By Andrew David Le M Asurier, B.Sc. a Thesis Subm Itted for the Degree of D
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1 CLUTCH SIZE AND FORAGING BEHAVIOUR IN APANTELES SPP. (HYMENOPTERA: BRACONIDAE) by Andrew David le Masurier, B.Sc. A thesis submitted for the degree of Doctor of Philosophy of the University of London and for the Diploma of Imperial College Department of Pure & Applied Biology, Imperial College at Silwood Park, A scot, Berkshire. May, 1987 2 ABSTRACT The aim of this thesis is to use the comparative method to study the adaptation of clutch size and foraging behaviour in the parasitoid, A p a n t e l e s g l o m e r a t u s (L.). A. glomeratus is native to Europe, where its principal host is Pieris brassicae (L.) (Lepidoptera: Pieridae), but in 1883 it was introduced to America as a biological control agent of Pieris rapae (L.). The optimal foraging behaviour of a parasitoid is affected by host distribution. Early instar larvae of P. brassicae are gregarious, whereas those of P. r a p a e are solitary. The searching efficiency of British A. glomeratus was found to be much higher for P. brassicae than for P. rapae. Apanteles rubecula (Marshall), in comparison with A. glomeratus , had a higher searching efficiency for P. r a p a e (its only known host). It was therefore predicted that selection should have improved the searching efficiency of American wasps towards that shown by A. rubecula. However, no evidence of such an improvement was found. The optimal clutch size of a parasitoid depends largely on host size. P. brassicae is larger than P. r a p a e , and the offspring fitness of American A. glomeratus ovipositing in P. r a p a e was found to be more severely density- dependent than that of British A. glomeratus parasitising P. brassicae. The optimum clutch size predicted for American A. glomeratus was consequently lower than that predicted for British A. glomeratus. However, no evidence of a lower clutch size in the American population was found. Brood size in gregarious A p a n t e l e s species was found to correlate with host size, but solitary species do not lie on the same relationship, and are more numerous than expected on the basis of host size. The possible role of evolutionary constraints on clutch size optimisation is discussed. It was predicted that the sex ratios of gregarious A p a n t e l e s species should be more female-biased than those of solitary species. A comparison of mean sex ratios provided evidence in support of this prediction. 3 CONTENTS Page A B S T R A C T 2 LIST OF FIGURES 8 LIST OF TABLES 19 CHAPTER ONE INTRODUCTION 26 CHAPTER TWO BIOLOGY OF HOSTS A N D PARASITOIDS; CULTURING METHODS; AND GENERAL EXPERIMENTAL METHODS 30 2.1. Biology of Hosts and Parasitoids 30 2.1.1. The hosts 30 2.1.1.1. Pier is brassicae 30 2.1.1.2. Pier is r a p a e 32 2.1.2. The parasitoids 33 2.1.2.1. Apanteles glomeratus 33 2.1.2.1. L Host range 33 2.1.2.1.2. Life cycle 33 2.1.2.1.3. Percentage parasitism 35 2.1.2.1.4. Clutch size and brood size 38 2.1.2.2. A p a n t e l e s r u b e c u l a 42 2.1.2.2.1. Host range 42 2.1.2.2.2. Life cycle 42 2.1.2.2.3. Percentage parasitism 47 2.1.2.2.4. Clutch size and brood size 48 2.2. Culturing Methods 48 2.2.1. The hosts 48 2.2.2. The parasitoids 49 2.3. General Experimental Methods 50 2.3.1. Experimental conditions 50 2.3.2. Standardisation of wasps 50 2.3.3. Host dissection method 50 CHAPTER THREE THE FORAGING BEHAVIOUR OF A. GLOMERATUS FO R P.BRASSICAE 52 3.1. Introduction 52 3.2. Materials and Methods 53 4 Page 3.3. R esu lts 55 3.3.1. The effect of host patch size on the number of hosts parasitised 55 3.3.2. The effect of time on patch on the number of hosts parasitised 57 3.3.3. Possible factors affecting the attack rate of A. glomeratus on P. brassicae 57 3.3.3.1. E g g depletion 57 3.3.3.2. Time spent