The Pocket Gopher

The Pocket Gopher
Article by Matt Oliver
Copyright © 2003 by ProGardenBiz
ProGardenBiz, an online magazine
http://www.progardenbiz.com

Pocket gophers (Tomomys spp.), so named for their fur lined
cheek pouches located outside the mouth on each side of the
face, are burrowing rodents. They are a serious and difficult
to control pest for both the Agricultural and Landscape
Industries, as well as the homeowner. They destroy vegetation,
damage machinery (such as mowers), damage irrigation systems
and underground wiring, and lower the aesthetic value of the
landscape. In addition, their burrowing activity on slopes
causes erosion and can be a major factor in slope weakening
and instability that may ultimately lead to a slope failure.
Pocket gophers were identified as a major contributing factor
to slope failure in a number of recent litigation cases in
California.

Biology

A thorough understanding of gopher biology and habits is
helpful, if not necessary, to a successful control program.
They are medium sized rodents with the head and body ranging
in size from 6-8 inches (15-20 cm) long. They have a powerfully
built upper body, short neck and legs, long clawed forefeet,
and two pairs of large incisors protruding beyond the mouth.
These fossorial features are tremendous adaptations for their
underground existence. They have a keen sense of touch, thanks
to their tail (short and sparsely haired) and vibrissae
(whiskers), which serve as sensory organs helping to guide the
gopher throughout its burrow system. Fur color is highly
variable, ranging from dark brown to very light tan.

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Pocket gophers do not hibernate and are thought to be active
year round even with snow on the ground, but do noticeably
decrease surface feeding and mounding in very hot weather.
Females produce 1-3 litters per year with an average size
surviving brood of 5-6. In unirrigated natural areas breeding
season is after the rains begin -- which may mean only one
litter per year. In irrigated, landscaped areas the continual
source of green foliage allows the female to raise 3 litters
per year.

They are territorial, anti-social, and live solitary except
during breeding periods and when the young are being raised.
Gophers live almost exclusively underground, venturing above
only to push excavated dirt from the burrow system, graze on
vegetation near burrow openings, or for the purpose of
migrating into new territory. Migration occurs both by adults
-- usually as a result of unfavorable environmental conditions
and/or habitat destruction (e.g. construction projects), and
young which the mother expels from her burrow system when they
are about half grown.

Burrow systems consist of a main tunnel, lateral runs, pop
holes, and various other functional tunnels and enlargements
which are used for nesting, storage (food caches), resting,
eating, etc. The main burrow is usually 2-4 inches in diameter
(averaging 2 1/2") and is 2-18 inches below and parallel to
the ground surface. Burrows of young may be small, covering
only one or two hundred square feet while those of older pocket
gophers may cover an area as large as three thousand square
feet. Lateral runs branch off the main run and are used
primarily to push excavated soil to the surface. The mounds
from these laterals are crescent shaped because the soil is
pushed with the forefeet out of the angled lateral to the front
and sides of the opening. Pop holes usually lead straight from
the surface to the main run and are used as an access for
feeding on nearby vegetation. All runs leading to the surface
end in a soil mound or plug which keeps the system completely
enclosed -- allowing no light to enter, and stabilizing burrow
temperature and humidity as much as possible. If a gopher dies,
irrigation or rain washes the loose soil plugs from lateral runs
and pop holes leaving open tunnels. Drainage tunnels are used
for water run-off, thus making it difficult to drown a gopher in
a well established system.

The rate of mound building varies with the season, tempurature,
and soil condition, but averages 1-3 mounds per day, during
active periods. The depth of active burrows is usually deeper
under hot conditions, especially in non-irrigated or
infrequently irrigated areas. Mounding often sharply decreases
in the heat of Summer followed by intensive mounding in the
Fall. This renewed Fall activity has often been blamed on
reinvasion of areas thought to have been controlled in the
early Summer.

Control

Although many different techniques have been used in gopher
control the most successful programs usually utilize one or
more of the following methods: trapping, fumigation, and
poison baits.

Trapping is an effective method in small areas such as a
homeowner situation, or as a follow up to fumigation or
baiting, but is time consuming and thus not very cost effective
in a large scale program. The most commonly used traps are the
Macabee trap and the box trap. Traps should always be placed in
the main burrow.

