Inhibition of algae and invertebrates by malathion from insecticide-diffusing substrata

The organophosphorus insecticide malathion has been used in insecticide-dif fusing substrata (IDS) to reduce aquatic insect colonization and grazing ac tivity, thereby facilitating study of stream ecosystem function. Although t here has been some concern that malathion may provide a source of P, previo us investigators have assumed that malathion has no direct inhibitory effec t on algae. In a seasonal study of 12 New Zealand streams, malathion exposu re frequently caused significant decreases in algal biomass accrual, Inhibi tion of algal biomass accrual by malathion was more severe under conditions of enhanced nutrient supply. Malathion exposure also substantially reduced insect densities. However, statistically significant invertebrate response s to malathion exposure were uncommon because of high variability of insect densities on IDS. These results suggest that 1) direct inhibition of algal biomass accrual by malathion in IDS should not be discounted, and 2) malat hion-induced reductions in insect density and grazing pressure on IDS may b e spatially variable and difficult to detect without extensive replication.