Revealing antagonistic interactions in the adverse effects of polystyrene and poly(methyl methacrylate) microplastics in bumblebees

Microplastics pose a significant ecological threat, yet their actual impact on terrestrial ecosystems and organisms remains poorly understood. This study investigates the effects of two common microplastics, poly(methyl methacrylate) (PMMA) and polystyrene (PS), on the pollinator <i>Bombus terrestris</i>, exploring their combined and sublethal effects at three different concentrations (0.5, 5 and 50 mg l^-1). PMMA and PS single exposure reduced bee survival in a concentration-dependent manner, whereas combined exposure (MIX) had no significant effect. PS reduced bee sucrose responsiveness, PMMA had no significant effect and MIX enhanced it. Learning and memory tests showed impaired mid-term and early long-term memory in bees exposed to PMMA and PS, with concentration-dependent effects. Interestingly, MIX exposure had no effect on memory retention. Our findings emphasize the differential effects of individual microplastics on bumblebee behaviour, suggesting potential risks to pollinator survival, cognitive function and possibly overall colony health, but also unexpected antagonistic interactions between these pollutants. The PS-PMMA antagonistic interactions highlight a challenge in assessing the toxicity of microplastics. Combined effects may not mirror the individual toxicity of PS and PMMA, highlighting the need for a careful assessment of polymer interactions, especially in environments or organisms contaminated by different microplastics.