Microplastics reach the brain and interfere with honey bee cognition

Scientific research exploring the impact of microplastics (MPs) in terrestrial systems is still at an early stage but has already confirmed that exposure to plastics leads to various detrimental health effects in several organisms. Although recent studies have shown the toxicological effects of single MP polymers on honey bees, the effects of different polymer combinations and their consequences on cognitive and behavioural performance remain unknown. To fill this knowledge gap, we investigated the effects of MPs, both individually and in combination, on the cognitive abilities of the honey bee Apis mellifera. We evaluated the acute oral toxicity of Polystyrene (PS) and Plexiglass (PMMA) MPs, as well as a combination of the two (MIX), at three different concentrations (0.5, 5 and 50 mg/L-1) and analysed their effects on sucrose responsiveness and appetitive olfactory learning and memory. We also explored whether these MPs could reach and accumulate in the insect brain using Two-Photon Fluorescence Microscopy (TPFM) in combination with an optimized version of the DISCO clearing technique. The results revealed that PS reduced the responsiveness of foragers to sucrose, whereas PMMA had no significant impact; however, the combination of PMMA and PS had a pronounced negative effect on sucrose responsiveness. In addition, both PMMA and PS, as well as MIX, impaired bee learning formation and memory retrieval, with PS exhibiting the most severe effects. Regarding our brain imaging analysis performed with TFPM, we found that after only three days of oral exposure, MPs could penetrate and accumulate in the brain. These results raise concerns about the potential mechanical, cellular, and biochemical damage that MPs may cause to the central nervous system.