“Plague of Rats” Threatens California Almond Orchards: Over $300 Million in Losses Feared

As roof rat populations explode across the San Joaquin Valley, growers face mounting economic damage, operational setbacks, and limited options for control.

California almond growers are sounding the alarm over a severe and fast-moving roof rat infestation that has already impacted more than 100,000 acres across Merced, Fresno, Kings, and Kern counties. The outbreak, believed to be the most widespread rodent issue in recent memory, is causing extensive damage to orchards and infrastructure—and early estimates suggest total losses could exceed $300 million.

According to CDFA's preliminary analysis, the infestation is targeting not only almond crops but also irrigation systems, orchard trees, and farming equipment, adding layers of complexity to an already challenging season. Reports gathered through Almond Board of California tours and meetings with growers confirm that roof rats are using irrigation canals and waterways as transit corridors, allowing them to rapidly spread across diverse agricultural regions.

An Escalating Crisis Below—and Above—Ground

Unlike typical rodent infestations, this outbreak involves roof rats (Rattus rattus), a species known for its climbing abilities and tree-dwelling habits. However, researchers have observed a shift in behavior: in almond orchards, these rats are burrowing underground, possibly due to a lack of canopy cover in winter months.

The result? Damage that hits orchards from every angle:

• Nuts consumed directly off trees

• Girdling of branches and limbs

• Chewed irrigation lines and electrical wiring

• Burrowing in root zones, compromising tree health

• Contamination of farm equipment and materials

  
  

One grower reported replacing an entire drip irrigation system at a cost of $20,000, while another described a 50% crop loss, despite killing 50–100 rats per day on-site.

The True Cost: Infrastructure Damage and Lost Production

CDFA’s fall 2024 survey estimated on-farm losses between $109 million and $311 million, driven largely by:

• Drip line replacements ($56M–$168M)

• Yield losses from disrupted post-harvest irrigation ($43M–$129M)

• Tree damage and equipment repair

• Increased labor and cleanup costs

  
  

These figures could underrepresent the full economic toll, as newly planted orchards and post-harvest maintenance issues were not fully quantified in the preliminary analysis. Even so, irrigation damage and yield reductions alone account for nearly 90% of the projected financial impact.

Conventional Control Measures Are Struggling to Keep Up

Growers, Pest Control Advisors (PCAs), and farm managers have ramped up rodent control strategies, but many are finding them costly, labor-intensive, and insufficient against the scale of the problem. 

Current management practices include: 

• Snap traps and bait stations

• Burrow fumigation using aluminum phosphide or carbon monoxide

• Predator support via owl boxes

• Bait rotations to combat rodent bait resistance

  
  

However, as many growers have noted, rats are becoming increasingly bait-shy—prompting the need for more strategic and science-based approaches.

Integrated Pest Management (IPM): The Path Forward

Researchers from UC ANR’s Vertebrate Pest Control Program, including Dr. Niamh Quinn and Dr. Roger Baldwin, are urging almond growers to implement a comprehensive Integrated Pest Management (IPM) plan tailored to the behavior of roof rats in tree nut systems.

Core IPM strategies include:

• Monitoring tools such as tracking tunnels, ink cards, and game cameras

• Elevated bait stations every 165 feet with diphacinone-treated oats during dormancy

• Snap traps in tamper-resistant tunnels, spaced roughly 245 feet apart

• Burrow fumigation during winter months using certified materials

• Zinc phosphide broadcast applications, though efficacy in California remains unclear

  
  

Applicators must follow rodenticide label regulations closely, and those using aluminum phosphide must now hold a Burrowing Vertebrate Pest Fumigation Certificate. Bait for use in bait stations must also be purchased from the County Agricultural Commissioner’s office.

The key, according to UC ANR, is overlapping control methods combined with frequent monitoring. Once bait consumption drops, growers should pivot to long-term trapping and surveillance to prevent re-infestation.

Industry Support and Regulatory Updates in Progress

To support growers amid this worsening crisis, the California Association of Pest Control Advisers (CAPCA), Almond Board of California (ABC), and other partners are working to distribute abatement tools, including owl boxes and bait stations, to affected regions.

Meanwhile, CDFA is pushing forward updates to pesticide labels to expand legal applications of diphacinone and zinc phosphide for almond orchards—a move that could significantly widen available tools if approved.

What Growers Can Do Now: Turn to Specialized Help

While regulatory changes and research continue, many almond growers are seeking immediate, field-ready support.

CaddyShack Rodent Services, a trusted partner of The Ag Center, offers non-toxic rodent control solutions for agricultural operations. Their team has been on the front lines of this outbreak, using techniques to safely and effectively reduce rat populations.

From individual orchard assessments to custom abatement programs, CaddyShack is equipping growers with practical strategies to manage infestations and protect long-term crop health.

Don’t wait until the damage is irreversible. Contact CaddyShack Rodent Services through The Ag Center to schedule a consultation or learn more about how they can support your operation.