Urban Rodents Are Evolving To Survive Common Poisons

Urban rodents may be evolving genetic defenses against widely used control methods.

A mouse that eats poison and survives does more than escape a trap. It may pass that survival advantage to the next generation.

That possibility is becoming increasingly relevant in cities across the northeastern United States, where pest control professionals have reported that some rodent infestations are no longer responding as expected to standard treatments.

Researchers at Rutgers University have now found evidence that genetic resistance may be part of the problem. In a study of urban rodents, 84% of the house mice tested carried at least one mutation associated with resistance to rodenticides.

The findings suggest that decades of chemical control may be shaping rodent populations in ways that make some poisons less reliable. The research was published in the international journal Pest Management Science.

“Pest management professionals often told us that rodent control was becoming more difficult in some areas, even though they applied the effective rodenticides,” said Jin-Jia Yu, a postdoctoral fellow in the Department of Entomology at the Rutgers School of Environmental and Biological Sciences and the first author of the study. “I wanted to find out whether this was occurring in the northeastern United States, especially the metropolitan areas, and how widespread the problem might be.”

Yu works in the laboratory of Changlu Wang, an extension specialist in the Department of Entomology and a leading expert on urban pests, including rodents, cockroaches, and bed bugs.

A Genetic Clue to Failed Treatments

The team examined DNA from 147 house mice and 143 Norway rats collected in urban areas of New York, New Jersey, Pennsylvania, and Washington, D.C.

The researchers focused on a gene called Vkorc1. This gene is involved in the biological pathway targeted by anticoagulant rodenticides, which interfere with blood clotting and are among the most commonly used rodent control chemicals in the U.S.

Certain changes in Vkorc1 can reduce the effects of these poisons, allowing an exposed animal to survive.

Among the house mice tested, 84% carried at least one mutation in the gene. Nearly 70% had mutations already known to help mice withstand commonly used rodenticides.

The pattern was less pronounced in Norway rats. About 35% carried mutations in the same gene, although the biological effects of many of those variants remain uncertain.

“We found that resistance appears to be much more widespread in house mice than many people realized,” Yu said. “Norway rats also carried genetic mutations, but scientists do not yet know whether most of those mutations affect Norway rats’ susceptibility to rodenticides.”

A rat steals bait from a trap without getting caught, illustrating the kind of behavior Rutgers researchers Changlu Wang and Jin-Jia Yu are studying as they investigate why some urban rodent populations are becoming more difficult to control. Credit: Wang Lab/Rutgers University

Mutations Scientists Have Not Seen Before

The researchers also identified several genetic variants that had not previously been documented in house mice or Norway rats.

Whether those variants increase resistance is still unknown. Genetic screening can reveal that a mutation is present, but additional laboratory work is often needed to determine how strongly it affects survival after exposure.

The study began after years of discussions with pest control professionals who described recurring infestations despite repeated treatments. Those reports raised a broader question: Were control failures caused only by bait placement, sanitation, and rodent behavior, or were the animals themselves becoming less vulnerable?

The new results suggest that, in many mouse populations, genetics may be an important part of the answer.

How Resistance Spreads

Rodenticide resistance is an example of evolution occurring under human pressure.

When a population is repeatedly exposed to the same poison, animals with protective mutations are more likely to survive and reproduce. Over many generations, those mutations can become increasingly common.

Not every treatment failure is caused by resistance. Rodents may avoid bait, find other food, enter through unsealed gaps, or recolonize an area after treatment. Poor sanitation and easy access to shelter can also allow an infestation to persist.

Resistance, however, can make those existing problems harder to solve. A baiting program that once worked reliably may remove susceptible animals while leaving more tolerant individuals behind.

Why Mice and Rats May Respond Differently

The Rutgers researchers found that house mice appeared more likely than Norway rats to carry known resistance mutations.

Behavior may help explain the difference. Mice tend to investigate unfamiliar foods quickly, which can increase their exposure to poison baits. That repeated exposure may create stronger evolutionary pressure on mouse populations.

Rats are often more cautious around unfamiliar food and objects. This behavior, known as neophobia, can make them difficult to trap or bait even without genetic resistance.

“Rats are very clever,” Yu said. “They will approach the novel food many times before they really take the food or the bait.”

That hesitation can protect rats from immediate exposure, while mice may encounter rodenticides more often and therefore face stronger selection for resistance.

A Public Health Challenge

Rodents are not simply an inconvenience. They can contaminate food, damage wiring and buildings, and carry pathogens and parasites.

In densely populated cities, even a modest decline in control effectiveness can have wider consequences. Infestations may last longer, require more labor to eliminate, and lead to greater use of chemical treatments.

“This research provides some of the first information on rodenticide resistance in the northeastern United States,” Yu said. “By understanding how prevalent the mutations are and where resistance exists, pest management professionals and public health agencies can make better decisions about how to control rodents.”

The findings may also help explain why a treatment that works in one neighborhood performs poorly in another. Urban rodent populations are often highly localized, and resistance may vary from block to block depending on previous exposure and breeding patterns.

Rethinking Rodent Control

Wang said the results reinforce the need to move beyond strategies that depend too heavily on poison.

“Rodents are more than a nuisance,” Wang said. “As resistance becomes more common, it becomes even more important to use science-based management strategies that protect both public health and the environment.”

A broader approach, often called integrated pest management, combines several methods instead of relying on a single chemical. These may include closing gaps around pipes and doors, removing food and water sources, improving waste storage, reducing clutter, monitoring activity, and using traps where appropriate.

The goal is not only to kill rodents already present, but also to make buildings and surrounding areas less suitable for them.

“Studies like this help us understand how rodent populations are changing and how our management strategies need to evolve with them,” he added.

The researchers hope genetic information will eventually help pest control professionals choose more effective strategies for specific locations while reducing unnecessary pesticide use.

“Ultimately, we want to help communities maintain effective rodent control, reduce unnecessary pesticide use and protect public health,” Yu said.

Reference: “Distribution and frequency of Vkorc1 polymorphisms in house mice and Norway rats in the northeastern United States” by Jin-Jia Yu, Alvaro Toledo, Adrienne E Kasprowicz, Megan V Phifer-Rixey, Xiaodan Pan, Babatunji Daramola and Changlu Wang, 21 April 2026, Pest Management Science.
DOI: 10.1002/ps.70833

Funding: National Institute of Food and Agriculture, U.S. Department of Agriculture, U.S. Department of Housing and Urban Development, U.S. Department of Housing and Urban Development, U.S. National Science Foundation

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