Factors affecting risk assessment of small mammals to pesticides

The quotient method (QM), a laboratory-based risk assessment methodology us ed by the U.S. Environmental Protection Agency to evaluate pesticides for r egistration and use, has not been thoroughly field-tested and its performan ce has not always been reliable. Our objective was to compare predictions o f risk using the QM with results from small mammal field studies to identif y factors that cause variation in the performance of the QM. We regressed e stimated mortality of herbivorous gray-tailed voles (Microtus canicaudus) a nd omnivorous deer mice (Peromyscus maniculatus) against predictions of mor tality based on LC50 probit analyses for pesticide concentrations in spray- tank mixtures and residues on the top and bottom 15 cm of alfalfa plants. P redictions of risk generally were correlated with estimated-mortality in th e field, but slopes often differed significantly from the predicted 1:1 rel ationship and were much greater than 1.0 for several relationships during t he 1993 experiment. Risk predictions may be influenced by a number of facto rs, including estimation of estimated environmental concentration (EEC), ve getation structure, weather patterns, species of concern, and demographic c haracteristics of the exposed populations. More accurate risk assessments m ay be accomplished by improving vegetation nomogram estimates of EEC and by incorporating vegetation structure and differences in natural history amon g species into QM predictions. Weather following application and the demogr aphic characteristics of the exposed populations are likely to remain unkno wn and will cause imprecision in risk assessments.