Unpacking the fitness consequences of a warmer spring on an overwintering butterfly

Declining butterfly abundance over recent decades coincides with both habitat loss and increasingly warmer springs. Warmer temperatures could pose a problem for ‘capital’ breeders, especially those species that enter winter diapause as a pupa or adult, since a higher metabolic rate pre-eclosion is expected to further deplete lipid stores needed for egg production. We undertook an experiment on Pieris napi, a butterfly that overwinters as a pupa, to assess to what extent access to nectar as an adult could compensate for a loss in body conditioning due to a warmer (6 °C vs. 2 °C) and or longer (195 vs. 145 days) post-diapause treatment. We found that a warmer and longer post-diapause time significantly increased the proportion of body mass lost between pupation and adult emergence, which in turn reduced lifetime fecundity for females in the water control treatment. However, when given access to a sugar/honey solution, a higher rate of egg laying and an extended lifespan allowed females to offset any fecundity disadvantage. The full consequences of a warmer post-diapause treatment were partially obscured by an elevated rate of egg laying during the first week, presumably due to a preceding effect of temperature on metabolic rate. Structural equation modelling suggests that the direct and indirect benefits of feeding on nectar as an adult double lifetime fecundity. We conclude that while potential reproduction may be limited by the availability of essential fatty acids acquired as a larva, other nutrients available in nectar are needed to realise a potential fecundity advantage. In this sense, butterflies could be considered an ‘income’ breeder, which can benefit from conservation actions that aim to provision more nectar resources in spring.