Abstract. The ubiquitous use of agrochemicals is one driver for the ongoing loss of insect biomass and diversity. Data show that field-realistic concentrations of neonicotinoid insecticides can negatively affect both population density and the fitness of solitary bees. However, the underlying mechanisms for these effects remain poorly understood. Here, using an established semi-field experimental set-up and Osmia cornuta as a solitary bee model, we examined the effects of field-realistic concentrations of a common neonicotinoid insecticide (clothianidin) on male larvae and adults. Besides measuring lethal (i.e., overwintering success and adult survival) and sub-lethal endpoints (i.e., emergence mass and emergence duration), we examined, for the first time, potential effects on the male reproductive physiology of a solitary bee (i.e., sperm quantity and viability). The data revealed no significant effects on any of the measured response variables. This may be due to the low degree of clothianidin exposure (0.56 ng g−1) and/or the apparent low susceptibility of solitary bee larvae to neonicotinoids. Furthermore, it is conceivable that ideal foraging conditions, combined with optimal weather and lack of other environmental stressors, may have improved the ability of bees to cope with the insecticide. To reliably assess and understand the environmental hazards of agrochemicals, a holistic approach, including laboratory, semi-field and field data is required. Knowledge of underlying mechanisms will help to mitigate the current global declines of insect populations.
