Seasonal variation rather than ectoparasitic fungi infection affects hemolymph parameters, thermal stress tolerance, and movement performance of an invasive ladybird.

Despite the prevalence of both Harmonia axyridis and its fungal ectoparasite Hesperomyces harmoniae across the globe, few studies have explored the impact of Hesperomyces harmoniae on the immune system and thermal performance of Harmonia axyridis, both of which are important traits for the continued geographic range expansion of this invasive species. Since He. harmoniae does impose a cost on Ha. axyridis (overwintering survival), we predicted that infected ladybirds collected from the field would have an increased immune response and decreased phenotypic performance compared to uninfected ladybirds. Moreover, these effects would depend on the season in which ladybirds were collected (summer growing vs autumn pre-overwintering season). In general, collection season had a significant effect on all physiological traits we assayed. All hemolymph parameters significantly decreased from summer to autumn. Ladybird movement activity and range of utilized temperatures increased from summer to autumn, whereas, average preferred temperature decreased from summer to autumn. Unsurprisingly, summer ladybirds were more heat tolerant and autumn ladybirds were more cold tolerant. He. harmoniae infection had almost no impact on any of the investigated traits. The only exception to this was with respect to cold tolerance, He. harmoniae infected ladybirds being less cold tolerant than uninfected ladybirds. Therefore, infection by this fungus does not appear to be physiologically severe enough to elicit a whole-body negative effect on its host ladybird, but instead, may deplete specific internal resources that negatively impact cold tolerance.