SQUIRRELS (Belding's, California and Rock Ground Squirrels)
Twenty-three species and 119 subspecies of ground squirrels exist in the United States (Hall 1981). At least 10 species can be of considerable economic importance to agriculture or have a significant impact on public health. This chapter covers the three species found in the far west and southwest. All three species range over extensive regions. While the California (Spermophilus beecheyi) and the Belding's (S. beldingi) ground squirrels are considered pests over large agricultural areas, they are not pests throughout their entire range. The rock ground squirrel (S. variegatus) is not a major pest but is important because of its involvement in the spread of plague. The California and rock ground squirrels are closely related, belonging to the same subgenus, Otospermophilus. They are similar in general size and body configuration. The Belding's ground squirrel, more commonly referred to as just the Belding ground squirrel, is substantially different in appearance from the California and rock squirrels.
Identification
The Belding ground squirrel is medium-sized with a stocky build and short, furry (but not bushy) tail. It is brownish gray to reddish brown in color, and has no stripes, mottling, or markings of any type. The underside of the body is dull cream-buff, paling on the throat and inner sides of the legs. Coloration varies somewhat with subspecies. The body is about 8 1/2 inches (21.6 cm) long, with a 2 1/2-inch (6.4-cm) tail. The ears are small and not prominent.
The California ground squirrel is 10 inches (25.4 cm) long and slightly larger than the Belding ground squirrel. It has a moderately long (6 1/2-inch [16.5-cm]) semi-bushy tail. Ears are tall and conspicuous, with some exceptionally long hairs at the tips. The fur is brownish gray and dusky, with a flecked or mottled and grizzly appearance. Fur markings vary with subspecies. The Douglas subspecies ( S. b. douglasii ), for example, has a blackish brown wedge-shaped patch in the middle of the back between the shoulders, which readily distinguishes it from the other subspecies.
The rock ground squirrel is a large-sized, heavy-bodied, ground squirrel (10 1/2 inches [26.7 cm] long) with a moderately long (8-inches [20.3-cm]) bushy tail. Large prominent ears extend above the top of the head. The fur is grayish, brownish gray, or blackish and is mottled with light gray or whitish specks or spots; coloration varies with subspecies. This ground squirrel resembles the California ground squirrel in many ways, but is somewhat larger and has a longer and bushier tail. The ranges of the rock and the California ground squirrels do not overlap; hence the two squirrels cannot be confused with one another.
Range
The Belding occupies the northeastern part of California, extending northward into eastern Oregon and eastward into the southwestern portion of Idaho. It also ranges into the north-central portion of Nevada. It is the most numerous and troublesome squirrel in Oregon and northeastern California.
The California ground squirrel's range extends along the far west coast from northern Mexico northward throughout much of California, the western half of Oregon , and a moderate distance into south-central Washington. This species is absent from the desert regions of California. It is the most serious native rodent pest in California, especially the subspecies S. b. fisheri and S. b. beecheyi, which occupy the Central Valley and the coastal region south from San Francisco.
The rock squirrel's range covers nearly all of Arizona and New Mexico. It extends eastward into southwestern Texas and northward into southern Nevada, and covers approximately two-thirds of Utah and Colorado. More than half of its range extends south into Mexico.
Habitat
The large ranges of these three species cut across highly varied habitat. The habitat discussed here is more or less typical and the one most often associated with economic losses.
Belding ground squirrels live mainly in natural meadows and grasslands but are adaptive to alfalfa, irrigated pastures, and the margins of grain fields. At higher elevations they may occupy meadows in forested areas, but they avoid forests or dense brushlands .
California ground squirrels occupy grasslands and savannah-like areas with mixtures of oaks and grasslands. They avoid moderate to heavily forested areas or dense brushlands . They generally prefer open space, but they are highly adaptable to disturbed environments and will infest earthen dams, levees, irrigation ditch banks, railroad rights-of-way, and road embankments, and will readily burrow beneath buildings in rural areas. They thrive along the margins of grain fields and other crops, feeding out into the field.
