COS 160-1 - Ecological predictors of pupal survival in a common North American butterfly

Background/Question/Methods

All holometabolous insects undergo a pupal life stage, a transformative period during which the insects are immobile and thus particularly vulnerable to both natural enemies and harmful abiotic conditions. For multivoltine species like the silver-spotted skipper [Epargyreus clarus (Cramer)], which produces both diapausing and non-diapausing generations throughout much of its range, both the duration of the pupal stage and the ecological challenges faced by pupae can differ among generations. We conducted a set of field and laboratory experiments to investigate the behavioral and ecological factors influencing the construction and persistence of pupal shelters by pre-pupal larvae, and measured the seasonal and annual variation in pupal mortality for E. clarus pupae experiencing different conditions.

Results/Conclusions

Surprisingly, measures of both cumulative and daily pupal predation were significantly higher during the relatively short (10-14 day) non-diapausing (summer) generations, compared with the diapausing (winter) generations, despite a nearly 20-fold longer pupal duration recorded for the latter. Indirect evidence from field censuses suggested that this intergenerational difference in mortality was due to seasonal variation in consumption of pupae by generalist vertebrate predators. The presence of a shelter increased survival in summer, though not in winter, perhaps because winter pupae were likely to be buried under autumnal leaf litter, regardless of initial shelter status. When constructing their shelters, pre-pupal E. clarus larvae did not prefer host leaves over non-host leaves, suggesting that induced preferences are unlikely to play an important role in this process. Despite finding marked differences in the decomposition rates of shelter leaves derived from host vs. non-host plants, several lines of evidence suggest that these differences are unlikely to impact E. clarus pupal mortality during either the summer or winter generations.