White-footed and deer mice (Peromyscus) tend to nest close to a food source by building in secluded ground-level cavities made with outdoor debris and abundantly lined with fur, feathers, or shredded cloth. They forage for food outdoors and indoors if there is an entry of at least ¼ inch. Over time, a successful ‘family’ will move and create other nests when their old one becomes unusable due to their waste. They store gathered food nearby in a type of larder. Seeds and grain are high on their list, but they will eat almost anything. Peromyscus reach sexual maturity in seven to eight weeks. Females gestate for three weeks, and their litters of three to five wean as early as two or three weeks later. In the northeast, females typically produce two to four litters per year (possibly more in warm climates).
White-footed mice and deer mice are difficult to tell apart but it’s not necessary as their habits are the same. Adults are 3–4 inches (not including their tails), and reddish-brown or gray with white undersides—a color pattern similar to deer. They have prominent (larger-than-house-mouse) black eyes. Because they’re active at night, you will likely see their signs more than you see them. Look for gnaw marks, damaged goods, urine stains, droppings, and smudges—dark grease stains left where their oily fur rubs along routes of repeated traffic on woodwork or cement. Droppings are small (1/8 to ¼ inch long) with pointed ends. (Rat droppings are 1/2 to ¾ inch long with blunt ends.)
You don’t want them thriving in or near your home because of contamination, risk of allergic reactions by members of your household, and because Peromyscus are involved in diseases that affect humans: multiple tick-borne diseases, including Lyme disease, and hantavirus. Lyme disease is a bacterial disease carried by mice and spread to other mammals by the blacklegged tick (Ixodes scapularis). Mice are carriers of the disease, and carriers of ticks, but don’t directly infect humans. Hantavirus, however, can be carried in excrement and saliva of rodents, and spreads by inhalation of dust where mice have lived and fed. Droppings often cause allergic reactions. In an outdoor setting, Peromyscus can be controlled by predators like fox, snakes, and large birds, but when they establish near homes—away from predators—their populations grow quickly.
They’ve likely found a food source in or near your home, and a conveniently protected nest site.
The answer is always exclusion and sanitation. Eliminate harborage, store food sources in rodent proof containers, and keep up with sanitation such as removing crumbs and food or grease residue. Here are the basic steps:
Find their entryways.
Look for outdoor paths such as small, well-worn trails along a building or through mulch, and smudge marks showing active entryways against or up the sides of buildings. Yes. Don’t forget to look up. Mice are good climbers. Smudges show routes of repeated travel. Do everything you can to block entryways by improving door seals and weather-stripping. If you find holes (they can be as small as a half inch) fill them with stainless steel mesh; additionally, add cement if needed to holes and gaps found in foundations. Look for gaps and holes around utility lines inside the home, including the basement and attic. Close gaps with hardware cloth or improved escutcheon plates (metal flanges) around holes in walls used for utilities like water lines. Scouting for gaps and holes and repairing them is a crucial step in reducing rodent access, and reduces insect pests as well.
Next, use trapping.
Snap traps vary. Find the ones that work best for you because they remain the best no-chemical way to efficiently kill rodents. Keep traps out of reach of pets and children—consider placing traps inside pieces of PVC pipe, or empty coffee cans, for instance, but place traps along those travel routes where scouting has shown activity: primarily along walls or utility lines. Peanut butter remains at the top of the bait list but if someone in your home has an allergy, try securing fibers coated with other fats like butter or bacon grease instead. Mice use fiber (yarn or strips of fabric) for nesting, and tugging on the fiber or string will set off the trap. Check traps daily and remove the carcass or the entire trap. But don’t stop trapping.
While rodenticides are an available option, they raise other concerns. Where will the poisoned rodent die, and what if other animals or birds ingest the poisoned rodent? Always read the label or consult a pest professional.
Don’t forget sanitation.
No amount of trapping will stop a mouse problem if there is a continued food supply luring them into your home. This is a two-fold process. First, proper food storage is key. Pest-resistant packaging such as very sturdy plastic containers or metal containers are much more useful in cupboards and pantries to prevent mice from feeding. Don’t forget to properly store pet food and bird seed. Secondly, sanitation. Mice (and cockroaches and ants) need very little to tempt them. Crumbs, dirty dishes, grease or sugar residues must be cleaned away to reduce future re-infestation. Pests easily discover scent trails leading to food sources used by prior generations. Clean garbage cans, and don’t forget those hard-to-reach places behind big appliances.
Be persistent. Exclude, trap, and clean. And repeat.