San Joaquin Farm Bureau Federation

By Vicky Boyd

An invasive rodent discovered knocking on the door of the Delta has raised worries among San Joaquin County farmers, who say any new pest is unwanted.

Of particular concern with nutria, the 20-pound semi-aquatic rodent now confirmed in five Central Valley counties, is its propensity to burrow, voracious appetite and high reproductive rate.

"It's just another thing for us to worry about," said Mary Hildebrand, a San Joaquin Farm Bureau board member who farms in the Delta. "But the disconcerting thing about this critter is it can reproduce so prolifically. It's not good, that's for sure. I think part of what's difficult is they look a lot like beavers, so it would be easy to think what you're seeing is just another beaver."

David Strecker, SJFB first president who also farms in the Delta, agreed. "It's another thing that causes damage that I have to be aware of when I'm out there. It's another non-native species – whether plant or animal – that can cause major effects." But nutria may be more than just a local problem if they begin burrowing into Delta levees, causing breaches that could threaten state water supplies, he said.

"This can blow up to be a very huge problem very quickly," Strecker said. 

Hildrebrand and Strecker's fears grew from the recent discovery of a single nutria on farmland west of Stockton. San Joaquin marks the fifth county in which the rodent has been found since the initial confirmation of nutria on private land in Merced County in 2017. The other counties where the invasive rodent has been confirmed are Stanislaus, Tuolumne and Fresno.

The San Joaquin County discovery was made after a dog dragged the dead animal back to a farmer, said Peter Tira, California Fish and Wildlife information officer. How the nutria wound up in San Joaquin County is a mystery, since the other populations are much farther south, he said. Whether the dead animal is part of a local breeding population also is unknown. "It's very concerning because it's right on the doorstep of the Delta," Tira said. "We had hoped to eradicate the population before it got to the Delta."

Until the first discovery in 2017, nutria had not been confirmed in California since the 1970s, when the state successfully eradicated the rodent.

Nutria actually were imported into California in the late 1800s, and the state department of agriculture licensed breeders into the 1930s and 1940s. When the fur market collapsed in the 1940s, many ranchers released their nutria because they couldn't afford to feed them.

A few theories have emerged about the source of the recent infestation, Tira said. One is the rodent was never really eradicated, and small populations remained in remote locations. He discounted that one, saying nutria probably would have been noticed because of the extensive damage they cause and their high reproductive rate.

Although nutria have been found in Washington and Oregon, Tira said mountains act as barriers to prevent migration. The most likely source is through humans transporting them.

"But we don't know and we may never know," he said.

With the first discoveries, Fish and Wildlife put together an interagency Nutria Response Team to draft an eradication plan. Because nutria have a steep reproductive curve, department officials say they have a narrow eradication window before rodent numbers grow out of hand.

Tira pointed to Louisiana's nutria problems as what California hopes to avoid. Instead, he said Maryland is an example of how a state can successfully eradicate nutria.

That state trapped more than 14,000 during the initiative and hasn't confirmed a single nutria in the past 2.5 years. "That's what we hope to do, and we're in constant contact with them," Tira said.

So far, most of the nutria trapped in California have been on private property, although the rodent also has been found on state and federal lands.

"So, farmers and landowners will be key in the fight to successfully eradicate it," he said. "We will provide free assistance to any landowner who asks for help to fight nutria."

Landowners in Merced County, where the rodent was first found, have been receptive and have granted Fish and Wildlife employees access so they can survey for and, if needed, trap the rodent, Tira said.

Hildebrand, who also sits on the South Delta Water Agency Board of Directors, said the group discussed nutria when it was originally discovered in 2017. But she said she suspects the board will revisit the subject because of the proximity of this latest discovery.

Nutria are classified as a non-game animal within California, meaning only property owners and their agents, such as employees, are allowed to kill them to protect their property, Tira said.

Nutria identification
Nutria are strong swimmers, frequenting both fresh and brackish water. At 20 pounds for a large adult, nutria loosely resemble a super-sized muskrat, which top out at about 4 pounds. Beavers, on the other hand, can grow to 60 pounds. The invader can be told from its native cousins by its white whiskers and round tail. Although muskrats may have a white muzzle, both the muskrats and beavers have black whiskers and flat tails.

Nutria also are voracious feeders, consuming up to 25 percent of their bodyweight in mostly riparian vegetation daily. But they may damage up to 10 times more in the process, leaving "eat out" areas that resemble an out-of-control lawnmower. In Louisiana where nutria are an endemic, Louisiana Wildlife and Fisheries leaders blame the rodent on denuding more than 40 square miles of coastline vegetation over a 16-year period. 

A predominately nocturnal animal, nutria tend to be elusive, swimming along ditch banks and avoiding open water. Female nutria reach reproductive age at 4 to 6 months old. Although litters average four to five young, females can have up to 13 young per litter and up to three litters per year.

For any landowner with water-control structures, burrowing rodents such as ground squirrels or muskrats are constant worries. With several pounds of body weight on them, nutria are an even larger concern because of their potential for eroding banks and weakening structural foundations, Tira said.

If you think you've seen nutria or nutria sign, photograph it and report it immediately to the Department of Fish and Wildlife's Invasive Species Program at This email address is being protected from spambots. You need JavaScript enabled to view it. or (866) 440-9530.