development

The apparatus used in these experiments was the same as

that described for the aphid studies in Section. Il (see Figure

2). Batches of newly--hatched coccineliid larvae (at least 10

in each experiment) were placed in in rearing Cells . Figure 44

The cage for maintaining cultures of adult Coccinellidae and for the experiments on fecundity and longevity. - 210 - FIGURE 44.

PE RS P EX BOX N arr.-a

COTTON WOOL

PERSPEX CONTAtNER. CORRUGATED CARDBOARD - 211 -

Each batch was given one of the aphid species and was inspected

at least daily to record times of moulting and to provide fresh aphids. Pre-pupal and pupal periods were recorded and the

increase in weight during each larval instar -of A. 2-punctata

was also determined.

(b) Weight of Food Eaten

The live weight of aphids taken daily by developing

coccinellid larvae was estimated by Blackman's technique,

slightly modified. Known fresh weights of appropriately-sized

nymphal aphids were provided every 24 hours (ensuring an excess

each time) to newly-hatched larvae in 2" x 1" glass tubes. A

thin film of "Fluon" was painted around the rim of each tube

and blotting paper fixed to the bottom by means of double-Sided

sellotape. This provided a rough surface for the insects to

crawl on and it also absorbed faeces and honeydew. The aphids

remaining at the end of each 24 hour period were weighed to

determine the weight ingested.

Another set of tubes was used each time to keep similar-

sized aphids in the absence of predators. From the weight loss of these unpredated aphids in the control tubes, a correction

factor was determined as follows:-

Correction Factor =

1 - Weight of control aphids after 24 hr. starvation Weight of control aphids before 24 hr. period 1/2

The apparent weight of aphids ingested was multiplied by the

correction factor appropriate to the species of aphid used.

This enabled a more accurate wet weight consumption to be

determined, although it involves two arbitrary assumptions

viz. that the predators feed uniformly throughout: the 24 hours

and that the loss in weight of starved, living aphids is the

same as aphid remnants. The latter would not: be important for - 212 - experiments with older coccinellid larvae which commonly eat whole aphids.

(c)Mortality Larval mortalities were recorded according to instar. It was difficult to obtain reliable figures, due to considerable differences between the mortalities of larvae from different egg batches. However, the mortality data still provide an indication of suitability of different aphids as food.

(d)Weight of Pupae and Adults

Pupal weights, recorded within 12 hours of the end of the pre-pupal stage, and weights of newly-emerged adults were measured on a torsion balance (Torbal Model EA-1) accurate to 0.001 gm.

(e)Fecundity and Longevity

The same cages were used as for adult cultures. Adults were weighed and sexed on emergence and placed in the perspex boxes, usually two pairs to each box. They were provided with excess food on fresh plant cuttings. Eggs were collected and counted daily, observations continuing until the death of all individuals.

(f)Fertility and Size of Eggs

Random samples of A. 2-punctata eggs (at least 100 eggs) were kept in moist conditions and the percentage hatch recorded.

Eggs were taken during the early, middle and late stages of the ovipositional period. The length and breadth of equivalent samples of about 20 eggs were measured under a low-power microscope with the aid of a micrometer eye-piece. - 213 -

3. RESULTS

(A) Adalia bipunctata

(a) Rate of Development at 20°C

The duration of the larval instars and the pre-pupal and

pupal stages on the three aphid diets is shown in Table 35.

Developmental and pupal periods were almost identical when

larvae fed on M. evansi and A. pisum. Both species proved

better for the larvae than A. fabae, on which their developmental

period increased by an average of 21%. All the instars were

relatively longer when fed A. fabae. Pre-pupal and pupal stages

were of similar length irrespective of diet.

(b)Weight of Food Eaten by Larvae

Table 36 compares the weights of food eaten when larvae

were offered the three species of aphids. The rate of food

uptake, like the rate of development, was slower on A. fabae.

The relative unsuitability of A. fabae as food for A. 2-

punctata larvae is therefore associated with its reduced rate

of intake. However, the total fresh weight of food ingested

per larva during development was similar for all the aphids

tested (Table 36) and except for the first two instars fed on

A. fabae, each instar consumed a relatively constant proportion

of the total aphids eaten irrespective of diet. Between 70 and

75% of the food eaten during development of larvae was taken

by the fourth instar.

(c)Mortality during Development

Mortalities in each instar are given in Table 37. Most

deaths occurred in the first instar, decreasina as the larvae

aged. Very few died in the final instar. It is notable that

mortality when larvae fed on A. uisum or A. fabae was more

than twice that when larvae led on M. evansi, on which aphid only 85 of the larvae failed to reach the pre-puoa1 stage.

- 214 -

Table 35 Mean Development Period of Larvae of A. 2-punctata fed on Various Aphids

MEAN DEVELOPMENT TIME (DAYS) AT 20°C Larval Diet INSTAR Total Pre- II IV ±S.E. Pupa Pupa

M. evansi 2.1 1.8 2.4 3.8 10.1+0.14 1.2 5.7 A. pisum 2.1 1.8 2.4 3.9 10.2±0.11 1.5 5.5 A. fabae 2.7 2.5 2.6 4.5 12.3+0.21 1.3 5.5

Table 36 Mean Fresh weight of Food eaten by larvae of A. 2-punctata

Instar Larval Diet I II III IV Total±S.E.

M. evansi Mean weight of 1.96 4.36 11.36 42.20 food taken(mg). 59.87+1.26 5 total intake 3.2 7.3 19.0 70.5

A. pisum Mean weight of 1.75 4.63 12.25 46.00 food taken(mg). 64.63+1.93 /0 total intake 2.7 7.2 18.9 71.2

A. fabae Mean weight of 0.48 2.07 11.12 42.86 food taken(mg). 56.53 1.33 5 total intake 0.8 3.7 19.7 75.8 - 215 -

(d) Weight of Pupae and Adults

Mean weights of newly-formed pupae and emergent adults,

together with the range of weights recorded, are given in Table

38. Larvae of A. bipunctata not only grew relatively slowly

on a diet of A. fabae but also developed to smaller pupae and

adults. Pupae and adults from a larval diet of M. evansi were

slightly, but not significantly (P=-->.- 0.05) heavier than those

from an A. pisum diet (Table 38).

Increases in weight during each instar are shown in Table

39. Larvae were similar in weight at the beginning of the

fourth instar, irrespective of diet. Differences in pupal

weight were therefore a consequence of differ enees in food

eaten by the final instar.

(e) Fecundity and Longevity

Most eggs were laid by individuals fed as adults on

./1: pisun,but numbers did not significantly differ (P 0.05)

from those laid by individuals fed on M. evansi. Egg production

was significantly reduced when adults were fed A. fabae; the

total number of eggs per individual on a diet of A. fabae was

less than half that of individuals fed M. evansi or A. pisum.

Blackman (1965) and El Hariri (1966) showed that the food given

to the larvae had no detectable influence on fecundity of the

adult, but in order to avoid any possible effect of larval

diet, all adults used in these experiments were reared as

larvae on a diet of A. pisnm. Thus, all individuals were of

similar weight when newly emerged.

Figure 45 shows the mean number of eggs laid at five day

intervals by female A. bipunctata civen various aphids as food.

Data on fecundity and longevity is shown in Table 40. Longevity

varied little in all the experiments so A. bipunctata led on

- 216 -

Table 37 Mortality of Larvae of A. 2-punctata

Larval Diet 5o Dying in each Instar Total I II III IV

M. evansi 5 2 1 - 8 A. pisum 9 7 3 1 20 A. fabae 12 8 4 2 26

Table 38 Effect of Larval Diet on Weights of Pupae and Adults of A. 2-punctata

Larval Diet Mean Weight of Mean Weight of 1-Day old Pupa (mg) Adult at emergence (mg)

M. evansi 16.1±0.61 (S.E.) 12.2+0.59 A. pisum 14.3+0.80 11.5±0.74 A. fabae 9.4±0.35 8.1+0.45 - 217 -

A. fabae were less fecund solely because they laid fewer eggs per day.

(f) Fertility and Size of Eggs

The fertility and sizes of eggs were determined (Table 40).

Over 90% of eggs from females fed A. pisum or M. evansi were fertile, but only 76% were fertile when A. fabae was fed and the eggs were smaller.

(B) Coccinella septempunctata

(a)Rate of Development at 22°C

Data for development times are given in Table 41. The duration of individual instars was strikingly similar when larvae were fed either A. fabae or M. evansi. Pre-pupal and pupal periods were also similar. The lengths of the three immature stages, i.e. larva, pre-pupa and pupa, in the life- cycle of A. 2-punctata and C. 7-punctata are compared in Table

42, which shows that the length of these stages in both species when fed on the same species of aphid was similar. Blackman

(1967b) found that the development of C. 7-punctata was generally longer than that of A. 2-punctata at 20°C, when given the same species of aphid as :food, but in the present study, the higher temperatures for the experiments with the former species appeared to eliminate this difference.

(b)Mortality during Development

There was very little mortality during the development of larvae fed on A. fabae or M. evansi (Table 43); no deaths occurred after the second instar when larvae were fed on the latter aphid.

(c)Weight of Pupae and Adults

Weights of both pupae and newly-emerced adults reared as larvae on either or the two aphid diets were not sionificantiv - 218 -

Table 39 Weight increase during development of Larvae of Adalia 2-punctata

INSTAR Larval Mean wet weight Diet I II III IV at emergence (mg) iw inc iw inc iw inc iw inc

M. evansi 0.1 0.3 0.4 0.6 1.0 3.0 4.0 8.2 12.2 A. pisum 0.1 0.3 0.4 0.8 1.2 2.8 4.0 7.5 11.5 A. fabae 0.1 0.2 0.3 1.2 1.5 2.2 3.7 4.4 8.1

iw = initial weight inc = effective increase in weight Figure 45

Fecundity of Adalia bipunctata fed as adults on various aphids. - 220 - 40- FIGURE 4S. A. ptsum DIET

30 -

20-

10- ) S

VAL n un TER 40 IN M. evansi DIET Y DA - (5 E AL M 20- FE ER

GS P to - G F E O

MBER 40 NU A. fobqe DIET N

MEA :20

10 20 70 3o 0 100 DAYS AFTER EMERGEMCE - 221 -

Table 40 Reproduction of A. 2-punctata fed as adults on various Aphid diets

ADULT DIET M. evansi A. pisum A. fabae

Mean Longevity of females (days) 78.0 82.0 75.0 Mean total Eggs Laid 831 947 385 Mean Eggs per day 10.6 11.5 5.1 Maximum Eggs per day 33.5 31.0 9.0 Mean Length of Eggs (mm) 1.26 1.24 0.93 Mean Breadth of Eggs (at widest 0.56 0.57 0.50 point) % Fertility of Eggs 95.8 91.7 75.7 - 222 -

different (P 0.05) (Table 44). Thus, as with the development and mortality data, the results indicate that A. fabae and M. evansi are equal in food value to the larvae of C. 7-punctata.

One-day old pupae were on average 4.8 mg (13.5%) heavier than one-day old adults (Table 44).

4. DISCUSSION

The results show that M. evansi is a suitable food for the development of larvae and the production of fertile eggs of A. bipunctata and C. septempunctata, on the basis that

A. pisum is entirely suitable (Blackman, 1967b). The effect of M. evansi on the fecundity of C. 7-punctata was not tested quantitatively, but the author and Michelakis (1973) found that abundant eggs were laid when cultures of this coccinellid beetle were fed solely on M. evansi.

Diets of M. evansi and A. fabae were of equal value for the development of C. 7-punctata larvae, M. evansi and A. pisum were suitable as food to both larvae and adults of A. 2- punctata, resulting in quicker development, heavier pupae and adults, and a higher fecundity than when A. fabae was given as food. The findings for development of larvae agree with those of Blackman (1967b) and El Hariri (1966). As several workers have pointed out (Smith, 1965; Blackman, 1967b; Mondal, 1972), the results of laboratory experiments, where predators are confined in relatively small areas with their prey, must be interpreted with caution. The predators have little difficulty in locating their prey, which are unable to elicit many of the escape responses observed in nature. Developmental periods and total egg production are no doubt an expression of complete satiation of appetite, which is often impossible in the field.

Nevertheless, these comparative studies in a constant, confined - 223 -

Table 41 Mean Development Period of Larvae of C. 7-punctata fed on M. evansi or A. fabae

Mean Development Time (Days) at 22°C Larval Diet Instar Total Pre- Pupa I II III IV ±S pupa

M. evansi 3.2 1.7 2.5 4.3 11.7±0.14 1.2 5.6 A. fabae 3.2 1.7 2.6 4.6 12.1+0.14 1.3 5.3

Table 42 Mean Duration of Various Life-Cycle Stages for A. 2-punctata and C. 7-punctata

Mean Development Time Species ! Larval Diet (Days) (°C) Larva Pre-Pupa Pupa Temp

A. 2-punctata M. evansi 10.1 1.2 5.7 20 A. fabae 12.3 1.3 5.5

C. 7-punctata M. evansi 11.7 1.2 5.6 22 A. fabae 12.1 1.3 5.3

- 224 -

Table 43 Mortality of Larvae of C. 7-punctata

% dying in each instar Total Larval Diet I II III IV 5

M. evansi 5 2 7 A. fabae 5 2 1 1 9

Table 44 Effect of Larval diet on Weights of Pupae and Adults of C. 7-punctata

Mean Weight of Mean Weight of Larval Diet 1-day old pupa (mg) Adult at emergence (mg) S.E. S.E.

M. evansi 40.9±1.6 36.5±1.4 A. fabae 39.8±1.5 34.61.5 - 225 -

environment are aimed primarily at detecting differences in the nutritional suitability of aphids and indeed, the use of small cages may be essential in order to minimise the influence of behavioural factors and thus to determine strictly nutritive value. Thus, Smith (1965) eliminated non-nutritional factors such as prey defence reactions completely by using dry, powdered aphids. Smith found that A. pisum and Rhopalosiphum maidis were both superior to A. fabae for growing several species of coccinellid larvae, including A. bipunctata.

