Long-Term Effects of Organic Insecticides on Two Species of Stonefly Naiads

Abstract

Two species of stonefly naiads, Acroneuria pacifica Banks and Pteronarcys californica Newport, were exposed to various organic insecticides for a period of thirty days in concentrations at or less than the 4-day TLm concentrations for each species. In general, the concentrations at or near the 4-day TLm concentrations were lethal to 50 per cent of the specimens within 4 to 10 days of continuous exposure. Less concentrated solutions produced toxic symptoms after 10 to 14 days. With some insecticides these symptoms appeared only after 21 days of continuous exposure. Normal molting of the specimens was inhibited in the aquaria at or near the 4-day TLm concentrations. In lesser concentrations molting was not reduced until after 21 to 25 days of exposure, at which time marked reductions in molting rates were observed.