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Chapter 12 IPM for Rats and Mice
Chapter 12




IPM for Rats and Mice
Introduction
Although the use of toxic baits is a common approach
to reducing rodent problems, they have been
overused in some areas, and associated populations
of rats and mice have developed a resistance to
anticoagulant baits. Even when baits remain
effective, poisoned rodents frequently die in
inaccessible places where their decomposing bodies
create unpleasant odors and feed pest insects such as
flesh flies and carpet beetles. Moreover, on school
grounds, there is always a risk that children or pet
animals will inadvertently come in contact with the
bait.

A better approach combines careful inspection,
regular monitoring, sanitation, garbage management,
rodent-proofing, trapping, and, if necessary, baiting
with toxicants. Unless the conditions that attracted
rodents in the first place are changed, new mice and
rats often move into the habitat vacated by the dead
ones, and the cycle will continue.

Identification and Biology
It is important to identify the rodent species that is
responsible for the damage. Several characteristics
can be used to distinguish between the Norway rat
and house mouse (Table 12-1 and Figure 12-1) and
the house mouse and several species of native mice
(Table 12-2). After trapping a rodent, you can use
information from these tables to identify it.

Rats
The Norway rat is also known as the brown, wharf,
house, gray, or sewer rat. It occurs in every state.
They were introduced from Europe in the 1700s and
are now found throughout North America. Several
aspects of their life history, such as litter size and age
to maturity (Table 12-3), influence how we manage
this species.

Characteristics of rats that can have an impact on
management


will feed on a wide variety of materials (see
Table 12-4)
usually search for food between dusk and dawn
but when hungry or living under crowded
conditions may be seen in the daylight
require water daily unless food items are
succulent
can travel several hundred feet from their nests
in search of food
prefer traveling along edges, e.g., the edge of the
floor next to the wall, along the outside or inside
of a foundation
travel along pipes and rafters
wary of crossing open spaces that provide no
cover
poor eyesight but are quite sensitive to patterns,
contrasts, and movements
Figure 12-1. Comparison of Norway Rat and House Mouse.
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Chapter 12 IPM for Rats and Mice
acute senses of smell, taste, touch, and hearing;
navigate using their whiskers and guard hairs
tend to be extremely wary (though temporarily)
of new objects in their environment

In general, Norway rats build their nests in
underground burrows or in ground level areas in
buildings (Table 12-4).

Rats have amazing physical abilities (Box 12-A).
Understanding what they can and cannot do is very
important when planning ways to prevent rat
problems or to reduce the number of rats present.

Mice
The house mouse is the most common species to
invade structures, but native species of mice, such as
meadow voles and deer mice, are also occasional
problems, especially when temperatures drop in the
autumn. Usually native mice can be trapped easily
and removed and rarely become chronic problems.
Deer mice, however, are known vectors of hantavirus
(Box 12-B).

Invasions of the house mouse can occur at any time
of the year, and once inside, house mice will continue
to reproduce, generation after generation, without
leaving the confines of the building. The
reproductive potential of house mice is high (Table
12-5), and their physical abilities, like those of the
Norway rat, are remarkable (Box 12-C).

Characteristics of mice that can have an impact
on management


can generally get all the water they need from
food; if dependent on dry food, they need some
free water
travel over their entire home range daily (about
33 feet), investigating changes and new objects
prefer to travel along edges and are wary of
crossing open spaces
poor eyesight; navigate using their whiskers
indoors- often live in false ceilings; in appliances
such as stoves, refrigerators, air conditioners, and
coolers; in wall and floor voids; and in similar
enclosed spaces
outdoors- prefer thickly vegetated ground level
areas

Indoors, populations of mice are limited by the
availability of food, by competition from other
animals, and by disease. The amount of available
shelter inside can limit the number of mice to a
certain extent; however, in spring, summer, and fall,
mice can establish themselves outdoors. They need
to live inside only during the severe conditions of
winter. Rats prey on mice and compete for the same
food and shelter; therefore, removing the rats often
results in higher and more visible populations of
house mice.

