Rats are some of the most troublesome and damaging rodents in the United States. They consume and
contaminate food, damage structures and property, and transmit parasites and diseases to other animals and
humans. Rats live and thrive under a wide variety of climates and conditions; they are often found in and
around homes and other buildings, farms, gardens, and open fields.
People do not often see rats, but signs of their presence are easy to detect. In Arizona, the most troublesome
rats are two introduced species: the roof rat and the Norway rat. It is important to know which species of rat is
present in order to place traps or baits in the most effective locations.
Norway rats, sometimes called brown or sewer rats, are stocky burrowing rodents that are larger than roof
rats. Their burrows are found along building foundations, beneath rubbish or woodpiles, and in moist areas in
and around gardens and fields. Nests may be lined with shredded paper, cloth, or other fibrous material.
When Norway rats invade buildings, they usually remain in the basement or ground floor. The Norway rat
occurs throughout the 48 contiguous United States. Generally it is founds at lower elevations but may occur
wherever people live.
Roof rats, sometimes called black rats, are slightly smaller than Norway rats. Unlike Norway rats, their tails are
longer than their heads and bodies combined. Roof rats are very agile climbers and usually live and nest
above ground in shrubs, trees, and dense vegetation such as ivy. In buildings, they are most often found in
enclosed or elevated spaces in attics, walls, false ceilings, and cabinets. The roof rat has a more limited
geographical range than the Norway rat, preferring ocean-influenced, warmer climates. In areas where the
roof rat occurs, the Norway rat may also be present.
While rats are much larger than the common house mouse, a young rat is occasionally confused with a
mouse. In general, very young rats have large feet and large heads in proportion to their bodies, whereas
those of adult mice are much smaller in proportion to their body size. While both rats and mice gnaw on wood,
rats leave much larger tooth marks than those of a mouse.
How to Spot a Rat Infestation
Because rats are active throughout the year, periodically check for signs of their presence. Once rats have
invaded your garden or landscaping, unless your house is truly rodent proof, it is only a matter of time before
you find evidence of them indoors. Experience has shown it is less time consuming to control rodents before
their numbers get too high, and fewer traps and less bait will be required if control is started early.
Inspect your yard and home thoroughly. If the answer to any of the following questions is yes, you may have a
- Do you find rat droppings around dog or cat dishes or pet food storage containers?
- Do you hear noises coming from the attic just after dusk?
- Have you found remnants of rat nests when dismantling your firewood stack?
- Does your dog or cat bring home dead rat carcasses?
- Is there evidence rodents are feeding on fruit/nuts that are in or falling from the trees in your yard?
- Do you see burrows among plants or damaged vegetables when working in the garden?
- Do you see rats traveling along utility lines or on the tops of fences at dusk or soon after?
- Have you found rat nests behind boxes or in drawers in the garage?
- Are there smudge marks caused by the rats rubbing their fur against beams, rafters, pipes, and walls?
- Do you see burrows beneath your compost pile or beneath the garbage can?
- Are there rat or mouse droppings in your recycle bins?
- Have you ever had to remove a drowned rat from your swimming pool or hot tub?
- Do you see evidence of something digging under your garden tool shed or doghouse?
BIOLOGY AND LIFE CYCLE OF THE RAT
Rats, like house mice, are mostly active at night. They have poor eyesight, but they make up for this with their
keen senses of hearing, smell, taste, and touch. Rats constantly explore and learn about their environment,
memorizing the locations of pathways, obstacles, food and water, shelter, and other elements in their domain.
They quickly detect and tend to avoid new objects placed into a familiar environment. Thus, objects such as
traps and baits often are avoided for several days or more following their initial placement. While both species
exhibit this avoidance of new objects, it is usually more pronounced in roof rats than in Norway rats.
Both Norway and roof rats may gain entry to structures by gnawing, climbing, jumping, or swimming through
sewers and entering through the toilet or broken drains. While Norway rats are more powerful swimmers, roof
rats are more agile and are better climbers.
Norway and roof rats do not get along. The Norway rat is larger and the more dominant species; it will kill a
roof rat in a fight. When the two species occupy the same building, Norway rats will dominate the basement
and ground floors, with roof rats occupying the attic or second and third floors. Contrary to some conceptions,
the two species cannot interbreed. Both species may share some of the same food resources but do not feed
side-by-side. Rats may grab food and carry it off to feed elsewhere.
Rats of either species, especially young rats, can squeeze beneath a door with only a 1/2-inch gap. If the door
is made of wood, the rat may gnaw to enlarge the gap, but this may not be necessary.
Norway rats eat a wide variety of foods but mostly prefer cereal grains, meats, fish, nuts, and some fruits.
When searching for food and water, Norway rats usually travel an area of about 100 to 150 feet in diameter;
seldom do they travel any further than 300 feet from their burrows or nests. The average female Norway rat
has four to six litters per year and may successfully wean 20 or more offspring annually.
