Pest control companies routinely use traps baited with rodenticide to kill rats and mice found in homes, restaurants, and businesses throughout the Philadelphia area, but a recent Rutgers University study suggests those companies face a gnawing problem.
Researchers discovered that mice in Philadelphia, Trenton, and suburbs like Levittown and New Hope harbor genetic mutations that shield them from standard chemical baits.
In fact, a majority of house mice sampled from Northeast urban areas, including Manhattan and other New York City boroughs, carried at least one mutation linked to rodenticide resistance — a clear sign that pests are actively evolving to survive common poisons.
Rats presented a different problem. While they lacked the chemical-resistant mutations found in mice, the study’s author suggests they possess the cognitive sophistication to outsmart and evade traps entirely.
Lead author Jin-Jia Yu, a postdoctoral researcher in Rutgers’ entomology department, said the findings indicate that pest control companies might need to develop different strategies.
Yu conducted his research with the supervision of another of the paper’s authors, Changlu Wang, an entomologist in the same department.
Published in the April issue of Pest Management Science, the peer-reviewed study was launched after frustrated pest control professionals repeatedly approached the Rutgers lab, reporting that rodents routinely survived multiple treatments.
“For the house mouse, we saw much more mutations rather than Norway rats,” Yu said. Norway rats are the common brown rat often seen in sewers. “Genetic mutation is not that special in these creatures. But we found that the house mouse shows a lot of genetic mutations related to rodenticide resistance.”
Rodents are a bigger problem in cities
This study focused on urban rodents. It found that mice in big cities such as Philly and New York had a high frequency of mutations of a certain gene.
Rodents are a bigger problem in cities than more rural areas. Data cited in the study indicate that an average of 12% of all households experience rodent sightings. But major metropolitan areas reporter higher rates, including Philadelphia (29%), Washington (20%), and Manhattan (15%).
Yu said that similar studies of mutations in house mice and Norway rats were conducted in Europe and that research in the U.S. has been limited. One study in 2009 did find some rats in England with mutations that made them resistant.
However, Yu said there had been no such studies in the Northeast.
It has long been known that rodents developed resistance to the rodenticides developed in the 1950s. So more potent compounds were created in the 1970s and include brodifacoum, bromadiolone, difenacoum, and difethialone.
The poisons contain anticoagulants that interfere with the activation of vitamin K reductase (VKOR), an enzyme essential for blood to clot. Eating the bait leads to fatal internal bleeding.
The Rutgers team looked for mutations in the gene known as VKORC1 that makes the enzyme.
Pest control companies, as well as the Philadelphia Department of Public Health, sent the researchers the tails of caught rodents. Yu said his research was possible only with their help.
A rare mouse mutation in Philly
The researchers analyzed DNA from 147 house mice and 143 Norway rats collected in New York, New Jersey, Pennsylvania, and Washington, D.C.
Among house mice, 84% carried at least one mutation in the VKORC1 gene. Nearly 70% carried mutations known to help mice survive rodenticides.
Of 24 mice collected in Philadelphia, the majority had a mutation and five had two. One mouse had a rare mutation.
Of 20 mice collected in Trenton, 10 had two mutations. Lansdale, Levittown, and New Hope had one mouse each with a mutated VKORC1 gene.
About 35% of the Norway rats also carried mutations. However, scientists do not yet know whether those mutations result in resistance in the rats.
Mice, Yu said, might be genetically adapting faster than rats because they are curious and more likely to eat unfamiliar food, including rodent bait.
However, rats will avoid new objects, including live traps, and learn from their encounters.
In other words, not only are mice mutating to survive, but rats may be learning to avoid entrapment.
“They’re pretty smart,” Yu said of rats.