Rats and mice can raise havoc with buildings and their contents, and burrowing rodents such as prairie dogs, ground squirrels and pocket gophers can damage fields, pastures and crops.Robert M. Timm, an extension wildlife specialist emeritus at the University of California Hopland Research & Extension Center, says some geographic regions have different problems; you might not have ground squirrels, but you might have pocket gophers.
Norway rats and house mice are problems nationwide, while roof rats tend to live in warmer climates such as the Pacific and Gulf Coast states. Sometimes the best advice on dealing with your specific type of rodent wildlife can be gleaned from local sources.
Some species of ground squirrels can cause serious losses in forage and other crops, while pocket gophers can be a major problem in perennial plantings such as alfalfa, orchards and vineyards. Even on pasture and range, these rodents can reduce forage productivity, competing with livestock.
In a hayfield or crops, rodent mounds can damage harvesting machinery. “This can happen with pocket gophers in alfalfa or grain fields. Machinery may run into those mounds and be damaged (or dirt ends up in the hay), and the burrows sometimes interfere with flood irrigation.
“Water may go down the burrows of ground squirrels or pocket gophers rather than where it’s intended,” says Timm. Rodent burrows can also create erosion problems, especially on slopes. Rodents can damage ditches, irrigation canals and the integrity of earthen structures, including roadways.
Damage from mice may go unnoticed until they have destroyed insulation in structure walls or ceilings. “Rats or mice may gnaw insulation off wires, causing electrical problems or fires. They may also damage chewable parts in vehicles, tractors and other equipment. They may disable machinery by chewing wires and creating shorts, or gnaw off drive belts, cooling system hoses, etc.
“Often by the time you notice a problem, there has already been a lot of damage done,” he says. For rats and mice in buildings, poison baits can work if you are able to eliminate their other food sources.
“Control of prairie dogs and ground squirrels can become complicated in habitats where there are endangered or protected species living in and around their burrows,” says Timm. For instance, black-footed ferrets live in prairie dog towns, and various species of endangered kangaroo rats share habitats with California ground squirrels. Burrowing owls may also use rodents’ burrows.
“Pesticides (baits and fumigants) registered for use against ground squirrels and prairie dogs have specific instructions on where such materials are prohibited and how to survey for endangered species in areas where rodent control is permitted,” says Timm. Appropriate federal and state agencies should be contacted for current recommendations.
Control of ground squirrels and prairie dogs is typically done by means of poison bait or by burrow fumigation. Trapping or shooting, which are more labor-intensive, are sometimes employed on a smaller scale or as follow-up to rodenticide application, says Matt Brechwald, who operated a gopher control service based in Kuna, Idaho, for many years before selling it in 2017.
“The person you hire to eliminate rodents may use several different methods. The one I used most is carbon monoxide injection into their tunnels to kill them. To do this, I purchased a piece of equipment called a PERC (pressurized exhaust rodent control) to get rid of gophers, voles and ground squirrels. Other products you can purchase or a hired operator might use include a Rodenator and Verminator. These put a combination of propane and oxygen into the tunnel. Then this is ignited, which creates an explosion that is supposed to kill rodents with the concussion and cave in the tunnel – to prevent re-infestation,” he says.
“Another method farmers sometimes use is an implement pulled behind a tractor, making a tunnel and dropping bait into that tunnel. As gophers are burrowing, they come across that tunnel, explore it and eat the bait. This is a way to put poison bait into a field and cover a wide area in less time,” says Brechwald.
Poisons
“The best poison available for gophers or voles is aluminum phosphide,” says Brechwald. “It comes in pellets you drop into the tunnel and get a little bit wet. This triggers a chemical reaction that produces fumes. This gas is extremely lethal and works well for killing rodents, but you need an applicator’s license to legally use this poison.” You must learn the proper way to handle this chemical and take a test to become certified to use restricted products.
“The aluminum phosphide works really well, but there have been human fatalities when gas came up through a crack in a house foundation. Now it can only be used for agricultural purposes by someone with an applicator’s license – with the education to use it. Even for agricultural purposes, it can’t be used within a certain distance from a residential structure,” he explains.
“Poisons you can buy over the counter generally contain cyanide in a bait for rodents to eat. In recent years, the EPA cut down the level of active ingredients that can be used in over-the-counter baits by about 50 percent, so they are not as lethal as they used to be and not as effective,” says Brechwald.
“If gophers or ground squirrels sample the bait and don’t consume enough to kill them, it just makes them sick for a while – and then they won’t touch it again,” he says.
Timm says some species of ground squirrels and prairie dogs will not consume grain bait when green forage is available, so timing of bait application is critical for success.
Traps
Trapping works well if a person is diligent. “There are a number of good pocket gopher traps on the market and, once you learn to use them, they can be very effective,” says Timm. “Ranchers trying to eliminate gophers carry traps around in their pickup. Whenever they see a fresh mound, they try to trap that gopher before it reproduces or causes major damage.”
“To trap gophers, you dig out the tunnel and insert the trap down into it, or use a box trap at the end of the tunnel that catches the animal as it comes out,” says Brechwald.
Smoke bombs
These can often be purchased at a farm supply store. “Their disadvantage is: You have to open up the tunnel system, light the smoke bomb, get it down into the tunnel then seal the tunnel up again. There are lower levels of success with these than when using a pressure system,” says Brechwald.
If the gopher is deep in the tunnel when the smoke bomb is lit, there may not be enough smoke to kill it. The gopher may smell the smoke coming and plug the tunnel with dirt, isolating itself until the smoke dissipates. “There’s also some risk for fire in a weed patch when everything is dry,” he says.
Injecting carbon monoxide
“The machine I used has a 14-horsepower motor and an air compressor powered by that motor,” Brechwald says. “All the exhaust from the motor goes through the cooling coil that goes into the air compressor. There, the carbon monoxide gets cooled, concentrated and goes into a pressure tank. I inserted a probe into the gopher tunnel through the ground surface to inject the carbon monoxide.”
Brechwald feels this is the most humane way to kill rodents. With the cooled gas going into the tunnels, they are rendered unconscious from oxygen deprivation.
Timm says recent research indicates this technique can provide moderate to good success in killing pocket gophers, depending on the location, time of year, soil type and who is doing the operation. “To achieve complete control may require a second treatment or follow-up using traps or rodenticide baits,” says Timm. 
PHOTO 1: Matt Brechwald. Photo provided by Matt Brechwald.
PHOTO 2: Adult house mouse
PHOTO 3: Norway rat
PHOTO 4: Pocket gopher mound
PHOTO 5: Pocket gopher (thomomys spp.)
PHOTO 6: California meadow mouse (vole), Microtus californicus
PHOTO 7: Meadow mouse burrow openings. Photos 2 - 7 by Jack Kelly Clark, courtesy of University of California Statewide IPM Program.
PHOTO 8: This photo shows how Matt Brechwald pumped the lethal gas into burrows under pressure. Photo provided by Matt Brechwald.
Heather Thomas is a freelance writer based in Idaho.
