Rats have caused more human suffering and more economic damage than any other vertebrate pest. From plague epidemics (the "Black Death" of Europe) to rat-bite fever, whether feeding on stored grain or gnawing electric wires, rats are enemies of humankind. Statisticians estimate that rats destroy 20 percent of the world's food supply every year -- directly by feeding and indirectly through contamination.
Yet, rats can be admired. They have adapted to nearly all human environments. They live in granaries, in fields, in city sewers, on ocean-going ships, on roofs, in attics, in basements, in street trees, on top of 30-story buildings, and inside subway tunnels.
Adept athletes, rats can leap three feet straight up and four feet horizontally. They can scramble up the outside of a pipe three inches in diameter, and climb inside pipes one-and-a-half to four inches in diameter. They can walk between buildings on telephone or power lines, and scramble on board a ship on its mooring line. Rats can swim through a half mile of open water, tread water for up to three days, swim against a strong current in a sewer line, and dive through a sewer trap to pop up inside a toilet. They can fall more than 50 feet and survive.
Rats gnaw constantly; their teeth are extremely hard. They commonly chew through building materials such as cinder block, aluminum siding, sun-dried adobe brick, wall board, wooden cabinets, lead sheathing, and plastic or lead pipes. After gnawing a hole, an adult rat can compress its body and squeeze through an opening only a half-inch high.
In most instances, rats are very wary. Hundreds may be living in a city block -- in underground burrows, in sewers, on roofs, inside buildings -- with few people in the area realizing it. Populations are dynamic: rats moving in, rats moving out, rats giving birth, and rats dying. Within a population, some rats will be easy to control, some difficult.
Successful long term rat control is not simple. The key is to control rat populations, not individual rats. Rat control requires an integrated approach that includes nonlethal tools such as careful inspection, upgraded sanitation, and rat-proofing structures. Lethal control often combines the use of rodenticides with nontoxic control measures such as snap traps or glue boards.
Rats are responsible for the spread of many diseases. Sometimes they transmit the disease directly, by contaminating food with their urine or feces. Sometimes they transmit disease indirectly, for example, fleas biting first an infected rat, then a person. Following are some of the more important diseases associated with rats:
The "Great Plague" of London killed half of the city's population. The "Black Death" of Europe lasted 50 years in the 14th Century and killed 25 million people. In the first quarter of this century, an estimated 11 million people died in Asia from plague.
The disease is transmitted primarily to man by the oriental rat flea. The flea bites an infected rat, and then, feeding on a human, inoculates them with the bacteria that cause the disease.
Although no major urban outbreak of plague has occurred since 1924, this is not a disease of the past. A reservoir of plague exists in some populations of wild rodents in several Western states, including Nebraska. Humans contacting these rodents could contract the disease. As suburbia expands into undeveloped areas, wild rodents can transmit the disease to urban rats. There is a danger that an outbreak of urban plague can occur in the United States.
Murine typhus occurs in California and in southeastern and Gulf Coast states. It is a relatively mild disease in humans. As with plague, murine typhus is transmitted from rats to humans by a rat flea. In this case, however, the disease organism enters the bloodstream when feces of infected fleas are scratched into a flea-bite wound.
Rats bite thousands of people each year; most bites occur in inner cities. [In some cases victims, particularly infants and bed-confined elderly, are bitten in the face while sleeping.] A small percentage of those bitten develop rat-bite fever. The bacteria that causes the disease is carried in the teeth and gums of many rats. Although the disease exhibits mild symptoms similar to flu in most cases, it can be fatal. It is of particular risk to infants.
Rats frequent sewers, rotting garbage, cesspools, and similar sites where Salmonella bacteria thrive. The bacteria also thrive in the intestinal tracts of rats. If infected rats travel to stored food, or dishes and silverware, or food preparation surfaces, their droppings can transmit Salmonella food poisoning to humans.
Human cases of this disease are seldom fatal. The disease organisms are spread from rat urine into water or food, and enter humans through mucous membranes or minute cuts and abrasions of the skin.
Trichinosis results from a nematode, or tiny roundworm, that invades intestines and muscle tissue. Both people and rats get the disease from eating raw or undercooked pork infected with the nematode. Rats help spread trichinosis when hogs eat food or garbage contaminated with infested rat droppings.
Rats have never been found to be infected with rabies in nature. Rabies transmission from rats to humans has never been documented in the United States. The U.S. Public Health Service recommends against anti-rabies treatments in the case of rat or mouse bites.
In the United States the two most important pest rats are the Norway rat (Rattus norvegicus) and the roof rat (Rattus rattus). The Norway rat is also called the brown rat, house rat, sewer rat, and wharf rat. The Norway rat is generally considered the most important rat in the U.S. It is found in every state.
