This blog is about the house mouse, not about the deer mouse which is the vector of hantavirus (HPS) and is in the news because of the camper exposure problem in Yosemite National Park.
In addition to snakes and large hairy spiders, few critters elicit fear and emotional distress like finding a mouse in your house. As the outdoor temperature cools in autumn, mice look for a warm place to spend the winter. For mice, heated homes are an ideal location to spend the cold winter months. All they need to do is to find a way to get into your home.
Despite their small size, mice can cause considerable visible and hidden damage in one's home. They are also carriers of diseases, have ectoparasites (fleas, mites) on them which may feed on humans, and contaminate many areas with their saliva, urine, and feces.
Identification of house mouse (Mus musculus). They are small and slender, light-weight, and furry. An adult's tail is about as long as the combined length of its head and body, for a total length of 5¼ to 7¾″. They weigh only about ½ to 1 ounce. Their fur is smooth and its color is darkish above and its belly is lighter, but their tail is semi-naked and uniformly dark. Their eyes are small and the ears are large.
Similar mice (deer mouse, Peromyscus maniculatus). They are very similar to the house mouse except that they are slightly larger, with their tail hairy and longer than half the length of the head and body combined. Most importantly, they are bicolored. They are pale grayish buff to deep reddish brown above and white below with a distinct line where the two colors meet, and their tail sharply bicolored.
Signs of infestation. (Besides the mouse itself, what to look for to determine mouse presence.)
- Gnaw marks. Chisel-like markings left on surfaces around a hole to gain entrance or food.
- Droppings. These are up to 1/8 to 1/4″ long, rod-shaped, and have pointed ends.
- Tracks/footprints. Front foot is 4-towed and print is left in front of the 5-towed hind print.
- Rub marks. These are dirty streaks left on vertical surfaces along runways.
- Runways. They frequently use the same paths, usually along walls and stacked materials.
- Damaged goods. Mice prefer seeds and cereals, but will readily feed on insects.
Where do they nest? Mice like to nest in soft, warm places. They will nest in wall or attic insulation, storage boxes or drawers containing paper or clothing, in unused upholstered furniture or bedding, etc. They will harvest soft materials such as shredded paper, cloth, cotton, etc. to line their nest area that is located in an undisturbed area or void.
Why are house mice dangerous?
Diseases are spread through their droppings, urine, and saliva (on their dander) which can become airborne. The most threatening disease is Salmonella, a cause of food poisoning spread via their droppings. Then there is infectious jaundice/leptospirosis/Weil's disease via their urine in water, plague and murine typhus via fleas, etc.
In addition, mice can spread airborne allergens that contribute to respiratory problems such as asthma. Back in 2004, 82% of urban, suburban, and rural US homes were found to contain mouse allergens (June issue of the Journal of Allergy and Clinical Immunology).
Finally, mice gnaw/chew because their incisor teeth continually grow and must be kept worn down. Unfortunately, they often gnaw on electrical wires causing shorts, which sometimes result in fires.
- Mice are opportunistic feeders and nibblers. There are 2 main feeding times, at dusk and just before dawn, with many mini feeding times in-between.
- Droppings are left wherever they spend time or travel. They also leave droplets of urine wherever they travel or spend time.
- Preferred nesting sites are dark and secluded with abundant nesting materials.
- They can squeeze through an opening of only 1/4″ to gain entry.
- Yard attractions.
- Keep grass mowed to less than 3″ in height. Higher provides harborage and seeds are mouse food.
- Eliminate clutter. Clutter is attractive mouse harborage.
- Create a 1-foot vegetation-free gap between the structure's wall and any vegetation.
- Eliminate bird feeders. Bird seed is very attractive to mice.
- Exclusion. Seal all gaps/openings on outside walls that will allow entry, any opening ¼″ or larger. As long as you're at it, it's best to seal so that no light can escape to the outside at night. Then you are also preventing the entry of overwintering insects such as stink bugs, cluster flies, etc.
- Trapping. Correct location and placement are key to their effectiveness. It involves using:
- Snap traps.
- Glue boards.
- Multiple-catch or live traps.
- Rodenticides. Routine use of rodenticides should be avoided. Why?
- Exposed rodents die in all kinds of places including inaccessible floor/wall voids, creating odors and attracting flies and dermestid/carpet beetles to feed on the carcass. All of these are objectionable to home occupants and hard to correct.
- Rodents tend to carry off bait and hoard/cash it. This stored bait is attractive to stored product pests, creating future problems from inaccessible areas.
- It does not compete well with human foods that are often abundant in homes.
- They can cause serious problems for non-targets such as pets and possibly infants.
Unfortunately, the US EPA has recently eliminated the use of all rodenticides for the control of all rodents except Norway rats, roof rats, and house mice. That is, at present, no rodenticides are labeled for or can legally be use for the control of deer mice (vectors of hantavirus/HPS) and white-footed mice (disease reservoir of Lyme disease; ticks vector it to humans for mice). A fine kettle of fish we find ourselves in these days!
Professional help. You can certainly try to solve the mouse problem yourself. If you're not successful or need help, then call a pest management professional. They will start by doing a thorough inspection to find and then advise you about attractive situations outside, where entry points exist and how to eliminate them, determine where the activity is occurring and why, and then devise/create a program to eliminate the mice based on their inspection and their knowledge of mouse biology and habits.
-Eric H. Smith, PhD, BCE