The Hazards of Secondary Poisoning from Zinc Phosphide to Selected Vertebrate Species

The objectives of this study were to determine: (1) the effects of weather upon toxicity of grains treated with zinc phosphide, (2) the acceptance by prairie voles of weathered bait, (3) the LD₅₀ of zinc phosphide on selected rodent species, and (4) the potential hazard of secondary poisoning to predators fed prey poisoned with zinc phosphide.

The rate of loss of zinc phosphide from the bait vehicle was significantly greater during the first week of exposure than in the succeeding three weeks; approximately 34 and 31 percent of the toxic material was lost in April and November, respectively, during the first week the bait was placed in the field.

White mice (Mus musculus) were fed weathered bait to evaluate the biological significance of weathering on the rodenticide. The number of mice dying was inversely correlated with the time the bait was exposed to weathering. Bait exposed four weeks was believed to be ineffective.

Acceptance of the bait by white mice apparently was not influenced by the length of its exposure in the field, Mice accepted bait which had been weathered one and four weeks as readily as fresh bait.

The LD₅₀ of zinc phosphide for white mice was determined to be 25.77 +/- 12.16 mg/kg. The LD₅₀ of zinc phosphide for prairie voles was 16.23 +/- 0 mg/kg.

The potential hazards of secondary poisoning from rodents killed with zinc phosphide were investigated for three species of predators, red fox (Vulpes fulva), gray fox (Urocyon cinereoargentus), and great horned owls (Bubo virginianus). No predators died from consuming poisoned voles, though some behavioral irregularities developed during the feeding trials.