Population Reduction of Richardson\u27s Ground Squirrels Using a Brodifacoum Bait

On 6 study plots (3 treated and 3 control of 2.47. acres--1 ha), we evaluated the efficacy of a 50 ppm brodifacoum grain bait on Richardson\u27s ground squirrels (spermophilus richardsonii Efficacy was measured by 3 methods: (1) radio-telemetry - 22 ground squirrels were live trapped on each treated plot and equipped with radio transmitters, (2) mark-recapture - by mark-recapture sampling methods and the computer program entitled CAPTURE, and (3) Tanaka\u27s formula - by using only marked survivors retrapped posttreatment. Treatment began the day following the pretreatment trapping period by systematically baiting each burrow entrance with approximately 16-19 g of bait. Overall efficacy of brodifacoum when measured by radio-telemetry was 98.1$, by mark-recapture 96.8% (SE x,1.5%) and by T-naka\u27s formula 99.1% (SE t 1.5%). Ground squirrels dying above ground varied between 18 and 64 among the 3 treated plots. The percentage of marked ground squirrels recovered dead above ground on the treated plots averaged 14.6% (range 6.8-24.5x). Non-target mortality on the treated plots included 3 vesper sparrows (Pooecetes gramineus and a white-tailed jackrabbit (L-enns t-ownsendi). Brodifacoum residues were 0.76 ppm for 1 vesper sparrow and at the level of detection (0.50 ppm) for the remaining vesper sparrows and the jackrabbit. The 50 ppm brodifacoum treatment tested on Richardson\u27s ground squirrels showed greater efficacy with less variability than zinc phosphide in previous studies on the same species