Overwintering instar plasticity and larval degree-day accumulation in the endangered Poweshiek skipperling butterfly (Lepidoptera: Hesperiidae)

Abstract

The Poweshiek skipperling, Oarisma poweshiek Parker, 1870 (Lepidoptera: Hesperiidae), is an endangered, univoltine grassland butterfly with extant populations in Michigan, United States and Manitoba, Canada. Larvae of this species are cryptic and have proven challenging to observe in situ . Features of Poweshiek skipperling life history, including the overwintering instar and larval degree-day requirements, have not yet been documented in Manitoba. In this study, we used macro photography to measure head capsule widths of more than 1000 Poweshiek skipperling larvae reared ex situ at Assiniboine Park Zoo in Winnipeg, Manitoba, Canada. Most larvae overwintered in the fourth instar and completed a total of six instars, but third and fifth instars also successfully overwintered. Two sixth instars failed to survive the winter. We produced pre-winter degree-day estimates for ex situ larvae and found significant differences in annual accumulations but no significant difference in overwintering instar between years. Our results address knowledge gaps related to Poweshiek skipperling biology that will benefit ongoing ex situ rearing efforts and in situ conservation actions. In addition, the noninvasive methods presented here would be ideal for studying the growth and development of other endangered insects.