print Font Size: small font medium font large font

Burrowing Animals

Despite an aggressive program by the Santa Clara Valley Water District to protect levees from erosion and other hazards, some levees in the county are threatened by ground squirrels and other rodents. Not to be confused with tree squirrels, ground squirrels dig burrows for shelter and safety. As burrows grow deeper and expand laterally, they weaken levees, ditch banks and earthen dams, as well as undermine roadways and buildings.

Frequently asked questions - below

Photo of Levee Repair
Temporary, emergency fix from burrowing animals on Sunnyvale West Channel during high tides and winter storms in 2005. Plastic sheeting was used to stop water from flowing through the levee.

1. How damaging can a ground squirrel or other rodent be?
Ground squirrels live in colonies that may include several dozen individuals in a complex of burrows. They can be very damaging to native vegetation along streams. Ground squirrels eat grain, nuts and fruits, devour plants in their seedling stage and gnaw on tree bark, twigs and leaves. Ground squirrels’ burrows — which can grow to 30 feet or more in length and may extend two to four feet below ground— undermine levees, roadways and structures, endangering public safety. They can also harbor harmful diseases, including bubonic plague, which is transmitted to humans by fleas carried by squirrels and other rodents.

2. What are the options for controlling burrowing animals? 
Control options are heavily influenced by the unique life cycle and behavior of ground squirrels. Baiting — using treated grain to poison squirrels — is most effective in summer and fall when squirrels primarily feed on seeds. Fumigation — using a gas pumped into burrows to kill squirrels — is most effective in spring when soil is moist and the squirrels have not yet begun to reproduce. Trapping is expensive and is only practical when dealing with very small squirrel populations. Predators — hawks, eagles, rattlesnakes and coyotes — eat ground squirrels, but are not able to keep squirrel populations low enough to prevent damage.

 Ground squirrel burrow in levee
Active ground squirrel burrow in levee.

3. What methods are used by the water district? 
The water district contracts with a qualified pest-management firm to use an integrated pest management approach to control burrowing rodents.  The control methods of baiting, fumigation, and trapping are used at the times of the year when they are most effective.  The control applications follow best management practices to safeguard sensitive wildlife and humans. 

4. Is bait used for squirrels affecting other wildlife? 
The anticoagulant baits could be poisonous to other wildlife, therefore special precautions are taken by the water district. Only areas with active ground squirrel activity are treated with bait. The anticoagulant rodenticides are only used in bait stations which prevent birds and larger animals from having access to the bait. The ground squirrels must enter the bait station to eat the bait. Bait stations are secured in place and have an inner lip to prevent the spilling of bait. Several feedings over a period of five or more days are required for a lethal dose for a ground squirrel. Carcass surveys are performed by the pest control firm to collect any squirrel carcasses to prevent secondary poisoning where a scavenger could eat a poisoned ground squirrel. The more expensive live trapping method is used in areas where endangered species could be affected by the baiting.

 Bait station
Secured bait station.

5. How do the bait stations affect the environment?
The water district’s rodent control program was thoroughly vetted with environmental stakeholders, including state and federal regulators, in the district’s Stream Maintenance Program Environmental Impact Report. It is one of the most environmentally sensitive programs of its type in the state. Other rodent-control methods are less selective and have a much higher potential of harming other animals. Bait stations are more effective because they can be modified to be species-specific.

6. Have burrowing animals caused problems in other areas?
A post-Katrina study by the University of California, Berkeley cited rodent burrows as a “pervasive problem” with earthen levees in New Orleans. The California Department of Water Resources 2005 white paper "Flood Warnings: Responding to California's Flood Crisis" lists animal burrows as one key factor in levee degradation statewide.  The U.S. Army Corps of Engineers routinely requires its local partners nationwide to perform burrow control on federally built and funded levees. 

If you further questions or concerns about this project, contact Raymond Fields at (408) 265-2607, ext. 3027, or by e-mail rfields@valleywater.org.