Integrated Pest Management Manual
Rats
Rats have caused more economic loss and more human suffering than any other vertebrate pest. From plague epidemics (the "Black Death" of Europe) to rat bites of inner- city children, from gnawing electrical wires in an attic to feeding on stored food in a warehouse, rats are a critical pest of humankind.
Rats have adapted well to living around people. So well, in fact, that rats are commonly called "domestic" rodents. They live and breed inside buildings and granaries, in city sewers and attics, in agricultural fields and warehouses, in ships and under concrete slabs. Although adapted to people, however, rats are wary. Hundreds can be living in, under, and around a complex of buildings with few people in the area aware of their existence.
Successful management of pest rats is not easy. It requires an integrated approach based on a good understanding of the biology and habits of pest rats, that combines effective inspection and monitoring with intelligent use of control tactics.
IDENTIFICATION AND BIOLOGY OF RATS
Major Pest Species of Rats
When we speak of rats, we are speaking of many different species, some pests, some not. In the United States, the two most important pest rats are the Norway rat and the roof rat. The two species look similar but are noticeably different both in appearance and in habits.
Norway rat (Rattus norvegicus Berkenhout). The most common pest rat in the United States, the Norway rat is also called the brown rat, sewer rat, or wharf rat. The adult is a large, stocky rat, from 12"-18" from the nose to the tip of the tail, weighing 12-16 ounces. In contrast to the roof rat, the Norway rat's tail is shorter than its head plus its body, its ears are small (cannot be pulled down to reach its eyes) and covered with hairs, and its nose is blunt.
Roof rat (Rattus (L)). A common rat of coastal areas, the roof rat is also called the black rat or the ship rat. The adult has a slender body, weighs 5-9 ounces, and measures 13"-18" from the nose to the tip of the tail. In contrast to the Norway rat, its tail is longer than its head plus its body, its ears are large (can be pulled down to reach its eyes) and nearly hairless, and its nose is pointed.
Geographic Distribution
Norway rat. Found in every state in the United States and common throughout much of populated North America, Norway rats absent from sparsely-inhabited areas, particularly in the western states.
Roof rat. Roof rats occur mainly in coastal areas, including California, Oregon, Washington, the Gulf states, and the Southeast and mid-Atlantic states.
Life Cycle
Rats are born hairless, with eyes and ears closed. They don't become furry and "rat-like" until they're about two weeks old. They begin eating solid food at three weeks and imitate their mother to learn to forage, escape, and watch for danger. They are weaned at four or five weeks. Rats become independent of their mother and ready to start families of their own, at three months.
Norway rat. Norway rats breed at any time during the year, but more frequently in warm months. Gestation lasts 22-24 days. The size of the litter is usually 8-10 pups. There are 3-4 litters per year. Lifespan in the wild is usually about 6 months.
Roof rat. Root rats may breed throughout the year, but more commonly in warm months. Gestation lasts 20-22 days. The size of the litter usually 4-8 pups. A female may breed at 2-5 months of age and have an average of 5.4 litters per year. Life span in the wild is usually 9-12 months.
Seasonal Abundance
Outdoor rat populations tend to peak in summer to early fall. They tend to be at their lowest levels in late winter to early spring. Indoors, rat populations may remain at the same levels throughout the year, limited only by periodic shortages of food, water, or nesting sites.
Nests
Rats are social. They live together in colonies with well-defined territories and a social hierarchy or "pecking order." Norway rats and roof rats prefer different types of nest sites.
Norway rat. Outdoors, Norway rats usually dig shallow burrows in the ground. The burrows are generally less than 18" deep and 3'long, with a central nest. The main burrow opening is 2"-4" in diameter. Hidden "bolt holes" are used for emergency escapes. Indoors, Norway rats will nest inside walls, underneath equipment, in cluttered storage areas, and similar sites, usually on the lower floors of a building.
Roof rat. The roof rat commonly nests above ground in trees, vine-covered fences, stacked lumber and woodpiles, and overgrown landscaping. Roof rats will sometimes nest in burrows if above-ground sites are limited. Indoors, roof rats prefer to nest in the upper levels of buildings in attics and ceiling and attic voids near the roof line. This species seems to be less dependent on man than the Norway rat and may live in forests far from human habitation, especially in warm areas.
Both species also nest in sewers and storm drains.
