Beyond generalists: The Brassicaceae pollen specialist Osmia brevicornis as a prospective model organism when exploring pesticide risk to bees

Bees are under threat from agricultural intensification, and species which are pollen specialists (oligolectic) are thought to have declined disproportionately compared to pollen generalists (polylectic). When assessing the risks of dietary pesticide (plant protection products) exposure to non-target beneficial insects such as wild bees, effects on pollen specialist species have seldom been considered. Research and risk assessment on pesticide risk to bees mainly use a small selection of model species, only representing pollen generalist species. Moreover, the foraging preferences of the existing model species are not always adequately matched to the crops investigated, which may lead to incorrect conclusions regarding the risks posed by pesticides in pollen and nectar. Here, we propose Osmia brevicornis, an oligolectic European wild bee species specialized on Brassicaceae pollen, as a new model organism suitable for assessment of how pesticides can impact specialist pollinators, especially in oilseed rape, a mass flowering Brassicaceae crop. We demonstrate that O. brevicornis can be successfully reared in the field next to oilseed rape and that its nesting success and offspring numbers can be increased by setting out a starting population. In our field assay, nesting tube diameter affected occupation rate and the sex ratio of O. brevicornis offspring. We describe a method for housing and controlled oral administration of sucrose solution in the laboratory, facilitating future studies on pesticide exposure. We conclude that O. brevicornis is a feasible model for assessing the risk of pesticides in the laboratory and in the field, especially for those compounds used in oilseed rape cultivation, as well as for investigating the general ecology of pollen specialists. By suggesting O. brevicornis as a potential model species, we aim to encourage diversification of the species used in agricultural ecology, especially to consider pollen specialists, and encourage attention to the foraging preferences and dietary needs of selected model species when considering pesticide exposure risk and effects