Aw, rats! Along with cockroaches and lima beans, most people put rats on their short list of "Things I Hate." And in this case, for some good reasons. Rats eat our food and contaminate it with urine and droppings, carry and spread diseases, and will bite if threatened. This fact sheet will bring you up to speed on rats and how to control them without using poisons.
There are two species of rats found in the Pacific Northwest, Norway rats and roof rats, and they are different enough in their habits that it can be important to know which is which.1
Norway rats, sometimes called brown or sewer rats, are large stocky burrowing rodents. Their burrows are found along building foundations, beneath woodpiles, and in moist areas in and around gardens and fields. When Norway rats invade buildings, they usually remain in the basement or ground floor. They are found throughout the United States.2
Roof rats, also called black rats, are smaller and sleeker than Norway rats and have much longer tails. Roof rats are agile climbers and usually live and nest above ground in shrubs, trees, and dense vegetation. In buildings, they are most often found above ground level in attics, walls, false ceilings, and cabinets. The roof rat prefers milder climates and is usually found in areas along the West Coast, the Gulf Coast, and the southern East Coast. There may be both Norway rats and roof rats in the same area. 2,3
Biology and Behavior
Both Norway rats and roof rats are mostly active at night. They make up for poor eyesight with keen senses of hearing, smell, taste, and touch. Rats constantly explore their environment and quickly detect and tend to avoid new objects, including traps. Roof rats are particularly wary.4
Rats gain entry to structures through cracks or by gnawing, climbing, jumping, or swimming through sewers. There is nothing like a rat swimming in your toilet to wake you up in the morning.2
Norway rats eat a wide variety of foods that follows closely the USDA recommended diet for humans--grains, meat, fish, nuts, and some fruit, (just a little weak on vegetables.) They may have four to six litters per year and may successfully wean 20 or more offspring annually.2
Roof rats also eat a wide variety of foods, but prefer fruits, nuts, berries, slugs, and snails. Their favorite habitats are attics, trees, and overgrown shrubbery or vines, riverbanks and streams. Roof rats prefer to nest in locations off the ground. They can sometimes be seen at night running along overhead utility lines or fence tops. The average litter size is three to five with from five to eight young in each litter.2
Rats cause damage in several different ways. Foremost, they eat and contaminate human food and animal feed, and damage containers and packaging materials in which they are stored. Rats can gnaw on and damage electrical wires, wooden, plastic, and wallboard building materials, and tear up insulation in walls and ceilings for nesting.2
Norway rats may undermine building foundations and slabs with their burrowing activities. If roof rats are living in the attic of a residence, they can cause considerable damage with their gnawing and nest-building activities. They also damage garden crops and ornamental plantings.2
Rats can spread diseases to people, among them murine typhus, trichinosis, salmonellosis, plague, and ratbite fever. The Black Plague in medieval Europe can be pinned on rats, and their fleas.
Other than seeing the rat itself, there are a number of ways in which you may become aware that there is a rat problem:
- Food is disappearing! Packages are being torn open.
- You may see rat droppings. These are up to 3/4 inch long and pointed at each end, (more than twice as big as mouse droppings).1
- Look for rat holes. Try stuffing them lightly with newspaper for 24 hours (or longer) to see if they are active.4
- After dusk, you may hear noises in the attic or see creatures that don't look like squirrels running on telephone wires or fences.
- You may find nests in firewood piles or burrows in compost piles.
- Smudge marks on rafters, joists and walls that rats rub against are a tell-tale sign.
- Rats gnaw to keep their teeth sharp and will leave teeth-marks on wood.
- Urine stains. These fluoresce under "black lights," useful to know if you have a problem in a warehouse.4
Managing Rats Problems
It takes a three-pronged approach to control rats: Rat-proofing, good sanitation and trapping. Rat-proofing is the most important of the three. If rats can't get in, you don't have a problem. Good sanitation is a close second. With nothing to eat, rats are unlikely to stay even if they are able to get in. Trapping should only be necessary if you haven't rat-proofed.
Rats can and do enter homes in every possible way. Any crack greater than 1/2 inch leaves your home vulnerable to rats.2 To be effective, rat-proofing must be thorough. Consider hiring a handyman if you don't have the time or interest to do it right.
The basic idea of rat-proofing is to seal all cracks wherever you find them on the outside of your home with rat -proof materials such as wire mesh, or sheet metal. Caulking and wood are rat-resistant but not necessarily rat-proof.
