How to Manage Pests
UC Pest Management Guidelines
Scientific Name: Spermophilus beecheyi
(Reviewed 9/08, updated 9/08)
In this Guideline:
Description of the Pest
The adult California ground squirrel has a head and body 9 to 11 inches long. Its somewhat bushy tail is about as long as its body. The fur is mottled dark and light brown or gray. Ground squirrels live in colonies that may grow very large if left uncontrolled. They are active during the cooler times on hot days and are usually most active in morning and late afternoon.
Each ground squirrel burrow system can have several openings with scattered soil in front. Individual ground squirrel burrows may be 5 to 30 feet long, 2.5 to 4 feet below the surface, and about 4 to 6 inches in diameter. Burrows provide the ground squirrels a place to retreat, sleep, hibernate, rear their young, and store food. Ground squirrels often dig their burrows along ditches and fence rows and on other uncultivated land. When uncontrolled, they frequently move into orchards and dig burrows beneath the trees.
The California ground squirrel can be active throughout the year in coastal areas of southern California. Especially in hot locations, adult ground squirrels become temporarily dormant (aestivate) when food is scarce or temperatures are extreme, primarily in late summer. Winter hibernation and summer aestivation are more typical among ground squirrels in inland areas where temperature variations are more extreme. Regardless of location, young squirrels tend to be active all summer.
Squirrels that do hibernate generally emerge around January when weather begins to warm. In late winter and spring, they feed on green vegetation but switch to seeds and fruit in late spring and early summer as the vegetation dries up. Females have one litter, averaging 8 young, in spring. Young squirrels emerge from their burrow when about 6 weeks old; they do not aestivate their first summer, and most do not hibernate during their first winter.
Ground squirrel damage is most prevalent in orchards adjacent to uncultivated areas where squirrels are not controlled. Ground squirrels gnaw fruit and bark and girdle trunks and scaffold limbs. They occasionally chew plastic irrigation lines and their burrows can contribute to soil erosion.
The type of direct control action needed for ground squirrels depends primarily on their activity patterns and feeding preferences during the time of year when control action is taken. The choice of tactics is also influenced by the location of the infestation and the number of squirrels present. Watch for signs of squirrel activity within the orchard, especially the appearance of burrows, during routine orchard activities. Check the perimeter of the orchard at least once a month during the times of year when squirrels are active. Midmorning usually is the best time of day for observing squirrels. For in depth information on managing ground squirrels, see Best Management Practices for California Ground Squirrel Control.
To make it easier to monitor and help limit the buildup of their populations, remove brush piles, debris, and stumps in and around the orchard. Ground squirrels quickly occupy abandoned burrow systems. If feasible, after controlling squirrel infestations outside your orchard use thorough cultivation or deep plowing to destroy burrow entrances and help slow the rate of reinvasion.
Keep records of when squirrels emerge from hibernation. Record the approximate number of squirrels you see and the location and number of burrows.
As soon as you see squirrels or burrowing activity within or adjacent to the orchard, be prepared to take action. Select the control method best suited for the time of year. The most effective time to control ground squirrels is in early spring when adults have emerged from their burrows but before they reproduce. For best control then, use burrow fumigation about 3 weeks after the first squirrels emerge from hibernation. Because squirrels feed almost exclusively on green vegetation early in the season, poisoned grain baits are generally not effective until late spring or early summer. Trapping can be used year round but is most effective when populations are small.
Treatment options for ground squirrels include the use of fumigants (gas cartridges and aluminum phosphide) and baiting with multiple-dose anticoagulants or zinc phosphide. Zinc phosphide and aluminum phosphide are restricted use materials that require a permit from the County Agricultural Commissioner for purchase or use.
Fumigation can be very effective against ground squirrel populations. The best time to fumigate is late winter or early spring when the squirrels are active and soil is moist. Fumigation is not effective when squirrels are hibernating or aestivating: at those times, they seal off their burrows. When the soil is dry, fumigation is much less effective because more of the fumigant escapes from burrows through cracks in the soil.
When using a fumigant, make sure to treat all active burrow systems in and around the orchard. Re-check all areas a few days after fumigation and re-treat any that have been reopened. For safety's sake, do not fumigate burrow systems that are adjacent to buildings or may open under structures.
