J^m *^o A Publication of The College of

\ X : UNIVERSITY OF CALIFORNIA

CONTROLLING
RATS AND MICE

TRACY I. STORER

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IHE RATS AND MICE of our homes and cities are real
pests. These rodents, their habits, and means for their control are discussed
in this circular. Brief attention also is given to native white-footed mice and
woodrats that often invade cabins in the hills and mountains.

Food and property worth millions of dollars are destroyed or damaged
each year by house rats and mice. These pests may also bring serious
diseases to man — plague, endemic typhus, and others — either directly or
through fleas or mites.

The best control is continuous and community-wide. Quiet day-by-day
application of control measures by private citizens and local government
officials is far better than a much-publicized "rat week" after which the
rodents are left undisturbed.

Exclusion and proper sanitation are the surest and most lasting defenses
against rats. This circular tells how to ratproof buildings, and describes
control by use of other methods, such as traps, poisoned baits, gases, and
dusts.

Rat poisons are dangerous to people and to livestock. Some gases used
in rat control are explosive. Stocks of these materials must be locked up,
and certain precautions are necessary in their use. A doctor should be called
immediately if an accident occurs.

Aid in control may be obtained from any county agricultural commis-
sioner or farm advisor, and in some cities and counties from the health
department. o ^, ^

This circular replaces the former Extension Circular 142.

Fig. 1. The Norway rat— the most destructive of all. Distinctive features (as compared with the
roof rat) are the blunt nose, moderate-sized, slightly haired ears, and the tail which does not
exceed the combined length of the head and body. The total length is up to 16 inches.

CONTENTS

Page

THE RAT PROBLEM 4

Kinds of rats and mice 4

Diseases carried by rats and mice 8

Rat bites and mites 9

Damage by rats and mice 9

Evidence of rats and mice 10

METHODS OF CONTROL 13

Exclusion 13

Traps 19

Poisoned baits 22

Deodorants for dead rats 29

Poisonous gases 29

Poisonous dusts 31

Other means of control 32

OTHER ASPECTS OF CONTROL 33

Legal requirements 33

Pest control operators 33

Native mice and rats 33

LITERATURE CITED 36

The Author:

Tracy I. Storer is Professor of Zoology and Zoologist in the Agricultural Experi
ment Station, University of California, Davis.

CONTROLLING
RATS and MICE

Tracy I. Storer

THE RAT PROBLEM

The "house" rats and mice have long
been unwelcome associates of man. They
followed his occupation and development
of new countries and now are present in
many parts of the world. In contrast to
native rats and mice they are "aliens,"
but well established. In California, as
elsewhere, these animals are all too com-
mon in cities and on farms, in stores and
warehouses, in slaughterhouses and on
garbage dumps, and sometimes along
streams or in fields well apart from man's
dwellings.

Rats and mice are objectionable in
many ways:

1) They eat and foul all sorts of foods.

2) They gnaw into packages, boxes, cabi-
nets, and buildings.

3) They damage furniture, clothing, and
other goods in homes and places of
business.

4) They carry diseases that menace human
society.

5) They give fright and annoyance to
many persons, and injure some by rat
bites.

Mankind unfortunately provides rats
and mice with their two essentials of
life — food and shelter. In consequence,
they live and multiply in almost every
suitable place to which they have access.
Rats and mice have been fought for cen-
turies, but the "rat problem" is still un-
solved.

This circular describes the kinds of
rats and mice, their habits, the kinds of
damage they may do, the evidence of their
presence, and the equipment, materials,

and methods used for control. To get rid
of these rodents, it is necessary to know
their habits, reactions, and food prefer-
ences, and then use this knowledge in
control operations. Often one must ex-
periment to find the most suitable bait,
the best places for traps, or other satis-
factory methods of control. No single
formula or method will give good results
in all cases.

Advice in control of rats and other
rodents may be had from various govern-
ment officials and agencies, including
county agricultural commissioners and
farm advisors; city, county, and state
health departments; state departments of
agriculture, and the U. S. Fish and Wild-
life Service and U. S. Public Health Serv-
ice. Specialists in these agencies will give
technical advice. Some will do control in
places where there is serious damage to
property or a health hazard. Moreover,
these agencies can use materials and
methods not available to the general
public.

