Preventing Rodent Damage in Greenhouses

October 17, 2016

Meadow Voles and Mice

Meadow voles can invade overwintering structures, and cause extensive damage to perennials. The head and body of meadow voles measure 3 to 5" long and the tail is 1½ to 2½" long. They have brown body hairs mixed with black guard hairs; belly hairs tipped gray and small eyes and ears. Meadow voles are active day and night, throughout the year and do not hibernate. They feed all winter long on tubers, bulbs, rhizomes, stems, roots, and seeds and occasionally on insects and animal remains. Meadow voles construct extensive tunnels and surface runways with numerous burrow entrances, often between or under pots. These surface runways are the most easily identifiable sign of voles, however, clippings or stems, stripped of foliage, in piles along their runways are also signs of their activity. By the time the runways are noticed, the damage is usually done. Voles are prolific breeders, especially in the spring and summer months however, they may breed throughout the year.

Outdoors, white footed mice often feed on seeds, berries and nuts. However, these nocturnal creatures can also enter greenhouses and feed on seeds and young seedlings. Often, growers will cover seedlings with glass or fiberglass sheets to prevent their feeding damage.  

Monitoring: The apple sign test has been used in orchards to monitor activity of voles, and can be used in greenhouses too. Use shingles or wood pieces that will blend into the surrounding soil. Slightly arch the shingle so that an animal can fit under it. Leave the shingles throughout the scouting area for five days, to allow rodents to adjust being there. After 5 days, place small cubes of apple under the shingles. Monitor the apple bits for signs of feeding. This will indicate the areas where control efforts should be practiced.

Rodent Proofing: The first step is to rodent proof the greenhouse (as best you can). Mice can get enter through openings as small as ¼ inch. Use fine mesh screen such as hardware cloth around the perimeter of the greenhouse at least 6 inches deep and bend outward at a 90ᴼ angle.

Sanitation: Sanitation will also do a great deal to control rodents. Eliminate protected areas for rodents around greenhouses. Most of our wildlife animals will not venture across a wide-open space because they are much more vulnerable to natural predators.

Remove all debris around the greenhouse, clean up the natural vegetation surrounding the greenhouse. Keep grass areas closely mowed or install geotextile fabric (weed cloth barrier) to prevent weed growth. Keep trash cans covered and avoid storing bird seed, pet food, or seeds in the greenhouse.

Traps and Baits: Snap traps can be effective if the populations are low. Traps should be set with a good understanding of the behavior of the rodents. Rodents are creatures of habit and prefer to follow the same runways. Identify the runways and place the traps perpendicular to the runways. Baits include: peanut butter for mice or a piece of an apple for voles (Cortland apple works well). Note that repellents do not seem to be consistently effective.

If trapping fails to reduce the population, growers find that using toxic baits (rodenticides) are effective.  Wear gloves when using traps and baits as they are sensitive to human scent. Rodenticide baits include the single feeding acute zinc phosphide bait and multiple feeding anticoagulant baits. Read and follow the label directions and precautions.  One of the most effective and common baits is zinc phosphide treated oats.  It is a single-dose toxicant available in pelleted and grain bait formulations and as a concentrate. Zinc phosphate is a restricted use pesticide purchased and used only by certified pesticide applicators.

Anti-coagulant baits are also effective in controlling voles. Anticoagulants are slow acting toxicants requiring from 5-15 days to take effect. Multiple feedings are needed for most anti-coagulants to be effective.

When using either type of bait, begin baiting about a month before the beginning of the overwintering of your perennials. Many growers will use homemade bait containers, using a 1.5 or 2 inch PVC pipe inserted inside another pot or use commercial bait stations. Keep baits dry, as moisture reduces their effectiveness.  Frequent use of baits can lead to “bait shyness” where voles that consume sublethal doses no longer eat the bait.

These toxic baits can be harmful to children, pets and wildlife and should be used with utmost caution. Read and carefully follow the directions and safety precautions on the label of any of these products. 

Owls: A new study reports that barn owls may reduce rodent populations on farms.
See: Are Barn Owls Nature’s Best Pest Control? and Barn Owl Research, University of Florida

Tina Smith, UMass Extension and Leanne Pundt, UConn Extension

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