by David Moore
Manager of Technical Services and Board Certified Entomologist
with contributions by Eric Smith, PhD, BCE
This blog is about the house mouse, not about the deer mouse and others typically found outdoors. Few critters elicit 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 their pets, and contaminate many areas with their saliva, urine, and feces.
IDENTIFICATION of house mouse (Mus musculus). They are small and slender, lightweight, 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. The deer mouse. They are very similar to the house mouse except that 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 is 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-toed and print is left in front of the 5-toed 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?
They can be much more than just a nuisance. Diseases can be 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. Find how the mouse got in and seal them out. During the day (mice are nocturnal or active at night) try to find how the mouse got inside; look around the outside of your house from the outside.The most common ways are via a door threshold (gap area on the bottom of a door) and around holes where utility lines come into the house. If you have a crawl space, then add around the crawl door and around vents. 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.
- Close the entryway(s):
- Door threshold. Install or have installed a new, tight-fitting threshold or a door sweep. Verify tightness by making sure that no light escapes to the outside at night.
- Utility line entrances. Tightly seal the gap with concrete, 1/8″ hardware cloth, or sheet metal.
- Carefully clean up or remove all of the droppings present:
- First, put on a pair of protective rubber/latex gloved.
- Then, lightly spray the droppings with Lysol.
- Pick them up with a paper towel slightly dampened with Lysol and dispose of them in a plastic bag.
- Put the bag in the outside trash.
- Did it work? Check the next few days for new droppings, those dark pellet-like things. If you find more, look again around the outside of your house for entryways. If you’re sure you found how it got in, then more droppings probably mean that it has already taken up residence in your home. Time to kill!
- Close the entryway(s):
- Trapping. Correct location and placement are key to their effectiveness. It involves using:
- Snap traps. These are the most efficient.
- Bait the trigger with what the mouse is feeding on (in this case, Cheerios) or with nesting material (a small amount of cotton tied to the trigger with sewing thread).
- Place with the trigger end against the wall where you see it running. Place out at least 4-6 snap traps.
- Available in stores like Lowe’s are self-contained traps. Follow the directions.
- Glue boards. Potential problems with children and pets getting stuck in them.
- Multiple-catch or live traps.
- Snap traps. These are the most efficient.
- 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 dermestidae/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 concerns and sometimes problems for non-targets such as pets and possibly infants.
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.