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From "Midwest Tree Fruit Spray Guide 2003," mice (more specifically known as voles) can cause serious damage to fruit plantings. Frequently, damage occurs but is not noticed until trees become weak, die or are removed. Damage can be anticipated each year, particularly from late summer to early spring, as they eat bark from the base of small saplings. Such damage can result in girdling death of the tree. Apple trees are most susceptible, but hungry voles will attack other fruit trees. Apple trees on dwarfing root stocks are particularly palatable to these mice.

Many plantings are being made in a hedgerow pattern; this does not permit cultivation between trees. Such plantings favor vole migration, as do mulches and vigorous sods. High populations also favor vole migrations. No single material or technique is effective for complete control of voles. It is therefore suggested that both the materials and the methods of control be varied during the season.

General Orchard Management Practices
A number of general orchard management practices can be employed to reduce the risk of injury and improve the effectiveness of control programs. Tree guards can be constructed from “hardware cloth” or similar materials with no larger than 1/4 inch mesh. These guards should enclose the tree and extend from several inches below the soil surface (voles dig in the top 2-3 inches of soil) to several inches above maximum snow line (about 18 inches). Pea-sized gravel or cinders, when placed around the trees, 4-6 inches wide and deep, also tend to discourage meadow voles from attacking the crown of the tree, but do not discourage other mouse species. To proliferate, voles must have abundant amounts of cover. Thus, maintaining a clean area 1 to 2 feet wide around the base of the trunk of the tree discourages surface feeding. This will also regulate vole populations in the long term. Chemical weed control in early spring significantly reduces the amount of labor involved in keeping the area around the tree clean.

The orchard cover or sod should be mowed short in late August and again after harvest. This reduces runway cover and aids baiting procedures. Cleaning out drainage ditches and fence rows, as well as either picking up or crushing all dropped fruit, discourages large mouse populations.

Orchard Mouse Control Program
Essential Knowledge
Determine species of vole (with snap traps). Three species may be found: Meadow Mouse (Microtus pennsylvanicus), Prairie Mouse (Microtus ochrogaster), Pine Mouse (Pitymys pinetorum). Materials for control may be the same, but control methods differ. Quick field identification may be made for both juveniles and adults based on the length of the tail.

  • Pine Vole: tail length about same length as the length of the hind foot.
  • Meadow and Prairie Vole: tail length about twice the length of the hind foot.
  • Determine infestations (with snap traps). There is a definite advantage in knowing when and where mice are most abundant. This makes control easier

Control
Control of mice in orchards can be accomplished using either zinc phosphide or chlorophacinone baits. Both baits will provide good control if used according to label directions.

Zinc phosphide is considered an acute bait and causes death of mice within 24 hours. It is available as either a weather-resistant pellet bait or mixed with prepared grains such as oats and corn. It is usually well accepted by mice. Zinc phosphide is not effective if applied more than two times.

Chlorophacinone (e.g.,RoZol ™) is an anticoagulant bait also available in some states (check your state regulations) as a weather-resistant pellet style bait. This bait is highly accepted by rodents, but death does not occur for several days. For effective control, a second application of chlorophacinone is needed within 20 to 40 days.

In general, all baits can be attractive to other wildlife including some birds and to domestic pets as well. Care must be taken that bait is applied directly in runs, bait stations (see below), or broadcast. Pick up all spilled materials to avoid consumption by non-target animals

Effectiveness of baits against meadow and pine voles
Chlorophacinone is more effective against pine vole than meadow vole, while zinc phosphide is more effective against meadow vole than pine vole. Consistent use of one of these chemicals will result in a shift of the vole population from one species to the other; therefore alternate baiting using zinc phosphide in the first application followed by chlorophacinone in the second application will likely reduce the populations of both species.

Techniques for Baiting

  1. Machine baiting: Expose bait in artificial trail (Trail Builder)
  2. Trail baiting: Expose bait in natural active runways only.
    Broadcast baiting: (NOT RECOMMENDED FOR PINE VOLE CONTROL). Broadcast bait by hand, cyclone type seeder, or tractor drawn equipment at recommended rates. When using zinc phosphide baits, the 2% concentration is recommended. OBSERVE SAFETY PRECAUTIONS. Zinc phosphide is a restricted use material. Read and follow all label directions and precautions.

Percentage Comparison of Control Methods

Method
Meadow or Prairie Voles
Pine Voles
Machine
90-95%
80-85%
Trail
80-85%
70-75%
Broadcast
78%
Not recommended

Timing
Apply on a sunny day in late fall when the mice are active. Mice begin to build up in early August, but baiting should be delayed to as late as possible in the fall. The most effective period for application is just before snow cover, after the grass cover has been reduced by frost, and when the fruit is rotted. Spot treatment during the winter and into early spring is recommended. Treat marginal lands to prevent re-invasion.

Pre-harvest Baiting Is Not Recommended
Application of poisoned bait before harvest to prevent orchard mouse damage to fruit in cold storage is not a sound practice for the following reasons:

  1. The recommended methods of orchard mouse control do not always result in 100 percent control of the species in the orchard. Therefore, some mice survive the pre-harvest control and may enter into the boxes of fruit on the ground and still be carried into the cold storage.
  2. The pre-harvest poison application will reduce the population of mice in the orchard; competition among the survivors will be greatly reduced, and food and cover will be more than ample. The survivors, under these favorable conditions, will breed and the number of young per litter may be as high as eight. In a very short time, the population will recover to its original level. A large number of young mice will be present during the recovery period. These mice, having a short home range, would not be exposed to poisoned baits applied during the normal control season.
  3. The recommended control season is just prior to freezing conditions. This is the best time to control the mice in an orchard and prevent their damage during the winter months, the season when population recovery is slowest.
    Check your control program with snap traps. Lack of visible damage does not indicate the effectiveness of your program.

Control in Storage

  1. Before Harvest
    a) Poison rats and mice in storage one month before picking; keep storage area baited, and free of debris.
    b) Clean up all outside debris, especially near loading door, one week before picking.
    c) Rodent-proof storage, seal all holes and cracks. Mice can fit through a hole the size of a dime.
  2. During Harvest
    a) Move filled boxes into storage quickly as any left overnight may have mice in them.
    b) AS YOU LOAD fruit into storage, bait storage. Place teaspoonful amounts in bait stations on floor along alleys, between rows of boxes, and under pallets. Do not place open baits on floors or any areas where contamination might occur. Commercial bait stations are available from agricultural supply companies. Always prevent contact with fruit.

Bait Stations in the Orchard
Bait stations can be prepared in several ways and eliminate or reduce the opportunity for non-target animals to contact the bait. Squares of heavy roofing shingles or other weather-resistant materials placed out of traffic areas between trees can serve as bait stations to provide protection for the bait and hiding places for the rodents. Some growers have constructed bait stations that require less refilling by building inverted T-shaped stations from PVC tubing and fittings that will provide bait storage and a protected feeding area. Place bait stations in the field 2 to 3 weeks before adding the bait.

 
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