Range
Rats commonly travel 100'-150' from their nests looking for food and water and patrolling their territory. They may have several "hotel" nest sites in an area and will move from home base to spend several days in one of these secondary nest sites.
Responses to Environmental Factors
Rat abundance is dependent on availability of food, water, and shelter. They need about an ounce of food and 1/2 fluid ounce of water daily, although the roof rat can get by on less. Both prefer to nest where water is easily available.
While the Norway rat prefers to feed on protein foods like meat, fish, insects, and pet food, the roof rat prefers a more vegetarian diet. It feeds on fruits, nuts, seeds, berries, vegetables, and tree bark. But the roof rat will also feed on garbage, pet food, and meat if it is readily available. Rats often cache or hoard food in hidden areas for use when food supplies run short.
Rats are wary of anything new that appears in their territory. This fear of new objects can make baiting and trapping difficult since rats will at first avoid baits, bait stations, and traps, and may come to associate them with danger as a management program proceeds.
When Norway and roof rats are found together, Norway rats will usually, but not always, displace roof rats.
Medical Importance
Rats have always been of medical importance due to their transmission of human diseases.
Direct effects. Rat bites, particularly in urban areas, may be a serious health problem. An estimated 14,000-24,000 bites to humans occur each year. Infants and helpless adults (unconscious, invalid, and elderly) are subject to attack by rats. A small percentage of those bitten develop rat-bite fever, a bacterial disease carried in the teeth and gums of many rats. All rat bites should receive medical attention.
Rats can spread Salmonella food poisoning, Weils disease (leptospirosis), trichinosis, and other diseases directly through contamination of food and water with their urine and feces.
Indirect effects. Rats may indirectly spread a number of serious human diseases by way of fleas and mites, most notably plague and murine (scrub) typhus fever. (See Pratt et al. [1986] for a complete discussion of rat-borne diseases).
Outbreaks of rat-associated diseases. Some of the diseases listed above can be fatal to humans. If disease transmission is suspected in your areas, contact your National Park Service Public Health Service representative.
Rabies. Rats have never been found to be infected with rabies in nature, and rabies transmission has never been documented in the United States. The United States Public Health Service recommends against anti-rabies treatment in the case of rat bite.
MONITORING AND THRESHOLDS FOR RATS
Periodic surveys of buildings and grounds can reveal the existence of rat infestation. Inspection visits should be made every other week and increased or decreased according to the severity of the problem. Evening inspection using a powerful flashlight is the best way to see rats, but there are many signs of rat infestation besides the animals themselves. Rat sounds, droppings, burrows, urine stains, smudge marks, runways, tracks, gnawing damage, nests, food caches, pet excitement, and rat odors are all signs of rat activity.
Sounds. Squeaks, gnawing sounds, clawing, and scrambling in walls are typical sounds of a rat infestation.
Droppings. A single rat may produce 50 droppings daily. Norway rat droppings are larger (3/4") than roof rat droppings (1/2"). Determine if an area is currently infested by sweeping up old droppings, then reinspect after a week. Fresh droppings have a putty- like texture; old droppings crumble easily.
Burrows. Estimates of relative abundance in a limited area can be made by counting, mapping, and loosely plugging burrow entrances on a weekly basis. Burrows which are reopened the following week are active.
Nests. Roof rat nests are often visible in attics, or they may be found when vegetation is trimmed.
Urine Stains. Under ultraviolet light (blacklight), rat urine will glow blue-white.
Runways. Rats regularly travel the same routes. Outdoor runways appear as beaten paths in the ground. Grass will be worn down.
Smudge Marks. Oil and grease that rub off a rat's fur build up on well-used runways.
Tracks. An adult rat's footprint is about 3/4" long. Rats may also leave a drag line (from their tail) in the middle of their tracks. A "tracking patch" can help determine the location and extent of rat activity. Place a light dusting of clay, unscented baby powder, or powdered limestone in suspected runways and near rat signs. Typical patch sizes range from 12"x4" to 6"x18". Examine the patch for tracks at regular intervals.
Gnawings. Rats constantly gnaw on hard surfaces. Gnawed holes may be 2" or more in diameter.
Food Caches. Rats may store surprisingly large quantities of food in protected areas.
Pet excitement. Cats and dogs often probe an area of floor or wall where rats are active, particularly if the rats only recently invaded.
