Effects of ingested microplastic particles on susceptibility of chironomid midges to the insecticide spinosad compared to Bacillus thuringiensis israelensis

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Abstract
The potential effects of microplastics (MPs, < 5 mm) in aquatic systems, especially through interactions with other environmental stressors, are a growing concern. In a prior study, we found that ingested polyethylene MPs altered the toxicity of Bacillus thuringiensis israelensis (Bti) to the midge Chironomus riparius, with larger particles (125 µm) increasing Bti toxicity and smaller particles (34–50 µm) reducing it. The current study describes the effects of the same MP particles on the toxicity of spinosad, an insecticide with a neurotoxic mode of action. Large and small plastic particles were fed to larvae that were simultaneously exposed to spinosad (0, 10, 20, 40, 53, and 67 mg/L) for 21 days. Natural kaolin particles (1–10 μm) were used as a nonplastic control particle. Spinosad alone reduced larval survival and adult emergence as expected, while ingested kaolin and MPs had no effect by themselves. In contrast to our findings with Bti, none of the particles altered spinosad toxicity to larvae at 7 days. However, the smaller-sized MP reduced adult emergence compared to spinosad alone. This pattern suggests that MPs can alter spinosad efficacy, but the effect depends on particle size, and the physiological mechanism appears to be different from those observed with Bti. These findings reinforce the importance of using a variety of sublethal toxicological endpoints and accounting for interactions with co-occurring contaminants for assessing risks associated with MPs in freshwater systems.

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