Effects of zinc phosphide bait coatings on bait consumption and mortality in gray-tailed voles (Microtus canicaudus).

BACKGROUND

Zinc phosphide (ZP) is one of the world's most-used rodenticides. However, ZP bait palatability can be poor and rodents may become averse to baits after ingesting sublethal amounts. Our primary objective was to determine if ZP bait coatings increased palatability and ultimately mortality of gray-tailed voles (Microtus canicaudus Miller). We offered six ZP formulations to 120 single-caged voles, including uncoated Prozap® and ZP AG® pellets and these same baits coated with powdered peanut butter or powdered cheese. A secondary objective was to determine if voles that received a sublethal dose of ZP maintained their ability to reproduce when compared to voles that were never exposed to ZP.

RESULTS

Mortality was high (75-100%) for each of the six ZP-bait formulations despite some variability in average ZP bait concentrations (2.17-2.45%). In total, 16 individuals (13.3%) survived the toxic trials, and average ZP bait consumption was alike for voles that died (0.24 g) and survived (0.23 g). All 10 pairings (five ZP-exposed pairs, five unexposed pairs) resulted in similar litter sizes and offspring mass.

CONCLUSION

Only one ZP bait formulation (ZP AG®-uncoated) was 100% efficacious and we found no evidence that our peanut butter or cheese coatings improved mortality despite voles seemingly consuming more coated than uncoated baits. Our bait coating methodology minimized ZP dilution and is useful for evaluating future ZP bait coatings. Our findings of similar fecundity between voles exposed and unexposed to ZP highlight the importance of having toxic baits that are palatable and lethal on first exposure. Published 2026. This article is a U.S. Government work and is in the public domain in the USA.