Mountain Beaver Damage Management

Damage Prevention and Control Methods

Exclusion

Mountain beaver, Aplodontia rufa

Small diameter plastic mesh seedling protectors (Fig. 8) will protect most conifer seedlings. Most are effective until the seedlings grow taller than the tube height. The relatively small (1-to 3-inch [2.5- to 7.6-cm]) diameter tubes do not protect much competing vegetation and also allow lateral branches to grow through the mesh. The advantage of plastic mesh protectors over some other control methods is that they provide protection from a variety of animals including deer (Odocoileus spp.), hares (Lepus spp.), elk (Cervus spp.), and voles (Microtus spp.). The cost of installation can be high, but can be reduced if done at the time of planting. Tree seedlings that become established and reach 30 inches (76 cm) or more in height are less susceptible to damage.

Figure 8. Plastic mesh seedling protector.

Plastic mesh seedling protectors photo degrade and deteriorate after several years. Although they expand with stem growth, they probably pro-vide little protection from girdling of large diameter stems by mountain beavers.
Wire mesh cages 1 to 3 feet (0.3 to 1 m) in diameter will protect individual trees but are expensive and may be climbed over and burrowed under. These cages also allow competing vegetation to be protected and often cause poor tree growth. The wire used in these cages may injure tree growth if cages are tipped or come into contact with the tree stem.

Cultural Methods

Plant large tree seedlings to improve survival of the trees in sites occupied by mountain beavers. Larger stems are less subject to being clipped at ground level. Although large seedlings may be seriously damaged, enough foliage often remains after damage to provide for regrowth and survival after later damage. Damage-resistant trees should be about 2 feet (0.6 m) tall and have 1/2-inch (1.3-cm) or larger diameter stems at the base. Trees should be planted away from burrow openings so that mountain beavers will find them less convenient to cut.

Prescribed slash burning before planting may reduce mountain beaver populations by reducing available forage and increasing predation. Extremely hot fires may cause some mortality, but most mountain beavers will remain protected in their burrows. Reduction in available forage after fire may cause mountain beavers to travel farther from burrows and subject them to higher levels of predation. Legal restrictions or other practices that inhibit prescribed burning may favor mountain beaver populations.
Mountain beaver burrow systems may be destroyed by tractor scarification on level or moderate slopes when done to remove logging debris for replanting or to convert brush fields to plantations. This method requires the use of toothed land clearing blades to rip soil and destroy burrows. It seldom removes the deeper nest chambers but can make the area unattractive to mountain beavers. Avoid piling soil and wood debris, both of which will attract mountain beavers. Wood debris piles should be burned when possible and soil leveled.

Removal of nest chambers after population reduction will reduce reinvasion of the burrow systems by 50% or more. Practical methods for locating and removing nest chambers need further study.

Localized control of plants such as sword fern, bracken fern, or salal may reduce the attractiveness of an area to mountain beavers, but more study is necessary before methods can be recommended. Use caution when applying herbicides to avoid causing increased feeding pressure on conifers by suddenly removing the availability of other forage plants. In such situations, tree seedlings may require protection with plastic mesh seedling protectors.

Repellents

Coniferous seedlings subject to mountain beaver damage may be treated with repellents, but they require special application procedures to assure the plant stem is treated near the base (Fig. 9).

Figure 9. Application of powdered repellent to conifer seedling.

The effectiveness of a repellent can be enhanced by conditioning the mountain beavers to the repellent. Treat cull seedlings with the same repellent and place them in active burrows. This practice has caused mountain beavers to avoid both treated and untreated planted seedlings for up to a year after planting. The only repellent that has been registered for mountain beavers in Washington and Oregon is 36% Big Game Repellent Powder (BGR-P ), originally registered only for big game. Thiram (tetramethylthiuram disulfide) is another repellent registered for hares, rabbits (Sylvilagus spp.), and big game that has been effective against mountain beavers. Repellents may be of most value where they cause a long-term avoidance. The placement of repellent-treated cull tree seedlings in burrows at time of planting and treating significantly improves repellent efficacy.

Toxicants

A pelleted 0.31% strychnine bait (Boomer-Rid®) has been registered in Oregon for control of mountain beavers. Recent field tests in Washing-ton and Oregon, however, showed marginal efficacy in late winter with Boomer-Rid®. Pelleted bait is placed by hand inside main burrows, using about five baits each in 10 burrow openings in each system. The registered label allows 1/2 to 1 1/2 pounds of bait per acre (0.6 to 1.7 kg/ha). The bait formulation contains waterproofing binders that tolerate wet burrow conditions.

