Are reports of reduced field efficacy of diphacinone and chlorophacinone in California ground squirrels (Otospermophilus beecheyi) due to elevated rodenticide metabolism?

Abstract

Anticoagulant rodenticides are commonly used by agricultural producers to control California ground squirrels, Otospermophilus beecheyi, in a variety of crops. With this widespread use, producers are reporting reductions in the efficacy of diphacinone and chlorophacinone for ground squirrel control. Reductions in efficacy have been shown to be related to increased metabolism of compounds to non-toxic metabolites. This study examined liver metabolism of diphacinone and chlorophacinone using microsome isolations from squirrels captured in areas that had never used anticoagulants and areas with a history of prior use. Only male squirrels from anticoagulant exposed areas had a significant reduction in metabolic activity when compared to males from unexposed locations. Based on these data, the cause of the decreased anticoagulant rodenticide efficacy in ground squirrels does not seem to be related to anticoagulant rodenticide metabolism, but may be a result of other physiological (i.e. genetic resistance) or behavioral (bait avoidance) factors.