Safe efficacy of three strychnine alkaloid bait concentrations for hand-baiting control of plains pocket gophers

In November 1990, field efficacy studies using milo baits formulated with 0.35%,0.75%, or 1.30% strychnine alkaloid were compared to a placebo (0.0% strychnine) for controlling plains pocket gophers (Geomys bursarius) near Pleasanton, Texas. These data were required by the US Environmental Protection Agency (EPA) as partial fulfillment for the maintenance of the rodenticide registrations of the US Department of Agriculture. Each of four treatment units (TUs) within a block (2) was randomly assigned one of the four baits. Within each TU, 15 gophers were captured (balanced roughly for gender) and instrumented with radio transmitters. Following a pretreatment acclimation averaging 4.1 days, bait (4g) was placed in active pocket gopher burrows by hand-baiting. Pocket gopher mortality was measured by monitoring the fate of radio-equipped pocket gophers (n=123) both pretreatment and posttreatment. Lack of gopher movement on two consecutive days indicated death, and the carcass was retrieved. Strychnine mortality was based on chemical analyses of carcasses, and it occurred in 0.0%,66.7%,96.3%, and 89.7% of gophers from the 0.0%,0.35%,0.75% and 1.30% TUs, respectively. Natural mortality was 7% on the placebo TUs. All three strychnine treatments provided significantly increased mortality over the placebo (P<0.0001) using Fisher's exact test for paired comparisons. A difference in gopher mortality occurred between the 0.32% and 0.77% strychnine treatments (P=0.003), but not between the other comparisons (0.32% vs 1.30%, P=0.18 and 0.77% vs. 1.30%, P=0.24). Gopher carcasses recovered posttreatment indicated 68 of 86 (79.1%) had strychnine alkaloid residues. The non-target strychnine hazard (using least-squares means) by treatment were 4.85ppm(0.35%),8.04ppm(0.75%), and 9.47ppm(1.30%). Carcass residue differences were not detected among strychnine treatments (F=2.48,df=2,3,P=0.23). Fortunately, non-target exposure was greatly decreased because all carcasses with strychnine residues were recovered underground at a mean depth of 0.51m(SE=0.027, range 0.15–1.17m). Placebo-baited TUs had 27 survivors and 2 deaths from unknown causes. None had detectable strychnine levels. No non-target mortalities were documented during carcass searches and radio-tracking activities.