Monitoring great horned owls for pesticide exposure in southcentral Iowa

Organophosphorus pesticide (OP) residues occur in mice (Peromyscus spp.) inhabiting Iowa cornfields, suggesting a possible route of OP exposure to mouse predators. In 1988 and 1989 we livetrapped and radiotagged great horned owls (Bubo virginianus), common predators of mice, near cornfields in Iowa before and after OP applications of COUNTER 15G® (active ingredient terbufos, phosphorodithioic acid S-[[1,1-dimethylethyl)thio]methyl] O,O- diethyl ester) and LORSBAN 15G® (active ingredient chlorpyrifos, phosphorothioic acid O-(3,5,6-trichloro-2-pyridinyl) O,O-diethyl ester). We captured 32 owls over the 2-year period and radiomonitored 7 owls in 1988 and 15 owls in 1989. We analyzed blood plasma from livetrapped owls for depression and reactivation of cholinesterase (ChE) activities and fecal- urate samples for excretory metabolites to determine if owls were exposed to OPs before capture. Plasma ChE activities from 3 owls captured post- application in 1989 were >2 standard deviations of the control mean, indicating OP exposure. However, no reactivation in enzyme activities occurred in plasma ChEs incubated in the presence of 2-PAM (pyridine-2- aldoxime methochloride), and alkyl phosphate residues were below quantification limits in fecal-urate samples. Most habitats within the home ranges of radiotagged owls were nontreated, and home range size estimates (min. convex polygon and 50% harmonic mean activity areas) and percent-use of treated areas did not differ (P > 0.05) between pre- and post-pesticide application periods. No radiotagged owls died during the 2-year study. We conclude that the relatively large proportion of nontreated habitats within home ranges and the diversity of prey consumed limited OP exposure in the great horned owls monitored.