Rodents are not a new problem for Central Valley growers.

However, the explosion in the roof rat population over the last 12 months has researchers and farmers scrambling to not only figure out what caused the boom, but also how to deal with it.

Calls to the state for help with rat infestations from western Fresno County growers — especially almond growers — spurred the California Department of Food and Agriculture to investigate the severity of the problem.

What the CDFA survey found: $109 million to $310 million in damage to trees, equipment, and irrigation lines across 112,000 acres in the western Central Valley.

Rats’ appetite for not only the nuts and tree bark but essential irrigation lines threatens the health of trees, said Fresno County Farm Bureau CEO Ryan Jacobsen.

High farming and water costs have caused farmers to abandon orchards. Similarly high costs to pull up those fields means thousands of abandoned acres that likely created ideal conditions for the rats, he said.

“Leaving it unchecked is not a possibility because I’ve seen firsthand where farmers have had significant reductions in production because of damage to the actual tree themselves or the irrigation supply,” Jacobsen said. “It’s just one of those very significant added costs that does hurt the success of the operation for the long term.”

Damage to Trees, Irrigation Lines Has Long-Term Effects on Orchards

While rats can be found in a variety of farms, they especially love orchards and vines, said UC Davis professor Roger Baldwin. They like to burrow and so with less soil tillage in permanent crops, the rodents can live undisturbed beneath dense tree canopies that protect them from predators.

Amongst the trees, they also have a steady supply of food. Not only will the rodents feed on the produce from the tree, but also the tree itself. The CDFA survey estimated 10% to 15% of trees per acre had direct damage, causing 20% yield reduction for those trees.

“They’re eating that bark or cambium layer of those branches, and if they girdle all or most of that branch, then that leads to mortality of that branch, which of course leads to loss of production. Not only for that year, but for the rest of the life of that tree,” Baldwin said.

The greatest damage, however, is with irrigation lines. If left unchecked, damaged lines mean trees don’t get water and that affects trees long term. Yields can be reduced by 73%, according to the CDFA survey.

Up to 75% of the acreage surveyed needed drip lines replaced, which cost farmers at the minimum $56 million. At the high end, it could $168 million.

“I’ve had instances where farmers would go out and put drip irrigation, brand new tubing out, and within 24 hours it was destroyed,” Jacobsen said.

Impact on Almond Prices

With such a formidable problem facing farmers, it’s natural to ask about the effect of the rat outbreak on crops such as almonds.

The USDA’s official estimate for the 2025 almond crop is 3 billion pounds — a 10% increase over 2024.

“Average prices fell 15% immediately following the report but stabilized soon after as international buyers reentered the market,” says AgWest Farm Credit.

Some growers feel the estimate may not reflect reality as they report anecdotally yields on the lighter side, Jacobsen said. But, with the problem concentrated in western parts of the Valley, damage caused by the rats may not be seen yet, he added.

Abandoned Acreage Creating Ideal Conditions for Rats

Solving the problem is no easy feat, Baldwin said.

Researchers think wet winters have contributed to the rise in the rats, but restrictions on rodenticides and abandoned orchards may have also created ideal conditions for the invasive species, he said.

Even without care, abandoned trees will still produce some food, and rats don’t have to fear traps or fumigants, he said.

Slow recovery to almond prices and higher farming costs have caused many farmers to abandon acreage. And pulling up trees is not cheap.

Of the 30,515 acres of abandoned almond orchards in California, Fresno County has 5,637, according to a 2024 study from the Almond Board of California.

“It’s gotten more challenging to remove an orchard for both large and small operations,” said a spokesperson from the Almond Board. “Ag waste burning has been phasing out since 2005 around the state. This year, that phase-out included small orchard removals, vineyard removals, and surface harvested prunings.”

Growers will do what’s called whole orchard recycling, where trees are ground down into chips and spread across the soil surface, the spokesperson said. That method has many benefits for the soil, but it is expensive. The growers who do it rely on incentives to help with the cost.

What’s more, clearing out the fields sends rats into neighboring orchards. Thus dealing with the problem requires short-term heartache, Baldwin said.

“Clearing fields would likely help reduce rat issues longer term, as it gets rid of some of the source populations from which the rats are able to invade into production fields,” Baldwin said. “In the immediate short-term, it could lead to an increase in rat populations if they move out of these fields and into fields currently in production.”

Aggressive Approach Needed Before Maintenance Treatment Can Be Effective: Baldwin

Getting a handle on the rodent populations will require a multi-pronged approach, Baldwin said. Unlike ground squirrels or voles, roof rats are non-native species, brought over centuries ago. They also compete with those native rodents, Baldwin said.

Because of the boom, it will take likely take aggressive action to bring them down to manageable numbers.

“In these kinds of situations, a rodenticide application may be warranted to knockdown initially high populations, but from that point then we would like to incorporate some other tools, perhaps trapping, perhaps the use of burrow fumigants,” Baldwin said. “In some cases we can utilize habitat modification to alter the desirability for those particular pest species.”

California has in recent years put restrictions on rodenticides, including Assembly Bill 2552, approved in 2024. Advocates say anti-coagulant rodenticides harm more than just rats, with up to 88% of raptors testing positive for rat poisons, according to the Centers for Biological Diversity.

While the broad restriction in AB 2552 — like many other California laws — have carveouts for ag, it can be difficult for farmers to navigate, Baldwin said.

Baldwin, along with researchers out of Cal Poly Humboldt, have proposals for bringing in barn owls to keep rodent populations in check.

“If we can incorporate these efforts over a larger scale, then that’s going to increase the utility of these kinds of management programs because then we can slow down that reinvasion aspect that we’re always concerned about,” Baldwin said.