CAN POPULATION EFFECTS OF PESTICIDES BE PREDICTED FROM DEMOGRAPHIC TOXICOLOGICAL STUDIES

Demographic toxicological analysis was evaluated as a method to estima te the total effect (lethal and sublethal) of pesticides on population s. Using the neem insecticide, Margosan-O and the pea aphid, Acyrthosi phon pisum (Harris), as a model, we questioned the necessity of evalua ting the response of different life stages to pesticides to estimate e ffects adequately at the population level, and whether pesticide persi stence should be considered. Population growth of the pea aphid was fo llowed after exposure of neonates in the Ist study and adults in the 2 nd study on broad bean, Vicia fava F., treated with Margosan-O. For al l of the parameters measured, the population exposed from birth was af fected more than the population exposed as adults. The intrinsic rate of increase (r(m)) for the population exposed from birth became negati ve when aphids were exposed to Margosan-O at the equivalent of 60 mg/l iter (ppm) azadirachtin. However, when adult aphids were exposed to Ma rgosan-O-treated plants, r(m) was not reduced even at rates as high as 100 ppm of azadirachtin. Thus, estimates of a population effect (r(m) ) of Margosan-O on A. pisum cannot be made by evaluating only one life stage even with a life table approach. The use of demography to predi ct population level effects of pesticides would therefore be misleadin g if done in the traditional way by exposing individuals from birth on ly. Persistence of Margosan-O, measured as mortality half-life, was 5. 8 d under the conditions of this study. The longer adult aphids surviv e exposure to Margosan-O and produce offspring, the less pesticide res idue will be available to cause to these offspring. Adults exposed to sublethal concentrations of a pesticide may act as a reservoir for the ir young. We argue that the actual effect of Margosan-O on a mixed age population of pea aphid cannot be determined with experiments that ev aluate only 1 animal life stage. We suggest that a stage-structured ap proach that includes the effects of temperature and pesticide persiste nce would improve the demographic method of toxicological analysis.