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Rodents (Latin rodere, to gnaw) are mammals of the order
Rodentia, characterized by a single pair of continuously growing incisors in
each of the upper and lower jaws. About forty percent of all mammal species are
rodents, and they are found in vast numbers on all continents except
Antarctica. They are the most diversified mammalian order and can be found in a
variety of terrestrial habitats, including human-made environments. There are
species that are arboreal, fossorial (burrowing), and semi-aquatic. Well known
rodents include mice, rats, squirrels, prairie dogs, porcupines, beavers,
guinea pigs, and hamsters. Other animals such as rabbits, hares and pikas,
which could be confused with rodents, were once included with them, but are now
considered to be in a separate order, Lagomorpha.
Most rodents are small animals with robust bodies, short
limbs and long tails, but there are exceptions to this. They use their sharp
incisors to gnaw food, excavate burrows and defend themselves. Most eat seeds
or other plant material, but some have more varied diets. They tend to be
social animals and many species live in societies with complex ways of
communicating with each other. Mating among rodents can vary from monogamy, to
polygyny, to promiscuity. Many have litters of underdeveloped, altricial young,
while others have precocial young that are relatively well developed at birth.
The rodent fossil record dates back to the Paleocene on the
supercontinent of Laurasia. They greatly diversified in the Eocene, as they
spread across continents, sometimes even finding means to cross oceans. Rodents
reached both South America and Madagascar from Africa; they were also the only
terrestrial placental mammals to reach and colonize Australia.
Rodents have been used as food, for clothing, as pets and as
laboratory animals in research. Some species, in particular the brown rat, the
black rat and the house mouse, are serious pests, eating and spoiling food
stored by humans, and spreading diseases. Accidentally introduced species of rodents
are often considered to be invasive, as they sometimes threaten the survival of
native species, such as island birds, previously isolated from land-based
predators.
CHARACTERISTICS
The distinguishing feature of the rodents is their single
pair of continuously growing, razor-sharp incisors. These incisors have thick
layers of enamel on the front and little enamel on the back.Because they do not
stop growing, the animal must continue to wear them down so that they do not
reach or even pierce the skull. As the incisors grind against each other, the
softer dentine on the rear of the teeth wears away, leaving the sharp enamel
edge shaped like the blade of a chisel. Most species have up to 22 teeth with
no canines or anterior premolars. There is a gap, or diastema, between the
incisors and the cheek teeth in most species. This allows them to suck in their
cheeks or lips to shield their mouths and throats from wood shavings or other
inedible material, and discard this waste from the side of the mouth.
Chinchillas and guinea pigs have a high fibre diet, and their molars have no
roots and grow continuously like the incisors.
In many species, the molars are relatively large,
intricately structured and highly cusped or ridged, though some, such as
Pseudohydromys, have smaller and simpler ones. Rodent molars are well equipped
to grind food into small particles. The jaw musculature is strong. The lower
jaw is thrust forward while gnawing and is pulled backwards during chewing.
Rodent groups differ in the arrangement of the jaw muscles and associated skull
structures, both from other mammals and amongst themselves. The Sciuromorpha,
such as the Eastern grey squirrel, have a large deep masseter, making them
efficient at biting with the incisors. The Myomorpha, such as the brown rat,
have an enlarged temporalis muscle, making them able to chew powerfully with
the molars. The Hystricomorpha, such as the guinea pig, have a larger
superficial masseter muscle and a smaller deep masseter muscle than rats or
squirrels, possibly making them less efficient at biting with the incisors, but
their enlarged internal pterygoid muscle may allow them to move the jaw further
sideways when chewing.
While the largest species, the capybara, can weigh as much as
66 kg (146 lb), most rodents weigh less than 100 g (3.5 oz). The smallest
rodent is the Baluchistan pygmy jerboa, which averages only 4.4 cm (1.7 in) in
head and body length, with adult females weighing only 3.75 g (0.132 oz).
