Leaf-Mining Insects and Their Parasitoids in Relation to Plant Succession

Leaf-mining Insects and their Parasitoids in relation to Plant Succession by Hugh Charles Jonathan Godfray A thesis submitted for the degree of Doctor of Philosophy of the University of London and for the Diploma of Membership of Imperial College. Department of Pure and Applied Biology Imperial College at Silwood Park Silwood Park Ascot Berkshire SL5 7PY November 1982 Abstract In this project, the changes in the community of leaf-miners and their hymenopterous parasites were studied in relation to plant succession. Leaf-miners are insects that spend at least part of their larval existence feeding internally within the leaf. The leaf-miners attacking plants over a specific successional sequence at Silwood Park, Berkshire, U.K. were studied, and the changes in taxonomic composition, host specialization, phenology and absolute abundance were examined in the light of recent theories of plant/insect-herbivore interactions. Similar comparisons were made between the leaf-miners attacking mature and seedling birch. The factors influencing the number of species of miner found on a particular type of plant were investigated by a multiple regression analysis of the leaf-miners of British trees and plant properties such as geographical distribution, taxonomic relatedness to other plants and plant size. The results are compared with similar studies on other groups and a less rigorous treatment of herbaceous plants. A large number of hymenopterous parasites were reared from dipterous leaf-mines on early successional plants. The parasite community structure is compared with the work of R.R.Askew and his associates on the parasites of tree leaf-miners. The Appendices include a key to British birch leaf-miners and notes on the taxonomy and host range of the reared parasites. Acknowledgements I would like to thank my supervisor, Dr.V.K.Brown, for her help and encouragement during the course of this study. I have gained much from many discussions on plant/insect- herbivore theory with people at Silwood, in particular Dr.Mick Crawley, Annette Duggan, Dr.Richard Hill, Dr.Jorge Soberon and Clive Stinson. Dr.Jeff Waage has taught me a lot about parasitoid biology. Dr.Mick Crawley and Dr.Alan Morton kindly helped me with statistical problems and Dr.Peter Muller with computing problems. Outside Silwood, a large number of people have given me invaluable advice on leaf- miner and parasite biology and the techniques of their study. I am extremely grateful to all these people and would like to especially mention Lt.Col.A.M.Emmet who has patiently answered many questions and whose researches into leaf-miner natural history, over the last twenty years, has made possible projects such as this one. I am also very grateful to those people who have identified specimens, or have given me reared material for examination. They are acknowledged individually in the relevant sections. Table of Contents Abstract 2 Acknowledgements 3 Table of Contents 4 Chapter One: Insect Communities & Succession. 1.1 Introduction 6 1.2 Plant Processes 8 1.3 Insect Herbivore Theory & Succession 12 1.4 Interactions between Insects & Succession 20 Chapter Two: Leaf-mining Insects. 2.1 Definition 29 2.2 Taxonomic Distribution of Leaf-miners 30 2.3 Life Histories of Leaf-miners 33 2.4 Morphological Adaptations to Leaf-mining 37 2.5 Ecological Studies of Leaf-miners 41 Chapter Three: The Leaf-miner Community over a Plant Succession. 3.1 Location of Fieldwork 52 3.2 Sampling Methods 60 3.3 Identification & Analysis 62 3.4 Presentation of Results 1 Herbaceous Plants 65 2 Birch 96 3.5 Leaf Dynamics 133 3.6 Discussion 138 Chapter Four: The Number of Leaf-miner Species on Different Plants. 4.1 Literature Review 171 4.2 The Factors Influencing the Leaf-miner Species Richness of British Trees 181 4.3 Early Successional Plants 206 4.4 Discussion 212 Chapter Five: The Parasites of Leaf-miners 5.1 Introduction 223 5.2 Materials & Methods 224 5.3 Tree Leaf-miner Parasites 229 5.4 Dipterous Leaf-miners of Herbaceous Plants 234 5.5 Discussion 240 Chapter Six: Conclusions. 256 References 261 Appendix One: A Key to British Birch Leaf-miners. 271 Appendix Two: Taxonomic Literature used to Identify Leaf-miners and their Parasitoids. 278 Appendix Three: The Leaf-miners of British Tree Genera. 279 Appendix Four: Taxonomic and Biological Notes on the Reared Parasite Species. 