Unlike anticoagulants, zinc phosphide is an acute toxicant that kills its target after a single feeding, and there is no antidote once it is ingested. It is both a federally and state restricted use material. On contact with stomach acid, zinc phosphide is hydrolyzed and produces phosphine gas. Death is much quicker compared to FGARs, usually within 12 hours. Because of its acute nature, primary toxicity (i.e. mortality of nontarget wildlife that directly consumes the bait) of nontarget wildlife or pets is a greater concern than with FGARs. However, secondary toxicity risks are low for zinc phosphide because the toxic gas quickly dissipates from the body after death, leaving no toxicant in the body to be consumed by scavengers.
Although the use of zinc phosphide is permitted in some urban areas, it is a restricted use material that can only be applied by a licensed pest management professional. Because of its acute toxic nature, a lack of an antidote, and because it cannot be used in bait stations, zinc phosphide is less likely to be used around homes.
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While zinc phosphide can be highly effective and a cheaper option compared to FGARs, efficacy of zinc phosphide is less predictable given its distinctive odor and taste that rodents do not always find palatable; if rodents do not consume the bait, it obviously will not work.
Non-toxic prebaiting prior to an application of zinc phosphide is strongly recommended by most rodenticide labels. This pre-bait application helps to determine 1) if ground squirrels are actively feeding on grain at the application site and 2) if any nontarget species are feeding on grain at the same site. If ground squirrels are observed feeding on the nontoxic grain, then a zinc phosphide application may commence. If not, there is no point in applying zinc phosphide as they are unlikely to consume the product. Additionally, if nontarget species are observed feeding on the nontoxic grain, then bait application cannot proceed unless some method of deterrence can be implemented to keep the nontargets from consuming the toxic bait (e.g. a bird hazing program if birds are found consuming the grain).
In addition to initial bait avoidance, ground squirrels can also develop a learned avoidance to zinc phosphide called “bait shyness.” Bait shyness occurs when ground squirrels consume a sublethal dose that causes them to get sick. They then associate this sickness to the novel food (i.e. zinc phosphide treated bait) that they consumed and learn to avoid it in the future. If zinc phosphide were frequently applied, many ground squirrels in a population would learn to avoid the product, rendering it ineffective. As such, zinc phosphide applications are allowed only once per year for ground squirrel control.
Zinc phosphide can be applied through broadcast applications or spot treatments . For broadcast applications, a hand or ground-driven spreader can be used to spread the bait. Aerial applications can also be used for some products. Be sure to read the label instructions thoroughly to determine the proper application rate (most products allow applications of between 6 and 10 pounds per acre). You must calibrate spreaders before application to ensure the proper rate of bait application.
Bait can also be applied by hand via the spot treatment approach. Check the label of the product you are using to determine the proper application rate. Baits applied to the ground adjacent to an active ground squirrel burrow system. Do not pile bait. Zinc phosphide is highly toxic. If bait is piled, nontarget species may ingest a toxic dose, but if bait is spread out, very few animals are inclined to pick up enough of the grain to get a toxic dose. Dispose of excess bait as per the label.
Due to its mode of action, zinc phosphide should not be applied when precipitation, fog, or morning dew are expected. Zinc phosphide should not be applied on top of snow, ice, or wet ground. Do not irrigate for 48 hours after application.