Rodents

Voles

Voles are small rodents that measure 4 to 8.5 inches long and weigh 0.8 to 3 ounces and vary in color from brown to gray. They are pudgy, with blunt faces and small eyes, small and sometimes inconspicuous ears, short legs, and a short (the long-tailed vole is an exception) and scantily haired tail.

Eight species of voles are found widely throughout various ecosystems of Colorado, in heavy ground cover of grasses, grass-like plants, and litter.

• Eight species of voles are found in Colorado. They often are called meadow, field or pine mice.
• Voles are small mammals that cause damage by girdling seedling and mature trees in orchards, shelterbelts and forests. They also damage field crops and frequently construct runways in lawns.
• Damage by voles can be reduced by habitat modification, exclusion, repellents, trapping, and poison grain baits.

Biology, Reproduction and Behavior
Voles are active day and night throughout the year and do not hibernate. They usually live between two and six months. Their home ranges usually are less than one-fourth acre and vary with season, food supply and population density. Voles construct many surface runways and underground tunnels with numerous burrow entrances. A single burrow may contain several adults and young.

Population densities of voles vary from species to species. Large population fluctuations that range from 14 to 500 voles per acre are common. Their numbers generally peak every three to five years. Population is influenced by dispersal, food quality, climate, predation, physiological stress, and genetics.

Voles have three to six young per litter and three to 12 litters per year. Their gestation period ranges from 20 to 23 days and they breed almost year around, although most reproduction occurs in spring, summer and fall. Females may become pregnant at three weeks of age.

Damage and Control
Voles can cause extensive damage to forests, orchards and ornamental plants by girdling trees and shrubs. They prefer the bark of young trees but will attack any tree, regardless of age, when food is scarce. Monitor orchards frequently so control measures can be implemented before appreciable damage occurs. Most damage occurs in the winter when voles move through their grass runways under the protection of snow. The greatest damage seems to coincide with years of heavy snowfall.

Damage to crops, such as alfalfa, clover, potatoes, carrots, beets and turnips is common and most evident when voles are at high population levels. Runways and tunnel systems constructed in agricultural fields can divert irrigation water. Voles often damage lawns and golf courses by constructing runways and burrow systems.

Vole damage to trees and shrubs is characterized by girdling and patches of irregular patterns of gnaw marks about 1/16 to 1/8-inch wide. Gnawed stems may have a pointed tip. Do not confuse vole damage with damage by rabbits, which includes stems clipped at a smooth 45-degree angle and wider gnaw marks. Stems browsed by deer usually have a rough jagged edge.Voles also girdle the roots of trees and shrubs.

Other signs of damage by voles include: 1) 1- to 2-inch-wide runways through matted grass and burrows; 2) visual sightings; 3) hawks circling overhead and diving into fields; and 4) spongy soil from burrowing activity. Trees that appear to suffer from disease or insect infestation may be suffering from unseen vole damage.

Methods to prevent and control damage are: habitat management, exclusion, repellents, trapping, and poison grain baits. Voles are classified as non-game wildlife in Colorado and may be captured or killed when they create a nuisance or cause property damage.

Habitat Management. Elimination of weed ground cover and tall grasses by frequent and close mowing, tilling, or herbicide application is the most successful and longest lasting method to reduce vole damage to orchards. This will diminish the amount of available habitat and reduce their numbers. Prunings left in orchards prevent proper mowing and provide a temporary food source, which may lead to damage by voles. Planting short grasses that do not mat or lodge, such as buffalo grass, blue grama, or dwarf fescues, will provide little protective cover and may reduce vole numbers.

Meadow voles are active during the day within their runways under thick grass and vegetation. Summer removal of vegetation (2 feet of trunks of fruit trees) provides some protection because voles avoid exposed areas.

Damage to lawns can be reduced by close mowing in the fall before snow arrives and by mowing and removing tall grassy cover near lawns. To repair damage to lawns from runway construction, rake, fertilize and water the effected area. Close mowing and weed control in grassy borders adjacent to agricultural crops will reduce the habitat for voles and should reduce damage. If suitable, plant crown vetch (a legume unpalatable to voles) in orchard and field boundaries to reduce vole populations.

Important predators of voles are: short-tailed shrews, badgers, coyotes, foxes, bobcats, barn owls, great horned owls, long-eared owls, short-eared owls, barred owls, screech owls, and some snakes. Predators will not likely keep an orchard vole-free, but they can help reduce the vole population. Orchardists should tolerate predators and protect them if they do not constitute a pest problem.

Exclusion
To protect against vole damage, encircle young trees and shrubs with 1/4-inch mesh hardware cloth or 3-inch diameter Vexar plastic-mess cylinders. This barrier should project 18 inches above the ground and 3 to 6 inches below the surface. Vegetable and flower beds may also be protected in this manner.

