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The Handbook: Prevention and Control of Wildlife Damage Management, Internet Center Wildlife Damage for

July 1994

Belding's, California, and Rock Ground

Rex E. Marsh University of California, Davis

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Marsh, Rex E., "Belding's, California, and Rock Ground Squirrels" (1994). The Handbook: Prevention and Control of Wildlife Damage. 26. https://digitalcommons.unl.edu/icwdmhandbook/26

This Article is brought to you for free and open access by the Wildlife Damage Management, Internet Center for at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in The Handbook: Prevention and Control of Wildlife Damage by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Rex E. Marsh Specialist in Vertebrate Ecology (retired) BELDING’S, Department of Wildlife, Fisheries and Conservation Biology University of California CALIFORNIA, AND Davis, California 95616 ROCK GROUND Fig. 1. Belding’s ground , beldingi (left) SQUIRRELS Fig. 2. California ground squirrel, Spermophilus beecheyi (right)

Damage Prevention and Frightening Fumigants Control Methods None are effective. Aluminum phosphide. Repellents Gas cartridges. Exclusion None are very effective. Trapping Limited usefulness and costly. Toxicants Box-type traps (kill and live catch). Cultural Methods Zinc phosphide. Conibear® traps. Flood irrigation and deep tillage may help discourage ground squirrels. Anticoagulants (diphacinone and Shooting chlorophacinone). Habitat Modification Limited effectiveness. Cholecalciferol (state registration only Eliminate brush, rock piles, and old Other Methods for rock squirrels). unused farm machinery that serve Burrow ripping following control. as harborage for the California ground squirrel.

PREVENTION AND CONTROL OF WILDLIFE DAMAGE — 1994 Cooperative Extension Division Institute of Agriculture and Natural Resources University of Nebraska - Lincoln United States Department of Agriculture and Plant Health Inspection Service Animal Damage Control B-151 Great Plains Agricultural Council Wildlife Committee Introduction Range

Twenty-three species and 119 subspe- The Belding occupies the northeastern cies of ground squirrels exist in the part of California, extending north- United States (Hall 1981). At least 10 ward into eastern Oregon and east- species can be of considerable econo- ward into the southwestern portion of mic importance to agriculture or have Idaho (Fig. 3). It also ranges into the a significant impact on public health. north-central portion of Nevada. It is This chapter covers the three species the most numerous and troublesome found in the far west and southwest. squirrel in Oregon and northeastern All three species range over extensive California. regions. While the California (Spermo- The California ground squirrel’s range philus beecheyi) and the Belding’s (S. extends along the far west coast from beldingi) ground squirrels are consid- northern Mexico northward through- ered pests over large agricultural Fig. 3. Range of the Belding’s (dark) and rock ground squirrels (light) in North America. out much of California, the western areas, they are not pests throughout half of Oregon, and a moderate dis- their entire range. The rock ground tance into south-central Washington squirrel (S. variegatus) is not a major (Fig. 4). This species is absent from the pest but is important because of its desert regions of California. It is the involvement in the spread of plague. most serious native pest in The California and rock ground squir- California, especially the subspecies rels are closely related, belonging to the S. b. fisheri and S. b. beecheyi, which same subgenus, . They occupy the Central Valley and the are similar in general size and body coastal region south from San configuration. The Belding’s ground Francisco. squirrel, more commonly referred to as The rock squirrel’s range covers nearly just the Belding ground squirrel, is sub- all of Arizona and New Mexico. It stantially different in appearance from extends eastward into southwestern the California and rock squirrels. Texas and northward into southern Nevada, and covers approximately two-thirds of Utah and Colorado. Identification Fig. 4. Range of the California ground squirrel in North America. More than half of its range extends The Belding ground squirrel (Fig. 1) is south into Mexico (Fig. 3). medium-sized with a stocky build and douglasii), for example, has a blackish short, furry (but not bushy) tail. It is brown wedge-shaped patch in the Habitat brownish gray to reddish brown in middle of the back between the shoul- The large ranges of these three species color, and has no stripes, mottling, or ders, which readily distinguishes it cut across highly varied habitat. The markings of any type. The underside from the other subspecies. of the body is dull cream-buff, paling habitat discussed here is more or less on the throat and inner sides of the The rock ground squirrel is a large- typical and the one most often associ- legs. Coloration varies somewhat with sized, heavy-bodied, ground squirrel ated with economic losses. (10 1/2 inches [26.7 cm] long) with a subspecies. The body is about 8 1/2 Belding ground squirrels live mainly in moderately long (8-inches [20.3-cm]) inches (21.6 cm) long, with a 2 1/2- natural meadows and grasslands but are bushy tail. Large prominent ears inch (6.4-cm) tail. The ears are small adaptive to alfalfa, irrigated pastures, extend above the top of the head. The and not prominent. and the margins of grain fields. At higher fur is grayish, brownish gray, or black- elevations they may occupy meadows in The California ground squirrel (Fig. 2) ish and is mottled with light gray or forested areas, but they avoid forests or is 10 inches (25.4 cm) long and slightly whitish specks or spots; coloration var- dense brushlands. larger than the Belding ground squir- ies with subspecies. This ground squir- rel. It has a moderately long (6 1/2- rel resembles the California ground California ground squirrels occupy inch [16.5-cm]) semi-bushy tail. Ears squirrel in many ways, but is some- grasslands and savannah-like areas are tall and conspicuous, with some what larger and has a longer and with mixtures of oaks and grasslands. exceptionally long hairs at the tips. The bushier tail. The ranges of the rock and They avoid moderate to heavily for- fur is brownish gray and dusky, with a the California ground squirrels do not ested areas or dense brushlands. They flecked or mottled and grizzly appear- overlap; hence the two squirrels can- generally prefer open space, but they ance. Fur markings vary with subspe- not be confused with one another. are highly adaptable to disturbed cies. The Douglas subspecies (S. b. environments and will infest earthen

