Denver House Mice

Identification

The house mouse (Mus musculus) is a small, slender rodent that has a slightly pointed nose; small, black, somewhat protruding eyes; large, sparsely haired ears; and a nearly hairless tail with obvious scale rings. House mice are considered among the most troublesome and economically important rodents in the United States.

Adult house mice weigh about 2/5 to 4/5 ounce (11 to 22 grams). They are generally grayish brown with a gray or buff belly. Similar mice include the white-footed mice and jumping mice (which have a white belly), and harvest mice (which have grooved upper incisor teeth).

 

Range and Habitat

Following their arrival on colonists’ ships, house mice spread across North America and are now found in every state, including coastal areas of Alaska, and in the southern parts of Canada.

House mice live in and around homes, farms, commercial establishments, and in open fields and agricultural lands. At times they may be found living far from human settlements, particularly where climates are moderate. The onset of cold weather each fall in temperate regions may cause mice to move into structures in search of shelter and food.

 

Fun Facts

Native to central Asia, this species arrived in North America with settlers from Europe and from other points of origin. A very adaptable species, the house mouse often lives in close association with humans and therefore is termed one of the “commensal” rodents along with Norway and roof rats. House mice are much more common in residences and commercial structures than are rats. Brooks (1973) regards them to be the most common mammal in cities, next to humans.

House mice are mainly nocturnal, although at some locations considerable daytime activity may be seen. Seeing mice during daylight hours does not necessarily mean that a high population is present.

Mice have poor eyesight, relying on their hearing and their excellent senses of smell, taste and touch. They are considered color-blind; therefore, for safety reasons, baits can be dyed distinctive colors without causing avoidance by mice, as long as the dye does not have an objectionable taste or odor.

House mice may burrow into the ground in fields or around structures when other shelter is not readily available. Nesting may occur in the ground or in any sheltered location. Nests are constructed of shredded fibrous materials such as paper, burlap, or other similar items, and generally have the appearance of a “ball” of material loosely woven together. They are usually 4 to 6 inches (10.2 to 15.2 cm) in diameter.

Litters of 5 or 6 young are born 19 to 21 days after mating. Mice are born hairless, with their eyes closed. They grow rapidly, and after 2 weeks they are covered with hair and their eyes and ears are open. They begin to make short excursions from the nest and eat solid food at 3 weeks.

Mice may breed year-round, but when living outdoors, they breed mostly in spring and fall. A female may have 5 to 10 litters per year. Mouse populations can therefore grow rapidly under good conditions, although breeding and survival of young decline markedly when population densities become high.

House mice have physical capabilities that enable them to gain entry to structures by gnawing, climbing, jumping and swimming.

Studies indicate that during its daily activities, a mouse normally travels an area averaging 10 to 30 feet (3 m to 9 m) in diameter. Mice seldom travel farther than this to obtain food or water.

Mice constantly explore and learn about their environment, memorizing the locations of pathways, obstacles, food and water, shelter, and other elements in their domain. They quickly detect new objects in their environment but do not fear them. The degree to which mice consume a particular food depends on the flavor of the food in addition to its physiological effect. Mice may reject baits simply because they do not taste as good as other available foods.

 

Damage Identification

When house mice live in or around structures, they almost always cause some degree of economic damage. In homes and commercial buildings, they may feed on various stored food items or pet foods. In addition, they usually contaminate foodstuffs with their urine, droppings, and hair. On farms, they may cause damage to feed storage structures and feed transporting equipment. A single mouse eats only about 3 grams of food per day (8 pounds [3.6 kg] per year) but destroys considerably more food than it consumes because of its habit of nibbling on many foods and discarding partially eaten items.

House mice living in fields may dig up and feed on newly planted grain, or may cause some damage to crops before harvest. Losses in stored foods are considerably greater. Mice commonly damage containers and packaging materials in warehouses where food and feeds are stored. Much of this loss is due to contamination with droppings and urine, making food unfit for human consumption.

