Disentangling plasticity from local adaptation in diapause expression in parasitoid wasps from contrasting thermal environments: a reciprocal translocation experiment

An important question in evolutionary ecology is to understand the drivers of phenotypic variation in contrasting environments. Disentangling plasticity from evolutionary responses in such contexts provides a better understanding of how organisms adapt to changing climates. Many aspects of the seasonal ecology of insect populations are essential for their persistence in temperate environments, including their capacity to overwinter. Phenotypic plasticity should result in locally adapted diapause levels to experienced environmental conditions. Using an outdoor reciprocal translocation experimental design, we compared diapause expression of Canadian and French populations of aphid parasitoid species of the genus Aphidius in both their native and foreign bioclimatic regions, the insects thus experiencing varying temperature conditions under similar latitude (i.e. same photoperiod). From June to December 2016, diapause and mortality levels were recorded every 2 weeks. We found both genotypic (population origins) and environmental effects (experimental locations) on diapause reaction norms of parasitoid species. The incidence of diapause was higher in Canadian populations (up to 90%) than in French populations (< 20%) at both locations, suggesting local adaptation to harsh (Canadian populations) or mild (French populations) winter climatic conditions in the area of origin. Phenotypic plasticity played an important role in mediating diapause incidence at different temperatures but similar photoperiods, as diapause was induced at higher levels in Canada than in France, independently of the origin of the parasitoid. We conclude that both plastic and evolutionary responses could be involved in the adaptation of parasitoid overwintering strategies to different thermal environments and to ongoing climate warming.