The roof rat is also called the black rat, ship rat, and Alexandrine rat. Roof rats are found primarily in coastal areas of the United States, including California, Washington, and Oregon, the Southeast and Middle Atlantic States, and the Gulf States.
The two species look much alike but there are noticeable differences. In general:
Rats must be understood to be controlled. Knowledge of their life histories, habitat and food requirements, patterns of behavior, range and other factors is essential to their management.
The Norway and roof rats have similar habits. Most of the discussions below apply to either kind of rat. Where differences are important for control purposes, however, the differences will be highlighted.
A mature female rat can give birth to 20 to 30 young in a year (4 to 6 at a time), if she lives that long. The average life span of a rat in the field is less than one year; females live longer than males.
The young are born in a nest. They are hairless, and their eyes and ears are closed. Within two weeks their eyes and ears open, they become furry and rat-like, and they begin exploring the burrow area. In the third week they begin to eat solid food, and imitate their mother to forage, escape, and watch for danger.
If the mother rat has become wary of rodenticides or traps, many of her young will learn to avoid them. This learning experience can make control difficult in sites where long term rodent control programs have been unsuccessful in the past.
Young are totally weaned at four or five weeks old. They then weigh about 1 ounces. At three months, the young are independent of their mother. They will mate and continue the cycle in the same location or will migrate to a new, unoccupied nest area.
Rats are social animals and live in colonies with well-defined territories that they mark with urine and glandular secretions. The colony has a complex social hierarchy with a dominant male leader and a "pecking order" of subordinate males and ranking females. The strongest and most dominant animals occupy the best nest and resting sites, and feed at their leisure. Weaker, subordinate rats are pushed out to less favorable sites, or forced out of the territory completely.
Rats are aggressive, and social conflicts are most common at feeding sites, prime resting areas, and territorial boundaries. Females fiercely defend their nest and young from other rats.
Rats have poor vision. They are nearly color blind, and react to shapes and movement rather than identifying objects by sight. Thirty to forty-five feet is the limit of their vision. Their eyes are adapted to dim light.
Other senses, however, compensate for poor vision. They use their sensitive nose to locate food, follow pathways, tell whether another rat is friend or foe, and identify new objects in their territory. They use long whiskers and guard hairs to "touch" their way through dark burrows, pipe chases, wall voids, and other runways. Their ears detect faint sounds that signal danger. Rats can taste certain chemicals at a parts-per-million concentration. [This explains why rats often reject baits or avoid traps that have been contaminated with insecticides.] Finally, rats have an excellent sense of balance which allows them to walk on wires and always land on their feet in a fall.
Rats are wary of anything new that appears in their territory. A bait station, a trap, a block of wood will be avoided for a few days until the rats become familiar with the new object; even then, they approach cautiously. This fear of new objects can make baiting and trapping difficult. Rats will avoid poison bait when it is first placed. Later, they may nibble warily. If the poison bait makes them ill, but doesn't kill them, they will avoid similar baits or stations in the future.
Rats need about one ounce of food daily. Norway and roof rats prefer different types of food. Norway rats prefer protein-based foods such as meat, fish, insects, pet food, nuts, and grain. Household garbage is ideal food for Norway rats. Roof rats prefer plant materials such as fruits, nuts, seeds, berries, vegetables, and tree bark. They occasionally feed on garbage and meats. Each rat species, however, will feed on nonpreferred food if nothing else is available.
Rats often cache or hoard food in hidden areas. This food may or may not be eaten when other food supplies run short. Hoarding is important for two reasons. First, rats may be moving a toxic bait into a location where the label does not permit it to be. Second, rats may be hoarding poison bait while feeding on their regular food; thus, a baiting program becomes ineffective.
Rats need water every day. The amount varies, depending on the moisture content of their food, but is usually around to one fluid ounce. Rats prefer to live where water is easily available.
Rats usually begin foraging just after dark. Most of their food gathering occurs between dusk and midnight, but short bursts of restlessness and activity can occur anytime, day or night. Rats commonly travel 100 to 150 feet from their nest looking for food and water and patrolling their territory. It is not unusual for a colony of rats that live outdoors to forage inside a building 100 feet away.
Outdoors, Norway rats usually dig burrows into the ground. The burrows are shallow (less than 18 inches) and usually short (less than three feet), with a central nest. Extra "bolt holes" are used for emergency escapes. They are hidden under grass or boards or lightly plugged with dirt. Burrow openings are two to four inches in diameter. Indoors, Norway rats nest inside walls, in the space between floors and ceilings, underneath equipment, between and under pallets, and in crawl spaces, storage rooms, and any cluttered area that is normally unoccupied. Norways prefer to live in the lower floors of a building.
