Two species of stonefly naiads were exposed to the chlorinated hydrocarbons DDT, aldrin, dieldrin, and endrin and to the organic phosphates parathion, malathion, guthion, Dylox, Di-Syston, and Bayer-29493. After the introduction of the pesticide, tests were run for 96 hours with observations made after each 24-hour period. There was a definite increase in mortality for both Pteronarcys californica and Acroneuria pacifica with increasing exposure time. Higher concentrations of insecticides produced higher death rates more rapidly than lower concentrations. There were definite symptoms of poisoning noted during the 4-day bioassays. The symptoms did not vary for the two species of stonefly naiads. Animals displaying tremors or convulsions usually died within 24 hours. Both species could recover from the effects of the poisons if they were removed from the solutions of toxicant before the tremors began. Removal from the test solution after the onset of tremors did not prevent death. With the same insecticide, the resistance of P. californica was greater than that of A. pacifica. Larger specimens of P. californica were more resistant than smaller specimens.
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