Zinc phosphide is a rodenticide used to kill gophers, moles, rats and squirrels. It is federally approved as a mole poison. Zinc phosphide-laced bait has a strong garlic-like odour, according to Michigan State Department of Natural Resources and Environment (DNRE). This is supposed to make it unattractive to other animals. Unfortunately, rodenticides like zinc phosphide don't just poison moles. They kill all kinds of wildlife, including bald eagles, bobcats, deer, foxes, freshwater fish, hawks, geese and owls, reports the Environmental Protection Agency (EPA). Zinc phosphide also poses risks to children and pets.
How Zinc Phosphide Works
Zinc phosphide turns into a gas when it reaches fluid in the stomach. This gas destroys cells throughout the bloodstream and body and results in organ and tissue damage. It takes from 15 minutes to four hours for the mole to die, according to DNRE. The death is a painful one and involves abdominal pain, convulsions, nausea, vomiting and paralysis.
Zinc Phosphide and Bait-Shyness
Wildlife specialist Robert Pierce of the University of Missouri says that zinc phosphide doesn't work well because it is normally impregnated in grain bait and moles do not eat grain, seeds or nuts. Dale Miller, editor of "National Hog Farmer Magazine," writes that the high concentration of poison in zinc phosphide baits makes them unpalatable. A mole may not eat enough of the bait for it to have lethal effects and then learns to avoid the bait in the future. This "bait shyness," writes Miller, is a feeding cue young moles take from their mothers.
Dangers of Zinc Phosphide Use
Zinc phosphide can kill dogs and cats through secondary exposure and birds are especially affected by the mole poison, according to DNRE. It takes several months for the bait to be degraded by weather or for the mole to degrade. Dry bait may stay toxic indefinitely. Zinc phosphide is classified as a Category I toxin by the EPA for oral intake and inhalation exposure. Wet conditions create the toxic phosphine gas. Inhaling this gas through examination of an infected animal or its vomit can poison people, warns veterinarian Amanda Schnitker.
EPA Rulings on Zinc Phosphide Use
University of California wildlife specialist Terry Salmon told "Hay and Forage Growers Magazine" reporter Eric McMullen that farmers are allowed to use two applications of zinc phosphide a year. The farmer must wear protective gear, including a mask and respirator, when applying the mole poison. In addition, zinc phosphide cannot be used in areas where grain-eating birds feed and waterfowl must be frightened away for 24 hours after treatment.
In 1998, the EPA ruled that manufacturers of zinc phosphide rodenticides must use indicator dyes and bittering agents in their products to minimise risk to children and pets. The mole poison must not be sold as loose pellets but in preloaded bait stations.
The Nebraska Department of Agriculture adds that only 0.454kg. or less of zinc phosphide mole poison may be sold to homeowners. Farmers may purchase 3.63kg. or more. Professional pest exterminators may buy 7.26kg. units of the mole poison.
National Wildlife Research Center (NWRC) scientists have found that adding anthraquinone repellent to rodenticides made of zinc phosphide can help protect wildfowl. Anthraquinone is a naturally occurring substance that absorbs near-ultraviolet light. This light spectrum is visible to many birds and also acts as a laxative. The NWRC scientists hope that the compound can teach wild birds to avoid the rodenticide.
- Environmental Protection Agency: Final Risk Mitigation Decision for Ten Rodenticides
- Michigan Department of Natural Resources and Environment: Zinc Phosphide
- Environmental Protection Agency: Zinc Phosphide
- Nebraska Department of Agriculture Pesticide and Noxious Weed Newsletter: EPA Rodenticide Ruling
- "Hay and Forage Grower Magazine"; Rodent Remedies; Eric McMullin; February 2004
- Missouri University School of Natural Resources: Controlling Nuisance Moles
- "National Hog Farmer Magazine"; Rodent Control 101; Dale Miller; June 2006
- DVM360: Toxicology Brief: A Case of Zinc Phosphide Toxicosis
- National Wildlife Research Center: New Combined Wildlife Repellent-Rodenticide Bait May Reduce Non-Target Hazards