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 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.