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The Richardson’s ground squirrel (AKA gopher) population can present an issue for farmers’ crops, cleaning entire fields if their numbers grow too large.
A rodenticide that is commonly used to control the Richardson’s ground squirrel population is zinc phosphide, which has been gaining more attention with the de-registration of strychnine in 2021 by Health Canada. Strychnine use was no longer allowed after March 2023.
Bait stations are used for zinc phosphide, with a treated grain product that could be palatable to non-target animals like pets or livestock.
“When [zinc phosphide] encounters a weak acidic environment, like you would see in an animal’s gut – the mammalian gut – it dissociates to produce phosphine gas, and the phosphine gas is really what causes the toxicity," explained James Tansey, provincial insect and vertebrate pest management specialist with the Ministry of Agriculture.
He said that the phosphine gas is thought to escape the animal and leave little in the way of residues. “If a dog or a cat or fox or badger encounters a cadaver, its likelihood of being poisoned by that cadaver is relatively low.”
He added that the poisoned Richardson’s ground squirrels tend to go into their burrows and die in the holes.
Tansey provided safety requirements for the bait stations that can keep non-target populations safe from the grain product containing zinc phosphide including:
-Construction with high strength material
-Entrance designed to keep non-targets and children out
-Internal structure to prevent bait from shaking out
-Securely closed so they cannot be easily opened by animals
-Nailed/fastened down so they cannot be blown over
-Properly labelled with a poison label
The application of zinc phosphide for the Richardson’s ground squirrel is meant to happen in the spring. “Once things begin to green up and you have lush green plants, then the likelihood of them taking the bait, or the relative palatability of the bait gets reduced,” explained Tansey.
“It should be used with caution, much like any pest-control product. Follow the label – the label is the law,” Tansey added, saying that when the products are used according to label directions, they are safe and efficacious, with little non-target mortality.
If pets do get into the bait, it is an emergency situation requiring immediate medical help, said Vanessa Cowan, assistant professor of veterinary biomedical sciences at the Western College of Veterinary Medicine.
Signs of zinc phosphide exposure include vomiting and diarrhea, and Cowan added the literature suggests the phosphine gas has a rotting fish smell. “I personally haven’t smelled that, and also would caution people from trying to smell for that odour, because you can become poisoned through inhalation.”
“The car that you transport them in should have the windows down and be very well ventilated, just in case the phosphine gas is formed, and the animal starts to vomit or burps up some of that gas,” added Cowan.
The gas has the potential to poison anyone in close proximity to the pet, and owners should notify the vet clinic if they suspect zinc phosphide poisoning.
It can also be helpful to bring in any packaging that shows active ingredients and their concentrations in products the pet may have consumed. Pet poison control centres can be called including the ASPCA Animal Poison Control Centre or the Pet Poison Hotline.
As with strychnine, there is no cure for zinc phosphide poisoning said Cowan. “Zinc phosphide targets many different organ systems in the animal, so it’s really just aggressive, supportive care to try and get the animal through. The prognosis is a little better if the animal survives the first 12 hours; however, sometimes animals can die much sooner than that.”
While there is a diagnostic test at the Western College of Veterinary Medicine for strychnine, they don't currently have one for zinc phosphide. There is a diagnostic test currently available at the Animal Health Laboratory in Guelph, Ontario.
“There have been talks about it at the Western College of Veterinary Medicine, but test development takes some time,” explained Cowan.
When making use of zinc phosphide as a rodenticide, ensuring that bait traps have all safety features and keeping an eye on pets can prevent unintended poisonings from happening.
**Information in this article from the Ministry of Agriculture was obtained before the writ was dropped Oct. 1, 2024**