Effects of Guthion 2S® on Microtus canicaudus: a risk assessment validation.
An assumption of the quotient method, a risk assessment methodology, was tested by determining if demographic responses of Microtus canicaudus to 5 application rates of Guthion 2S (azinphos-methyl) were dose-dependent in field enclosures in Oregon. Vole population sizes in enclosures with application rates of 1.55-4.67 kg/ha were depressed below levels in enclosures with application rates of 0.0 and 0.77 kg/h for one trap period. Male and female survival rates were lower in enclosures with 1.55-4.67 kg/ha than in enclosures with rates of 0.0 and 0.77 kg/ha, and these lower rates persisted for 2-6 weeks depending on sex and application rate. Populations were female-biased, and no effects due to pesticide application were detected in the proportion of females in the population. No pesticide effects were detected in reproductive performance, recruitment, activity, or male mass change, but a pesticide-related trend was evident in the cumulative number of recruits. Predictions of effects increasing with application rate were generally supported, but effects on M. canicaudus were of short duration, and the higher application rates may have caused a compensatory response in those populations.