Some Components of Competition in Parasitoid
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SOME COMPONENTS OF COMPETITION IN PARASITOID WASPS by Laura Mary Ridout October, 1978 A thesis submitted for the degree of Doctor of Philosophy of the University of London and for the Diploma of Imperial College Department of Zoology and Applied Entomology, Imperial College Field Station, Silwood Park, Ascot Berkshire 2 ABSTRACT Experiments within and between four species of parasitoid wasps were run in the laboratory. The species used were the Ichneumonid Nemeritis canescens, and the Braconid Bracon hebetor, two parasitoids of the Indian meal moth Plodia interpunctelZa, and two aphid parasitoids, Diaeretiella rapae and Aphidius matricarias, Braconids attacking the peach-potato aphid Myzus persicae. Diaeretiella and Nemeritis show a decline in searching efficiency as the density of conspecifics increase, as does Bracon when searching for healthy or paralysed- hosts. In Diaeretiella, the proportion of females in the offspring declines as the initial density rises. Diaeretiella, Aphidius and Nemeritis larvae compete predominantly by contest competition. In Bracon there is no overall trend in mortality with density, but the lengths of resulting adult wasps is reduced as the number of eggs developing on one host is increased, suggesting that some scramble competition is occurring. The distribution of Bracon pupae per host becomes signi- ficantly more clumped, and the mean pupae recovered per parasitized host increases, as the parasitoid density increases. This may be due to a decrease in egg predation by active host larvae. The contribution of behavioural effects to intraspecific competition is examined for Nemeritis canescens. Under the conditions used here, the percentage of time spent searching does not decline with density. This is due to a complex nggA,j4ve feedback system operating principally via walking. It is suggested that the decline in Nemeritis searching efficiency is due to eggs lost between expulsion from the ovipositor and safe arrival in a healthy host, due to the defence reactions of the host larvae. 3 The overall effects of the presence of a competing species are examined and discussed. An increasing density of Aphidius causes a decline in the proportion of females in the Diaeretiella offspring. The number of Nemeritis offspring and its searching efficiency, decline with the density of Bracon. A Nemeritis larva cannot survive in a host also attacked by Bracon. Intraspecific competition in Nemeritis is also affected by the presence of a constant number of Bracon. The number of offspring, and searching efficiency relationships are shifted downwards, and depressed disproportionately at high Nemeritis densities. The number of hosts paralysed by Bracon is stimulated by the presence of an increasing number of Nemeritis, although the efficiency with which Bracon finds paralysed hosts for oviposition declines with Nemeritis density. The proportion of females in the Bracon offspring declines simultaneously. Encounters with Bracon individuals have a similar effect upon the behaviour of a Nemeritis as encounters with a conspecific (i.e. they can cause changes in behaviour, and affect the direction of change), but do not explain the decline in Nemeritis searching efficiency with Bracon density. 4 TABLE OF CONTENTS Page ABSTRACT ... ... ... ... ... ... ... ... ... 2 INTRODUCTION 6 CHAPTER ONE. THE BIOLOGY OF THE PARASITOIDS STUDIED AND THEIR HOSTS ... ... ... ... ... ... ... ... 10 1.1. The Peach-potato aphid Myzus persicae (Sulz.) and its parasitoids Diaeretiella rapae (M'Int.) and Aphidius matricariae (Hal.) ... ... ... ... ... ... 10 1.1.1. Myzus persicae (Sulz.) ... 10 1.1.2. Diaeretiella rapae (M'Int.) ... 16 1.1.3. Aphidius matricarias (Hal.) ... 20 1.2. The Indian meal moth Piodia interpunctella (Hubner), and its parasitoids Nemeritis canescens (Gray.) and Bracon hebetor (Say) ... ... ... ... ... ... 22 1.2.1. PZodia interpunctella (Hubner) 22 1.2.2. Nemeritis canescens (Gray.) ... 26 1.2.3. Bracon hebetor (Say). ... 30 CHAPTER TWO. INTRASPECIFIC COMPETITION: AN INTRODUCTION ... 33 CHAPTER THREE. COMPETITION WITHIN SPECIES I ... 54 3.1. Introduction ... ... ... ... ... ... 54 3.2. Aphid Parasitoids ... ... ... ... ... ... ... 54 3.2.1. Materials and Methods 54 3.2.2. Analysis of Results and Discussion ... 58 3.3. Meal Moth Parasitoids ... ... ... ... ... ... 71 3.3.1. Materials and Methods 71 3.3.2. Analysis of Results and Discussion ... 76 3.4. Summary ... ... ... ... ... ... ... 106 5 Page CHAPTER FOUR. COMPETITION WITHIN SPECIES II. BEHAVIOURAL OBSERVATIONS ON NEMERITIS CANESCENS ... 109 4.1. Introduction ... ... ... ... ... ... 109 4.2. Materials and Methods ... ... ... ... ... ... 109 4.3. Analysis of Results and Discussion ... 111 4.4. Summary and Conclusions ... 