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GL0423-UC-squirrels1_BT_Edits.jpg Roger Baldwin/UC Davis
A ground squirrel is spotted. The creatures can cause lots of problems in vineyards.

In vineyards, squirrely is not funny

A single female ground squirrel can cause problems in a vineyard for a year or more.

You do not want these critters in your vineyards because their favorite menu items include large servings of young grape vines, to the point where a sizeable contingent of them can consume the same amount of forage as a very large steer.

Pest management experts will pretty much acknowledge that all you need is one female California ground squirrel and you have the beginning of a problem for a year and more as the next generation breeds.

Litters of ground squirrels emerge from breeding burrows around early May, normally in groups of six to eight, and they’re born hungry.

In the case of vineyards, squirrels will travel from the protection of nearby brush and debris piles to chow down on — and sometimes kill — young vines by stripping the bark or perhaps mortally damage older vines by burrowing through their root systems.

While they consume shoots and fruit, they more often condition their teeth by gnawing on drip tape irrigation lines and emitters. When they’ve finished feeding and want to bed down for a nap, their burrowing activity benefits no one.

Pest Management Specialist Roger Baldwin was slated to speak on that subject — How to Control California Ground Squirrels in Commercial Agriculture — at a University of California Cooperative Extension workshop in San Luis Obispo and then along came the coronavirus and conference cancellations.

“Estimates vary, but in general, the species inflicts upwards of $50 million in damage throughout California agriculture,” says the wildlife specialist at the University of California, Davis. “I did a survey several years back that showed a 4.6% loss of revenue in grape fields where squirrels are found.

“From a management perspective, it’s all problematic, and we encourage growers to utilize an integrated approach involving multiple mitigation strategies. Don’t just rely on one tool because not all individual animals in a population are equally susceptible to one effort. You’ll take out some, but leave others, so it’s good to mix and match strategies.”

Deterring ground squirrels

Initially, to deter ground squirrels from hanging around vineyards, the first recommendation is prevention through habitat modification.

“Sometimes just removing pruning piles on the periphery of vineyards takes away a place for them to live, although truth be told, ground squirrels can hang out in a burrow in the vineyards themselves,” Baldwin says. “But doing everything you can too try and detour their presence is good strategy.”

When it comes to removing an established presence, trapping is one method although it is labor intensive and usually not practical for large populations. “If you have localized areas with a relatively small squirrel population, trapping is one way to go. But there are two other strategies generally more effective, rodenticides and burrow fumigants.”

Fumigants are generally used about this time of year as squirrels emerge from hibernation before soil moisture levels drop too much. “When the soil dries out, it gets cracks and pores and the gas tends to dissipate too rapidly for it to work properly,” he says.

“Fumigants can be extremely effective, like the gas cartridge type, little smoke bombs designed specifically for burrowing rodents. Their efficacy rate in getting rid of ground squirrels is about 75%. Another nice attribute is they are not a restricted-use product. On the negative side, they’re a bit more expensive and labor intensive.”

What happens if you miss the early eradication window of opportunity? “There are several different kinds of rodenticides with various application strategies. Common baits are two anticoagulants, grain- or pellet-based diphacinone and chlorophacinone, generally used in bait stations, but they can be broadcast or used in spot treatments also.

“Zinc phosphide is an acute toxicant that kills after a single feeding.  You get a more rapid knock down of a population, although there is a distinctive odor and taste that some squirrels will avoid.”

First generation anticoagulants have the potential for secondary exposure and product labels advocate daily carcass searches to remove kill before scavengers or predators show up. 

“Phosphides are more acutely toxic and can be lethal to anything else that feeds directly on the baits — read the product label,” Baldwin advises.

Bottom line: “Mix and match the most common mitigation methodologies — burrow fumigation, trapping, and rodenticides. If you get rid of 50% of ground squirrels in a given year, that’s not going to give you long term relief because of their reproductive capabilities.  Ninety percent removal is the desired goal.”

For more news on pests, disease management and other issues affecting vineyards, subscribe to the bi-monthly newsletter The Grape Line.

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