Winter represents a challenging period for insects inhabiting temperate regions. A plethora of studies have investigated how abiotic environmental conditions such as temperature affect insect overwintering success. However, only a few studies have focused on biotic factors and the mechanisms affecting the overwintering performance of insects. Here, we investigated the effects of the parasitic fungus Hesperomyces virescens on the overwintering performance and immune system functioning of the invasive ladybird Harmonia axyridis. Winter survival was significantly lower for infected than for uninfected beetles. Body mass loss during overwintering tend to be higher for infected individuals compared to uninfected ones and for larger beetles. In addition, parasitic infection reduced post-winter longevity without food in male but not female ladybirds. Total haemocyte and protein concentration as well as antimicrobial activity against Escherichia coli significantly decreased during ladybird overwintering. However, haemolymph parameters were only poorly affected by Hesperomyces infection, with the exception of antimicrobial activity against Escherichia coli that tended to be higher in infected ladybirds. Interestingly, none of the pre-winter haemolymph parameters were good predictors of ladybird winter survival. Overall, our results indicate that energy exhaustion unrelated to immune system challenge is the most probable explanation for increased overwintering mortality in infected beetles.

- two laboratory overwintering experiments using computer-instructed climatic chambers

- comparison of uninfected and Hesperomyces virescens infected ladybirds

- ladybird survival and spring longevity measurements

- haemolymph sampling (haemocyte concentration, protein concentration, antimicrobial activity, microsporidia counts)