B-152 dams, levees, irrigation ditch banks, The Belding ground squirrel feeds always left open and never plugged railroad rights-of-way, and road extensively on the leaves, stems, and with soil. The California and rock embankments, and will readily bur- seeds of wild and cultivated grasses. ground squirrels are more colonial in row beneath buildings in rural areas. Its diet, more than that of the other their habits. A number of squirrels oc- They thrive along the margins of grain species discussed in this chapter, tends cupy the same burrow system. The fields and other crops, feeding out into to change less dramatically and Belding ground squirrel is somewhat the field. remains heavily slanted toward green less colonial and its burrows are more succulent vegetation rather than seeds. widely dispersed. Rock squirrels inhabit rocky areas, This, in part, is because of a short hence their name. They live in rocky Ground squirrels are rapid runners active period (from February to July) canyons or on rocky hillsides in arid and good climbers. Of the three spe- at higher elevations where food is of environments, but they adapt to dis- cies, the California and rock ground high quality and plentiful, and few turbed environments and will live squirrels are the most prone to climb- seeds may have matured by the date along stone walls and roadside irriga- ing. When scared by humans or preda- the squirrels start into hibernation. The tion ditches, feeding out into culti- tors, ground squirrels always retreat to lack of seeds in their diet creates sig- vated fields. their burrows. nificant squirrel control problems because commercial squirrel baits use Ground squirrels are hibernators. Food Habits cereal grains as the base of their bait, Most or all of the adult population hence the bait may be poorly accepted goes into hibernation during the cold- Ground squirrels are essentially herbi- by the squirrels. The Belding also con- est period of the year. Squirrels born vores, but insects sometimes make up sumes flowers, stems, leaves, and the previous spring may not go into a very small portion of their diet. The roots of herbaceous plants, depending complete hibernation during the first California ground squirrel, and possi- on its habitat. It consumes seeds and winter. In hot arid regions they may bly the other two, will consume eggs fruit of mature plants in greater quan- estivate, which is a temporary summer of small ground-nesting birds, such as tities in regions where the hibernation sleep that may last for a few days to a quail. Ground squirrels are known to period is delayed until late summer or couple of weeks. cannibalize their own kind and some- fall. times scavenge on road kills of squir- Male California and Belding squirrels rels or other vertebrate species. This, The California ground squirrel feeds generally emerge from hibernation 10 however, represents a very small part extensively on the leaves, stems, and to 14 days prior to the females. The of their overall diet. seeds of a wide variety of forage reverse is reported for rock squirrels. grasses and forbs, depending on the Breeding commences shortly after All three species do well in the absence availability in the area. In oak savan- emergence from hibernation. Breeding of free water, even in the drier regions nah habitat, acorns are a favorite food. is fairly well synchronized, with the of the west. They obtain needed water Thistle seeds are also highly preferred. vast majority of the females in the area from dew or succulent vegetation, All grains and a wide variety of other bred over about a 3-week period. plant bulbs, and bark. If water is avail- crops are consumed in cultivated areas Exact breeding dates may vary from able, they will sometimes be seen by this opportunistic feeder. region to region depending on drinking, but the presence of a stream weather, elevation, and latitude. Those The food of the rock squirrel is varied, or stock reservoir does not offer any farthest north and at the higher eleva- depending on the native vegetation of special attraction for the squirrels. tions are latest to emerge from hiber- the region. It eats many kinds of nation and to breed. Gestation is 28 to Ground squirrels feed almost exclu- grasses and forbs. Acorns, pine nuts, 32 days, and the young are born in a sively on green vegetation when they juniper berries, mesquite buds and nest chamber in the burrow system. emerge from hibernation and through- beans, and fruit and seeds of various The young are born hairless with their out their gestation and lactation native plants, including cactus, make eyes closed. They are nursed in the period. As the grasses and herbaceous up much of its diet. vegetation begin to dry up in arid cli- burrow until about 6 to 7 weeks of age mates and to produce seed, the squir- (about one-third adult size), when they rels switch to eating fruit or seed for General Biology, begin to venture above ground and the majority of their diet. With the Reproduction, and start feeding on green vegetation. Only 1 litter is produced annually. California ground squirrel this switch Behavior is dramatic; a complete change occurs The litter size of the California ground over as short a period as 2 weeks. squirrel averages slightly over 7, while Using their cheek pouches for carrying All species of ground squirrels dig burrows for shelter and safety. The that of the rock and Belding squirrels food items, the California and rock average 5 and 8, respectively. The ground squirrels are highly prone to burrow systems are occupied year after year and are extended in length rodent’s relatively slow annual hoarding and caching food. The reproductive rate is compensated by a Belding is rarely seen in this activity. and complexity each year. Each system has numerous entrances which are relatively long life span of 4 to 5 years.