To locate the main burrow, look for the freshest mounds since
they indicate an area of recent gopher activity. You will
usually see a small circle or depression representing the
plugged lateral tunnel. This plug is generally surrounded on
one side by soil, making the mound form a crescent shape. The
main tunnel is usually found 3-1/2 inches from the plug side of
the mound, and is most often between two mounds. Locating the
main burrow usually requires practice, but your skill will
improve with experience.

After locating the main tunnel, open it with a shovel or garden
trowel and set the traps in pairs facing the opposite
directions. This is necessary in order to intercept the gopher
coming from either end of the burrow. The box type is easier
for most inexperienced trappers to set, but requires more
excavation. Box traps are useful when the diameter of the
gopher's main burrow is small (less than 2 1/2 inches) since
small burrows will need to be enlarged to accomodate the box
traps. All traps should be wired to stakes to prevent loss.
After setting the traps, exclude light from the burrow by
covering the opening with dirt clods, sod, cardboard, or some
other material. Fine soil can be sifted through the edges to
ensure a tight seal. If light enters, the gopher may plug the
burrow with soil, filling the traps in and making them
ineffective. Check traps often and reset when necessary. If no
gopher is caught within 3 days, reset the traps in a different
location.

Poison baits offer the quickest and most effective method of
controlling a large gopher infestation. The most commonly used
toxicants are chloraphacinone, strychnine, and zinc phosphide
pelleted bait. Chloraphacinone, the lesser used of the
toxicants, is a multiple dose anti-coagulant that prevents the
normal process of blood coagulation ultimately causing death
from internal bleeding. It has limited field use because of the
necessity of making multiple applications in the same burrow
system, but may be useful where an extra margin of safety is
desired. The acute toxicants, strychnine and zinc phosphide,
are the most used and most effective. Most baits are prepared on
hulled wheat, barley, or milo grains, with wheat seeming to be
the most preferred by the common Battae (T. bottae) gopher. Zinc
phosphide baits are only accepted adequately in blended pelleted
bait. Strychnine alkaloid bait comes in various formulations
ranging from .25% to 3.0%. In instances where a tractor pulled
mechanical bait applicator is used, formulations from 1.8% to as
high as 3.0% can be utilized. The burrower building mechanical
bait applicator is seldom used in urban situations. Zinc
phosphide can be obtained in 1.0% to 2.0% formulations.

One registered burrow fumigant, aluminum phosphide, is very
effective when used under ideal conditions. Soil should be moist
to accomodate gas formation and to provide a good soil seal.
Even though the gopher often detects burrow fumigation efforts
and trys to plug the system, the use of aluminum phosphide can
still be very effective if at least 2 points within the burrow
system are treated at the same time. The material is used in
pellet form with the pellets being placed into the runway using
a 5/8 to 3/4 inch probe to open the system and a gloved hand to
drop them in. A dirt clod, rock, or plant material is then
placed over the probe hole. This product can be very hazardous
and must be used according to label directions, as with all
pesticides, and requires a restricted materials permit.

Note: Use of strychnine and zinc phophide baits and the fumigant
aluminum phosphide require restricted material permits and user certification.

Many factors influence the success of a baiting program; proper
bait placement within the gopher system, environmental factors
such as soil type, soil moisture, and availablity of green
forage. All can enhance or hinder bait acceptance, and control
results. For instance, dry sandy soils often will collapse when
probed, preventing any bait application, while overly wet soils
may cause the bait to become soggy, muddy, and quickly mold,
thus making it unacceptable to the gopher.

The types of available plants affect how quickly gophers accept
bait. For example, gophers are controlled more easily in turf
than in O'Connor's Legume as the latter is the preferred host.

Finally, gophers may become "bait shy" if they ingest sublethal
amounts of a bait and become sick. Because the animal
associates the sickness with the taste of the bait, it will no
longer feed on it. Once this occurs, another type of bait or
alternative control method should be used.

Any gopher population can be controlled and in many situations
even eliminated. Succesful programs in large scale situations
generally require an initial clean-out of intensified treatment
to bring the existing population to a maintainable level (90%
or better). Once control is achieved a continuous maintenance
program will most often be required to prevent reinfestation
problems from developing as a result of migration from heavily
infested surrounding areas.
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About the Author:

Matt Oliver is General Manager at Agricultural Pest Control
Services, Inc., a company that specializes in controlling
vertebrate pest problems. Matt is a Contributing Editor for
ProGardenBiz Magazine, an online magazine for professional
gardeners and landscape contractors. Visit ProGardenBiz to
find out how you can get a free subscription, start-up
guidance, business ideas and inspiration at
http://www.progardenbiz.com.
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