How to Manage Pests
UC Pest Management Guidelines
Scientific Name: Thomomys spp.
(Reviewed 11/09, updated 11/09)
In this Guideline:
Description of the Pest
Adult pocket gophers are 6 to 8 inches in length with stout brown, gray, or yellowish bodies and small ears and eyes. They rarely are 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, you rarely find more than one gopher per burrow system.
The conspicuous, 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 aboveground 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 three litters each year. 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 the orchard is favored by extensive weed growth or the presence of many cover crops, especially perennial clovers and legumes.
Pocket gophers can be serious pests, particularly in young orchards. While herbaceous cover crops are their preferred food, pocket gophers will also feed on the bark of tree crowns and roots. When cover crops or weeds dry up, gophers bark consumption may become extensive enough to girdle and kill young trees or reduce the vigor of older trees. Damage to trees is always underground and usually not evident until the trees show signs of stress. Gophers sometimes gnaw on plastic irrigation lines, and mounds can lead to a loss of irrigation water down their tunnel system.
Take action as soon as you see any sign of gopher activity in the orchard. For infestations that cover a limited area, use traps, aluminum phosphide fumigation, or hand-applied poison bait. Trapping and hand-baiting can be used at any time of year, but they are easier when the soil is moist and not dry and hard; aluminum phosphide must be used when soil moisture is relatively high. In addition to control within orchard, consider controlling gophers in adjacent areas, thus reducing the potential for further gopher problems. Gopher control is often best done in late autumn and early winter when mounding activity is high. Additionally, because population densities are usually lowest during early winter, control actions during this time of year can be more effective than after gophers have reproduced.
The best times to monitor for gophers are after irrigation and when mound building peaks in autumn and spring. Monitor monthly in spring, paying close attention to orchard perimeters to determine whether gophers are invading the orchard. Monitor more closely in weedy areas such as roadsides and in young orchards with extensive weed growth or ground cover. This type of vegetation is more likely to support gophers, and low-growing vegetation makes signs of burrowing activity more difficult to see. Look for darker-colored mounds, which indicate newly removed soil.
Treatment options for pocket gophers typically include the use of baiting with multiple-dose anticoagulants, strychnine, or zinc phosphide; trapping; or fumigating with aluminum phosphide. Control of vegetative cover can also reduce the attractiveness of orchards to gophers by removing preferred food sources (e.g., clovers and legumes). Often, a single approach is not sufficient to effectively control gopher populations. An integrated approach that utilizes more than one control option should provide greater control.
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. However, restriction criteria of baits and fumigants often change, so it is best to consult your local Agricultural Commissioner before using any baits or fumigants to assure full compliance with current laws and regulations.
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.
Bait must be applied below ground. For small infestations or where the use of a mechanical burrow builder is not feasible, use a probe to find the main tunnel next to a fresh mound or between two fresh mounds. Once you find the main tunnel, drop bait into the burrow and then put a dirt clod, stone, or other cover over the hole to keep out light and prevent soil from falling onto the bait. Place bait in two or three places along the tunnel.
This hand-application method can be used for single-dose or multiple-dose baits. Reservoir-type hand probes designed to deposit single-dose baits are available. Bait application is faster with these devices because they eliminate the need to stop and place the bait by hand.
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 may be feasible in situations such as unplanted borders or between widely spaced young trees when the terrain is relatively level and the soil is not too rocky. However, because the burrow builder creates an extensive network of burrows throughout the orchard, only use it when gopher densities are high as these new burrows will increase the speed with which gophers can reinvade the orchard.
Traps are effective against small numbers of gophers but are labor intensive and therefore relatively expensive to use. However, trapping often results in greater control of gophers than baiting, so the cost may be offset by effectiveness. You can use either pincer-type or box-type kill traps.
To place traps, probe near a fresh mound to find the main tunnel, which usually is on the lower side of the mound. The main tunnel usually is 8 to 12 inches deep, and the probe will drop quickly about 2 inches when you find it. Place two traps in the main tunnel, one facing each direction. Be sure to anchor the traps to a stake with wire. After placing the traps, cover the hole to keep light out of the tunnel. If there is no evidence that a gopher has visited the trap within 48 hours, move it to a new location.
Pincer-type traps can also be placed in lateral tunnels. To trap in laterals, remove the plug from a fresh mound and place the trap into the lateral tunnel so that the entire trap is inside the tunnel. Gophers will come to the surface to investigate the tunnel opening and will be caught. This approach is quicker and easier to implement than trapping in the main tunnel. However, 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).
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* can be effective if applied underground into tunnels during a time of year when soil is moist enough to retain the toxic gas, typically in late winter to early spring. Application of aluminum phosphide is similar to hand baiting. Use a probe to locate the main tunnel. Once the tunnel has been found, wiggle the probe a bit to enlarge the probe-hole to a size large enough to allow for the aluminum phosphide tablets to be dispensed into the tunnel. Follow label instruction on the number of tablets to place into the tunnel. Finally, cover the probe hole with a rock or dirt clod being careful not to bury the tablets under loose dirt. Treat each tunnel system twice. When using aluminum phosphide, be sure to carefully follow all label directions and safety instructions.
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 potentially slowing reinvasion rates. However, studies on the efficacy of this device have yet to be concluded.
No scientific data has been reported to show that chemical repellents effectively keep gophers from inhabiting orchards. Frightening gophers with sound or vibrations also does not appear effective.
Snakes, owls, and hawks are usually not sufficient to effectively control gophers. These predators consume a number of gophers but usually not enough to eliminate the need for additional control measures.
Gophers are not aquatic, so if flood irrigation is a possibility, it can help control gopher populations.
*Permit required from county agricultural commissioner for purchase or use.
UC IPM Pest Management Guidelines:
R. A. Baldwin, UC IPM Program/Kearney Agricultural Center, Parlier