Get Up To Speed On Voles

For commercial fruit growers, the control of voles is necessary, but can be costly and time consuming. Many agronomists and crop protection advisers say that any successful vole control program involves the right combination of advanced rodenticides and best practices.

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Throughout the years, commercial growers have tried many things to control voles including baiting, installing mesh barriers, and repellents. In general, the most practical vole control technique involves the use of rodenticides, either an acute — primarily zinc phosphide (ZP) — or an anticoagulant with an active ingredient such as chlorophacinone.

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There are a number of variables to take into account when considering the use of rodenticides.

Ensuring bait acceptance. It’s difficult to achieve control of a vole population if they are reluctant to eat the rodenticide because they don’t like the taste. Anticoagulant rodenticides work at much lower concentrations of active ingredient than acute rodenticides such as ZP, so there is less bad tasting toxicant in the bait. As such, rodents are more likely to eat a lethal dose.  Equally important is the need to avoid a condition known as “bait shyness,” which is when a rodent eats less than a lethal dose and experiences the rapid onset of pain or discomfort. Anticoagulant rodenticides such as Rozol Pellets (chlorophacinone, Liphatech) work slowly and without pain or discomfort.

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To be palatable to rodents, a rodenticide must be tastier than their current food source. Some rodenticides, experts say, are less attractive to voles, while others appeal to the pests. Rozol Pellets, for example, are formulated with food-grade grains for excellent acceptance by rodents. “If you apply a bait like zinc phosphide, you have to keep in mind that it’s not very palatable,” says Jim Cowin, crop advisor for J.R. Simplot Co., Prosser, WA. Cowin and others also point out that zinc phosphide use is often predicated on pre-baiting with non-treated oats, which adds a second step to the baiting process. “Pre-baiting can add time and cost, which is a definite factor to keep in mind,” Cowin says.

Dealing with poor weather. The best time to treat for voles is during late fall, after harvest but before snow cover. Doing so prevents voles from attacking trees when normal food sources are in short supply. Early spring is also an opportune time for treatment.
Experts widely recommend against the use of zinc phosphide when the potential for wet weather exists. They say the active ingredient in zinc phosphide is not resistant to moisture and will dissipate if exposed to it. “If it’s dry weather, and there’s not a threat of rain or a heavy dew, I might go with zinc phosphide,” say’s G.S. Long’s Director of Technical Services Glenn Dahmen. “If there’s any chance for moisture in the forecast, however, we recommend against it.”

Additionally, many growers don’t treat for voles during the growing season, but prefer to wait until after harvesting late varieties, says Ted Dietrich, a senior fieldman with G.S. Long Co., based in Union Gap, WA.  “That means they’re treating for voles between October and November when frequent rain is likely,” Dietrich says.

A two-pronged approach is often a wise choice, according to Gregg Richards, an agricultural consultant with Chamberlin Distributing Co., in Wenatchee, WA. “If you put out zinc phosphide too late it will melt into the ground with the first rain. The best approach is to use zinc phosphide if the weather stays dry and follow it up two or three weeks later with a weather-resistant bait.”

Getting bait down quickly and efficiently. Another key factor is the small window of opportunity that often exists for baiting during uncertain weather conditions. There are alternatives to traditional application methods, such as hand baiting, and the use of standard fertilizer spreaders or pull-behind units.

One practice is broadcasting bait by air. Aerial application is especially key for growers with large acreage, according to Nate Squires, field service manager for Northwest Wholesale, Wenatchee, WA. “When you’re talking upwards of a thousand acres, it’s much more practical to have the bait flown on to get it completed quickly,” says Squires. “Given the added application costs, it dictates a bait with good field life that won’t break down in poor weather conditions,” adds Ron Benitz, also a crop advisor with J.R. Simplot Co.

Another recommendation of many experts is to consider the use of ATV-mounted electric broadcast spreaders.  “Broadcast spreaders are particularly well suited to high-density plantings where larger tractors won’t fit. They will distribute the pellets more quickly and further, improving control and profitability,” says Squires.

Traditional methods of applying vole bait will also continue, says Ron Benitz, as growers constantly tackle a host of variables specific to each orchard. “It’s often a matter of tailoring the approach to each situation,” he says.

Growers and advisers alike fully realize the need to ensure safety when it comes to the use of rodenticides. Uppermost is the safety of employees and applicators, as well as risks to non-target animals. The use of toxicants has come under increased scrutiny as of late due to concerns related to goose kills in the Pacific Northwest.

Fortunately, some active ingredients have lower risks to non-targets, such as game birds. The primary toxicity of chlorophacinone to birds, for example, is 20 to 2,000 times less than zinc phosphide, according to EPA’s Office of Prevention, Pesticides, and Toxic Substances.

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