154 CHAPTER FIVE. INTERSPECIFIC COMPETITION: AN INTRODUCTION ... 156 CHAPTER SIX. COMPETITION BETWEEN SPECIES I. 190 6.1. Introduction ... ... ... ... ... 190 6.2. Materials and Methods ... .. ... ... ... ... 190 6.3. Analysis of Results and Discussion ... 191 6.4. Summary ... ... ... ... ... ... ... 221 CHAPTER SEVEN. COMPETITION BETWEEN SPECIES II. BEHAVIOURAL OBSERVATIONS ON NEMERITIS CANESCENS IN THE PRESENCE OF BRACON HEBETOR ... 223 7.1. Introduction ... ... ... ... ... ... 223 7.2. Materials and Methods ... ... ... ... ... ... 223 7.3. Analysis of Results and Discussion 225 7.4. Summary and Conclusions ... ... ... ... ... 260 CHAPTER EIGHT. GENERAL DISCUSSION 262 ... ACKNOWLEDGEMENTS ... ... ... ... ... ... ... 272 REFERENCES ... ... ... ... ... ... ... ... 273 APPENDICES ... ... ... ... ... ... ... ... 311 ... ... PLATES ... ... ... ... ... ... ... 399 6 INTRODUCTION The primary objective of this study was to investigate competition at the third trophic level i.e. at the predator or parasitoid level, placing special emphasis on behavioural interactions between individuals from different species. Considerable knowledge of conspecific interactions was thought necessary for comparison, and to give an idea of the ways in which individuals of a species may react towards others. For the present, competition will be taken to mean the use of a common resource such that there is a detrimental effect upon one or both users (individuals or species) involved. Investigations of interspecific competition of one sort or another, from strictly controlled and replicated laboratory systems, to simple field observations have been undertaken for a wide variety of plant and animal species. They range from studies on planktonic algae and Protozoa (Sykes, 1974; Tilman, 1977; Gause, 1934; Vandermeer, 1969), yeast and bacteria (Gause, 1932; Levin et al., 1977) through triclads, molluscs, crayfish, copepods, crabs, spiders and mites, and starfish (Lock- and Reynoldson, 1976; Branch, 1976; Bovbjerg, 1970; Hebert, 1977; Bach et al., 1976; Uetz, 1977; Croft and Hoying, 1977; Menge and Menge, 1974) to many higher plants and animals. Fish, frogs, salamanders, lizards, rodents, birds and flowering plants (Robertson et al., 1976; Wilbur, 1972; Fraser, 1976a, b; Lister, 1976; Levin and Anderson, 1970) have also attracted study. Among the insects, the most exhaustive competitive studies have been carried out on flour and grain-eating beetles, usually Tribolium species (Coleoptera: Tenebrionidae) (e.g. Park, 1948; Birch, 1953; Crombie, 1945; Nathanson, 1975) and on Drosophila species (Diptera: Drosop hilidae) by a 7 number of authors (e.g. Ayala, 1971; Budnik and Brncic, 1974; Futuyma, 1970; Merrell, 1951; Ranganath and Krishnamurthy, 1975; Richmond et al., 1975; Wallace, 1974). A similar series of experiments have been done by Fujii and Utida on bean-weevils, Callosobruchus species (Fujii, 1967, 1968; Utida, 1953, 1961). It is striking that most of the entomological examples of controlled laboratory experiments have been performed on insects belonging chiefly to the second trophic level. (There are also a number of mostly field- based examples; the controversy over the Erythoneura leafhoppers (Ross, 1957, 1958; Savage, 1958; McClure and Price, 1976), Rathcke's Study on stemborers (1976) and some interesting recent work on bees by Heinrich (1976) and Johnson and Hubbell (1974, 1975)). At first these experiments were concerned with testing Cause's hypothesis that two species cannot coexist for any length of time sharing the same limited resource. Later it became clear that this approach was too simplistic; and the emphasis was shifted to a consideration of how organisms manage to avoid severe competitive effects when they appear to be sharing a limited major resource, and how much real sharing can be tolerated. The mechanism of the interaction (e.g. by exploitation or interference etc.), as far as it is known, is sometimes mentioned in passing. However, there is a great deal of scope for a closer look at such mechanisms. In many cases, more than one method may be operating, and, where one species is better in one way and another in a different way, both may persist. Zwglfer (1971) gives some neat examples of such "counter-balanced" competition. On the whole, very little has been done on the behavioural inter- actions between adult heterospecific insects, although' behaviour which 8 could be labelled "contest" has been demonstrated between conspecifics in insects further up the food chain (Hassell, 1971b). This raises the question of interspecific encounters at higher trophic levels: do insect predators and parasitoids exhibit behaviour along the lines of vertebrate territoriality (e.g. as in cichlid fish (McKaye, 1977), chipmunks (Meredith, 1977) and blackbirds (Orians and