A summary of a presentation by Roger Baldwin, UC Davis, Wildlife Management Specialist. Summary by Tianna DuPont, WSU Extension; Cameron Burt, WSU Tree Fruit Research Center Orchard Manager. Reviewed by Roger Baldwin, UC Davis.
Pocket Gophers
Gophers are burrowing rodents 6 to 8 inches in length. They spend the majority of their time below ground in their burrows. Gophers can cause extensive damage because they feed directly on the roots and the cambium layer of trees.
Identification
Pocket gopher mounds are generally horseshoe shape shaped in appearance with a plug toward the lower end of one side. In contrast mole mounds tend to be more conical in shape with a plug right in the middle or no visible plug. Girdling damage to trees or vines at the ground level or below is often a sign of pocket gopher damage.
Damage
The most common damage we see from gophers is direct feeding on taproots which can weaken and kill trees. Gophers can girdle trees when they feed on the bark and the cambium layer. This girdling generally occurs below ground. Girdled trees lose growth and vigor and the girdling causing the damage is only visible when the soil is pulled away from the base of the tree.
Meadow Voles
Voles are dark grayish-brown and 4 to 6 inches in length. Vole populations tend to cycle which can be challenging. Populations can remain at low levels for extended periods of time and then populations can multiply quickly. In some settings populations can reach several thousand per acre.
Identification
Voles have open burrow systems. Burrows are shallow with a diameter of 1 to 1.5 inches. Voles tend to create well-worn trails which go back and forth from the openings of their burrows. To determine if burrow systems are active look for fecal pellets and fresh grass clippings.
Damage
Voles can damage trees by girdling. Generally, vole damage is above ground in contrast to below ground damage from pocket gophers. Vole damage generally extends up to 6 to 7 inches above ground. Voles generally do not climb well and so damage tends to be only in the lower area of the tree.
Controls
Rodent control is most effective when it is part of an integrated program. If focus is only one tool some individuals will not be affected by the tool and as those individuals multiply the population will become more and more resistant to that single management strategy.
Habitat modification
Habitat modification is altering the habitat of a given area to make it less attractive for the target species. Habitat modification can be quite effective for voles. Voles are very cover dependent. Thick vegetation in between rows of trees is attractive to voles. Keep alleyways well mowed to 2 inches of stubble height or less to substantially reduce your risk of girdling damage.
Eliminate preferred food sources. In particular, nitrogen fixing plants like clover are ideal food sources for pocket gophers and voles. Removing these food sources will reduce the carrying capacity of the area.
Burrow destruction. During orchard removal, deep ripping will remove old tunnel systems which will slow the reinvasion of gophers after planting.
- Keep alleyways mowed to 2 inches or less to reduce cover.
- Eliminate preferred food sources.
Biocontrols
Barn owl boxes can be installed to encourage owl presence. Barn owls are not very territorial and so potentially they can be used to keep rodent populations in check. Currently there is not a large amount of data about the effectiveness of owls to manage gophers and voles, although limited investigation has shown some benefit of barn owls for managing gophers.
Exclusion
Using tree protectors around the base of trees can reduce damage from voles. It is important to bury tree protectors several inches into the ground. If tree protectors are not buried, they can create a good habitat for voles which can enter under the tree protector into a protected space around the tree and feed.
Fencing around areas has not been effective or practical on a field scale for pocket gophers. Fencing can be effective at slowing the movement of voles from natural areas into crop fields. If used, fencing should extend 6 inches below ground and 8 to 12 inches above ground. Aluminum flashing is an example of fencing that has been effective. Plastic mesh fencing has also been used, but has to be replaced annually. It is critical to maintain a two-foot-wide vegetation free area on each side of fencing to keep voles from lingering in the area around barriers and finding ways to invade.
- Tree protectors can limit damage caused by voles.
- Fencing has been effective at slowing movement of voles into fields.
Baiting and Rodenticides
Pocket gophers
For pocket gophers, available active ingredients include strychnine, chlorophacinone, diphacinone, and zinc phosphide. Strychnine and zinc phosphide are acute toxicants which means they kill after a single feed. These are restricted-use pesticides with many rules that must be followed for safe use. Chlorophacinone and diphacinone are first generation anticoagulants which are chronic feed materials which means the animal needs to consume the feed material over the course of several days to get a lethal dose.