Studies by Blackman (1967b) and Michelakis (1973) illustrated the importance of eliminating or at least reducing the effects of non-nutritional factors when assessing the food value of aphids for coccinellids. Michelakis found the life- span of C. 7-punctata larvae fed on Myzus persicae at 20°C to be three days longer than that found by Blackman. This was largely attributed to the larger feeding area provided in

Michelakis' work (63 square cm base against 3.9 square cm base), so that more time and energy were spent searching for and capturing prey. The development period of C. 7-punctata fed on M. persicae, an_aphid considered high in food value, in the larger arena was in fact almost identical to that when fed on

Brevicoryne brassica, an aphid considered low in food value, in the restricted arena used by Blackman, i.e. approximately 16 days. The environmental area may be of much greater importance to an aphid like Lipersicae, a non-acyiregating species avoiding heavy predation through its low population density, than to an aphid like B. brassicae, an aggregating aphid (Way, 1963) forming dense colonies on suitable host leaves, Thus the findings of Michelakis and Blackman suggest that the development period of C. 7-punctata when fed. B. bras5;icae might be s:imilar irrespective of the size of experimental arena, whereas when - 226 -

fed M. persicae, larvae may develop much quicker when confined with their prey in a small container compared to a large one.

Therefore, in a comparison of M. persicae and B. brassicae as a larval diet for coccinellids, the nutritional superiority of the former might only be revealed when dispersal of the prey is largely prevented, as it was in Blackmants experiments.

Similarly in the present study, it was shown that M. evansi was a more suitable aphid than A. fabae for development and reproduction of A. 2-punctata in the confines of a small rearing cell. However, as stated previously, under natural conditions there exists a balance between the nutritional value of a prey and its ease of capture in sufficient numbers and this may often result in aphids like A. fabae and B. brassicae being as high in food value as A. pisum, M. persicae or M. evansi in the field.

A wide variety of predators feed on M. evansi on stinging nettles (see Section III). Experiments described in this section indicate that this aphid may be a valuable source of food in terms of predators! rate of development and fecundity, but the question remains as to how much benefit is derived from large colonies of M. evansi by predators in the field? Small predators such as early instars of Coccinellidae and Heteroptera may experience great difficulty in capturing this aphid (Dixon,

1958). It is an agile, long-legged species capable of kicking away attackers, smothering their mouthports in wax, or dropping from the plant as a last resort. Kaddou (1960) found that aphids with long appendages were more difficult for larvae of

Hippodamia quinquesignata to capture than the more compact and tenacious aphids with short appendages. Most adult predators and older larvae seem better equipped to overcome prey escape - 227 -

activities, even though the larger the attacker, the stronger the defence reactions may be elicited.

Camouflage is seemingly also part of M. evansi's defensive barrier. It is green in colour on most of the aerial parts of its host, U. dioica, but often develops a yellowy tinge against the background of senescing leaves. The distinctive dark green line mid-dorsally on adult apterae is thought to be "disruptive colouration" changing the apparent outline of the insect

(Heathcote, 1969).

Thus, although M. evansi on nettles may be a potentially useful food source to many natural enemies, behavioural and morphological characteristics appear to restrict its availability and therefore its value as an alternate or alternative prey close to crops. Field studies recorded in Section III indicated that Anthocoridae fed on M. evansi and bred on nettles for most of the growing season, whereas Coccinellidae occurred mainly as adults and there appeared to be little oviposition by this group of predators on nettles. The efficiency of the aphid's defence mechanisms against young larvae of Coccinellidae may partly explain these observations; perhaps a madibulate predator may be more readily repelled or dislodged than one with piercing stylets. Coccinellid larvae are generally

"clumsier" than most larvae of Heteroptera and so may be more affected by cornicle secretions. In contrast, early inst.ars of Anthocoris nemorum were often observed in the field with large individuals of M. evansi impaled helplessly on their stylets.

The habitat can also affect the degree of exploitation of any prey. Adult Coccinellidae are fregneptly attracted to nettle patches during spring and early summ r. The young stems - 228 -

in April are at a height preferred by adult C. 7-punctata, and the more mature stems, usually 1 metre or more in height, provide vegetation at a suitable level for both C. 7-punctata and A. 2-punctata. During the present studies, however, adult

Coccinellidae appeared to visit nettle patches mostly in search of food rather than egg-laying sites. The stinging hairs of U. dioica may present mechanical difficulties to females attempting to deposit eggs. This could also account for the scarcity of syrphid eggs observed during field sampling. Thus aphid prey which prove valuable as food in the laboratory may not be exploited by immature stages of certain predators in the field due to the avoidance of particular host plants by egg- laying females.

It can be concluded that M. evansi is apparently a suitable food for both A. bipunctata and C. septempunctata and probably for other natural enemies commonly observed on stinging nettles, but that this suitability may be tempered by the agility of the aphid when disturbed and its colour variation according to host plant condition. Predators searching for food or oviposition sites must overcome the defence reactions of M. evansi as well as any physical or chemical obstructions of the plant. These factors, together with the temporal pattern of abundance of the aphid, seemingly set a limit to the range of natural enemies which feed on M. evansi. - 229 -

SECTION V

STUDIES ON POPULATIONS OF COCCINELLIDAE ON STINGING NETTLES

1. INTRODUCTION

Few insects associated with arable ecosystems can depend exclusively on crop plants, unless these are perennials or are grown continuously. Wild areas usually provide essential shelter and food during fallow periods, hibernation sites for most species during the winter and a supplementary diet during the growing season for a wide range of phytophagous, carnivorous and parasitic insects. Thus, many weeds serve as hosts to both pests and natural enemies, and the question facing the applied entomologist is whether, on balance, a particular species of wild plant favours natural enemies of potential importance in pest control more than the pests themselves. In addition to its role as a habitat for insects, non-crop land may have other diverse influences, both biological and physical in nature, and these are now briefly reviewed as a background to assessment of the possible economic importance of stinging nettles which •is undertaken in the General Discussion,

(A) Influence of Non-crop Land on Pests and their. Natural Enemies

(a) Reservoir of Pests

Dependence of certain pests on hosts outside the crop has been fully reviewed (Jepson and Southwood, 1958; van Emden,

1965a,b; Way and Ranks, 1968; Pollard, 1971). Many economically important aphids display a heteroecious life-cycle in temperate regions which involves alternation between a woody winter host and one or several herbacious summer hosts which often include weeds (e.g. Iluonymus europaeus - winter host !nd ChenoLodium album (weed) or field beans or sugar beet (crops) - summer hosts for AT) his fabae). This is a soasonal movement between hosts, - 230 -

coinciding with the active growth and senescence of winter and summer food plants, but dispersal to alternative hosts also occurs within a summer season. In summer there is often a general increase in infested area with time, caused partly by lack of suitable food, partly by changes from localised crops to widespread weeds and partly by increased numbers dispersing over the terrain. For example, Shepherd's purse, black night- shade, charlock and fat hen are common weed hosts of Myzus persicae; Young and Garrison (1949) found Aphis gossypi Glov. on 21 species of plants, most of which were common hedgerow weeds. Spread of many aphids to a variety of wild plants is probably a vital factor in their survival, particularly during the familiar trough in numbers during late July and August in temperate regions.

Cereal pests, including Oscinella frit (L) (Jepson and Southwood, 1958), Hylemyia coarctata (Fall.) (Gough, 1946),

Cephus cinctus Nort. (Post, 1946) and several species of aphid (Gair, 1953; R Dransfield,pers. comm.) often colonise wild grasses which can be important alternate or alternative hosts. Several fruit pests also feed on wild plants; Blair and Groves

(1952) reported that fruit tree red spider mite, Metatetranychus ulmi, feeds on more than 80 species of plant, including 12 hedgerow trees and 11 orchard weeds.

Apart from providing food, weeds serve as sites for shelter and overwintering. Flea beetles, Phyllotreta spp. and the carrot fly, Psila rosae (F.), overwinter and shelter on hedgerow plants and chemical control by insecticides applied to field edges in early spring has been recommended (Wright and Ashby, 1945; Jones and Jones, 1964).

The economic importance of wild hosts is enhanced by their function as reservoirs of virus and bacterial diseases. These - 231 -

are usually transmitted to crops by insects and can depress yield much more than direct feeding by the insects concerned.

Thus M. persicae, a relatively scarce aphid, is the vector of over 100 viruses in about 30 plant families, and these include many important crop diseases such as tobacco rosette and potato leaf-roll (Smith, 1968). Weeds are often symptomless carriers of disease, so that their capacity as reservoirs is disguised. Thus Poos (1955) found that five species of symptomless grasses growing near maize showing bacterial wilt were infected with the disease. The vector appeared to be the flea beetle Chaetocnema pulicaria Melsh.

(b) Reservoir of Natural Enemies.

Movement of phytophagous insects between habitats to utilise different food supplies during the growing season is often mimicked by their predators and parasites. Some natural enemies hibernate in very similar sites to the pests upon which they feed. Wild plants thus harbour beneficial as well as harmful species of insect, both of which begin to disperse to other sites in spring. Hedgerows and perennial plants are particularly attractive to predators at this time, due to the large invertebrate fauna occurring there which exploit the early flush of leaves. An early generation of many species of

Coccinellidae, Anthocoridae and Neuroptera may develop on prey or hosts on hedgerow shrubs and wild plant's beneath them, while later generations appear on arable crops (Pollard,

1971). Coccinellidae of the overwintering generation occur on nettles in spring, breeding there before migra ting to nearby crops such as field beans (Banks, 1955). Bombosch (1966a) found that many natural enemies changed habitat through the growing season from the edges of woodland to arable field and - 232 -

roadside ditches. Bonnemaison (1964) recorded the dispersal of Coccinella septempunctata from winter retreats to wild habitats between mid-January and early April in the Paris region; temperature appeared to be the most important factor determining the peak of this dispersal. Skuhravy and Novak (1966) noticed in Czechoslovakia that migration of coccinellids to sugar beet fields after emergence from hibernation was rarely direct, but was via wild plants. Iperti (1965) and

Hodek. (1973) suggested that predators are forced to leave

"typical" spring sites due to food shortage and many arrive on young crops to utilise the abundance of prey developing there. Lusis (1961) followed the gradual change of habitat in the coccinellid beetle Adalia 2-punctata. This species first appeared on Padus and fruit trees in parks and orchards, then on ornamental shrubs and trees (e.g. Rosa and Ulmus) and subsequently on other trees such as Tilia and Salix. After the disappearance of aphids from trees, A. bipunctata moved to weeds and cultivated crops. Yakhontov (1966) found a similar change of habitat in Synharmonia conglobata and Coccinella undecempunctata. Before mid-July they fed on Hyalo terus arundinis and Myzus persicae in orchards, but left when the aphids departed to secondary hosts. The predators then appeared on various herbaceous weeds, cotton and'alfalfa fields or on other aphid-infested trees such as Salix spp. and poplars.

During summer Blepharidopterus angulatus (Fall.) invaded unsprayed orchards at East Mailing, infested with red spider mite, from neighbouring alder windbreaks where they were feeding on Psyllidae. Proximity of alder trees greatly influenced abundance of this predator in an orchard (N G Solomon, pers. comm.). - 233 -

Changes in habitat result not only from the search for adequate numbers of prey. Mobile natural enemies visit

"atypical" hosts to supplement their diets with food of plant origin or to deposit their eggs. - Flowering plants are important to adult parasites and syrphids (van Emden, 1965a; Pollard, 1971).

Remaudiere and Leclant (1971) observed the attraction of

Syrphidae to orchards containing an abundance of pollen-bearing plants. Impact of natural enemies on M. persicae was much greater in badly-attended or abandoned peach orchards, over-run with wild plants, than in those well maintained and subject to periodic cultural practices. van Emden (1965b) found that cabbage aphids on a brussels sprouts crop were most heavily attacked by syrphid larvae near flowers at the edge of the field. This was attributed to increased oviposition adjacent to feeding sites of the adult syrphids. Pollard (1971), on the other hand, investigated two areas with a contrast in abundance of hedges and concluded that syrphids significantly reduced numbers of Brevicoryne brassicae only in the mainly arable area, where there were less hedgerows and wild plants.

However, flight conditions close to the brussels sprouts were not the same in the two areas and there was a greater aerial abundance of syrphids in the more diverse habitat. Dixon (1959) attributed the peak of syrphid eggs on broom during the flowering period. of the shrub but before the peak in aphid numbers to attractiveness of the flowers. Leius (1960) showed that umbelliferous flowers were frequently visited by adult parasitic Hymenoptera and suggested that knowled-je of adult feeding habits was a pre-requisite of successful introductions for biological control purposes.

Honeydew produced by Homoptera is another important food for some natural enemies, which may have to vfsit non-crop - 234 -

plants in order to obtain such food. Honeydew forms most of the diet of the predatory ant Formica ruf a (MUller, 1956;

Zoebelein, 1956), which has been used in the biological control of European forest pests (E Elton, 1958).

The value of wild habitats to pest control is partly determined by their degree of attractiveness to pests and natural enemies and how this changes through the season. Weed species may constitute a source of pests at the beginning of the growing season, but later on they may be beneficial in diverting pests away from the crop. Similarly natural enemies could find adequate numbers of prey on wild plants at a time when their impact is most required on nearby crops. Wild habitats are thus needed which not only act as sources of natural enemies but also promote on the crop either early arrival of natural enemies or a paucity of pests during its vulnerable stages, or preferably both.

There are several reports of weeds acting as diversionary hosts to pests. Stride (1970) suggested that plants such as Cissus adenocaulis grown in hedgerows could be used to protect cotton against large immigrations of Lygus vosseleri. Dysdercus spp., another pest of cotton, occur sparingly on the scarce alternate wild hosts in South Africa, where the insect is a pest, but in Swaziland there may be large populations on the abundant wild hosts from which damaging numbers migrate to cotton. However, these wild hosts can elsewhere divert

Dysdercus immigrants from cotton provided they become attractive at the right time. In some years, populations of D. superstitiosus in Zambia are kept away from cotton crops by several species of

Hibiscus (Pearson, 1958). The interplanting of cotton with maize or alfalfa may also divert damaging populations of - 235 -

Heliothis and Lygus from the main crop (Pearson, 1958 ;Stern,

1969). Maize planted at the 'wrong' time can, however, be a

source of damaging infestations on cotton.

