Population responses of Peromyscus resident in Iowa cornfields treated with the organophosphorus pesticide COUNTER registered

We examined the effects of a field application of 1.41 kg/ha active ingredient COUNTER registered insecticide on resident small mammal populations in a corn agroecosystem in Iowa. The majority of individuals captured were Peromyscus maniculatus and Peromyscus leucopus; preliminary results showed no differential effects by species and further analyses were done at the generic level. The study design was a treatment-reference and before-after application comparison. Demographic parameters examined were: minimum number known alive, total number of captures as an index of activity, captures by habitat type, number of first captures (recruitment) and last captures (mortality plus emigration), sex ratio, age class structure, and several measures of reproductive activity. Brain ChE levels were also examined for treatment and reference animals. COUNTER registered residues were detected only in treatment site mice. There were no significant differences in most demographic parameters. There was a significant interaction between reproductive activity and time (three factor loglinear analysis); however the effect was consistent on the treatment and reference sites. There was a significant increase in activity measured as number of captures on the reference site relative to the treatment site post-application as well as a lower than expected number of captures in the cornfield habitat on the treatment site post-application; this did not appear to be related to mortality on the treatment site. The only possible treatment effect appeared to be some depression of activity, especially in the corn field; this is consistent with other published research on the effect of organophosphorus pesticides on small mammals in agroecosystems.