The fate of orally ingested microplastics during cooperative brood care in two social Hymenoptera species

Anthropogenic pollutants contribute to insect decline in terrestrial ecosystems. Microplastics (MP), a major pollutant, are already present in all ecosystems and expected to further accumulate. In social insects, negative effects of MP could not only manifest on individual but also colony level due to cooperative brood care. However, food is transferred differently during cooperative brood care among social Hymenoptera species. Therefore, it can also vary whether particles are passed on with food. Consequently, depending on the strategy of food transfer, the different life stages of a species could be affected by MP to varying degrees. To better assess interspecific differences, we comparatively investigated the fate of polystyrene MP particles during cooperative brood care in colonies of the Japanese carpenter ant ( Camponotus japonicus ) and the buff-tailed bumblebee ( Bombus terrestris ). Here we show that both, ant and bumblebee workers, ingested MP particles with their food. However, we only found MP particles in the digestive system of bumblebee workers and larvae, but not in ant larvae and only occasionally in workers. This is likely due to the infrabuccal pocket (IBP), only present in the ants, that effectively prevents the transfer of particulate matter within colonies during cooperative brood care. In contrast, the unobstructed transfer of MP from bumblebee workers to larvae may entail negative effects on larvae or carry-over effects during development. Thus, negative colony-level effects of MP particles on social Hymenoptera may be exacerbated in those species that lack an IBP.