Experimental zinc phosphide-treated apple bait was poorly accepted by mountain beavers and was potentially hazardous to bait handlers. The treated bait was readily eaten by black-tailed deer (Odocoileus hemionus colum-bianus) and could present a hazard.

Baiting is severely restricted in areas frequented by endangered species such as northern spotted owls (Strix occidentalis caurina), and bald eagles (Haliaeetus leucocephalus).

Fumigants

Fumigants are generally ineffective because of the open, well-ventilated structure of the mountain beaver bur-row systems. Aluminum phosphide that was activated when mountain beavers pulled pellets attached to vegetation into the nest area was only partially effective. The use of carbon monoxide gas cartridges and carbon monoxide gas have been unsuccessful in controlling mountain beavers. No fumigants are registered for mountain beaver control. The use of smoke bombs or similar material is effective in locating the numerous openings in a mountain beaver burrow system.

Trapping

Mountain beavers are routinely kill trapped for damage control on many forest lands scheduled for planting. Trapping is usually done just prior to planting and repeated 1 or 2 years afterward. Trapping is also repeated when damage is found in established plantations. Set kill traps in older stands where stems and roots are being girdled and undermined. Live trapping is seldom done in forest lands except for research purposes, but it is used where there are urban damage problems.
Kill trapping is normally done using unbaited Conibear^® No. 110 traps set in main burrows. Anchor traps with three sticks, with either two in the spring (Fig. 10)

Figure 10. Method for setting a kill trap in a mountain beaver burrow.

or with one in the spring and one at the far end of the jaws, in a vertical position with the trigger hanging. The trap should take up most of the space in the burrow, and when properly anchored, is readily entered by the mountain beavers. This trap is sometimes not immediately lethal because of the mountain beaver’s thick short neck. Stronger double-spring traps may be more effective, but are more difficult to set in the limited burrow space.

Teams of trappers are normally used when trapping large acreages. Individual trappers should be spaced about 30 to 50 feet (9.1 to 15.2 m) apart, depending on habitat conditions. Extra searching may be required in areas with many small drainages that may have many burrows. Active burrows have fresh soil and vegetation piled at burrow entrances or in burrows. Bur-rows can often be visually inspected through openings to determine if there is recent use. Set two or three traps in each active burrow system. All trap sites should be marked with flags and mapped so they may be relocated; a crew of trappers should use several colors of flagging so that individuals can relocate their own traplines by color. Trapping in older stands of conifers can be very difficult because traps are not easily relocated when branches hide the flagging. Mapping and flagging travel routes in this type of habitat may be necessary. The trap lines are usually checked after 1 day and again checked and pulled after about 5 days. Traps are usually reset during the first check even where mountain beavers are captured, because the systems may be quickly invaded by other mountain beavers. If trapping is un-successful, move traps to burrows with fresh activity. During the breeding season (January to March), male mountain beavers may be more commonly trapped than females because of their greater activity.

During subfreezing temperatures, trapping should be postponed or trap-ping periods lengthened to include warmer periods when mountain beavers are more active. Trapping during periods of snow is also usually less successful than during snow-free periods because trap sites are difficult to locate and set, and animals are less active.

Trapping may take nontarget species such as weasels, spotted skunks (Spilogale putorius), mink, squirrels (Tamiasciurus spp.), rabbits, and hares that use the mountain beaver burrows. Nontarget losses may be reduced by positioning the trap trigger near the side of the trap so that it is less likely to be tripped when small animals pass through.

Live trapping is recommended where domestic animals may enter the bur-rows. Double-door wire mesh live traps such as Tomahawk traps (6 x 6 x 24 inches [15 x 15 x 61 cm]) should be set nearly level in main burrows. Suit-able vegetation should be placed inside and along the outside of the trap. Wrap the trap with black plastic and cover it with soil to protect animals from the weather. Placement should assure that animals enter rather than go around the ends of the trap. Traps must be checked once or twice daily, preferably in early morning and again in the late afternoon, to minimize injury and stress to mountain beavers held in the live traps. Live-captured mountain beavers should be placed in dry burlap sacks and, if necessary, euthanized with carbon dioxide.

Shooting

Shooting is not a practical control method.

Other Methods

Habitat manipulation by increasing or decreasing favored vegetation has been evaluated only indirectly. Where native forbs were seeded to reduce deer damage to Douglas-fir plantations, mountain beaver damage did not significantly decrease or increase. In another area, where red huckleberry was abundant and extensively cut, mountain beaver damage to Douglas-fir was insignificant.