Microplastics (<5 mm) are emerging freshwater contaminants that can have a wide range of effects on aquatic biota. One concern is that combined effects of microplastics (MPs) with other stressors, such as co-occurring contaminants in urban or agricultural runoff may be significant even when the direct effects of MPs may be modest. Despite the frequent detection of both insecticides and MPs in freshwater ecosystems, there is a lack of co-exposure studies of insecticides (especially Bacillus thuringiensis israelensis (Bti)) and MPs. Here we tested the effects of ingested MPs and Bti individually and in co-exposure using the aquatic midge Chironomus riparius as a model organism. First instar larvae were fed two sizes of white polyethylene particles (34-50 and 125 μm diameter) at 106 mg/L in an artificial diet and simultaneously exposed to increasing concentrations of Bti (7, 13, 27, 53, and 89 ng/L Active Ingredient) in the water column for 21 days. For comparison, a trial was also conducted with naturally occurring kaolin clay particles (1-10 μm diameter) at 106 mg/L in the artificial diet. Bti alone reduced 7-day larval survival at higher concentrations (53, and 89 ng/L). Dietary PE-MPs and kaolin did not affect the survival of C. riparius larvae. However, when exposed in combination, PE-MPs modified the toxicity of Bti. This modification was size-dependent, with smaller particles (34-50 μm) increasing survival of Bti-exposed larvae and larger particles (125 μm) reducing survival. Our results show the potential for microplastics to alter the efficacy of an insecticide widely used to control nuisance midges and mosquitoes and add to a growing body of literature describing how the toxicological effects of microplastics are influenced by the size and shape of particles.