KINDS OF RATS AND MICE

General Features. Rats and mice are
rodents or gnawing animals (mammals
of the zoological order Rodentia). Like
all rodents, they have, at the front of the
mouth, two pairs of chisel-like teeth that
grow continuously and are self-sharpen-
ing. These incisors are used to cut food,
to remove shells or coverings of nuts and
seeds, to make nests by shredding cloth,
paper, or grasses, and to gnaw wood,
plaster, or softer materials so that the
animals may enter places giving food or
shelter. Rats and mice have no canine

[4]

teeth (tusks). Their check or grinding
teeth have small projections used to chew
food finely before swallowing it.

The toes end in sharp claws that help
in climbing and digging. When cornered,
a rat uses its incisor teeth and claws, and
may inflict severe injuries. The long tail
serves as a counter-balance to the body
in running, jumping, and climbing.

Most rodents have scent glands which
leave odors on their droppings, trails, and
nests. In rats and mice these glands are
just inside the vent or anal opening, be-
low the base of the tail. The odor from
house rats is mild to the human nose, but
that from the house mouse is strong and
unpleasant — the "mousy odor."

Rats and mice have rather poor vision,
but the senses of smell, taste, hearing, and
touch are keenly developed. Their fre-
quent sniffing movements tell them much
about their surroundings through odors
received. Their choice in foods is un-
doubtedly based upon taste preferences.
They are frightened by unusual sounds,
which may cause them either to stop
abruptly or to hurry to safety. They be-
come used to ordinary noises, however,
and are often active where people, do-

mestic animals, or machines are close by.

The long "whiskers," or vibrissae, on
the nose, and other long hairs above the
eyes, serve the sense of touch. There are
sensory nerves about the base of each
hair. It is the habit of a house rat or
mouse to run close beside a wall, against
which these sensory hairs touch to give
the animal information about its sur-
roundings. In the laboratory, rats with
the vibrissae removed have been found
less skillful in running and finding their
way.

Three kinds of rats and one kind of
mouse, all "aliens" from the Old World,
are now abundant and of great economic
importance in California and many other
parts of the United States. They are the
Norway rat, the roof rat, the black rat,
and the house mouse. The alien rats may
be distinguished from the native woodrat
(p. 35) by their scaly tails and usually by
their coarser hair.

Norway Rat. The largest of the alien
rats, and the one which does most dam-
age, is the Norway rat {Rattus norveg-
icus) , also known as brown rat, house
rat, wharf rat, and sewer rat (fig. 1) .

Full-grown adults are about 16 inches

Fig. 2. The roof rat. Important characteristics are the slender snout, large naked ears, and long
slender tail which usually exceeds the combined length of head and body. The total length is
up to 15 inches. The black rat is identical in form except for its black coat of hair.

[5]

in total length, with the tail 7% inches
long, and weigh 11 or 12 ounces; excep-
tional individuals weigh as much as 24
ounces. The nose is rather blunt, and the
ears are of moderate size and slightly
haired. The tail is scaly, nearly naked,
and blunt-ended; when laid forward it
usually does not reach to the end of the
nose, and is never longer than the head
and body. The coarse body fur is gen-
erally brown, with scattered black hairs,
and is darkest along the middle of the
back. The under parts are pale gray to
yellowish-white.*

The Norway rat is distributed generally
throughout lowland California, along the
seacoast and in the interior, both in cities
and in the country, but has not invaded
the mountains to any extent. It lives about
residences, stores, warehouses, slaughter-
houses, barns, pigpens, and chicken
yards, on garbage dumps, in sewers and
tunnels, and along the banks of streams
and ditches.

This rat stays mainly at the ground
level, seldom going above the first floor
of a building. It climbs, but not as much
as the other alien rats. It burrows and
makes nests under buildings or platforms
on the ground, beneath piles of lumber or
stones, in garbage dumps, and in marshy
places along both fresh and salt water.
The nest, indoors or out, is of trash and
not always neatly or well formed as with
some other rodents.

The tunnels are 2 to 3 inches in diam-
eter and of various lengths, often with
more than one opening on the surface. In
addition, each burrow commonly has sec-
ondary exits or "bolt holes" which may
be hidden under grass or boards. Norway

* Additional more technical characters are:

(a) norvegicus, length of ear from notch, 16-20
mm; ear when laid forward normally not ex-
tending more than halfway to eye; tail more or
less bicolor; hind foot 38-46 mm long; mammae
(teats), 12; first molar (cheek tooth) without
distinct small cusp or style on first row of cusps.

