The Ecology of Southern African Wild Silk Moths (Gonometa Species, Lepidoptera: Lasiocampidae): Consequences for Their Sustainable Use
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University of Pretoria etd – Veldtman, R (2005) The ecology of southern African wild silk moths (Gonometa species, Lepidoptera: Lasiocampidae): consequences for their sustainable use by Ruan Veldtman Submitted in partial fulfilment of the requirements for the degree Philosophiae Doctor (Entomology) in the Faculty of Natural & Agricultural Science University of Pretoria Pretoria July 2004 University of Pretoria etd – Veldtman, R (2005) University of Pretoria etd – Veldtman, R (2005) vir Ma University of Pretoria etd – Veldtman, R (2005) ACKNOWLEDGEMENTS A special thanks to Antoinette Botes (University of Stellenbosch) for assisting with fieldwork; Frank and Margaret Taylor, (Development Consultancies) for making it possible to collect data in Botswana; farmers of the Northern Cape and North West Provinces for their hospitality and assistance during fieldwork; and the late G. Bailey, and S. Worth for comments on events and facts related to the study. I would also like to thank E. Veldtman of Emerald Utility Systems Ltd. for development of tailored data base software and cover page illustration, P. le Roux for assistance in data processing; H. Geertsema (University of Stellenbosch), G. Prinsloo (Plant Protection Research Institute), E. Kioko (International Centre of Insect Physiology and Ecology) and D. Barraclough (Natal Museum) for supplying information on parasitoid species; G. Olivier for supplying cocoons from Namibia; S. Chown for funding SEM examination of cocoons; P. Cloete (Roediger Agencies) for advising and assisting in impact tests and the Department of Polymer Science (University of Stellenbosch) for use of their impact tester; and L. Erasmus (University of Stellenbosch) for help with interpolation graphics. P. le Roux (University of Stellenbosch) and M. Warren (University of Pretoria) for valuable discussions. To my supervisors I express great appreciation for their time and effort: Melodie McGeoch, who constantly motivated me to do better and assisted me during difficult stages of my write up, as well as inspiring a deep interest in spatial ecology; and Clarke Scholtz for his supervision during the fieldwork stage of the project, support with resulting logistic problems and a for being a source of taxonomic expertise. To my family I am thankful for support and understanding throughout the duration of my study, as well as those academic years preceding it. To my wife I thank you for helping in almost every facet of the project as well as providing constant words of encouragement during its duration. The University of Pretoria, Liberty Life Trust and the National Research Foundation are thanked for funding this study. i University of Pretoria etd – Veldtman, R (2005) The ecology of southern African wild silk moths (Gonometa species, Lepidoptera: Lasiocampidae): consequences for their sustainable use Student: Ruan Veldtman1 Supervisors: Prof. Melodie A. McGeoch2 & Prof. Clarke H. Scholtz1 Departments: 1 Zoology & Entomology, University of Pretoria; 2 Conservation Ecology, University of Stellenbosch Degree: Doctor of Philosophy (Entomology) ABSTRACT The pupal cocoons of two southern African wild silkmoth species, Gonometa postica and G. rufobrunnea (Lepidoptera: Lasiocampidae), are composed of high quality silk and have potential as a commercially viable resource. However, limited ecological research has been done on these species, and their population dynamics is especially poorly known. A steady and predictable supply of cocoons is paramount to the economic sustainability of a wild silk industry. There is thus an urgent need for documenting and understanding the population dynamics of southern Africa’s Gonometa species. Here, the temporal and spatial variation of pupal (and thus cocoon) abundance, as well as associated natural enemies, are described for both Gonometa species for the first time. The larval parasitoid species emerging from parasitised pupae were quantitatively associated with species-specific emergence holes, making field-identification of these species possible. Eleven sites in total were sampled, over four generations, across the region where both species have historically reached high population densities. Apparent spatial synchrony in pupal abundance found between sites suggests that climate is responsible for observed population size fluctuations. As predicted from their life history traits, temporal variability was lower than expected for classically eruptive species. Gonometa species thus have an intermediate position on the population dynamics gradient. In turn, the responses of natural enemies were not predictable from Gonometa species defensive traits, but appear to be mediated ii