Winter temperature predicts prolonged diapause in pine processionary moth across its geographic range

Prolonged diapause occurs in a number of insects and is interpreted as a way to evade adverse conditions. The winter pine processionary moths (Thaumetopoea pityocampa and Th. wilkinsoni) are important pest of pines and cedars in the Mediterranean region. They are typically univoltine, with larvae feeding across the winter, pupating in spring in the soil and emerging as adults in summer. Pupae may, however, enter a prolonged diapause with adults emerging one or more years later. We tested the effect of spatial variation in winter temperature on the incidence of prolonged diapause, using a total of 64 individual datasets related to insect cohorts over the period 1964-2015 for 36 sites in 7 countries, covering most of the geographic range of the species. We found high variation in prolonged diapause incidence over the species’ range. Insect cohorts exposed to average winter temperatures lower than 0°C were associated with higher prolonged diapause incidence than cohorts exposed to intermediate temperatures. Prolonged diapause may represent a risk-spreading strategy although it is associated with high mortality because of a longer exposure to mortality factors, desiccation, and energy depletion. Climate change, and in particular the increase of winter temperature, may reduce the incidence of prolonged diapause at the colder sites whereas it may increase it at the warmer ones, with consequences on the population dynamics.