Rodents have wide-ranging morphologies, but typically have squat bodies and
short limbs. The forelimbs usually have five digits, including an opposable
thumb, while the hind-limbs have three to five digits. The elbow gives the fore-arms
great flexibility. The majority of species are plantigrade, walking on both the
palms and soles of their feet, and have claw-like nails. The nails of burrowing
species tend to be long and strong, while arboreal rodents have shorter,
sharper nails. Rodent species use a wide variety of methods of locomotion
including quadrupedal walking, running, burrowing and climbing, bipedal hopping
(kangaroo rat and hopping mouse), swimming and even gliding. Scaly-tailed
squirrels and flying squirrels, although not closely related, can both glide
from tree to tree using parachute-like membranes that stretch from the fore-limbs
to the hind-limbs. The agouti is fleet-footed and antelope-like, being
digitigrade and having hoof-like nails. The majority of rodents have tails,
which can be of many shapes and sizes. Some tails are prehensile, as in the
Eurasian harvest mouse, and the fur on the tails can vary from bushy to
completely bald. The tail is sometimes used for communication, as when beavers
slap their tails on the water surface or house mice rattle their tails to
indicate alarm. Some species have vestigial tails or no tails at all. In some
species, the tail is capable of regeneration if a part is broken off.
Wood mouse with its long whiskers
Rodents generally have well-developed senses of smell,
hearing and vision. Nocturnal species often have enlarged eyes and some are
sensitive to ultraviolet light. Many species have long, sensitive whiskers or
vibrissae for touch or "whisking". Some rodents have cheek pouches,
which may be lined with fur. These can be turned inside out for cleaning. In
many species, the tongue cannot reach past the incisors. Rodents have efficient
digestive systems, absorbing nearly 80 percent of ingested energy. When eating
cellulose, the food is softened in the stomach and passed to the cecum, where
bacteria reduce it to its carbohydrate elements. The rodent then practises
coprophagy, eating its own fecal pellets, so that the nutrients can be absorbed
by the gut. Rodents therefore often produce a hard and dry fecal pellet. In
many species, the penis contains a bone, the baculum; the testes can be located
either abdominally or at the groin.
Sexual dimorphism occurs in many rodent species. In some
rodents, males are larger than females while in others the reverse is true.
Male-bias sexual dimorphism is typical for ground squirrels, kangaroo rats,
solitary mole rats and pocket gophers; it likely developed due to sexual
selection and greater male-male combat. Female-bias sexual dimorphism exists
among chipmunks and jumping mice. It is not understood why this pattern occurs,
but in the case of yellow-pine chipmunks it may be that males selected larger
females due to their greater reproductive success. In some species, like voles,
sexual dimorphism can vary from population to population. In bank voles,
females are typically larger than males, however male-bias sexual dimorphism
occurs in alpine populations, possibly because of the lack of predators and
greater competition between males.
DISTRIBUTION AND HABITAT
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One of the most widespread groups of mammals, rodents can be
found on every continent except Antarctica. They are the only terrestrial
placental mammals that have colonized Australia and New Guinea without human
intervention. Humans have also allowed the animals to spread to many remote
oceanic islands (e.g., the Polynesian rat).Rodents have adapted to almost every
terrestrial habitat, from cold tundra (where they can live under snow) to hot
deserts. Some species such as tree squirrels and New World porcupines are
arboreal, while some, such as gophers and mole rats, live almost completely
underground, where they build complex burrow systems. Others dwell on the
surface of the ground but may have a burrow into which they can retreat.
Beavers and muskrats are known for being semi-aquatic,but the rodent
best-adapted for aquatic life is probably the earless water rat from New
Guinea. Rodents have also thrived in human-created environments such as agricultural
and urban areas.
Some rodents, like this American beaver with its dam of
gnawed tree trunks and the lake it has created, are considered ecosystem engineers.
