About Rats and Mice - MSPCA-Angell

Mice and rats are widespread throughout North America. They both breed year-round, with mice having about eight litters of four to seven young annually, and rats producing about twenty young each year. Both rats and mice are nocturnal and can enter dwellings through tiny spaces — mice can squeeze through holes the size of a dime and rats through holes the size of a quarter.

While often thought of as “pests,” mice and rats, like all animals, have rich social and emotional lives. Multiple studies, for example, have demonstrated empathy in rats, and other studies have revealed both rats and mice to be intelligent creatures. That said, it is understandable that people do not want to share their spaces with mice and rats. It’s critical to note, however, that killing mice and rats is not a humane or effective solution. Doing so merely opens up vacant territory for another animal to move in. Instead, the best solution to mice and rat conflicts is prevention and exclusion.

Two kinds of mice are most likely to cause problems for homeowners: native mice including the white-footed mouse and deer mouse (more common in rural areas), and the house mouse (anywhere there are buildings). House mice prefer to live in comfortable areas between walls, behind appliances, and in unused drawers. Often the only sign of them is their small droppings (the size and shape of grains of rice) or any gnawed food items. Native mice will often seek the shelter of homes in early fall or winter. All mice are omnivorous, preferring grains and seeds, and can live without water if the food they eat is moist.

As with mice, two kinds of rats most commonly come into conflict with people: the Norway, or brown rat, and the black, or roof rat. Norway rats are common in dense areas of human settlement, and live in eighteen-inch-deep burrows underground or in lumber piles or similar environments. In the United States, black rats are most often found in coastal areas of the South, Southeast, and West. Black rats are excellent climbers and live higher off of the ground, usually in the upper levels of buildings or nesting in trees or vines.

Rats are good swimmers and jumpers, capable of leaping three vertical and four horizontal feet. Like mice, rats are omnivorous, eating a variety of plant and animal foods. Unlike mice, however, rats do need access to a water supply. Signs of rats in and around your home can include one-half to three-quarter-inch droppings, gnawed holes in baseboards or door frames, and the presence of burrows (although these can be confused with burrows of squirrels and chipmunks).

Possible Conflicts & Solutions

The most common problems resulting from rats and mice are contamination of human food sources from urine and feces, and possible gnawing damage to electrical wiring. With both mouse and rat conflicts, it is important to locate and eliminate their food and shelter sources, and then permanently exclude them.
Following are some suggestions, and visit the MSPCA’s Intruder Excluder for more tips:

Clean kitchen areas well, clean up spills quickly, and store food in the refrigerator or in sealed metal, glass, or heavy plastic containers. A diluted bleach solution will get rid of any scent trails that are present.
If possible, human or pet food should not be stored outside, and if it is in a garage or basement it should be placed in sturdy plastic, glass, or metal containers. Always feed pets indoors and thoroughly clean up the area afterward.
Trimming and clearing away brush and debris within 18 inches of house or building foundations can help eliminate protective cover and expose the animals’ burrows and entry points. To find entry points for mice inside, sprinkle powder along the perimeters of walls. This will show where there is mouse activity and where exclusion is needed.
Eliminate indoor mouse nesting areas, such as old clothing, books, or papers in bags or boxes. Store them in plastic containers.
The entryways that rats and mice use must be sealed for permanent exclusion from dwellings. To avoid trapping the animals in your home as you seal it, try to deter them before beginning any exclusion work. This is especially important if their entryway does not lead outside but goes into a wall. The most effective deterrents are cleanliness, placing cat hair around entryways, and moistening rags with pure peppermint oil (a natural repellent to mice and rats). To exclude mice, find their entryways (making sure to check in hard-to-reach places like behind the dishwasher and stove, under the sink and cabinets, near where utility pipes and wires lead into the house, and cracks in the foundation) and seal them. Steel wool, copper wire mesh, or quick-drying cement works well for smaller openings. For larger openings, balling and stuffing galvanized window screening and covering it with caulking or cement can do the trick. Expanding foam insulation is often also effective. Attach rubber or metal runners at the bottom of doors if that is where the mice are gaining access.
After cleaning areas well and removing old woodpiles, ground cover, and trash, rats can be excluded using heavy 1/4-inch hardware cloth or heavy-gauge screening. Check all accessible areas such as heating vents and the openings where electrical or utility lines enter a building. Indoor holes in walls and floors can be sealed with caulking and foam sealants; however, rats can chew through these materials, so they need to be combined with copper wire mesh or aluminum window screening. If a rat has an obvious exit-way, place food outside of it for them, and seal the hole after the animals have left. When rat infestation is a problem around buildings, blocking the foundation with a hardware cloth or a concrete L-shaped footer can prevent burrowing. Be sure to bury the footer at least a foot deep and extend it at a 90 degree angle outward for another foot.
Live trapping is another option for eliminating mice and rats, though it should be used only as a last resort. Trapping can break up family groups, trapped-and-relocated animals can find it hard to survive in new surroundings, and, unless conditions are made less appealing, new animals will simply move into the home or building to take the place of those caught in traps. If all other efforts to deter mice or rats fail, humane box traps will catch mice without harm and enable them to be released elsewhere. (Note that relocating wildlife is illegal in Massachusetts, so you can only release the animals on your own property).
Only trap and release mice in warm weather, and release them in an area close by the home or building where they were caught. This increases their chances of surviving both the elements and unfamiliar territory. Traps should be placed in areas where mice are present and close to walls since mice like to travel along a wall or barrier. The traps can be baited with appealing substances such as peanut butter, popcorn, or sunflower seeds. Traps can be cleaned with a mild bleach solution (1:30 ratio of bleach to water) and used again.

As with most conflicts with animals in buildings, remember that unless proper exclusion, habitat management, and sanitary measures are taken, killing rats and mice will simply be an endless cycle as new animals move in to occupy vacant territory. Poisons and glue traps are especially inhumane, cause intense suffering of target and non-target animals, and should not be used. Rodenticides are also dangerous to companion animals and wildlife. In just the past few years, here in Massachusetts, rodenticides have caused the deaths of family pets as well as bald eagles, owls, hawks, and other wild animals, and legislation to better regulate use of these poisons is pending in the Massachusetts Legislature.

Public Health Concerns

Both mice and rats can carry diseases that are transmittable to humans, such as hantavirus and salmonellosis. It is important to clean areas with a mild bleach solution that have come into contact with mouse or rat droppings and to see a doctor if ever bitten by a rat.

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