THE EVOLUTION OF EGG CLUSTERING IN BUTTERFLIES - A TEST OF THE EGG DESICCATION HYPOTHESIS

Females of many insect species cluster their eggs. Egg clustering by l epidopteran species usually results in aggregation of larvae that are more often conspicuously coloured and apparently distasteful or unpala table than larvae of solitary species. While the costs and benefits of aggregation in terms of larval survival and growth are well documente d, the evolutionary ecology of egg clustering has been long debated an d is still unresolved. We tested the egg desiccation hypothesis, first proposed by Stamp (1980), which to our knowledge has never been exami ned experimentally. The egg desiccation hypothesis proposes that egg c lustering is adaptive per se (i.e. increases fitness of females) by re ducing egg mortality via desiccation. We tested this hypothesis for th e Nymphalid butterfly, Chlosyne lacinia, an egg-clustering species on its sunflower host plant, Helianthus annuus. We first documented natur al Variation in batch size for this butterfly. We then tested experime ntally hatch success of varying batch sizes and egg-layering arrangeme nts under controlled humidity levels. Hatch success was positively rel ated to relative humidity. Eggs in larger groups with greater number o f layers had greater hatch success than smaller, monolayered egg batch es, especially when relative humidity was low. Our results indicate th at, not only number of eggs, but also the arrangement (i.e. layering a nd density), increase batch survival by protecting eggs from desiccati on. However, despite increased hatch success in dense, multilayered cl usters, we found wide variation in layering and density in natural pop ulations of C. lacinia. This variation is probably maintained by trade -offs in egg survival, such as increased cannibalism of eggs by siblin gs, in dense clusters. Nevertheless, protection from egg desiccation p rovides an alternative explanation for the origin and maintenance of e gg clustering in lepidopterans and possibly other insects. The pattern of egg deposition in the Nymphalidae supports this hypothesis, since most North American species cluster their eggs tightly,whereas most sp ecies in tropical regions lay eggs singly or in loose monolayers.