The period of attractiveness of wild habitats partly

influences the abundance of natural enemies on crops and their

effects on crop pests. It was stated earlier that sites are

needed for overwintering, and for post-emergence and post-

harvest feeding by natural enemies in agricultural areas, but

where these sites continue to provide sufficient food and shelter during the summer, they are merely competitors with

cultivated areas for local populations of predators and parasites.

Then natural enemies are diverted to or retained on weeds,

establishment of adequate natural enemy/pest ratios, vital to

significant impact on the pest, may be prevented. Thus

Bombosch (1963, 1966b) found many natural enemies on six

common weeds in Germany, but many of these continued to support

abundant aphids at times when pest infestations were developing

on crops. Banks (1955) considered Coccinellidae to be

ineffective predators of Aphis fabae because of their continued

presence on stinging nettles when A. fabae arrived on the beans,

while Galecka (1966) observed a considerable time-lag between

the build-up of aphids on potatoes and the arrival of predators from adjacent woodland. This frequent asynchrony between

arrival of pests and their natural mortality acy7mt§ on annual field crops could sometimes be attributed, therefore, to

availability of food in wild habitats delaying their departure

to crops.

(B) Other Influences of Non-Crop Land on Agriculture

Apart from the role of non-crop plants in the biology of

insects, consideration should be given to their physical, - 236 -

aesthetic and economic contributions in farming areas. Physical effects of shelter, extending over much shorter distances than

the biological ones cited above, have been examined by Lewis

(1965, 1966, 1969, 1970) and by Marshall (1967, 1974). Small weakly-flying insects are deposited in the lee of hedges and

stronger fliers also tend to congregate in this area of calmer

air. This may augment the chances of a pest arriving on a

crop or concentrating on part of it but presumably it can

afford the same chances to the predators and parasites (Pollard,

1968). Shelter also affects the physiological condition of the

crop and van Emden (1965b) found that this reduced the

reproductive capacity of Brevicoryne brassicae and could nullify

the effects of greater initial deposition of the aphid in sheltered areas.

Despite many investigations into the effects of shelter on crops, the manner in which the modified micro-environment

influences growth is still incompletely understood (Marshall,

1967). Marshall (1974) found that swede turnips and sugar beet

grown in sheltered conditions had higher dry matter production

and leaf area than plants in exposed conditions, although no

significant difference in dry matter was recorded at the end of

the crop growing seasons. The mid-season difference in the growth

of both crops appeared to be caused by a difference in plant

water stress. Aphids appear to show varying responses to

changes in water stress; reduced watering of plants can increase

or decrease numbers of aphids, or have no effect (McMurtry, 1962;

Wearing, 1967; Wearing and van Emden, 1967).

In many parts of Britain areas of wasteland and hedgerows

are frequently considered as incompatible with modern farm

technology. In 1962 an estimated 448,000 acres of hedgerow . existed in Great Britain (Moore et al, 1967), of which about - 237 -

7,000 acres or 14,500 miles was being lost annually (Shrubb,

1970). Agriculturalists argue that hedgerows depress growth of adjoining crops and divert labour and machinery from more important tasks in order to maintain them. They can. also be a hindrance to the efficient use of farm machinery, particularly the combine harvester. In field margins and along roadside ditches, tall growing "tweeds" are also being eliminated by cutting or herbicide application (Southwood, 1972). Conversely, uncultivated land outside the crop is defended by conservationists as an integral part of the farming landscape, providing essential habitats for wildlife including game birds and some of our most colourful butterflies (Pollard, 1971; Southwood,

1972; Davis, 1973a). The farm management experiment at Silsoe

(Barber, 1970) demonstrated the possibility of compromise; the internal hedges of a farm could be drastically manipulated without significant effects on wild fauna, if certain uncultivated areas were maintained.

Table 45 summarises the role of uncultivated land in agroecosystems and illustrates the need for extensive studies on individual components of such land. Even when its contribution has been assessed in a variety of situations, the decision to eliminate or preserve a wild plant species rests finally with individual farmers. Campaigns aimed at altering the farm community's attitude towards "weeds" in harmless situations may prove futile in the face of prejudice and tradition. Farm economics may also dictate whether some areas are left to provide a balanced countryside. Unfortunately it is virtually impossible, on present knowledge, to define whether a particular situation is beneficial. Even in cases where elimination of a wild plant is considered desirable, it is not always practically possible. Table 45

Summary of the Role of Uncultivated Land in Agroecosystems

BIOLOGICAL EFFECTS PHYSICAL EFFECTS

Positive Negative Positive Negative

Source of natural enemies Source of pests and diseases Windbreaks Altering micro-climate early in season early in season at edge of crop

Overwintering and Overwintering and sheltering Deposition of Deposition of pests on sheltering sites for sites for pests natural enemies on crops increased natural enemies crops increased

Supplementary food for Supplementary food for pests Hindering efficient use adult predators and of farm machinery parasites e.g. nectar, pollen

Alternative supply of Alternative hosts for pests Economic problems of prey when pest is scarce when crop is unsuitable management e.g. cutting, herbicide treatment Diversionary hosts for Diversionary hosts for pests during growing natural enemies season

Habitats for wildlife Aesthetic Value - 239 -

Attempts to eradicate spindle, the winter host of A. fabae, for example, would undoubtedly result in time-consuming failure due to the difficulty of locating all the spindle trees in an area and suckering from roots left behind in the hedgerows (Jones and Jones, 1964).

Although a few biological and integrated control schemes in Britain are likely to prove economic in protected cultivation,

(Hussey and Bravenboer, 1971; Wyatt, 1974), there has been little success with the organised use of predators and parasites in field conditions. Vagaries of climate affecting survival of predators and their searching and breeding efficiency result in the relative insignificance of natural mortality agents as a sole control measure against most of our native field pests. Cultural practices during the growing of annual crops also fail to provide the stability vital for an equilibrium to establish between prey and natural enemy populations. Nevertheless, predation, parasitism and fungal infection in field situations partly influence both short- and long-term abundance of pests and thus at least lessen the economic burden for growers of repetitive chemical insecticide application. Our aim should be to further the impact of indigenous natural enemies whenever possible and in this respect appropriate manipulation of non-crop reservoirs could sometimes be vitally important.

There is some evidence of indigenous populations of

Coccinellidae being manipulated to help in biological control.

Destruction of ground cover by cutting grass in orchards in south-eastern Kazakhstan and by cultivating the ground under walnut trees in California successfully drove beetles on to - 240 -

orchard trees (Savoiskaya, 1966; Siuss and Hagen, 1966).

Strip-harvesting improved the impact of coccinellids on

alfalfa aphids (Schlinger and Dietrick, 1960) while defoliants

concentrated Myzus persicae on the remaining foliage of peach

•trees, making them easier prey for predators migrating into

the orchards in autumn (Tamaki and Weeks, 1968a). Although

there have been no field attempts in Britain to enhance the

impact of coccinellids there are several reports in the

literature of this predator group constituting a seemingly

important part of the natural enemy complex associated with

pest aphids (e.g. Dunn, 1949, 1960; van Emden, 1961; Smith

and Hagen, 1966; van Emden et al., 1969; Dean, 1971, 1974;

Milne, 1971; Brown, 1972). This suggests that they might be

used to influence levels of pest damage, provided a sufficient

number of uncultivated areas are maintained as sources of

food in spring and then manipulated in order to force the

predators on to appropriate crops at the correct time. This

was the concept examined in the present study.

(D) Aims of Work described in this Section

Work described earlier in the thesis has indicated that

a reservoir of natural enemies exists during spring and early

summer on stinging nettles, where their aphid prey, Microlophium

evansi, forms a valuable source of food for rapid development

and reproduction. To build on this information, there is a

crucial need to determine at what time and to what extent

natural enemies migrate from nettles to aphid-infested crop

plants.

Experiments described in this section aimed firstly to

follow the movement of a representative predator group, the

Coccinellidae, between a patch of nettles and neighbouring

bean plots, in relation to the development: of aphid infestations - 241 -

on both the weed and the crop. An attempt was made to discover

whether or not stinging nettles serve as a diversionary

habitat for both the feeding and oviposition of Coccinellidae,

by offering a choice of nettles and field beans to small

populations in cages. Furthermore, the effect on predators

of cutting nettles, a common farm practice disrupting the

M. evansi/U. dioica system, was investigated. It was hoped

that this might suggest a means of manipulating nettles as a

contribution to the biological control of pest aphids. On the

basis of these studies the overall value of nettles as a

non-crop habitat in agricultural areas is assessed.

2. MATERIALS AND METHODS

(A) Experiments using Radiotracers on the Habitat Preferences of Coccinellidae

(a) Laboratory Tests

In order to study the movements of a common aphid predator

group, the Coccinellidae, between nettles and field beans, a

rapid and efficient marking technique was required. It was

considered that the use of a radioactive isotope might prove •

satisfactory, if this could be applied to a nettle patch and

subsequently taken up by the aphids and ladybirds feeding at

the patch, and therefore preliminary investigations into the

feasibility of radio-labelling nettles were made in the

laboratory. Experiments were done with sulphur-35 labelled

sodium sulphate solution and phosphorus-32 labelled sodium

phosphate solution according to their availability. 5-35

and P-32 are weak beta emitters, commonly used for ecological

studies because of their suitable half-lives (87 days for

S-35, 14.2 days for P-32), effective incorpoation and

•retention in insects, low toxicity, ease of detection and

handling and low cost (Jenkins, 1983). A simpl,? study - 242 -

made of the uptake of radioactive solutions into nettle stems and subsequently into Microlo hium evansi and its coccinellid predators.

( 1) Uptake of Radioactivity by U. dioica and M. evansi

Two cut stems of U. dioica, taken from the field, were each placed in a solution of S-35 labelled sodium sulphate which had an activity of 6/uCi (micro-Curies). About 20 individuals of M. evansi were placed in clip-on cages on the upper leaves of each stem. The cages were lined with filter paper sprayed with acidified bromo-cresol to catch and identify spots of honeydew produced by the aphids. After 48 hours, leaf-discs of the plant, removed with a cork borer, ten aphids and the honeydew-soaked filter paper were placed in turn under a Geiger counter (Mullard MX168 Detector and Panax 102ST Scaler) and activity was recorded. Each sample was counted three times for a period of one minute and the background count subtracted from the mean value. Leaf discs and aphids were then ground up separately in distilled water, diluted to 10 ml and an 0.5 ml aliquot mixed with 5 ml of scintillant (NE240). Activity of these solutions was then measured in a scintillation counter

(Panax P7103 Scaler and P7201 Timer). The filter paper was put in a small beaker containing about 10 ml of distilled water.

After mixing well and allowing to stand for 30 minutes, the solution was made up to 20 ml and an 0.5 ml aliquot counted in the scintillation counter. From counts of a standard solution _12 . (1 Ci = 2.2 x 1U disintegrations per minute), the efficiency of the scintillation counter and consequently of the Geiger counter was estimated.

The experiment was repeated using 5 uCi/stem and making counts one week and two weeks after treatnent. - 243 -

(ii) Uptake of Radioactivity by Potted Plants

Uptake of radioactive solution applied to the soil around nettle stems was studied to determine whether this might prove satisfactory as a labelling technique in the field. Three nettle plants in small plastic pots were given 2, 4 and ayuCi of S-35 labelled sulphate solution respectively and watered with 25 ml of distilled water every 3 days. Activity in upper and lower leaf discs was measured by the Geiger counter 3, 7,

10 and 21 days after treatment. A sample of 10 aphids caged • on an upper leaf during the experiment-was measured for activity after 7 and 21 days.

(iii) Biological Half-life of P-32 in Adults of Coccinella 7-punctata

One of the important factors determining the efficiency of a radio-isotope as a label in ecological studies is its rate of uptake and its biological half-life or decay in insect tissue. Laboratory experiments were done, therefore, which aimed to measure rate of uptake of P-32 by adult C. 7-punctata which fed on radioactive aphids and the rate at which activity was lost when this species switched to a diet of non-radioactive aphids.

P-32 labelled phosphate gave much higher counts in foliage than did S-35 sulphate when administered to cut stems of

U. dioica and Vicia faba. Radioactive phosphate was therefore used in estimating uptake and decay of radioactivity in

C. septempuncta preying on labelled aphids. AC y r thos iphen pisum was the aphid used in this experiment, as it was the only species available in sufficient numbers. Six cut stems of V. faha, infested with populations of A. DisUM, were each labelled with 5 Ci of P-32 phosphate solution. 48 hours later samples of 10 to 15 aphids were taken, counted by the Geiger - 244 -

counter and offered to four adults of C. 7-punctata caged individually in 3" x 1" glass tubes. Every 24 hours fresh aphids of known activity were given, after the ladybirds and aphid remains had been counted by the Geiger counter.

After one day's feeding on labelled aphids, two of the coccinellids were transferred to a diet of unlabelled A. pisum in order to measure the loss of activity with time. The experiment continued for one week, by which time the radioactivity detected in ladybirds transferred to unlabelled diet was at or near the background level.

(b) Field Tests

P-32 phosphate was applied to nettles in the field in an attempt to label predators which fed on M. evansi on the labelled nettles. A suitable natural stand of nettles did not exist at Silwood Park, so a plot was artificially established in the middle of a ploughed field and infested with M. evansi in late April with infested stems from a 15°C constant environment room. Small plots of beans were sown around the nettle patch, up to a distance of 30 metres, on April 20th.

These were artificially infested with A. fabae, where necessary, in the third week of May. The experimental design is shown in

Figure 46. It was hoped that the beans would trap coccinellids leaving the nettles.

For safety, - a maximum dosage of 1 mCi of P-32 phosphate was applied to the nettle patch of about 600 stems. The phosphate was applied in dilute solution from a watering can, care being taken not to contaminate clothing or skin.

Applications were made from May 2nd at 14 day intervals (i.e. alter the lapse of one half-life). Samples of aphids and nettle leaves were taken throughout May and their activity recorded by the Geiger counter. Adult Coccinellidae were - 245 -

captured at approximately 3 day intervals from June 7th onwards on the nettles and the bean plots and their level of radioactivity also measured with the Geiger counter. The

Coccinellids were lightly anaesthetised with CO2 from a

"Corkmaster" containing a Sparklets syphon bulb in order to facilitate rapid counting. An individual-specific cellulose paint-marking system was adopted to supplement the radiotracer technique (Banks, 1955). Paint marks were made in various positions on the elytra using a small pin. After detection of radio-label and paint-marking, individual adult coccinellids were released at the place of capture. This was done only on a weekly basis after the end of June.

