Can Population Effects of Pesticides Be Predicted from Demographic Toxicological Studies?

Abstract

Demographic toxicological analysis was evaluated as a method to estimate the total effect (lethal and sublethal) of pesticides on populations. Using the neem insecticide, Margosan-O and the pea aphid, Acyrthosiphon pisum (Harris), as a model, we questioned the necessity of evaluating the response of different life stages to pesticides to estimate effects adequately at the population level, and whether pesticide persistence should be considered. Population growth of the pea aphid was followed after exposure of neonates in the 1st study and adults in the 2nd study on broad bean, Vicia fava F., treated with Margosan-O. For all of the parameters measured, the population exposed from birth was affected more than the population exposed as adults. The intrinsic rate of increase (rm) for the population exposed from birth became negative when aphids were exposed to Margosan-O at the equivalent of 60 mg/liter (ppm) azadirachtin. However, when adult aphids were exposed to Margosan-Otreated plants, rm, was not reduced even at rates as high as 100 ppm of azadirachtin. Thus, estimates of a population effect (rm) 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 predict population level effects of pesticides would therefore be misleading if done in the traditional way by exposing individuals from birth only. Persistence of Margosan-O, measured as mortality half-life, was 5.8 d under the conditions of this study. The longer adult aphids survive exposure to Margosan-O and produce offspring, the less pesticide residue will be available to cause damage to these offspring. Adults exposed to sublethal concentrations of a pesticide may therefore act as a reservoir for their young. We argue that the actual effect of Margosan-O on a mixed age population of pea aphid cannot be determined with experiments that evaluate only 1 animal life stage. We suggest that a stage-structured approach that includes the effects of temperature and pesticide persistence would improve the demographic method of toxicological analysis.