CHRONIC EFFECTS OF LOW INSECTICIDE CONCENTRATIONS ON FRESH-WATER CADDISFLY LARVAE

The insecticide load in surface waters does not ordinarily reach conce ntrations acutely toxic to aquatic fauna. The effects of the low insec ticide concentrations typical of natural habitats are still not clear, for they often appear only after relatively long exposure times. To t est such a situation, the insecticides lindane and parathion were intr oduced into a static-with-renewal outdoor aquaria system at concentrat ions about four and five orders of magnitude lower than their respecti ve 96-h LC(50)s, and their chronic (about 90 days) effects on the surv ival rate of freshwater caddisfly larvae were observed. The emergence and hence survival rate of Limnephilus lunatus Curtis was significantl y reduced by lindane at 0.1 ng l(-1), a value nearly five orders of ma gnitude lower than the 96-h LC(50) Parathion, with acute and subacute toxicity similar to that of lindane, did not significantly alter the e mergence rate of this species. In contrast, this substance did produce a significant reduction in emergence rate of the closely related spec ies Limnephilus bipunctatus Curtis at 1 ng l(-1), even though this spe cies was significantly less susceptible than L. Lunatus to parathion a t high concentrations. We conclude that chronic insecticide exposure c an be hazardous to freshwater macroinvertebrates even at unexpectedly low concentrations. The low-concentration effects may depend on both s pecies and substance and therefore cannot be predicted from toxicity d ata at higher concentrations.