The location of marked individuals when captured was recorded so that movement might be determined when recaptured.

The last application of P-32 phosphate was made on June 27th and capture and marking by paint of the coccinellids ended on

August 31st. (B) Experiments using Laboratory and Field Cages on the Habitat Preferences of Coccinellidae

Adult coccinellidae are mobile insects capable of dispersing over distances of several miles during their search for food etc. Cage experiments were designed therefore to enable the relative attractiveness of stinging nettles and field beans, infested and uninfected with aphid prey, to adults of A. 2-punctata and C. 7-aInctata to be studied while preventing long-distance flight of the species concerned. Records Vicl,Te made of searching, feeding and oviposition by the coccinellids when offered a choice of the two species of plants in controlled. laboratory and field experiments.

(a) Experiments in Constant Environment Room at 15°C.

(1) In Petri--Dishes Figure 46

The radio-labelling and paint-marking of adult Coccinellidae design of field experiment. - 247 -

FIGURE 46.

0 PLOUGHED FIELD APPROX.4 ACRES IN AREA WILD GRASS i

0 I *.

' ar ri 7: E> 1 n NETTLE 11 i .. 129 10 PATCH 9 St74.MET:1::F.1 I I L+ L.1 1 1 7 15 2A zi-ras5

0 18 FIELD ZEAL 0 9

0 0 0 0 0 0 0 0 0

IRE G - 248 -

Half of the base of a petri-dish, with the sides coated in "Fluon" was lined with bean leaves and the other half lined with nettle leaves. The leaves were held down, under surface uppermost, with double-sided sellotape to prevent the adult coccinellids from crawling under them. When aphid-infested leaves were used, 20 individuals of A. fabae or M. evansi were placed on the relevant host leaves and allowed to settle. The combinations are given in Table 46. Table 46

Combinations of Beans and Nettles for Experiments in Petri-Dishes

A. Nettles and Beans - both infested. B. Nettles (uninfested) and Beans (infested).

C. Nettles (infested) and Beans (uninfested).

D. Beans only (uninfested and infested).

Two female C. 7-punctata, starved previously for 24 hours, were introduced and allowed to settle for two minutes. At 3 minute intervals for 15 minutes, the position of each female and its state of activity - resting, searching or feeding - was recorded. There were at least two replications for each combination.

(ii) In Small Cages

Nylon mesh-covered cages measuring 2' x 2' x 2', containing two similar sized potted plants, one each of U. dioica and V. faba, were used. Combinations A, B and C as in Table 46 were used, the plants in A being approximately equally infested with 200 to 300 M. evansi and A. fabae or A. pisum.. 10 to 15 adults were kept in each cage. Each experiment ran for 5 days and records of the position of adult C. 7-punctata were made ten times daily between 9.30 and 18.30 hours.

Similar experiments were done with adults of A. 2-punctata using potted plants suspended by wire from the tops of the cages. - 249 -

This was done because this species is associated with relatively tall vegetation (Iperti, 1965; Blackman, 1967a).

As the A. 2-punctata were somewhat inactive only two observations were made each day for seven days. Combinations used are shown in Table 47.

Table 47

Combinations of Beans and Nettles for Experiments with - A. 2-punctata in Small Cages

A. Nettles and Beans - both infested.

B. Nettles (infested and uninfested) and Beans (infested and uninfested).

All the adult coccinellids in these laboratory tests came from cultures reared at 20°C in a 16 hour photoperiod. (b) Experiments in the Field

Field cages measuring 3' x 2,1/2' x 5' high and covered in fine nylon mesh were used to study resting, feeding and oviposition preferences of adult coccinellidae. The floor of each cage was covered with a layer of sand into which 12 large water-filled coffee jars with metal lids were embedded. A cut plant stem was inserted through a hole in each lid; six nettle and six field bean stems were therefore set up in each cage.

A small piece of cotton wool was placed around the base of each stem to prevent insects falling into the coffee jars.

Twelve to twenty adults of C. 7-punctata, approximately equal numbers of each sex, were placed in each cage. A. 2- punctata adults spent the majority of the time at the tops of cages and rarely oviposited on the stems, even when these were suspended near the roofs. This difference in behaviour of the two species in cage conditions was contrary to that found by

Blackman (1967a). - 250 -

(i) Resting and Feeding Preferences Adults of C. 7-punctata were taken from shrubs and from wild herbaceous plants. Nettle stems, mostly uninfested, were taken from the field and bean stems from potted plants. Twenty daily bbservations were made for two-days between 10.00 and

18.00 hours. The position of all adult coccinellids was recorded each time; any not on a stem were classified as "on the cage". Four separate experiments were done between May 3rd and May 25th 1973 with four different sets of C. septempunctata.

(ii) Egg-laying Preferences

These were investigated in field cages during June and

July 1973, when adult C. 7-punctata were actively ovipositing.

In 3 experiments the adult beetles were taken from a nearby bean field, while in another two experiments they were selected from nettle patches. Six nettle and six bean stems were taken from the field and placed in the water-filled jars in each cage. The combinations of beans and nettles are shown in

Table 48.

Table 48

Combinations of Beans and Nettles for Experiments on oviposition of C. 7-punctata

A. Nettles and Beans (both infested). )

B. Nettles (infested) and Beans ) Adult Beetles from (uninfested). ) Bean Field. C. Nettles only (infested).

D. Nettles and Beans (both infested). ) Adult Beetles E. Nettles only (infested). from Net tales.

Infested plants were selected which appeared to support in excess of 1,000 aphids. The experiments, each of which was replicated four times, lasted 4 days, when stems began to wilt. in the heat. The position of all adult and the numbers of - 251 -

eggs laid on nettles, on beans and on the cage were recorded at the end of each experiment.

A uniform nettle patch measuring 42 metres by 3 metres, sheltered on the east side by the brick wall of a "ha-ha" was marked out into 7 sub-plots early in 1972. Stems were cut down on one sub-plot each month, leaving 2 uncut ones as

'controls'. The design used in 1972 is shown in Figure 47.

Cutting as close to the ground as possible with a hook was done during the second week of the months May to September.

Cut stems were raked off the sub-plot to minimise colonisation of remaining stumps by aphids from the tops.

Beginning on May 9th 50 stems, randomly selected in each sub-plot, were individually scored for level of infestation of

M. evansi, the categories being the same as described in

Section III. This was repeated at fortnightly intervals until the second week of September. Larval, pupal and adult

Coccinellidae found on the sample stems were counted and identified.

(b)Experiments - Year 1973

The same nettle patch was used in 1973 with a modified experimental design (Figure 48). An April 'cut' was included and each sub-plot was divided in two. One half was cut at the same time as in the previous year but the other half was left uncut. This was done to indicate the possible effects of a previous years' cut as well as the effects of a cut in the current year. Populations of aphids and coccinellids were

scored as in 1972, except that only 25 stems per sub-plot were inspected and the scoring was clone weekly rather than fortnightly Figure 47

Cutting of nettle stems - lay-out of plots 1972.

Figure 48

Cutting of nettle stems - lay-out plots 1972_3.

1 G R .5_ .47.

1 2 3 4 5 6 7

UNCUT EISPT. CUT AUG. CUT :YU LY CUT SUNE-. CUT MAY CUT UNCUT

42. rn

F I GU RE .

1 7,:t 1 i2 1 L i to I 9 3 1 7 6 I 5 4 I 3 2 I i 1 I I I I I sr-.:-D-r. AUG. , TLMS I :TUNE I MAI I APW.t. UNCUT 1i IY.o.- CI.IT 1 c,-ur IINCUT i cur UNCUT 1 ' CUT UwcuT I CUT UNCUT I CUT UNCUT I CUT L. I ._

CUT BETWEEN THE AN! D 12.TH DAY OF EACH MONTH - 254 -

when aphids and coccinellids were abundant. Results are given in terms of numbers per 100 stems.

3. RESULTS

(A) Experiments using Radiotracers

(a)Uptake of Radioactivity by U. dioica and M. evansi

There were detectable amounts of radioactivity after-48 hours in leaf discs from upper nodes of the U. dioica stems each given 6 micro-Curies of S-35 sulphate. Using the Geiger counter, levels in aphids and their honeydew were not significantly different from background activity i.e. less than twice background activity (Table 49). Using the scintillation counter significant activity was however detected in the samples of M. evansi but not in the honeydew

(Table 49). The efficiency of the Geiger counter when counting aphids labelled with S-35 was estimated from this experiment to be only 0.50. When the experiment was repeated with a dose of 5/uCi per stem, significant activity was found in the nettle • aphids after one week and two weeks, but not in offspring produced during this time (Table 50). Using the scintillation counter it was found that radioactivity in the honeydew rase sharply during the second week, from an average count of 47/ minute to 612. There was insignificant activity in exuviae.

(b)Uptake of Radioactivity by Potted Plants

Much S-35 sulphate was taken up by all three plants, particularly by upper leaves, within three days of applying the solutions of S-35 labelled sodium sulphate to the soil

(Table 51). Levels of activity remained detectable throuabout the three week period and peaked after about 10 days. It was notable that towards the end of the experiment morn activity was present in lower leaves, a reversal of the trend in the - 255 -

first two weeks (Table 51). Most radioactivity was recorded in the plant given 8/uCi of S-35 labelled sulphate, but more was recorded in the plant given only 2/uCi than the one given

4/uCi.

The aphids had levels of activity more than 40 counts per minute above background level one week after soil treatment.

After three weeks, activity in aphid samples from all three plants had dropped to background level. Thus as might be expected, biological half-life of S-35 in the aphid and nettle tissue was much shorter than its natural half-life of 87 days.

P-32 phosphate was used in the present experiment due to its stronger beta emission and thus its easier detection.

Much more activity was recorded in aphids from P-32- than from S-35-labelled Vicia faba, frequently over 1,000 counts/ minute for 10-15 aphids from a bean stem given 5/uCi of labelled phosphate solution (Table 53). Levels of activity of C.7- punctata adults fed continuously on 10-15 labelled aphids per day and of those fed on unlabelled aphids after 24 hours on labelled ones are shown in Table 52. Daily intake of activity by the coccinellids was estimated by subtracting the radioactivity of aphid remains determined by the Geiger counter from the activity of aphids supplied a day earlier.

Uptake of radioactivity by three of the adult C. 7- punctata was rapid during the first 24 hours, but much slower in the fourth adult (Table 52). There was an increase of activity with time of coccinellids continually fed with labelled aphids, reaching more than 1000 counts/minute within four to five days. Radioactive label was lost from adults - 256 -

Table 49 Levels of Radioactivity in M. evansi-colonised U. dioica given 6,/uCi of S-35 Sulphate

*Mean Activity in Counts/Minute (after 48 hrs) SAMPLE Geiger Counter Scintillation Counter

Leaf Discs 55 80 M. evansi (Sample of 10) N.S. 46 Honeydew N.S. N.S.

*Counts corrected for background activity.

N.S. = Counts less than twice the background count.

Table 50 Levels of Activity in M. evansi-colonised U. dioica given 5/uCi of S-35 Sulphate

Mean Counts/Minute after:- SAMPLE 1 Week 2 Weeks 3 Weeks

Leaf Discs 59 42 N.S.

M. evansi 41 53 (Sample of 10) N.S.

Offspring N.S. N.S. N.S.

Honeydew* N.S.(47) 110(612) N.S.(NS. )

*Counts in Brackets are with the Scintillation counter. - 257 -

Table 51 Uptake of Soil-applied S-35 Sulphate by potted U. dioica

Mean Counts/Minute by Dosage of Sample Geiger Counter after:- S-35 Sulphate 3 days 7 days 10 days 21 days

2/uCi Upper leaf 124 264 435 58 Lower leaf N.S. 42 168 165 M. evansi N.S. 48 44 N.S.

4/uCi Upper leaf 32 92 225 136 Lower leaf N.S. 40 57 170 M. evansi N.S. 45 57 N.S.

81uCi Upper leaf 260 374 549 108 Lower leaf 157 119 184 310 M. evansi N.S. 84 76 N.S.

Table 52 Uptake and Decay of P-32 Phosphate in adults of C. 7-punctata fed labelled M. evansi

Mean Counts/Minute by Geiger Counter from:- Day Coccinellids on labelled Coccinellids on unlabelled diet diet after day 1 1 2 1 2

1 408 157 614 689 2 721 592 189 212 3 762 952 108 95 4 805 1141 32 84 5 984 1568 N.S. 54 6 1106 1704 N.S. 26 7 1013 1810 N.S. 30 - 258 -

transferred to a diet of unlabelled aphids after 24 hours very rapidly on the first day, but at a slower rate for the subsequent five or six days until background levels of activity were reached (Table 52). One of the adult coccinellids transferred to unlabelled aphids laid eggs from the third day onwards, but significant levels of activity (i.e. at least 2 x background level) were not detected in these eggs.

Estimated biological half-life of P-32 in adult C. 7- punctata is graphically shown in Figure 49. Despite its rapid incorporation into coccinellid body tissues, disappearance of P-32 from this species was about seven times faster than that expected from its natural rate of decay. Figure 49 ignores the large amount of decay on day 1 which was probably due to ejection of labelled material from the gut, which had therefore not been incorporated in body tissues. These results suggest that a label of P-32 in adult C. 7-punctata might be lost in less than a week of feeding on non-radioactive aphids.

(d) Labelling of Adult Coccinellidae in the Field

Development of M. evansi populations on the labelled nettle patch and of A. fabae populations on the bean plots is shown in Table 54. Peak populations of 8600 M. evansi/100 stems and 130,000 A. fabae/100 stems were reached and these were exploited by several species of coccinellid during the season, chiefly Coccinella septempunctata, Adana bipunctata and Propylea quatordecimpunctata.

