Larval development and diapause induction under climate change: an experimental test with the wall brown butterfly

Under climate change, organisms with complex life cycles like insects are confronted with environmental cues that may interfere with their life cycle regulation, including diapause induction. Maladaptive responses to novel conditions may contribute to local or regional population declines and extinctions. In northwest Europe, the wall brown butterfly Lasiommata megera is a multivoltine species whose regional decline has been hypothesised to be related to recent warming in late summer and autumn increasing the probability of producing a late third generation instead of going into larval diapause to hibernate. To test this lost generation hypothesis, we exposed young larvae of different population origins to outdoor conditions and in situ warming (i.e. increased temperature at the level of the host plant by 0.5–3°C depending on the month). The warming treatment resulted in shorter larval developmental times and reduced adult body size. However, the population origin and the temperature treatment did not affect the probability to produce a third generation of adult butterflies at the end of summer. Yet, families showed substantial variation for the probability to induce a third generation. Overall, we observed reduced larval survival in the warming treatment. We discuss the implications of these results for the conservation of the wall brown in northwest Europe and, in a broader manner, for life cycle regulation in insects under rapid climate change.