Microsporidian parasite impairs colony fitness in bumblebees

Emerging infectious diseases can have a major impact on the fitness of
novel hosts, thereby contributing to ongoing species declines. In social
insects, collaborative brood care by workers and successful mating of male
sexuals are key to colony fitness. The microsporidian
endoparasite, Nosema ceranae, has spread almost
globally, shifting across honeybee species and now to bumblebees. However,
despite N. ceranae being linked to recent population declines,
its possible impact on bumblebee colony fitness remains poorly understood.
Here, we show that N. ceranae infections can significantly impact
Bombus terrestris worker feeding glands, as well as the
longevity, sperm quality, and mating abilities of drones. In the
laboratory, workers and drones were exposed to either the parasite or not.
Then, parasite infection rates and loads, as well as lethal and sublethal
parameters were assessed. Infected drones revealed higher parasite
infection rates and spore titres as well as mortality compared to female
workers, suggesting sex-specific susceptibility. Furthermore, infections
impaired feeding glands, sperm traits, and altered mating behaviour - all
of which are key to colony fitness. Our findings provide a mechanistic
explanation on how N. ceranae contributes to the ongoing decline
of wild bumblebee populations, calling for respective mitigation measures.