There are 23 species of voles and lemmings in North America (Hoffmann and Koeppl, 1985).  Two species, meadow and pine voles, are serious pests in eastern U. S. apple orchards (Tobin and Richmond, 1993).  Meadow voles are sometimes referred to as meadow mice or field mice and pine vole as pine mice or woodland voles.  In the eastern U. S., the distribution of meadow voles extends from Maine to South Carolina and pine voles from New Hampshire to south Georgia and west into central Texas.  Meadow voles may be a potential pecan pest in North and South Carolina, but they should not be a problem in the remainder of the southeastern U. S. pecan region based on their geographic distribution.

Both vole species prefer heavy ground cover including grasses and clovers (Tobin and Richmond, 1993).  Meadow voles are mostly active aboveground creating surface runways  through the vegetation that are 1 to 2 inches wide.  These runways are littered with droppings and grass cuttings.   Even though meadow voles normally build grass nests on the ground surface, they occasionally will occupy underground burrows of other animals.  In contrast, pine voles mainly live underground and create underground burrow systems 3 to 4 inches beneath the soil surface.  Accompanying the burrows is an extensive subsurface trail system that is 1 to 2 inches beneath the surface liter.  Surface runways are narrow (0.75 to 1.0 inches wide) and the runways are normally clean of grass cuttings and other debris in contrast to the littered runways of meadow voles.  Openings to the surface from underground tunnels and dirt piles excavated from the underground tunnels also indicate the presence of pine voles.  In apple orchards, pine voles often build their nest near the base of tree trunk 3 to 12 inches below ground.

Adult meadow voles have a coat with an overall coarse texture resulting from long hairs (guard hairs) mixed with an under coat of dense soft  hairs.   The color of the fur is dark brown mixed with black on the back and dusky gray on the belly (Tobin and Richmond, 1993).  The feet and tail are dusky above and pale below, and the tail is vaguely bicolored.  Mature meadow voles weigh about 1.0 to 2.5 ounces and overall length (snout to tail tip) is about 5 to 8 inches.  Tail length is more than twice the length of the hind foot, a characteristic which readily distinguishes meadow voles from pine voles. Other than the short tail, additional characteristics of adult  pine voles distinguish them from adult meadow voles.  The short, soft fur lacks guard hairs.  The fur is chestnut-brown on the back, paler on the sides, and silvery to slate gray on under parts.  Winter color is darker than summer color (Hamilton, 1938).  Mature pine voles are smaller than meadow voles.  The pine vole weighs less at only 0.8 to 1.3 ounces and shorter with a length of about 3.5 to 5 inches and, as a result, the body shape is more stocky.  In addition, pine voles have smaller eyes and shorter ears than meadow voles.

Pine vole damage to the roots of young transplanted pecans was observed during the spring of 2005 in an orchard on NILO Plantation, Albany, Ga.  The damaged trees were 1 to 4 years old.  Damage was spotty among the transplanted trees.              

Damage was extensive to the taproot (Fig. 1).  The tree dies because it does not have sufficient roots.  Dead trees could easily be lifted from the soil if the tree had not already been knocked over.  If voles cause damage to mature pecan trees, it has not been reported.

Voles create a nonuniform pattern of gnaw marks on roots which was evident on the taproot of the young pecan trees (Fig. 1).  In contrast, marks made by most other gnawing animals, are characteristically uniform.  Vole gnaw marks occur at various angles and in irregular patches.  Irregularity of the marks is the result of the animal feeding at preferred sites on the roots or abandoning a feeding site after initial feeding.   This feeding pattern indicates voles are discriminating feeders as demonstrated with carrots.  Meadow voles consistently preferred certain carrot breeding lines and hybrids over others (Lane et al., 1969).  In pecan, most of the carbohydrates are stored in the roots (Lockwood and Sparks, 1978).  Carbohydrates provide nutrition to the voles. Thus, young pecan taproots would provide a desirable feeding site for pine voles (Fig. 1).

