By Tom BurfieldRodents may be relatively tiny creatures, but they can cause big damage to just about anything you grow anywhere you grow it.
How to control them depends on the type of rodent you’re dealing with and the type of crop you’re growing.
“There are a variety of rodents that can cause serious problems,” says Vincent Lazaneo, urban horticulture adviser for the University of California Cooperative Extension.
“A single gopher feeding at the base of a mature citrus tree can girdle larger roots, chewing the bark off and basically kill a mature tree,” he says.
Citrus and almond trees are among the favorite snacks of gophers in California, he says.
Ground squirrels are another major pest. They don’t feed on roots directly, but they’ll steal fruit from trees, especially almonds and other nut crops. And their burrowing activity can divert irrigation water.
Roof rats like to munch on evergreens like avocado and citrus trees. They’ll feed on the fruit itself and then gnaw on the bark.
If there’s a lot of dense vegetation around your trees, you can expect meadow mice to take up residence there and chew on the tree bark.
In California pocket gophers and ground squirrels probably are the two most damaging rodents, Lazaneo says.
Gophers are a pain year-round, but squirrels typically cause more damage as crops mature.
Baits, traps do the job
Poison baits and traps are the most effective means of rodent control, Lazaneo says.
He emphasizes that quick action is imperative. All rodents have one characteristic in common: “They multiply very rapidly.”
Once you start a baiting or trapping program, you must be consistent and monitor it regularly. And many poison baits have restrictions, so read the label.
For large fields, you can use a mechanical bait applicator to make an artificial burrow. Then place toxic bait at regular intervals and allow pocket gophers to dig around and intersect these artificial burrows and find the bait.
For smaller areas, place bait in burrows or use metal kill traps. They’re labor intensive but very effective, Lazaneo says. Baits are more effective for ground squirrels, mice and rats, while traps work best for gophers.
Providing habitat for raptors, such as owls, can reduce a rodent population, but that’s not likely to completely control them, he says.
“Dropping the population by 25 percent to 30 percent isn’t going to help you if the rest of the population is decimating your crop,” he says.
Squirrels chewing on irrigation hoses cause the most trouble for Jeff Fontana, field manager for Cal-Citrus Packing Co. of Lindsay, Calif.
Even orchards that use furrow irrigation are affected by seemingly bottomless holes that squirrels dig in the middle of the furrows that can divert water and prevent trees past the hole from getting sufficient nourishment.
One of the tools Fontana uses to fight the rodents is a Rodex rodent control system that he bought for $1,500.
It has two cylinders of oxygen, a cylinder of propane, a 4-foot wand and a regulator to control the gas mix. He puts the wand in a hole and fills it with gas, which is touched of by an igniter. It blows up the hole, the tunnel and whatever’s inside. There’s a similar device called the Rodenator.
Fontana complements the Rodex treatment with a bait system he makes by forming a T with 3-inch PVC pipe, which he fills with Ramik Green poison bait pellets.
Help also is available from the Tulare County Agricultural Commissioner’s office in the form of what Fontana calls “blue oats”—the rodenticide diphacinone mixed with oats and dyed blue. In late summer it was selling for $30 for a 50-pound bag.
Chucking the woodchucks
On the East Coast, woodchucks are a primary threat to vegetable crops, while squirrels occasionally help themselves to corn, and sometimes meadow or pine voles will get into vegetable crops or damage nursery ornamentals, says Paul Curtis, Extension wildlife specialist and associate professor at Cornell University in Ithaca, N.Y.
Most growers prefer traps to control woodchucks. If growers are in a rural area, shooting is a favored method to eliminate the large rodents.
For a relatively small field, electric fencing is effective. Several acres can be covered for a few hundred dollars, but electric fences have to be taken down when you spray.
Ground cover management is important to control voles in orchards and nursery settings by eliminating places for the rodents to hide, he says.
Many fruit growers use herbicide treatments under their trees to keep the vole population down, and rodenticides often can be used to effectively control voles in orchards and vineyards, Curtis says.