(b) rattus and alexandrinus, length of ear from
notch, 24-26 mm; ear when laid forward reach-
ing halfway to eye; tail not bicolor; hind foot,
36-40 mm long; mammae, 10.

rats take to water when necessary, and
can swim easily.

Female Norway rats average 8 to 9
embryos (extremes of 2 and 17 have been
found) at a pregnancy, and may breed 4
or more times during a year. Pregnancy
lasts 21 to 22 days, and it is thought that
the young probably can shift for them-
selves when about 3 weeks old. A female
may first breed at the age of 4 to 5
months, when her weight is about 5
ounces (140+ grams).

It is not known how long wild rats live,
but much of the population probably is
replaced by new individuals each year.
The possible rate of increase is suggested
by one laboratory experiment with cap-
tive Norway rats, which began with one
pair; more than 1,500 were produced by
the end of a year!

Because of its large size and bold na-
ture, the Norway rat has in many places
replaced the black rat. When rats are re-
duced in numbers by trapping, house
mice often increase, suggesting some kind
of competition between these various ro-
dents. Yet Norway rats and house mice
often inhabit the same buildings.

Roof and Black Rats. The roof rat
(Rattus rattus alexandrinus), sometimes
called Alexandrine, or gray rat (fig. 2),
grows to a total length of 15 inches, with
the tail measuring 8V2 to 10 inches; its
weight seldom exceeds 8 ounces. The nose
is sharp and slender, the ears are rather
large, with little or no hair, and the thin,
tapering, scaly tail is almost always
longer than the combined length of head
and body. The back and sides are gray
or gray-brown, and the belly white or
nearly so.

Roof rats live in much of lowland Cali-
fornia, coastwise and inland, in cities and
rural areas, and have been found scatter-
edly in the mountains to altitudes of
5,000 feet. They inhabit homes, ware-
houses, packing sheds, and feed stores,
often living in attics or upper stories, but
may also be found at ground level, espe-
cially when Norway rats are absent. In

[6]

Fig. 3. The house mouse. The eyes are small, the body is brown, and the tail scantily haired.
The head-and-body length is 3 to 4 inches, and the tail 3 to 31/2 inches.

some inland localities they live along
stream banks. Roof rats climb readily and
travel on the exteriors of rough-surfaced
buildings, on electric wires and cables,
and in trees. In California they often nest
in trees, especially palms, and in dense
hedges or vines growing on fences. These
rats can also swim easily.

The roof rat averages about 6 embryos
per brood and may breed several times
per year. The rate of development and
ages at which young become independent
are thought to be about the same as for
the Norway rat. Female roof rats become
sexually mature at a weight of about 3
ounces (90 grams).

Except for its almost solidly black
color, the black rat {Rattus rattus rattus)
resembles the roof rat in size and struc-
ture. In California this rat is found only
near salt water, in seaports and adjacent
towns along the entire coast, but inland
only about as far as Martinez, Contra
Costa County.

Roof and black rats are far more com-
mon on ships than the Norway rat, but
all these alien rats travel in vessels, es-
pecially cargo ships.

House Mouse. The common house
mouse (Mus musculus) is 3 to 4 inches
in head-and-body length, and the tail is 3
to 3% inches long (fig. 3) . The ears and
eyes are small (as compared to those of
the native white-footed mice, fig. 16) , and
the tail is scantily haired. The upper sur-
face of the body is almost uniformly
brown, and the under parts pale brown to
whitish.

House mice have followed settlers into
almost all parts of California. The mice
live in any structure they can enter, and
also entirely apart from man in the fields
of some interior lowland districts. Those
living outside during the summer tend to
move into buildings with the first cold
weather.

Since house mice in nests are easily
transported in bales of hay, household
goods, crates, and boxes, they are con-
stantly being moved about and into new
localities. These mice live at all levels
in buildings, from basements to attics.
They climb readily, and can pass through
holes % inch square, but seldom burrow
in the ground. They often make compact
nests of cloth, sacking, or other materials

[7]

that are shredded finely and worked into
a round hollow ball used especially to
shelter the young.

