The disadvantage of having a big mouth: the relationship between insect body size and microplastic ingestion

Abstract

Plastic pollution is ubiquitous, and animals are exposed to diverse plastic shapes and sizes. When plastics enter natural environments, they break down into microplastics (MPs; <5 mm) and likely become more accessible to smaller animals. Insects play critical environmental and economic roles, ingest plastics in the wild, and can physically degrade ingested MPs into smaller and more harmful nanoplastics. While particle size and body size undoubtedly impact plastic ingestion, we have no predictive understanding of how these factors interact to influence which plastics are a threat to which animals. To uncover these potential interactions, we studied how a model cricket species (Gryllodes sigillatus) interacts with plastics of differing sizes throughout a twentyfold change in body size during growth and development. We fed crickets a range of MP sizes of 38 to 500 {micro}m with clearly defined particle size thresholds. We investigated whether crickets would avoid MPs when given a choice and found that they do not; instead they gradually began to consume more of the plastic diet over time. We then studied how MP ingestion is influenced by body size and mouth size, and the extent of breakdown that occurs once MPs are ingested. We found that crickets would only consume whole beads when their mouth size was larger than the MP. While small MPs were more likely to be excreted whole, larger MPs were more extensively broken down as crickets grew. We conclude that crickets do not exhibit avoidance behaviour towards plastic and ingest it once a particle can be consumed whole. These effects of insect behaviour and body size on the likelihood of plastic ingestion and the degree to which MPs are degraded have important implications for regulating the size classes of plastic particles entering natural environments and how plastics move through those environments once discarded.