Reduced risk anticoagulant baiting strategies for California ground squirrels

The anticoagulants diphacinone and chlorophacinone (0.01% and 0.005% concentrations) are used extensively for control of California ground squirrels (Spermophilus beecheyi). Because of concerns of risks to nontarget wildlife, there is a need to develop baiting strategies that minimize the amount of bait applied, while still providing a high level of control. In 1997 and 1998, we conducted laboratory studies to determine the effect of timing and number of bait applications, and amount of bait given per application on the efficacy of 0.01% diphacinone for California ground squirrels. Results suggest that only two applications with 4 or 5 d between applications are necessary to achieve control. This is in contrast to the label recommendation of three to four applications with 48 h between applications (spot baiting), and two applications with 2 or 3 d between them (mechanical broadcast baiting). Furthermore, in our study, squirrels only required a small amount (less than 4 g of bait) per application to receive a lethal dose. Field studies are underway to test this baiting strategy and various bait application methods in the field

Use of zinc phosphide for California ground squirrel control

Zinc phosphide (ZnP) is the only acute rodenticide currently registered for control of the California ground squirrel (Spermophilus beecheyi). Research has shown ZnP to give excellent control of sciurid rodents, but operational control programs in California have reported poor and inconsistent control. We examined the literature and conducted 34 field trials between 1996 and 1999 in order to identify factors affecting the field efficacy of ZnP. Important factors identified from the literature include bait acceptance, prebaiting, and timing of control operations in relation to ground squirrel and vegetation phenology. We used ground squirrel counts or active burrow counts to assess the efficacy of ZnP in the field trials. Treatments were either mechanical broadcast or spot baiting of 2% ZnP-treated oat groats. The first field trials in 1996 and 1997 were conducted without bait acceptance tests and prebaiting, and control was inconsistent, ranging from none on one plot, poor on three plots (45% to 63%), to good on two plots (84% to 87%). Field trials in 1998 and 1999 were conducted with bait acceptance tests and pre-baiting. In 1998 control was excellent (88% to 100%) on all plots. However, control was variable in 1999 trials with good control (80% to 90%) on five plots, but poor control (60% to 79%) on two plots, and no control on one plot. In our studies, pre-baiting had little effect on the efficacy of the ZnP for controlling California ground squirrels