House mice average 5 to 6 young per
litter; the young can run about when 21
days old and can breed at 42 days. Indi-
vidual captive female mice have produced
100 young per year. This species is short-
lived; most individuals probably do not
live much more than one year, hence the
population "turnover" is rapid. At times
house mice increase to very large num-
bers (Hall, 1927; Piper, 1928; Storer,
1931).

DISEASES CARRIED BY
RATS AND MICE

The alien rodents and their insect para-
sites carry several important human dis-
eases (Hull, 1941) . This is a most impor-
tant reason for control of rats and mice.

Plague is a bacterial disease of wild
field rodents and rats, and is usually
transmitted by fleas. With the spread of
rats by ship-borne commerce, plague in-
fections have occurred in several United
States seaports. Two epidemics of flea-
transmitted bubonic plague in San Fran-
cisco (1900, 1909) resulted in 281 cases
with 191 deaths. In Los Angeles during
October, 1924, plague from rats took the
pneumonic form in human beings. This
form is spread by coughing (droplet in-
fection) from one person to another. Of
32 human cases, 30 died.

Plague is now well distributed among
field rodents (as sylvatic plague) in many
parts of the western United States, and
some human cases have resulted. Rats in
cities or towns may be reinfected from the
surrounding field rodents, and thus be
a health hazard. A few plague-infected
rats have been found in California cities
during recent years. Often when neither
rats nor field rodents show visible signs
of plague, the fleas from these animals
prove to contain plague bacteria, thus
maintaining the "reservoir" of this dis-
ease among rodents.

Endemic or murine typhus is a New

World disease of rats and man carried by
rat fleas. It is related to the louse-borne
typhus of Europe, and to the spotted fever
carried by ticks, but is not as deadly.
Human cases of murine typhus have in-
creased greatly during recent years in
states of the Atlantic seaboard and Gulf
Region.

It was first reported in California in
1916, and to the end of 1945 there were
362 human cases and 17 deaths in the
state. Here it occurs mainly in Los An-
geles, San Diego, Orange, San Bernar-
dino, and Santa Barbara counties. Cases
have been reported in all months, but are
more numerous in late summer and
autumn. The largest percentage of human
cases is among food industry workers
in large cities, but some cases have been
found in other kinds of business estab-
lishments, motion picture theaters, and
private homes (Beck and Van Allen,
1947). Control of rat fleas by DDT, and
control of rats, have been effective in re-
ducing the number of human cases.

Leptospirosis. This disease, also
called Weil's disease, infectious jaundice,
and Stuttgart disease of dogs, affects the
kidneys in rats, and is spread by rat urine
contaminating food or water. Workers in
mines, rice fields, sewers, etc., in the
Orient suffer a heavy mortality, and there
are some human cases in the United
States. About 40 per cent of the rats in
San Francisco show evidence of this dis-
ease. Dogs also are susceptible, and one
or more epidemics have occurred among
city dogs in California.

Food Poisoning. Certain bacteria
common in rats and their droppings cause
food poisoning in man, and epidemics
have resulted from human food contam-
inated by rats. Food dirtied by rats is
always a health hazard.

Rat-bite Fever. Another bacterial
disease of rats transmissible to man is
rat-bite fever, of which several cases have
been reported in the United States during
recent years.

[8]

Trichinosis. This disease of rats,
swine, and man is also present in cats,
bears, and some other animals. It is
caused by a microscopic trichina worm
that lives as a larva in the muscles of in-
fected animals. When flesh of one infected
animal is eaten by another of the suscep-
tible animals, the larvae taken in with the
flesh become mature, breed, and produce
many new larvae. If the latter are abun-
dant, the poisonous substances they se-
crete lead to illness or death of the new
host. Rats aid in spreading this disease
among pigs, and man becomes infected
by eating pork or bear meat which has
been imperfectly cooked or cured and
therefore contains many living trichina
larvae.

Rickettsialpox. A disease of house
mice, transmitted by a tiny blood-sucking
mite, was first recognized in 1946, when
about 80 human cases occurred in two
housing centers in New York City. Some
cases have occurred in other places.

Because of the danger of these
rodent-borne diseases, no one ex-
cept a trained public health official
should handle alien rats or mice or
keep them in cages.

