Aug 26, 2025

Rodent attack: How growers can battle rat, squirrel, gopher damage

Battling burrowing rodent wildlife pests is more challenging today than a couple of decades ago. Last year saw a spike in vole numbers throughout the western U.S., said Roger Baldwin, a University of California Cooperative Extension (UCCE), Davis, wildlife specialist.

Since 2023, rodent damage has been worse than normal. Expanding rodent populations have spurred more requests for management information from Baldwin, a professor who works in UCCE’s Department of Wildlife, Fish and Conservation Biology. 

Orchards and vineyards in California’s Central Valley experienced a dramatic increase in problems linked to roof or black rats, while ground squirrels caused increased issues in many cropping systems throughout the state, he said. Over the last year, roof rat losses to drip lines and irrigation systems are estimated to exceed $100 million, Baldwin said.

Rat threat

In addition to voles, roof rats and ground squirrels, pocket gophers and Norway rats have caused destruction in orchards and fields. Baldwin recently visited an almond orchard where squirrel damage didn’t leave a single surviving almond on a tree. 

While annual systems, including tomatoes and watermelons, typically experience less damage than permaculture crops, rodents can still puncture subsurface drip lines and chew on melons and pumpkins.

Locating and repairing rodent-caused irrigation leaks takes significant effort and becomes even more challenging when gopher populations are high. Mice and voles can crawl into cracked and dried heavy clay soil, potentially damaging subsurface irrigation.

Destroying habitat

Deep soil ripping to destroy burrows can be an effective control tool. However, adjacent populations may rapidly repopulate vacated burrow systems. Research has shown that destroying burrow systems at a depth of 1.5 feet can reduce reinvasion rates by up to 90% for California ground squirrels.

Habitat modification is the first tool Baldwin considers. While voles like to hide in plant cover, eliminating the cover can markedly reduce and, in some cases, eliminate potential vole issues, Baldwin said.

Orchard brush and pruning piles can harbor and become staging sites for a variety of animal pests like rabbits and birds, so removal is recommended.

Ground cover

Cover cropping for rodent control remains a balancing act. Aside from typical cover crop benefits, the protein in nitrogen-fixing plants like clovers and legumes can become ideal rodent food resources. Growers avoid the nitrogen species and instead focus on grasses and cover crops will find that gophers prefer large, fleshy taproots and nitrogen-fixing plants over fibrous root systems, Baldwin said. 

For vole populations, Baldwin recommends maintaining minimal vegetation through a variety of strategies, including ensuring bare ground a couple of feet around tree or vine bases. Growers should also keep cover crops mowed to a low stubble height  of 2 inches or less, he said.

Trapping aid

Trapping offers greater utility for some species and can reduce pocket gopher populations. Baldwin’s studies show trapping could be as cost-effective as most other management strategies. 

“We’ve done a lot of research on trapping for gophers, and we find it to be highly efficacious,” Baldwin said. “We’ve seen anywhere from 92% to 94% removal rates utilizing trapping as a primary tool in both alfalfa and in vineyards. It can be more labor-intensive, but I think individuals would be surprised at the cost effectiveness of it in a lot of cases. Even if it does take a little bit more time, we’re getting rid of a much greater number of those gophers out there, which can make it a more viable tool to use.”

Baldwin recommends trapping as a follow-up to other gopher management strategies. Trapping also works well for rat control and can be valuable for managing roof rats, particularly in cropping systems. Because trapping requires more effort for ground squirrels, Baldwin advises trapping as a later approach. Trapping can be beneficial for organic production systems, which are restricted in options.

Multiple tools

“Effective management relies on a suite of tools,” Baldwin said. “An integrated approach to controlling rodents is almost essential, at least for long-term management. We have these tools  that can be effective in certain situations, but we really need to do our best to utilize a combination of those tools.”

Growers can sometimes fall into the trap of thinking that the tool they’re using is all they need. While a particular tool may work for several years, animal learning eventually renders the tool less effective.

Traps, however, can’t catch all gophers, as some become leery. If the only tool a grower uses over time is trapping, the proportion of the trap-shy population will increase substantially and render trapping far less effective, Baldwin said. Periodically mixing in other tools can catch trap-shy animals and maintain trapping longer. 

Rodent resistance

With repeated use, rodents can develop resistance to anticoagulant rodenticides, passing resistant genes to future generations. To minimize resistance, growers need to add other tools to eliminate the genetic-resistant populations, Baldwin said.

“It becomes very important to mix and match those tools in order to hit or effectively target all of those individuals in a population that might not be susceptible to one technique or the other,” he said. “It also tends to lead to more efficacious control, which minimizes numbers far more extensively and for far longer periods of time, which ultimately means that we’re spending a lot less on management programs because they have just a few individuals to treat rather than a large reoccurring population.” 

Though it varies by species, rodents reproduce rapidly, and can produce several litters a year.  Integrated pest management(IPM) also relies heavily on species monitoring.

Diligence required

To limit populations, growers should monitor and adjust management programs throughout the year. 

“It’s important that growers don’t think about rodent control as being a once-a-year process,” he said. “If you’re getting rid of a rodent population one time, say in the springtime, but then you’re not going to do anything again until next spring, it’s easy for those populations to rebound to numbers essentially equivalent to— and in some cases even more than — what they had the previous year.”

Reduced effort on management programs could increase crop production and save money, Baldwin said.

— Doug Ohlemeier, Assistant Editor