California almond orchards are under siege from an unprecedented rat infestation, creating widespread damage and heavy financial losses for growers.
Rodent expert Roger Baldwin, a professor at the University of California, Davis, said that until recently he received fewer than one inquiry a year about rats. Now his phone is inundated with calls from farmers and journalists after the state’s $4.7 billion almond industry was struck by the rodents.
According to the Almond Board of California, a newsletter this month reported that more than 100,000 acres in the San Joaquin Valley have been affected, with estimated damages of $109 million to $311 million over the past year.
The infestation began last fall, catching growers off guard. Baldwin said the shift has been dramatic. “There are inquiries and questions about doing interviews on rat control and giving seminars on rat control, and can I write articles about this,’’ he said. “The problem went from very, very low to being the No. 1 issue I’ve dealt with in my 17 years with UC.’’
California produces all of the nation’s commercial almonds, most of them in the Central Valley, which includes both the San Joaquin and Sacramento valleys. The state also provides more than 75% of the global almond supply, with India as the top importer.
The reasons for the sudden surge remain uncertain, but experts cite three main factors. More than half a million acres of farmland were left unplanted in the early 2020s due to drought and limited water, leading to no pest control in those areas. The return of average rainfall in 2022–23 created increased vegetation and food supplies for wildlife. In addition, state pesticide restrictions have reduced growers’ options for managing rodents.
Other pests such as squirrels and deer mice have contributed to the destruction, but rats have been the most damaging. Some farmers have reportedly spent hundreds or thousands of dollars per acre attempting control measures. “At those levels, that’s extreme,’’ Baldwin said. “That’s a very substantial cost.’’
Much of the loss is linked to rats chewing through irrigation systems, including drip lines, sprinklers, and even electrical wiring, sometimes causing fires. The rodents also consume almonds and gnaw tree bark, leaving trees vulnerable.
The Almond Board’s report, citing a February survey from the California Department of Food and Agriculture, stated that infrastructure damage often outweighs crop loss. One grower spent $20,000 replacing a drip system, while another lost half of his crop despite killing 50 to 100 rats daily.
The pests spread through irrigation canals and burrow beneath nut trees, making them harder to detect. Roger Isom, president and CEO of the Western Tree Nut Association, said they often shelter in abandoned fields or vineyards during the day before moving into orchards at night.
He described finding the remains of a rat feast last fall. “Down in the base of a tree where it branches out, it was kind of like that was their dining table,’’ Isom said. “You could see a bunch of empty shells where they were eating.’’
Baldwin has been conducting seminars on pest management, while the state offers training for fumigation certification. Farmers are also using bait stations, snap traps, and owl boxes to encourage predators, though costs remain high and results mixed.
Isom said owl boxes show promise, but it is too early to gauge their effectiveness. Growers are bracing for what is expected to be the second-largest almond harvest on record, even as the infestation threatens yields.
“We’ve never seen anything like this,’’ Isom said. “I’ve had growers who have lost more than half their yield. We’re not going to know until this year’s crop is in whether it was just weather-related or it was due to rat damage. It has certainly affected orchards. Hopefully, we can get ahead of this thing and prevent it from becoming a big, big issue.’’
