NEW YORK, NY— For decades, the playbook for fighting rodents has been simple: set bait, kill the pests and repeat.
But researchers say the rodents are evolving.
A Rutgers University study found that many house mice collected across New York, New Jersey and Pennsylvania carry genetic mutations linked to resistance against the anticoagulant poisons commonly used to control them.
Those mutations can allow some mice to survive treatments that once worked, forcing property owners and pest control companies to rethink how they battle infestations.
"We really should focus on alternatives, rely less on the chemicals, so that you wouldn't end up having higher resistance and more features in future," Rutgers entomologist Changlu Wang, who led the study, said in an interview with Patch.
Researchers analyzed 147 house mice collected between 2021 and 2025 from communities across New York, New Jersey and Pennsylvania.
More than 84 percent carried at least one genetic mutation associated with resistance to commonly used anticoagulant rodenticides.
New York produced some of the highest rates in the study.
Only two of the 34 mice collected statewide showed no known resistance mutations.
Samples came from Manhattan, the Bronx, Brooklyn, Goshen and Syracuse, with Manhattan accounting for the largest share of New York City specimens.
The findings do not mean every resistant mouse will survive every bait application.
But they do suggest that populations are adapting after decades of widespread poison use.
"Previously, a susceptible population, they can be killed by this one feeding," Wang said. "Now, maybe they have feed more, or they may even don't die because they have that mutation allow them to counteract the effect of the poison."
House mice have several advantages when it comes to evolution.
"They have a shorter life cycle, they mature faster than rats, so naturally they're more likely to mutate," Wang said.
He said mice also appear to carry greater natural genetic variation than rats, giving resistance more opportunities to emerge.
Their behavior helps, too.
"Mice have what we call the neophilia, so they like to explore for new things, new objects, so they're more likely to expose to the poison compared to rats," Wang said. "The rats are very smart, they tend to avoid new things, new food."
Because mice encounter bait more often, individuals with resistance mutations survive and pass those traits to future generations.
No.
But Wang said many resistant mice require larger doses or multiple feedings before dying. Some survive treatments altogether.
That can leave pest control companies using more bait or switching repeatedly between different products.
The problem, he said, is that changing poisons does not stop evolution.
"Rodents can evolve," Wang said. "As a result, those chemicals become less effective."
Instead, he recommends rotating products while emphasizing sanitation, sealing entry points and other non-chemical strategies.
The study arrives as New York City begins testing an entirely different strategy.
Last year, the City Council approved a pilot program that uses rat contraceptives in designated rat mitigation zones rather than relying solely on traditional rodenticides.
The program requires the Department of Health and Mental Hygiene, working with the Department of Sanitation and rodent-control experts, to establish at least two residential pilot areas, each covering at least 10 city blocks.
Officials must also identify similar comparison neighborhoods that will not receive contraceptives so researchers can measure whether the treatment actually works.
Unlike traditional poisons, the bait used in the pilot does not kill rats.
Instead, it contains cottonseed oil, which reduces fertility after repeated feeding.
The goal is to shrink rat populations gradually over time while reducing reliance on rodenticides.
For at least 12 months after deployment, inspectors will visit each neighborhood every month, documenting burrows, droppings, runways, gnaw marks and live rats.
They will also monitor how much contraceptive bait rodents consume.
Within six months after the pilot ends, the City must publish a public report detailing whether the program reduced rat activity, what it cost, whether wildlife suffered unintended effects and whether contraceptives could replace some traditional rodenticide use.
Although Wang's study focused on house mice rather than rats, he said the broader lesson applies across rodent management.
"I think all the public residents, property owners, pest control companies, they all need to pay attention to the poison resistance issue," he said.
Researchers say resistant rodents may create another problem.
When mice survive after eating poison, they continue moving through neighborhoods carrying toxic chemicals in their bodies.
That makes them more likely to expose predators to larger doses.
"Mice may have higher poison in their body and they are still alive," Wang said. "Therefore the predators, owls, eagles, vultures...skunks, foxes, etc., they would more likely to be exposed to higher dose of poisons."
That concern echoes one of the goals built into New York City's contraceptive pilot.
The law requires the city to document any harm to non-target wildlife and identify opportunities to reduce the use of traditional rodenticides if the fertility-control program proves successful.
Not yet.
While the study found resistance throughout the Northeast, Wang said researchers need far more samples collected systematically over time before they can compare neighborhoods block by block.
"We continue to collect samples if we want, say, compare neighborhood with neighborhood," he said. b
He said some European countries routinely monitor rodent resistance, giving pest managers better information about where different control methods are needed.
"In the U.S. we don't have much federal interest in funding this type of research," Wang said.
Researchers tested: 147 house mice from New York, New Jersey and Pennsylvania.
They found resistance-linked mutations in:
New York samples came from:
The researchers say resistance appears throughout the region rather than in one isolated location.
It doesn't poison rats.
Instead, the bait contains cottonseed oil, which reduces fertility after repeated feeding.
The City will:
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