RAT BITES AND MITES

In the poorer housing areas of large
cities some persons are bitten by wild
rats. In Baltimore, Maryland, 87 people
were treated for rat bites at one hospital
during 1939-43. Most of the victims were
infants under one year of age, and nearly
all were bitten at night while asleep.

House rats carry mites and several
kinds of fleas, some of which also attack
human beings, and may carry disease.
The tropical rat mite {Liponyssus bacoti)
is so small that it can just be seen by the
unaided eye. It is common on rats in Cali-
fornia, and at times bites people, causing
serious discomfort to both adults and in-
fants. The spots of attack are reddish,

with slight swelling and persistent itching
that may be mistaken for flea bites.

DAMAGE BY RATS AND MICE

The total amount spent for rat and
mouse control is small when compared
with the amount of damage done by these
animals.

No one knows how many rats exist. One
guess, widely reprinted, placed the rat
population as equal to the human popu-
lation ; but recent careful surveys in sev-
eral cities fail to support this idea. The
numbers of rats vary from place to place.

If rats did equal the number of persons
in the United States and if each rat ate
one ounce of food per day, the daily needs
would be about 4,690 tons. If they ate
only wheat — at $1.00 per bushel — the
total annual levy would cost over 57 mil-
lion dollars.

Assumptions aside, it is well known
that rats consume much food intended for
man and domestic animals, that they often
take high-priced foods, and that they foul
or damage far more than they eat. Hence
rats (and mice) are expensive nuisances.

The alien rats and mice may eat prac-
tically anything used as food by human
beings and by livestock (except hay).
State and federal food inspectors are giv-
ing increased attention to foods contami-
nated by rats and mice, and can recognize
hair, droppings, or urine stains in pack-
ing houses. Contaminated food is con-
demned for human use, and must be dis-
posed of at a loss.

Even more important economically
than the food actually consumed is the
damage to sacks, boxes, and other con-
tainers, causing the loss of packaged ma-
terials. Many thousands of such food
containers are thus rendered unsalable
in packing houses, warehouses, and retail
stores. In homes, cardboard cartons and
other packages of food are torn open,
spilled, and dirtied so that many must be
discarded. Slabs of bacon and other
meats may be eaten and gnawed exten-
sively. House mice sometimes live con-

[9]

tinuously in cold-storage warehouses
where they eat nothing but high-priced
meat.

To get nesting materials, rats and mice
often gnaw and pull apart clothing, dry-
goods, blankets, mattresses, upholstered
furniture, and carpets, so that the damage
is too great to repair. Cloth or paper bags
of grain or cereals in feed warehouses
often are so chewed that much labor is
needed to resack the contents and to re-
pair the sacks. Piles of empty bags some-
times are riddled with holes and made
useless by the work of rats and mice.

On poultry and pigeon farms, in addi-
tion to destroying feedstuffs and sacks,
rats often eat eggs, and at times kill num-
bers of young birds.

Rats, and less often mice, gnaw into
buildings, usually at the doors or win-
dows, and when inside they cut holes in
cabinets and other interior fittings as
they search for food or for places to nest.
Both indoors and out, rats may put elec-
tric circuits out of operation by gnawing
away insulation (either lead or fabric),
exposing, and sometimes cutting the
wires. Mice may enter wiring conduits
and travel within them; they also may
gnaw off the insulation of wires and pro-
duce short circuits.

Rats and mice have been accused of
causing fires by short-circuiting power
wires (also by gnawing matches), but
conclusive evidence of such damage is
difficult to obtain. At times they build
nests inside the frames of electric motors.
Short circuits caused by nest materials
may burn out motors.

EVIDENCE OF RATS AND MICE

Rats and mice may be seen or heard,
or their presence may be discovered
through other common signs, such as
droppings, runways or foot marks,
smears, holes, piles of earth from bur-
rows, nests, damage to food or property,
gnawings, and disappearance of bait.

Sight. Rats and mice try to avoid being
seen. One may have quite an infestation

without seeing a single live rodent.
Gunderson (1943) , in Iowa, believes that
the number of rats present, particularly
on farms, may be roughly guessed on the
following basis : none seen, 0 to 20 pres-
ent; occasionally seen, 20 to 200; often
seen, more than 200.

