Inert agricultural spray adjuvants may increase the adverse effects of selected insecticides on honey bees (Apis mellifera L.) under laboratory conditions

Abstract Currently, more than 350 spray adjuvants are registered in Germany (January 2021). Unlike plant protection products (PPPs), adjuvants are not subjected to regulatory risk assessment. In practice, numerous combinations of PPPs and adjuvants are therefore possible. Thus, tank mixtures containing insecticides that are classified as nonhazardous to bees and approved for use in bee attractive crops may raise pollinator safety concerns when mixed with efficacy increasing adjuvants. This study analyzes whether selected “PPP-adjuvant” combinations result in increased mortality and pose an elevated risk to honey bees. To answer this question, we chose six common spray adjuvants of different classes for laboratory screening. These were then tested in a total of 30 tank mixtures, each with a neonicotinoid (acetamiprid), pyrethroid (lambda-cyhalothrin), diamide (chlorantraniliprole), carbamate (pirimicarb), and butenolide (flupyradifurone) formulation. We followed OECD test guideline 214 (acute contact test) but adopted the use of a professional spray chamber for more realistic exposures. Our results showed that, in total, 50% of all combinations significantly reduced the lifespan of caged honey bees in comparison to individual application of insecticides. In contrast, none of the adjuvants alone affected bee mortality (Cox proportional hazard model, p > 0.05). With four of the five insecticide formulations, the organosilicone surfactant Break-Thru® S 301 significantly increased bee mortality within 72 h (for all insecticides except chlorantraniliprole). Furthermore, acetamiprid yielded the highest and second-highest mortality increases from a tank mixture with the crop oil surfactants LI-700 (hazard ratio = 28.84, p < 0.05) and Break-Thru® S 301 (hazard ratio = 14.66, p < 0.05), respectively. In the next step, field trials should be performed to provide a more realistic exposure scenario under colony conditions to verify these findings.