RODENTS: Squirrels, Belding's, California,
and Rock Ground
Belding’s ground squirrel, Spermophilus beldingi (left)
Fig. 2. California ground squirrel, Spermophilus
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 Belding ground squirrel (Fig. 1) 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 (Fig. 2) 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.
The Belding occupies the
northeastern part of California, extending northward
into eastern Oregon and eastward into the southwestern
portion of Idaho (Fig. 3). It also ranges into the
north-central portion of Nevada. It is the most numerous
and troublesome squirrel in Oregon and northeastern
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 (Fig. 4). 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 (Fig. 3).
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
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.
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
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
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.
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 channeling rain or
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
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.
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
Squirrels can be excluded from buildings with the
same techniques used to exclude commensal rats (see
Rodent-proof Construction and Exclusion Methods). Use
sheet metal cylinders around tree trunks to prevent loss
of fruit or nut crops.
While fences can be
constructed to exclude squirrels, they aren’t usually
practical because of their expense. Ground squirrels can
readily dig beneath fences that are buried several feet
(m) deep in the soil. Sheet metal caps atop a 4-foot
(1.2 m) wire mesh fence will prevent them from climbing
over. For a fence to remain squirrel-proof, the
squirrels that burrow near the fence should be
eliminated. Experiments with a temporary low electric
fence have been shown to seasonally discourage
California squirrels from invading research or small
garden plots from outside areas.
Cultural Methods and
Flood irrigation, as opposed to sprinkler or drip
irrigation, discourages ground squirrels in orchards,
alfalfa, and pasture land. It does not, however, get rid
of them completely. Ground squirrels are limited by
frequent tillage, especially deep discing or plowing.
Squirrels compensate by living at the margins of
cropland and then feeding inward from the field borders.
Keep fence lines vegetation-free by discing as close as
possible to them to limit the area where squirrels can
Eliminate piles of orchard
prunings from the margins of the orchard to reduce cover
sought by the California ground squirrel. Remove
abandoned irrigation pipes or farm equipment from field
margins, as well as piles of rocks retrieved from
fields, to reduce sites beneath which the squirrels
prefer to burrow.
Ground squirrels cannot be frightened from their
burrow sites by traditional frightening methods such as
propane exploders or flagging.
Chemical taste and/or odor repellents are
ineffective in causing the squirrels to leave or avoid
an area or in preventing damage to growing crops. Seed
treatment repellents may offer some limited protection
to newly planted crops and may be state registered for
special local needs. Thiram is an example of a taste
repellent sometimes used as a seed protectant.
Rodenticide-treated baits are the most economical of all
approaches to population reduction and, hence, have
traditionally been the mainstay of ground squirrel
control. Currently, zinc phosphide is the only acute
rodenticide that is registered by EPA for the control of
Belding and California ground squirrels. In addition,
the anticoagulants diphacinone and chlorophacinone are
registered (some of these labels are state registrations
only). Cholecalciferol has a New Mexico state
registration for rock squirrels but not for any other
squirrel species. Zinc phosphide, for the most part, has
replaced 1080 and strychnine 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 consumed at the initial feeding.
The chronic slower-acting
anticoagulants are more expensive to purchase and
require more bait because multiple feedings are
necessary to produce death. Also, death is delayed. On
the other hand, these accumulative poisons do not
produce bait shyness, thus providing more latitude than
zinc phosphide in the timing of baiting programs.
Zinc phosphide baits are
most often hand applied with a tablespoon (4 g) of bait
scattered on bare ground over about 3 or 4 square feet
(0.3 m2) next to the burrow entrance. Zinc phosphide is
a Restricted Use Pesticide when used in large
quantities; follow label instructions as to methods and
rates of application. Some labels permit broadcast
application of zinc phosphide and anticoagulant baits.
Use hand-cranked cyclone seeders or vehicle-mounted
tailgate seeders for such applications.
baits, depending on the label directions, may be hand
applied like zinc phosphide but require somewhat more
bait as well as repeated applications. Three or 4
applications a day on alternate days is a commonly used
schedule for the California ground squirrel. Double
strength diphacinone or chlorophacinone (0.01%) is most
effective for broadcast applications.
