Pocket Gopher Management: Don’t Wait Too Long!

Roger A. Baldwin, Cooperative Extension Wildlife Specialist, Dept. of Wildlife, Fish, & Conservation Biology, UC Davis

Pocket gophers are short, stout burrowing rodents, usually 6–8 inches in length. They spend most of their time below ground where they use their front legs and large incisors to create extensive burrow systems. Common forms of damage include consumption of roots and girdling of stems and trunks that result in a loss in vigor of the plant, loss of irrigation water down burrow systems, and chewing on subsurface irrigation lines. Mounds can also result in additional problems including serving as weed seed beds, causing damage to farm equipment, serving as a hazard to farm laborers, interfering with harvest operations, and causing channeling that can lead to substantial soil erosion.

In California, pocket gophers (Thomomys spp.) may be responsible for more damage to orchards than any other mammal species given their widespread distribution, yet many growers choose to ignore them assuming that they will not cause substantial losses. To be sure, there are many orchards where pocket gophers are found, yet damage is not apparent. However, damage to root systems may still be present, potentially reducing yields; this needs to be studied further. Additionally, pocket gophers can be present in an orchard for several years without causing apparent mortality, yet within a short period of time they can switch to feeding on tree crops leading to substantial losses. The only way to ensure that pocket gophers will not cause substantial concerns is to minimize their presence in orchards. This is particularly important for young trees which are highly susceptible to pocket gopher damage.

Pocket gophers can breed at different times throughout the year, although there is typically a pulse in reproduction toward late winter through early spring. Management efforts implemented before this reproductive pulse will often be more effective as there will be fewer individuals to remove at that time. Additionally, pocket gophers mound more frequently during this period given high natural soil moisture. This makes identification of active tunnel systems easy, thereby reducing the time required to treat an orchard while also increasing the efficacy of these management efforts. It should be pointed out that if you intend to use burrow fumigants, high soil moisture is also key for effective control. All of this points to the importance of focusing management efforts on the winter and early spring seasons to minimize pocket gopher damage.

A number of options are currently available for managing pocket gophers although most control programs center on trapping, burrow fumigants, and toxic baits. Given space limitations, I will focus on these three options. For additional information on managing pocket gophers, I suggest checking out the UC IPM Pocket Gopher Pest Note (http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7433.html).

Trapping. Trapping is safe and one of the most effective methods for controlling pocket gophers,  with recent studies showing that a 90% reduction in pocket gopher density is possible after two trapping sessions separated by 1 to 2 weeks. A third trapping session has resulted in complete removal of pocket gophers from some fields. Although a bit more time consuming than burrow fumigation and rodenticide baiting, recent research has shown that trapping is actually a very cost-effective approach when soil conditions are ideal for trapping efforts (i.e., moist, friable soils with relatively shallow burrow systems) given the high efficacy observed with trapping. Trapping becomes a less practical large scale management tool when treating hard, dry soils, but it still can be a good follow-up approach to alternative management options even in more difficult trapping conditions because it allows you to target remaining individuals that other tools might miss. In short, I think trapping should be a tool that all growers employ to some extent, even if it is not the primary tool they prefer to use.

The most common type of trap is a two-pronged, pincher trap such as the Macabee, Easy Set, or Gophinator, which the pocket gopher triggers when it pushes against a flat, vertical pan. Another popular type is the choker-style box trap, although these traps require extra excavation to place and maybe a bit bulky to be practical in a large field setting. All pocket gopher traps can be effective, although the Gophinator has proven to be the most effective in recent trials.  We have not seen a substantial benefit to covering trap sets.   As such, it is generally easier to leave trap sets uncovered to speed up the trapping process. We have not observed any impact of human scent on traps, nor have we been able to identify an attractant that increases capture success.

Fumigation. Burrow fumigants can be effective at managing pocket gopher populations. Primary burrow fumigants have historically included aluminum phosphide and gas cartridges. However, as of January 1, 2012, carbon monoxide producing machines can now be used to apply pressurized exhaust to burrow systems.

Aluminum phosphide is the primary fumigant used for pocket gopher control; it is quite effective (around a 90% removal rate after two treatment periods) and has a low material cost, although labor costs can be higher. The primary method for applying aluminum phosphide is similar to that of hand baiting. You use a probe to find a pocket gopher tunnel, and drop the label designated number of tablets into the probe hole. The opening is then sealed to eliminate light from entering and the toxic gases from exiting the tunnel. Typically, you treat each burrow system twice to maximize efficacy. The key with aluminum phosphide treatments is to only apply when soil moisture is relatively high. Because of this, fumigation is typically most effective in late winter and early spring. However, fumigation after irrigation can also be a good strategy. Please note that aluminum phosphide is a restricted-use material. Applicators must be licensed and trained on its proper use.

