The following research was conducted and summarized by Chicago Bird Alliance’s Rodenticide Research Team
Impacts of anticoagulant rodenticides on target rodents and non-target wildlife populations
Anticoagulant rodenticides (ARs) are a common way to control commensal rat populations in cities around the world (Jacob and Buckle, 2018). ARs kill rats, but by extension harm non-target wildlife, by preventing blood clotting (Vandenbroucke et al., 2008). Since the development of Warfarin in 1950, ARs have become more potent as rats evolved resistance to first generation ARs (hereafter, FGARs) and therefore, second generation ARs (SGARs) were developed to require fewer feedings to administer a lethal dose (Jacob and Buckle, 2018). Warfarin, chlorophacinone, and diphacinone generally require that an animal eat multiple doses of the bait over several days. These are known as multiple-dose or "first generation" anticoagulants. SGARs or single-dose anticoagulants are more toxic because they bind more tightly to the enzyme that makes bloodclotting 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.
As a result, newer products containing SGARs are particularly harmful to non-target wildlife because they can cause biomagnification, which is the accumulation of toxicants in predator tissues after consuming poisoned prey (Elliott et al., 2014). This biomagnification of ARs also cause morbidity and mortality in many non-target wildlife species that consume poisoned prey, notably predatory birds (Thorton et al., 2022, Okoniewski et al., 2021), top mammalian carnivores, and endangered species (Sánchez-Barbudo et al., 2012, Riley et al., 2007, Cypher et al., 2014). The efficacy of ARs for controlling rat populations is not well understood, but 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.
Alternative methods of controlling rat populations
Contraception (ContraPest, GoodBites)
ContraPest/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 the manufacturer’s Senestech’s own research.
The Good Bites form of this contraceptive comes in pellets filled with fat and salt which taste sweet and delicious to rats. It is claimed rats prefer the taste of Good Bites/ContraPest(liquid) to other food sources.
ContraPest is a low cost product at about $5/lb, per Dr. Mayer the scientist who developed the product and is a founding principal of Wisdom Good Works.org
Many cities have, are or are planning to, conduct pilot projects using ContraPest to reduce rat populations. Those include New York City, Boston, Columbus, Ohio, Hartford, Conn., and Ferndale, Michigan. Seattle, Washington ran a successful trial in 2022 with a 90% reduction of the city’s rat population.
Laboratory Evaluation of the Effectiveness of the Fertility Control Bait ContraPest on Wild-captured Black Rats
The use of contraceptives to control animal populations is not novel: contraceptives have been a method in use for deer populations in the U.S., monkeys in Thailand, and pigeons in Canada and several countries in Europe.
Crucially important, GoodBites/ContraPest is a non-toxic alternative: safe for both the target species (rats) and non-target species (organisms that consume the rats; raptors, opossums, foxes, coyotes, etc.) and more economical than ARs.
Birth control currently has higher financial costs. The cost of rodenticides is lower — in the case of ContraPest with its expensive active ingredients, probably intrinsically so (though SenesTech CSO Nicole Williams says that the cost of synthesizing active ingredients 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.
Good Bites - 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 without poisons that could harm the environment. More about these efforts can be found in the organization’s most recent newsletters:
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:
Seattle, Queen Anne Pilot https://raptorsarethesolution.org/poison-free-by-2023/
Boston, Jamaica Plain https://wisdomgoodworks.org/jamaica-plain/
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 (Fogarty, 2019).
Urban cities have been examining alternatives to rodenticides. Newton, Massachusetts, a suburb of Boston, uses an integrated approach which 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 (Elmore, 2022). These alternatives have shown they can have an effect when used in a holistic strategy (Chan 2023; Elbeshbishi, 2022)
Burrow Rx fumigation
Prompt attention to trouble spots
New York City
Recycling bins to accommodate pizza boxes
Burrow Rx fumigation
Urban environmental conditions conducive to rat infestations
Many experts on urban rats have highlighted the link between thriving rat populations and accessible garbage (e.g. Parsons and Munshi-South, 2020). 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 (Murray et al. 2018). 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 1) residents being aware of the reporting system, 2) aware that their receptacles are damaged, and 3) residents being 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 (Block Club Chicago, 2023).