A relatively easy way to fumigate is with the use of gas cartridges. Use one or two cartridges for each burrow that shows signs of activity. A large burrow system may require more than two. Quickly shove the ignited cartridges into the burrow using a shovel handle or stick and seal the burrow entrance with soil. Watch nearby burrow entrances; treat and seal any that begin to leak smoke. The larger and more complex the burrow system, the more smoke it takes to be effective.
Poison bait is usually the most cost-effective method for controlling ground squirrels, especially for large populations. Bait consists of grain or pellets treated with a poison registered for ground squirrel control. To be effective, the bait must be used at a time of year when ground squirrels are feeding on seeds and will readily accept baits such as in late spring or early summer. In fall, squirrels store a lot of the seed instead of eating them, so it may require more bait to control the population.
Before you use baits, place a small amount of untreated grain near burrows in the morning and check in the late afternoon to see if the squirrels have taken it (this ensures that nocturnal animals have not eaten the grain). If the grain is taken during the day, proceed with baiting. If it is not taken, wait several days or a week and try again. Remember: bait is not effective unless it is eaten by the target pest. When using poison baits, make sure to follow label directions carefully to reduce hazards to nontarget species.
Multiple-dose anticoagulant baits can be applied in bait stations, as spot treatments near burrows, or broadcast over larger infested areas. Check the label to make sure that the bait you plan to use is registered for the method or bait station you intend to use. For a multiple-dose bait to be effective, animals must feed on it over a period of several days.
Various kinds of bait stations are commonly used; all are designed to let squirrels in but to exclude larger animals. Special types of stations must be used within the ranges of the San Joaquin kit fox or endangered kangaroo rats to ensure that these species are excluded. Consult you local agricultural commissioner or the DPR Web site recommendations on use of poison baits in areas that are within the range of endangered species.
Place bait stations near runways or burrows and secure them so they cannot easily be tipped over. If squirrels are moving into the orchard from adjacent areas, place bait stations along the perimeter of the orchard where squirrels are invading, one station every 100 feet. Use more stations when the number of squirrels is high.
Check bait stations daily at first, then as often as needed to keep the bait replenished. If bait feeding is interrupted, the bait's effectiveness will be greatly decreased. Make sure to pick up any bait that spills and to replace bait that is wet or moldy. Successful baiting usually requires 2 to 4 weeks. Continue to supply bait until feeding ceases and you observe no squirrels; then properly dispose of unused bait.
When specified on the label, anticoagulant baits can be applied as spot-treatments, which are economical and effective for small populations. Reapply according to label directions to make sure there is no interruption in exposure to the bait. Scattering the bait takes advantage of the ground squirrels' natural foraging behavior and minimizes risks to nontarget species that are not as effective at foraging for seeds. Never pile the bait on the ground because piles increase the hazard to livestock and certain nontarget wildlife.
After treatment, pick up and dispose of any carcasses whenever possible to prevent secondary poisoning of dogs or other scavengers. Burial is a good method for disposal as long as the carcasses are buried deep enough to discourage scavengers. Do not touch dead animals.
Assess the potential hazard to humans, livestock, and nontarget wildlife before you use baits; if it is risky, use another method for ground squirrel control.
Trapping controls small populations any time of year when squirrels are active. Trapping is especially effective from mid-spring through fall. Ground squirrel traps include Conibear traps and modified gopher box traps.
Conibear kill traps are usually placed unbaited in the burrow entrance, where squirrels are trapped as they pass through. Trap effectiveness can be increased by putting a tunnel of roofing paper (24 inches long) at the entrance of the burrow. The squirrel will mistake the light at the end of the tunnel for the burrow opening and run full speed through the trap. The tunnel also minimizes any sun reflection off the metal trap.
If you are using this type of trap within the range of the San Joaquin kit fox, you must place the trap in a covered box with an entrance no larger than 3 inches wide to exclude the fox, or you must spring the traps at dusk and reset them again in the morning.
Modified Gopher Box Traps
Modified box traps consist of a pair box traps that have been joined together by removing the backs, connecting the two traps with wire mesh, and them to a board. The traps are baited with foods such as almonds, barley, melon rinds, oats, or walnuts. Place bait in traps well behind the trigger or tied to the trigger without setting the traps for several days, until the squirrels become used to taking the bait. Then put in fresh bait and set the traps. Place traps so that nontarget animals are not likely to be caught. For example, place traps inside a larger box with openings no larger than 3 inches wide, just large enough to allow ground squirrels to enter.
UC IPM Pest Management Guidelines:
Acknowledgment for contributions to Vertebrates: