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 representative of mosquito communities from alpine hydrosystems (Culicidae, Chaoboridae, Chironomidae, and Simuliidae) together with a nonindigenous vector competent Culicidae species. Bioassays indicate that exposure to tannic acid at concentrations from 0.25 to 4 mM is deleterious for Culex pipiens, Simulium variegatum, and Chironomus annularius, but not for Aedes, Anopheles, Culiseta, and Chaoborus species. Histopathological observations reveal that, among the target organs of tannic acid, mainly the midgut epithelium is affected by treatment. However, the extent of degeneration varies according to the taxon, the duration of the treatment, and the concentrations assayed. The vulnerability of epithelial cells differs among cell types, clear cells of the anterior midgut showing symptoms of intoxication before dark cells of the posterior midgut. The toxic effects of tannic acid are discussed, particularly in comparison to those of insecticidal bacteria, in order to evaluate the potential for use of tannins in the regulation of larval populations of dipteran pests.