Combined effects of commercial insecticides on survival of the red mason bee Osmia bicornis

Insecticides are widely used to control pests, but can also be toxic to ecosystem service providers, including bees. Moreover, their efficacy can increase if they are applied in mixtures. These mixtures can be intentional, e.g., two or more active substances formulated in one product or created by farmers as tank mixtures (i.e., a mixture of different, individually formulated plant protection products (PPP) applied together in one application event or applied at short intervals), or unintentional, when exposure to a mixture results from bees foraging on different crops, each of which is treated with a different PPP. Although studies on the combined toxicity of various PPPs are increasing, the impact of mixture is still not adequately addressed in environmental risk assessment, posing a potential threat to bees. Here, we assessed the interactive effects of five insecticides belonging to different chemical classes: organophosphate Dursban 480 EC (a.s. chlorpyrifos), two pyrethroids – Sherpa 100 EC (a.s. cypermethrin) and Karate Zeon 050 CS (a.s. lambda-cyhalothrin), neonicotinoid Mospilan 20 SP (a.s. acetamiprid), and sulfoximine Closer (a.s. sulfoxaflor)) applied topically alone and as binary mixtures, on survival of Osmia bicornis females. Contrary to expectations, the experiment revealed either no interaction (in ‘Dursban × Sherpa’ experiment) or antagonistic interactions, particularly in mixtures of insecticide belonging to pyrethroids (Sherpa or Karate) with the one belonging to either neonicotinoids (Mospilan) or sulfoximines (Closer). Moreover, the mixture ‘Karate × Closer’ suggests a potential antagonistic effect on the survival of O. bicornis already at field-relevant concentrations. Such unexpected results emphasize the need for longer-term testing of cumulative toxicity effects via different exposure routes and on different bee species, as well as the need to consider the risk of exposure to multiple pesticides when assessing the safety of pesticides for bees.