The Problem With Rodenticides – The Mountain Times


By Anna Morris

Last fall, around the same time I was laying the winter duvet on our bed, my cat took a great interest in the space under the kitchen sink. Unsurprisingly, a mouse was huddled there, seeking shelter in the heat. Although I was friendly and all wild animals are welcome in our yard, I would prefer them to stay outside the house. What to do?

As I was considering how to handle our unwanted guest, the pesticides were immediately ruled out. As an educator at the Vermont Institute of Natural Science (VINS), I frequently speak about rodenticides – poisons for rats and mice – and implore our visitors to look for non-toxic alternatives when trying to control pests. Although these poisons target rodents, raptors – and other animals, both tame and wild – that eat these rodents can be injured or killed by secondary poisoning, known as relay toxicosis.

Twice after giving this talk, a visitor got in touch to ask if a particular rodenticide their exterminator uses is “safe” for raptors, as they had been assured. It comforted me to hear that people are concerned about this issue, and I wanted to investigate this possibility – are there any “raptor safe” rodenticides?

Modern rodenticides can be divided into three categories: first generation anticoagulants, second generation anticoagulants and non-anticoagulants. Both types of anticoagulant rodenticides work by blocking the action of an enzyme that recycles vitamin K, reducing the availability of a key component of the blood clotting pathway and causing fatal internal bleeding. First-generation anticoagulants require rodents to feed on the poisoned bait several times. Second-generation blood thinners, however, can kill with a single dose and are toxic even by inhalation or skin contact. Both can also kill birds of prey by relay toxicosis, as both are stored in the liver of rodents and remain there even after death.

There is a large literature on the presence of anticoagulant rodenticides in the tissues of wild raptors in North America. A three-year study in New York State found that of 265 birds of prey tested, 49% had an anticoagulant rodenticide in their systems. Among the species affected were some of the most common and cosmopolitan raptors: the great horned owl, the red-tailed hawk, the Cooper’s hawk and the mountain owl.

The three second-generation anticoagulant rodenticides that pose the greatest risk to raptors are brodifacoum, difethialone and bromadiolone. Fortunately, the EPA restricts their use to licensed pest control companies, so they are not sold “over the counter.” However, all three can still be used by pest control companies in Vermont and New Hampshire.

Any chemical designed to be highly toxic to mammals is likely to be at least somewhat toxic to birds, which have much faster metabolisms. The risks of secondary poisoning from non-anticoagulant rodenticides, such as cholecalciferol and zinc phosphides, have not been studied extensively, but are thought to pose a low risk to birds – but not no risk . These poisons work in unique ways, causing muscle spasms, paralysis, or organ failure.

For a wildlife rehabilitator, diagnosing rodenticide poisoning is difficult because symptoms can overlap with other illnesses. At VINS’ Wild Bird Rehabilitation Center in Quechee, Vermont, birds that show lethargy, bruising and excessive bleeding are suspected of poisoning. Rehabilitators treat these birds with supportive nutrition and a vitamin K supplement to replace what the rodenticide has depleted. This therapy can take three to four weeks and depends a lot on the courage of the bird.

So, are there rat poisons that are safe for birds of prey and other non-target wildlife? The answer is no. If you are looking for a rodenticide that will only target mice and leave owls completely alone, there is currently no such miracle. All chemicals on the market for killing rodents are also toxic to birds to some degree.

However, there is a wide variety of non-chemical alternatives. Integrated pest management is all about gaining an understanding of the natural history and behavior of your pest species. Be sure to seal off any places they might enter your home, secure food sources (especially compost and trash), and encourage natural predators by managing your garden to be environmentally friendly. wildlife. Spring traps and Havahart traps are also safer alternatives to chemicals. (Avoid glue traps – they are also responsible for many untargeted wildlife deaths, from dehydration or starvation.) For these small efforts, your local hawks and owls will thank you by continuing their natural pest control and helping to maintain the balance of biodiversity. .

Anna Morris is an environmental educator at the Vermont Institute of Natural Science. Illustration by Adelaide Murphy Tyrol. The Outside Story is attributed to and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation:


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