Brain cholinesterase activity and brain and liver residues in wild birds of a forest sprayed with acephate

Abstract

AbstractIn May 1981, acephate (O,S‐dimethyl‐acetylphosphoramidothioate) was applied at 0.85 kg [a.i. (active ingredient)]/ha in a ponderosa pine (Pinus ponderosa Douglas) forest of northern Arizona for suppression of a pandora moth (Coloradia pandora Blake) infestation. Brain cholinesterase (ChE) activity was determined in six species of wild birds, and brain and liver residues were determined in five species. Brain ChE was not inhibited on the day of spraying, but ChE was significantly inhibited 2 d after acephate application. The species of birds which presumably had the smallest territories and therefore would be more likely to be confined to the relatively small spray plots (about 250 ha) had the greatest ChE inhibition, both in magnitude and in numbers of birds with inhibition. Acephate and methamidophos (O,S‐dimethyl‐phosphoramidothioate) residues in brain and liver were highest on the day of spraying and decreased to undetectable concentrations 14 d after application. No correlation between brain ChE activities and brain or liver residue concentrations was found. The results of this study suggest that brain ChE inhibition found after acephate application is a better indicator of exposure than is tissue residue analysis. Brain ChE inhibition is probably due to a metabolite of acephate that is not detected by the methods used. Birds exposed to acephate in the wild may suffer as yet undetermined sublethal effects, but it is unlikely that acephate applied at 0.85 kg/ha or less will kill many birds.