Comparative histopathology of some Diptera and Crustacea of aquatic alpineecosystems, after treatment with Bacillus thuringiensis var. israelensis

Species of Diptera (larval Culicidae, Chironomidae and Simuliidae) and Crus tacea (cladoceran adults) that are representative of the fauna of aquatic a lpine ecosystems are routinely treated with Bacillus thuringiensis var. isr aelensis (Bti) for mosquito control. The toxicity of Bti on these organisms was investigated. Bioassays indicated that Bti used at the concentration f or operational field application is deleterious to all dipteran species, bu t not to Cladocera. Histopathological observations reveal that, in all case s, the midgut epithelium is affected by Bti treatment. However, the vulnera bility of epithelial cells to the microbial insecticide is different from o ne cell-type to another, clear cells being more vulnerable than dark cells. Because of the concentration of clear cells within the anterior midgut of dipteran larvae, this region appears to be the first to show symptoms of in toxication through enhancing a natural process of cell turnover. The relati ve importance of such a vulnerable region in the midgut of dipteran larvae may account for the general efficacy of Bti to these species. Its harmlessn ess to Cladocera may be correlated to the relative scarcity of clear cells and their patchy distribution along the whole midgut together with importan t cellular renewal capacity of the epithelium.