Symptomatic Infection with IVairimorpha/I spp. Decreases Diapause Survival in a Wild Bumble Bee Species

Our work shows that bumble bee queens that grew up in a colony infested by parasites are less likely to survive the winter diapause. However, infection levels in overwintering queens have little impact on survival length. The factor that impacts the survival length of infected queens the most is their body size, reflected in both body mass and head capsule size. Larger queens from infected colonies survived for longer periods than smaller queens, whereas no such effect was observed in queens from healthy colonies. The variation in body size among queens who grew up in infected colonies was much larger than among those from healthy colonies, suggesting inconsistency in nutrition availability in the maternal colony. Overall, our findings suggest that parasite infection can have indirect detrimental effects on diapause survival that can be mediated via nutrition. Vairimorpha, a microsporidian parasite (previously classified as Nosema), has been implicated in the decline of wild bumble bee species in North America. Previous studies examining its influence on colony performance have displayed variable results, from extremely detrimental effects to no observable influence, and little is known about the effects it has on individuals during the winter diapause, a bottleneck for survival in many annual pollinators. Here, we examined the effect of Vairimorpha infection, body size, and mass on diapause survival in Bombus griseocollis gynes. We demonstrate that gyne survival length in diapause is negatively affected by symptomatic Vairimorpha infection of the maternal colony but does not correlate with individual pathogen load. Our findings further indicate that increased body mass offers a protective effect against mortality during diapause in infected, but not in healthy, gynes. This suggests that access to adequate nutritional resources prior to diapause might offset the harmful effect of Vairimorpha infection.