Aquatic environments receive multiple stressors rather than individual entities from surrounding environments. These stressors not only pollute aquatic environments but have deleterious impacts on various aquatic biota. This study assessed the impact of multiple stressors using chironomid midges as the target organism. Here we exposed chironomid midges to two concentrations (50, 150 ng/L) of Bacillus thuringiensis israliensis (Bt), a bacterial insecticide, three concentrations (250, 500, and 750 mg/L) of a microplastics mixture, and combinations of Bt x microplastics. The microplastic mixture consisted of a 33:33:33 ratio of polyethylene (34-50 µm): polyethylene (125 µm) and a range of particle sizes of poly (ethylene terephthalate) prepared in the lab. Before the assay, a natural biofilm was cultivated on the microplastics by exposing them for 22 days to artificial water inoculated with stones from a local stream and maintained under plant grow lights (14:10). Triton X was used as a surfactant to sink the microplastics. Assays were initiated with first-instar larvae with 7-day survival and adult emergence as endpoints. The results showed that Bt and biofilm-covered microplastics at reduced the 7-day survival of larvae. Biofilm-covered microplastics at higher concentrations did not affect survival but delayed emergence time. When presented together, Bt augmented the microplastic toxic effects on chironomids at lower concentrations but not higher. The mode of action of Bt is primarily through lesions and disruption of the gut epithelium, but the likelihood of ingesting and succumbing to a lethal dose is also influenced by feeding rate and larval size. The effects of co-ingested microplastics on Bt toxicity to midges appear to be complex and concentration-dependent, but not in a linear way.