Though some species are common pests for humans, rodents
also play important ecological roles. Some rodents are considered keystone
species and ecosystem engineers in their respective habitats. In the Great
Plains of North America, the burrowing activities of prairie dogs play
important roles in soil aeration and nutrient redistribution, raising the
organic content of the soil and increasing the absorption of water. They maintain
these grassland habitats, and some large herbivores like bison and pronghorn
prefer to graze near prairie dog colonies due to the increased nutritional
quality of forage. Prairie dogs can, however, also contribute to regional and
local biodiversity loss, increased seed depredation and the establishment and
spread of invasive shrubs. Burrowing rodents may eat the fruiting bodies of
fungi and spread spores through their feces, thereby allowing the fungi to
disperse and form symbiotic relationships with the roots of plants (which
usually cannot thrive without them). As such, these rodents may play a role in maintaining
healthy forests.
In many temperate regions, beavers play an essential
hydrological role. When building their dams and lodges, beavers alter the paths
of streams and rivers and allow for the creation of extensive wetland habitats.
One study found that engineering by beavers leads to a 33 percent increase in
the number of herbaceous plant species in riparian areas. Another study found
that beavers increase wild salmon populations.
TYPES OF RATS
NORWAY RATS
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Identification Habitat
The Norway rat (Rattus norvegicus, ) is a stocky burrowing
rodent, unintentionally introduced into North America by settlers who arrived
on ships from Europe. Also called the brown rat, house rat, barn rat, sewer
rat, gray rat, or wharf rat, it is a slightly larger animal than the roof rat .
Adult Norway rats weigh an average of 1 pound (454 g). Their fur is coarse and
usually brownish or reddish gray above and whitish gray on the belly. Blackish
individuals occur in some locations.
Range
First introduced into the United States around 1775, the
Norway rat has now spread throughout the contiguous 48 states. It is generally
found at lower elevations but may occur wherever humans live.
Habitat
Norway rats live in close association with people. In urban
or suburban areas they live in and around residences, in cellars, warehouses,
stores, slaughterhouses, docks, and in sewers. On farms they may inhabit barns,
granaries, livestock buildings, silos, and kennels .They may burrow to make
nests under buildings and other structures, beneath concrete slabs, along
stream banks, around ponds, in garbage dumps, and at other locations where
suitable food, water, and shelter are present. Although they can climb, Norway
rats tend to inhabit the lower floors of multistory buildings.
Food Habits
Norway rats will eat nearly any type of food. When given a
choice, they select a nutritionally balanced diet, choosing fresh, wholesome
items over stale or contaminated foods. They prefer cereal grains, meats and
fish, nuts, and some types of fruit. Rats require 1/2 to 1 ounce (15 to 30 ml)
of water daily when feeding on dry foods but need less when moist foods are
available. Food items in household garbage offer a fairly balanced diet and
also satisfy their moisture needs.
General Biology, Reproduction, and Behavior
Norway rats are primarily nocturnal. They usually become
active about dusk, when they begin to seek food and water. Some individuals may
be active during daylight hours when rat populations are high.
Rats have poor eyesight, relying more on their hearing and
their excellent senses of smell, taste, and touch. They are considered
color-blind. Therefore, for safety reasons, baits can be dyed distinctive
colors without causing avoidance by rats, as long as the dye does not have an
objectionable taste or odor.
Rats use their keen sense of smell to locate food items and
to recognize other rats. Their sense of taste is excellent, and they can detect
some contaminants in their food at levels as low as 0.5 parts per million.
Norway rats usually construct nests in below-ground burrows
or at ground level . Nests may be lined with shredded paper, cloth, or other
fibrous material. Litters of 6 to 12 young are born 21 to 23 days after
conception. Newborn rats are hairless and their eyes are closed, but they grow
rapidly. They can eat solid food at 2 1/2 to 3 weeks. They become completely
independent at about 3 to 4 weeks and reach reproductive maturity at 3 months
of age.
