Ecological predictors of pupal survival in a common North American butterfly

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Allison Brackley and Martha R. Weiss, Biology, Georgetown University, John Lill, Biological Sciences, George Washington University, Washington, DC

Background/Question/Methods
All holometabolous insects undergo a pupal life stage, a transformative period during which most pupae are immobile and unable to flee from natural enemies or harmful abiotic conditions. For multivoltine species with both diapausing and non-diapausing generations, each experiencing different ecological challenges, pupal mortality is likely to vary across seasons. The use of protective microhabitats may also influence pupal mortality by mediating the risk of discovery by natural enemies, and/or buffering temperature and moisture extremes experienced by pupae resting within.
Epargyreus clarus, a native North American butterfly, pupates in leaf litter after constructing a leaf litter shelter for itself. Early-season generations of E. clarus produce continuously developing pupae while late-season generations produce diapausing pupae. We conducted a set of field and laboratory experiments to investigate 1) how pre-pupal larvae select litter leaves for shelter creation; 2) how long shelters last in the field before decomposing; 3) if leaf litter shelters improve pupal survival; and 4) how pupal survival varies over the season (comparing non-diapausing and diapausing pupae).
Results/Conclusions
Pre-pupal larvae did not preferentially select litter from their larval host plant to construct a pupal shelter; however, they may use other factors, such as leaflet size or leaf toughness, when choosing a substrate.
Litter from the leguminous host plants used by E. clarus decomposed quickly, while litter from the non-host canopy tree Quercus rubra had a much slower decomposition rate. Survival curves from two winter field experiments and three summer field experiments were similar within each seasonal category. Leaf litter shelters boosted survival by about 20% in the summer, but did not affect winter survival. Surprisingly, during the relatively short (10-14 day) summer pupal duration, daily per capita mortality rate (5% ± 1.6 SD) was an order of magnitude higher than that of diapausing (winter) pupae (0.3% ± 6-3 SD), despite a nearly 20-fold longer pupal duration recorded for the latter. We attribute the increased pupal mortality for the summer cohorts to higher mammalian predation during the warmer months.
Our results provide some of the first illustrations of the importance of protected microhabitats and seasonality in determining pupal survival, and add to our understanding of this neglected yet essential stage of the lepidopteran life cycle.