Rodent Management

 
Voles
Voles can injure or destroy trees.

Rodents, primarily voles, can significantly damage orchard trees.  In the winter when other food is scarce, voles will gnaw the bark from trees up to the snow line.  In severe cases hundreds of trees can be completely girdled in just one winter.  Managing the orchard floor to reduce habitat is a critical part of keeping vole populations below action thresholds.

Both meadow voles and pine voles can be found throughout orchard areas in the Intermountain West. Voles rarely live for more than 1 year.  Litters contain up to 12 pups and females can reproduce in any season.  Females reach reproductive maturity in 3 weeks and a fecund female can have about four litters per year.  Under favorable conditions, vole populations can increase rapidly.

An effective vole management plan in orchards consists of five components:

  • Habitat destruction
  • Scarce food supply
  • Exclusion
  • Facilitating predators
  • Baiting with rodenticide

Destroy Habitat

Like all mammals, voles require shelter from the elements and from predators.  Vegetation in orchards and nearby fence rows or brush piles provide adequate habitat.  These types of habitats, however, also encourage predators that could feed on voles, as well as beneficial pollinators, predatory insects, and desirable game birds.  Depending on the site and landowner’s personal preferences, these areas could have more benefits than negative effects.

If necessary, the most efficient means of destroying habitat is to mow the orchard close.  The last mowing in the fall should cut remaining vegetation short.  Orchards that are clear cultivated sometimes leave tufts of vegetation around each tree.  This is prime vole habitat and may encourage trunk girdling in the winter.  Chipping or burning brush piles reduces habitat.  Keeping fence rows clear also removes rodent habitat.  Equipment “bone yards” can also provide shelter for rodents.  Leaving no place to hide will reduce vole population.

Mulches can also be vole habitat and their potential as habitat must be weighed against their desirable characteristics such as moisture retention and weed emergence.

Remove Food

Voles feed on a variety of materials.  Their preferred diet is high energy content foods such as green tissues, seeds, nuts, and fruit.  When preferred foods are absent, they will eat woody materials and bark.  Fruit that falls to the ground as part of the harvest process provides voles with their preferred diet.  With adequate food, populations can skyrocket in the fall.  Flail chopping dropped apples will cause them to decompose faster and provides less food for rodents.

Exclude

It is not possible to exclude voles from entire orchard blocks, but it is possible to exclude access to the trunks of individual trees.  Trunk guards will help protect trunks against vole feeding.  Various commercial products are widely available.  An inexpensive trunk guard can be made using ¼ inch mesh galvanized hardware cloth.  Cut an 18 inch square and make a cylinder around the trunk and fasten the two edges with wire.  Place the cylinder 2-3 inches into the soil to discourage burrowing under the guard.

Facilitate Predators

Kestrel and Owl Boxes
Barn owl boxes (left) should have the appropriately-sized entrance hole, and be mounted to a building or tall, sturdy post. Kestrels are excellent predators of small mammals. Their nest boxes (right) must be cleaned yearly. 

Birds of prey are important predators of voles and gophers.  The kestrel is a small hawk that is an excellent predator for mouse, vole, and large insect control.  Kestrels will return to maintained boxes year after year, but are highly territorial. 

The barn owl, because of its voracious appetite for gophers, voles, and mice, is a valuable friend to the orchardist.  During a 4 month nesting season, a barn owl family may consume about 1000 rodents per year.  The barn owl population in the West is dwindling partially due to lack of nesting sites, which orchard growers can improve by providing boxes.  If rodenticides are used, select the least toxic options, since predator birds may die from consuming dying rodents.

Attracting Birds of Prey

  • Kestrels:  Screw nesting boxes to power poles, trees, or freestanding posts 10-20 feet above ground, away from human activity. Install up to 1 per 5 acres to increase chances of nesting, but note that a pair may defend up to 250 acres. Adding a bit of nesting material (twigs, wood shavings) can help attract the birds. Monitor each box weekly and remove starling nests. Clean boxes each year.
  • Barn owls:  To attract/keep birds on the farm, keep old wooden barns; they will not nest in metal barns. Nest boxes on trees or 15-30 ft steel posts facing east and away from roads or busy orchard activities can be used in place of cavity trees or abandoned buildings. A tall tree within 50 yards of the nest box is necessary to provide cover. Owls may patrol up to 200 acres per nesting site. The boxes must be cleaned yearly and kept free of starlings.
  • Other large raptors:  Perches are necessary for owls and raptors to spot their prey. Retain old trees in edge plantings or install posts with a 2x4 across the top to facilitate hunting activities. Adequate perching structures will encourage raptors to remain on site year around and can provide valuable winter predation.

See A Guide for Attracting Wildlife for Pest Control on Farmland in Utah and related designs for nest boxes, perches, and promoting wintering raptors on farmland.

Rodenticide

Baiting may be necessary if populations are still high—especially in the fall. Population action thresholds are determined by placing 50 to 100 apple slices in a block of trees. Slices should be placed under some sort of cover such as a wooden shingle or small plank supported by a stone. After 24 hours return to the orchard and count the number of slices that are missing or that show feeding. When 20% to 25% of slices are missing or show feeding, the potential exists for serious vole damage and further action is warranted.

Comparison of Two Rodenticide Groups

Anticoagulants Zinc Phosphide
Comes in various formulations and more widely used Comes in various formulations and not as widely used
Dangerous to raptors and scavengers Somewhat safer for raptors and scavengers
Broadly toxic to mammals and must be handled with care Broadly toxic to mammals and must be handled with care
Kills slowly; may engender bait shyness Kills quickly
Simple rodent bait station made from PVC pipes
A bait station made from PVC pipe laid in an orchard at the transition from grass to vegetation-free zone.  A bait station protects the bait from weather and from non-target consumption.

Effective baiting requires presenting the bait in a setting so rodents will feed and so non-target animals don’t have access to the bait. An inexpensive bait station is made using PVC pipe and tees. Use 1½ or 2 inch PVC pipe. Cut the pipe into 4-5 inch segments and push a piece of pipe into each opening of the tee. Place the bait stations in the orchard unbaited for a week or two to allow habituation. To place the bait, lift up the sidearm of the tee and place a small quantity of bait in the tee, then lay it back down flat. Voles are likely to feed on bait when they can see all the way through the tube and when they don’t have to “back out” of the pipe. The openings are small enough to keep non-target animals from the bait and the bait is protected from moisture.

When using rodenticides read and follow the label directions. Store them in a dry place where other animals and children do not have access. Wear appropriate personal protective equipment (especially rubber gloves) to protect yourself.


Common Rodenticides Used In Orchards

Rodenticide Type Days of Feeding Bird Toxicity Risk Mammal Toxicity Risk
Brodifacoum Anticoagulant Single Highest Highest
Bromadiolone Anticoagulant Single Moderate Moderate
Chlorophacinone Anticoagulant Multiple Slight Highest
Diphacenone  Anticoagulant Multiple Moderate Highest
Defethialone Anticoagulant Single Highest Moderate
Warfarin Anticoagulant Multiple Slight Low
Bromethalin Non-anticoagulant Single Low Low
Cholecaciferol Non-anticoagulant Multiple Low Low
Strychnine Non-anticoagulant Single Possible Possible
Zinc phosphide Non-anticoagulant Single Low Slight