Proceedings of the Vertebrate Pest Conference

Three subspecies of the California ground squirrel (Spermophilus beecheyi) were studied. In the first test of Experiment 1, Beechey (S. b. beecheyi), Douglas (S. b. douglasii), and, for comparative purposes, Sierra (S. b. sierrae) ground squirrels were each offered a drinking tube containing one of three H20 solutions of strychnine sulfate (0.01, 0.05, or 0.5%) after being deprived of water for 23 hr. In Test 2 the survivors of Test 1 were offered for 24 hr a free choice of distilled water and two of the above concentrations of strychnine solutions. In Test 3 the survivors of Test 2 were offered for 24 hr a free choice of the same three concentrations of strychnine, but plain water was available. In the three tests, lethal amounts of strychnine solutions were consumed by 11 (79%) of the Douglas, none of the 14 Beechey, and 2 (18%) of the Sierra ground squirrels. In Experiment 2 the Douglas ground squirrels again proved to be the subspecies most susceptible to strychnine when compared with Beechey. In a third experiment, the squirrels did not reject strychnine bait on the basis of odor; however, in the fourth experiment Beechey ground squirrels that had been trained to reject strychnine-treated oat groats, a preferred grain, subsequently displayed much less interest in the olfactory cues from oat groats, suggesting that both previous experiences and olfactory cues are relied upon, at least in part, in their subsequent rejection of toxic food, even though strychnine sulfate may be odorless. These data help explain some of the problems associated with the use of strychnine as a rodenticide for the Beechey ground squirrel.