Confronting the Rodent Crisis: Challenges and Interventions for Almond Growers

California almond growers are facing an unprecedented challenge this season as a severe roof rat infestation sweeps through orchards across the Southern and Western San Joaquin Valley. An early survey indicates the outbreak has impacted well over 100,000 acres, a number that has likely grown, causing widespread damage and significant economic losses. 

What ABC Has Learned 

Almond growers across Merced, Fresno, Kings, and Kern counties have reported an alarming spike in rodent populations. Over the last several months, ABC has participated in tours and meetings about the severity of this rat infestation, and field observations indicate that these rodents are using irrigation canals and other waterways as corridors, enabling them to rapidly spread between orchards and diverse agricultural fields. This mobility is exacerbating the extent of damage and complicating containment efforts.

Roof rats are arboreal, meaning they spend a considerable amount of their life above ground, often building nests in trees. However, for several years, researchers have noted that in almonds and other tree nuts, they use burrows extensively.  Although it’s undetermined as to why, some believe it’s the lack of cover in the winter months that drives this behavior. 

According to preliminary findings from a CDFA-led survey conducted in the fall of 2024, monitoring efforts revealed particularly high densities of rats in the reported areas, with up to 32 rats captured per night at some locations. Direct damage to the trees occurs through consumption of nuts, girdling of small and large limbs, and burrowing in the root zone. Additionally, damage to irrigation systems, equipment and other infrastructure within orchards is often greater than direct crop loss.

Economic analyses from the CDFA report estimated total on-farm losses ranging from $109 million to $311 million. Major expenses contributing to these figures included drip line replacements, which alone 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. Additional significant costs stemmed from direct tree damage, farm equipment repairs, machinery cleaning due to rodent contamination and replacement of damaged trees.

The lower-end estimates assumed less extensive damage, such as 25% of affected acreage needing drip line replacement and 10% tree damage. Conversely, the higher-end scenario accounted for more severe conditions, including drip line replacements on 75% of impacted acreage, 15% tree damage, and substantial farm equipment repairs and maintenance.

Drip line damage and yield reductions alone represented nearly 90% of the anticipated financial impact. Furthermore, CDFA cautions that these numbers may underestimate the true financial burden, as broader impacts, including newly planted orchards and extensive post-harvest cleanup beyond farm machinery, were not fully quantified in the preliminary analysis.

During ABC’s learnings, one grower noted the replacement of an entire drip irrigation system was estimated at $20,000, and the rodents also chewed through irrigation wiring causing fires. A second almond grower detailed extreme damage that was estimated at a 50% crop loss, even while he was exterminating between 50-100 rats a day. 

UC ANR’s IPM Recommendations

Farm managers, PCAs, and growers are expressing growing frustration and concern over their ability to sustain operations. Management strategies currently in use include bait stations and aluminum phosphide treatments of burrows in the winter, and less conventional methods such as snap traps, owl boxes and applying carbon monoxide or carbon dioxide into burrows as another fumigation option. However, many growers report these tactics are labor-intensive, costly, and insufficient for the scale of this infestation. Additionally, growers noted that rotating bait flavors has become necessary as rodents quickly grow wary and "bait shy.”

Based on research led by UC ANR scientists Niamh Quinn and Roger Baldwin, growers are advised to implement a comprehensive Integrated Pest Management (IPM) strategy tailored specifically to roof rat behavior in tree nut orchard environments. The cornerstone of effective IPM is rigorous monitoring with the use of tracking tunnels and ink cards. Strategically placed game cameras can pinpoint rat activity hotspots, while also providing information on how rat numbers are changing over time.

Using that information, growers can then implement targeted baiting programs. Elevated bait stations containing diphacinone-treated oats, positioned every 165 feet, align with roof rats’ natural climbing behavior while minimizing risks to non-target species. For tree nuts, bait stations strategically placed on the ground could also help, although this application is limited to dormant seasons. To have a significant impact, growers must keep stations full of fresh bait over a period of about four weeks. In situations where rat densities are very high, a longer period may be needed. 

Again, monitoring is important during the bait program period and growers should stop the program when bait consumption becomes minimal. As a reminder, rodenticide regulations are ever-changing, and applicators should always read the label carefully to ensure compliance.

Strategically placed snap traps in protective trapping tunnels, spaced approximately 245 feet apart, form a crucial component of long-term rat control post-baiting. Regular maintenance and monitoring of these traps are necessary to sustain effectiveness. Collaboration among neighboring orchards can further enhance the efficacy of these integrated pest management measures.

During the dormant season, broadcasting zinc phosphide bait or applying zinc phosphide within burrows is also an option, although the efficacy of this bait for rats in California orchards is unknown. Burrow fumigation with aluminum phosphide, gas cartridges or carbon monoxide and carbon dioxide injection devices are also options. UC ANR advises growers to be aware that if using aluminum phosphide, in addition to normal requirements that apply to all restricted-use fumigants, you also now need to have a Burrowing Vertebrate Pest Fumigation Certificate (PCA license) to use this product.  Also note that any diphacinone-treated bait for use in bait stations must be bought from a County Ag Commissioner’s office.

Based on the level of infestation learned from using the tracking tools, multiple overlapping strategies may be necessary to reduce the populations.  And if possible, rotating to a non-diphacinone option is a good decision to avoid diphacinone-resistance.

Additional Support and Resources on the Horizon

Industry groups are mobilizing to provide tangible support for growers facing ongoing rodent pressures. CAPCA, in collaboration with the Almond Board of California and other agricultural partners, is pooling funds to purchase and distribute abatement stations or owl boxes to affected regions, depending on CDFA recommendations. 
Additionally, CDFA is actively working to facilitate updates to pesticide labels for rodent control materials. Specifically, updates and approvals for the use of zinc phosphide and diphacinone are in progress, which could expand application options for growers if finalized. These advancements would provide growers with additional chemical tools to integrate into their rodent management programs.
Furthermore, the latest information and assistance opportunities are being planned for informative sessions at the 51st Annual CAPCA Conference in October and The Almond Conference in December. UC Cooperative Extension’s Roger Baldwin will be presenting Monday, October 20, at the annual CAPCA conference in Reno, Nevada. ABC’s Gabriele Ludwig will be leading a discussion about rodent control in the face of changing regulations at The Almond Conference December 10-12 in Sacramento, CA.