Rodenticide Research

The following research was conducted and summarized by Chicago Bird Alliance’s Rodenticide Research Team and Communications Team

What is rodenticide and why is it a problem?

Anticoagulant rodenticides (ARs) are a common way to control rat populations in cities around the world. ARs kill rats and harm nontarget wildlife by preventing blood clotting.

There are many different active ingredients registered as rodenticides in the United States. They can be grouped together according to how they work. Here are some common rodenticides:

FGARs (first-generation ARs)

●  chlorophacinone

●  diphacinone (and its sodium salt)

●  warfarin (and its sodium salt)

SGARs (second-generation ARs)

●  brodifacoum

●  bromadiolone

●  difenacoum

●  difethialone

Non-ARs

●  bromethalin

●  cholecalciferol

●  strychnine

●  zinc phosphide

FGARs generally require that an animal eat multiple doses of the bait over several days. SGARs are more toxic because they bind more tightly to the enzyme that makes blood-clotting agents. They can also interfere with other steps in vitamin K recycling. SGARs are not easily excreted from the body and they can be stored in the liver. Since the development of warfarin in 1950, ARs have become more potent, as rats evolved resistance to FGARs and, therefore, SGARs were developed to require fewer feedings to administer a lethal dose. 

As a result, newer products containing SGARs are particularly harmful to nontarget wildlife because they can cause biomagnification, which is the accumulation of toxicants in predator tissues after consuming poisoned prey. This biomagnification of ARs causes morbidity and mortality in many nontarget wildlife species that consume poisoned prey, notably predatory birds, top mammalian carnivores, and endangered species. The efficacy of ARs for controlling rat populations is not well understood but is up for debate, as their continued use has not led to reductions in rat populations in cities around the world. 

Scientists at Lincoln Park Zoo’s Urban Wildlife Institute recently completed a study of rodenticide exposure in rats, raccoons, opossums, and skunks. Of the 101 rats tested, 74% tested positive for at least one type of AR, suggesting that many rats survive long enough to be eaten by predators such as owls and other raptors. Of the 93 raccoons, opossums, and skunks tested, 100% had been exposed to at least one type of AR, mainly brodifacoum (≥80%), and the majority (53%) had been exposed to at least 3 types, suggesting widespread exposure in non-target urban mammals. One opossum had rodenticide pellets in its stomach, and opossum joeys who had not yet weaned tested positive for the same AR as their mother, suggesting transfer through milk. Gray and red foxes and coyotes are other local wildlife that were not part of the study but that may be similarly affected, along with avian predators. These results suggest that the use of ARs to control rat populations are harmful to Chicago’s wildlife.

Alternatives to rodenticide

Contraception (ContraPest, GoodBites)

• The nonprofit organization WISDOM Good Works has developed a fertility control product to control rats without poisoning the environment. The product initially came in a liquid form known as ContraPest. The scientists at WISDOM Good Works recently developed a new pellet formulation called GoodBites. This product, when ingested by rats, renders both male and female rats infertile. More about these efforts can be found in the organization’s most recent newsletters.

• ContraPest and Good Bites are EPA-registered oral contraceptives that rats consume simply by eating it. ContraPest disturbs reproductive functions in rats by targeting sperm cell production and ovarian function. It works via two active compounds, one of which induces a kind of menopause, reduced ovulation, in females, while the other, called triptolide, limits sperm production in males. One dose of the contraceptive prevents the reproduction of rats for 45 days. Delivered cumulatively over time, the product has reduced populations by 95%, according to research by the manufacturer, SenesTech.

• The Good Bites form of this contraceptive comes in pellets filled with fat and salt that taste good to rats. According to SenesTech, rats prefer the taste of Good Bites and ContraPest to other food sources.

• Crucially important, GoodBites and ContraPest are nontoxic alternatives: safe for both the target species (rats) and nontarget species (organisms that consume the rats: raptors, opossums, foxes, coyotes, etc.). 

• Birth control currently has higher financial costs. However, ContraPest is a relatively low-cost product at about $5/lb, per Dr. Loretta Mayer, the scientist who developed the product and is a founding principal of WISDOM Good Works.

• SenesTech CSO Nicole Williams says that the cost of synthesizing active ingredients in ContraPest will probably go down as the company explores alternative sources and methods. However, the negative externalities of rodenticides are higher in terms of accidental poisonings, secondary ingestion, and suffering of the target rodents.

Contraceptives for animal population control is not novel

• Contraceptives have been a method in use for deer populations in the U.S. (with nixed results), monkeys in Thailand, pigeons in Canada and several countries in Europe.

• Many cities have conducted, are conducting, or are planning to conduct pilot projects using ContraPest to reduce rat populations. Those include New York City; Boston; Columbus, Ohio; Hartford, Connecticut; and Ferndale, Michigan. Seattle, Washington, ran a successful trial in 2022 with a 90% reduction of the city’s rat population.

Efforts are underway in other cities and states to use alternative, more humane methods of rat control that do not harm wildlife, including rat contraception: Evanston, New York, Queen Anne in Seattle, and Jamaica Plain in Boston.

Fumigation

Some cities, such as Evanston and New York City, have had success eliminating rat burrows using carbon dioxide fumigation. This method does not affect other wildlife and can efficiently remove many rats simultaneously in burrow systems.

Success stories in controlling rat populations

There are a limited number of success stories in controlling rat populations, which points to the challenge of the problem and that rodenticides have limited effectiveness. Rat control has been successful in some cases that are not replicable in urban environments with existing populations. For example, Alberta, Canada, began working on this in the 1950s and successfully stopped rats from becoming entrenched by addressing the issue aided by strong policies, geography, and climate, while small islands have been able to remove entrenched populations. 

Urban cities have been examining alternatives to rodenticides. Newton, Massachusetts, a suburb of Boston, uses an integrated approach that focuses on garbage containment strategies, eliminating outdoor nests and burrows with carbon monoxide, birth control for interior spaces, and reducing habitat by increasing the amount of rodent-proofing required in new construction and increasing inspections to improve compliance. These alternatives have shown they can have an effect when used in a holistic strategy.

Why are urban environments conducive to rat infestations?

Many experts on urban rats have highlighted the link between thriving rat populations and accessible garbage. In Chicago, Lincoln Park Zoo scientists found that alleys with more uncontained garbage had more rats and more rat complaints relative to alleys with less uncontained garbage. As such, it is imperative that garbage be secured in rat-proof containers. The current 311 reporting system to replace damaged garbage receptacles is a great step in reducing rat access to food waste. However, it relies on residents being 1) aware of the reporting system, 2) aware that their receptacles are damaged, and 3) motivated to make reports. To increase the rate of damaged receptacle replacement, it would be more efficient for Streets and Sanitation staff collecting garbage to report the need for replacement cans directly.

In addition to accessible garbage, clutter from overgrown vegetation, discarded items, and other debris can provide shelter and harborage for rats. To minimize these conditions, cluttered property owners could be given educational material about rats and later fined if conditions are not improved. Crucially, fines must be enforced, as a recent investigation found many unpaid rat-related fines.