Becky Wheeler-Dykes, Orchard Systems and Weed Ecology Farm Advisor, UCCE Glenn, Tehama and Colusa Counties
Luke Milliron, Orchards Advisor, UCCE Butte Glenn, Tehama Counties

 

In February 2025, Dr. Roger Baldwin gave an excellent presentation on vertebrate pest management at the North Sac Valley Prune Day. Dr. Baldwin discussed aspects of the basic biology, life cycles, and best management tactics for moles, voles, and ground squirrels. This article is a recap of his presentation.

Highlights:

• It’s critical to correctly identify the vertebrate species present to effectively manage populations
• Barn owls can help maintain low populations of gophers
• Burrow destruction, weed management, and tree protectors can dissuade vertebrates from making your orchard their home
• Baits can be highly effective when used properly and reapplied when necessary
• Trapping provides excellent control of gophers, and can be a good strategy for ground squirrels
• Fumigation is effective in wet soils when animals are becoming active
• LABEL IS LAW – always consult with your PCA and/or ag commissioner when considering fumigating or applying any rodenticide 

 

Pest biology and identification
Ground squirrels can be identified by their mottled gray-brown fur and bushy tails (Fig. 1A) and mostly move on the ground instead of in trees. Social in nature, ground squirrels are diurnal and live in burrow systems that have many large, visible holes on the orchard floor (Fig. 1B). Ground squirrels cause damage by girdling of trees (Fig. 1B), causing tree blow over by damaging root systems, chewing irrigation lines (Fig. 1C), eating fruit, and creating safety hazards for workers. Ground squirrels typically hibernate during the winter and emerge in the spring when they begin eating vegetation. Later in the summer their food preference transitions to seeds until they enter hibernation again.

Pocket gophers are about 6-8 inches long, gray-brown  (Fig. 2A) and typically spend most of their time below ground. Their tell-tale plugged C-shaped mounds can help distinguish them from other burrowing pests (Fig. 2B). Gophers feed directly on roots and plants and may also girdle trees below ground (Fig. 2C). Additionally, gophers create excellent weed seed bed habitat, as well as tripping hazards and soil erosion.

Meadow voles are the smallest of the common vertebrate pests discussed, typically 4-6 inches long with dark grayish fur (Fig. 3A). Unlike ground squirrels and pocket gophers whose populations increase steadily over time, meadow voles may have small populations for several years and then have a sudden population boom, becoming unmanageable seemingly overnight. Mole mounds are conical with no plug (Fig 3B), and there are often well-worn trails between their shallow burrows. Primary damage from voles is the girdling of trees typically 6-8 inches above ground level (Fig 3C). Voles sometimes chew on wires and other plant material as well.

Multiple species of vertebrate pests may be present in an orchard, and control tactics differ for each. Correct identification of the pest(s) causing damage in an orchard is critical for effective management. Integrating multiple methods of population management is typically most successful in maintaining low numbers and avoiding severe damage.

 

Biocontrol
Attracting birds of prey is a helpful tool in managing gopher populations. Barn owls are good candidates as they are not territorial, so multiple owls may be attracted to a field using owl boxes. Barn owls are best used to keep low populations from growing to be unmanageable but are unlikely to provide complete control or be very effective with large populations. Ground squirrels are diurnal and therefore will not be controlled well by barn owls. Voles reproduce so quickly that barn owls are not typically able to keep up and provide adequate population management. Owl boxes should be placed about 8-10 feet above ground and cleaned every 2-3 years to maintain viability as a good shelter option.

 

Habitat modification
Habitat modification reduces the desirability of a field or orchard for establishment of vertebrate pests. Destroying burrows and even ripping fields in extreme cases can be effective at reducing vertebrate populations. While there will likely be a reinvasion after burrow destruction, it is typically slow. Flood irrigation forces vertebrate pests to leave their burrows, where predators like coyotes, herons, and raptors can feast and do some of the hard work for you, although dogs and a sharp shovel maybe the best follow-up to flooding. The most effective habitat modification for vole control is to minimize weed cover, keeping cover crops mowed to 2 inches and maintaining a vegetation-free zone of 2-3 feet on either side of trees. Without adequate weed cover for voles to move between holes, they will often move on to ‘greener pastures’. 

 

Baiting
Baits fall into one of three categories: anticoagulants, zinc phosphide, or strychnine. All three types are considered restricted use materials.

Anticoagulants require the animal to feed multiple times on the bait to be effective, and the material can be used as spot treatments, broadcast applications, or in bait stations. Do not leave piles of the bait as this can easily deliver a lethal dose to non-target vertebrates like pets and children. Anticoagulant bait should be reapplied 4 days after initial spot or broadcast application, and bait stations should be checked regularly to make sure there is still enough bait. 

Zinc phosphide is an acute toxin, typically effective with a single dose. This product is used in spot and broadcast treatments and should not be used near buildings or in bait stations. The strong odor and taste of zinc phosphide can cause bait shyness; pre-baiting with a non-toxic bait can improve efficacy of the product.

Strychnine is the most effective rodenticide against gophers and is good for initial knockdown of populations before trapping. Of these three common vertebrate pests, strychnine is only registered for use against gophers. For small populations, using a funnel and spoon to apply the strychnine-laced bait to burrows can be effective.  Larger populations may necessitate using a probe dispenser or even a burrow dispenser on a tractor. Dispensing through a shanked tunnel dug by a tractor is very hit and miss because you have to connect with the gopher’s tunnel network. Gophers can develop resistance to strychnine making it critical that you integrate it with other approaches. 

 

Trapping
Trapping can be effective for ground squirrels. Kill traps are excellent options where there is no concern of non-target captures. Live traps are a good alternative, but keep in mind that squirrels caught in live traps must be euthanized with a CO2 chamber or by shooting the animal. Drowning is not a legal method of euthanasia in California.

Trapping is an excellent control tactic for managing gopher populations. Popular traps like the McAbee and the Gophinator can be set either at the entrance of tunnels or can be covered with soil after baiting and setting; little difference in efficacy has been reported, but covering is a safer option in areas where non-target animals may access the traps. Good control is often achieved with two successive rounds of trapping. Studies have shown no difference in trap success with different attractants, and human scent does not affect efficacy. While trapping is very labor intensive, it is so efficacious against gophers that most growers find it worth the effort.

 

Fumigation
Burrow fumigation uses poison gases injected into burrow systems to kill vertebrate pest populations. Fumigation works best when soil moisture is high, trapping the gas in the burrow. Ground squirrel fumigation is best timed after squirrels emerge from hibernation and is ineffective during hibernation. Late winter to early spring is the best time to fumigate for gophers, before they begin to reproduce for the year. Gas cartridges, essentially a smoke bomb, have a 62-86% efficacy against ground squirrels, but are not effective against gophers. These are easily available with no restrictions against purchase, but care must be taken to avoid starting fires. Aluminum phosphide is highly effective against both ground squirrels and gophers but is highly restricted. With large populations it may be worth it to use aluminum phosphide products, but it’s critical that you understand the restrictions and required documentation; consult with your ag commissioner’s office when planning an application. Pressurized exhaust machines essentially force exhaust from a small engine into the burrow system. These devices (not classified as a pesticide) work well against ground squirrels and are moderately effective against gophers. Carbon dioxide canisters are similar in efficacy to exhaust machines but are considered a pesticide.

Overall, the best set of management strategies employed in any given orchard will depend on many factors, including species, population levels, primary damage and logistical constraints. Integrate multiple approaches for the best control. It’s important to consult with your PCA and/or ag commissioner to ensure proper application and always adhere to pesticide labels – LABEL IS LAW!