Rats & Mice

There are six major problems caused by rats and mice:

1. They eat food and contaminate it with urine and excrement.
2. They gnaw into materials such as paper, books, wood or upholstery which they use as nest material. They also gnaw plastic, cinder blocks, soft metals such as lead and aluminum, and wiring which may cause a fire hazard.
3. Rats occasionally bite people and may kill small animals.
4. They, or the parasites they carry, (such as fleas, mites and worms) spread many diseases.
5. Rats can damage ornamental plants by burrowing among the roots, or feeding on new growth or twigs. They also eat some garden vegetables, such as corn and squash.
6. Rats and mice are socially unacceptable. These rodents have been a problem for centuries, chiefly because they have an incredible ability to survive and are so difficult to eliminate. In addition, they are extremely compatible with human behavior and needs.

Mighty Mouse and Super Rat!

Here are just a few of the abilities that have enabled rats and mice to survive people's constant attempts to eliminate them:

A female rat can have up to 84 young in her life span, which averages about a year in the wild. They can burrow long distances from nest to food sources, reducing their exposure to predators. The tunnels may extend 4 vertical feet into the earth. They can scale walls and walk across telephone wires with ease. They are excellent swimmers--capable of navigating a half mile through open water. They are amazingly resilient, easily surviving falls up to 50 feet.

Rats - Description:

There are two primary species of rats present in the Pacific Northwest: The Norway rat and the roof rat. The Norway rat is both larger and heavier than the roof rat. It has a wider distribution and is usually more common, although the roof rat may be abundant in some localities, usually near coastal areas. Norway rats build their nests in burrows under buildings, low shrubs or ground cover, wood piles, yard accumulations of junk, and garbage dumps. The roof rat, on the other hand, is a better climber than the Norway rat and is more likely to build its nest in walls, attics, vines or trees.

Norway rat (Rattus norvegicus) - is 13-18 1/2 inches total length, with its tail being shorter than its head and body combined. An adult Norway rat weighs about 3/4 to 1-1/4 pounds. It is mostly brown, with a lighter colored stomach. The tail is semi-naked and darker above than below, giving it a two-toned effect.

Roof rat (Rattus rattus) - is also 13-18 1/2 inches total length, with its tail being longer than its head and body combined. An adult roof rat weighs about 3/4 to 2/3 pound. It is mostly black with some gray below, although there are some variations. The tail is also semi-naked, but of one color.

Mice - Description - We also have two species of mice that cause problems in the Pacific Northwest. The House Mouse and the Deer Mouse.

House mouse (Mus musculus) - The house mouse looks somewhat like a young roof rat, but smaller. It is approximately 5-1/2 to 7-1/2 inches total length. Like the roof rat, its tail is as long or longer than the head and body combined. However, mice have proportionately smaller heads and feet than those of a roof rat. The color of the house mouse depends upon its habitat; if it lives indoors it will usually be dark gray with a light gray stomach; outdoors it will usually be a sandy brown color. House mice do not pose as serious a problem to the householder as rats, but they can be quite a nuisance. They also eat and contaminate food with their urine and droppings; may gnaw on wiring creating a fire hazard, and they can transmit some diseases. Spread of diseases by mice, however, is not considered a serious health hazard in our area.

Deer Mouse (Peromyscus maniculatus) - This wide-spread, native rodent is another medium-size mouse, averaging 7 inches total length. The tail is longer than the head and body combined. Upper body is varying shades of brown with white sides and underparts (including chin and throat). Tail is strongly bi-colored. Deer mice have been identified as occasional vectors of Lyme Disease and Hanta Virus and should be controlled around human habitation where these diseases are prevalent. Check the CDC website or your local health department for more information on these diseases.

Rodent Control - the most important steps in controlling rodents involve sanitation and elimination of their home sites.

Sanitation first:

1. Store garbage in rodent-proof containers. Make sure your garbage cans have tight fitting lids so dogs or wildlife can't tip them.
2. Clean up spilled or unused pet food. Store pet food in rodent-proof containers. Failure to do this may lead to serious insect infestations of the feed that rodents often carry into the walls. Also, clean up pet excrement, as rats will eat it if they have to.
3. Do not scatter food for wildlife such as birds or squirrels. Use rodent-proof bird feeders. If rats are a serious problem, eliminate all bird feeders.
4. Compost piles should be properly managed to ensure rapid decomposition of potential rat foods.
5. Clean up fallen fruits or nuts from trees. Prune seed pods from lilac and other shrubs.
6. Store garden/lawn seed, bone meal, etc. in rodent-proof containers.

