Weatherability and acceptance of selected commercial zinc phosphide rodent baits

We evaluated the chemical and physical characteristics of five commercial zinc phosphide (Zn3P2) rodenticide baits at selected intervals during 16 days of exposure to prevailing weather under simulated field conditions, and conducted laboratory feeding trials to assess the effects of weathering on the acceptance and toxicity of one of the baits to Norway rats (Rattus norvegicus), roof rats (R. rattus), and Polynesian rats (R. exulans). In bait weatherability trials, the concentration of Zn3P2 declined most rapidly in an oat bait that was overcoated with this toxicant. The Zn3P2 incorporated into pelleted formulations was less susceptible to physical weathering. All baits softened as they absorbed moisture but, with the exception of Ridall-Zinc® Field & Agricultural Bait, hardened again as they dried. Mold colonization was not apparent until after at least 4 days of exposure and occurred earlier and was more prevalent under wetter conditions. Hopkins® Zinc Phosphide Pellets were most durable in terms of retaining both their physical form and Zn3P2 concentration. Applying 2·5 cm of simulated rain (with or without drying) did not affect consumption of 1·0 cm Hopkins Zinc Phosphide Pellets by roof rats and Polynesian rats, but may have enhanced consumption by Norway rats. Mortality was similar among treatments for Norway rats and Polynesian rats, but was lower for roof rats offered pellets that received added moisture. Mortality for all species and treatment groups was inadequate for operational control purposes.