Females may come into heat every 4 or 5 days, and they may
mate within a day or two after a litter is born. Breeding often peaks in spring
and fall, with reproductive activity declining during the heat of summer and
often stopping completely in winter, depending on habitat. These seasonal
trends are most pronounced in more severe climates. The average female rat has
4 to 6 litters per year and may successfully wean 20 or more offspring
annually.
Norway rats have physical capabilities that enable them to
gain entry to structures by gnawing, climbing, jumping, swimming, and other
tactics. For more detailed information on their physical abilities and the
resulting need to design rodent-proof structures, see the chapter Rodent-Proof
Construction and Exclusion Methods.
Studies indicate that during its daily activities, a rat
normally travels an area averaging 100 to 150 feet (30 to 45 m) in diameter.
Rats seldom travel farther than 300 feet (100 m) from their burrows to obtain
food or water.
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
bait stations often are avoided for several days or more following their
initial placement.
Norway rat burrow system
Place baits and bait stations near, but not on, rat runways.
Rats will quickly find them and after a short period of avoidance, will
cautiously investigate them. Baited but unset traps will aid in overcoming
rats’ fear of them; expanded-trigger traps set directly on travel routes may
immediately catch rats.
Rats will at first avoid novel food items placed in their
environment. They may eat very small amounts, and subsequent feeding will
depend on the flavor of the food and its physiological effect. If the food
contains poison or some other substance that soon produces an ill effect but
not death, the food will often be associated with the illness. This “bait
shyness” was a major problems when single-dose acute toxicants were the main
rodenticides in use. Today, only two rodenticides registered for Norway rat
control, red squill and zinc phosphide, possess characteristics that make bait
shyness a potential problem.
Bait shyness can persist for weeks or months and may be
transferred to nontoxic foods of similar types. Pre-baiting, that is, training
rats to feed repeatedly on nontoxic bait for a period of days prior to applying
the toxicant in the bait, will largely prevent sublethal doses and thus bait
shyness. It will also lead to successful control, with very few rats left to
become bait-shy. Prebaiting will almost always increase control success when
zinc phosphide or red squill baits are used.
Because anticoagulant rodenticides are slow-acting, the
rats’ subsequent illness is not associated with the bait even if a sublethal
dose is consumed; thus, bait shyness does not usually occur. These baits serve,
in effect, as their own prebait.
Damage and Damage Identification
Norway rats consume and contaminate foodstuffs and animal
feed. They may damage crops in fields prior to and during harvest, and during
processing and storage. Rats also damage containers and packaging materials in
which foods and feed are stored.
Rats cause structural damage to buildings by burrowing and
gnawing. They undermine building foundations and slabs, cause settling in roads
and railroad track beds, and damage the banks of irrigation canals and levees.
Rats also may gnaw on electrical wires or water pipes, either in structures or
below ground. They damage structures further by gnawing openings through doors,
window sills, walls, ceilings, and floors. Considerable damage to insulated structures
can occur as a result of rat burrowing and nesting in walls and attics.
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 a variety of
rodents, but it is more commonly associated with roof rats (Rattus rattus) than
with Norway rats.
Rat Sign
The presence of rats can be determined by a number of signs
described below: Droppings may be found along runways, in feeding areas, and
near shelter. They may be as large as 3/4 inch (2 cm) long and 1/4 inch (0.6
cm) in diameter. Fresh droppings are soft in texture.
Tracks, including footprints or tail marks, may be seen on dusty
surfaces or in mud . A tracking patch made of flour can be placed in pathways
overnight to determine if rodents are present.
Urine, both wet and dry, will fluoresce under ultraviolet
light. Urine stains may occur along travelways or in feeding areas.
Runs or burrows may be found next to walls, along fences,
next to buildings, or under bushes and debris. Rats memorize pathways and use
the same routes habitually.
Smudge marks (rub marks) may occur on beams, rafters, pipes,
and walls as a result of oil and dirt rubbing off rats’ fur along frequently
traveled routes .