Levels of radioactivity as determined by the Geiger counter in randomly-taken leaf discs and aphids from the treated nettle plot are given in Table 55. The radioactivity of aphid samples was rarely more than twice background although activity in leaves, which varied considerably, was generally - 259 -

Table 53 Levels of Radioactivity in samples of 10-15 A. pisum from P-32 labelled Bean Stems

Sample Mean counts/minute by Geiger Counter

1 708 2 1246 3 1062 4 1605 5 1338 6 1130 7 664 8 930 - 260 -

significantly higher than background level. Adult Coccinellidae were first sampled systematically and removed to the laboratory for detection of radioactivity on June 7th. Radioactivity of coccinellids was in most cases very low; 53.6(M of adults captured on the nettles had a detectable radio-label and only

12.75 of adults captured on the beans (Table 56). Total numbers of coccinellids captured each week are given in Table

57. The peak of adults on nettles occurred on May 22nd, ten weeks before that on the bean plots. Significantly more coccinellids (17)-<.- 0.05) were. observed on the bean plots either adjacent to or 15 metres away from the treated nettle patch than on bean plots 30 metres away or on the nettle patch itself (Table 57). There was limited oviposition by coccinellids on the nettles early in the season, but after June 12th, by which time large infestations of A. fabae (mean of 300 aphids/ stem) had developed on the beans, egg batches were found only on the bean plots.

A paint marking system was also used to study the movement of adult Coccinellidae within the experimental area.

Only eight recaptures of marked individuals were made (Table 58), perhaps partly due to the paint being removed by the predators secretions, but probably due laraely to frequent emigration of adults from the study area to alternative habitats e.g. nearby orchards.

(B) Experiments using Laboratory and Field Cages

(a) Habitat. Preferences of Coccinellidae in Petri-Dishes

No discernible preference was shown when aphid-infested bean and aphid-infested nettle leaves wel offered to :female

C. 7-punctata i.e. combination A, but when there was a choice between infested and uninfcsLed host plants i.o_ combin,7cions B, C and D, females quickly settled and fd on the aphid prey. Figure 49

Radioactive decay of Phosphorus-32 in adult C. 7-punctata. - 262 -

FIGURE 49. NATURAL OECA`f — — BIOLOGICAL ID,-4.-CAY

100

LE) A C 25 OG S TY (L IVI CT F A O

AY EC D %

10 25

TIME IN DA'15

ESTIMATED BIOLOGICAL HALF-LIFE Or: P-'62 2 DA\I'S - 263 -

Table 54 Development of Populations of M. evansi and A. fabae during Field Experiment using radiotracers

Date Nos. of M. evansi/100 stems Nos. of A. fabae/100 stems (Mean of all plots)

May 8 1200 - May 22 2100 - June 3 8600 5600 June 17 7500 30000 June 30 3200 130000 July 11 750 52000 July 24 30 3500 Aug 7 0 600

Table 55

Levels of Radioactivity in U. dioica and M. evansi following field applications of P-32 Phosphate

Date Mean Counts/Minute Sample of 10 M. evansi Leaf Discs

May 9 N.S. N.S. May 15 61 39 May 22 29 20 May 29 25 33 June 2 N.S. 25

June 10 N.S. N.S.

-264 -

Table 56 Levels of Radioactivity in Adult Coccinellidae during Field B)_Teriment using radiotracers

(Mean Counts/Minute by Geiger Counter)

Beans Nettles Date Numbers Mean Highest Numbers Mean Highest Counted Activity Activity Counted Activity Activity

June*7 - - - 9 64 121

17 7 N.S. 33 1 25 25

24 8 N .S . 24 2 47 48

26 11 N .S . 28 4 19 42

30 14 N .S . 31 2 N.S. N .S . July 5 28 18 26 3 31 62

11 24 N.S. 26 2 N .S . N .S .

17 10 N.S. N .S . 2 N .S . N .S .

*3 adults captured on the nettle patch on Nay 31 gave mean

counts/minute of 36.

% of total captures on beans with significant label = 12.7

% of total captures on nettles with significant label = 53.6 - 265 -

Table 57 Populations of Adult Coccinellidae on Beans and Nettles during experiment using Radiotracers

ADULT COCCINELLIDS ON:- DATE Nettles Beans adjacent Beans 15 metres Beans 30 metres to Nettles from Nettles from Nettles

May 17 13 24 34 - 31 12 June 7 14

14 7 6 1

21 2 1 1 28 3 6 2 2 July 5 2 17 10 1 12 17 11 4 19 8 6 1 26 2 43 37 10 Aug 2 1 73 71 7 9 78 70 3 16 9 8 1 23 3 6 4

TOTAL: 93 264 219 31 - 266-

Table 58 Recaptures of Paint-marked adult Coccinellids on Beans and Nettles

Date & Place of Date & Place of Species Original Capture Recapture

C. 7-punctata June 26 Nettles July 3 Nettles A. 2-punctata June 26 Bean Plot 4 July 5 Bean 5

A. 2-punctata June 30 Bean 3 July 5 Bean 2

A. 2-punctata June 17 Bean 1 July 24 Bean 5

C. 7-punctata June 21 Bean 10 July 31 Bean 11 C. 7-punctata July 24 Nettles Aug 7 Bean 2 C. 7-punctata July 17 Bean 3 Aug 7 Bean 4

C. 7-punctata July 11 Bean 7 Aug 7 Bean 5

Total number of adults marked = 340. - 267 -

The predators actively searched the experimental area for the first few minutes, often rejecting any prey encountered. Most of the experimental period was then spent in the capture and ingestion of food. Total numbers of observations for the various combinations are given in Table 59.

Table 59

Experiments in Petri-dishes: Observations of adult C. 7-punctata on Bean and Nettle Leaves

TOTAL OBSERVATIONS COMBINATION OF LEAVES TOTAL OBSERVATIONS OF ADULTS ON NETTLE OF ADULTS ON BEAN (in two 15 minute (in two 15 minute NETTLE BEAN periods) periods)

A. Infested Infested 17 19 B. Uninfested Infested 7 17

C. Infested Uninfested 18 6

D. Infested and UNINF:3 INF:21 Uninfested

(b) Habitat Preferences of Coccinellidae in Small Cages (i) C. septempunctata

. The response to aphid infestations on the plants was again evident (Table 60) although there were more adult beetles recorded on the sides, roof and floor of the cages than on the plants. Long inactive periods were spent, particularly at the tops of cages, by both C. 7-punctata and A. 2-punctata. There were twice as many occurrences of C. 7-punctata on infested as on uninfested plants. However, uninfested plants were visited and thoroughly searched.

When both infested beans and infested nettles were offered, no significant preference 0.05) was shown for feeding on either bean aphid, A. fabae, or nettle aphid, M. evansi. - 268 -

Table 60

Experiments in small cages: Observations of adult C.7-punctata on Beans and Nettles

COMBINATION OF PLANTS TOTAL OBSERVATIONS (during 5 days) OF ADULTS ON:- NETTLE BEAN NETTLE BEAN CAGE

A. Infested Infested 170 138 701

B. Uninfested Infested 60 118 274

C. Infested Uninfested 98 59 186

(ii) A. bipunctata

A. bipunctata was retained on aphid-infested rather than uninfested potted plants (Table 61). Table 61

Experiments in Small Cages: Observations of adult A. 2-punctata on Beans and Nettles

COMBINATION OF PLANTS TOTAL OBSERVATIONS TOTAL OF ADULTS ON NETTLE OBSERVATIONS ON:- NETTLE BEAN (during 7 days) BEAN CAGE

A. Infested Infested 42 26 165

B. Infested Infested T:26 U:8 1:22 U:6 88 and and Uninfested Uninfested

When both beans and nettles were aphid-infested, more adult

A. 2-punctata were observed on nettles during the 7 day period, although differences in the daily totals of observations were not significant (P>. 0.05). The longer time spent on infested nettles could not be explained by differences in plant height, since all plants were kept at the same height: within the cages.

(i ) Preferences when Resting or Feeding Results of the four experiments with C. 7-punctata are given in Table 62. The difference between tfle numb -c crC - 269 -

observations of adults on the bean and nettle stems was not significant 0.05), although more time was spent searching and feeding on the latter. As in the laboratory tests, adult

C. 7-punctata spent most of the experimental period inactively on the cages. A small number of egg batches were laid on beans and nettles as well as on the cages. Table 62

Experiments in Field Cages: Observations of adult C:7--1punctata when resting or feeding on Beans and Nettles

NUMBER OF OBSERVATIONS (during 2 days) OF EXPERIMENT ADULTS ON:- NETTLES BEANS CAGE

1 37 16 326

2 62 47 266

3 74 58 361 4 36 38 180

TOTALS 209 159 1133

(ii) Preferences when Egg-laying

During the experiments in which ovipositing C. 7-punctata taken from a nearby bean field were caged with stems of field beans and stinging nettles, no eggs were found on the nettles, even when these were heavily infested with Microlophium evansi.

Adults, either resting or feeding on M. evansi, were frequently observed, however, on the nettles. Eggs were laid on stems of field beans, whether infested or uninfested with aphids, and on the sides and roof of the cage (Table 63). - 270 -

Table 63 Experiments in Field Cages: Observations of ovipositing C. 7-punctata on Beans and Nettles

COMBINATION OF STEMS AT END OF EXPERIMENT NUMBERS ON:- EXP. NETTLES BEANS CAGE NETTLES BEANS ADULTS EGGS ADULTS EGGS ADULTS EGGS

A Infested Infested 10 0 16 341 54 259

B Infested Uninfested 21 0 18 146 36 257

*Totals of 4 replicates.

Similar results were obtained when cut stems of. nettle were placed in water-filled containers amongst a crop of field beans in late June and early July No coccinellid eggs were found on the nettles but they were found frequently on adjacent bean plants.

All the C. 7-punctata used in experiments A and B (Table

63) were originally taken from a field bean crop at Silwood

Park. When adults were taken instead from nettle patches and offered nettle and bean stems in a field cage, they similarly oviposited on the beans and on the cage only (Table 64), When the same batches of adults were subsequently offered only infested nettle stems, no eggs were laid on nettles and 60 eggs (7 batches) were laid on the cages. When C. 7-punctata taken from the bean field were given infested nettles only in the cages, there was no oviposition during the 4 day experimental period (Table 64). It was notable that when one batch of these same adults was then given a choice of infested nettles and infested beans, egg-laying recommenced to a slight degree; 15 eggs (2 batches) were laid on beans and 21 eggs (2 batches) on the cage.

ej,

TOTAL cr FIELD PLANTS INFESTED STEMS NOS. FOUND TOTAL NOS. ON TOTAL NOS. ON

FROMWHICH OFFERED IN ON NETTLES AT END BEANS AT END CAGE AT END a T EXP. OF EXPERIMENT OF EXPERIMENT OF EXPERIMENT 9

ADULTS WERE THE CAGE , 7 TAKEN 1 ADULTS EGGS ADULTS EGGS ADULTS EGGS

c-+ Nettles and 14 0 12 184 30 P '11 C Nettles 54 e■ H. Beans (D • H D Nettles Nettles 12 0 - - 44 60 • n o P.) E Beans Nettles 24 0 - - 30 0 RJ w

• 0 C7' h W M M <

P Os 0

r+ 0 in M O

O U) - 272 -

(a) Experiments - Year 1972

The development of populations of M. evansi and of

Coccinellidae on nettle plots cut at various times during the

growing season is graphically presented in Figure 50. The

abundance of the two common coccinellid species, C. 7-punctata

and A. 2-punctata, is shown in Figure 51. Table 65 shows the

weekly numbers of aphids and adult Coccinellidae on each plot.

Stems cut in May or June showed rapid regrowth and by

mid-August the May plot had stems which were mostly less than

10 cm shorter than stems on the uncut plots. Stems on the

plot cut in July regrew slightly, but they remained less than

50 cm high. Little or no regrowth was seen in stems which

were cut in August or September.

Cutting in June and July had the only striking effect on

aphid abundance, causing complete :Loss of aphids during the

period of peak numbers. There was slight recovery in the aphid

population on the July cut plot in early August following

regrowth of the stems. Nettles were cut in June just as aphid

numbers were approaching their maximum. However, rapid regrowth

of the stems enabled quick recolonisation and build-up of the

aphid population and within a month of cutting numbers were

almost back to the level on uncut plots (Figure 50).

Cutting nettle stems in May increased the abundance of

C._7Lplinctata (Figure )1). The peak of overwintering generation

adults was much greater and. occurred about two weeks earlier

than on uncut plots. There was much 1110,7P oviposition by

Coccinellids on this plot than elsewhere, resulting in a peak

in numbers of pupae of 56/100 stems in mid-July. Consecluently.

the peak of second generation adults was h 7aber than on any Figure 50

Populations of aphids and adult coccinellids - Cutting experiment 1972. - 274 -

F IGURE .50. TUNE CUT 30000 Suns 94h A 40 // \ -10000 / \

35 I, \ • --, \ • ... \ APHIDS

// V 'N 3o \ -1000

20- - 100

15-

10- 10

0 tr) S 40 MAY CUT 10000

EM may 94-h

T I-

S 35 tn

E ui

TL 30- - 1000

ET w

N 25- z

0 0 0 0 20 100 / 1

S 15 5" LI D

E L 10 10 N I C S

COC 4- 0 -100oo t.9 UNCUT 0

1000

- 100

•I 0 9 23 11 3i -141 2:a it MAY 3M1S JULY AUG. - 275 -

other plot. All these adults emigrated from the plot by the middle of August. Cutting nettles in May also resulted in fewer overwintering adults of A. bipunctata and less oviposition by this species than on plots left uncut. Thus, while cutting in May changed the relative proportions of the commonest species of Coccinellidae, overall numbers of this predator group were little affected (Figure 50).

Nettle stems cut in June similarly attracted fewer numbers of overwintering A. 2-punctata but seemingly were no more attractive to the overwintering generation of C. 7-punctata than uncut stems. Cutting of stems in July, August or September appeared to have no effect on the abundance of coccinellids.

(b) Experiments - Year 1973

Populations of aphids and Coccinellidae are graphically presented in Figure 52,and Table 66 shows their weekly numbers on each plot. Abundance of adults and pupae of the two commonest species of Coccinellidae are shown in Figure 53.