University of Pretoria etd – Veldtman, R (2005) by between-species cocoon strength differences. Using data on the number of G. postica pupae per tree and associated parasitism at several sites, the importance of the degree of spatial explicitness in the quantification of aggregation and the detection of density dependence was illustrated. The spatially explicit method gave different results and more information regarding the spatial pattern of pupal abundance and parasitism than non- and semi-explicit methods. Similarly, the detection of density dependence in parasitism rates was affected by the use of spatially explicit data, with the spatial explicit approach giving different and more biologically informative results than traditional, non-spatially explicit methods. This has marked implications for previous insect-host - parasitoid studies aimed at detecting density dependence. The variability in cocoon size, a surrogate for larval performance, adult fecundity and silk yield, revealed that gender, followed by species, contributed most to observed size differences, with no clear differences between generations or localities. Finally, the between-host plant and within- host plant distribution of G. postica and G. rufobrunnea pupae was quantified, chiefly investigating the deterministic nature of the choice of pupation site. The distribution of both species at these scales was found to be markedly non-random, with pupae generally preferring specific tree characteristics and micro-sites. These results now provide the basis for recommending an appropriate utilisation strategy for southern Africa’s wild silk moths. Based on the spatial and temporal variability in pupal abundance observed, a constant and predictable cocoon supply for natural harvesting is unlikely. Long-term, broad-scale documentation of Gonometa species population cycles may make it possible to predict cocoon availability in the future. Until such research is done, it is recommended that the current practise of only collecting cocoons from which moths have emerged be continued. Simultaneously, artificial rearing and seeding as alternative utilisation strategies should be experimentally explored based on the information gathered and patterns identified here. iii University of Pretoria etd – Veldtman, R (2005) TABLE OF CONTENTS Acknowledgements………….…………………………………………………………. i Abstract…………………………..………………….…………………………………. ii Table of contents…………………………………………….…………………………. iv GENERAL INTRODUCTION……………………………………………………… 1 References………………………………………………………….. 15 CHAPTER 1: Predicting population dynamics and natural enemy responses from 19 herbivore life history and defensive traits Introduction………………………………………………………… 19 Methods…………………………………………………………….. 24 Results……………………………………………………………… 30 Discussion………………………………………………………….. 46 References………………………………………………………….. 52 CHAPTER 2: The parasitoids of southern African wild silkmoths (Lepidoptera) 58 References………………………………………………………….. 67 CHAPTER 3: Aggregation and spatial heterogeneity: from one dimension to the 69 next Introduction………………………………………………………… 69 Methods…………………………………………………………….. 80 Results……………………………………………………………… 84 Discussion………………………………………………………….. 93 References………………………………………………………….. 99 Appendix…………………………………………………………… 105 iv University of Pretoria etd – Veldtman, R (2005) CHAPTER 4: Spatially explicit host-parasitoid relationships: density dependence 106 revisited Introduction………………………………………………………… 106 Methods…………………………………………………………….. 114 Results……………………………………………………………… 119 Discussion………………………………………………………….. 129 References………………………………………………………….. 134 CHAPTER 5: Variability in cocoon size in southern African wild silk moths: 141 implications for sustainable harvesting Introduction………………………………………………………… 141 Methods…………………………………………………………….. 144 Results……………………………………………………………… 147 Discussion………………………………………………………….. 155 References………………………………………………………….. 157 CHAPTER 6: Fine-scale pupal abundance and distribution patterns of Gonometa 159 postica and G. rufobrunnea (Lepidoptera: Lasiocampidae) Introduction………………………………………………………… 159 Methods…………………………………………………………….. 162 Results……………………………………………………………… 170 Discussion………………………………………………………….. 194 References………………………………………………………….. 199 GENERAL CONCLUSION………………………………………………………… 206 References………………………………………………………….. 214 Appendices…………………………………………………………. 216 v University of Pretoria etd – Veldtman, R (2005) GENERAL INTRODUCTION “Spatial aggregation, or non-random search, by natural enemies, both predators and parasitoids, in response to patchy distributions of prey or hosts has a profound effect upon the population dynamics of victim and hunter populations.” Heads & Lawton 1983 “Claiming that an activity is sustainable requires us to predict the future. Reliable prediction of the future requires an especially profound understanding of the past and present. No qualifications