Odor. Heavy infestations have a distinctive odor. Experienced pest managers can smell the difference between a rat and a mouse infestation.
Learn to differentiate between fresh rat sign and old sign which may indicate old (non- active) infestation.
Evaluation of population size. Rat signs may be interpreted visually as follows.
Rat-free area or low rat population: no sign of rat presence.
Medium population: old droppings and gnawing common, one or more rats seen by flashlight at night, none during the day. Each rat seen at night usually indicates 10 or more elsewhere.
High population: fresh droppings, tracks, gnawing evident, three or more rats seen at night, one or more in daylight.
Estimates of rats present can also be made by placing premeasured, ground, nontoxic cereal bait in various locations to determined how much is eaten each night. Double the amount each night until the amount taken in one night levels off. Divide the amount by 1/2 oz. This will provide a very rough estimate of the minimum number of rats present.
In most circumstances the injury (threshold) level is one rat as determined by rat sighting or sign. The action level is one rat for population reduction programs and zero rats for prevention programs.
NON-CHEMICAL CONTROL OF RATS
Successful rat management programs use a combination of tools, procedures, and strategies. Some are lethal to the rat, some are not. Lethal procedures include the use of rodenticides, snap traps, and glue boards to quickly reduce a population. Nonlethal procedures include improving sanitation, reducing harborage, and rat-proofing buildings. Long- term, the most important tactics for reducing rat problems are in this second, nonlethal category, because the procedures reduce the environment's capacity to support rats or block the rats' access to buildings.
Before a park manager can decide what combination of strategies would be best for a particular situation, he or she needs to determine where the rats are nesting and feeding, locate their travel routes, and determine the extent of the infestation. This information is obtained through inspection and regular monitoring.
Improved Sanitation
Rats are attracted by food spills, open garbage, and food stored in accessible sites. Baiting and trapping programs often fail because the bait can't compete with the rats' regular food. Reducing the rats' food will reduce the rat-carrying capacity of the site, as well as making lethal control programs more effective.
In urban settings, rats feed largely on garbage. Regular trash pickups at the end of each day, rather then storing trash overnight, and the use of rat-proof trash containers are relatively simple methods of reducing rat food sources. Damaged dumpsters and containers should be repaired or replaced and should always be kept closed overnight.
Pet food dishes and water dishes should not be left full overnight. Bird seed is a favorite rat food. Bird feeders should be equipped with seed catchers, or the dropped seed should be cleaned up every evening.
Food in warehouse-style storage should be rotated properly-first in, first out. Food should be stored on pallets, not on the ground, and there should be about 2' of space between pallets and the side walls to permit inspection.
Harborage Reduction
Landscaping should not include thick hedges or bushes which obscure the ground. Ground covers such as ivy, which provide cover or runs for rats, should not be planted adjacent to buildings. High grass, weeds, wood piles, and construction debris should not be permitted near foundation walls. Dumpsters and outside garbage containers should sit on a paved or concrete pad. Indoors, reduce clutter in rarely-used rooms and organize storage areas.
Rat-Proofing
Building rats out of a structure, and keeping them out, is called rat-proofing.
- In roof rat areas, cables, trees, and pipes leading to or touching a structure should be rat- proofed with galvanized metal barriers.
Trapping
The snap trap is an effective method of killing rats when used correctly. Traps are especially useful when you wish to avoid the use of poisons, to eliminate bait shy or bait resistant rats, to avoid odors from dead rats in inaccessible places, or to collect live rats for ectoparasite or resistance screening. The best traps are those with expanded triggers (treadles) set for a light touch. Set the traps along runways with the trigger towards the wall, or tie the traps to pipes or rafters or wherever droppings, gnawing, grease marks, and other evidence of activity is found. The number one mistake in using traps is not using enough. (See Environmental Protection Agency [1991] for information on trapping).
Another way to trap rats is with glue boards. Glue boards are used much like snap traps. Secure the glue board with a nail or wire so it can't be dragged away. Be aware that some people may protest the use of glue boards as inhumane, since the rat may struggle for some time.
Natural Enemies
Rats may be preyed upon by many other animals including dogs, cats, weasels, snakes, and owls. Rats are susceptible to a variety of diseases and parasites. Some natural enemies ranging from ferrets to bacterial toxins have been used in the past with varying degrees of success in rat control programs.