The legacy of cutting in July, August and September in the previous year was evident early in the season, particularly during May when growth of stems was noticeably behind other plots. However most stems on plots cut from July onwards in

1972 were at the height of previously uncut stems by the end of June. The plot cut in April 1973, which had been left uncut the previous year, regrew very rapidly and by early June stems were almost as tall and vigorous as those on plots left uncut. As in 1972, cutting in May or June resulted in new growth, but stems remained slightly more stunted and flowering occurred up to 4 weeks later than elsewhere. Cutting stems in

July resulted in regrowth to less than 40 cm and there was little recovery of stems cut in August or September. Figure 51

Populations of A. 2-punctata and C. 7-punctata -

Cutting experiment 1972. - 277 -

ADULTS C.7 - punciaia PUPAE

----ADULTS 1 FIGURE. 51. A. 2- punc4-ata PUPAE

0 0 tti

0 cn

UNCUT

i5-

10-

• • \ 17, 1-3 a 1 14 2." AUG. s;;EPT.

- 278 - Table 65 Populations of Aphids and Adult Coccinellidae during Cutting Experiments 1972

i May 9 May 23 June 5 June 19 July 3 July 17 July 31 Aug 14 Aug 28 Sep 11 SITE 1TREATMENT 1 A C A C A C A C A C A C A CACAC A C I 1 Uncut 100 4 920 4 8750 18 9800 46 8700 8 5400 14 300 22 50 8 - - - - I 2 1May cut - - 1350 4 10750 40 12200 38 13930 14 5050 2 850 34 50 4 20 - 10 - , 13une cut 1150 4 3800 4 14760 12 1200 16 5150 4 13850 4 1200 16 100 - 40 - 10 - 4 I1July cut 350 - 1650 2 5350 16 7880 24 11800 8 - - - 4 20 2 - - 10 - 5 pAucl cut 200 - 450 - 7450 8 10550 14 13800 8 3750 2 400 22 50 2 - - - -

6 iScbt cut 200 - 1550 - 6050 12 8450 28 10850 10 5450 8 330 14 20 - 10 - - - 1Uncut 50 4 1200 4 4800 10 6400 34 8660 8 5100 10 400 14 20 4 - - - - f

A = Numbers of M. evansi/100 stems

C = Numbers of adult Coccinellidae/100 stems Figure 52

Populations of aphids and adult coccinellids

Cutting experiment 1973.

Figure 53

Populations of A. 2-punctata and C. 7-punctata

Cutting experiment 1973. - 282 -

A DuLT5 FIGURE .53. C-7. - punc+'n-i-a

----ADULTS A.2. - p n c:Vai-cx PUPAS.

(r) 2 w

30 MAY cuT O 0 25

Q. 15 cc 0 to /-A — — •I P

D 0 i-----1-3\ 1-1

20 UN CUT

10-

,1 • •• • • ;N., 21 4 10 25 2 0 14' 2 M A'1 :TUNE A ti - 283 - Table 66

Populations of Aphids and Adult Coccinellidae during Cutting Experiments 1973

TREATMENT May 21 June 4 June 18 June 25 July 2 July 9 July 16 July 23 Aug 6 Aug 2 ITE PREVIOUS CURRENT YEAR YEAR A*- C+ A C A C A C A C A CA CA CA C A C

1 Uncut Apr cut 200 - 3000 - 7500 4 1200 10 6300 16 800 12 150 6 50 - 50 - 2 2 Uncut Uncut 300 - 2600 - 3000 - 10000 6 9900 22 1300 12 40 ------3 May cut May cut - - 3000 6 4800 20 17200 26 8400 40 1500 8 150 10 50 2 - - 20 - 4 May cut Uncut 550 - 2500 - 7400 8 18000 18 12400 20 800 16 50 - - - - 2 10 - 5 June cut June cut 250 - 2200 4 - - 1000 - 900 6 600 16 400 10 60 4 100 4 20 2 6 June cut Uncut 150 - 3000 - 4600 6 15500 24 12600 22 1600 20 100 2 ------7 July cut July cut 50 - 3000 - 4800 8 7200 - 1800 16 200 4 - - - 2 - - 10 - 8 July cut Uncut 100 - 5700 2 13800 18 16200 22 5000 18 400 4 50 4 - - - - 10 - 9 Aug cut Aug cut 100 - 2500 - 2500 - 4100 4 1400 4 100 - - LO Aug cut Uncut 50 - 3000 - 2700 12 3600 20 500 4 100 4 - 2 20 - 50 - - - L1 Sept cut Sept cut 100 - 1000 - 3000 8 650 - 350 4 50 2 50 ------L2 Sept cut Uncut - - 1100 - 1400 - 1700 - 600 2 - 2 - L3 Uncut Uncut 50 - 550 - 1000 - 300 4 200 - 50 - 50 - 20 - - - - -

*A = M. evansi/100 stems +C = Adult Coccinellidae/100 stems - 284 -

Trends in populations of M. evansi were similar to those

of 1972, with cutting in June producing the most notable effect (Table 66). Aphids quickly recolonised stems cut in May and

the peak population on this plot was as high as that on the

neighbouring uncut plot. Aphids also recolonised the plot cut in June following the regrowth of stems, but numbers of

M. evansi were at least ten-fold less in late June than on

most of the plots left uncut. Cutting in July 1973 was done when the aphid population was already declining rapidly, not

while numbers were still near the peak as in 1972. Consequently

little change in the pattern of abundance compared to the adja-

cent uncut plot was caused by cutting stems in July, numbers

dropping apparently to zero in mid-July and resurging slightly in mid to late August. Development of the aphid population

on stems cut in April was almost identical to that on the adjacent uncut plot (Table 66). it was notable that the four

plots in the experimental lay-out for 1973 which had been cut

in August or September 1972 all supported fewer aphids than

other plots. This did not appear to he due to differences

in initial populations in spring or to differences in the

impact of natural enemies and thus suggests that nettles cut

late in the season may be less suitable for development or

reproduction of aphids in the following season.

Changes in the pattern of abundance of C. 7-punctata and

A. 2-punctata resulting from cutting of nettles in early May

or early June were similar to those reported for 1972 (Figure

53). Short stems which appeared on the plot cut in May were particularly attractive to adults of C. 7-bunctata and frequent

oviposition was reflected in the large peaks of pupae and

•second generation adults of this species. Numbers of - 285 -

A. bipunctata in the study area were generally much lower than in the previous year and therefore effects of cutting nettles on this species were less clear. However, reduction in numbers of overwintering adults on nettles cut in May was again evident. No eggs or pupae of A. bipunctata were found on the May-cut plot. Cutting stems in June produced slightly more oviposition by the overwintering generation of C. 7- punctata than on uncut plots, but adult numbers were seemingly not affected. Only four A. 2-punctata of the overwintering generation were observed on nettles cut at this time.

Cutting in April, in addition to minimal effects on the growth of both nettle stems and aphid populations, as already described, did not appear to alter the abundance of Coccinellidae.

Cutting from July onwards was too late to affect the overwintering brood of adult coccinellids, although cutting and removing of infested nettle stems during July may have caused the death of many larvae and thus the small number of second generation adult coccinellids observed on this plot were probably immigrants following slight regrowth of the stems.

4. DISCUSSION (A) Movement of Adult Coccinellidae in the field

(a) Labelling of Coccinellidae with radioactivity

Laboratory studies demonstrated that adult Coccinellidae can be radio-labelled by applying radioactive phosphate or sulphate solution to the host plant of their aphid prey.

Easily detectable activity, i.e. counts of more than 1800/ minute in the Geiger counter, was obtained when C. 7-punctata was given daily samples of Acyrthosiphon pisum from radioactive bean stems. Detectable activity was infrequently obtained in the field however, either in aphids or in adult Coccinellidae - 286 -

following fortnightly application of P-32 phosphate to a small

patch of nettles. This was partly due to the short half-life

of P-32 in the predators (approximately 2 days) and to the

relative inefficiency of the Geiger counter as a means of

detecting the label, but mainly due to the small dosage of

radioactive solution placed on the nettle patch. One milli-Curie

followed by 0.5 mCi every 14 days was applied and unfortunately

this proved insufficient to create high levels of radioactivity

in the plants and in the aphids. Several authors have suggested

that Phosphorus-32 remains detectable in insects for at least

two to three weeks (Anon., 1961; Bennett, 1963), but this

depends on both the strength of initial label and method of

detection. Detection by autoradiographic and scintillation

techniques can be very sensitive but involves the killing of

labelled animals. The less sensitive Geiger counter was used

in the present study so that captured predators could be

re-released (Godfrey, 1954).

Pendleton and Grundmann (1954) obtained average counts

of 250/minutes in a Geiger counter for the aphid Anuraphis

feeding on a radioactive thistle plant and counts from 214 to

2,683 in ladybird predators. However, this resulted from the

introduction of 1 mCi of P-32 into the pith of a single plant,

. the same amount applied in dilute solution from a watering can

to about 600 nettle stems in the present experiment. Better

labelling might have been achieved by either increasing the

dosage of P-32 or by employing an isotope with a stronger

beta emission e.g. Cerium-144. Nevertheless, 12.7%, of adult

coccinellids (13 in 102) captured on the bean plots during

June and July did have a detectable radio-labei. Thus some

localised movement of adult coccinel lids between the nettle patch and the bean plots did occur. - 287 -

(b) Labelling of Coccinellidae with paint The paint marking method revealed a major problem in the study of trivial flight by active predators such as Coccinellidae.

There were very few recaptures of paint-marked individuals during sampling on the bean plots, probably due partly to occasional loss of paint from the elytra but largely to the ability of these insects to disperse over a large area. Many alternative habitats, e.g. orchard trees, shrubs, were situated around the 4-acre field used for the experiment and it was thus unlikely that sufficient information on the localised movements of coccinellids would be obtained without recaptures being made over a wide area after the marking of individuals. With only eight recaptures from more than three hundred marked adults it appears that when adult ladybirds left the nettle patch or bean plots they frequently flew out of the experimental area completely. Populations of adults on the beans in June were apparently immigrants from outside the field and a small number which had migrated from the central patch of nettles.

However, the small proportion of paint--marked coccinellids on the beans was not only a reflection of the emigration of most coccinellids from nettles to habitats outside the experimental area. Eggs were laid on the nettles and on the beans which eventually resulted in a second generation of adults. This new generation of adults emoroing from pupae, together with the death of overwintering generation adults, may have reduced greatly the proportion of marked individuals in the coccinellid population.

The apparent tendency for adults to disperse over a wid e area supports the suggestion of Pollard (1971) that unlike physical effects of wild areas, biological effects often - 288 -

extend over considerable distances. Thus mobile predators feeding on M. evansi at a particular nettle patch in spring may appear several miles away rather than in an adjacent habitat later in the season. According to Banks (1955) numbers of coccinellids coming to bean fields at Rothamsted Experimental Station were largely determined by the proximity of nettle patches or the remoteness of the crop.from such places, or the shelter afforded by trees and buildings, hindering the free access of beetles to the beans. Proximity and remoteness are relative terms and it seems that ladybirds may contribute to the biological control of pests on crops well away from their source on wild plants in spring. While changes in prey abundance often enforce movement of predators to new feeding areas, it may not always be successively adjacent habitats that are visited. Short- and long-distance migration by different individuals of a population of strong-flying predators probably increases the chances of survival of the population by minimising infra-specific competition for food and the danger of extinction due to localised catastrophe.

The apparent movement of many adult Coccinellidae from nettle patches to somewhat distant habitats implies that one growerts efforts to conserve wild plants as a source of natural enemies might largely benefit other growers in the region. The ideal is therefore for every grower to conserve possible reservoirs of natural enemies even though this may often represent an altruistic act by each grower on behalf of his neighbours! Such widespread action might refute the statement of Dempster and Coaker (1974) that "most (natural enemy) species are probably too mobile for pest control to be achieved by the management of habitats surrounding the crop." - 289 -

(B) Selection of Plants by Adult Coccinellidae

Experiments done in a constant environment room at 150C and in the field indicated that Coccinella 7-punctata did not distinguish between aphid-infested nettles and aphid-infested field beans as a source of food, although too much reliance on these results must be guarded against in the knowledge that climatic conditions are markedly affected inside cages of fine mesh (Woodford, 1973). Iperti (1965) suggested that coccinellids distinguish between height rather than type of vegetation and the results of experiments in which a choice was offered between two infested plant species and of experiments where regrowth of cut stems at different heights was present support this view.

Adult C. 7-punctata spent more time on infested than uninfested stems. A habitat satisfying nutritional requirements may not, however, provide suitable sites for oviposition and subsequent development of immature stages of the life-cycle.

There are two major factors which appear to determine suitability of a site for oviposition; firstly the proximity of a prey population which is palatable and sufficiently abundant as a larval food and secondly the presence of a substrate on which eggs can be laid (Chandler, 1966; Hodek, 1973). Female coccinellids do not evidently require the immediate presence of prey as a stimulus for oviposition (Hanks, 196; Hodek, 1973), hence the frequent occurrence of egg batches on the sides and roof of cages in the experiments. Nevertheless egg-laying on cages may have been the result of stimuli received from adjacent aphid-infested plants either at the time of oviposition or during previous searching activity on the stems. Laboratory tests on the food value of Ailhis fabae and M. evansi (Section TV) suggested that both these specie are an accolAable diet fo - 290 -

C. 7-punctata and could have stimulated egg-laying by females of this species. However, it is probable that females respond not only to the presence or absence of prey but also to density of prey and to the condition of their host plant. Wratten

(1973) showed that females of A. bipunctata appear to respond to aphid density by laying eggs only where the density of lime aphids can support a generation of larvae. Thus females may cease oviposition when current aphid density is insufficient for the survival of their larvae, but they might equally respond to current host plant condition, particularly to whether or not it is likely to prove suitable for the reproduction of aphid prey throughout the larval development period. Mature stems of

U. dioica, the lower leaves of which usually showed signs of senescence, which were used in the cage experiments on oviposition may therefore not have stimulated egg-laying by female coccinellids despite being heavily infested with M. evansi and it is notable in this respect that when nettles only were placed in the cages, oviposition frequently ceased or was reduced to a low level until field beans were also added. It appears that mature, infested field beans, unlike mature, infested nettles, provided all the necessary stimuli for oviposition to take place either on the plants or on the adjacent structure of the cage.

