To pass on or not to pass on: The fate of orally ingested microplastic during cooperative brood care in two social Hymenoptera species

<title>Abstract</title> Anthropogenic pollutants contribute to insect decline in terrestrial ecosystems. Microplastic (MP), a major pollutant, is already present in all ecosystems and expected to further accumulate. However, studies on effects of MP on the ecologically important social insects are still scarce. In social insects, negative effects of MP could not only manifest on the individual but also on the colony level due to cooperative brood care. We tested whether MP particles taken up by workers of the Japanese carpenter ant ( <italic>Camponotus japonicus</italic> ) and the buff-tailed bumblebee ( <italic>Bombus terrestris</italic> ) are transferred to larvae during brood care. Both, ant and bumblebee workers, ingested MP particles with food. However, we only found MP particles in the digestive system of bumblebee larvae, but not ant larvae. This is likely due to the infrabuccal pocket (IBP), only present in the ants, that effectively prevents the transfer of particulate pollutants within colonies during cooperative brood care. In contrast, in bumblebees the unobstructed transfer of MP from workers to larvae may entail negative effects on larvae or carry-over effects during development. Thus, negative colony-level effects of pollutants on social Hymenoptera may be exacerbated in those species that lack an IBP.