Secondary poisoning of birds of prey by the organophosphorus insecticide, phorate

Abstract

As a part of a monitoring study, five dead or debilitated bald eagles (Haliaeetus leucocephalus) and a red-tailed hawk (Buteo jamaicensis) from the Fraser River Delta of British Columbia (BC), Canada tested positive for residues of the organophosphorus insecticide, phorate. The first group of three birds was found during January and February 1992, from which two eagles were confirmed to have been poisoned by phorate. The second poisonings took place from December 1993 to March 1994, when three eagles and a hawk were confirmed as phorate poisoned. The brain and plasma cholinesterase results are presented along with data on chemical residue analyses of ingesta. Granular phorate, used in the area to control pests on potatoes and other root crops, appears to have a prolonged half-life similar to carbofuran and fensulfothion, two other granular insecticides associated with secondary mortality of raptors in the Fraser Delta. The soil persistence of phorate combined with a high acute toxicity poses a risk to waterfowl which are attracted to flooded fields in the autumn and winter and, thus, to raptors scavenging on dead waterfowl. The manufacturer has voluntarily withdrawn phorate, marketed as Thimet 15G, from the BC market as a result of the evidence presented here