A field test of the quotient method for predicting risk to Microtus canicaudus in grasslands

We used gray-tailed voles, Microtus canicaudus, as an experimental model species to field test a prediction of the Quotient Method (QM) for risk to small mammals of an insecticide in grasslands. In May 1997, we placed voles into 12 0.2-ha enclosures planted with a mixture of pasture grasses. In late July, we applied 1.55 kg/ha of the insecticide Guthion(R) 2S (azinphos-methyl) in three treatments: a control (all habitat sprayed with water), full spray (all of the habitat sprayed with Guthion 2S), and half-spray (one-half of the habitat sprayed with Guthion 2S and one half with water). Five replicates were used for the half-spray and control, and two replicates for the full-spray. The Guthion 2S treatment did not depress population size, growth rate, or survival of voles in half-spray or full-spray enclosures. Our results were inconsistent with the QM prediction for use of Guthion 2S in grass habitats. These results also differed from our previous studies in alfalfa habitats in which measurable responses were detected at the same application rate. The differences probably are due to the different types of vegetation between current and previous studies. The grass habitat in our enclosures was an average of 60 cm high and was very thick from ground level to about 40 cm. Much of the residue likely accumulated in the upper strata of vegetation and did not reach ground level at this application rate. Under the conditions of this study, voles were less affected in grassland habitat than they were in alfalfa habitat.