When rats are numerous on a garbage
dump, some can usually be seen in day-
light. A dump after dark, seen by a flash-
light, often is a spectacular sight.

Sound. In any place where fair num-
bers are present, rats and mice may usu-
ally be heard running and squealing after
dark. In a home, even one or two mice or
rats scampering and dragging food over
the rough plaster "keys" in an attic often
make enough noise to disturb the people
below — and often lead to prompt efforts
toward control.

Droppings. These signs commonly are
blackish, soft, and somewhat shiny when
fresh, but become hard and dull later
(fig. 4) . Undisturbed droppings last for a
long time; hence only those in places
often swept are useful to indicate recent
activity, and whether few or many ro-
dents are present. A Norway rat (cap-
tive) may produce 25 to 180 droppings
per day, more being voided with fruit or
vegetable diets. Droppings are found
scattered wherever rats run, but may be
concentrated in places where the animals
stop to feed. Those of the Norway rat
often are in groups, are spindle-shaped,
and % to % inch long; those of roof or
black rats are scattered, more sausage-
shaped, and slightly smaller; droppings
of the house mouse are scattered, spindle-
shaped, and about Y16 by ^ inch.

Runways. Paths regularly used by
numbers of rats may become beaten run-
ways on the ground or over piles of
refuse, and smoothed routes over board
surfaces indoors. On soft earth, dust, or
flour, the foot prints and tailmarks often
show clearly; the fresher the print, the
sharper the pattern. Flour, whiting, or
other light-colored powder may be dusted
from a sifter can on places where rats are

[10 1

Fig. 4. Evidences of rats: Droppings and damage to grain sacks; tracks in dust; a freshly gnawed
post, and footprints and tail marks in flour. Courtesy of U. S. Public Health Service.

believed to be running; then any tracks
made during the night can be seen the
next day, and one may estimate the
number of animals using the runway. A
small mirror set at an angle on the end
of a stick can be used to search for tracks
on overhead pipes or other structures. A
flashlight is useful in dark corners.

Smears. Grease, soot, and dirt from
the belly hairs will leave smears on well-
travelled runways. These can best be seen
where the animals press their bodies
down in changing direction over the sides
of beams and on ladders or along pipes
or beams. A run along a beam below floor
or ceiling joists is often marked by a
series of U U U shaped marks between
the joists (fig. 5). Smears are most evi-
dent as dark marks on whitewash or non-
glossy light-colored paint, or as whitish
markings where flour is stored. Shiny
smears indicate recent use.

Holes. Entrances to burrows in open
ground or under pavements, foundations,
or piles of rubbish are common signs,
and, if they show foot prints, are in active
use. Not all holes show mounds of earth
excavated from burrows. Old holes may

last long after disuse. Stopping all holes
in an area with plugs of crumpled news-
papers or soft earth, and later noting
which are reopened, is an easy way to rec-
ognize those being used.

The nests of Norway rats (fig. 6) are
not easily found, since they are hidden in
burrows, under floors or foundations, in
piles of goods or rubbish, or in disused
packing boxes. Nests of roof or black rats
may be in similar places (rarely in bur-
rows) , but some are built out-of-doors in
dense trees or in bushes and vines. The
small nests of house mice are placed in
any sort of shelter: under piles of sacks
or paper, in loose or baled hay, in boxes,
in disused clothing, inside upholstered
furniture, and in office desks. When living
in fields, house mice nest in burrows.

Urine Stains. These are not usually
easy to find, although they are present in
many places inhabited by rats or mice,
even on sacks of food materials. Such
stains fluoresce under ultra-violet light.
Unfortunately, certain other materials
are also fluorescent, but identification of
rodent stains is possible by trained in-
spectors.

Fig. 5. Smears made by rats traveling along a supporting floor beam. The rats must crawl
along the side of the beam under each joist, and thus leave the u u V u-shaped markings.

Courtesy of U. S. Public Health Service.

[12]

^f, bolt holes***

entrance

TOP VIEW

Fig. 6. Burrow of Norway rat in a poultry pen. In addition to the open entrance, there are
emergency exits or bolt holes, lightly covered by soil, and not easily seen from the surface.

(Pisanoand Storer, 1948.)