Gnawing may be visible on doors, ledges, in corners, in wall
material, on stored materials, or other surfaces wherever rats are present.
Fresh accumulations of wood shavings, insulation, and other gnawed material
indicate active infestations. Size of entry holes (often 1 1/2 inches [4 cm] in
diameter or less for mice, 2 inches [5 cm] or larger for rats) or tooth marks
can be used to distinguish rat from mouse gnawing. Rats keep their paired
incisor teeth, which grow continuously at the rate of about 5 inches (13 cm)
per year, worn down by gnawing on hard surfaces and by working them against
each other.
Sounds such as gnawing, climbing in walls, clawing, various
squeaks, and fighting noises are common where rats are present, particularly at
times of the day when they are most active.
rat prints and rub marks Estimating Rat Numbers
Rat sign and visual sightings are of limited value in
accurately estimating rat numbers, but they are the simplest and often the only
practical method available. Search premises thoroughly when looking for rats.
In structures, searches should include attics, basements, around foundations,
crawl spaces, and behind and under stored materials. The following estimates
can then be made:
No sign: no rats or few present. If only a few rats are
present they may have invaded only recently.
Old droppings and gnawing common, one or more rats seen by
flashlight at night, or no rats observed in daytime: medium numbers present.
Fresh droppings, tracks, and gnawing present, three or more
rats seen at night, or rats seen in daytime: large numbers present.
Since rats are normally nocturnal and somewhat wary of
humans, usually many more rats are present than will be seen in the daytime.
Under certain conditions, rats may become quite bold in the presence of humans,
and then a high percentage of the population may be visible.
A conservative estimate of rat numbers can be made from
measuring their food consumption. You can do this by feeding the rats for a
while on finely ground grain (whole grains or pelleted foods may be carried off
uneaten). When offered over a period of time, the ground grain will usually be
accepted and eaten by rats. Consumption may gradually increase to a maximum
level over the period of a week or so as the rats’ natural fear of novel foods
is overcome. Divide the total amount of food eaten per day by 1/2 ounce (15 g);
this will give a minimum estimate of the rats present. Some rats eat more than
1/2 ounce (15g) daily, but rats will probably also be using other foods in
their environment. If too much alternative food is available, this technique
will not give an adequate estimate.
ROOF RATS
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Appearance
Black or brown, can be over 40 cm long, with a long tail,
large ears and eyes, and a pointed nose. Body is smaller and sleeker than the
Norway rat’s. Fur is smooth.
roof rat image
Behavior, Diet & Habits
Nests inside and under buildings, or in piles of rubbish or
wood. Excellent climber that can often be found in the upper parts of
structures.
Omnivorous, but shows a preference for grains, fruits, nuts
and vegetables.
Reproduction
Becomes sexually mature between two and five months,
producing four to six litters per year that consist of six to eight young each.
Lives up to one year.
Roof rats are prodigious breeders. Females can breed
year-round. Within a year, one female may be responsible for up to 40 new
rodents.
Signs of Roof Rat Infestation
Visual sightings of live or dead rodents indicate rodent
activity. If roof rats are seen exposed, it often indicates their hiding spaces
are all filled by other rats or that they have been disturbed, such as by
construction. Droppings are another good indicator of roof rat activity. Roof
rat droppings are 12 to 13 mm with pointed ends, whereas Norway rat droppings
are 18 to 20 mm and capsule shaped. Other indicators can include grease marks
along surfaces as well as nests. Grease
marks are produced as the rodent travels along an edge, and the oils in their
fur are deposited. Indoor nests usually are constructed in insulation such as
in attics.
More Information
Black rats have long been named as carriers of the fleas
responsible for the Black Death in the Middle Ages. While this plague is no
longer as serious a threat to humans, roof rats are still potential carriers of
disease.
The first step in controlling a roof rat infestation is to
properly identify the rodents. Roof rats have hairless, scaly tails that are
longer than their heads and bodies. These rats are nocturnal and are excellent
climbers.