Archived content:
• What are the different forms of rodenticide, how toxic are they to animals (raptors, other birds, other animals), how effective are they
There are many different active ingredients registered as rodenticides in the United States. They can be grouped together according to how they work.
Anticoagulants: stop normal blood clotting
Bromadiolone, chlorophacinone, difethialone, brodifacoum, and warfarin are all anticoagulants.
Non-anticoagulants: work in different ways
Includes zinc phosphide, bromethalin, cholecalciferol, and strychnine.
Warfarin, chlorophacinone, and diphacinone generally require that an animal eat multiple doses of the bait over several days. These are known as multiple-dose or "first generation" anticoagulants.
Single-dose anticoagulants are more toxic because they bind more tightly to the enzyme that makes bloodclotting agents. They can also interfere with other steps in Vitamin K recycling. Second-generation, or single-dose anticoagulants, are not easily excreted from the body, and they can be stored in the liver.10 Most of the single-dose rodenticides are not allowed to be marketed to non-licensed applicators.11 Instead of classifying anticoagulants into "first generation" or "second generation", many sources refer to them as single-dose or multiple-dose rodenticides because this is less confusing.
Anticoagulant Rodenticides are a particularly deadly category of rodenticide that have been implicated in many of the raptor fatalities that we have seen. There are 11 rodenticides (7 anticoagulent rodenticides and 4 nonanticoagulent rodenticides) that have recently undergone the registration review process as required by FIFRA. They are as follows:
FGARs (first generation anticoagulant rodenticides)
● chlorophacinone,
● diphacinone (and its sodium salt),
● warfarin (and its sodium salt)
SGARs (second generation anticoagulant rodenticides)
● brodifacoum
● bromadiolone
● difenacoum
● difethialone
Non- ARs
● bromethalin
● cholecalciferol,
● strychnine,
● zinc phosphide[1]
More research is needed on rodenticide efficacy
ALTERNATIVES: what are they and how effective are as compared to Single/Multi-Dose rodenticides
Integrated pest management
Table 1: General classification of pest control methods by their means of population control.
EPA is also in the midst of doing a Biological Evaluation (BE) of these 11 rodenticides as required by the ESA. https://www.regulations.gov/docket/EPA-HQ-OPP-2023-0567/document
The EPA has already issued a draft BE and solicited comments re the draft. https://www.regulations.gov/document/EPA-HQ-OPP-2023-0567-0001/comment
November 2024 is the deadline for the final BE.
In the meantime, you can learn more at the EPA link above and also at the Regulations.gov website. Each of the above chemicals has a “docket” (file) which you can review at Regulations.gov. You can retrieve the docket simply by typing in the name of the chemical in the search box. The agency’s registration review is considered a “nonrulemaking” action. Within the docket, you will see documents including rules, proposed rules, notices and supporting materials. You will also see comments from members of the public, including companies, nonprofit organizations and everyday people. Submitting comments is one way we can make our voices heard. Comments often refer to studies and scientific data.
How do rodenticides affect animals locally? Affect populations?
Rodenticide baits are made to attract animals. Pets and wildlife may take the bait if they find it. When an animal eats the bait directly, it is called primary poisoning. Secondary poisoning is caused by eating poisoned prey. It may also be called relay toxicosis. See the fact sheet on Ecotoxicology. For ways to prevent exposures, see the information below about what you can do to reduce risks.
The rodenticides with high secondary poisoning risks to birds such as hawks and owls include difethialone, brodifacoum, and possibly bromadiolone (see Table 3).23 The rodenticides that pose the greatest secondary poisoning risks for wild mammals, dogs and cats include chlorophacinone, diphacinone, bromadiolone, and brodifacoum. Bromethalin and cholecalciferol may pose secondary risks but these risks have not been measured.2