Large Invasive Rodents Are Wreaking Havoc in California. New Research Suggests Someone Deliberately Introduced Them

Genetic testing revealed that nutria living in California since 2017 are most closely related to a population in central Oregon—too far for the creatures to have traveled on their own

Sarah Kuta | Daily Correspondent

April 16, 2026

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Someone appears to have intentionally introduced an invasive rodent to California, decades after the creatures were eradicated from the state.

For the past nine years, wildlife officials have been battling nutria, a large, semiaquatic rodent with a long tail and beaverlike teeth that’s native to South America. The destructive critters, which can wreak havoc on wetlands, crops and water infrastructure, were discovered in 2017 in San Joaquin Valley, an agricultural area in the middle of the state. That was a bit of an unwelcome surprise, because nutria were thought to have been wiped out in California in the 1970s.

Now, authorities have come up with a possible explanation for how the animals ended up there again. Recent genetic testing conducted by the California Department of Fish and Wildlife (CDFW) has revealed that the state’s nutria are closely related to a population in central Oregon.

The findings suggest that the pests got a little help crossing state lines from humans, who deliberately—and illegally—transported them to California, CDFW announced on April 7. Scientists detailed their research in a study published last November in the journal Evolutionary Applications.

“Given where nutria were rediscovered in California, it is nearly impossible that they could have migrated there on their own,” Michael Buchalski, a geneticist for CDFW, writes in an email to SFGate’s Ariana Bindman. “It’s too far of a distance, and we don’t find any nutria in the areas in between. That makes human introduction the most likely scenario.”

For now, officials say they can only speculate why someone might have brought the animals into the state. But perhaps the perpetrators “thought they could be an effective natural way to manage aquatic vegetation on their private property,” Buchalski tells SFGate. “Also, some people just really like nutria. … Or it could have been malicious, in hopes that they would cause environmental damage. It’s hard to know.”

To get to the bottom of the state’s rodent mystery, scientists collected nutria DNA data from around the nation and the world. They also tracked down decades-old nutria skins and skulls housed at museums and universities all over California, including a taxidermy specimen at the Butte County Weights and Measures Department office. DNA collected from these pre-eradication remains allowed them to directly compare the state’s past and present nutria.

Their investigation showed that California’s historical nutria descended from multiple diverse lineages, whereas modern-day nutria all came from a single lineage that’s most genetically similar to populations in central Oregon. The findings suggest that nutria were recently reintroduced, rather than having survived undetected for decades.

As part of the study, researchers built a comprehensive, global reference database of nutria DNA. This not only helped them identify the likely source of California’s nutria but should also help other land and wildlife managers compare populations and trace invasion lineages in the future.

Nutria first entered the United States when they were brought to Elizabeth Lake, California, in 1899. People wanted to use the animals for their pelts, thinking they could be sold in the lucrative fur trade. Initially, the nutria did not thrive in the state. But animals introduced later gained a foothold; historical records suggest that they were living in the Central Valley and South Coast by the 1940s and ’50s.

However, the invasive creatures began damaging the environment, prompting state officials to take action to curb the population. The species was declared eradicated from California by the late ’70s.

After that, officials did not detect any evidence of nutria in the state for years. Then, in 2017, they found a pregnant female in Merced County. Concerted eradication efforts have been underway ever since, with wildlife managers using visual observation surveys, motion-detection cameras and scat-detection sniffer dogs to locate and trap the creatures. Since the rediscovery, they’ve captured 7,841 nutria.

The state says it’s now spending about $5 million per year on eradication efforts. The work spans hundreds of thousands of acres of wetland and riparian habitat in the Sacramento-San Joaquin Delta and Central Valley.

The animals pose a major threat to the state’s aging levees and earthen dams, which are already strained by atmospheric rivers, drought and groundwater pumping.

“Nutria can weaken levees to the point where even minor stresses or environmental changes can trigger a breach,” Farshid Vahedifard, now an engineer at Tufts University, told New Scientist’s James Dinneen in 2023. “While nutria may not be the only contributing factor to levee failures, they can act as the final straw that breaks the camel’s back.”

Under state law, members of the public are allowed to kill nutria “by any legal means” if the animals are damaging their property or crops. They must report their kills to the state as soon as possible, ideally by providing both photos and the carcass.

However, wildlife officials urge caution, because nutria are easily mistaken for other aquatic mammals, including beavers, muskrats, river otters and mink.

“The majority of nutria reports received by CDFW have been muskrats, as have been some ‘nutria’ featured in the media,” according to the state.

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.