To prevent a colony from nesting in your home, make sure
that all the windows and vents are screened. Roof rats can also enter openings
in walls, eaves and roof from the branches of trees. Trim all tree branches to
further prevent entry.
definition
The house mouse (Mus musculus,) is a small, slender rodent
that has a slightly pointed nose; small, black, somewhat protruding eyes; large,
sparsely haired ears; and a nearly hairless tail with obvious scale rings.
House mice are considered among the most troublesome and economically important
rodents in the United States.
Adult house mice weigh about 2/5 to 4/5 ounce (11 to 22
grams). They are generally grayish brown with a gray or buff belly. Similar
mice include the white-footed mice and jumping mice (which have a white belly),
and harvest mice (which have grooved upper incisor teeth). For more details on
species identification, see a field guide such as that by Burt and
Grossenheider (1976).
Native to central Asia, this species arrived in North
America with settlers from Europe and from other points of origin. A very
adaptable species, the house mouse often lives in close association with humans
and therefore is termed one of the “commensal” rodents along with Norway and
roof rats. House mice are much more common in residences and commercial
structures than are rats. Brooks (1973) regards them to be the most common
mammal in cities, next to humans.
Range
Following their arrival on colonists’ ships, house mice
spread across North America and are now found in every state, including coastal
areas of Alaska, and in the southern parts of Canada.
Habitat
House mice live in and around homes, farms, commercial
establishments, and in open fields and agricultural lands. At times they may be
found living far from human settlements, particularly where climates are
moderate. The onset of cold weather each fall in temperate regions may cause mice
to move into structures in search of shelter and food.
Food Habits
House mice eat many types of food but prefer seeds and
grain. They are not hesitant to eat new foods and are considered “nibblers,”
sampling many kinds of items that may exist in their environment. Foods high in
fat, protein, or sugar may be preferred even when grain and seed are present.
Such items include bacon, chocolate candies, butter, and nutmeats.
Unlike Norway and roof rats, house mice can survive with
little or no free water, although they readily drink water when it is
available. They obtain their water from the food they eat. An absence of liquid
water or food of adequate moisture content in their environment may reduce
their breeding potential.
General Biology, Reproduction, and Behavior
House mice are mainly nocturnal, although at some locations
considerable daytime activity may be seen. Seeing mice during daylight hours
does not necessarily mean that a high population is present, although this is
usually true for rats.
Mice have poor eyesight, relying on their hearing and their
excellent senses of smell, taste, and touch. They are considered color-blind;
therefore, for safety reasons, baits can be dyed distinctive colors without
causing avoidance by mice, as long as the dye does not have an objectionable
taste or odor.
House mice may burrow into the ground in fields or around
structures when other shelter is not readily available. Nesting may occur in
the ground or in any sheltered location. Nests are constructed of shredded
fibrous materials such as paper, burlap, or other similar items, and generally
have the appearance of a “ball” of material loosely woven together. They are
usually 4 to 6 inches (10.2 to 15.2 cm) in diameter.
Litters of 5 or 6 young are born 19 to 21 days after mating,
although females that conceive while still nursing may have a slightly longer
gestation period. Mice are born hairless and with their eyes closed. They grow
rapidly, and after 2 weeks they are covered with hair and their eyes and ears
are open. They begin to make short excursions from the nest and eat solid food
at 3 weeks. Weaning soon follows, and mice are sexually mature at 6 to 10 weeks
of age.
Mice may breed year-round, but when living outdoors, they
breed mostly in spring and fall. A female may have 5 to 10 litters per year.
Mouse populations can therefore grow rapidly under good conditions, although
breeding and survival of young decline markedly when population densities
become high.
House mice have physical capabilities that enable them to
gain entry to structures by gnawing, climbing, jumping, and swimming. For more
detailed information on their physical abilities and the resulting need to
design rodent-proof structures, see the chapter Rodent-Proof Construction and
Exclusion Methods.