Damage
Rodents damage food, clothing, documents, and
structures through gnawing, urination, defecation,
and nesting activities. The damage to food from
contamination is probably ten times greater than the
damage by direct feeding. Feces and urine raise the
humidity of enclosed spaces, promote wood
deterioration, and provide a medium for the
proliferation of microorganisms. Rodents cause fires
by chewing through the insulation on electrical wires,
and they are involved in spreading human pathogens
(Table 12-6). Hantavirus is transmitted by mice and
is a cause for (Box 12-C).

Detection and Monitoring
Inspect an area thoroughly to find as many sites of
activity as possible. At the same time, note all
possible harborage sites, sources of food and water,
and holes that provide access to the building. Many
signs can be used to detect the presence of rodents
(Box 12-D), and inspections should be conducted
both inside (Box 12-E) and outside (Box 12-F) of
structures to determine where problem areas occur.
Make detailed notes about problem areas on a map of
the building. Remember to inspect any outbuildings
on the property.

Effective monitoring requires several steps,
including the following:


make a site plan of the school with separate
drawings of each floor so you can accurately
record information;
lightly dust smooth surfaces near suspected
harborage, runs, or entry points with unscented
talcum powder or powdered chalk to gain further
information. Footprints and drag lines (made by
tails) across powdered surfaces indicate rodent
traffic. The powder can also be dusted onto a
heavy, smooth material such as a piece of floor
tile that can be moved around. Hold a flashlight
at a low angle to illuminate tracks on dusty
surfaces;
inspect at night with a strong flashlight. Look
for movement and listen for squeaking,
scrambling, and gnawing sounds;
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Chapter 12 IPM for Rats and Mice
Table 12-1. Differences Between the Norway Rat and House Mouse

Norway Rat
House Mouse
Scientific name
Rattus norvegicus
Mus musculus
Adult weight
3 to 21 ounces
1/2 ounce
Snout
blunt pointed
Ears
small & thick
with short hairs
large, some hair
Tail coloration
dark above, pale
underneath
all dark
Fur
brown with black;
shaggy
gray to light brown
Droppings

capsule-shaped,
pointed
rod-shaped, pointed
Food requirement
about 1 ounce/day
about 1/10 ounce/day
Water source
free water*
water from food; also need
free water if dependent on a
diet that is dry or high in
protein
Climbing ability
can climb
good climber
Nest locations
mainly in burrows
walls, attics, basements
near/in stored material
Swimming ability
excellent can
swim

* Water present by itself and not simply a constituent of the food eaten by the rodent. Free water is unnecessary when feeding on
succulent foods but needed if diet is dry and/or high in protein.

Table 12-2. Distinguishing the House Mouse from Other Similar Species. Common Name Scientific Name
Description
House Mouse
Mus musculus
small feet and head in proportion to body; long ears for body size;
relatively small eyes; tail nearly hairless and equal or longer than
the head and body combined; tail is uniformly dark
Deer Mouse or
Peromyscus spp.
same size or slightly larger than house mouse; distinctive
White-footed Mouse
bicolor coat, pale gray to reddish brown above white belly;
tail brown or gray on top and white underneath; large eyes;
invades buildings near fields or wooded areas
Vole or Meadow
Microtus spp.
large, robust body; weighs about twice as much as the house mouse;
Mouse

smaller, heavily furred ears; short tail; sometimes invades buildings, but
commonly found outdoors under boards, boxes, etc.
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Chapter 12 IPM for Rats and Mice
Table 12-3. The Biology of Norway Rats
Breeding
(estrous)
cycle
polyestrous, every 4-5 days; in subtropical
climates, rats can reproduce year around; in
cooler climates, populations peak in spring &
autumn

Litter size
average of 5 to 12

Litters per year
up to 9, depending on food
availability (average - 4)

Age at weaning
around 30 days

Gestation period
20 to 25 days

Sexual Maturity
75-