Rainfall and Guthion 2S interactions affect gray-tailed vole demography

The "quotient method" (QM), a pesticide risk-assessment model used by the U .S. Environmental Protection Agency,, assumes that the expected exposure co ncentration of: a contaminant is a function of application rate immediately after pesticide application. The QM does not take into account weather con ditions (eg, rainfall) at the time of spray. We used gray-tailed voles (Mic rotus canicaudus) as an experimental model species to field-test this assum ption of the QM by simulating a 0.25-cm rainfall. In June 1999, we placed v oles 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 (azinp hos-methyl) in four treatments; a dry control, a wet control ("rain"),a dry treatment (sprayed with Guthion 2S, no "rain"), and a wet treatment (spray ed with Guthion 2S and "rain" within 24 h). We used four replicate populati ons for each treatment. Survival rates of male voles in dry-treatment enclo sures declined throughout the rest of study following pesticide application , while male survival rates displayed short-term increases in other treatme nts. Rainfall improved male survival and may have mitigated the adverse eff ects of Guthion 2S. We also detected significant time X treatment interacti ons on population size and population growth rates of voles. Guthion 2S tre atment depressed population size and growth rate in the dry treatment; howe ver, rainfall may have reduced the risk of Guthion 2S to voles. The interac tion between rainfall and Guthion 2S application resulted in a deviation fr om the risk predicted by the QM.