Friday, August 27, 2010

Voles: What's A Gardener To Do?


They'll devour your bulbs, gnaw on your vegetables, strip the bark off your fruit trees and eat the plant roots. Who could ever believe such a cute little mouse could do such damage? If your garden has ever been invaded by a horde of voles, you are a believer.

Voles, also known as meadow mice (Microtus californicus and Microtus montanus), should not be confused with house mice. Voles do all their damage outdoors. According to the UC Davis Integrated Pest Management Program:

"Voles are mouse-like rodents somewhat similar in appearance to pocket gophers. They have a compact, heavy body, short legs, a short-furred tail, small eyes, and partially hidden ears. Their long, coarse fur is blackish brown to grayish brown. When fully grown they can measure 5 to 8 inches long, including the tail. Although voles spend considerable time above ground and you occasionally can see them scurrying about, they spend most of their time below ground in their burrow system. The clearest signs of their presence are the well-traveled, above-ground runways that connect burrow openings. A protective layer of grass or other ground cover usually hides the runways. The maze of runways leads to multiple burrow openings that are each about 1-1/2 to 2 inches in diameter. You can locate the runways by pulling back overhanging ground cover. Fresh clippings of green grass and greenish-colored droppings about 3/16 inch long in the runways and near the burrows are further evidence of voles. With age, the droppings lose the green coloring and turn brown or gray."

Besides feasting on just about every food plant in your garden, voles are busy reproducing. Several thousand voles per acre is not unusual in a heavy outbreak year. These outbreaks, fortunately, don't happen every year, usually reaching maximum numbers every three to six years. And can they pump out the babies! Female voles mature in 40 days and can have up to 10 litters per year, with each litter numbering between three and six baby voles. They live for about 12 months.

They are herbivores, and not nearly as picky eaters as your kids. Voles cause damage by feeding on a wide range of garden plants including artichokes, beets, Brussels sprouts, cabbage, carrots, cauliflower, celery, lettuce, spinach, sweet potato, tomatoes, and turnips. They also can damage turf and other landscape plantings such as lilies and dichondra. 

Voles will gnaw the bark of fruit trees including almond, apple, avocado, cherry, citrus, and olive. Vole damage to tree trunks normally occurs from a few inches aboveground to a few inches below ground. Although not known as climbers, if they can reach the lower branches of a fruit tree...up they go.

That UC site on voles isn't too helpful if you are currently dodging an army of  these hungry critters: "To prevent vole damage, you need to manage the population in your area before it reaches high numbers." Oh, thanks.

So, what is a gardener to do? Think like a vole. 

One of the reasons voles dart around so fast: to avoid predators such as coyotes, foxes, cats, hawks and owls. And it's why they like to move under the cover of mulches and weeds. Modify their habitat, and you can reduce vole populations.
Mow, till, weed whack or destroy any grassy areas or mulched areas that are adjacent to a garden. The wider the strip, the less likely the voles will dart across. 15 feet should be the minimum cleared area; use a greater width if the vole numbers are high. Around trees, clear an area at least four feet in diameter. Voles won't munch on trees in an open area.

Quarter-inch mesh hardware cloth or fencing can stop voles, in combination with a cleared area. The fence should be 12 inches above the ground, and 10 inches below ground level. Attractive, no?

Put a cylinder, such as flexible drain pipe, around young trees. Make sure the cylinder is wide enough to allow the tree to grow.

Trapping. This will work best when the vole population is small. You will still need at least 12 wooden mouse traps, baited with peanut butter, oatmeal and apple slices, for a small garden. 50 is better. Set the traps along their runways. The problem with this: how are you going to keep the kids and pets away from the traps?

Baits. Anticoagulants are effective against voles, but again...make sure the kids and pets can't get at it. The California Department of Food and Game warns you to choose your vole poison carefully. Indoor mouse bait should not be used on outdoor voles. Their advice: "Rodent bait users must follow label directions carefully. Some rodent baits, for example those that contain the active ingredients chlorphacinone and diphacinone, are legal to use in outdoor areas. These products can be used to control field rodents such as gophers, voles and ground squirrels. Other rodent bait products, such as those that contain the active ingredients broadifacoum, bromodialone or difethialone, can only be used to control rodents found within structures, like rats and mice. Read product labels carefully before using any pesticide, and follow directions exactly. Check daily for dead rodents. Wearing gloves, collect the carcasses as soon as possible, place in plastic bags and dispose in garbage cans with tight lids that other animals can't open. Always wear protective gloves when handling any dead animal."

Bells, Whistles, Gases, etc. According to UC Davis: "Burrow fumigants such as gas cartridges aren’t effective for controlling voles, because their burrow system is shallow and has numerous open holes.Commercial repellents are available for protecting plants from voles, but their effectiveness is questionable and their use often isn’t practical. Electromagnetic or ultrasonic devices and flooding also are ineffective against voles."

Do nothing. Vole populations are cyclical; let nature take its course. In the meantime, cringe.

And now, the geeky part. Why do vole populations soar? It could be what they are eating early in the spring: wheat and other grasses. 
From one study: "It had been long suspected that compounds in plants affect the seasonal reproductive output of wild rodents. In 1981, 6-MBOA became the first naturally occurring compound in a plant verified as impacting seasonal reproductive cycling. Since then, a substantial body of work has accumulated on 6-MBOA as an initiator of seasonal breeding and an effector of population size for many rodents and a few birds. Compounds related to and possibly co-occurring with 6-MBOA remain unexplored in this regard.

6-MBOA is passed from adult females to offspring during gestation and lactation, with increased growth and greater gonadal size in the recipient young. Juveniles rely on the interaction of maternal photoperiod history and 6-MBOA to time the onset of growth and puberty. Adults fed a diet containing 6-MBOA produce more female progeny. When 6 MBOA is fed to pregnant females, gonadal development in the male offspring is enhanced". Nature's Viagra!

1 comment:

  1. They certainly were a problem earlier this year -- when Precious the Cat was catching two or three per week. But lately? The yard has been a "Vole Free Zone" and I really can't figure out why. The cats always get their share -- but never knock them out completely. Perhaps the new dog has something to do with it? I can't tell. What I can tell you is that I haven't lost a melon yet this summer (to Voles that is) -- which is highly unusual...