Comparative temporal response of toxicity for the neonicotinoid clothianidin and organophosphate dimethoate insecticides in two species of solitary bee (Osmia bicornis and Osmia cornuta)

Solitary bees provide essential pollination services. Concerns for the decline of these wild bee species have led to calls for their inclusion in pesticide risk assessment. Solitary bees differ from honey bees in their physiology and ecology and this may affect how they respond to pesticide exposure. Here we investigate the life-time toxicity of two insecticides, the organophosphate dimethoate and neonicotinoid clothianidin, for two mason bee species, Osmia bicornis and O. cornuta using a toxicokinetic/toxicodynamic stochastic death model taken from Dynamic Energy Budget (DEBtox) theory. Both species showed concentration and exposure duration dependent effects for each chemical. LC₅₀ values estimated from the model parameters at 48 h were ≥ 14 fold and 6 fold those at 480 h for dimethoate and clothianidin respectively. Survival modelling indicated greater sensitivity in O. bicornis than for O. cornuta to dimethoate, whilst for clothianidin, O. cornuta females but not males, were more sensitive than both sexes of O. bicornis. These sensitivity differences were not related to body size. Toxicokinetic and toxicodynamic traits derived from modelling indicated lower elimination rates in O. bicornis and higher killing rates for O. cornuta females for dimethoate and lower elimination rates for clothianidin in O. cornuta females that were related to sensitivity. This study shows the near life-time testing is possible for solitary bees and that combining adult life-time toxicity tests with toxicokinetic/toxicodynamic modelling provides a more mechanistic understanding of pesticide effects in solitary bee species.