Inert Ingredients in Agrochemicals Threaten Bee Health

This systematic review in *Proceedings of the Royal Society B* reveals that 'inert' ingredients in agrochemicals—commonly assumed non-toxic—pose significant, under-researched risks to honeybee health. Despite widespread use, regulatory frameworks do not require toxicity testing for these substances, unlike active ingredients. Only 19 studies have examined their effects, revealing mortality, impaired learning, reduced queen rearing, and synergistic interactions with other stressors. Key co-formulants like trisiloxanes and N-methyl-2-pyrrolidone persist in pollen and wax, with some surfactants blocking bee spiracles. Field-realistic concentrations increase insecticide toxicity. Chronic exposure leads to sublethal effects, including developmental issues in larvae. Current testing protocols, focused on acute neurotoxicity, fail to capture risks from inert ingredients, which may have higher field exposure levels. The term 'inert' is misleading; regulatory reforms are needed to mandate toxicity testing, adopt systems-based assessments, and address ecological impacts. Legislative barriers, including trade secret protections, hinder research. The European Commission has proposed banning certain co-formulants, but progress remains limited. Recommendations include prioritising research on sublethal and synergistic effects, revising terminology, and updating policies to protect bee health and support sustainable agriculture.