Resident viruses, but not honeybee-associated viruses, impair solitary bee fitness in the field.

Viruses can impact individual host fitness and host population dynamics, especially following host shifts. Thus, the decline of wild solitary bee populations over the last few decades may be linked to viruses or other pathogens. However, evidence for the impact of viruses-transmitted from other genera or resident in solitary bees-on their fitness remains scarce. Here, by assessing solitary bee (Osmia cornuta) foraging, offspring sex ratio, survival and body mass across seven locations in northern Switzerland, we show that resident viruses-but not honeybee-associated viruses-can impact fitness proxies in the field. Loads of Osmia-resident viruses (Ganda bee virus-GABV; Scaldis River bee virus-SRBV) and honeybee-associated viruses (black queen cell virus-BQCV; deformed wing virus B-DWV-B) were quantified in foraging females. Prevalence and loads of GABV and SRBV were higher than BQCV and DWV-B. Females with high SRBV or GABV loads had reduced offspring survival or lower male offspring body mass, respectively. Honeybee-associated viruses had no impact on O. cornuta fitness proxies. We demonstrate that viruses can affect solitary bee fitness negatively, but the degree of impact varies with viral species and provenance. Further research is needed to unravel the dynamics of multi-host pathogens in pollinator communities.