Pay special attention to utility entry points. Make sure vent covers and all windows and screens are tightly in place and that there is good flashing around doors. Chimneys should be covered with spark arrestors.2 Pet doors make an easy entryway for rats. Consider closing them at night.
There are many good resources available for tips on how to rat-proof. Start with your local Cooperative Extension. The University of Nebraska has available online and in print a publication called, "Rodent-proof Construction and Exclusion Methods" with many good ideas.5
2 . Sanitation
Rats like many of the same foods you and your pets do. If you are relaxed about your housekeeping, the bad news is you are going to have to get serious.
- Store all food in tightly sealed containers. Keep fresh fruit and vegetables in the refrigerator. Do the dishes every night, if not immediately after each meal. Don't leave pet food bowls out overnight.1
- Have garbage and garden debris collected frequently. Make sure garbage cans have tight fitting lids. Compost piles are giant buffet tables for rats. If you compost food scraps, rat-proof a bin or buy a rodent-resistant backyard composter. 6
- Sanitation also needs to take place in the yard. Woodpiles should be stored off the ground. Shrubbery can be thinned and pruned up off the ground too. Trees should be pruned back at least two feet from roofs. Dense ground covers like ivy should also be thinned or removed.
3 . Trapping
When it comes to choosing between trapping and baiting, trapping wins hands down. Baiting can lead to poisoning of non-targeted animals, including your pets and children, and can lead to odor problems when rats die in inaccessible areas.1
That said, trapping rats is not necessarily easy. Never underestimate the craftiness of a rat! Rats have a natural suspicion of new things and will not just jump into your traps and die. It may take several days before they even sniff at the trap, so don't be impatient.
There are several tricks to be successful trapping. First, use lots of traps. A dozen or more snap traps around the home is not excessive. Multiple traps provide multiple opportunities to trap rats. Make sure you buy the bigger rat traps and not mouse traps. Both the traditional snap trap and the newer trap with a plastic expanded treadle work fine. Live traps are not recommended for rats. (Do you really want to release a rat?)
Be careful handling rat traps. They can hurt you if you get snapped. Place traps where children and pets will not go. Traps can be enclosed in boxes with "doors" cut into them.
Placement is key. The closer to where the gnawing, droppings, and food-thievery is taking place, the better. Traps should usually be placed against and perpendicular to walls, so they can catch a rat traveling in either direction. The exception is with roof rats where the trap may need to be screwed into a tree or in a rafter to get it close to the high-climbing roof rat.3
The type of bait matters too. Peanut butter is the reliable bait of choice. Nuts, dried fruit, bacon and gum drops can be used with or instead of peanut butter. Tying the bait to the trap may help rats with a "light touch" from stealing the bait. Also, the bait can be attached to the underside of the trigger device to make it even harder to steal.2
The most important part of trapping is to not set the trap initially! Let the rats explore the unset trap for a while, and take the bait a couple times, before you set the trap. This will increase your odds dramatically.
What Not To Do
Other methods for the most part should be avoided. Traps that electrocute are more expensive but no more effective than snap traps. Glueboards are considered inhumane by many people. Repellants generally don't work. Nor do various electronic and sound emitting devices.2 The bottom line: Stick to trapping.
Homeowners should not use baits. If there is a very serious pest problem, it may be time to call in a handyman or a pest control professional to help "tighten the ship" and set up and maintain a trapping program. The Health Department can help deal with neighborhood-wide problems.
- Hovanic, Cathi et al. 2007. Rats and mice. Gardening in Western Washington. Wash. St. Univ. Extension. http://gardening.wsu.edu/library/comm001/comm001.htm
- Salmon, T. P. et al. 2003. Rats. Pest Notes Publ. 74106. U.C. Agriculture and Natural Resources. http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74106.html
- Marsh, R. 2006. Roof rats: Damage, prevention, and control methods. Internet Center for Wildlife Damage Management. http://icwdm.org/handbook/rodents/RoofRats.asp
- Olkowski. W. et al. 1991. Common-sense pest control. Taunton Press. Newtown, CT.
- Multnomah County Health Dept. 2003. About rats. Portland, OR. http://www.co.multnomah.or.us/health/vector/rats.htm
- Baker, R.O. et al. 1994. Rodent-Proof Construction and Exclusion Methods. Cooperative Extension Division. University of Nebraska-Lincoln. http://digitalcommons.unl.edu/icwdmhandbook/27/ [click on "Download the Document"]
Stein, Dan. "Controlling Rats Without Poisons." Eugene, OR: Northwest Center for Alternatives to Pesticides.