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 lepidopteran species usually results in aggregation of larvae that are more often conspicuously coloured and apparently distasteful or unpalatable than larvae of solitary species. While the costs and benefits of aggregation in terms of larval survival and growth are well documented, the evolutionary ecology of egg clustering has been long debated and is still unresolved. We tested the egg desiccation hypothesis, first proposed by Stamp (1980), which to our knowledge has never been examined experimentally. The egg desiccation hypothesis proposes that egg clustering is adaptive per se (i.e. increases fitness of females) by reducing egg mortality via desiccation. We tested this hypothesis for the Nymphalid butterfly, Chlosyne lacinia, an egg-clustering species on its sunflower host plant, Helianthus annuus. We first documented natural Variation in batch size for this butterfly. We then tested experimentally hatch success of varying batch sizes and egg-layering arrangements under controlled humidity levels. Hatch success was positively related to relative humidity. Eggs in larger groups with greater number of layers had greater hatch success than smaller, monolayered egg batches, especially when relative humidity was low. Our results indicate that, not only number of eggs, but also the arrangement (i.e. layering and density), increase batch survival by protecting eggs from desiccation. However, despite increased hatch success in dense, multilayered clusters, we found wide variation in layering and density in natural populations of C. lacinia. This variation is probably maintained by trade-offs in egg survival, such as increased cannibalism of eggs by siblings, in dense clusters. Nevertheless, protection from egg desiccation provides an alternative explanation for the origin and maintenance of egg 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 species in tropical regions lay eggs singly or in loose monolayers.