Almond farms battle Central California rodent surge. ‘An infinite sea of rats’

An infestation of roof rats in the central San Joaquin Valley of California has caused more than $300 million in damage as the population of the hungry rodents has spiked and farmers struggle to stop the vermin from causing more damage.

University of California researchers and almond industry officials said the rats have chewed through irrigation tubing, gnawed on trees, and chewed up nuts throughout Merced, Fresno, Kings and Kern counties.

Also troubling is that the city rats appear to have adapted well to their newfound home in almond country. In Fresno County alone, almonds are farmed on more than 300,000 acres

In a newsletter published by the Almond Board of California, officials reported meeting with growers about the severity of the problem. What they learned is the rats have switched from their traditional habitat of living above ground in nests to burrows in the ground.

The rats have also figured out that by scurrying along irrigation canals and other waterways they can connect to more orchards and vineyards. The increased mobility has exacerbated the extent of damage, complicating containment efforts, almond officials said.

“We live in an infinite sea of rats,” said Niamh Quinn, University of California Cooperative Extension Human-Wildlife Interactions Advisor. “They are everywhere.”

Quinn said it’s unclear what has caused the rise in the Valley’s rat population but she suspects it could be attributed to several factors, including favorable weather, additional food resources and an increase in the number abandoned orchards.

“The abandoned orchards are like little refuges for the rats,” Quinn said.

One grower told almond board officials that he had replace his entire drip irrigation system at a cost of $20,000. A second almond grower estimated rats destroyed 50% of his crop, even while he was exterminating between 50-100 rats a day.

A 2024 survey by the California Department of Food and Agriculture found that in some locations, as many as 32 rats per night were recorded.

The state department of agriculture estimated that farm losses ranged from $109 million to $311 million. Among the biggest losses was the replacement of irrigation drip lines that accounted for losses between $56 million and $168 million, and yield losses resulting from disrupted post-harvest irrigation, estimated at $43 million to $129 million.

Westside farmer Don Cameron said this latest infestation is beyond compare.

“This is the worst I have ever seen,” Cameron said.

The last significant spike in rats was in 2018 when wet weather created a perfect environment for the quick-breeding rats to flourish.

Growers have used nearly every tool they have available, including bait stations and aluminum phosphide, a highly toxic chemical used in fumigation to kill rodents. But insecticide treatments are labor intensive and costly.

University of California scientists recommend growers use a comprehensive Integrated Pest Management strategy that involves rigorous monitoring with the use of tracking tunnels and ink cards to identify what type of rodent they are battling. Field cameras can also spotlight hotspots.

Other options for growers include non-chemical solutions such as owl boxes or snap traps.

Quinn said one of the biggest challenge farmers face is that rats are prolific at what they do, “causing damage and making babies.” The best growers can hope for is finding a pest management plan that works.

“You are never going to kill all the rats,” she said. “And it will be a constant battle of monitoring and managing.”

This story was originally published August 7, 2025 at 7:30 AM with the headline "Almond farms battle Central California rodent surge. ‘An infinite sea of rats’."