Anticoagulant baits are
most often exposed in bait boxes, where a continuous
supply of bait will be available to the squirrels. Bait
boxes may be made of rubber tires, or metal, plastic, or
wood containers. Many are made of sections of 4-inch
(10-cm) plastic irrigation pipe designed in an inverted
“T” configuration (Fig. 5). Squirrels are often
reluctant to enter the bait boxes or stations for a few
days, and it may take several additional weeks before
all the squirrels are killed and bait consumption
ceases. Caching of bait does occur, especially with
California ground squirrels, and is more prevalent in
the late summer and fall of the year. Apply baits
earlier in the year to save bait.
The timing of baiting
programs is critical to good control. For maximum
effectiveness, bait only when all the squirrels are out
of hibernation or estivation and are actively feeding on
seed. Commercial baits are prepared on grain or
To assure good bait
acceptance prior to an extensive control program,
acceptance should be tested by scattering tablespoons of
bait next to a few burrows. If all of the bait is gone
the next day, good bait acceptance is indicated. Bait
acceptance is especially important with zinc phosphide
or cholecalciferol, both of which require just a single
feeding to produce death. Good acceptance avoids poor
control and possible bait or toxin shyness, which will
adversely affect repeat control efforts.
If acceptance of cereal
baits is less than adequate (either prebait or test
baits are not consumed), then zinc phosphide application
should be delayed until bait acceptance is improved, or
not applied at all in favor of other control options.
Anticoagulant baits placed in bait stations can
sometimes be an effective option where zinc phosphide
acceptance is marginal. Squirrels may learn to take the
anticoagulant bait over time and, since they are
accumulatively poisoned with no bait shyness, control
will not be jeopardized by marginal feeding as long as
feeding continues over a number of days.
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 hibernation. Hibernating squirrels
plug their burrows with soil to separate themselves from
the outside, whereby they are safe from the lethal
consequences of the toxic gas.
Burrow fumigation has a
distinct advantage over toxicants and trapping in that
it is linked to no behavioral trait other than that
squirrels seek the cover of their burrows when
disturbed. Fumigation is most effective following ground
squirrel emergence from hibernation and before the
squirrels have time to reproduce. Recently born
squirrels, too young to venture above ground to be
baited or trapped, are effectively controlled by
Gas cartridges are easy to
use and are available from commercial manufacturers and
distributors or from the USDA supply depot at Pocatello,
Idaho. They consist of cylinders of 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 as possible. Quickly cover the
burrow entrances with soil or sod and tamp tight to seal
in the toxic gases. The best results are obtained when
soil moisture is high, because less gas will escape the
system. Do not use near buildings, because high
temperatures may cause fires.
The method for using
aluminum phosphide differs considerably from that for
gas cartridges. Place the prescribed number of aluminum
phosphide tablets or pellets as far back into the burrow
opening as possible. Then insert a wad of crumpled
newspaper into the burrow and seal it tightly with soil.
The newspaper plug
prevents the soil from covering the pellets or tablets,
permitting them to react more readily with the
atmospheric and soil moisture to produce the lethal
phosphine gas. Aluminum phosphide is a Restricted Use
Pesticide. Knowledge of its proper handling is required.
Although labor-intensive, trapping can be highly
effective in reducing low to moderate squirrel
populations over relatively small acreages or where
poison baits may be inappropriate. Trapping can be
conducted any time the squirrels are out of hibernation.
For humane reasons, avoid the period when the females
are lactating and nursing their young. Trapping prior to
the time the young are born is biologically most sound
from a control point of view.
An initial investment of
an adequate number of traps is required, but, if
properly maintained, traps will last many years. In
agricultural situations, 100 or more traps may be needed
to start with. A good rule of thumb is one trap for
every 10 to 15 squirrels present. If too few traps are
used, the trapper becomes discouraged long before the
squirrel population is brought under control. Several
types of traps are used for ground squirrels. A modified
pocket gopher kill-type box trap has been used to trap
the California ground squirrel for many years (Fig. 6).