B-153 Damage and Damage wide variety of other crops, are con- fence will prevent them from climbing Identification sumed in agricultural regions by this over. For a fence to remain squirrel- opportunistic feeder. Almonds, pista- proof, the squirrels that burrow near Two of the three species included in chios, walnuts, apples, apricots, the fence should be eliminated. Experi- this chapter, the California and the peaches, prunes, oranges, tomatoes, ments with a temporary low electric Belding, are considered serious agri- and alfalfa are subject to extensive fence have been shown to seasonally cultural pests where they are found in damage. Certain vegetables and field discourage California squirrels from moderate to high densities adjacent to crops such as sugar beets, beans, and invading research or small garden susceptible crops or home gardens. peas are taken at the seedling stage, plots from outside areas. Rock squirrels overall are relatively and orchard trees are sometimes injured by bark gnawing. Cultural Methods and Habitat insignificant as agricultural pests even Modification though their damage may be economi- Rock ground squirrels consume peas, Flood irrigation, as opposed to sprin- cally significant to individual growers. squash, corn, and grains of all kinds. kler or drip irrigation, discourages All three are implicated in the trans- They also feed on various fruit, includ- ground squirrels in orchards, alfalfa, mission of certain diseases to people, ing apples, cherries, apricots, peaches, and pasture land. It does not, how- notably plague. This is the major rea- pears, and melons, primarily to obtain ever, get rid of them completely. son that rock squirrels are included in their seed. They sometimes dig up and Ground squirrels are limited by fre- this chapter. They are all adaptive and consume planted seed. Rock squirrels quent tillage, especially deep discing feed on a variety of crops, depending are not major pests, however, because or plowing. Squirrels compensate by on the ones grown in proximity to their preferred natural habitat infre- living at the margins of cropland and their natural habitat. Since ground quently adjoins cultivated crops. squirrels are active during daylight then feeding inward from the field hours, and their burrow openings are Legal Status borders. Keep fence lines vegetation- readily discernible, damage identifica- free by discing as close as possible to them to limit the area where squirrels tion is generally uncomplicated. The three species of ground squirrels can thrive. Their burrowing activities, particularly discussed in this chapter are generally those of the California and Belding regarded as pests and, as such, are not Eliminate piles of orchard prunings ground squirrels, weaken levees, ditch protected. Local laws or regulations from the margins of the orchard to banks, and earthen dams, and under- should, however, be consulted before reduce cover sought by the California mine roadways and buildings. Bur- undertaking lethal control. ground squirrel. Remove abandoned irrigation pipes or farm equipment rows can also result in loss of irrigation Be aware that several of the numerous from field margins, as well as piles of water by unwanted diversions, and in ground squirrel species are on the rocks retrieved from fields, to reduce natural habitats they may cause accel- threatened or endangered species lists. sites beneath which the squirrels prefer erated soil erosion by channelling rain Any control of pest species must take to burrow. or snow runoff. into consideration the safeguarding Burrow entrances in school play- and protection of endangered ground Frightening squirrels and other rodent species. grounds, parks, and other recreational Ground squirrels cannot be frightened areas are responsible for debilitating Damage Prevention and from their burrow sites by traditional falls, occasionally resulting in sprained frightening methods such as propane or broken ankles or limbs. Burrows in Control Methods exploders or flagging. horse exercising or jumping arenas or on equestrian trails can cause serious Exclusion Repellents injuries to horses and to their riders if Squirrels can be excluded from build- Chemical taste and/or odor repellents thrown. ings with the same techniques used to are ineffective in causing the squirrels The Belding ground squirrel, under exclude commensal rats (see Rodent- to leave or avoid an area or in prevent- favorable conditions, reaches incred- proof Construction and Exclusion ing damage to growing crops. Seed ible densities, often exceeding 100 per Methods). Use sheet metal cylinders treatment repellents may offer some acre (247/ha). Extensive losses may be around tree trunks to prevent loss of limited protection to newly planted experienced in range forage, irrigated fruit or nut crops. crops and may be state registered for pastures, alfalfa, wheat, oats, barley, special local needs. Thiram is an While fences can be constructed to and rye. example of a taste repellent sometimes exclude squirrels, they aren’t usually used as a seed protectant. The California ground squirrel, where practical because of their expense. numerous, significantly depletes the Ground squirrels can readily dig Toxicants forage for livestock, reducing carrying beneath fences that are buried several capacity on rangeland as well as irri- feet (m) deep in the soil. Sheet metal Rodenticide-treated baits are the most gated pasture land. All grains, and a caps atop a 4-foot (1.2 m) wire mesh economical of all approaches to

