MONITORING GREAT HORNED OWLS FOR PESTICIDE EXPOSURE IN SOUTH-CENTRAL IOWA

Organophosphorus pesticide (OP) residues occur in mice (Peromyscus spp .) inhabiting Iowa cornfields, suggesting a possible route of OP expos ure to mouse predators. In 1988 and 1989 we livetrapped and radiotagge d great horned owls (Bubo virginianus), common predators of mice, near cornfields in Iowa before and after OP applications of COUNTER 15G(R) (active ingredient terbufos, phosphorodithioic acid S-[[(1,1-dimethyl ethyl)thio]methyl] O,O-diethyl ester) and LORSBAN 15G(R) (active ingre dient chlorpyrifos, phosphorothioic acid O-(3,5,6-trichloro-2-pyridiny l) 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 p lasma from livetrapped owls for depression and reactivation of choline sterase (ChE) activities and fecal-urate samples for excretory metabol ites to determine if owls were exposed to OPs before capture. Plasma C hE activities from 3 owls captured post-application in 1989 were >2 st andard deviations of the control mean, indicating OP exposure. However , no reactivation in enzyme activities occurred in plasma ChEs incubat ed in the presence of 2-PAM (pyridine-2-aldoxime methochloride), and a lkyl phosphate residues were below quantification limits in fecal-urat e samples. Most habitats within the home ranges of radiotagged owls we re 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 per iods. No radiotagged owls died during the e-year study. We conclude th at the relatively large proportion of nontreated habitats within home ranges and the diversity of prey consumed limited OP exposure in the g reat horned owls monitored.