Roof rats commonly nest above ground in trees -- particularly untrimmed palm trees, and in piles of wood or debris, vine-covered fences, and stacked lumber. Overgrown landscaping is also a prime nesting area. Roof rats will sometimes dig burrows if above-ground sites are limited and Norway rats are not occupying the area. Indoors, roof rats prefer the upper levels of a building in the attic and in ceiling and attic voids near the roof line. But at times they also live in the lower levels of a building as do Norways.
Both species also occupy sewers and storm drains, and both on occasion can be found in highly unusual nest sites. Both Norway and roof rats can have several "hotel" sites in an area. A rat may spend a week in its home base and then move for a day or two into a secondary "hotel" site. Norway rats have been shown on occasion to have a home range of up to 20 acres when these secondary sites were included in the calculations.
Rats give many signs that they are infesting an area. Inspection will determine if a site is infested, and will identify where rats are feeding and nesting, their patterns of movement, the size of the population, and the extent of the infestation. This helps the pest control technicians decide what control measures to use, where and how to use them, and how much effort is needed to put the program in place.
An inspection using a powerful flashlight just after dark is the best way to see rats. Dead rats are signs of a current or past infestation. If all that are found are old dried carcasses and skeletons, it may mean an old infestation. Many fresh carcasses are an indication that someone may be baiting the area currently. If rats are actively observed during the day, the rat population is probably high.
When a building is quiet, squeaks and fighting noises, clawing and scrambling in walls, or gnawing sounds may be heard.
A single rat may produce 50 droppings daily. Roof rat droppings are generally smaller (half-inch) than Norway rat's (three-quarter inch). The highest number of droppings will be found in locations where rats rest or feed.
Both wet and dry urine stains will glow blue-white under an ultraviolet light (blacklight).
Oil and dirt rub off of a rat's coat as it scrambles along. The grease marks build up in frequented runways and become noticeable.
Outdoors, rats constantly travel the same route; their runways appear as beaten paths on the ground.
A rat's foot print is about three-quarter inches long, and may show four or five toes. Rats may also leave a "tail drag" line in the middle of their tracks.
A rat's incisor teeth grow at a rate of about five inches per year. Rats keep their teeth worn down by continuously working them against each other and by gnawing on hard surfaces.
Roof rats, in particular, often nest or store food in the attics of buildings. Roof rat nests may also be found when dense vegetation is trimmed.
Outdoors, rat burrows may be found singly or in groups along foundation walls, under slabs and dumpster pads, in overgrown weedy areas, beneath debris, and in embankments.
Cats and dogs may excitedly probe an area of floor or wall where rats are present, especially if the rats have only recently invaded.
Heavy infestations have a distinctive odor which can be identified with practice. The odor of rats can be distinguished from the odor of mice.
It's not easy to tell how many rats are infesting a site. As a rough guide, you can use rat signs to characterize the population as low, medium, or high.
Most successful rat control programs use a combination of tools and procedures to knock down the rat population, and to keep it down. Methods used combine habitat alteration and pesticide application. Some of the tools, such as baiting and trapping, are lethal to the rat. Some tools are not; rat-proofing, for example. Sometimes applicators recommend changes that their customers need to make, such as increasing the frequency of garbage pickup or making building repairs.
The following sections describe some of the major techniques and tools used in controlling rats:
Food. Like all animals, rats need food to survive. Baiting programs often fail because the bait can't compete with the rats' regular food. The rats simply ignore the baits or cache them. Reducing the rats' normal food encourages them to feed on any rodenticide baits placed in their territory.
Eliminate hiding places.
Outdoors
Indoors
Long term, the most successful form of rat control is to build them out. Also called rat-proofing, this technique makes it impossible for rats to get into a building or an area of a building. Rat-proofing prevents new rats from reinfesting a building once it has been cleared.
Building Exterior.
Building Interior.
Snap Trap. The snap trap is an effective method of killing rats when used correctly. Trapping is advised for use in places where rodenticides are considered too risky or aren't working well, if the odor of dead rats in wall or ceiling voids would be unacceptable, or when there are only a few rats infesting a limited area.
Trapping has several advantages. There is less nontarget risk than from a toxicant. The technician knows instantly whether or not the trap has been successful. Traps also allow for disposal of the carcass so that there are no odor problems.
Careful attention to detail is necessary to ensure proper placement in adequate numbers or rats will simply pass them by.
The best traps are those with expanded triggers (treadles) set for a light touch.
Glue Boards. Another way to trap rats is with glue boards. Glue boards use a sticky material that captures rodents. Although most often used against mice, they are sometimes effective against rats. Be sure to use larger glue boards that have been designed to trap an animal the size of a rat. Be aware that some consider glue boards inhumane, since they often kill the rodents.
A rodenticide is a pesticide designed to kill rodents. There are four major formulations of rodenticides used to control rats: food baits, water baits, tracking powders and fumigants.
