Polystyrene Microplastics Modulate the Toxicity of the Hydrophilic Insecticide Thiacloprid for Chironomid Larvae and Also Influence Their Burrowing Behavior

As there is still little knowledge of interactions between microplastics (MP) and hydrophilic compounds, we propose ways the toxicity of hydrophilic pesticides can be modulated by MP, when sorption can be excluded. Larvae of Chironomus riparius were exposed to thiacloprid (TH, 1 µg/L) and polystyrene microplastic particles (PS; <50 µm; 150,000 and 1,000,000 particles/L) for 96 h, solely or in co-exposure. Burrowing behavior and mortality were observed. Larvae in treatments containing PS established themselves quicker in the sediment and kept the ability to rebury for a longer time compared to control and TH, respectively. While TH elevated the mortality, exposure to PS alone did not affect the survival of the larvae. In co-exposure of TH and PS, a concentration of 150,000 particles/L significantly reduced the toxicity of 1 µg/L TH after 96 h, an effect that was not observed at 1,000,000 particles/L. Therefore, we hypothesize that this modulation of the toxicity of TH eventually may have resulted from a combination of a ‘protective MP layer’ in the gut and a higher retention time of particles in larvae exposed to 150,000 particles/L than in those exposed to 1,000,000 particles/L due to the lower number of ingestible particles in the former.