Products marketed to kill rodents are instead threatening the lives of the wildlife that eat them as poisons travel up the food chain.


People who consider rodents to be pests often turn to an array of products, known as anticoagulant rodenticides, which are marketed to lethally “solve” the issue with poisoned bait. But researchers have been collecting evidence for years showing that it’s not just nuisance rats that can end up dead.

Some of the most recent studies, conducted in California, found that everything from Pacific fishers to bobcats to northern spotted owls often become victims of rodenticides. The list of potentially affected wildlife is long — basically anything that preys on a rodent could be at risk, because the poisons are so toxic they travel up the food chain, and in some cases, can remain in an animal’s body for years. It can even leapfrog in utero from one generation to the next.

“If you have a very poisoned rat, you’re going to have a very poisoned hawk,” says Kelle Kacmarcik, director of wildlife solutions and advocacy at WildCare, a wildlife rehabilitation center in Marin County, California.

And that’s a huge problem.

Spotted owl
The threatened northern spotted owl on the Klamath National Forest in Siskiyou County, Calif. (Photo by Kendra Knight, CC BY 2.0)

“Food web contamination is a red flag indicator for not just for one species but all other species dependent on that prey item,” says Mourad Gabriel, a research associate faculty at the One Health Institute, Wildlife Heath Center at the University of California, Davis and co-director of the nonprofit Integral Ecology Research Center. His research has looked at the risk to fishers and northern spotted owls from rodenticide exposure from cannabis farms, a growing problem in California. But “other species may start to become imperiled because of this emerging threat,” he says.

Rodenticides affect wildlife around the world and have been used since the 1940s, but recent studies in California have helped illuminate the impacts and ignite calls for regulatory or policy changes.

California took an important first step in 2014 by increasing restrictions on some rodenticides and the state is currently evaluating whether it will take more action. But that’s still not enough, say some experts, like those at WildCare, who want to see a state ban on the products and more public education and behavior changes to reduce the use of the poisons. How to target those messages though, depends on the audience.

In urban and suburban areas, the biggest users of rodenticides are regular consumers who buy the products at hardware stores for home use or hire pest-control applicators to treat their properties. In agricultural areas, it’s applied by farmers to keep rodents away from crops. In forested parts of the state, it’s been dumped by the pound at illegal and trespass cannabis operations on public, private and tribal lands.

A bill introduced in the California legislature last year would have prohibited the use of anticoagulant rodenticides except in public health emergencies, but it stalled in committee. A new bill was just introduced in February, providing hope for some legislative action on the issue.

While the state figures out what to do the poisons are traveling up the food chain, endangering more and more wildlife.

A Widespread Problem

Anticoagulant rodenticides are sold under dozens of brand names with a number of different active ingredients. They work by affecting the animal’s processing of vitamin K, which inhibits clotting and coagulation, ultimately leading to uncontrolled internal bleeding. One of the most common first-generation anticoagulant rodenticides uses the blood thinner called warfarin as its active ingredient, but rats appear to have developed a resistance to it.

Manufacturers responded by developing a new line of more toxic products, using active ingredients like brodifacoum or bromadiolone, which have no medicinal uses. These second-generation rodenticides pack a bigger punch in a small amount and are designed to deliver a lethal dose to a rodent with just one feeding. They also have longer half-lives, which means the poison can stay in the animal’s liver for years, compared to just days or months with the first-generation poisons.

Both of these factors contribute to an increased possibility of nontarget wildlife eating poisons intended for rodents. Second-generation rodenticides don’t kill animals right away. It can still take days, and in that time a sick animal often consumes more of the poison and exhibits signs of illness, making it an easier target for predators.

Documentation in California shows that the problem of rodenticides moving up the food chain and affecting nontarget wildlife is widespread.

A 2017 study found that hawks hunting for prairie dogs preferred those that had been treated with rodenticides to those which hadn’t because “the poisoned prairie dogs were easier to capture due to lethargy and decreased awareness,” according to an analysis of the research by California’s Department of Pesticide Regulation.

A study by Gabriel and colleagues tested the livers of two distinct populations of fishers (Pekania pennant) collected between 2007-2014 from the mountains of Northern California and the Southern Sierra Nevada. The researchers found that 85 percent of the fishers had been exposed to one or more anticoagulant rodenticide. That’s bad news for fishers, which are already imperiled by habitat loss and are currently being reviewed for listing under the Endangered Species Act.

More recently, in a 2018 study, Gabriel looked at two owl species in California and found anticoagulant rodenticides in 70 percent of northern spotted owls (Strix occidentalis caurina), which are listed on the Endangered Species Act. And for barred owls (Strix varia), which rely less on rodents in their diet, the researchers found 40 percent had anticoagulant rodenticide exposure.

In both studies, cannabis operations were the likely sources of exposure.

Rodenticides found at a cannabis farm in California. (Photo by Integral Ecology Research Center)

“Rodents love cannabis plants because of the high water and sugar content that these plants have,” says Gabriel. “And so in order to dissuade them from damaging their cannabis plants, cultivators are placing the anticoagulant rodenticides out there.”

Another study by different researchers published in 2015 in Ecotoxicology found that 88 percent of bobcats (Lynx rufus) living in the mountains near Los Angeles had exposure to anticoagulant rodenticides.

