Sublethal effects of the organosilicone surfactant Silwet L-77 and its interactions with acetamiprid and sulfoxaflor on honeybee (Apis mellifera) gustatory responsiveness and olfactory learning

Pesticides remain indispensable for global food security, yet their use must be reconciled with the preservation of biodiversity. Despite advances in developing "safer" pesticides, their sublethal effects and synergistic potential with co-formulants in tank-mixtures, including "inert" spray adjuvants, often remain poorly understood in beneficial insects like bees. In this study, we assessed the effects of acute oral exposure to both single compounds and adjuvant-insecticide mixtures on gustatory responsiveness, associative olfactory learning, and 90-minute short-term memory retention in honeybee foragers (Apis mellifera L.). Specifically, individual foragers were exposed to either single compounds (0.1%, 0.5%, 1% or 2.5% SilwetTM L-77; 6, 30 or 150 ng acetamiprid; 3, 9 or 27 ng sulfoxaflor), adjuvant-insecticide mixtures (0.5% or 1% SilwetTM L-77 combined with 6 or 30 ng acetamiprid, or 3 or 9 ng sulfoxaflor), a positive control (6 ng imidacloprid), or negative controls (0%, 0.1%, and 1% acetone). While we found no significant interaction effects for these adjuvant-insecticide mixtures, single-compound exposure to the highest doses of 2.5% SilwetTM L-77 and 27 ng sulfoxaflor significantly reduced gustatory responsiveness. Furthermore, all three agrochemicals alone weakly affected associative learning without impacting short-term memory retention. Although adjuvant-insecticide mixtures did not significantly impair learning ability, these adjuvants may impact sensory perception. Moving beyond mortality-based assessments and including behavioural assay to test sensory perception and learning ability is essential for gaining a more holistic understanding of pollinator health.