Eliminate shelters second:

1. Employ rodent-proofing features in new buildings.
2. To rodent-proof existing buildings, you should do the following:

• repair any cracks or small holes in the foundations
• repair broken windows and doors
• repair broken sewers
• seal any holes, where pipes or wires enter the building, with sheet metal collars or concrete. Repair screens and cover foundation vents with rodent proof screen material.

Also...
3. Elevate compost, lumber and wood piles at least 12 inches above ground.
4. Remove unmanaged blackberries or brush near buildings.
5. Prune ornamental shrubs away from the ground and avoid planting ground covers that afford shelter (ivy).

Population Control third:

Choosing between traps and poison for rodent control will depend upon the severity of the problem, the location and the experience of the householder.

Traps are often preferred over poison baits for the following reasons:

1. Poison baits, carelessly used, can harm children, pets and non-target animals.
2. Sick rodents may escape to areas between walls or under floors where they die and decompose, causing odor and insect problems.
3. Traps are often especially preferred for controlling easy-to-trap mice, since they eat less than rats and sometimes do not ingest enough rodenticide to make it effective.

Snap traps are recommended and should be placed in areas that are frequented by rodents. These areas can be identified by looking for gnaw marks, rodent tracks, droppings, urine stains, burrows or grease smudges along walls. An effective method of baiting snap traps is to bind a small wad of gauze into the trigger with thread and then work peanut butter into the gauze until saturated. The gauze acts to entangle the rodent's teeth so they can't escape before tripping the trap mechanism. Set the trap AFTER you place the bait on the trigger!

Rat traps should be baited and left UNSET until rodents begin feeding. This will help insure success. Traps should be tied down or anchored in some way, as rats may drag the trap away if they are only partially (non-fatally) caught.

Gum drops, bacon, nuts, oats and dried fruit may be used in addition to peanut butter for bait. When trapping mice, make the bait in each trap about pea-size and attach it to the tripping mechanism with thread or thin wire if necessary. Traps should be baited with fresh material regularly to remain attractive to the pests. There should be many more traps than rodents for trapping to be effective. It is best to place traps close to each other (every 5 - 10 feet for mice - no more than 20 feet for rats) and move them every few days if no rodents are caught. Be aware, though, that older rats MAY avoid a newly placed trap for over a week so, for rats, you may want to leave traps in place for two weeks before trying another place. Success is enhanced by placing traps with the trigger-end against walls where the rodents like to run. This allows the pests to run across the trigger from both directions. Traps can be reused without special cleaning.

Rodenticide Baits last:

Always read the label before using any pesticide! There are many rodent baits available on the market. These are usually ready-to-use baits containing an anticoagulant poison. When using rodent baits, they should be kept out of reach of children, pets and wildlife. The best way to make them inaccessible is to use well-constructed bait boxes with two access openings, just large enough to admit rats or mice (about two inches).

With any bait, warfarin based baits in particular, stations should be placed so they are constantly presented to the pests. Follow the directions on the label for the correct dosage.

If poisons are used, eliminate all other food sources, i.e. garbage, dog food, fallen fruit, compost, dog droppings, etc. If the bait is not touched for a few days, wait. Rats sometimes avoid a new bait for long periods before starting to feed.

Whether you choose traps or poison baits, sanitation and elimination of hiding places are a must! If rat populations are high, trapping, poisoning or both may be considered.

If the rodent problem appears to involve your entire neighborhood, consult with neighbors and your local Health District before attempting to control the problem.

Professional pest control operators also handle baiting or trapping programs. Look for Exterminators in the yellow pages.

For more information contact your local WSU Extension office.

Sources of Additional Information:
(Some of the following links contain pesticide recommendations. It is your responsibility, as the reader, to find out whether the pesticides mentioned in these links are registered for this use in Washington State before attempting to use such products. )

Rat and Mouse Control - University of Florida WSU Extension
Controlling House Mice U. of Nebraska WSU Extension
Bait Stations for Controlling Rats and Mice U. of Missouri Extension
Roof Rat Control around Homes and Other Structures (Arizona WSU Extension - PDF)
Rodenticides for Control of Norway Rats, etc. - U.C. Davis WSU Extension
Non-Chemical Rodent Control - U. of Florida
The Veterinarian's Guide to Managing Poisoning by Anticoagulant Rodenticides
Urban Entomology, by Walter Ebelling
Scientific Guide to Pest Control by: Bennett
Community Pest and Related Vector Control, by Don J Womeldorf & Thomas D Peck
Individual County Health Districts

Written by Cathi Hovanic, Sharon J. Collman, Dave Pehling with additional information provided by King County Health Department and Snohomish Health District. Updated links 04/21/08 by Dave Pehling.