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The “quotient method” (QM), a pesticide risk-assessment model used by the U.S. Environmental Protection Agency, assumes that the expected exposure concentration of a contaminant is a function of application rate immediately after pesticide application. The QM does not take into account weather conditions (e.g., rainfall) at the time of spray. We used gray-tailed voles (Microtus canicaudus) as an experimental model species to field-test this assumption of the QM by simulating a 0.25-cm rainfall. In June 1999, we placed voles into 16, 0.2-ha enclosures planted with a mixture of pasture grasses. In early August, we applied 2.44 kg/ha of the insecticide Guthion 2S (azinphos-methyl) in four treatments; a dry control, a wet control (“rain”), a dry treatment (sprayed with Guthion 2S, no “rain”), and a wet treatment (sprayed with Guthion 2S and “rain” within 24 h). We used four replicate populations for each treatment. Survival rates of male voles in dry-treatment enclosures declined throughout the rest of study following pesticide application, while male survival rates displayed short-term increases in other treatments. Rainfall improved male survival and may have mitigated the adverse effects of Guthion 2S. We also detected significant time × treatment interactions on population size and population growth rates of voles. Guthion 2S treatment depressed population size and growth rate in the dry treatment; however, rainfall may have reduced the risk of Guthion 2S to voles. The interaction between rainfall and Guthion 2S application resulted in a deviation from the risk predicted by the QM. |
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