Microplastics (MPs) are small plastic particles of less than 5 mm that may negatively affect aquatic organisms. Aquatic systems receive multiple pollutants, especially MPs and insecticides, from wastewater, urban stormwater runoff, and agriculture. Though a few studies have reported the combined effects of pesticides and MPs, interactions between MPs and spinosad have not been investigated. We hypothesized that ingested polyethylene microplastics (PE-MPs) both at smaller (34-50 µm) and larger (125 µm) sizes at 106 mg/L would alter the toxic effects of spinosad (0, 10, 20, 40, 53, and 67 mg/L) to sediment-feeding midge larvae as we observed for MPs and Bacillus thuringiensis israelensis in prior experiments. We monitored 7-day larval survival and 21-day emergence in controlled laboratory conditions and larvae were fed an artificial diet (DECOTABs) with and without PE-MPs. Dietary PE-MPs in different sizes did not affect the survival or emergence of C. riparius but reduced the toxicity of spinosad. This reduction might be due to MPs adsorbing chemicals on their surfaces, making them unavailable for midges to digest, and showing toxic effects. However, spinosad alone and combined with dietary PE-MPs significantly affected both larval survival and emergence since spinosad effects on feeding, digestion rates, locomotion, and oxidative stress. This study concludes that the interaction of multiple stressors is important due to the amplification/reduction of pollutant toxicity and altering the availability of pollutants that makes it difficult to assess the risk posed by multiple pollutants.