1) Laboratory techniques are described for testing the reactions of representative stream invertebrates to known time/concentrations of pesticides under conditions of water flow and continuous circulation and replacement.(2) A 1-h exposure to serial dilutions of pesticide, followed by a 24-h holding period in clean water, formed the first basis or comparison between the reactions of different species.(3) DDT was found to have a marked differential effect on predator invertebrates such as dragonfly naiads and Nepa on the one hand, and prey organisms such as Baetis naiads and Simulium larvae on the other. Many dragonfly naiads could survive an exposure to 20 ppm DDT for I h (and live long enough to produce adults), while concentrations as low as 0-05 ppm for 1 h could produce near 100% mortality in Baetis naiads and Simulium larvae.(4) Lebaycid (fenthion) produced strikingly different reactions in that its lethal effect on dragonfly naiads, Baetis naiads and Simulium larvae was nearly uniform with no obvious differential effect. Dragonfly naiads proved to be even more susceptible to fenthion than were net-spinning larvae of the caddis fly, Hydropsyche.(5) The uniform impact of Lebaycid (fenthion) on test invertebrates was not evident in other organophosphorus compounds tested, all of which^as well as the carbamate Baygon-showed varying degrees of differential effect.(6) In experiments in which concentration x period of exposure was kept constant, dragonfly naiads were found to be more tolerant to long exposures of low concentrations of Lebaycid (fenthion) than to short exposures (I h) of correspondingly higher concentrations.(7) Whereas both agrionid and libellulid dragonfly naiads were equally affected by short (I-h) exposures to Lebaycid (fenthion), agrionids appeared to be rather more tolerant than libelluiids to long exposures (24 and 48 h) at correspondingly lower concentrations.In the case of DDT, the situation was reversed in that libellulid naiads were more tolerant to prolonged exposures to very low concentrations than were agrionids.(8) The implications of these findings are discussed with particular reference to pesticide impact on predator/prey relationships in freshwater ecosystems.
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