Effects of Guthion 2S∘ledR on Microtus canicaudus: A Risk Assessment Validation

Abstract

An assumption of the quotient method, a risk assessment methodology, was tested by determining if demographic responses of Microtus canicaudus Miller to five application rates of Guthion 25® were dose-dependent in field enclosures. Vole population sizes in enclosures with application rates of 1.5 5-4.67 kg ha -1 were depressed below levels in enclosures with application rates of 0.0 and 0.77 kg ha -1 for one trap period. Male and female survival rates were lower in enclosures with 1.55-4.67 kg ha -1 application rates than in enclosures with rates of 0.0 and 0.77 kg ha -1 , and these lower rates persisted for 2-6 weeks depending on sex and application rate. Populations were female-biased throughout the study, and no effects due to the 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. Our 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.