METHODS OF CONTROL

The principal means of controlling rats
and mice are (1) exclusion, (2) traps,
(3) poisoned baits, and (4) poisonous
gases or dusts. Various other control
measures have been tried, but many are
of little use. Exclusion is the real solution,
and all other methods give only tempo-
rary relief.

EXCLUSION

Exclusion of rats and mice means es-
sentially only two things: leaving no
opening more than % inch in width into
a building, or other enclosure, and pro-
tecting all food supplies from these ani-
mals (see figs. 7-10).

House mice can pass easily through a
crevice % inch wide, and young rats
need little more space. If buildings are
constructed and maintained with these
two principles constantly in mind, no rats
or mice can find shelter or food inside.
Unfortunately this is not always done,
and in some places it is difficult to do.

[

In practice, exclusion of rodents re-
quires the following:

1) Concrete floors and exterior curtain
walls around foundations.

2) Solid exterior walls with no holes or
large cracks.

3) Elevation of small buildings 12 to 18
inches above ground.

4) Screens over all openings, both at the
ground level and above, of hardware
cloth of Vz- or 14-inch mesh, or metal
grille with equally small apertures.

5) Cement or metal sheathing around all
holes where pipes or conduits pass
through walls.

6) Tight-fitting doors and window screens
throughout the premises.

Rat proofing alone is not enough; such
construction must be frequently inspected
and kept in good repair. Leaving un-
screened doors or windows open over
night, or failing to replace sheathing after
a small plumbing job, can give rodents a

13]

Gal. iron,
flaskinq

Coaduif

Fig. 7. Structural details of buildings in relation to exclusion of rats and mice. Undesirable fea-
tures: A, ventilator to basement with widely spaced bars; B, space about entrance of pipe; C,
ventilator with wide-mesh grille; F, hole in wall around entrance of conduit; H, free passage for
rats from below the floor into the walls.

Corrective measures: D, filling space around entrance of pipe with concrete; E, covering ven-
tilator with hardware cloth of Vz- or 14-inch mesh; G, covering entrance of conduit with close-
fitting sheet-metal flashing; I, stop of wood at floor level; J, header block between joists, completely
closing the space between sill and floor; K, sheet-metal "header"; filling space between studs with
brick L, or concrete M.

At the right is shown the usual "western" type of framing which, when walls are in place,
prevents rodents in a basement from gaining access to spaces between studs and walls.

chance to reinfest the building. Crates,
baled goods, and other packaged ma-
terials brought into a building should be
immediately examined for rodents or
young in nests.

Foundations. Rats burrow readily
under concrete slab floors laid on the
ground. Interior openings in such floors,
as around pipes or sumps, may be used
as entrances. Rats also tunnel under the
foundations of exterior walls, even when
the foundations extend 24 inches below
the ground surface. To prevent such bur-
rowing, a thin curtain wall, L-shaped in
section, may be poured around the foun-
dation or slab. It need be only 2 or 3
inches thick. The foot of the L should
project outward to discourage rats from
tunneling down beside the wall.

To make a curtain wall, dig a trench
12 inches wide and 24 inches deep around

the edge of the foundation or slab. Brace
an upright retaining form (thin boards
or even corrugated box cardboard will
do) with stakes, so that it stands 2 or 3
inches from the inner side of the trench,
and fill the space with concrete. Then
pour the foot of the L. Later, remove the
form and refill the trench with earth.

Exterior Walls. Concrete, masonry,
or sheet metal walls will exclude rats.
When corrugated metal sheets are used,
mice may enter at the bottom unless stops
are added to close the small semicircular
spaces where the corrugations join the
foundation or sill. Wooden walls can be
kept rodent proof if knotholes are cov-
ered, cracks battened tightly, and the
bottom boards are nailed down securely.
The lower edge should be sheathed with
strips of light weight (28 gauge) gal-
vanized iron sheeting. It is of prime im-

[14

portance to leave no "gnawing edge"
where the animals can start an entry hole.
For outbuildings and farm structures
of frame construction on wooden studs,
single walls are preferable. Double walls,
especially if sheathed on both sides with
wood or wall board, provide ideal shelters
for rats and mice, and often serve as
passageways by which the animals reach
upper floors or attics. Old buildings with
double wooden walls often may be made
rodent proof by tacking strips of sheet
iron along the outside base and removing
the lowermost inside boards to place stops
of one kind or another, as shown in fig-
ure 7.