Rock squirrels inhabit rocky areas, hence their name. They live in rocky canyons or on rocky hillsides in arid environments, but they adapt to disturbed environments and will live along stone walls and roadside irrigation ditches, feeding out into cultivated fields.
Food Habits
Ground squirrels are essentially herbivores, but insects sometimes make up a very small portion of their diet. The California ground squirrel, and possibly the other two, will consume eggs of small ground-nesting birds, such as quail. Ground squirrels are known to cannibalize their own kind and sometimes scavenge on road kills of squirrels or other vertebrate species. This, however, represents a very small part of their overall diet.
All three species do well in the absence of free water, even in the drier regions of the west. They obtain needed water from dew or succulent vegetation, plant bulbs, and bark. If water is available, they will sometimes be seen drinking, but the presence of a stream or stock reservoir does not offer any special attraction for the squirrels.
Ground squirrels feed almost exclusively on green vegetation when they emerge from hibernation and throughout their gestation and lactation period. As the grasses and herbaceous vegetation begin to dry up in arid climates and to produce seed, the squirrels switch to eating fruit or seed for the majority of their diet. With the California ground squirrel this switch is dramatic; a complete change occurs over as short a period as 2 weeks. Using their cheek pouches for carrying food items, the California and rock ground squirrels are highly prone to hoarding and caching food. The Belding is rarely seen in this activity.
The Belding ground squirrel feeds extensively on the leaves, stems, and seeds of wild and cultivated grasses. Its diet, more than that of the other species discussed in this chapter, tends to change less dramatically and remains heavily slanted toward green succulent vegetation rather than seeds. This, in part, is because of a short active period (from February to July) at higher elevations where food is of high quality and plentiful, and few seeds may have matured by the date the squirrels start into hibernation. The lack of seeds in their diet creates significant squirrel control problems because commercial squirrel baits use cereal grains as the base of their bait, hence the bait may be poorly accepted by the squirrels. The Belding also consumes flowers, stems, leaves, and roots of herbaceous plants, depending on its habitat. It consumes seeds and fruit of mature plants in greater quantities in regions where the hibernation period is delayed until late summer or fall.
The California ground squirrel feeds extensively on the leaves, stems, and seeds of a wide variety of forage grasses and forbs, depending on the availability in the area. In oak savannah habitat, acorns are a favorite food. Thistle seeds are also highly preferred. All grains and a wide variety of other crops are consumed in cultivated areas by this opportunistic feeder.
The food of the rock squirrel is varied, depending on the native vegetation of the region. It eats many kinds of grasses and forbs. Acorns, pine nuts, juniper berries, mesquite buds and beans, and fruit and seeds of various native plants, including cactus, make up much of its diet.
General Biology, Reproduction, and Behavior
All species of ground squirrels dig burrows for shelter and safety. The burrow systems are occupied year after year and are extended in length and complexity each year. Each system has numerous entrances which are always left open and never plugged with soil. The California and rock ground squirrels are more colonial in their habits. A number of squirrels occupy the same burrow system. The Belding ground squirrel is somewhat less colonial and its burrows are more widely dispersed.
Ground squirrels are rapid runners and good climbers. Of the three species, the California and rock ground squirrels are the most prone to climbing. When scared by humans or predators, ground squirrels always retreat to their burrows.
Ground squirrels are hibernators. Most or all of the adult population goes into hibernation during the coldest period of the year. Squirrels born the previous spring may not go into complete hibernation during the first winter. In hot arid regions they may estivate , which is a temporary summer sleep that may last for a few days to a couple of weeks.
Male California and Belding squirrels generally emerge from hibernation 10 to 14 days prior to the females. The reverse is reported for rock squirrels. Breeding commences shortly after emergence from hibernation. Breeding is fairly well synchronized, with the vast majority of the females in the area bred over about a 3-week period. Exact breeding dates may vary from region to region depending on weather, elevation, and latitude. Those farthest north and at the higher elevations are latest to emerge from hibernation and to breed. Gestation is 28 to 32 days, and the young are born in a nest chamber in the burrow system. The young are born hairless with their eyes closed. They are nursed in the burrow until about 6 to 7 weeks of age (about one-third adult size), when they begin to venture above ground and start feeding on green vegetation. Only 1 litter is produced annually.