Like Norway rats, roof rats eat a wide variety of foods, but their food preferences are primarily fruits, nuts,
berries, slugs, and snails. Roof rats are especially fond of avocados and citrus and often eat fruit that is still
on the tree. When feeding on a mature orange, they make a small hole through which they completely remove
the contents of the fruit, leaving only the hollowed out rind hanging on the tree. The rind of a lemon is often
eaten, leaving the flesh of the sour fruit still hanging. Their favorite habitats are attics, trees, and overgrown
shrubbery or vines. Residential or industrial areas with mature landscaping provide good habitat, as does
riparian vegetation of riverbanks and streams. Roof rats prefer to nest in locations off the ground and rarely
dig burrows for living quarters if off-the-ground sites exist.
Roof rats routinely travel up to 300 feet for food. They may live in the landscaping of one residence and feed
at another. They can often be seen at night running along overhead utility lines or fence tops. They have an
excellent sense of balance and use their long tails for balance while traveling along overhead utility lines.
They move faster than Norway rats and are very agile climbers, which enables them to quickly escape
predators. They may live in trees or in attics and climb down to a food source. The average number of litters a
female roof rat has per year depends on many factors, but generally is three to five with from five to eight
young in each litter.
Rats consume and contaminate foodstuffs and animal feed. They also damage containers and packaging
materials in which foods and feed are stored. Both species of rats cause problems by gnawing on electrical
wires and wooden structures (doors, ledges, in corners, and in wall material) and tearing up insulation in walls
and ceilings for nesting. Norway rats may undermine building foundations and slabs with their burrowing
activities. They may also gnaw on all types of materials, including soft metals such as copper and lead as well
as plastic and wood. 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.
Among the diseases rats may transmit to humans or livestock are murine typhus, leptospirosis, trichinosis,
salmonellosis (food poisoning), and ratbite fever. Plague is a disease that can be carried by both roof and
MANAGING A RAT PROBLEM
Three elements are necessary for a successful rat management program: sanitation measures, building
construction and rodent proofing, and, if necessary, population control.
Sanitation is fundamental to rat control and must be continuous. If sanitation measures are not properly
maintained, the benefits of other measures will be lost, and rats will quickly return. Good housekeeping in and
around buildings will reduce available shelter and food sources for Norway and, to some extent, roof rats.
Neat, off-the-ground storage of pipes, lumber, firewood, crates, boxes, gardening equipment, and other
household goods will help reduce the suitability of the area for rats and will also make their detection easier.
Garbage, trash, and garden debris should be collected frequently, and all garbage receptacles should have
tight-fitting covers. Where dogs are kept and fed outdoors, rats may become a problem if there is a ready
supply of dog food. Feed your pet only the amount of food it will eat at a feeding, and store pet food in rodent-
For roof rats in particular, thinning dense vegetation will make the habitat less desirable. Climbing hedges
such as Algerian or English ivy, star jasmine, and honeysuckle on fences or buildings are very conducive to
roof rat infestations and should be thinned or removed if possible, as should overhanging tree limbs within 3
feet of the roof to make it more difficult for rats to move between them.
The most successful and long lasting form of rat control in buildings is to "build them out." Seal cracks and
openings in building foundations, and any openings for water pipes, electric wires, sewer pipes, drain spouts,
and vents. No hole larger than 1/4 inch should be left unsealed to exclude both rats and house mice. Make
sure doors, windows, and screens fit tightly. Their edges can be covered with sheet metal if gnawing is a
problem. Coarse steel wool, wire screen, and lightweight sheet metal are excellent materials for plugging gaps
and holes. Plastic sheeting, wood, caulking, and other less sturdy materials are likely to be gnawed away.
Because rats (and house mice) are excellent climbers, openings above ground level must also be plugged.
Rodent proofing against roof rats usually requires more time to find entry points than for Norway rats because
of their greater climbing ability. Roof rats often enter buildings at the roof line area so be sure that all access
points in the roof are sealed. If roof rats are travelling on overhead utility wires, contact a pest control
professional or the utility company for information and assistance with measures that can be taken to prevent
Rodent Proofing Your Home
- Repair or replace damaged ventilation screen around the foundation and under eaves.
- Provide a tight fitting cover for the crawl space.
- Seal all openings around pipes, cables, and wires that enter through walls or the foundation.
- Be sure all windows that can be opened are screened and that the screens are in good condition.
- Cover all chimneys with a spark arrester.
- Make sure internal screens on roof and attic air vents are in good repair.
- Cover rooftop plumbing vent pipes in excess of 2 inches in diameter with screens over their tops.
- Make sure all exterior doors are tight fitting and weatherproofed at the bottom.
- Seal gaps beneath garage doors with a gasket or weather-stripping.
- Install self-closing exits or screening to clothes dryer vents to the outside.
- Remember that pet doors into the house or garage provide an easy entrance for rodents.
- Keep side doors to the garage closed, especially at night.