Em. Schauber et al., INSECTICIDE EFFECTS ON SMALL MAMMALS - INFLUENCE OF VEGETATION STRUCTURE AND DIET, Ecological applications, 7(1), 1997, pp. 143-157
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.