283 Chapter 1 Insect Communities and Succession 1.1 Introduction In the last ten years much active research into the theory of insect-plant relationships has led to a wealth of predictions relating to the pattern of insect her- bivory (Gilbert 1975, Crawley 1982). A large body of this theory relates to how the characteristics of plants, as they vary over a successional gradient, will in- fluence the evolution and ecology of the insects that attack them and vice-versa. Empirical studies are now needed to test theory and also to allow theory to advance further. An illustration of this would be the relationship between the degree of herbivore polyphagy and the successional status of their foodplants. Depending on the emphasis attached to different processes, completely different predictions can be made. Further theoretical exploration of this issue cannot go ahead until empirical studies have shown which initial assumptions are correct (Rhodes and Cates 1976, see also 1.3). There are two approaches to the empirical verification of insect-plant interac- tion theory. The first is a detailed examination of a limited number of species using field observation with experimental manipulation in the field and laboratory. A second approach is to examine a much wider selection of species and to compare the trends observed with the predictions of theory. It is the second approach that is taken here, though it is realised that there, are limitations to this method. For ex- ample the methodological problems of statistical analysis where it is not known what the correct independent unit of observation is, and also problems of causality. Nevertheless it is encouraging to note the crucial role played by the comparative approach in another ecological field; behavioural ecology (e.g. Crook 1964) which had a similar theoretical advance about ten years earlier (Krebs and Davies 1978). In 1977, Professor T.R.E.Southwood and Dr.V.K.Brown initiated a long term study into the relationships between insects and plants during secondary succession, work that has been led by Dr.Brown since 1979. The main part of this work has been an examination of the connection between herbivorous insect numbers and diversity and botanical factors such as plant taxonomic and stuctural diversity. The Inscct Communities and Succcssion 7 initial results were published by Southwood, Brown and Reader (1979) and further results are either in preparation or in press (Brown 1982, Brown 1983, Brown and Southwood 1983, Southwood, Brown and Reader 1983, Stinson and Brown 1983, Stinson in prep., Hyman in prep.). The study described here, while forming part of the general scheme, is a divergence in that a restricted guild of herbivores; leaf- mining insects, is examined over the successional gradient. Though a more restricted assemblage of insects is studied, this work still falls firmly in the comparative empirical approach. However, the more restricted nature of this study enables the assemblage to be examined at the level of individual insect-plant interactions and for account to be taken of the third trophic level, neither of these things being possible when the complete, or a large section, of the herbivore community is under study. In the remainder of this chapter, the main areas of insect-plant theory that are relevant to changes in insect community structure over a succession are discus- sed and their predictions stated. Chapter 2 defines the leaf-miner guild and reviews previous ecological studies of leaf-miners. In Chapter 3 the structure of the leaf- miner community found attacking the plants over a specific succession at Silwood Park is described and interpreted in the context of the predictions in Chapter 1. One particular question is examined in more detail in Chapter 4; the factors deter- mining the number of herbivore species attacking a particular species of plant. The study is based not only on the Silwood leaf-miner community but also on a wider literature based study of the complete British fauna. A more detailed review of work in this particular field is also given. In Chapter 5 the parasitoid community of the leaf-miners of early and late successional plants are compared. The data for the early successional plants were collected during the study while data for late successional plant miners comes chiefly from the work of Dr.R.Askew and his group (see Chap 5.3). Chapter 6 comprises the final discussion and conclusions. Inscct Communities and Succcssion 8 1.2 Plant processes 1.2.1 The theory of plant succession Probably the most important structuring force affecting the phytophagous insect community is the nature of the underlying plant community. The course of plant succession determines in large part the changes in the insect community over succession. Plant succession is one of the oldest concepts in ecology, though in recent years it has been the subject of much renewed active research. The problem has been to provide a mechanistic explanation of the almost universally observed sequence in plants on a newly cleared site, from colonizing herbs, to normally a "climax" community of woody perennials. Running parallel to the community approach is the attempt to understand the characteristics of the individual species of plants of different successional stage in terms of selection operating on their life- history strategies. The theory of succession proposed by Clements (1916) dominated plant ecology until about 1970.

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