How to Manage Pests

UC Pest Management Guidelines

Asparagus

Pocket gophers

Scientific Name: Thomomys spp.

(Reviewed 7/13, updated 7/13)

In this Guideline:


DESCRIPTION OF THE PEST

Adult pocket gophers are 6 to 10 inches long; have stout brown, gray, or yellowish bodies; and have large clawed front paws, small ears and eyes.

They are rarely seen above ground, spending most of their time in a tunnel system they construct 6 to 18 inches beneath the soil surface. A single burrow system can cover several hundred square feet. It consists of main tunnels with lateral branches used for feeding or for pushing excavated soil to the surface. Gophers are extremely territorial; except for females with young, or while searching for mating opportunities, you rarely find more than one gopher per burrow system.

Fan-shaped soil mounds over tunnel openings are the most obvious sign of gopher infestation. These tunnel openings are almost always closed with a soil plug. Gophers feed primarily on the roots of herbaceous plants. They may also come above ground to clip small plants within a few inches of their burrow and pull vegetation into the burrow for feeding.

Gophers breed throughout the year on irrigated land, with a peak in late winter or early spring. Females bear as many as 3 litters each year, but typically only 2 per year in asparagus. Once weaned, the young travel to a favorable location to establish their own burrow system. Some take over previously vacated burrows. The buildup of gopher populations in fields is favored by extensive weed growth, especially perennial clovers, legumes, and nutsedge.

DAMAGE

Primary damage from pocket gophers occurs through chewing of subsurface drip tape. These holes lead to uneven water distribution, with some areas receiving too much water, and other parts not receiving any. Identifying where gophers puncture buried drip tape can be difficult. Water pools on the surface do not necessarily pinpoint the puncture, as water can travel down gopher tunnels for quite some distance before rising to the surface. Fixing pocket gopher punctures of subsurface drip tape can be time-consuming and quite expensive.

Pocket gophers also feed on the roots of asparagus plants. However, given the fibrous nature of asparagus roots, the direct damage to the plant is often minimal.

MANAGEMENT

Often, a single approach is not sufficient to effectively control gophers. An integrated approach that utilizes more than one control option provides greater control. Management options for pocket gophers include baiting with multiple-dose anticoagulants (e.g., diphacinone, chlorophacinone), strychnine, or zinc phosphide; trapping; or fumigating with aluminum phosphide. Control of vegetative cover can also reduce the attractiveness of fields to gophers by removing preferred food sources (e.g., nutsedge and legumes).

Gopher control is often most effective in winter when mounding activity is high. Additionally, because population densities are usually lowest during winter, control actions during this time of year can be more effective than after gophers have reproduced.

Monitoring
  • Monitor for gophers after irrigation
  • Monitor when mound building peaks between late autumn and early spring.
  • Monitor monthly in spring, particularly along field perimeters, to determine the presence of invading gophers from adjacent fields.
    • Be sure to monitor more closely in weedy areas. This type of vegetation is likely to support gophers, and low-growing vegetation makes signs of burrowing activity difficult to see.
    • Look for darker-colored mounds, which indicate newly removed soil.
Treatment Decisions

Take action anytime gopher activity is observed in a field. For infestations that cover a limited area, good options to use are:

  • Traps
  • Aluminum phosphide fumigation
  • Hand-applied poison bait

Trapping and hand baiting can be used at any time of year, but they are easier to implement when the soil is moist, not dry and hard. Aluminum phosphide must be used when soil moisture is relatively high.

After management actions in asparagus fields, consider controlling gophers in adjacent areas to reduce the potential for further gopher reinvasions.

Strychnine (1.8% active ingredient) and aluminum phosphide are currently restricted-use materials that require a permit from the county agricultural commissioner for purchase or use. Be aware that restrictions for use of baits and fumigants around buildings may exist. Since restriction criteria of baits and fumigants often change, consult your local agricultural commissioner before using to assure full compliance with current laws and regulations.

Baits

While multi-dose anticoagulants are available for gopher control, single-dose acute baits (i.e., strychnine and zinc phosphide) have historically been the most effective.

Apply bait below ground.

  1. Use an all-in-one probe and bait dispenser to locate the gopher tunnel.
  2. Once located, deposit bait into an open, fresh tunnel.
  3. Cover the opening left by the probe with a dirt clod or rock to prevent light from entering the burrow. When using this method, be sure not to bury the bait with loose dirt, as this will limit access to the bait.
  4. Treat burrow systems at least twice to maximize efficacy.

Gophers often back-fill old tunnels with loose soil and these backfilled tunnels can feel like open tunnels to inexperienced bait applicators. Applying bait in these backfilled tunnels will greatly limit the efficacy of this management approach; gophers will not find bait placed here.

