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Rodent management requires an integrated approach combining multiple tactics. Start with a detailed inspection to determine the source of the problem and to accurately identify the rodent
Determine why rodents are present, where they nest, and how they access the building.
Rats and mice prefer to be active in dark, protected places, especially when no one in around. Your inspection should include all areas low, such as under furniture and appliances, to high, such as inside drop ceilings and on top of pipes. Look for droppings, gnaw marks, footprints, and sebum marks (an oily, brown substance that accumulates on pathways that rodents use frequently). It might be necessary to conduct inspections at night when rodents are active. This involves sitting quietly to watch rodent activity patterns or using wildlife cameras.
Rodents don’t need large openings. Rats can squeeze through a hole 3/4 inches wide (about the size of a quarter) and under a gap 1/2 inch tall. Mice can enter a hole 3/8-inch-wide (size of a dime) and under a gap 1/4 inch tall (the thickness of a #2 pencil). Look for openings that allow rodents to enter or move through your home. Frequently used entry points will have sebum marks.
Food and moisture
Rodents often enter a building to find food, water, or shelter. When inspecting your living space, think about how food and water might be available. Is food stored in a chew-proof container or location? Are spills cleaned up, even in hard-to-reach areas like the sides of the stove or under its lid? Look in the back of your pantry for food spillage, or signs that a leaky faucet is providing water.
Rodents, especially house mice, may build a nest indoors near a food source. They prefer areas that are dark, undisturbed, and warm, like under cabinets, next to or inside stove walls, and near appliance motors (ex. refrigerator).
Droppings provide important information about rodent activity. However, there is a lot of room for misinterpretation when conducting an inspection.
- Number of Droppings: the sudden appearance of a large number of droppings can be alarming, but doesn’t prove you have a full-blown infestation. According to some estimates, rats can produce 40 to 50 droppings per day, while mice can create between 50 and 60. Droppings may be dispersed as cues to navigate a new area, or to identify a food source.
- Size of Droppings: taking a close look at droppings can help determine if your rodent problem is an individual, or a reproducing population. While there is some variation in dropping size from one rodent, finding large and very small droppings next to each other can indicate that you have both adult and juvenile rodents.
- Dropping Color: rodent baits contain a dye that changes the color of droppings. If you are using rodent bait but do not see a change in color of droppings, this could mean they're not eating it. Droppings that show the color of the bait, suggest the rodent has eaten a lethal dose and may already be dead.
Safe Cleanup of Droppings
Removal of droppings is an important part of management. First, because rodents use droppings to communicate about resources, removing them—and cleaning surfaces—can eliminate these cues. In addition, appearance of new droppings after cleanup indicates that control methods aren’t yet working. The Centers for Disease Control and Prevention (CDC) provides clean up recommendations that reduce the risk of exposure to rodent pathogens.
Pro Tip - Non-Toxic Monitoring
Rodent baits are available without a rodent-killing chemical, but instead have a product that makes droppings and urine fluoresce under UV light. Monitoring with non-toxic blocks can provide information about rodent movement. For example, if non-toxic blocks are used on the exterior of the building, and fluorescent droppings are found indoors, this indicates that an entry point is present. Similarly, if non-toxic blocks are used in the basement, but fluorescent droppings are found on the second floor, the rodents are moving up vertically within the building.
In general, rodent droppings are cylindrical, with pointed to rounded edges. They are typically black, but can differ in color based on food that was eaten. For example, green or red droppings may indicate that the rodent fed on a rodenticide bait. There are no lines on rodent droppings (compared to the vertical lines found on American cockroach droppings). Mouse droppings are typically 1/8 to 1/4 inches long, Norway rat droppings are 3/4 to 1 inch long, and roof rat droppings are about 1/2 inch in length. The CDC offers a pictorial key to identifying droppings that can help determine which pest is present.
Rodents may leave footprints when walking through dirt or dust. Mouse footprints often look like a collection of dots, and the length of the actual footprint is 3/8 to 3/4 inches long.
Rat footprints look like an actual foot, measuring from 3/4 to 1 ¼ inches in length, depending on whether it’s the front or back paw. Adult rats may also leave a ‘tail drag,’ a wavy line between the paw prints.
Impressions left behind after a rodent chews on something are often easy to see. For a pair of teeth (two grooves), gnaw marks from an adult mouse measure 1 to 2 millimeters wide, while adult rat gnaw marks are 3.5 to 4 millimeters wide.
You’ll want to reduce the number of rodents quickly. Trapping and baiting are two options, each with their own benefits. Some key considerations:
Rats exhibit this behavior, which is a “fear of new.” To overcome this fear, put out traps with plenty of bait, but do not set the trap. Eventually, most rats will approach the trap and take bait. This should be repeated until rats have taken bait about three times. Then, a minimal amount of bait should be used and the trap should be set/deployed. If a rat attempts to feed on the bait but is not caught after “pre-baiting,” this can complicate control. See Capturing Smart Rats Requires Common Sense and New Ideas for tips on catching these rats. NOTE: Pre-baiting may help to improve trap capture of mice too.
This can occur when snap traps are left in the same place for a long period of time. Rodents, especially mice, become accustomed to the trap and do not explore, or actively avoid it. If you have an active rodent infestation but traps are not catching anything, remove traps for a few days, then replace them in new areas that might have activity.
A benefit of trapping is knowing where a rodent has died, and avoiding decomposition in hard-to-reach places and secondary pest problems associated with carcasses. In addition, traps can be placed systematically (evenly spaced at first) to show high activity areas including entry points and harborage. Numerous trap types are available, and some traps perform better than others in certain setting.
