NEW JERSEY— 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 Jersey 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.
Researchers analyzed 69 house mice and six Norway rats collected from communities across New Jersey, including Trenton, Paterson, New Brunswick, Branchville, Newton and Runnemede.
New Jersey house mice carried several genetic changes linked to resistance against anticoagulant rodenticides.
Researchers found resistance-linked genetic changes in many New Jersey house mice, with some of the highest concentrations appearing in urban areas.
The most common change appeared in 59 percent of samples, while other resistance-related changes appeared in smaller groups of mice.
In Trenton, researchers found multiple mice carrying the same resistance-related traits, suggesting those changes are circulating within the local population.
In Paterson, researchers found mice carrying more than one resistance-linked change at the same time, showing that some animals have developed multiple genetic traits associated with reduced sensitivity to anticoagulant rodenticides.
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 also increases their exposure to rodenticides.
"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.
New Jersey Norway rats showed a different pattern than house mice. Researchers found no resistance-linked nonsynonymous mutations in the state’s rat samples. Most New Jersey rats retained wild-type sequences across key gene regions examined.
Across the broader rat dataset, 35 percent of Norway rats carried at least one Vkorc1 mutation, but New Jersey populations did not show the resistance-linked variants identified elsewhere.
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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 alone 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 nonchemical strategies.
New Jersey focuses on integrated pest management, which combines multiple strategies instead of relying only on pesticides.
The state’s approach aim to reduce the conditions that allow rodents to survive, including removing food sources, sealing entry points and preventing buildings from becoming nesting sites.
Cities across New Jersey have increased efforts to limit access to trash, one of the biggest sources of food for urban rodents.
In Hoboken, for example, residents and businesses must use rodent-resistant trash containers to reduce curbside feeding opportunities.
New Jersey requires buildings to remain free of conditions that attract rodents, including structural gaps and areas where rodents can hide.
Sealing cracks, repairing damage and eliminating nesting areas can reduce infestations before chemical treatments are needed.
Rodenticides remain available in New Jersey, but are regulated under the state’s Pesticide Control Code.
The law requires rodent bait to be placed in tamper-resistant bait stations or areas inaccessible to children, pets, domestic animals and wildlife. Bait stations must also display the registered pesticide label.
Certain rodenticides, including second-generation anticoagulant products, require licensed applicators when used in regulated settings.
The New Jersey Department of Environmental Protection oversees pesticide use and certification requirements.
The state also allows newer rodent-control options, including EPA-approved fertility-control products that private property owners and pest control companies can use. These products aim to reduce rodent reproduction rather than kill animals through traditional poisoning methods.
Researchers say combining these approaches with sanitation, exclusion and targeted treatments can help slow the spread of resistance.
Researchers said resistance may create another concern beyond controlling rodents.
When mice survive after consuming poison, they continue moving through neighborhoods with toxic chemicals still in their bodies, increasing the chance predators will consume contaminated prey.
"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."
For Wang, the findings point to a broader shift in how communities should think about rodent control.
"I think all the public residents, property owners, pest control companies, they all need to pay attention to the poison resistance issue," he said.
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