Carbon monoxide producing machines are increasing in popularity for managing pocket gopher populations. The most common and best-studied device is the Pressurized Exhaust Rodent Controller (PERC) machine. Efficacy with this device (~55 to 65%) has been lower than with aluminum phosphide, trapping, and strychnine baiting. Additionally, purchase costs for the machine are quite high. That being said, multiple burrow systems can be treated at once (up to 6), allowing applicators to treat fields much more rapidly. If the PERC machine is used very extensively, it appears to provide cost-effective results, but it must be used very extensively to be considered as cost-effective as burrow fumigation with aluminum phosphide, trapping, or strychnine baiting.

Burrow fumigation with gas cartridges is generally ineffective and expensive for pocket gophers, although their efficacy may be somewhat increased if a blower is used to diffuse the smoke throughout the burrow system.

Toxic baits. There are three baits for pocket gopher control: 1) strychnine, 2) zinc phosphide, and 3) anticoagulants (e.g., chlorophacinone and diphacinone). Both strychnine and zinc phosphide are considered acute toxicants. This means that they kill after a single feeding. Strychnine has historically been available in two concentrations in California: 0.5% and 1.8%. However, due to supply issues, strychnine importation into the U.S. is currently very low. As such, the 1.8% strychnine bait is no longer available for purchase. That being said, a recent investigation showed that 0.5% strychnine is still highly efficacious, with 100% removal rates observed across three fields. Keep in mind that pocket gophers can develop a behavioral resistance to strychnine if repeatedly used over time. As such, strychnine baiting should be supplemented with other management approaches to reduce this potential.

Zinc phosphide is also available for pocket gopher control; it comes in a 2.0% concentration. Bait acceptance can be low with zinc phosphide, as it has a distinctive odor and taste that pocket gophers are often averse to. Anticoagulants such as chlorophacinone and diphacinone are multiple feeding toxicants. With these rodenticides, individuals must consume the bait multiple times over the course of 3 to 5 days to receive a toxic dose. This means larger amounts of bait are required to maintain a ready bait supply over this time period. Because of this, acute toxicants are typically preferred over anticoagulants for pocket gopher control although none of these products have proven as consistently effective as strychnine.

There are two primary methods for baiting in fields: 1) hand baiting with an all-in-one probe and bait dispenser, and 2) a burrow builder. Hand baiting can be effective if you have relatively few pocket gophers in a field. For this approach, an all-in-one probe and bait dispenser is used to locate a tunnel. Once the tunnel is located, bait is directly deposited via a hand-crank or lever. Typically, it is recommended that each burrow system be treated at least twice to maximize efficacy.

Although hand baiting can be effective for smaller pocket gopher populations, the burrow builder can be a more practical method for treating larger areas. The burrow builder is a device that is pulled behind a tractor on a 3-point hitch and creates an artificial burrow at a set depth. Bait is then deposited at set intervals along the artificial burrow. While engaging in normal burrowing activity, pocket gophers will come across these artificial burrows and consume the bait within. This device must be used when soil moisture is just right. If the soil is too dry, the artificial burrow will cave in, but if it is too wet, the burrow will not seal properly and will allow light to filter in; pocket gophers will not travel down burrows if they are not sealed. Although convenient, the efficacy of this method has varied extensively among growers. Experimentation is key to determining the applicability of this approach for each grower.

All of the techniques listed previously can be effective at removing pocket gophers from orchards. However, it is important to understand that most, if not all, techniques will require multiple applications to maximize removal rates. Not all individuals in a population will be actively creating mounds at a given time; you will not be able to target treatment applications if you do not know that a pocket gopher is present. As such, it is strongly recommended that you treat fields at least twice, preferably separated by 1 to 2 weeks, so as to maximize the likelihood that you will encounter all, or almost all, pocket gophers in the field. Your ultimate goal should be a reduction in population size of at least 90%. Even with effective removal, reinvasion into orchards will occur. As such, long-term monitoring will be required to remove reinvaders before populations have a chance to reestablish.

It is important to utilize pocket gopher management tools in an integrated manner. Continued reliance of one technique will ultimately result in lower efficacy as pocket gophers will adapt to avoid the management tool (e.g., strychnine behavioral resistance). Incorporating these tools with other management options such as flood irrigation and habitat manipulation will further increase the effectiveness of pocket gopher management programs.