rejecting previously-parasitised hosts and cleaning 64 3.3.4. The effect of host patch size on the duration of patch visits 76 3.4. D iscu ssio n 76 CHAPTER FOUR A COMPARISON OF THE SEARCHING EFFICIENCIES OF A. GLOMERATUS A N D A. RUBECULA F O R P. BRASSICAE A N D P. R A P A E 89 4.1. Introduction 89 4.2. Materials and Methods 92 4.3. R e su lts 93 4.3.1. Experiment 1: the functional response of British A. glomeratus to P. brassicae 93 4.3.2. Experiment 2: the functional response of British A. glomeratus to P. ra p a e 96 4.3.3. Experiment 3: the functional response of British A. rubecula to P. ra p a e 96 4.3.4. Experiment 4: the functional response of American A. glomeratus to P. ra p a e 97 4.3.5. Factors affecting the asymptote of the functional response curves 97 4.4. D iscu ssio n 103 5 Page CHAPTER FIVE THE EFFECT OF HOST QUALITY ON THE CLUTCH SIZE OF A. GLOMERATUS 105 5.1. Introduction 105 5.2. H o st Size 111 5.2.1. The effect of host species on parasitoid development 111 5.2.1.1. Materials and methods 111 5.2.1.2. R e su lts 113 5.2.1.2.1. Developm ent tim e 113 5.2.1.2.2. Juvenile su rvivo rsh ip 113 5.2.1.2.3. A d u lt size 116 5.2.1.2.4. C lu tch size 116 5.2.2. Comparison of the optimal clutch sizes of British and A m e rican A. glomeratus 116 5.2.2.1. Materials and methods 116 5.2.2.2. R e su lts 120 5.2.2.2.1. The effect of clutch size on juvenile survivorship 120 5.2.2.2.2. The effect of clutch size on adult size 120 5.2.2.2.3. E g g load 120 5.2.2.2.4. The effect of clutch size on development time 124 5.2.2.2.5. Comparison of the clutch size fitness functions 124 5.2.2.2.6. Comparison of the optimal clutch sizes 127 5.2.3. Comparison of the observed clutch and brood sizes of British and American A. glomeratus 134 5.2.3.1. Materials and methods 134 5.2.3.2. R e su lts 135 5.2.3.2.1. C lu tch size 135 5.2.3.2.2. Brood size 135 5.3. Host Instar Attacked 136 5.3.1. Materials and methods 136 5.3.2. R esults 136 6 Page 5.4. Previous Parasitism 141 5.4.1 Preliminary experiment 142 5.4.2. M a in experim ent 142 5.4.2.1. Materials and methods 142 5.4.2.2. R e su lts 149 5.4.2.2.1. Frequency of oviposition and probing behaviour in parasitised and unparasitised hosts 149 5.4.2.2.2. The relationship between observed wasp behaviour and egg laying 149 5.4.2.2.3. C lu tch sizes 152 5.4.2.2.4. Duration of oviposition behaviour 152 5.5. D iscu ssion 153 CHAPTER SIX HOST SIZE AND THE DISTRIBUTION OF SOLITARY AND GREGARIOUS BROOD S IZ E S I N APANTELES 160 6.1. Introduction 160 6.2. M ethods 161 6.3. Results 163 6.3.1. Brood size frequencies 163 6.3.2. The effect of host size on brood size 163 6.4. D iscu ssion 172 CHAPTER SEVEN REPRODUCTIVE STRATEGIES OF SOLITARY AND GREGARIOUS APANTELES S P E C IE S 177 7.1. Introduction 177 7.2. M ate rials and M e thods 180 7.3. Results 181 7.3.1. Sex ratio 181 7.3.1.1. Sex ratios of solitary and gregarious A p a n te le sspecies 181 7.3.1.2. Field sex ratios of A. glomeratus and A. rubecula 184 7.3.1.3. Factors contributing to the female-biased sex ratio o f A. glomeratus 184 7 Page 7.3.2. E g g complement 187 7.3.2.1. Comparison of the egg complements of A. glomeratus and A. rubecula 187 7.3.2.2. Comparison of the fecundities of solitary and gregarious A p a n te le sspecies 192 7.4. D iscu ssio n 192 CHAPTER EIGHT GENERAL DISCUSSION 198 ACKNOWLEDGEMENTS 212 R E F E R E N C E S 213 A P P E N D I X I 248 APPENDIX II 249 8 LIST OF FIGURES Page Figure 2.1 Frequency distribution of the size of egg batches laid by captive P. brassicae on potted Brussels sprouts plants. 31 F igu re 2.2 Frequency distribution of clutch sizes (number of eggs laid per oviposition) laid by British A. glomeratus in one day- old P. brassicae larvae presented in patches of fifty on potted Brussels sprouts plants. 40 F igu re 2.3 Frequency distribution of the size of A.