Studies indicate that during its daily activities, a mouse
normally travels an area averaging 10 to 30 feet (3 m to 9 m) in diameter. Mice
seldom travel farther than this to obtain food or water. Because of their
limited movement and feeding behavior, both of which differ from those of
commensal rats, they are much more difficult to control in some situations.
Mice 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 new objects in their environment
but, unlike rats, do not fear them. Thus, they will almost immediately enter
bait stations and sample new foods (baits). The degree to which mice consume a
particular food depends on the flavor of the food in addition to its
physiological effect. Mice may reject baits simply because they do not taste as
good as other available foods.
If the bait contains poison or some other substance that
produces an ill effect (but not death) within a few hours, the bait will often
become associated with the illness. Bait shyness can persist for weeks or
months and may be transferred to nontoxic foods of similar types. Prebaiting,
that is, training mice to feed repeatedly on nontoxic bait for a period of days
prior to applying the toxicant in the bait, will largely prevent sublethal
doses and thus bait shyness. It will also reduce the number of mice left to be
bait shy. Prebaiting is especially recommended with zinc phosphide baits. All
of the other toxic baits currently registered for house mice are chronic or slow-acting.
Because of this slow action, the mice’s subsequent illness is not associated
with the bait even if a sublethal dose is consumed; thus, bait shyness does not
usually occur. These baits, in effect, serve as their own prebait.
Damage and Damage Identification
When house mice live in or around structures, they almost
always cause some degree of economic damage. In homes and commercial buildings,
they may feed on various stored food items or pet foods. In addition, they
usually contaminate foodstuffs with their urine, droppings, and hair. On farms,
they may cause damage to feed storage structures and feed transporting
equipment. A single mouse eats only about 3 grams of food per day (8 pounds
[3.6 kg] per year) but destroys considerably more food than it consumes because
of its habit of nibbling on many foods and discarding partially eaten items.
House mice living in fields may dig up and feed on newly
planted grain, or may cause some damage to crops before harvest. But losses in
stored foods are considerably greater. Mice commonly damage containers and
packaging materials in warehouses where food and feeds are stored. Much of this
loss is due to contamination with droppings and urine, making food unfit for
human consumption.
House mice cause structural damage to buildings by their
gnawing and nest-building activities. In livestock confinement facilities and
similar structures, they may quickly cause extensive damage to insulation
inside walls and attics. Such damage also occurs in homes, apartments, offices,
and commercial buildings but usually at a slower rate because mouse populations
in such structures are smaller. House mice often make homes in large electrical
appliances, and here they may chew up wiring as well as insulation, resulting
in short circuits which create fire hazards or other malfunctions that are
expensive to repair. Mice may also damage stored items in attics, basements,
garages, or museums. Damaged family heirlooms, paintings, books, documents, and
other such items may be impossible to replace.
Among the diseases mice or their parasites may transmit to
humans are salmonellosis (food poisoning), rickettsialpox, and lymphocytic
choriomeningitis. Mice may also carry leptospirosis, ratbite fever, tapeworms,
and organisms that can cause ringworm (a ungal skin disease) in humans. They
have also been found to act as reservoirs or transmitters of diseases of
veterinary importance, such as swine dysentery, a serious bacterial disease of
swine often called “bloody scours.” . Tracks left in dust by (a) Norway rat and
(b) house mouse.
Mouse Sign
The presence of house mice can be determined by a number of
signs described below:
Droppings may be found along runways, in feeding areas, and
near shelter. Differentiating between mouse droppings and those of certain
insects may be difficult. Mouse droppings are about 1/4 inch (0.6 cm) long,
whereas those of cockroaches are usually 1/8 to 1/4 inch (0.3 to 0.6 cm) long
and under a magnifying glass show distinct longitudinal ridges and squared-off
ends. In comparison, droppings of bats contain insect fragments and are more
easily crushed between the fingers.