The litter size of the California ground squirrel averages slightly over 7, while that of the rock and Belding squirrels average 5 and 8, respectively. The rodent's relatively slow annual reproductive rate is compensated by a relatively long life span of 4 to 5 years.
Damage and Damage Identification
Two of the three species included in this chapter, the California and the Belding, are considered serious agricultural pests where they are found in moderate to high densities adjacent to susceptible crops or home gardens. Rock squirrels overall are relatively insignificant as agricultural pests even though their damage may be economically significant to individual growers. All three are implicated in the transmission of certain diseases to people, notably plague. This is the major reason that rock squirrels are included in this chapter. They are all adaptive and feed on a variety of crops, depending on the ones grown in proximity to their natural habitat. Since ground squirrels are active during daylight hours, and their burrow openings are readily discernible, damage identification is generally uncomplicated.
Their burrowing activities, particularly those of the California and Belding ground squirrels, weaken levees, ditch banks, and earthen dams, and undermine roadways and buildings. Burrows can also result in loss of irrigation water by unwanted diversions, and in natural habitats they may cause accelerated soil erosion by channelling rain or snow runoff.
Burrow entrances in school playgrounds, parks, and other recreational areas are responsible for debilitating falls, occasionally resulting in sprained or broken ankles or limbs. Burrows in horse exercising or jumping arenas or on equestrian trails can cause serious injuries to horses and to their riders if thrown.
The Belding ground squirrel, under favorable conditions, reaches incredible densities, often exceeding 100 per acre (247/ha). Extensive losses may be experienced in range forage, irrigated pastures, alfalfa, wheat, oats, barley, and rye.
The California ground squirrel, where numerous, significantly depletes the forage for livestock, reducing carrying capacity on rangeland as well as irrigated pasture land. All grains, and a wide variety of other crops, are consumed in agricultural regions by this opportunistic feeder. Almonds, pistachios, walnuts, apples, apricots, peaches, prunes, oranges, tomatoes, and alfalfa are subject to extensive damage. Certain vegetables and field crops such as sugar beets, beans, and peas are taken at the seedling stage, and orchard trees are sometimes injured by bark gnawing.
Rock ground squirrels consume peas, squash, corn, and grains of all kinds. They also feed on various fruit, including apples, cherries, apricots, peaches, pears, and melons, primarily to obtain their seed. They sometimes dig up and consume planted seed. Rock squirrels are not major pests, however, because their preferred natural habitat infrequently adjoins cultivated crops.
Legal Status
The three species of ground squirrels discussed in this chapter are generally regarded as pests and, as such, are not protected. Local laws or regulations should, however, be consulted before undertaking lethal control.
Be aware that several of the numerous ground squirrel species are on the threatened or endangered species lists. Any control of pest species must take into consideration the safeguarding and protection of endangered ground squirrels and other rodent species.
Damage Prevention and Control Methods
Exclusion
Limited usefulness and costly.
Habitat Modification
Flood irrigation and deep tillage may help discourage ground squirrels. Eliminate brush, rock piles, and old unused farm machinery that serve as harborage for the California ground squirrel.
Repellents
None are very effective.
Toxicants
Zinc phosphide. Anticoagulants ( diphacinone and chlorophacinone ). Cholecalciferol (state registration only for rock squirrels).
Fumigants
Aluminum phosphide . Gas cartridges.
Trapping
Box-type traps (kill and live catch). Conibear ® traps.
Shooting
Limited effectiveness.
Other Methods
Burrow ripping following control.
ACKNOWLEDGEMENTS
The above information was adapted from PREVENTION AND CONTROL OF WILDLIFE DAMAGE with permission of the editors, Scott E. Hygnstrom, Robert M. Timm, and Gary E. Larson (Cooperative Extension Division, Institute of Agriculture and Natural Resources University of Nebraska-Lincoln, United States Department of Agriculture Animal and Plant Health Inspection Service, Animal Damage Control, Great Plains Agricultural Council Wildlife Committee).
