Complex drivers of phenology in the pine processionary moth: Lessons from the past

Climate change affects the life cycle of many species. Yet, responses to yearly variation of weather can either help species track optimal conditions or be maladaptive. We analysed phenological data of 46,479 pine processionary moths (Thaumetopoea pityocampa) during 15 years along an altitudinal gradient in southern France. These larvae were sampled in situ and allowed to pupate in a common garden at lower elevation. Individuals originating from higher elevation emerged earlier than those sampled at low elevation, which suggests local adaptation. Yearly variations in temperature also affected phenology. Warm springs caused an earlier adult emergence, while autumn temperatures had an opposite effect. Environmental cues could thus induce contradictory plastic responses. Synchronization mechanisms were identified. Variability in the duration of the pupal phase is a key parameter to synchronize adult emergence in spite of different larval development rates that only marginally influenced emergence dynamics. Semivoltine individuals experiencing prolonged diapause were synchronized with univoltine individuals emerging the same year. These data highlight some contradiction in the effect of spatial versus temporal variations of the temperature on adult emergence. This suggests that phenological responses to the current climate change cannot easily be anticipated by space-for-time substitution designs.