It can be set near burrow openings, in trails, or in
trees where nut or fruit crops are being damaged. Bait
traps with walnuts, almonds, slices of orange, or pieces
of melon. With all types of squirrel traps, the control
period will be more decisive and maximum results
obtained if the traps are left unset or tied open and
baited for several days to permit the squirrels to get
used to them. Then rebait and set all the traps.
Unbaited Conibear® traps
(No. 110 or No. 110-2) with a 4 1/2 x 4 1/2-inch (11.4 x
11.4-cm) jaw spread are effective when set over the
burrow entrances. This method is not useful where
squirrels are living in the rocks or in rocky situations
where burrow entrances are inaccessible. A special trap
box (Fig. 7) will facilitate the use of Conibear® traps
that cannot be set over burrow openings. These make the
Conibear® traps more versatile as they can be set in
trails or near burrow openings. Conibears in trap boxes
must be baited to entice the squirrels into the trap. If
the squirrels are readily eating seed, then wheat, oats,
or barley can be used as bait. The Conibear® trap has
virtually replaced all uses of leghold traps in the far
west for ground squirrel control.
Live-catch wire or wooden
traps can be used to trap ground squirrels in
residential areas where kill-type traps are considered
inappropriate from a public relations point of view. The
captured squirrels should be removed from the site and
humanely euthanized with carbon dioxide. Releasing live
ground squirrels elsewhere is illegal in some states,
uneconomical, and rarely biologically sound in any
holistic approach to pest management or disease
If local laws permit, shooting with a .22 rifle may
provide some control where squirrel numbers are low, but
it is very time-consuming. For safety considerations,
shooting is generally limited to rural situations and is
considered too hazardous in many more populated areas,
even if legal. Ground squirrels that are repeatedly shot
at become very hunter/gun-shy. Rarely can one get close
enough to use a pellet gun effectively, and the noise of
a shotgun scares the squirrels sufficiently that after
the first shot, the remaining squirrels will be very
hesitant to emerge from their burrows.
Once ground squirrels have
been removed from a crop area, their reinvasion can be
substantially slowed by ripping up their old burrow
sites to a depth of at least 20 inches (51 cm),
preferably deeper. One to three ripping tongs mounted on
the hydraulic implement bar of a tractor works well.
Spacing between rips should be about 3 feet (1 m). This
approach is not suitable where the burrows are beneath
large rocks or trees.
Economics of Damage and Control
In one experimental study,
12 California ground squirrels were found to consume
about 1,000 pounds (454 kg) of range forage. In another
study, it was calculated that 200 ground squirrels
consumed the same amount as a 1,000-pound (454-kg)
steer. In spite of control, the California ground
squirrel has caused an estimated 30 to 50 million
dollars of agricultural and other damage annually in
A northern California
study of the Belding’s ground squirrel showed that 123
squirrels per acre (304/ha) destroyed 1,790 pounds of
alfalfa per acre (2,006 kg/ha) over one growing season.
Little seems to be
recorded concerning the extent or amount of economic
damage caused by the rock squirrel. Economic loss is
believed to be relatively low, but the rock squirrel’s
role in the transmission of plague makes it important
from a public health viewpoint.
The cost of control varies
with the situation, squirrel density, and methods
employed. Baiting with an acute toxicant like zinc
phosphide is the most economical method, with 1 pound
(454 g) of bait ample for placement adjacent to 60
burrow entrances. The use of anticoagulant baits is
considerably more expensive, requiring anywhere from 1/2
to 1 1/4 pounds (227 to 568 g) of bait per squirrel. The
expense of bait stations would be an added cost.
The use of burrow
fumigants is about 8 to 10 times more expensive for
materials and labor than the use of zinc phosphide
baits. Trapping is half again more expensive than burrow
Figure 5, 6, and 7 adapted
from R. E. Marsh by David Thornhill.
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
mammals 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 mammal 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.
Tomich, P. Q. 1982. Ground
squirrels. Pages 192208 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.
Scott E. Hygnstrom, Robert
M. Timm, Gary E. Larson
PREVENTION AND CONTROL OF
WILDLIFE DAMAGE — 1994
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