Comparison of 0.005% and 0.01% diphacinone and chlorophacinone baits for controlling California ground squirrels (Spermophilus beecheyi)

Diphacinone and chlorophacinone, first-generation anticoagulant rodenticides, are frequently used for control of California ground squirrels (Spermophilus beecheyi) in agricultural and rangeland areas in California, USA. Owing to growing concerns over the risks to non-target species associated with the use of these rodenticides, the USA Environmental Protection Agency (EPA) proposed that the concentration of baits for above-ground use should be reduced from 0.01% to 0.005% active ingredient. We conducted field trials to compare the efficacy of 0.005% and 0.01% chlorophacinone and diphacinone baits in broadcast and spot applications for control of California ground squirrels on rangeland. We found no significant difference in efficacy owing to bait type, concentration or application method. Repeat testing is needed in other habitat types (e.g. crop areas) where alternative foods might reduce the effectiveness of a 0.005% bait application.