Tracks, including footprints or tail marks, may be seen on dusty
surfaces or in mud . A tracking patch made of flour, rolled smooth with a
cylindrical object, can be placed in pathways overnight to determine if rodents
are present.
Urine, both wet and dry, will fluoresce under ultraviolet
light, although so will some other materials. Urine stains may occur along
travelways or in feeding areas.
Smudge marks (rub marks) may occur on beams, rafters, pipes,
walls, and other parts of structures. They are the result of oil and dirt
rubbing off mice’s fur along frequently traveled routes . They may be less apparent
than rub marks left by rats.
Gnawing may be visible on doors, ledges, in corners, in wall
material, on stored materials, or on other surfaces wherever mice are present.
Fresh accumulations of wood shavings, insulation, and other gnawed material
indicate active infestations. Size of entry holes (often 1 1/2 inches [3.8 cm]
in diameter or less for mice, 2 inches [5 cm] or larger for rat) or tooth marks
can be used to distinguish rat gnawing from mouse gnawing. Mice keep their
paired incisor teeth, which grow continuously, worn down by gnawing on hard
surfaces and by working them against each other
. Rub marks along
beams, rafters, or other travel routes give evidence of rodent activity. Mouse
rub marks can be distinguished from those of rats by their smaller size.
Sounds such as gnawing, climbing in walls, running across
the upper surface of ceilings, and squeaks are common where mice are present.
Visual sightings of mice may be possible during daylight
hours, and mice also can be seen after dark with the aid of a flashlight or
spotlight.
Nests frequently are found when cleaning garages, closets,
attics, basements, and outbuildings where mice are present. They consist of
fine, shredded fibrous materials
Odors may indicate the presence of house mice. A
characteristic musky odor is a positive indication that house mice are present,
and this odor can be used to differentiate their presence from that of rats.
Estimating Mouse Numbers. Mouse sign and visual sightings
are of limited value in accurately estimating mouse numbers, but they are the
simplest and often the only practical method available. Search premises
thoroughly when looking for mice. In structures, searches should include
attics, basements, around foundations, crawl spaces, and behind and under
stored materials.
One method to detect the presence of mice is to make
nontoxic tracking-dust patches of flour or talc at 20- to 30-foot (6- to 9-m)
intervals throughout a structure. The number of patches showing tracks after 24
hours, and the abundance of tracks in each patch, indicate the size of the
population. Because house mice, unlike rats, do not travel far from their nests
or shelter, the percentage of patches showing tracks is a good indicator of the
relative size and distribution of the mouse population.
Snap trapping is also an excellent way to determine the
presence of mice. A relative index of mouse abundance can be calculated from
the number of mice trapped for a certain number of traps set during 1 or more
nights (for example, 35 mice caught per 100 trap nights).
Legal Status
House mice are not protected by law. They may be controlled
using any pesticide registered by federal or state authorities for this
purpose, or they may be controlled by use of mechanical methods such as traps.
Damage Prevention and Control Methods
Effective prevention and control of house mouse damage
involves three aspects: rodent-proof construction, sanitation, and population
reduction by means of traps, toxicants, or fumigants. The first two are useful
as preventive measures, but when a house mouse infestation already exists, some
form of population reduction is almost always necessary. A flow chart outlining
steps in controlling house mice is found .
Control of house mice differs in important ways from the
control of Norway or roof rats. Mice are smaller and therefore can enter
narrower openings, making rodent-proofing more difficult. They have limited
areas of movement (home range) and require little or no free water. While
having a reproductive capability that is higher than that of rats, house mice
are usually less sensitive (often far less sensitive) to many rodenticides.
Persons who do not take these differences into account when attempting house
mouse control may expect poor results.
After rats are controlled at a given location, house mice
may increase in numbers by moving in from elsewhere or by reproduction. This
may be expected because habitats suitable for rats are usually even more
suitable for mice. One should anticipate that following rat control, the potential
for house mouse problems may increase, and control measures should be taken
before mouse numbers reach high levels.
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