B-154 population reduction and, hence, have PVC Inverted-T traditionally been the mainstay of anticoagulant bait station ground squirrel control. Currently, zinc phosphide is the only acute ro- 1. Make the bait station from PVC pipe denticide that is registered by EPA for no smaller than 4" in diameter for the control of Belding and California ground squirrels. ground squirrels. In addition, the anti- 2. The long, upright end, 24" to 30", of coagulants diphacinone and chloro- the “T” is a bait reservoir. Keep this end upright by attaching it to a fence phacinone are registered (some of post, building, tree, stake, etc. After these labels are state registrations filling with bait, place a plastic cap only). Cholecalciferol has a New on the end to keep moisture from Mexico state registration for rock reaching bait. squirrels but not for any other squirrel 3. Keep bait in the station at all times species. Zinc phosphide, for the most during the control program. part, has replaced 1080 and strychnine 24" to 30" for squirrel control, since the latter are no longer registered for these species. Zinc phosphide is not always highly efficacious, but efficacy is improved if prebaiting is conducted. Bait shyness occurs when sublethal doses are con- sumed at the initial feeding.

The chronic slower-acting anticoagu- 12" to 15" lants are more expensive to purchase and require more bait because mul- o 4" tiple feedings are necessary to produce 45 PVC (4") T death. Also, death is delayed. On the other hand, these accumulative poi- Fig. 5. Commonly used inverted “T” ground squirrel bait station made of 4-inch (10-cm) PVC pipe. sons do not produce bait shyness, thus Stake secures station in upright position. providing more latitude than zinc phosphide in the timing of baiting Anticoagulant baits are most often To assure good bait acceptance prior programs. exposed in bait boxes, where a con- to an extensive control program, Zinc phosphide baits are most often tinuous supply of bait will be available acceptance should be tested by scatter- hand applied with a tablespoon (4 g) to the squirrels. Bait boxes may be ing tablespoons of bait next to a few of bait scattered on bare ground over made of rubber tires, or metal, plastic, burrows. If all of the bait is gone the about 3 or 4 square feet (0.3 m2) next to or wood containers. Many are made of next day, good bait acceptance is indi- the burrow entrance. Zinc phosphide sections of 4-inch (10-cm) plastic irriga- cated. Bait acceptance is especially is a Restricted Use Pesticide when tion pipe designed in an inverted “T” important with zinc phosphide or used in large quantities; follow label configuration (Fig. 5). Squirrels are cholecalciferol, both of which require instructions as to methods and rates of often reluctant to enter the bait boxes just a single feeding to produce death. application. Some labels permit broad- or stations for a few days, and it may Good acceptance avoids poor control cast application of zinc phosphide and take several additional weeks before and possible bait or toxin shyness, anticoagulant baits. Use hand-cranked all the squirrels are killed and bait con- which will adversely affect repeat con- cyclone seeders or vehicle-mounted sumption ceases. Caching of bait does trol efforts. occur, especially with California tailgate seeders for such applications. If acceptance of cereal baits is less than ground squirrels, and is more preva- adequate (either prebait or test baits Anticoagulant baits, depending on the lent in the late summer and fall of the are not consumed), then zinc phos- label directions, may be hand applied year. Apply baits earlier in the year to phide application should be delayed like zinc phosphide but require some- save bait. what more bait as well as repeated until bait acceptance is improved, or applications. Three or 4 applications a The timing of baiting programs is criti- not applied at all in favor of other day on alternate days is a commonly cal to good control. For maximum control options. Anticoagulant baits used schedule for the California effectiveness, bait only when all the placed in bait stations can sometimes ground squirrel. Double strength squirrels are out of hibernation or esti- be an effective option where zinc phos- diphacinone or chlorophacinone vation and are actively feeding on phide acceptance is marginal. Squirrels (0.01%) is most effective for broadcast seed. Commercial baits are prepared may learn to take the anticoagulant applications. on grain or pelletized cereals. bait over time and, since they are

B-155 accumulatively poisoned with no bait 1/2" hardware cloth shyness, control will not be jeopar- dized by marginal feeding as long as Baiting area feeding continues over a number of days.

Fumigants Ground squirrels can be killed in their burrow systems by introducing one of several toxic or suffocating gases, such as phosphine gas or carbon monoxide. Fumigation should be conducted when the squirrels are out of hiberna- tion. Hibernating squirrels plug their burrows with soil to separate them- Fig. 6. Two modified pocket gopher traps mounted on a single board with a common bait area are selves from the outside, whereby they very effective for trapping ground squirrels. are safe from the lethal consequences of the toxic gas. 1/2" hardware Burrow fumigation has a distinct cloth is stapled over the back end advantage over toxicants and trapping of the box. in that it is linked to no behavioral trait Box is constructed out of other than that squirrels seek the cover 1" wood stock. of their burrows when disturbed. Fumigation is most effective following ground squirrel emergence from hiber- nation and before the squirrels have 3" time to reproduce. Recently born squirrels, too young to venture above ground to be baited or trapped, are Notch in trap box leaves space for 1.75" effectively controlled by fumigants. spring to protrude.

