Description of the Pest
Deer are large mammals, typically around 3 to 3.5 feet at the shoulder. Deer are often not seen, in large part because they forage primarily during low-light hours (i.e., early morning and late evening). However, in residential and residential interface areas, deer often habituate to people, and are seen regularly during light hours.
When not visible, other forms of deer sign, such as hoof prints, must be observed to verify their presence. Deer hoof prints are split and about 2 to 3 inches in length. They are easily distinguished from pig and sheep prints in that deer prints are more pointed at the front and rounded at the back; the opposite is true for pigs and sheep. Deer droppings can also be used to verify deer presence. The appearance of droppings vary, but commonly each fecal pellet is oblong, somewhat pointed at one or both ends, and 0.25 to 0.5 inch long.
Deer occur in many foothill and coastal areas and sometimes in the Central Valley near riparian habitats.
Mule deer, including the subspecies called black-tailed deer (O. hemionus columbianus), can be serious pests to tree and vine crops. They are also occasional pests to alfalfa and a variety of row crops.
Deer can completely defoliate young trees or vines. They can stunt, distort, or kill plants by repetitive browsing. Although relatively rare, buck deer can severely scar the bark when they rub their antlers on trunks and lower limbs. Deer may browse on older trees or vines, but the damage to them is usually less severe than that caused to young plants.
Deer can cause significant damage in areas where adjacent habitat supports moderate to high deer populations. Foothill and coastal districts with brush or woodlands that provide cover for deer usually experience the most frequent damage. Some valley orchards or vineyards near stream bottoms may also suffer. State game management laws limit the control methods available to growers and make combating deer damage expensive. Primary control methods include exclusion, repellents, frightening devices, and shooting.
Fencing is the most effective method for excluding deer from a field. Fencing also substantially reduces crop theft and vandalism from humans. Fencing is costly, but if you are planting where deer and uninvited people are likely to present continuing problems, it will likely pay for itself in the long run.
Check deer fences periodically to be sure they remain intact; damaged wire, broken gates, soil washout beneath fences, etc., permit access and must be repaired immediately. Deer that manage to circumvent the fence and get inside may have to be removed by shooting if they cannot be driven out.
Woven Wire Fences
A fence made of woven wire exclude deer if the fence is tall enough. Use a 6-foot (1.8 m) fence of woven wire with several strands of smooth or barbed wire along the top to extend the height to 7 or 8 feet. Be sure the fence is tight to the ground or deer will crawl under.
Wire mesh cylinders around individual trees may be effective where only a few new trees are being planted in a location subject to deer damage. Make the cylinders at least 6 feet tall and large enough in diameter to keep deer from reaching over them to eat the foliage. Secure the cylinders with stakes so they cannot be tipped over.
Electric fencing is less expensive to install than woven mesh fencing but it costs more to maintain. High-tensile wire is the best choice, as it is more resilient than other types; it can absorb the impact of deer, falling limbs, and farm equipment without stretching or breaking. Use a high-voltage, low-impedance power source that provides sufficient voltage to repel deer while being less likely to short out when vegetation touches the wires. Control vegetation around the base of the fence; in wet weather, contact with wet foliage can drain enough voltage from the fence to render it ineffective.
Eliminating suitable cover for bedding and other survival needs rarely results in less crop damage. Deer are highly mobile and many travel 1/2 mile or more to reach a desired area, especially when they have become accustomed to feeding there.
Monitoring and Treatment Decisions
In brushy areas, deer usually stay out of sight during the daytime. They move into vineyards or orchards at night to browse, although in areas where hunting is not allowed, deer can habituate to humans; consequently, they are often seen in the daytime in these areas. Feeding evidence and hoof prints indicate their presence. In areas where deer are not visible, you may also use spotlights to check for deer at night.
Growers can use chemical repellents for tree and vine crops over relatively small areas. Repellents are not always effective for a variety of reasons including:
- lack of alternative food sources for deer
- habituation to the field
- need for frequent reapplication after rainfall or irrigation events
- need to reapply to new vegetative growth
As such, repellents should not be relied upon to eliminate damage, but rather to reduce damage to acceptable levels.
Depending on the active ingredient, chemical repellents elicit a fear, pain, or discomfort response in deer. Those repellents that elicit a fear response (e.g., repellents based on rotten egg formulations, dried blood, etc.) are typically most effective. Repellents that elicit a pain response (e.g., those containing capsaicin) are also effective if applied at high concentrations. Efficacy from urine and fecal matter from predators tends to be highly variable; more effective options are available.
Noise-making devices such as propane exploders and electronic distress calls are not often reported to be effective for more than a day or so because deer rapidly habituate to frightening devices. A relatively new electronic distress call (Deer Shield) is effective for white-tailed deer (Odocoileus virginianus). This product has not been sufficiently tested on mule deer in California, but may work given its reported success on white-tailed deer.
If only a few deer are involved, having someone patrol newly planted areas at night with a spotlight to frighten deer away may prove effective, though expensive.
For small fields (less than 80 acres), dogs can be effectively used to frighten deer away. Dogs must be fenced in to keep them from roaming away from the target area. Options for fencing include a woven wire fence, electric fencing, or invisible fencing. Effective breeds will vary but could include retrievers and other medium-sized active dogs. The dogs do not require extensive training, but not all dogs are appropriate for field protection; they must be out in the elements at all times, and must show an interest in chasing offending animals away.
In some circumstances depredation permits may be obtained from the California Department of Fish and Wildlife, but shooting is rarely a successful solution to a significant deer problem. Encouraging deer hunting in the area can lower the overall deer population and thus reduce damage from deer.