The problem isn’t just in rural areas. At WildCare, located 20 miles north of San Francisco in suburban Marin County, tests from 2015-2016 identified 13 species with rodenticide exposure. Raptors tested had a 78 percent poisoning rate. For mammals like bobcats, coyotes, foxes and racoon it was 86 percent.

Many of the animals tested in these studies didn’t die from poisoning, but even these sublethal amounts can still be problematic. California Department of Pesticide Regulation scientist Deborah Daniels reported in a 2013 letter from the department that it can “reduce the fitness of wildlife at a time when wildlife are already meeting numerous challenges.”

For example, in one study, researchers found that rodenticide exposure in bobcats may have led to an increased susceptibility to mange, a parasitic infection that’s increasingly fatal to the species in California.

“No matter how large or small, you’re looking at a toxic poison in the systems of these animals,” says Alison Hermance, director of communications of WildCare. And anticoagulants mean that even a small scrape can cause an animal to bleed out, turning an otherwise minor injury into something fatal.

Researchers have found animals in California with up to five different kinds of rodenticides in their systems at one time. Gabriel says that understanding all the ways sublethal doses of poison affect animals, including the build-up of multiple kinds of poisons, is “a new field of research that needs to be tackled extensively.”

Mixed Results

The fact that wildlife are dying from rodenticides has been documented for years. But what to do about it is still being debated.

Daniels concluded there was enough data to find that “exposure and toxicity to nontarget wildlife from second generation anticoagulant rodenticides is a statewide problem.” And that problem, she wrote, persisted in both urban and rural areas. Testing on 492 animals between 1995 and 2011 found that 73 percent had some traces of at least one second-generation poison.

So California took action to limit the sale of the second-generation products, which it deemed a bigger health threat. In 2014 it banned their sale to ordinary consumers — people could no longer just stop at the corner store to buy some — but they continue to be available to licensed pest-control applicators and those in the agriculture industry.

Unfortunately, the regulation hasn’t been very effective yet. Kacmarcik at WildCare says they haven’t seen the amount of second-generation rodenticides in wildlife decreasing; in fact they found a small uptick during the first two years after the regulation went into effect.

There are a couple of reasons. The first is that some of these long-lived poisons persist in animals for years. The second is that it is still legal to pay someone to apply the poison. And third, people stocked up the products as many stores unloaded their supplies for cheap just before they were banned. So even though the sale of them is no longer legal, they’re still being used.

Mourad Gabriel
Dr. Mourad Garbiel at a cannabis grow site clean up. (Photo by USFS, CC BY 2.0)

For forested rural areas where cannabis operations are often the biggest culprit, Gabriel says, “We are seeing a decrease in second generations, but a major uptick in first generation being applied.” Despite the legalization of cannabis cultivation in California, thousands of operations still flout the regulations and many operate illegally on public lands. Many of these poisons are left behind when operations are abandoned, endangering wildlife and costing the state millions in cleanup costs.

The state, in a November 2018 report, admitted that its “limited data” showed that exposure rates to the second-generation anticoagulant rodenticides had not decreased. The report concluded that these second-generation poisons have “significant adverse impact to non-target wildlife.” But the state doesn’t think the first-generation products pose a significant risk. Gabriel, however, says the risks are still there even though the products are less toxic and there is extensive documentation that the poisons can still transfers to predators. At WildCare, nearly 30 percent of the animals tested had been exposed to a first-generation anticoagulant rodenticide.

Searching for Solutions

The Department of Pesticide Regulation is currently considering whether to reevaluate its regulations for the second-generation products and what specific data it would need if did, says Charlotte Fadipe, the agency’s assistant director. “In general, the department is considering data related to second generation anticoagulant rodenticide exposure rates, as well as any resulting risk of adverse impacts to non-target wildlife.”

Finding solutions to curb the use of anticoagulant rodenticides at cannabis farms will be extra tough. “You’re looking at a new, emerging threat and risk,” says Gabriel. “Current regulations don’t seem to be sufficient enough to keep it in check.” Since those who are running illegal grows aren’t likely to follow stricter pesticide regulations, more resources for enforcement are also needed, he says. Without that many of these cannabis operations will continue to pose a threat to other efforts to conserve wildlife populations in the state.

Hermance says that WildCare would like to see a complete restriction on anticoagulant rodenticides. “It’s such a tragedy that these poisons kill or injure the animals that nature has provided to give us rodent control,” she says. “Hawks, foxes, owls, racoons are out there eating rodents every single day — knock them out and you’re going to have a much bigger problem down the road.”

And in most cases, poisons simply aren’t needed to solve a problem, they’re just easier. “I’m amazed at just how quickly people jump to poison as a solution to a nuisance problem,” she says.

Red-tailed hawk
Raptors like red-tailed hawks have been more effective in Ventura County, Calif. at eliminating pests than poisons. (Photo by Jon D. Anderson, CC BY-NC-ND 2.0)

Until there’s more state action, public education on the risks to wildlife and on alternatives to poisons is needed.

In one recent example, Ventura County in Southern California has turned to raptors to decrease the number of ground squirrels burrowing in its dirt levees and dams. The birds have proven to be better and cheaper than poisons.

That’s the type of lesson that the experts hope will resonate not just in California but around the world. “Stop and think about what’s causing the problem and how you can solve it without poison,” says Kacmarcik. “If we could get more people to do that, you’d see a significant reduction in the poisoning that’s happening.”

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Tara Lohan

worked as The Revelator's deputy editor from 2018-2024. She is the editor of two books on the global water crisis and is working on a book about dam removal.

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