The house mouse is the most successful rodent in adapting to life with people. It's found almost everywhere people are, feeding on human food, sheltering in human structures and reproducing at a rapid rate. The house mouse is the most troublesome and economically important vertebrate pest, contaminating millions of dollars worth of food, damaging property and causing electrical fires with its constant gnawing. Mice may enter a building from the outside and spread through a structure along pipes, cables and ducts. Although large numbers can build up in food service areas or trash rooms, one or a few mice can survive practically anywhere.
Many control failures against house mice are due to a lack of understanding of mouse biology and habits. A pair of mice can produce 50 offspring in one year. Because they seek food over a range of only 10 to 30 feet, traps, glue boards and bait must be placed close to the nest to be effective. Remember that good inspections are critical for successful mouse control.
Sounds - Sounds are common at night where large numbers of mice are present; listen for squeaks, scrambling and sounds of gnawing.
Droppings - A house mouse produces many droppings per day; mouse droppings are frequently the first evidence that mice are present. However, be aware that large cockroaches and bats may produce droppings similar to house mice. Look along runways, by food near shelters and in other places mice may frequent.
Urine - House mice occasionally make small mounds known as "urinating pillars." These consist of a combination of grease, urine and dirt and may become quite conspicuous. Look for many small drops of urine using a blacklight. Urine stains will fluoresce under ultraviolet light. (Mouse urine spots are not as easy to detect as those made by rats.)
Grease marks - Like rats, mice produce greasy smears where dirt and oil from their fur mark pipes and beams.
Runways - Most mouse runways are indistinct trails free of dust and are not readily detectable.
Tracks - Look for footprints or tail marks on dusty surfaces or on mud; use a nontoxic tracking dust (like talc) to help locate mice within buildings.
Gnawing damage - Newly-gnawed areas on wood are light in color, turning darker with age. Look for small tooth marks and enlarged cracks beneath doors. Mice make wood chips with a consistency like coarse sawdust around baseboards, doors, basement windows and frames and kitchen cabinets.
Visual sightings - Mice are often active in daylight and this may not indicate a high population (as it does with rats). Use a powerful flashlight or spotlight at night to confirm mouse presence.
Nest Sites - Inspect garages, attics, basements, closets and other storage places for evidence of nests. Be alert to fine shredded paper or similar materials; these are common nest-building materials.
Mouse Odors - Mice produce a characteristic musky odor.
Estimating Numbers of Mice
The number of mice observed or food consumed is not reliable as a census technique with mice. Unlike rats (which may travel widely within a building leaving tracks on many patches of dust) mice do not range widely.
- Read natural signs such as droppings, urine stains, tracks and damage.
- Make nontoxic tracking patches of talc at 20- to 30-foot intervals throughout a building. The more tracks seen in each patch and the more patches showing tracks, the larger the population is. The percentage of patches showing tracks will reflect the extent of the local infestation.
- Tracking patches are also an excellent means to evaluate a control operation. Compare the number of tracks or patches with mouse tracks before and after a control program.
- Control and Management
Control and prevention of mice is a three-part process, which includes sanitation, mouse-proofing and population reduction with traps or baits. Sanitation and mouse-proofing will help prevent mice from entering buildings. When a mouse population already exists, some kind of lethal control is necessary. Otherwise, mice, which reproduce rapidly and can find food almost anywhere, will continue to be a problem.
Habitat and Harborage Reduction
Sanitation - Good sanitation makes it easier to detect signs of mouse infestation. It also increases the effectiveness of baits and traps by reducing available food. However, the best sanitation will not eliminate mice; they require very little space and small amounts of food to survive and reproduce.
- Store bulk foods in mouse-proof containers or rooms. In storerooms, stack packaged foods in orderly rows on pallets so that they can be inspected easily. A family of mice can live in a pallet of food without ever having to leave the immediate area.
- Keep stored materials away from walls and off the floor. A 12-18 inch yellow or white painted band next to the wall in storage areas permits easier detection of mouse droppings. This band and the areas around pallets should be swept often so that new droppings can be detected quickly.
Mouse-Proofing - Completely mouse-proofing a building is difficult because mice are reported to be able to squeeze through an opening as little as 1/4 inch high. To mouse-proof a building:
- Seal large holes to limit the movement of mice into and through a building.
- Plug holes in foundation walls with steel wool or copper mesh.
- Caulk and fit doors and windows tightly.
- Seal holes around pipes, utility lines, vents, etc., to make it difficult for mice to move in and out of wall and ceiling voids. (This limits mice to a smaller area and may make snap traps and glue boards more effective.)
- Do not prop open kitchen doors; install screen doors wherever possible.
