Anticoagulant Rodenticides – a 21st Century Silent Spring?
To learn more about how rodenticide affects wildlife, check out National Parks Service for more info HERE.
See enlarged image HERE.
Anticoagulant rodenticides (ARs) are a toxic pesticide designed to kill rodents. Inadvertently, ARs are making their way through the food web, and the same characteristics that make ARs lethal to rodents also poison wildlife and cause disease and death.[1], [2]
You've probably seen these rodenticide bait boxes around businesses and even private residences.
The concept is pretty simple: a rodent enters the box and consumes the poison—the problem is, the rodent does not die inside the box. The poisoned rodent exits the box and dies out in the open where owls, eagles, bobcats, foxes and other wildlife consume the easy prey. They then become poisoned as well. Dogs and cats are also in danger of secondary poisoning.
What are Anticoagulant Rodenticides?
Anticoagulant Rodenticides (ARs) contain toxins that work by preventing an animal’s blood from clotting, causing internal hemorrhaging and eventually death. There are two main types of ARs - first generation (FGARs) or second generation (SGARs) based on the specific toxins involved. SGARs were developed in the 1970’s because rodents were developing immunity to FGARs. SGARs have greater toxicity and a longer half-life, and so the toxins remain in the tissues of affected animals longer, and can bio-accumulate over time. These traits which make SGARs more effective at killing rodents are precisely what presents risks to wildlife.
How ARs Affect Wildlife
ARs in general, and SGARs in particular, poison wildlife in several ways: 1) primary poisoning when an animal eats the rodenticide bait and dies several days later; 2) secondary poisoning when a predator or scavenger eats prey that has eaten poisoned bait; or 3) tertiary poisoning when an animal preys on another species that has been secondarily poisoned, e.g., “a mountain lion eats a coyote with secondary poison that ate a poisoned squirrel.” [3] ARs can be an underlying cause of disease normally not associated with them, e.g., by suppressing an animal’s immune system and it can become more susceptible to other disease such as mange.[4] Rodenticides have also been found to affect wildlife at the genetic level, though the long-term consequences of this are unknown at this time.[2]
Impacts in Vermont
The Vermont Fish and Wildlife Department (FWD) tests carcasses of some wildlife turned in by trappers. Multiple studies on fisher in Vermont have resulted in alarmingly high levels of rodentcide with the most recent test showing 100% of all fisher testing positive for rodenticide. Read HERE. More on that in our white paper HERE.
Upon learning of this, a Protect Our Wildlife Board member, who is also a biologist, petitioned the Fish & Wildlife Department and Board to place a moratorium on the recreational trapping of fisher. They denied her petition on December 18, 2014. See our email in response to their decision HERE.
As per FWD's records, 31% of bobcats tested positive for three types of rodenticides, yet they still allow bobcats to be hunted and trapped for recreation.
Upon receiving a complaint from a business owner in Middlesex who noticed red fox kits dying around the property, FWD had necropsies performed. Test results revealed all of the kits were poisoned by rodenticide. See report and photos HERE.
Wildlife rehabilitators report cases of poisoned wildlife, like this Great Horned Owl who was cared for by Otter Creek Wildlife Rescue.
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You can learn about this owl's story HERE.
Rodenticides may be a 21st century version of Silent Spring, and fisher are the canary in the coal mine. Inaction or resistance to change are not options that will preserve Vermont’s wildlife.
AR Bait Boxes
AR baits are manufactured as chunks or pellets and placed inside a tamper-proof “bait box.” Rodents can enter the box, feed on bait, and then leave to die somewhere away from the bait station – and where predators can find and eat them. Many boxes are black, and they are often placed outside along the foundation walls of buildings.
Vermont Regulations
ARs are regulated in Vermont by the Agency of Agriculture and Food Markets. Currently FGARs and SGARs are still allowed to be used in Vermont, with only minimal restrictions on SGARs. SGARs are now classified as a “class A” pesticide meaning they can only be purchased and used by certified applicators who must pass an exam with the Agency of Agriculture.[5], [6] SGARs are no longer allowed to be sold in stores to the general public. However, unfortunately it is still possible to purchase SGARs on-line at some sites.
SGARs contain any one of the following: brodifacoum, bromadiolone, difenacoum, or difethialone.
FGARs contain any of the following: warfarin, coumatetralyl, diphacinone, or chlorophacinone.
Resources for Alternatives to ARs
Raptors Are the Solution offers helpful information on alternatives to ARs. Learn more HERE.
What You Can Do!
Education is the first step in changing laws. Please share this information with your Vermont legislators found HERE (insert your town to find your legislators).
We would like to mirror California's law that was passed that enacted a moratorium on ARs. See fact sheet HERE.
Articles / Studies of Interest
Harvard Law School's PETITION TO SUSPEND THE REGISTRATIONS OF ANTICOAGULANT RODENTICIDE PRODUCTS IN MASSACHUSETTS HERE
Anticoagulant rodenticides in urban bobcats: exposure, risk factors and potential effects based on a 16-year study HERE
High Prevalence of Anticoagulant Rodenticide Exposure in New England Fishers (Pekania
pennanti) HERE
Greater predisposition to second generation anticoagulant rodenticide exposure in red foxes (Vulpes vulpes) weakened by suspected infectious disease HERE
[1] Fisher, Penny et al, 2019. Anticoagulant Rodenticides, Islands, and Animal Welfare Accountancy. National Library of Medicine (NLM), National Center for Biotechnology Information.
https://pmc.ncbi.nlm.nih.gov/articles/PMC6912481/
[2] Fraser, Devaughn et al, 2018. Genome-wide Expression Reveals Multiple Systemic Effects Associated with Detection of Anticogulant Poisons in Bobcats (lynx rufus). Molecular Ecology, 2018.1-18. https://raptorsarethesolution.org/wp-content/uploads/2018/04/Fraser_et_al-2018-Molecular_Ecology_Multiple-systemic-effects-associated-with-anticoagulant-poisons-in-bobcats.pdf
[3] National Park Service, Santa Monical Mountains. Avoiding Unintentional Poisoning. https://www.nps.gov/samo/learn/management/rodenticides.htm
[4] National Park Service, Santa Monical Mountains. Bobcats Living on the Urban Edge. https://www.nps.gov/samo/learn/nature/bobcats.htm
[5] Act S.301, An Act Relating to Miscellaneous Agricultural Subjects. 2024. See p. 15, section (29). https://legislature.vermont.gov/Documents/2024/Docs/ACTS/ACT141/ACT141%20As%20Enacted.pdf
[6] State of Vermont, Agency of Agriculture and Food Markets, Public Health & Agricultural Resource Management Division. Vermont Rule for Control of Pesticides. Effective February 24, 2023. https://legislature.vermont.gov/Documents/2024/Docs/ACTS/ACT141/ACT141%20As%20Enacted.pdf