Factors Affecting Risk Assessment of Small Mammals to Pesticides

The quotient method (QM), a laboratory-based risk assessment methodology used by the U.S. Environmental Protection Agency to evaluate pesticides for registration and use, has not been thoroughly field-tested and its performance has not always been reliable. Our objective was to compare predictions of risk using the QM with results from small mammal field studies to identify factors that cause variation in the performance of the QM. We regressed estimated mortality of herbivorous gray-tailed voles (Microtus canicaudus) and omnivorous deer mice (Peromyscus maniculatus) against predictions of mortality based on LC50 probit analyses for pesticide concentrations in spray-tank mixtures and residues on the top and bottom 15 cm of alfalfa plants. Predictions of risk generally were correlated with estimated mortality in the field, but slopes often differed significantly from the predicted 1:1 relationship and were much greater than 1.0 for several relationships during the 1993 experiment. Risk predictions may be influenced by a number of factors, including estimation of estimated environmental concentration (EEC), vegetation structure, weather patterns, species of concern, and demographic characteristics of the exposed populations. More accurate risk assessments may be accomplished by improving vegetation nomogram estimates of EEC and by incorporating vegetation structure and differences in natural history among species into QM predictions. Weather following application and the demographic characteristics of the exposed populations are likely to remain unknown and will cause imprecision in risk assessments. Keywords—Azinphos-methyl Microtus canicaudus Peromyscus maniculatus Quotient method Risk assessment