The vole damage at NILO occurred on young interplanted trees in a mature orchard.  The trees were replacements for mature trees that had been cut down.  The replacement tree was  planted adjacent to the decaying stump.  Ground cover was clover, primarily Crimson and Ball, followed by common Bermuda grass later in the season.  Vole damage was not evident in a four-year-old orchard planted adjacent to the mature orchard.  Other than being a new orchard established in a previously cultivated field, this orchard differed from the mature orchard in that the ground cover was sparse and without litter and the trees had been individually treated with Temik for four consecutive years.  Both orchards were irrigated, the new orchard with microjets and the mature orchard with sprinklers.

Toxic baits (mainly zinc phosphide) can be used for vole control. The pellet formulation is generally broadcasted (Tobin and Richmond, 1993) but can be placed by hand in runways and burrow openings.  Because pine voles are mostly subterranean, the toxin is expected to be more effective when placed in burrows.  Burrow placement is not practical and especially because vole presence is not readily evident until the pecan tree dies.  The more practical approach, when damage is scattered as at NILO, may be to drop a few pellets (consult the label) into the transplant hole at the time the dead tree is replaced with a new one.  Voles are classified as non-game animals and control is allowed when they cause damage.  Application code and regulations can be obtained from the state wildlife agency

In apple orchards, managing ground vegetation is important in minimizing vole damage (Tobin and Richmond, 1993).  Ground covers provide food, concealment from predators, protection from unfavorable weather, and enhance reproduction opportunities.  Consequently, maintaining a vegetation-free zone under the tree is a priority.  A vegetation-free zone discourages voles from living near the base of the tree where they cause the most damage.  A free zone may be especially important with pine voles as they nest below ground near the base of the tree.  In addition, to discouraging pine voles, a vegetation-free zone (6 ft diameter circle) in itself maximizes growth of young pecan trees (Smith et al., 2002).  Thus, the vegetation-free zone is dual purposed.

Pine vole damage has not been a problem at NILO in the past.  The appearance of pine vole damage may be related to the recent establishment of clover as a cover crop.  Tall dense clovers and the resulting thick thatch produced when clover dies provide an ideal habitat for the vole (Tobin and Richmond, 1993).  The lack of obvious vole damage in the young orchard at NILO may be directly related to its sparse ground cover.  Vole activity increases with ground clover density and when density is below 40% little activity occurs (Tobin and Richmond, 1993).   However, the trees had been treated annually with Temik which may have deterred the voles. 

A mature orchard in Leary, Ga. has had Ball clover as a cover crop for more than10 years.  During the last 4 years, replacement trees have been planted as at NILO.  Cultural practices are similar to NILO with the exception that thatch build up has been prevented by maintaining a wide herbicide strip.  Pine voles have not been a problem, suggesting vole damage to young trees can be controlled by a wide vegetation-free zone.     

Literature Cited

Hamilton, W. J., Jr.  1938.  Life history notes on the northern pine mouse. J. Mammal. 19:163-170.

Hoffmann, R. S. and J. W. Koeppl.  1985.  Biology of new world Microtus, p.84-115.  In: R. H. Tamarin (ed.). Zoogeography.  Amer. Soc. Mammal. Spec. Pub. 8.

Lane, R. P., C. E. Peterson, and F. C. Elliott.  1969.  Feasibility of using the meadow vole, Microtus pennsylvanicus, to bioassay the nutritive value and culinary quality of some carrot inbred lines and hybrids.  J. Amer. Soc. Hort. 94:96-98.

Lockwood, D. W. and D. Sparks. 1978. Translocation of ¹⁴C from tops and roots of pecan in the spring following assimilation of ¹⁴CO₂ during the previous growing season.  J. Amer. Soc. Hot. Sci. 103:45-49. 

Smith, M. W., B. S. Cheary, and B. L. Carroll. 2002.  Fescue sod suppresses young pecan tree growth.  HortScience 37:1045-1048.

Tobin, M.. E. and M. E. Richmond.  1993.  Vole management in fruit orchards. U. S. Dept. Interior, Fish and Wildlife Serv., Biol. Rpt. 5.