House mice cause structural damage to buildings by their gnawing and nest-building activities. In livestock confinement facilities and similar structures, they may quickly cause extensive damage to insulation inside walls and attics. Such damage also occurs in homes, apartments, offices, and commercial buildings but usually at a slower rate because mouse populations in such structures are smaller. House mice often make homes in large electrical appliances, and here they may chew up wiring as well as insulation, resulting in short circuits which create fire hazards or other malfunctions that are expensive to repair. Mice may also damage stored items in attics, basements, garages or museums.

Public Health
Among the diseases mice or their parasites may transmit to humans are salmonellosis (food poisoning), rickettsialpox, and lymphocytic choriomeningitis. Mice may also carry leptospirosis, ratbite fever, tapeworms, and organisms that can cause ringworm (a ungal skin disease) in humans. They have also been found to act as reservoirs or transmitters of diseases of veterinary importance, such as swine dysentery, a serious bacterial disease of swine often called “bloody scours.

Mouse Sign
The presence of house mice can be determined by a number of signs described below:

Droppings may be found along runways, in feeding areas and near shelter. Differentiating between mouse droppings and those of certain insects may be difficult. Mouse droppings are about 1/4 inch (0.6 cm) long.

Tracks, including footprints or tail marks, may be seen on dusty surfaces or in mud (Fig. 2). A tracking patch made of flour, rolled smooth with a cylindrical object, can be placed in pathways overnight to determine if rodents are present.

Urine, both wet and dry, will fluoresce under ultraviolet light, although so will some other materials.

Rub marks along beams, rafters or other travel routes give evidence of rodent activity. Mouse rub marks can be distinguished from those of rats by their smaller size. Stains may occur along travelways or odors may indicate the presence of in feeding areas. A characteristic musky odor is a positive indication. They are used to differentiate their presence from that of rats. They may be less apparent than rub marks left by rats. Mouse sign and visual sightings are of limited value in accurately estimating mouse numbers.

Sounds such as gnawing, climbing in because house mice, unlike rats, do walls, running across the upper sur-not travel far from their nests or shelface of ceilings, and squeaks are com-ter, the percentage of patches showing where mice are present.

 

Legal Status

House mice are not protected by law. They may be controlled using any pesticide registered by federal or state authorities for this purpose, or they may be controlled by use of mechanical methods such as traps.

 

Damage Prevention and Control Methods

Exclusion
Seal all openings larger than 1/4 inch (0.6 cm) wide.

Habitat Modification
Good sanitation practices reduce sources of food, water, and shelter. Store foodstuffs in rodent-proof structures or containers. Control weeds and remove debris from around structures.

Frightening
Ultrasonic devices have not been proven to control mice.

Repellents
Ro-pel® Moth flakes (naphthalene) not specifically registered, but may be of some value.

Toxicants
Anticoagulant rodenticides (slow acting chronic-type toxicants). Brodifacoum (Talon®). Bromadiolone (Maki®, Contrac®). Chlorophacinone (RoZol®). Diphacinone (Ditrac®). Pindone(Pival®, Pivalyn®). Warfarin (Final® and others). Toxicants other than anticoagulants (may be acute or chronic poisons). Bromethalin (Assault®, Vengeance®). Cholecalciferol (Quintox®). Zinc phosphide (Ridall Zinc®, ZP®).

Fumigants
Practical use is limited to structures, containers, and commodities; for use only by trained personnel.

Trapping
Snap traps. Live traps (Sherman-type, Ketch-All®, Tin Cat®, and others).
Glue boards.

 

 

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ACKNOWLEDGEMENTS
The above information was adapted from PREVENTION AND CONTROL OF WILDLIFE DAMAGE with permission of the editors, Scott E. Hygnstrom, Robert M. Timm, and Gary E. Larson (Cooperative Extension Division, Institute of Agriculture and Natural Resources University of Nebraska-Lincoln, United States Department of Agriculture Animal and Plant Health Inspection Service, Animal Damage Control, Great Plains Agricultural Council Wildlife Committee).