Bait Applicator Training

Before initiating a baiting program, train all bait applicators to identify backfilled tunnel systems. An effective way to conduct this training is to:

  1. Have novice bait applicators probe for open (non-back-filled) tunnel systems
  2. Once they have found a tunnel, they dig down into these tunnel systems to verify whether they are open or backfilled
  3. Repeat until the bait applicator successfully identifies open tunnel systems with at least 90% accuracy

Following these methods should result in consistently more efficacious control efforts.

Mechanical burrow builder

For infestations that cover a large area, a mechanical burrow builder can be effective and economical. This device is pulled behind a tractor to construct artificial gopher tunnels into which it places bait (All baits used in burrow builders are restricted-use materials). Use of a mechanical burrow builder is best suited for situations such as unplanted borders or before planting a field. Soil moisture is also important. Tunnels created in dry soil will cave in, while tunnels created in wet soil may not form properly.

Traps

Traps are effective against small numbers of gophers but are more labor-intensive than baiting. However, trapping often results in greater control of gophers than baiting, so the added effort may be offset by effectiveness. You can use either pincer-type or box-type kill traps. The smaller size and lower cost of pincer traps typically makes them a more practical choice in a field setting.

  1. Probe near a fresh mound to find the main tunnel, which usually is on the lower side of the mound. The main tunnel is 8 to 12 inches deep, and the probe will drop quickly about 2 inches when you find it. Tunnels typically run in only one or two directions. Occasionally you will have tunnels running in three or more directions.
  2. Place as many traps as tunnels because it is not known which part of the tunnel the gopher is using.
  3. Be sure to anchor the traps to a stake with wire.
  4. Trap holes may be covered with sod, canvas, or some other material, or they may be left uncovered. A recent study has shown that covering trap holes to block light has at best a minor effect on capture success; if trapping a large area, substantial time can be saved by leaving trap holes uncovered. However, covering trap holes may keep children and pets out of traps.
  5. Move the trap to a new location if there is no evidence that a gopher has visited the trap within 24 to 48 hours.
Pincer-type Traps

Pincer-type traps can also be placed in lateral tunnels.

  1. Remove the plug from a fresh mound.
  2. Place the trap into the lateral tunnel so that the entire trap is inside the tunnel.
  3. Gophers will come to the surface to investigate the tunnel opening and will be caught.

Benefits and issues with this approach include:

  • This approach is quicker and easier to implement than trapping in the main tunnel.
  • Trapping in lateral tunnels may be less effective at certain times of the year (e.g., summer) and for more experienced gophers (e.g., adult males).
  • In many areas, the plugs in these lateral tunnels are quite extensive; in these situations, trapping laterals becomes counterproductive given the extensive period of time required to remove these plugs.
Fumigants

Most fumigants, such as gas cartridges, are not effective because gophers quickly seal off their tunnels when they detect the smoke or poison gases. However, aluminum phosphide (permit required from county agricultural commissioner for purchase or use) can be very effective if applied to tunnels during a time of year when soil is moist enough to retain the toxic gas. This is typically in winter to early spring.

Application of aluminum phosphide is similar to hand baiting.

  1. Use a probe to locate the main tunnel.
  2. Once the tunnel has been found, wiggle the probe a bit to enlarge the probe hole to a size that allows the aluminum phosphide tablets to be placed into the tunnel.
  3. Follow label instruction on the number of tablets to apply.
  4. Cover the probe hole with a rock or dirt clod, being careful not to bury the tablets under loose dirt.
  5. Treat each tunnel system twice.
  6. Be sure to carefully follow all label and safety instructions.
Other approaches
Deep tilling

Deep-till the field before replanting if extensive gopher populations are present. This will slow reinvasion rates into asparagus fields and provide more time to get gopher populations under control. When taking a field out of production, deep tilling of soil will kill some gophers and destroy most or all burrow systems in a field.

Flooding

Gophers are not aquatic, so if flood or furrow irrigation is a possibility, it can help control gopher populations.

Gas explosive device

The use of a gas explosive device that combines propane with oxygen has been used to kill gophers through concussive force. This device has the added benefit of destroying part or all of the gopher's tunnel system, thereby slowing reinvasion rates. Be sure to exercise caution when using these devices because of the potential for:

  • Unintended damage to property
  • Injury to users and bystanders
  • Starting fires in dry environments
  • Destruction of turf

Additionally, these devices can by very loud, making them unsuitable in residential areas. This device is generally not as effective as trapping, aluminum phosphide fumigation, or baiting.

Repellents

Unfortunately, no research shows that chemical repellents keep gophers from inhabiting fields. Currently a repellent is being marketed for use in subsurface drip tape, but efficacy for this product has not yet been determined. Frightening gophers with sound or vibrations also is not effective.

Predators

Snakes and owls are usually not sufficient to effectively control gophers. These predators do consume a number of gophers, but usually not enough to keep numbers low enough to eliminate the need for additional control measures.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Asparagus
UC ANR Publication 3435

Vertebrates

  • R. A. Baldwin, Department of Wildlife, Fish, and Conservation Biology, UC Davis

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