The question remains as to why no egg batches appeared on nettle stems during experiments in which both bean and nettle stems were present and in which both the beans and the cage were used as an egg-laying site by C. 7-punctata. If eggs were laid on the cage as a result of stimuli received from infested beans, why were they not also laid on adjacent nettles as a result of the same stimuli? The appearance of eggs on the - 291 -

mesh and wood surfaces of the cage as well as on bean leaves suggested that females were relatively unselective during their search for a substrate on which to oviposit. Absence of egg batches on nettle sterns thus infers that this species of plant was actively rejected in caae conditions. The texture of the

plant surface or the stinging hairs might have mechanically obstructed the act of oviposition, but this would presumably apply also to young. nettle stems on which oviposition by coccinellids did occur. The few eggs recorded during earlier

experiments on resting and feeding behaviour by adult coccinellids

were laid on young nettle stems as well as on potted bean plants.

It was only fully-mature nettle stems selected for the experiments

on oviposition in June and July that were rejected as an egg-

laying site. The apparent inadequacy of fully-grown nettles as

an egg-laying site for females of C. 7-punctata requires more

detailed investigation, as well as the possibility that similar

rejection of many crop plants in late July and August in this

country is masked by the influence of shortening day-length on

egg production.

Experiments which involved the cutting of small patches

of nettles showed how disruption of a habitat at critical

times can affect insect abundance, although only tentative

conclusions can be drawn in the absence of sufficient repli-

cation. Davis (1973b) similarly showed how the time of mowing

Meadow cranesbill, Geranium pratense U. influenced populations

of the weevil Zacladus geranii (Payk); cutting this roadside

plant between June and the end of August disrup ted larval

development within the seeds. Davis recommended that some

areas of cranesbill should be left uncut during this period to maintain a slrong population of the 1 - 292 -

The effects of cutting nettles must be viewed against the background of coccinellid abundance and egg-laying behaviour on nettles left uncut. The overwintering generation of adults appeared commonly on nettles in May and eggs were laid for most

of the month. From late May/early June onwards, however, very few egg batches were found on nettles. Pupae began to appear in late June, giving rise to a second generation of adults in

July which eventually migrated to other plants, possibly as a

result of the crash in populations of its prey, M. evansi. Of

the two commonest species of coccinellid on uncut nettles,

A. 2-punctata was often more abundant than C. 7-punctata but

numbers of the two species were sometimes similar. Cutting in May had little effect on populations of M. evansi but reduced numbers of A. 2-punctata while simultan- eously increasing the abundance and egg-laying of C. 7-punctata on the plot. This was due to the preference of the latter species for aphids infesting low plants i.e. 0-50 cm (Iperti,

1965; 1966). The large peak of second generation adults of

C. 7-punctata quickly dispersed to alternative sites. Cutting

during May may thus be detrimental to the short-term aims of

pest control by maintaining the attractiveness of nettles to

C. 7-punctata at a time when migration of predators to nearby

crops is most required, although this would be partly counter-

balanced by a reduction in attractiveness of nettles to the

less voracious A. 2-punctata, which would fly to taller plants

i.e. above 1 metre (Iperti, 1965; 1966). Furthermore the

retention of C. 7-punctata on nettles cut in early May would

result in less oviposition on plants in alternative habitats,

which might include crops and thus the impact of larvae and of

newly-emergent second generation adults on pest infestations - 293 -

would be reduced. The latter is especially important in the

longer-term in that the numbers of aphids which persist into autumn and eventually colonise overwintering hosts partly

determine pest abundance in the following season (Way, 1967;

Way and Cammell, 1973).

Cutting nettle stems in June did not appear very beneficial

in terms of pest control. Numbers of eggs laid by A. bipunctata

were reduced, so that at least oviposition by this species was

mainly diverted elsewhere, but there was little change in the

abundance of C. 7-punctata compared to uncut plots. Cutting of

nettles in early July had different effects in the two years.

In 1972 there was no egg laying by Coccinellidae on nettle

stems cut at this time. Thus oviposition in other habitats,

including crops, was presumably increased. In 1973 egg-laying.

by Coccinellidae began much earlier and cutting in July was

done when the development ac most coccinellid larvae was

nearing completion and many pupae were on the stems. Larvae

unable to crawl to neighbouring plots were probably killed but

rapid departure by the new generation of adults was enforced.

The optimal time for destruction of nettles, either by cutting

or by herbicide treatment might be early July in most years.

Such a practice might maintain the relative unattractiveness

of nettles as an egg-laying site in Late May and June, thus

ensuring oviposition on other host plants, while the new

generation of emerging adults resulting from any small amount

of oviposition earlier in_the season would he forced to emigrate

to other areas in search of food as quickly as possible at the

time of cutting. The effectiveness of indigenous populations

of Coccinellidao in preventing damaging increases in the

abundance of pest aphids both in the current and in the sneceedino season might thus be enhanced. - 294 -

SECTION VI

GENERAL DISCUSSION

1. The Case for Conservation of the Stinging Nettle The role of the perennial stinging nettle, Urtica dioica as a source of beneficial natural enemies has been partly examined by experiments described in this thesis, but prior to making any practical suggestions regarding the conservation of U. dioica in situations close to arable crops, there follows an attempt to discuss more completely its various biological attributes in terms of possible effects on the abundance of pests and natural enemies on crops and its aesthetic importance as a food source for wild fauna.

(A) Nettles as a source of alternative and alternate prey for Natural Enemies

Nettles support aphid and psyllid prey for a wide range of natural enemies. Populations of Microlophium evansi were found to increase during April and May, peak about mid-June and then drop to low levels in late June and July. Alternate prey are therefore available early in the season after the emergence of many natural enemies from hibernation and prior to the invasion and build-up of most pest aphids on young crops.

Limited numbers of prey, particularly the larvae of Psyllidae, may also be present on nettles in August and September after the harvest of many common field crops.

M. evansi remained abundant on most patches of nettles until the end of June and may therefore have been a diversionary prey for some natural enemies when pest populations were beginning to increase rapidly on adjacent crops. Thus there appeared to be no defined period of emigration from nettles by adult Anthocoridae and other predatory Heteroptera, perhaps due to the availability of sufficient aphids and psyllids for - 295 -

most of the season. Certain predators, however, e.g. the overwintering generation of Coccinellidae, appeared to disperse from nettles in June irrespective of the numbers of M. evansi present, seemingly in search of more suitable sites for egg- laying. In years when pest numbers are low during June, due to their late arrival on crops from spring feeding areas or due to climatic conditions being unfavourable for their multiplication, continued abundance of aphids on nettles around field edges may be advantageous in retaining some predators in the area which could eventually disperse while their impact on crop aphids might still be significant. This is particularly important where crop quality is threatened right up to harvest and possibly beneficial in the long-term by contributing to the decline in pest populations in late summer/early autumn and thus reducing the numbers successfully recolonising overwintering hosts. Experiments on the cutting of nettle plots at certain times indicated that the rapid movement of some natural enemies to cultivated areas might be artificially manipulated.

Nettles were also found to harbour a source of beneficial hymenopterous parasites and pathogenic fungi, although the latter were recorded very infrequently as infecting M. evansi.

Aphidius ervi, the most abundant primary parasite during this study, has been recorded from Acyrthosiphon pisum (Stary, 1970). and the species of fungi Entomophthora aphidis and

E. planchoniana from aphids on cereals (Dean, 1971) and on broad beans (Milne, 1971). Rates of parasitism fell rapidly in July as numbers of M. evansi reached low levels and there was probably a dispersal of adult parasites to alternative habitats at a time which might only have contributed to the decline of aphid populations on most crops. - 296 -

(B) Nutritional Value of M. evansi

M. evansi was shown in experiments described in Section

IV to satisfy the nutritional requirements of larvae of Adalia bipunctata and Coccinella se tempunctata and of the adult stage of the former species. Many other natural enemies in the field feed and reproduce on nettles infested with M. evansi. The food value of this aphid is tempered by its ability to resist capture, particularly by small predators with chewing mouthparts.

(C)Oviposition Site for Coccinellidae

Mature infested stems were rejected as an egg-laying site by female C. 7-punctata in cage experiments, although both the present author and Banks (1955) recorded eggs on young nettle stems in May. Other predator groups e.g. the Anthocoridae oviposited on mature nettles. The apparent change in attractiveness of nettles as they age to egg-laying coccinellids may be economically important in increasing the number of egg batches on crops during the summer and therefore the impact of coccinellid larvae on pest aphids.

(D)Source of Pests

U. dioica does not appear to harbour important crop pests, apart from an isolated report of M. evansi reaching damaging levels on.wheat, oats and barley in Argentina (Tapia, 1969) and the occurrence of the weevil Phyllobius pomaceus Gyll. on nettles near strawberry beds (Nronenberg, 1941). Southey (1947) suggested that nettle may be a symptomless carrier of certain

hop viruses which could be transmitted to hops by the ilea

beetle Psylliodes attenuate. Aphis fabae was found occasionally during sampling, but as individuals rather than as flourishing colonies.

(B) Aesthetic Value

Most people would consider Urtica dioica an obnoxious 'weed, - 297 -

and in amenity areas it may be a nuisance to picnicers and ramblers. It is arguable, nevertheless, that all wild plants contribute to the natural beauty of an undisturbed habitat. The stinging nettle, in particular, is a food for the larvae of several colourful butterflies, including the small tortoise shell, Aglais urticae L. and the Red Admiral, Vanessa atalanta L.

(Newman, 1967). Many naturalists would be alarmed at any further reduCtion in the numbers of these species in Britain.

Almost fifty species of insects are associated with Urtica and more than half of these are virtually confined to it (Davis,

1973a).

(F) Direct Economic Value

U. dioica was formerly a plant highly favoured for its culinary uses. Farmers once regarded nettles as indicators of both lime and nitrogen conditions in the soil (Anon., 1939).

Nowadays it is rarely considered of any practical value, except by wine-making enthusiasts.

2. The Feasibility of Conserving Nettles in Farming Areas

How worthwhile and how feasible is the conservation of perennial stinging nettles in agricultural areas? Hedgerow removal and the use of herbicide sprays have generally resulted in large-scale disappearance of the herbaceous flora along field edges (Pollard, 1968), which includes U. dioica; many of the intensively farmed areas of southern and eastern England have thus become somewhat bereft of wild plants. In the case of an 80 acre field completely surrounded by hedgearowth, the ratio of crop to non-crop is at least 10 to 1, so that populations of natural enemies, on dispersal 1 i"om wild plants, are diluted over formidable areas of farm-land. Nowc,ver, it is the temporal pattern of arrival by predators on crop:, which must begin sufficiently early to prevent the notorious eplosion - 298 -

of many pest populations, which has been shown to be important

(Hodek et al., 1965; Crawley, 1973) and the proximity of predator reservoirs may largely determine this. A small percentage of uncultivated land, whether as hedgerows or copse areas in field corners can be maintained without loss of production on most farms (Barber, 1970) and this can ensure that even in low numbers, the impact of natural enemies on pest populations is optimised. Weeds are generally considered as competitors to farm production, whether or not they are actually within the crop.

Farmers believe that much seed is blown from plants around buildings and along field boundaries on to distant crop areas and that runners of twitch or couch, for example, are often picked up from the edges of fields and carried by mechanically- propelled implements. Furthermore, as discussed in Section V, many wild plants may be important alternate or alternative hosts of pests and diseases. Weeds in this way are quite rightly regarded as sources of new infestation and infection and should be eradicated (H. Watt, pers. comm.). What is needed, however, is discrimination between wild plants which indeed constitute a threat to efficient crop production and those, like the perennial nettle, which support beneficial natural enemies without the danger of harbouring pests or of spreading and establishing within the crop. To this end the Nature

Conservancy is informing farmers through mobile displays and advisory leaflets of the potential value of certain 'weeds'.

A gradual change of attitude is undoubtedly occurring and it is to be hoped that pure stands of stinging nettles around buildings and.along field borders will be more frequently avoided during general weed clearing operations. - 299 -

SUMMARY

1. The work aimed to evaluate the importance of Microlophium

evansi (Hemiptera : Aphididae) as a prey for beneficial

natural enemies. Literature on the diversity/stability concept, the role of diversity in preventing pest outbreaks,

population studies of aphids, and the special characteristics of nettle aphids and their host plant is briefly reviewed.

2. The response of nettle aphids to constant temperatures o between 6 and 27°C was investigated. There was an inverse relationship between developmental period and temperature

up to 25°C for M. evansi and up to 27°C for Aphis urticata.

Net reproductive rate of M. evansi was greatest at 12° and this aphid appears to be adapted to lower ambient temper-

atures than most other species which have been studied.

Optimal temperatures for population increase and for the

growth of its host plant seem to be between 12 and 15°C.

3. The population dynamics of M. evansi on perennial stinging nettle, Urtica dioica L. is described. Techniques used

in sampling and scoring infestations of this aphid are

explained. populations increased rapidly during May to

reach a peak during mid or late June, followed by an

equally rapid crash to low levels of abundance by late

July. The size of infestations varied enormously between

patches and appeared to correspond not only with the size

and visually-assessed vigour of the nettle stems, but

also with their nutritional quality as determined by aphid

mean relative growth rate. There was some evidence of

relatively large and relatively small populations of

M. evansi developing on nettle patches in alternate years, - 300 -

perhaps due to inherent or aphid-induced changes in the

nutritional quality of the host plant.

4. A wide range of natural enemy groups were sampled on stinging nettles. The most important numerically were the Anthocoridae, the Coccinellidae and the Braconidae,

all of which commonly appear on crops during the summer.

The relative importance of various factors in determining aphid abundance was evaluated by constructing a computer

simulation model. The impact of natural enemies was

seemingly unimportant for most of the season. The model

suggested that the decline in aphid abundance was largely caused by deterioration in the food quality of nettle

stems.