5.5" 2" Gas cartridges are easy to use and are available from commercial manufac- turers and distributors or from the 5.5" 12" to 14" USDA supply depot at Pocatello, Idaho. They consist of cylinders of 7" combustible ingredients with a fuse. Place the cartridge at the entrance of the burrow and light the fuse; then, with a shovel handle or stick, push the lit cartridge as far back into the burrow Conibear® trap No. 110 or No. 110-2 is as possible. Quickly cover the burrow placed inside the box opening with the entrances with soil or sod and tamp chain secured to the box with a staple. tight to seal in the toxic gases. The best results are obtained when soil mois- Fig. 7. Wooden trap box constructed to hold a ture is high, because less gas will Conibear® (No. 110) for trapping ground escape the system. Do not use near squirrels. buildings, because high temperatures may cause fires. Trapping The method for using aluminum phos- The newspaper plug prevents the soil from covering the pellets or tablets, phide differs considerably from that Although labor-intensive, trapping can permitting them to react more readily for gas cartridges. Place the prescribed be highly effective in reducing low to with the atmospheric and soil moisture number of aluminum phosphide tab- moderate squirrel populations over to produce the lethal phosphine gas. lets or pellets as far back into the bur- relatively small acreages or where poi- Aluminum phosphide is a Restricted row opening as possible. Then insert a son baits may be inappropriate. Trap- Use Pesticide. Knowledge of its proper wad of crumpled newspaper into the ping can be conducted any time the handling is required. burrow and seal it tightly with soil. squirrels are out of hibernation. For

