Species-specific effects of microplastic-contaminated pollen on larval development in two wild bee species (Apidae, Megachilidae; Apiformes)

Plastic production has increased for centuries, with a similar increase in the amount of microplastics released into and accumulating in soils and water bodies, detrimentally affecting organisms. One common transition is via air, during which microplastic particles likely also sediment onto plant parts, e.g., flowers, potentially contaminating floral resources and exposing pollinators to microplastics. In the laboratory, the detrimental effects of such contaminations in sugar water were shown in adult bees, usually honeybees (Apis mellifera), while corresponding experiments with larvae are still scarce. In this study, we tested the effect of pollen contaminated with a microplastics mixture in two concentrations (Low 0.5 mg/kg, High 50 mg/kg) on the development and survival of larvae of two wild bee species (Osmia bicornis and Bombus terrestris). Results indicate that Bombus terrestris larvae fed low microplastic concentrations consumed up to 20% less pollen and developed into significantly fewer pupae (up to 26%) as compared to larvae fed high microplastic concentrations and non-contaminated control pollen. Similarly, Osmia bicornis larvae fed low concentrations of microplastics developed more slowly, hatched later, and showed higher body mass and a higher conversion of pollen to body mass than larvae fed high concentrations of microplastics or the control. Responses differed between treatments and species, suggesting a non-monotonic pattern that warrants confirmation across a broader dose range. A possible explanation might be that higher concentrations activate immune and protective reactions not triggered by lower concentrations. The effects of microplastics on pollinators and other terrestrial organisms clearly warrant further research, as we are only beginning to understand the magnitude of the impacts these pollutants have on our ecosystems.