- Rat vs. Mouse Traps: traps for rats and mice are different, and selecting the correct trap is important. Rat traps may be too slow to kill a mouse, whereas a mousetrap will not inflict enough force to kill a rat. Identify your target pest to select the correct trap.
- Traps differ in the ease of setting, disposal, cost, reusability, and other factors. These details were reviewed for the homeowner market in a 2018 article by WireCutter: The Best Mousetrap.
How to Set a Snap Trap
Tips for Using Snap Traps
These play an important role in monitoring insect pests. However, they are not ideal for rodent management for a few reasons. First, adult rodents have specialized guard hairs that are used to detect changes in ground texture. These hairs alert adult rodents to the presence of a surface that could trap them, such as mud in nature. Because of this adaptation, glue traps primarily catch juvenile rodents that do not have fully formed guard hairs. A second consideration with glue traps is that some rodents can escape by pulling off fur or urinating and making the glue less tacky. A final concern is the humaneness of this control tactic, as animals expire from exposure to environmental conditions, starvation or stress.
Trap Selection and Placement
Trapping provides useful information about the rodent population, such as accurate identification, described in What do Mice and Rats Look Like? Knowing the mouse species, for example, can tell you if they are primarily living indoors (house mouse) or if they enter the home from exterior entry points (white-footed and deer mouse) such as high up on a building where electrical wires connect, or where tree branches provide a bridge. Rodent age can provide insight about the proximity of the nest, because pups (juvenile rodents) don’t travel far from the breeding site. Finally, the sex of rodents is important to determine if the problem is ongoing. If captures are consistently male, but not an adult female, it’s possible you are not capturing the breeding female. You may change your management program by switching from a food bait to a nest-material bait such as string, dental floss or fabric/cotton. You can distinguish between males and females by looking at the distance between the genitals. Male mice have a large distance, while female mice have a short distance.
In general, use of baits requires less effort than trapping because it can target more than one rodent at a time. Baits are used extensively by the pest management industry and some are available to the public. However, baits are pesticides, and thus pose some level of risk. Baits must be applied according to the label, often in a tamper-resistant station that prevents removal of the bait by children or pets. Loose bait should never be placed in areas where children or pets can access the poison. Other considerations include:
A common misconception about baits is that rodents will actively search them out as a food source. However, if baits (or traps) are placed in areas where rodents are not active, they are unlikely to interact with the device. Therefore, information gained during the inspection should be used to place bait in active rodent runways to increase success.
If rodents have access to fresh produce, high protein items, or other appealing food sources, they are unlikely to feed on a bait. This highlights the importance of sanitation to remove other food options.
Professionals have more formulation options than homeowners due to concerns about the safety of applications. No matter what product is selected, it’s important to read and follow label instructions to determine how that product can and should be used most effectively.
Cryptic (unseen) Death
Feeding on baits does not cause rodents to leave the home in search of water. Instead, they tire and look for protected areas. Their death inside walls and other voids, produces a smell that can last for days (mice) to weeks (rats). Dead rodents that are not removed can attract later-stage decomposers such as carpet beetles and clothes moths, which will feed on the hair and skin of rodent carcasses and eventually your wool rugs and clothing.
Rodent baits (rodenticides) can harm non-target animals by exposure to bait or active ingredients. For example, predators such as raptors (birds of prey) and coyotes have experienced health problems, including death, from the presence of rodenticides in their blood, likely from feeding on poisoned rodents. Children and pets can also be exposed to rodenticides.
When a group of organisms is constantly exposed to a pesticide, natural variation in the population can give rise to resistance and reduce effectiveness. This emphasizes the need for an integrated approach to rodent management that relies on more than baiting.
In 2020, the state of California approved a ban on the use of common rodenticides (with a few exceptions) due to documented effects on wildlife and larger ecological concerns. This law sets a precedent for other states to follow, but the ban’s implications on rodent populations and consequent effects on rodent-borne pathogens are not known. See this 2019 article.
Reducing the number of rodents is important, but long-term prevention with a good management plan is the ultimate goal. This is achieved through sanitation to remove food and water sources (mentioned above) and exclusion to prevent new rodents from entering the building.
Sanitation, also referred to as housekeeping, eliminates sources of food and water and makes the site less attractive. Rodents aren’t fussy and will eat packaged food, spills in hard to clean areas, pet food left in bowls, discarded food items, compost, and even pet feces. Water sources can be condensation from motors (refrigerator) and water pipes, leaks from poor pipe connections, puddles of any kind, or natural water sources.
Seal openings that allow pests to enter or move from room to room. There are numerous considerations when performing exclusion, including pest pressure (the attractiveness of the area and the likelihood that rodents are present), opening size, and what exclusion material to use. Visit the Scientific Coalition On Pest Exclusion’s website for more details.
- The SCOPE Exterior Inspection Instruction Form provides details on how to conduct an inspection to identify entry points, and is used in combination with the Exterior Inspection Form.
- The General Inspection Form can be used to identify not only exclusion issues, but also sanitation concerns. The benefit of this form is that observations can be ranked and prioritized depending on their relative contribution to the overall pest problem at a site.
How Do I Keep Rodents Out?
- Monitoring & Record Keeping: The final part of a rodent management program is to keep records on the success of your efforts—what worked and what didn’t as evidenced by rodent captures. Use our Rodent Monitoring Log. This can help you to interpret the issue and determine if your efforts are successful.