Ground Squirrels

General Biology, Reproduction, and Behavior
Ground squirrels construct and live in extensive underground burrows, sometimes up to 6 feet (2 m) deep, with many entrances. They also use
and improve on the abandoned burrows of other mammals such as prairie dogs and pocket gophers. Most return to their nests of dried vegetation
within the burrows at night, during the warmest part of summer days, and when they are threatened by predators, such as snakes, coyotes, foxes, weasels, badgers, and raptors. The squirrels generally enter
their burrows to estivate, escaping the late summer heat. They hibernate during the coldest part of the winter. Males usually become active above ground 1 to 2 weeks before the females in the spring, sometimes as early as late February or early March. A few may be active above ground throughout the year.
Breeding takes place immediately after emergence. The young are born after a 4- to 5-week gestation period with 2 to 10 young per litter. Generally only 1 litter is produced each year. Densities of the ground squirrel populations can range from 2 to 20 or more per acre (5 to 50/ha).

Damage Prevention and Control Methods Exclusion
Exclusion is impractical in most cases because ground squirrels are able to dig under or climb over most simple barriers. Structures truly able to exclude them are prohibitively expensive for most situations. Sheet metal collars are sometimes used around tree trunks to prevent damage to the base of the trees or to keep animals from
climbing trees to eat fruit or nut crops.

Cultural Methods/Habitat Modification Flood irrigation of hay and pasture lands and frequent tillage of other crops discourage ground squirrels somewhat. Squirrels, however, usually adapt by building the major part of their burrows at the margins of fields, where they have access to the crop. During the early part of the season they begin foraging from the existing burrow system into the field until their comfort escape zone is exceeded.

When this zone is exceeded and as the litters mature in the colony, tunnels will be extended into the feeding area. Late in the summer or fall, tillage will destroy these tunnels but will not disturb or destroy the original system at the edge of the field. Some research has been conducted on the effect of tall vegetation on ground squirrel populations and movements. The data, while sketchy, indicate that the squirrels may move out of tall vegetation stands to more open grass fields. The addition of raptor (hawk, owl, and kestrel) nest boxes and perches around the field border or throughout the colony may reduce colony growth, but is not a reliable damage control method.

Pack Rats

Woodrats – familiarly known as “pack rats” – are beautiful and interesting animals that hardly deserve the negative stereotype most people have of “rats”. Desert woodrats (12 inches long, 4 ½ ounces) are the smallest of Coloradan species; the bushy-tailed woodrat of mountain talus slides and mining camps (to over 16 inches long and 11 ounces) is the largest. These are fastidious, beautiful animals. Color differs from gray (gray woodrat) to blackish brown (Mexican woodrat), to rich reddish tan (bushy-tailed woodrat), and most have white to grayish bellies.

Woodrats are known as Pack rats because they pick up any curious object they find in their habitat and “pack” it home to the den, where bits of rock or wood, cow chips, bones and hardware all add to the strength of the house. A peculiar characteristic is that if woodrats find something they want, they will drop what they are currently carrying, for example a piece of cactus, and “trade” it for the new item. They are particularly fond of shiny objects, leading to tales of rats swapping jewelry for a stone.

Range:
Most parts of Colorado have one or another of the state’s six woodrat species. The eastern woodrat lives in riparian woodlands on the eastern plains, and the gray woodrat occurs in southeastern Colorado.

Habitat:
Woodrats prefer the rough, broken terrain of canyon and mesa walls of the Colorado Plateau; building dens in holes around cholla cactus, the cactus’ spines act as protection against preditors.

Many generations of woodrats may use a choice den site, adding to the house, latrine, and a sticky accumulation of urine. The urine catches pollen and other debris blown from the surrounding area and thus captures a sample of the environment of a particular time. Dated by carbon-14, fossil woodrat middens, or manure piles, have provided information to reconstruct changing habitats of the Southwest over the past 40,000 years.

Reproduction:
Inside the house is a grass-lined nest where the animals spend cold weather and rear young. Females produce up to four litters of one to six young after a gestation period of four to five weeks. Woodrats are fairly protected in their dens, but still owls and coyotes take a toll as do gopher snakes and rattlesnakes. Adults may live several years.

House Mice

• The house mouse is the most common rodent pest in most parts of the world. It can breed rapidly and adapt quickly to changing conditions.
• A female house mouse gives birth to a litter of approximately six mice about 19 days after mating and is capable of mating again in two days.
• Can produce six to 10 litters a year.
• Are able to mate two months after birth.
• Can produce 2,500 heirs in six months, with ideal conditions and no mortality.
• Establish a nesting site near sources of food and feed 15 to 20 times a day.

Habitat:
House mice live in structures, but they can live outdoors.

Threats:
Micro droplets of mouse urine can cause allergies in children. Mice can also bring fleas, mites, ticks and lice into your home.

Prevention:
To keep mice and other rodents out, make sure all holes of larger diameter than a pencil are sealed. Mice can squeeze through spaces as small as a nickel. Seal any cracks and voids. Don’t overlook proper drainage at the foundation and always install gutters or diverts which will channel water away from the building.