Comparison of mixing methods and associated residual levels of zinc phosphide on cabbage bait for rodent management

Zinc phosphide is a toxicant that is used extensively for rodent management throughout many parts of the world. Some rodent species, such as Belding\u27s ground squirrels (Urocitellus beldingi Merriam, 1888), often avoid zinc phosphide grain baits, leaving green vegetation such as cabbage as the only viable carrier for rodenticides. However, to date, ambiguity has existed as to the most appropriate mixing strategy for zinc phosphide-coated cabbage baits, and it is unknown how rapidly zinc phosphide degrades on these green carriers. Following laboratory and field-enclosure trials, we detected no significant difference in mean zinc phosphide concentrations or variability in zinc phosphide concentrations between mechanical and hand mixing strategies. However, the use of a mechanical mixer was determined to be the more practical option given that it is quicker and requires less effort for mixing large quantities of bait, it minimized worker exposure to phosphine, and because it yielded mean concentrations that were closer to target values. Both the moisture content of cabbage and zinc phosphide concentrations diminished over time, resulting in a fairly minimal window of exposure for non-target wildlife. Field investigation of this exposure risk, as well as an assessment of efficacy of zinc phosphide-coated cabbage baits for Belding\u27s ground squirrel management, are warranted