These products can be applied in several depending on specific label instructions. One method is the funnel and spoon method. This is when you use a probe to find the tunnel system, insert the funnel into the hole leading to the tunnel, spoon a label-specified amount of bait into the opening of the funnel, cover the opening careful not to bury the bait, and then move to another tunnel. This is effective with a small numbers of burrows. All-in-one probe and bait dispensers are a more efficient tool for application in larger areas. The probe is inserted into the tunnel system, and when operators pull a lever, a pre-determined amount of bait is dispensed. In the case of extensive gopher problems throughout an orchard, operators can use a burrow builder, which is a device pulled behind a tractor which creates an artificial tunnel and dispenses bait at a set interval in an artificial tunnel. This strategy can be variably effective. To be effective the depth of the shank must be set to the depth of other burrows in the adjoining area of the field such that gophers will intercept the artificial burrow. Measure the average depth of burrows in the orchard you plan to treat in order to determine the appropriate depth. Keep in mind this depth may change in different areas of the field. Soil condition is also important. If soil is too wet the machine does not cover the top of the artificial tunnel properly, which allows light and air to enter which the gophers don’t find attractive. If the soil is too dry the artificial tunnel will cave in. The machine can also plug and must be regularly checked to make sure it is functioning properly.
Of the active ingredients strychnine has been most effective. However, strychnine is less available. Baldwin’s group has recently tested alternative products in the lab. Unfortunately, none of the alternative products met the 80% efficacy goal. However, the strychnine product also did not have as high of efficacy as expected. Strychnine efficacy was highly variable between rodents from one area and another. In one area where strychnine had been repeatedly used the rodents were highly resistant to strychnine. Baldwin’s group is also looking at other toxicants registered for other targets including cholecalciferol and bromethalin to learn if they might be effective on pocket gophers and voles (not currently registered for use on pocket gophers and voles). These products were not effective on their own. However, in other parts of the world cholecalciferol has been combined with anticoagulants. The combination can act as a synergist increasing the potency of the cholecalciferol while allowing a lower dose of each. These combinations were effective in both the lab and the field. Currently Baldwin’s group are looking for a registrant to work with these products and hopefully they will be available in the future.
- Strychnine works best.
- Chlorophacinone, diphacinone, and zinc phosphide also registered.
- Use probe to find tunnel.
- Dispense bait in tunnel.
Voles
Zinc phosphide and anticoagulants are labeled for voles. Zinc phosphide is generally most widely used. Broadcast and spot treatment applications are used most widely. Bait stations can be used but as voles do not move great distances, many bait stations have to be used. Consider that most labels will only allow broad cast applications during non-bearing time periods. Another consideration is that zinc phosphide has a flavor to it that is not attractive to rodents (garlicy). To increase bait acceptance, pre-bait with a non-toxic bait and make sure that rodents are consuming the bait and then follow up with the toxic bait. This is critical because in most cases only one application of zinc phosphide per calendar year is allowed. Zinc phosphide is also sensitive to moisture. If it comes in contact with moisture it will off-gas and lose efficacy. Use when there is no rain anticipated. Heavy dew and fog can also be sufficient moisture to reduce efficacy.
- Zinc phosphide and anticoagulants available.
- Zinc phosphide is primarily used.
- Broadcast and spot applications are the primary application methods.
- Pre-bait with nontoxic bait to increase consumption of zinc phosphide baits.
- Zinc phosphide limited to 1 application per year.
- Use zinc phosphide in dry conditions.
Burrow fumigation
Aluminum phosphide tends to be effective for pocket gophers. These are tablets or pellets which are inserted into the tunnel system and react with the moisture to create a poisonous gas. This is a restricted-use pesticide with many rules that must be followed for safe use. Gas cartridges, while they can be effective for ground squirrels, are generally not effective for pocket gophers.
Pressurized exhaust machines are another tool. These tools create exhaust which is primarily carbon monoxide which can be injected into the burrow system which will asphyxiate the animal. Examples include the BurrowRX, PERC Machine and the CO-Jack. Baldwin has tested these machines and seen 60 to 65% efficacy and as such they can be part of an integrated program. These machines can also be very effective on ground squirrels. Carbon dioxide injection devices are now available for use, as well. They have not been extensively tested, but efficacy is expected to be similar to pressurized exhaust machines.
- Can be effective on pocket gophers.
- Work best when soil moisture is high.
- Do not use close to buildings.
Repellents
Repellents use odors or taste to repel the rodent from feeding. There are many of these products, many of which are not effective. However, in recent studies anthraquinone has shown promise as a vole repellent. Anthraquinone is a post-ingestive repellent which means the animal has to consume the repellent. When they ingest the material they get sick and hopefully learn to avoid feeding on that particular crop. When the bottom 12 inches were treated on newly planted trees girdling was greatly reduced in studies. Currently registration is in process but not currently available for use.
- Commonly available but often ineffective.
- Anthraquinone may show some promise for voles.
- Limited investigation has shown some reduction on gopher activity following application of Protec-T.
Resources
Vertebrate Pest Control Handbook
Meadow Voles and Pocket Gophers: Management in Lawns, Gardens, and Cropland
Use pesticides with care. Apply them only to plants, animals, or sites listed on the labels. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.
YOU ARE REQUIRED BY LAW TO FOLLOW THE LABEL. It is a legal document. Always read the label before using any pesticide. You, the grower, are responsible for safe pesticide use. Trade (brand) names are provided for your reference only. No discrimination is intended, and other pesticides with the same active ingredient may be suitable. No endorsement is implied.
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