Urban rodents may be evolving against common poisons

For years, pest control professionals throughout the Northeast have reported a troubling pattern. In some neighborhoods, rodents seemed increasingly more difficult to eliminate, even when standard control methods were used.

Now researchers at Rutgers University believe they may know one reason why.

A study found that 84% of house mice sampled from urban areas in the Northeast carried at least one genetic mutation linked to rodenticide resistance, suggesting many mouse populations may be evolving ways to survive the poisons commonly used to control them. 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 one of the nation's leading experts on the management of urban pests, including cockroaches, bed bugs and rodents.

The researchers analyzed DNA from 147 house mice and 143 Norway rats collected from urban areas in New York, New Jersey, Pennsylvania and Washington, D.C. They focused on a gene called Vkorc1, where certain mutations have been associated with resistance to anticoagulant rodenticides, the most widely used rodent-control chemicals in the U.S.

The results were striking.

Among the house mice examined, 84% carried at least one mutation in the Vkorc1 gene, and nearly 70% carried mutations already known to help mice survive common rodenticides. About 35% of the Norway rats also carried mutations in the same gene.

“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.”

The team also identified several genetic variants that had never before been reported in house mice or Norway rats. Scientists don’t yet know whether those newly discovered mutations contribute to rodenticide resistance.

The study emerged from several years of conversations between Rutgers researchers and pest-management professionals, many of whom reported persistent rodent problems despite repeated treatments.

The findings point to a long-running evolutionary contest between humans and one of their oldest urban adversaries. Anticoagulant rodenticides have been used for decades to suppress rat and mouse populations. Over time, rodents carrying mutations that help them survive exposure to those chemicals may gain an advantage, allowing resistance traits to spread through populations.

Researchers found that house mice appear to be adapting more rapidly than rats. One possible explanation involves behavior. Mice are naturally curious and more likely to investigate and consume unfamiliar food sources, including poison baits, Yu said. Rats, by contrast, tend to be cautious and suspicious of new objects.

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

The findings have important implications for public health. Rodents, which contaminate food, damage buildings and infrastructure, can spread diseases and parasites. If commonly used rodenticides become less effective, communities may face greater challenges controlling infestations.

“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.”

Wang, a coauthor of the study, said the findings underscore the need for a broader approach to rodent management.

“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.”

The scientists’ goal is to help communities manage rodent populations effectively while reducing environmental risks. “Studies like this help us understand how rodent populations are changing and how our management strategies need to evolve with them,” he added.

Rather than relying exclusively on chemical controls, researchers recommend combining multiple strategies, including sealing entry points, improving sanitation, modifying habitat and using traps when appropriate.

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

Other Rutgers researchers who contributed to the study included: Alvaro Toledo, an assistant professor; Xiaodan Pang, a postdoctoral associate, and Babatunji Daramola, a graduate student, all in the Department of Entomology in the School of Environmental and Biological Sciences.

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