Snap Traps If used correctly, snap traps are very effective in controlling mice. They must be set in the right places, in high numbers and in the right position or mice will miss them entirely. Always place traps in areas that are inaccessible to students. Here are some tips to keep in mind when trapping mice:
- Remember that mice rarely go further than 30 feet from the nest, only 10 feet in most cases. If mice are sighted throughout a building, it means that there are many locations where you will have to set traps. Place snap traps not only wherever you see obvious signs of mice, but also in a three-dimensional sphere about ten feet in diameter around those signs. (Mice are good climbers.)
- Mice can be living above their main food supply in suspended ceilings, attics, inside vertical pipe runs and on top of walk-in coolers. Or they can be below, in floor voids, crawl spaces, or under coolers or other equipment.
- The best sites are those with large numbers of droppings since that means the mice are spending a lot of time there. Other good sites are along walls, behind objects, and in dark corners, particularly where runways narrow down, funneling the mice into a limited area.
- Successful trapping requires good mouse baits. Peanut butter, bacon, cereal and nuts are attractive to mice. Food baits must be fresh to be effective. Another bait is a cotton ball, which the female mice like to use for nest material. It must be tied securely to the trigger.
- Two or more traps placed next to each other will capture more mice than single traps.
- Probably the biggest mistake made in mouse trapping is not using enough traps. Use enough to quickly eliminate the mice.
- Great care must be taken to place traps out of the public view and to check them regularly.
- Mice can carry several diseases, so technicians should wash their hands after handling traps or other items that come in contact with mouse urine and feces. Use disposable latex gloves or tongs to handle dead mice. A bleach/water solution of at least three tablespoons household beach per gallon can be used to sanitize traps.
Multiple-catch mouse traps catch up to 15 mice without being reset. Some brands are called "windup" traps; the windup mechanism kicks mice into the trap. Others use a treadle door. Live mice must be humanely killed.
Mice are curious and like to investigate new things. They enter the small entrance hole in the trap without hesitation. Odor plays a role too; traps that smell "mousy" catch more mice. Place a small dab of peanut butter inside the tunnel entrance to improve the catch.
- Mice are captured alive but may die in a day or two; dead mice may cause odors or attract insects. Some traps have a clear plastic end plate or lid so you can see if any have been captured.
- Check traps frequently; mice can get hung up in the mechanism and render the trap inoperative.
- Place the traps directly against a wall or object with the opening parallel to the runway, or point the tunnel hole toward the wall, leaving one or two inches of space between the trap and the wall.
- If mice are active, place many traps 6-10 feet apart. After the mouse infestation is eliminated, maintenance traps may be placed where mice have been numerous before. Additionally, traps can also be placed at potential entry points such as storerooms, loading docks, near utility lines and at doorways.
Glue boards can be effective when other methods have failed against a "bait-shy" mouse or when food is abundant. As with other traps, placement is the key. Locations that are good for other types of traps are good sites for glue boards.
- Place glue boards in hidden locations away from areas where staff can view them. (One method is to place the glue board inside a tamper-resistant bait station.)
- Use the larger "rat-size" glue boards, which are more difficult for mice to escape from.
- Do not put glue boards directly above food products or in food preparation areas.
- Set glue boards lengthwise and flush against a wall, box, or other object that edges a runway.
- Move objects around; create new, narrow runways six inches wide to increase the effectiveness of glue boards.
- Put peanut butter or a cotton ball in the center of the board.
- Place the glue boards 5 to 10 feet apart in infested areas (closer if the population is large).
- If no mice are captured in three days, move the boards to new locations.
- If a trapped mouse is alive, kill it humanely before disposal. Replace the boards if they become covered with dust. Glue boards do not work well in cold areas.
"Building out" mice and trapping are the most effective control methods. Rodent baits should be used only in emergency situations to supplement these methods. If there is a repeated need to use baits, it is likely that sanitation and mouse-proofing should be improved. Remember that rodent baits are poisons. Additionally, use of baits in schools represents special problems because of incidents where students have moved or tampered with rodent baits. In schools, baits should be used only after nonchemical control measures have been instituted.
Children, pets, wildlife and domestic animals must be protected by putting the bait in tamper-resistant bait boxes in inaccessible locations. Using baits alone will not provide long-term control of mice.
- Apply bait at several locations rather than relying on a few large placements.
- Use fresh baits labeled for mouse control. (Never store baits with other pesticides; mice can detect tiny amounts of repellant chemicals that may cause mice to reject the bait.)
- Place the baits in favorite feeding and nesting sites as determined by large numbers of droppings.
- Place the baits between hiding places and food, up against a wall or object to intercept the mice.
- Make bait placements 10 feet apart or closer in infested areas if they can be adequately secured.
- If bait is refused, try switching to a different type and replace the baits often.
- Use small bait stations which are more attractive to mice than the larger rat-type stations.
- Make sure that sanitation is such that no other food is readily available to mice.