B-156 humane reasons, avoid the period eating seed, then wheat, oats, or barley Economics of Damage and when the females are lactating and can be used as bait. The Conibear® Control nursing their young. Trapping prior to trap has virtually replaced all uses of the time the young are born is biologi- leghold traps in the far west for In one experimental study, 12 Califor- cally most sound from a control point ground squirrel control. nia ground squirrels were found to of view. Live-catch wire or wooden traps can consume about 1,000 pounds (454 kg) An initial investment of an adequate be used to trap ground squirrels in of range forage. In another study, it number of traps is required, but, if residential areas where kill-type traps was calculated that 200 ground squir- properly maintained, traps will last are considered inappropriate from a rels consumed the same amount as a many years. In agricultural situations, public relations point of view. The 1,000-pound (454-kg) steer. In spite of 100 or more traps may be needed to captured squirrels should be removed control, the California ground squirrel start with. A good rule of thumb is one from the site and humanely has caused an estimated 30 to 50 mil- trap for every 10 to 15 squirrels pre- euthanized with carbon dioxide. lion dollars of agricultural and other sent. If too few traps are used, the Releasing live ground squirrels else- damage annually in California alone. trapper becomes discouraged long where is illegal in some states, A northern California study of the before the squirrel population is uneconomical, and rarely biologically Belding’s ground squirrel showed that brought under control. sound in any holistic approach to pest 123 squirrels per acre (304/ha) management or disease prevention. Several types of traps are used for destroyed 1,790 pounds of alfalfa per ground squirrels. A modified pocket Shooting acre (2,006 kg/ha) over one growing gopher kill-type box trap has been season. used to trap the California ground If local laws permit, shooting with a Little seems to be recorded concerning squirrel for many years (Fig. 6). It can .22 rifle may provide some control the extent or amount of economic be set near burrow openings, in trails, where squirrel numbers are low, but it damage caused by the rock squirrel. or in trees where nut or fruit crops are is very time-consuming. For safety Economic loss is believed to be rela- being damaged. Bait traps with wal- considerations, shooting is generally tively low, but the rock squirrel’s role nuts, almonds, slices of orange, or limited to rural situations and is con- in the transmission of plague makes it pieces of melon. With all types of sidered too hazardous in many more important from a public health view- squirrel traps, the control period will populated areas, even if legal. Ground point. be more decisive and maximum squirrels that are repeatedly shot at results obtained if the traps are left become very hunter/gun-shy. Rarely The cost of control varies with the unset or tied open and baited for sev- can one get close enough to use a pellet situation, squirrel density, and eral days to permit the squirrels to get gun effectively, and the noise of a shot- methods employed. Baiting with an used to them. Then rebait and set all