Food Baits. Rat baits combine a poison effective against rats with a food bait attractive to rats. At one time, applicators mixed their own baits. Now baits are mostly purchased ready-made and packaged as extruded pellets, in a dry meal, or molded into paraffin blocks for wet sites. Baits may be obtained in 45-pound bulk tubs, in place packs containing less than one ounce of bait, or anything in between.
Some baits kill rats after a single feeding, some require multiple feedings. Some are anticoagulants [causing rats to bleed to death], some affect respiration, and others have totally different modes of action. Some are only slightly toxic to people or pets, some moderately toxic, and some very toxic.
Many of the old, ancient poisons that were toxic to humans were also used to poison rodents. Experimentation with poisons for killing rodents, produced rodenticides made of arsnics, cyanide, strychnine, etc.: stomach poisons, that were mixed with food and had such extreme toxicity that they killed any animal that ingested them in sufficient amounts. Rats that did not eat a lethal dose, however, recovered, became "bait shy" and communicated their preference -- or revulsion -- to others in the colony. Because of this, these poisons were undependable.
A new type of rodenticide was developed in the 1940's that reduced the clotting ability of the blood. This material became Warfarin, the first anticoagulant rodenticide. Others followed: warfarin, coumafuryl, chlorophacinone, diphacinone, pindone, valone. The anticoagulants were effective and did not cause bait shyness. Several factors overcame the risks of acutely toxic poisons. While the anticoagulants could be lethal to warm-blooded animals, many species including poultry, farm animals, pets, and humans would have to consume large quantities over several days to cause fatalities. As well an antidote, vitamin K, was developed.
Evidence of resistance to anticoagulants and a desire for quicker results drove the successful search for single dose anticoagulants -- brodifacoum and bromadiolone. In recent years non-anticoagulant rodenticides with different modes of action, such as bromethalin or cholecalciferol, have been proven effective. Zinc phosphide, used as a single dose non-anticoagulant, is somewhat poisonous to all vertebrates. It is often used as a tracking powder and is licked from the fur when rodents groom themselves. It is also incorporated in dry baits. Zinc phosphide should never be mixed with bare hands nor applied without wearing gloves.
Remember, rodenticides must be used very carefully: they are made to kill animal species of the same class as humans.
Several general guidelines should be followed when using a poison bait. First and foremost, protect children, pets, wildlife, and domestic animals from eating the bait. All rodenticides have warnings on the label telling the applicator to place the bait "in locations not accessible to children, pets, wildlife, and domestic animals, or place in tamper-proof bait boxes." What are safe, inaccessible areas is determined by evaluating each case. Ask questions like these:
If so, change your placement or put the bait inside a tamper-proof bait box.
Bait boxes. A tamper-proof bait box is designed so that a child or pet cannot get to the bait inside, but the rat can. [Bait trays and flimsy plastic or cardboard stations are not tamper-proof bait boxes.] Tamper-proof boxes differ in the type and quality of construction, but they are usually metal or heavy plastic. Rat bait stations are normally larger than those used for mice. Most designs are not considered to be truly tamper-proof unless they can be secured to the floor, wall, or ground.
Water baits. Rats drink water daily if they can. When rat water supplies are short, water baits -- specially formulated rodenticides that are mixed with water -- can be extremely effective. Several types of liquid dispensers are available. The best are custom designed for toxic water baits, but plastic chick-founts can also be used in protected sites.
Tracking Powders. Rats groom themselves by licking their fur. Tracking powder makes use of this behavior. This formulation is a rodenticide carried on a talc or powdery clay, 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. The major advantage to tracking powders is that it can kill rats even when food and water is plentiful, or if rats have become bait or trap shy.
Fumigants. Aluminum phosphide tablets and ignitable gas cartridges are sometimes used for burrowing rodent control. Both can be placed into rat burrows to exterminate all inhabitants of the burrow. Burrow entrances must be sealed to achieve maximum success. Fumigants should not be used in the immediate vicinity of buildings that are occupied by humans or livestock. Burning gas cartridges can pose a fire hazard if used in association with wooden frame buildings.
Rats have adapted to nearly all human environments. Along the way, they have caused more human suffering and economic damage than any other vertebrate pest. But they are marvelous athletes and successful survivors as well. Successful long-term rat control is not simple. The key is to control rat populations, not individual rats.
The two most common pest rats are the Norway rat and the roof rat. To be controlled they must be understood. Two of the most important biological factors to help control rats are:
Successful rat control programs usually use a combination of tools and procedures to knock down a rat population and keep it down. Long-term, the most successful form of rat control is to build them out, also called rat-proofing. Other control tactics include trapping and poisons. When using rodenticide baits and tracking powders, care must be taken to avoid risks to people, children, pets, and nontarget animals.
Thanks to: Pesticide Education Resources @ University of Nebraska-Lincoln
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