When it comes to control of ground squirrels, California agriculture’s leading destructive pest, 90 percent is the minimum, says Fred Rinder of the Fresno County Department of Agriculture.
Rinder, a wildlife management supervisor, outlined best practices to deal with ground squirrels in a talk during a San Joaquin Valley grape symposium at Easton.
In the case of vineyards, squirrels can kill young vines by stripping the bark and damage older vines by burrowing through root systems.
They consume shoots and fruit and their gnawing to condition their teeth causes significant losses in drip irrigation lines and emitters. Their burrowing interferes with ditches and levee banks and can damage foundations of buildings.
After hibernation during the winter, they breed during February and March, producing litters of six to eight that normally emerge in April.
In describing controls, Rinder said “your goal for control should be a minimum of 90 percent. All you need is for one female to survive and you have the beginning of a problem for the year.”
Once breeding is complete, he said, the females prefer a diet of plants such as fillaree because it is high in protein to support lactation.
“That becomes important because if you try to use treated grain baits, you have to remember they do not readily go to eat grain. So you need to pre-bait, or chum, to get them away from what they naturally want to eat.”
Using the example of practices in the Mendota area of western Fresno County, Rinder said in April young squirrels may need a week or two to get off their diet of vegetation shoots and switch to foraging for grains. Examination of stomach contents of shot squirrels can reveal their feeding activity.
Most effective control, whether with bait stations, traps, or fumigants, varies with the season, animal life-stage, population density, and location, although their can be some overlap of the three methods during April and May.
“Before you do anything, try to get rid of brush or debris piles that are shelter for squirrels,” Rinder said. The rodents can readily travel a quarter-mile from such protection to feed in vineyards and orchards.
Another preparatory step before timing of treatment is to sample the squirrel population. Biologists check shot squirrels for a ratio of either 60 percent male and 40 percent female, or vice versa, to determine the breeding activity. During that period, the maximum number of squirrels is above ground. Afterward, females retreat to burrows.
“We use a pre-bait of just rolled oats without any dye or anything added to it. If they eat this, we know they will eat the treated grain,” he said.
Common baits for squirrels are two anticoagulants, diphacinone and chlorophacinone, and zinc phosphide (a restricted material requiring a permit for purchase and use).
Use of bait stations made of PVC is important to exclude non-target species and plans for construction of them are available at agricultural commissioners’ and farm advisors’ offices.
The anticoagulants require multiple feedings over a period of several days. Although relatively large amounts need to be consumed for these compounds to be effective, there is potential for dogs that might consume multiple squirrel carcasses to be harmed, so carcasses should be collected and buried.
Stations should be placed every 100 feet near runways and burrows or at the perimeter of the vineyard. They should be anchored to a fence post or stake to keep the bait from spilling out. They should be monitored for two to four weeks and repeat applications can be made if necessary.
Spot treatments by hand (using anticoagulants only) can be made near burrows. Bait should be scattered and not clumped, since scavenging squirrels will naturally forage and find the bait.
Rinder cautioned that zinc phosphide bait is an acute poison that results in death of the pest in six to eight hours. “Five or six treated kernels can kill a squirrel.”
After the bait applications are made and the population is reduced, gas cartridges can be used on surviving pests. Fumigants can be used any time of year, but treatment is most successful in late winter to early spring when hibernation is ending and moist soil retains the gas.
“Put the cartridge into the burrow opening, light the fuse, and push it into the burrow fuse-first. Allow it to ignite and shovel soil into the burrow opening to seal it. Plug any openings where you see smoke, and then move on to the next opening showing no smoke,” Rinder said.
Although traps work well against squirrels, they are time-consuming for preparation and monitoring and are most suited to control of low populations. The best designs for squirrels are the Conibear #110 traps or modified gopher traps.
Conibear traps should be placed directly in the opening of the burrow. Traps should be secured to a stake so they can be monitored and retrieved and to prevent them being carried off by predators.
Most useful during early morning and late afternoon, Conibear traps have to be sprung during the evening to avoid non-target animals. Traps need to be monitored and reset as necessary.
Use of modified gopher traps involves baiting for several days at the unset traps to encourage squirrels to forage near them. Afterward, traps can be set and monitored daily.
To avoid catching non-target animals, Rinder recommends setting a larger box over traps with a 3-inch opening 6 inches above the soil line, allowing squirrels to gain access.
Regardless of which method of control is used, it is best for the grower or rancher to become thoroughly familiar with regulations available at respective county agricultural commissioners’ offices before carrying out a control program.
The California ground squirrel, Spermophilus beecheyi, also feeds on almonds, pistachios, walnuts, applies, apricots, peaches, prunes, oranges, tomatoes and alfalfa. It consumes certain vegetable crops at the seedling stage.
A study done several years ago estimated that, in spite of controls, the species inflicts $30 million to $50 million in agricultural and other damage each year in California.
Another study calculated that 200 ground squirrels consumed the same amount of forage as a 1,000-pound steer.