Proceedings of the Vertebrate Pest Conference

The U.S. Department of Agriculture (USDA), Animal Plant and Health Inspection Service (APHIS) maintains six federal registrations for zinc phosphide (Zn3P2); three of these address the control of eight rodent species using steam-rolled oats (SRO) or wheat grains in diverse applications. Optimal foraging theory (OFT) and "particle dose analysis" (PDA) afford predictions relevant to the efficacy, acceptance, bait shyness, and non-target hazards of these Zn3P2 baits. For PDA, numbers of SRO groats or whole wheat grains associated with acute oral median lethal (LD50) or approximate lethal (ALD) doses of Zn3P2 were compared among nine target rodent and eleven non-target avian species. Key outcomes were: 1) mean (±S.D.) SRO groats and wheat grains weighed 23 (±9) and 18 (±9) mg [assumed to carry ≈0.46 (2.0%) and ≈0.33 (1.82 %) mg Zn3P2, respectively; 2) published acute oral LD50 values for the target rodents ranged between 16.2 and 18.0 mg/kg, with a 42.0 mg/kg ALD cited for the white-footed mouse (Peromyscus maniculatus); 3) estimated minimum-maximum number of SRO groats and wheat grains needed for ingestion of the LD50/ALD doses by target species were 1.6 to 39.1 and 1.7 to 3.8, respectively; and 4) estimated minimum-maximum number of SRO groats and wheat grains associated with primary LD50/ALD hazards to nontarget avian species were 1.3 to 175.8 and 1.8 to 245.1, respectively. Theoretical implications of OFT and PDA to efficacy, acceptance, bait shyness, and specificity of Zn3P2 baits in rodent control are provided; the potential effects of food-handling time, bait-search time, predator density, social-dominance hierarchy, food-intake pattern, and bait-distribution pattern are discussed.