Declining ring-necked pheasants in the Klamath Basin, California: I. Insecticide exposure

A study of organophosphorus (OP) insecticide exposure was conducted on a de clining population of ring-necked pheasants (Phasianus colchicus) associate d with agricultural lands at Tule Lake National Wildlife Refuge (TLNWR) dur ing the summers of 1990-92. Findings at TLNWR were compared with a nearby p heasant population at Lower Klamath National Wildlife Refuge (LKNWR) not su bjected to intensive farming or OP insecticide applications. Direct toxicit y of anticholinesterase (antiChE) compounds (in this case methamidophos) ki lled 2 young pheasants (91 and 92% brain acetylcholinesterase [AChE] inhibi tion), but no deaths of adult radio-equipped hens were ascribed to direct i nsecticide intoxication. However, within 20 days postspray of OP insecticid es, 68% (28 of 41) of the adult pheasants collected at TLNWR were exposed t o antiChE insecticides, and exhibited brain AChE inhibition of 19-62%, with 15% (6 of 41) showing greater than or equal to 55% brain AChE inhibition. The lack of radio-equipped hens dying was unexpected because >50% brain ACh E inhibition has been frequently used as a 'diagnostic tool' for evaluating cause of death from antiChE insecticides. No young were radio-equipped, so the extent of the effects of insecticide exposure on the survivorship of y oung was unknown. It is concluded that insecticide exposure was not the maj or factor impacting the pheasant population (see Grove et al., in press), a lthough some young were acutely intoxicated. However, the loss of insects k illed by insecticide use may have contributed to food shortages of young ph easants, indirectly influencing survival.