5. Laboratory experiments showed that M. evansi was an adequate diet for the completion of development by the

coccinellid beetles Adalia bipunctata and Coccinella

septempunctata. A diet of M. evansi in the adult stage enabled A. bipunctata to lay twice as many eggs as when

fed Aphis fabae. Experiments done in laboratory and

field cages aimed to examine the feeding and egg-laying

behaviour of adult Coccinellidae. When A. 2-punctata and C. 7-punctata were offered aphid-infested stems of nettle

and field bean, they fed on both M. evansi and A. fabae,

although C. 7-punctata appeared to oviposit preferentially

on mature bean stems.

6. Attempts were made to follow the movements of marked

Coccinellidae between nettles and small plots of field

beans. P-32 labelled phosphate was used as a radiotracer,

applied to the nettles in dilute solution and this was - 301 -

supplemented by paint-marking the elytra of adult

beetles. The coccinellids appeared to disperse over a much wider area than that to which recapturing was confined.

7. The effects of cutting small patches of nettles at various times on populations of Coccinellidae are described. Cutting in early May attracted greater numbers of

C. 7-punctata than uncut plots, but fewer A. 2-punctata.

It is suggested that cutting nettles in June or July may

increase their value as a reservoir of beneficial natural

enemies, largely by ensuring the rapid departure of second

generation Coccinellidae to crop habitats.

8. The influence of the perennial stinging nettle on the abundance of pests and natural enemies on agricultural

crops and its aesthetic importance as a food source for

wild fauna are discussed. It is concluded that patches of perennial nettle should be conserved around farm

buildings and along field borders, especially since there

is no threat of potential pest species being harboured

or of the spread and establishment of this 'weed, within annual crops. - 302 -

ACKNOWLEDGEMENTS

I am particularly grateful to my supervisor, Professor

M J Way for his encouragement and constructive criticism of my work and to Dr G Murdie for his advice on sampling and analysis.

I should like to thank Professor T R E Southwood for use of the facilities of the field station; Dr M J Crawley and

Mr D P Duckett for helpful discussions; the technicians and gardening staff at Silwood Park for providing equipment and plants; and the Ford Foundation for supplying the computer terminal link to the college, CDC 6600.

Special thanks are due for the expert professional assistance of Rita Gibbs (graphical work), Jak Carroll (typing) and my wife, Pat Perrin (translating). Also thanks to

John W Oram for the use of the office facilities at

Independent Recording Studios, Gravesend, Kent. - 303 -

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WAY, M J (1973) Objectives, methods and scope of integrated control. In: Insects: studies in population management, edit. P W Geier, L R Clark, D J Anderson and H A Nix, 137-152. Ecol. Soc. Aust (Memoirs 1), Canberra. WAY, M J and BANKS, C J (1968) Population studies on the active stages of the black bean aphid, Aphis fabae Scop., on its winter host Euonymus europaeus L. Ann. appl. Biol. 62, 177-197 WAY, M J and CAMMELL, M E (1971) Self-regulation in aphid populations. In: Dynamics of Populations edit. P J den Boer and G R Gradwell, 232-242. Proc, Adv. Study Inst. Dynamics Numbers Popul. (Oosterbeek, 1970). Wageningen. WAY, M J and CAMMELL, M E (1973) The problem of pest and disease forecasting - possibilities and limitations as exemplified by work on the bean aphid, Aphis fabae. Proc. 7th. Brit. Insect. Fung. Conf. 1973, 3, 933-954 WAY, M J and MURDIE, G (1965) An example of varietal variations in resistance of Brussels sprouts. Ann. appl. Biol. 56, 326-328 WEARING, C H (1967) Studies on the relations of insect and host plant II. Effects of water stress in host plants on the fecundity of Myzus persicae (Sulz.) and Brevicoryne brassicae (L). Nature, Lond. 213, 1052-1053 WEARING, C H and van EMDEN, H F (1967) Studies on the relations of insect and host plant I. Effects of water stress in host plants on infestation by Aphis fabae Scop., Myzus persicae (Sulz.) and Brevicoryne brassicae (L). Nature, Lond. 213. 1051-1052 WILLIAMS, C B (1922) Co-ordinated rhythm in insects; with a record of sound production in an aphid. Entomologist 55, 173-176 WILLIAMSON, M (1972) The Analysis of Biological Populations. Arnold, London, 180 pp. WILSON, A G L, HUGHES, R D and GILBERT, N E (1972) The response of cotton to pest attack. Bull. ent. Res. 61, 405-414 WOODFORD, J A T (1973) The climate within a large aphid-proof field cage. Entomologia exp. appl. 16, 313-321 WRATTEN, S D (1973) The effectiveness of the coccinellid beetle, Adalia bipunctata (L) as a predator of the lime aphid, Eucallipterus tibiae L. J. anim. Ecol. 42, 785-802

WRATTEN, S D (1974) Aggregation in the Birch aphid Euccraphis punctipennis (Zett.) in relation to food quality. J. anim. Ecol. 43, 191-198 - 319 -

WRIGHT, D W and ASHBY, D G (1945) The control of the carrot fly (Psila rosae Fab.) (Diptera) with DDT. Bull. ent. Res. 36, 253-268

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APPENDIX : Computer listing

There follows a computer listing of the model discussed in Section III, called PROGRAM EVANSI. All the sub-routines the model calls are included and the program should run as it stands on CDC FORTRAN machines.

Sub-routines SCALE and PRTPLT for output of results in graphical form were devised by Dr M J Crawley. - 321 -

r-- ■•■••■ PROGRAMEVANSI(INPuT7OUTPUT,TAPE5=1:WU19TAPE6=OUTPUT) COW.ION/bDu1/bLANKI1)019COSSINUMb(o) 6IMENSIGN IhEADCZL.),LACILLS(o93) ,LAEWUT(4,2) DIMENSION UIV(6)95NO3(i) DIMENSICN +Ap1ERAL(5u), 4-FFCmAX1(41), +i- ECMAX(4 1)/

+FECMAX4(4i)/ + j (1) UIMENSIGN +1AGEAD((1), 4-INSTAP(1))1 4- DAYDEG(SG), +UFGThR(4), 41- E1P(1 -6.A UIt"ENSICiN +DAT1(4)) UAT2(4), 41)4T3(4), +DAT4(4)1 4- UAT5(4) DIMENSIGN +PAkA(12C),

1-PROP (J +PPED(1?(i) uiviENSIGN -1- A1(3)9‘Yi() , 4-42(4) , Y(4)9 4-X3(4)9Yi(4), :‘\ 43(4),Y5(i-F) , 4- Ao(3)9Yd(i)9 +X4(5)04C0) DATA B,41.:S//-9U.9/..«(.ii, DATA +APTEkALP2.*U./9 -FIAGEADPJ0- ,:/q 1 -1- DAYDED/'3uu./, 4- INSTAP/500/ DATA FLCOAA/4iu./ DATA

DATA

Li; IA

- 322 -

DATi4 4- A4/19U.9.91900.900.906.J0./7 .4- Y4/1.9.997.49t.86,/ DATA 1- X'-3/200.,10JU./1`D00..,,f006./9 +Y.5/0.9.37,0/1./ OAIA 4- A6/2000U,.,-)b006.+4DUJO.,/, 4- Yb/1.0.8,/ UATA +i--1- 14.914^10.18*i5../ D.ATA +IJATI/1/54.6b0917.9-3./1

+DAT3/4-5.,74b.91b.9./1u 4- UAT4/55.90,7./3./9 +Dcd5/60.' l,(.5(. .1 9 1u./ - UAFA PRcO/K.10.,(*144-0971/0,79./Y--'13./Y*29.9(*(5.9(*3*97*6., -1- 7.9(5L;;7*1697*144.1711(.17:-,-515.974151.041e2.950./ DATA PAkA/14*1.6(*.Y5,19:$0*.9b,,l*.',;97:-'.900*.997iq*.96,- 4- 7:,.943(*.'297,;-.939/*.t.1097*.9(),0*1.7 DATI, FUNG/04*1.91(,*.VL,'“*.I./ - - **Ir***,-**,-*********,,,i;,**,w***i:w**wi,.***i,*******

INITIAL INFLSTATIUN OF NL--TTLES it-

t1wiLkk+t(31(J)!o

IAGEAD(j0)=63 AY -Ffs= i'AFOAYS=, 2+,.! 1- oi4HAT(L) r.*:1Ar(0.(._:A=4)) h ;)-;;W-1.1 ) ) kEAD(5$1)(1171.7..ftL“JJ=.-i)

i.-:.AL.)(59L(Ji)t(L,4!i0U1(19,.))?j=isL),1:1-i,40 CALL SC;ALL( 1-6SA)IV rO__AbLL,5,L.r001) Or) 99 1 7-iti 1F(APTiA. ,) ki)

1..)!:Y=FLO,'f(i)

,r Ci

:3 0 Lrr ( L.. ) t `f ) .1 — 323 —

...... _.____,...... , ...... , -**ii.e,--;.;;;;;**-:;--**********- *** **- -;:* * -;ei:- ie * ,- *;:e i:--;:- -;:*-re.rel-***.-14:=.4-i:-* **--;-.-- , . ...,...._..... i'e - LTikN E HCTS Ur PLANT OUALITY AND APHID DESirY ON kLPIROGU(JIV -7_ kAIE

************************************************* . ■ PLOH:=h(DAYY.Y.9Y94) PLINF=F(Ai'DYS,XtyY1 , 3) DENSA=4.(Aell-17A6,,Yb,3)

************************************************** • _

COii,PUIL IRE NO. or .LPH1L) bUrN TOUAy 1 ******************************-w***************** - . • _ DO 200L=Ilt0 1F(INSTilk(L).Nt.5) GO [U LOu K=L—IAGLAU(L) IF(TErld(I).(L7.1 ./.) t:ELmAX(K)=FE(:MAA-i-(K) IF(TElviP(I).LE.1/0) f.trCr,',X(K)=(-LUHAXI(K) INTEMP(1).LL.13,,) FE_CmAX(1:)=FLLITA!(K) IF(TEkP(I).LE..) FEC:4(K)=tELi,:AX3(K) 1 b0P.N=30kN+AFTEkAL(L)*rtLNA)*t-1_0(Lr*PLINF*(A-NSA 2uu CONIINUL I *******—**.* 1 * * UPbAft 1 IRNS Or RPHIL:r3 r,h01-_. Itt-_kAlukc. * T t-1:-CLICA_L:S- H:0JEn'cILH pA55L0 - ' * 1 *

Lip i00 L=1;z30

LU TO 1 iFtv,,,,L;,)001 (.:0 10 IF(DAY5GAL)„LI.u.L6In.;v» v.,() 40 • i USTi - 1F(11,1S1;kk(L).LI!')) 00

J._ ALL I ri(i,; LS si 4

(L. )==.(ki ( ) ( . — 324 —

ALATE=F(APT, X 40. 415) SEARCh=■'(,1-03 TA,A5,y5,4) 'RD (I )=PkLi, ( i)* AT L AI * EARcH

DETERMiNE LOSS oi= :-;PHIUS UOE TU Pc.DATION_ . PAi:ASIIISH, FUNGUS INFECTION, + EMIGRAF1ON 01' AL,-;1 AL

- *- -. 00 51 J=1950 51 APTERAE(j)=PPIERAE(J)*HAHA(1)*t- oNG(1) DO 80 K=1,50 80 PROP(K)=APTtRAE(K)/APTA DO 52 J=I,56 APTERAE(J)=PTERAt(J)—(PRED(1)*HR0P(J)) DO 53 J=1,5b IF(INSTAR(J).NE.4) GO 10 53 APIEAE(J)P1A(J)ALAIE 5-) CONTINUE AOTOT= ***********************,,****,.:-****-;.:**4i.***********

COPAPOit TOTALS

00 /0 J=1,50 1F(INSTP(J).N..-_.5) 00 fl; AfJOT=ALTONAP)EAL(J) iv CONTINUL A2TA=u., Un APTA=APIA-i-AHLAc..(J) IF(APTA0LL.',:.) 00 40 AI=AL061u(PT) A2=0. A3=0.

AL-1,7!=o c

! 111

r Li FukCii0c: F

I ( ) i oU

— 325 —

CONTIUE F=YVALt1) RETWAN F=YVAL(NDL ) kEIURN E.■,;L) SUbPOUT1NL SCiA,r_(1730U,Niu39u1V9ILAULAbELS/LAiJOUT) DIMENSICN it- E,w(2it)4L,n,L-JLLS(o/J)9LAtiUUT(.-:,12) DI'mENS1CN HOUN63(12), DIV(o) HO6=b WP1TE (b/dUO) (imEAU(r)q =i)LU) WRITE (c9oCI) N=0 Do 9 K=29.1692 N=N+1 ',,:i-;ITE(6v802)BOUNDS(K),(LAdELS(N,J)4J=11.:1 ) , HUUNOSCK —1) 9 CONTINUL WRITE (9bijd) ((LlAijOOT(i?J)7J=11.2)91=1/4.) . WITE (09':7)1(J) DO i ISLALL = i9 NO6 IF X = 2 J.SCLE 1 DIV(ISLfiLt) = (30Uis)US(Itc.i\—i)—b6uNUSt1K.'',))/70. RETUP FnRHAT (,t();=)- 8;_iu FORMAT (*1**1J3sr2u-i) FOWf-1A4 6■:,2 1- 0HhAl (ft 7-v -izi3c:_11.4is11 -9luer,T,, V9f7197:11,1):_ii0,-i- 6,B FOHiij-

LILuCis C!W';k("N UAlA LJAIA :5L'I',r,,/u1- DATA DOT/in./ HATA L.1) L. V s 9 • 0 ) 1 )

XV(::))'s

1)1v(:.-.)9 Fr;n41:(1K--) - 326 -

XV(7) = Wi XV(6) = DO 1 1=1,76 1 ALINE( 1 )=6Lr=INK ALINE(1 ) = DOT ALINE(iU) = DOT Do 2 I = I, NOG J=2*I NDIV = (Y\v(1)-bNU(J))/u1v(I) IF ) NUIV IF (14_)1V.6:7) NUIV = 7C) 2 ALINE(NO1v1=NuNoti) OiRITE(613) 19,XV(i),(ALINE(11)111=1,70)9Xv(7)9AV(8) 3 FoiR'MAT (A A 914 91 4 9T 17 9E11.,-, 91- 3i00(id)711059E11.47 T119,E11.4) RFTURN ENO