Entomologists and pest-management researchers at Rutgers University in New Jersey had a mystery on their hands. Over the course of several years, many local pest control crews had told them similar horror stories of persistent rodent infestations defying their best efforts.
“Pest management professionals often told us that rodent control was becoming more difficult […] even though they applied the effective rodenticides,” Jin-Jia Yu, a postdoctoral fellow with the school’s department of entomology, explained in a press statement.
Yu and his colleagues now know why. Detailed genomic analysis of DNA from 147 house mice (Mus musculus domesticus) collected from urban areas across New York, New Jersey, Pennsylvania and Washington, D.C. has discovered that 84% of house mice carry a mutation that helps these pests resist anticoagulant rodenticides. And over a third (35%) of the team’s 143 Norway rat specimens (Rattus norvegicus), all sourced from those same northeastern cities, carried these mutations too—polymorphic alterations to the creatures’ Vkorc1 gene.
Anticoagulants are the most widely used rodent-control compounds in the U.S., favored in part because these slow-acting poisons lessen the likelihood that rat and mouse populations will associate the poison traps with danger and learn to avoid them.
These evolving rodent species are “more than a nuisance,” according to Rutgers urban pest specialist Changlu Wang, who oversees the lab where Yu works. “As resistance becomes more common, it becomes even more important to use science-based management strategies that protect both public health and the environment.”
Fear & scurrying
Wang and Yu also probed the divergence between greater house mouse resistance to these poisons and the still alarming, but less dramatic, presence of these mutations in the northeast’s Norway rats. One key difference, they theorized, was rats’ heightened food paranoia.
“Apart from habitat differences, house mice are often characterized as neophilic (attracted to novel objects), whereas rats exhibit neophobic tendencies (showed wariness and avoidance of novel objects),” the researchers wrote in their study for the journal Pest Management Science.
“In some cases,” they noted, “rats have evolved heightened neophobic behaviors in response to intensive rodent control efforts, such as avoidance of bait containers.” So, contrary to what you might recall from Pixar’s Ratatouille, house mice are a much more culinary adventurous species than urban rats, which probably made Remy even more of an outlier really.
Yu believes that textbook natural selection on the more curious, bait-tasting mice have helped speed up anticoagulant poison resistance within the mice’s Vkorc1 gene. The team found that 69% of these mice held one of two specific mutations, L128S or Y139C, that have already been documented as poison resistance genes.
Many of both these rodents species’ other mutations, however, were brand new, meaning researchers still have more work to do investigating the exact impact of this altered DNA.
“We found that resistance appears to be much more widespread in house mice than many people realized,” Yu noted, “[…] but scientists do not yet know whether most of those mutations [also] affect Norway rats’ susceptibility to rodenticides.”
Life finds a way
According to Wang—one of the nation’s leading experts on the behavior, ecology, and management of urban pests—the new study should be a wake up call to city planners and urban pest control professionals.
“Studies like this help us understand how rodent populations are changing and how our management strategies need to evolve with them,” Wang said.
Specifically, he hopes that urban pest managers will try to reduce their reliance on chemical rodenticides—pairing more targeted use with greater attention to sealing spaces that rodents use to gain entry, improving waste removal practices, deploying traps, and altering habitats. The researchers said this shift could reduce both the health risks of disease-carrying rodents and the environmental risks of increased rat poison chemicals—which can leech into waterways and impact people and wildlife.
“Ultimately,” as Yu put it, “we want to help communities maintain effective rodent control, reduce unnecessary pesticide use and protect public health.”
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