gun scares the squirrels sufficiently acute toxicant like zinc phosphide is the traps. that after the first shot, the remaining the most economical method, with 1 squirrels will be very hesitant to pound (454 g) of bait ample for place- Unbaited Conibear® traps (No. 110 or emerge from their burrows. ment adjacent to 60 burrow entrances. No. 110-2) with a 4 1/2 x 4 1/2-inch The use of anticoagulant baits is con- (11.4 x 11.4-cm) jaw spread are effec- Other Methods siderably more expensive, requiring tive when set over the burrow anywhere from 1/2 to 1 1/4 pounds entrances. This method is not useful Once ground squirrels have been (227 to 568 g) of bait per squirrel. The where squirrels are living in the rocks removed from a crop area, their expense of bait stations would be an or in rocky situations where burrow reinvasion can be substantially slowed added cost. entrances are inaccessible. A special by ripping up their old burrow sites to trap box (Fig. 7) will facilitate the use a depth of at least 20 inches (51 cm), The use of burrow fumigants is about of Conibear® traps that cannot be set preferably deeper. One to three rip- 8 to 10 times more expensive for over burrow openings. These make the ping tongs mounted on the hydraulic materials and labor than the use of Conibear® traps more versatile as they implement bar of a tractor works well. zinc phosphide baits. Trapping is half can be set in trails or near burrow Spacing between rips should be about again more expensive than burrow openings. Conibears in trap boxes 3 feet (1 m). This approach is not suit- fumigation. must be baited to entice the squirrels able where the burrows are beneath into the trap. If the squirrels are readily large rocks or trees.

B-157 Acknowledgments

Figures 1 and 2 from T. I. Storer (1958). Figures 3 and 4 adapted from E. R. Hall (1981), by David Thornhill, University of Nebraska- Lincoln. Figure 5, 6, and 7 adapted from R. E. Marsh by David Thornhill.

For Additional Information

Beard, M. L., G. O. Maupin, A. M. Barnes, and E. F. Marshall. 1987. Laboratory trials of cholecalciferol against Spermophilus variegatus (rock squirrels), a source of human plague (Yersinia pestis) in the southwestern United States. J. Environ. Health 50:287-289. Clark, J. P. 1986. Vertebrate pest control handbook (rev.). Div. Plant Industry, California Dep. Food Agric., Sacramento, California. 350 pp. Hall, E. R. 1981. The of North America. Vol. 1, 2d ed. John Wiley and Sons, New York. 600 pp. Marsh, R. E. 1987. Ground squirrel control strategies in California agriculture. Pages 261-276 in C. G. J. Richards and T. Y. Ku, eds. Control of pests. Taylor & Francis, London. Salmon, T. P. 1981. Controlling ground squirrels around structures, gardens, and small farms. Div. Agric. Sci., Univ. California, Leaflet 21179. 11 pp. Salmon, T. P., and R. H. Schmidt. 1984. An introductory overview to California ground squirrel control. Proc. Vertebr. Pest Conf. 11:32-37. Tomich, P. Q. 1982. Ground squirrels. Pages 192- 208 in J. A. Chapman and G. A. Feldhamer, eds. Wild mammals of North America: biology, management, and economics. The Johns Hopkins Univ. Press, Baltimore, Maryland.

Editors Scott E. Hygnstrom Robert M. Timm Gary E. Larson

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