In abnormally crowded conditions or other stress situations, rats may display aggressive behavior toward each other, including cannibalism and abandonment of young.
CHEMICAL CONTROL OF RATS
Rodenticides are commonly used to provide rapid reduction of rat populations. There are three major formulations of rodenticide: toxic baits, water baits, and tracking powders. Fumigants are also used occasionally to fumigate burrow systems.
Toxic baits. These combine a poison with a food bait attractive to rats. Today, most baits are obtained ready-made as extruded pellets, or in a dry meal, or molded into paraffin blocks for wet sites. Some baits kill rats in a single feeding, some require that a rat feed a number of times. Some are anticoagulants (causing rats to bleed to death), some affect respiration, and others work by entirely different modes of action. They range in toxicity to people from very toxic to slightly toxic. Be sure to read the label and supporting information that comes with each product to ensure safe use.
Every rodenticide has a warning on the label to place the bait "in locations not accessible to children, pets, wildlife, and domestic animals, or in tamper-proof bait boxes." What qualifies as a safe, inaccessible area needs to be determined on a case-by-case basis.
If you believe there is a risk to children or nontarget animals, the bait should be placed inside a tamper-proof bait box. A bait box is tamper-proof if a child or a pet cannot get to the bait inside. It is usually made of metal or heavy plastic. But a bait box is not truly tamper-proof unless it can be secured to the floor, wall, or ground.
In parks, there is the additional problem that there may be nontarget rodents that can find their way into bait stations (or traps, for that matter). Be sure to survey for these and adapt your management tactics to avoid harming them.
Water baits. These specially-formulated rodenticides are mixed with water and dispensed from "chick-founts," or custom toxic-water dispensers. Since rats drink daily, water baits are effective when free water is in short supply. Water baits are less effective against roof rats. Be sure to only use water baits where no other animals or children can get to them.
Tracking powders. These are rodenticides mixed with a talc or powdery clay and applied into areas where rats live and travel. The powder sticks to the rats' feet and fur and is swallowed when the rats groom themselves. Tracking powders are effective even where food and water are plentiful.
The rodenticide in tracking powders is 5 to 40 times more concentrated than in baits. Avoid applying tracking powder where the powder could become airborne and drift into nontarget areas, or where other nontarget animals may come in contact with it.
Fumigants. Several fumigants are available for burrow fumigation. Most are extremely hazardous and should only be used by experienced professionals. National Park Service policy for rat management emphasizes rodent-proofing rather than the use of rodenticides. Consult your regional Integrated Pest Management coordinator when considering their use.
REFERENCES
1. Environmental Protection Agency. 1992. Integrated Pest Management: Vertebrates. A Guide for Commercial Applicators. United States Environmental Protection Agency, Office of Pesticide Programs.
2. Howard, W.E., and R.E. Marsh. 1981. The rat, biology and control. Leaflet #2896. Univ. Calif. Div. Ag. Sci.
3. Pratt, H.D., B.F. Bjornson, and K.S. Littig. 1986. Control of domestic rats and mice. HHS Publ. #(CDC) 86-8396. United States Department of Health and Human Services, Public Health Service, Center for Disease Control, Atlanta, GA.
Sources of additional information
1. Bennett, G.W., Owens, J.M., and R.M. Corrigan. 1988. Truman's Scientific Guide to Pest Control Operations. (4th Edition). Edgell Communications, Duluth, MN.
2. Centers for Disease Control. 1980. CDC pictorial keys; arthropods, reptiles, birds, and mammals of Public Health Significance. United States Dept. Health, Education, and Welfare. Public Health Service, Atlanta, GA.
3. Ebeling. W. 1975. Urban Entomology. Univ. Calif., Div. Ag. Sci.
4. Mallis, A. 1990. Handbook of Pest Control (7th ed.). Franzak and Foster Co. Cleveland, OH.
5. Olkowski, W., S. Daar, and H. Olkowski. 1991. Common-Sense Pest Control. The Taunton Press, Newtown CT.
6. Pratt, H.D., and R.Z. Brown. 1986. Biological Factors in Domestic Rodent Control. HHS Publ. #(CDC) 86-8396. United States Department of Health and Human Services, Public Health Service, Centers for Disease Control, Atlanta, GA.
7. Prevention and Control of Wildlife Damage. 1992. Great Plains Agricultural Council, Nebraska Cooperative Extension Service.