INSECTICIDE EFFECTS ON SMALL MAMMALS - INFLUENCE OF VEGETATION STRUCTURE AND DIET

We tested whether differences in the diets of nontarget organisms or i n vegetation structure could result in adverse ecological effects not predicted by the Quotient Method, a laboratory-based risk assessment m ethodology used by the Environmental Protection Agency to evaluate pes ticides for registration and use. We established populations of herbiv orous gray-tailed voles (Microtus canicaudus) and omnivorous deer mice (Peromyscus maniculatus) in 24 0.2-ha enclosures planted with alfalfa (Medicago sativa) and monitored the populations by live trapping. Alf alfa in twelve enclosures was mowed on 22 June to reduce vegetation he ight, and 3 wk later we applied azinphos-methyl at 0, 0.88, and 3.61 k g/ha. We compared predictions of risk with observed effects on the sma ll mammals. Treatment with azinphos-methyl at 3.61 kg/ha reduced popul ation density and growth, survival, recruitment, and body growth of vo les in both mowed and unmowed enclosures. Survival of female voles exp osed to 3.61 kg/ha was lower in mowed than in unmowed enclosures, and the 0.88 kg/ha treatment affected body growth of male voles only in mo wed enclosures. Vole densities in 3.61 kg/ha enclosures remained depre ssed for greater than or equal to 6 wk after spraying. Azinphos-methyl did not detectably affect reproductive activity of female voles. Prec ipitation after spraying may have increased exposure of the mammals to azinphosmethyl, resulting in greater effects on voles than reported i n a previous, similar experiment. Deer mouse densities in mowed enclos ures receiving 3.61 kg/ha decreased 47% within 5 d after spraying. No other adverse insecticide effects on deer mice were significant, but c onfidence intervals for such effects were wide. Analysis of deer mouse feces indicated that consumption of arthropods just after spraying wa s greater in insecticide-treated enclosures than in controls. In gener al, the Quotient Method adequately predicted effects on the small mamm als, but its performance may be affected by vegetation structure and p recipitation.