An invasive, elusive 20-pound rodent threatens California agriculture

Wearing chest-high waders, biologist Sean McCain tries to tiptoe to the edge of a marsh in California’s Central Valley. A biologist with the state Department of Fish and Wildlife, McCain squints as he searches the shoreline.

“I don’t think they know we’re here yet,” he whispers. Then, excitedly, he points. “Is that one right there? I think we’re looking at one right there.”

What he sees is a 20-pound rodent that looks like a cross between a rat and a beaver, with a scaly tail, webbed feet, and big orange bucked teeth. It’s a nutria and it’s poking its head out of a dark cave of tule reeds.

McCain’s first saw a family of nutria here two weeks ago, and since then he’s returned to this spot almost every day to check on them.

Nutria can eat up to 25 percent of their body weight in one day. They munch on the roots of green duckweeds, cattails, and tule reeds. If they clear cut an entire marsh like this one, they put all the birds and frogs and other species that depend on it at risk. “I’ve been watching the vegetation recede away from the middle of the pond,” McCain says.

They can also tear up crops and levees, damaging the state’s water infrastructure, and threatening farms.

California is trying to eradicate nutria, an invasive rodent that is damaging irrigation ditches and farms.

Nutria aren’t native to California, or the United States. Fur farmers brought the South American rodent to Southern California in the late 1800s as an attempt to make an affordable mink alternative. After multiple attempts, the nutria fur business never took off, but the rodents went feral. California’s Department of Food and Agriculture determined they were eradicated in the 1970s.  

Last year, though, a few were spotted again in Merced County, and now they’re multiplying. In just over a year, a female nutria and her offspring can have up to 200 babies. This April, the state created a task force to fight them. Now, the rodents are on the move, heading north towards the San Joaquin Delta, California’s most important water source.

The nutria problem is potentially so big that the Department of Fish and Wildlife is pulling staff from all over the state for on-the-ground training in finding and eradicating the pest.

At another pond, the team hasn’t seen any nutria in the flesh, so they’re setting up a wildlife camera to try and capture them on tape, creating a feeding platform within the camera’s range.

They wade into chest deep water, yank reeds out of the marsh to make a little nest, and put bait on a wooden platform: bright orange slices of sweet potato. It turns out, nutria love its color and taste. Because these are unlikely weapons in the fight against nutria, local farmers are donating barrels of sweet potatoes. They fear a nutria scourge could wreak havoc on California’s entire ag industry.

Sweet potato farmer Stan Silva hadn’t even heard the word “nutria” until a few months ago. He’s still never seen one, but he’s worried about the damage nutria could do if they aren’t  eradicated.

“It would be devastating,” Silva says. “They can basically ruin the ag industry here — they get in your fields, burrow into your canal ways, your waterways. They’re just a menace.”

Aaron and Stan Silva donate tons of sweet potatoes to help fight the invasive nutria rodent.

Stan Silva’s grandparents came to the Central Valley from the Azores and Lisbon in Portugal in the early 1900s, and started farming on small plots. Now, three generations of Silvas grow sweet potatoes on 850 acres and supply the largest retailers in the country.

In the back is a huge gyro machine, which processes 30 bags of sweet potatoes in a minute. Grandson Rueger Silva’s in charge of its operation.

“I’m only 18, but my dad had a heart attack so I had to come back and help with everything,” he says as he darts around the machine, pressing buttons and checking the conveyor belt.

In late summer, 44-year-old Aaron Silva had a massive heart attack that left him hospitalized for a month.

“[Rueger] left college to come back to help on the ranch until his father gets further along here,” Stan Silva says proudly.

Aaron Silva’s still recovering from brain injury that was a side-effect of the heart attack. He says he’s having to re-learn customers’ names and faces. But, he says, he didn’t lose any memory of the process of farming and packing sweet potatoes. Farming’s in his blood, and he doesn’t want anything to threaten it.

For this family, giving away five tons of sweet potatoes is a drop in the bucket compared to the work Fish and Wildlife employees like Sean McCain are doing to fight nutria. “We’re fortunate that these guys are out on the battle lines doing this for ag,” Stan Silva says. “Not just for us, but or all of ag.”

McCain does this kind of work all day long: trying to get eyes on the rodents, assessing habitat, and checking cameras to determine the right place for traps. After climbing out of the marsh, he plugs an SD card from one of the wildlife cameras into his laptop.

“There we go,” he says, as a big family of nutria sneak onto the screen. “Those are adult nutria, and that one just stole our sweet potato.”

Now that the team’s confirmed a big family of nutria in this swamp, they’ll send in trappers, who will start to eradicate this population one by one.

It’s part of a larger plan rolled out at the Nutria Incident Command Center at an old hunting check station near Los Banos, California. Greg Gerstenberg holds down the fort as the incident command chief.

In the deep freezer outback, Gerstenberg rummages around for frozen nutria. He pulls out nutria heads and bottles of their urine — helpful for trapping. They’re keeping these for training, and for display, so farmers can see what they look like.

They’re also studying how other states have dealt with the pest. Louisiana’s offering a bounty to hunters to counter their out-of control nutria problem. Gerstenberg’s modeling California’s efforts on the Chesapeake Bay’s, where nutria were eradicated.

On the walls of the command center, huge maps of the Central Valley and San Joaquin Delta are covered with pink, yellow and blue dots showing where nutria have been spotted. After capturing nutria on camera in the southern Delta, Fish and Wildlife will pull their resources from Merced County and create a sort of fire line just north of this sighting, limiting and pushing the nutria out before they start to multiply in the Delta.

The state’s trapped more than 330 nutria since April. Gerstenberg says he hopes they’re not in the thousands, but he has no way to estimate the number. “Our goal is to remove them quicker than they’re reproducing,” he says.

Correction – This story has been corrected to make clear that a female nutria and her offspring can have up to 200 babies in just over a year.

Top photo: Department of Fish and Wildlife biologist Sean McCain seeking out nutria in a marsh full of tule reeds. (Photo by Lisa Morehouse)

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