For small organic farms, voles can easily be caught using mousetraps baited with apples. Trapping used in conjunction with ground cover control can be an effective means of controlling voles without using poisons, he says.
Voles build up their population during the summer, when there’s a lot of green grass to eat, says Jonathan Kays, Extension specialist, natural resources, at Western Maryland Research and Education Center in Keedysville. When frost comes, they start eating roots and girdling trees.
The best time to eliminate them is late summer and early fall when they’re hungry and readily take bait and before they start causing damage to trees.
There are two kinds of voles—the meadow vole that lives on the surface and the subterranean pine vole.
The pine vole is more shy and requires better bait placement to control, so Kays recommends setting out traps first to determines which kind you’re dealing with.
Trapping or shooting are the best methods to control squirrels, Curtis says.
Fencing is not effective and no rodenticides are registered in New York to control them, he says.
Woodchuck bombs that can be put in the borrow system are a short-term fix to control woodchucks.
“Those can be very effective for killing the animals that are in the burrows, but the burrows usually are quickly reactivated by other woodchucks,” he says.
Victory over voles
Voles are the biggest problem Bob Fix of Fix Bros Inc. faces in his apple, pear, peach and cherry orchards in Livingston, N.Y.
He clears undergrowth from around the base of the trees and puts out a pelletized zinc phosphide material, such as Pro Zap, as bait either by hand or by aerial application after the fall harvest.
Application costs about $50 to $60 per acre, he says.
During the winter, under cover of snow, is when the voles can do the most harm by chewing on tree bark.
“You won’t know that there’s damage until the snow melts,” he says.
Other effective treatments to control rodents include Contrac rat and mouse bait blocks.
“It has been head and shoulders better than anything we’ve seen,” says Dennis Galt, a salesman at Grangetto’s Farm & Garden Supply Co. in Escondido, Calif.
For squirrels, he recommends an anticoagulant like PCQ, which he terms “a top-notch bait.”
And there’s a new squirrel trap called the Squirrelinator that he says is “unbelievable.”
“It will catch eight, nine or 10 squirrels at a time,” he says. “They enter from sides, and they can’t get out.”
Home sweet home for barn owls
Although some experts question the value of using raptors—or birds of prey—to control rodents, Steve Simmons of Merced, Calif., says he has first-hand knowledge of their worth, especially when it comes to controlling pocket gophers.
Simmons, a retired teacher at Merced High School and selfmade bird expert, says he and his students built more than 10,000 owl boxes in a nine-year period in a wood shop class that sort of doubled as a business.
Students sold the boxes to local farmers, with the money going toward college scholarships.
Simmons continues to monitor activities of a variety of birds, especially barn owls, and has collected a wealth of information about owls and their prey.
Over a one-year period that began with 48 boxes with two adults per box, 87,600 rodents were killed, 36,792 of which were gophers.
Making an owl nesting box is not difficult, and, for about $20 worth of plywood and other materials, it’s cheaper than buying most ready-made ones, Simmons says. Some sell on the Web for up to $175.
He suggests mounting the boxes on a pole about 10 feet high, allowing one for every 20 to 25 acres.
If all the boxes become occupied and rodents persist, put up more.
Clean out the boxes once a year while wearing a dust mask. And he suggests putting wood shavings on the bottom to keep eggs from rolling around and to prevent fluids or excrement from sticking to the box.
His boxes are 11 inches wide, 23.5 inches long and 15 inches deep.
Although Simmons has retired from the owl box-making business, he sells plans and a brochure listing do’s and don’ts, and at least one of his students, Larry Fister, now a firefighter, continues to make them.
Owl-box expert Steve Simmons will send two informational brochures and plans for his owl box to anyone who sends $7 to Barn Owl Box Plans, 2499 5th Ave., Merced, CA 95340.
Larry Fister, who actually makes the boxes, can be reached at firstname.lastname@example.org.