Histopathological effects of tannic acid on the midgut epithelium of some aquatic Diptera larvae

The impact of tannins on larval Nematocera was investigated by an extensive survey of the relative toxicity of tannic acid in Diptera larvae represent ative of mosquito communities from alpine hydrosystems (Culicidae, Chaobori dae, Chironomidae, and Simuliidae) together with a nonindigenous vector com petent Culicidae species. Bioassays indicate that exposure to tannic acid a t concentrations from 0.25 to 4 mM is deleterious for Culex pipiens, Simuli um variegatum, and Chironomus annularius, but not for Aedes, Anopheles, Cul iseta, and Chaoborus species. Histopathological observations reveal that, a mong the target organs of tannic acid, mainly the midgut epithelium is affe cted by treatment. However, the extent of degeneration varies according to the taxon, the duration of the treatment, and the concentrations assayed. T he vulnerability of epithelial cells differs among cell types, clear cells of the anterior midgut showing symptoms of intoxication before dark cells o f the posterior midgut. The toxic effects of tannic acid are discussed, par ticularly in comparison to those of insecticidal bacteria, in order to eval uate the potential for use of tannins in the regulation of larval populatio ns of dipteran pests. (C) 1999 Academic Press.