Openings. Black and roof rats easily
climb vines, trees, and rose trellises
against outside walls, and can run along
electric cables or wires, or even the an-
tenna for a home radio. Rats of all kinds
and mice can go up rough stucco or board
walls. They seldom climb smooth walls of
sheet metal or finished concrete, but may
find a foothold on outside pipes, drains,
or rain ducts. Rats may cut fly screen, par-
ticularly near the ground or roof, al-
though window screens at intermediate
levels are rarely damaged. A top-hinged
screen frame left unhooked at the bottom
may give rodents a chance to get in.

All outside openings above ground

Fig. 8. Permanent rat-proofing for windows and doors. Left, four folded channels of galvanized
sheet iron bent to cover edges of hardware cloth (3 meshes per inch); a narrow lip (3/16 to 1/4
inch) is bent out along one margin of each channel to stiffen the frame; the 4 channels are riveted
together at the corners and the framed screen is fastened to the window casing with screws.
Detail of the channel is shown below. Right, three rectangular channels bent from galvanized sheet
iron to fit closely around the bottom and lower edges of a door and nailed or screwed into place.
Channels add strength to the door, prevent damage to the lower edge, and will stay in position
better than flat strips of sheet metal tacked to one side of the door.

[15]

must be screened completely with hard-
ware cloth of %- or ^4-inch mesh. This
includes all attic ventilators and louvers,
tops of air shafts, elevators wells, rooftop
stairway entrances, windows, and vent
pipes.

Gaps left by plumbers and electricians
around pipes or conduits entering walls
(fig. 7, B) are often the commonest paths
of entry for rats and mice, and may be
closed in three ways.

1) In brick, tile, or concrete walls the space
is filled with concrete, using reinforcing
of old wire screen if necessary (fig. 7,D).

2) On wooden walls a sheet-metal collar is
fitted closely around each pipe, extend-
ing several inches beyond the edges of
the opening, and nailed to the wall (see
fig. 7,G).

3) At entrances to steam tunnels, air ducts,
or other places where ventilation is nec-
essary and where workmen must enter
from time to time, close-fitting but re-
movable hardware cloth screens, framed
with sheet metal, are bolted or screwed
into place.

Doorways. Every outside door should
fit closely in its frame when closed, and
the clearance over the floor or sill should
never be greater than % inch. A strip may
be nailed to the bottom edge if there is
a wider gap. Doors to restaurants, gro-
cery and feed stores, and granaries or
feed rooms should be sheathed along the
bottom and up 6 to 12 inches on each
side, with a 3-inch width of galvanized
sheet iron, securely nailed. An even bet-
ter practice (fig. 8) is to form U-shaped
channels of sheet metal, of a width to fit
closely over the thickness of the edge and
bottom of the door, and fasten them with
wood screws. A rat or mouse needs a free
edge on a door to start gnawing and can-
not begin if the edge is covered with metal
or fits closely in the casing.

Sliding doors to warehouses seldom
fit tightly, and rats often enter in the
space between the door and wall. Often

strips of wood covered with metal can be
fitted to the doors or casings to keep the
animals out.

At receiving and shipping depart-
ments in the rear of stores, the main doors
must be open throughout the day and
often after dark. An extra pair of half-
height "Dutch" doors 3 to 4 feet high may
be installed to stop rodents. They should
be light in weight, sheathed completely
on sides and edges with smooth sheet
metal, and hung on double-swing spring
hinges. They should fit closely over the
floor and come together with no more
than % inch between the free edges.
Loaded warehouse hand trucks or dollies
can easily be pushed against and past the
doors, and the doors will close at once be-
hind the person pushing the truck. No rat
can run in at floor level or climb the
smooth surface of the doors.

Corn Cribs. The ordinary wooden
corn crib, set on the ground, provides a
good home for rats and mice. Space be-
low the floor gives shelter for nesting, and

Fig. 9. Construction for a corn crib to exclude
rats. The ends of studs and joists are nailed
together to resist lateral pressure when the crib
is filled. Spaces between studs are closed by
wooden stops at the floor level. Hardware cloth
is placed between joists and flooring and be-
tween studs and outside slats. The whole crib
is elevated on concrete, field stone, or wooden
sills so that the floor is at least 12 inches above
ground.

[16]

the open construction above makes food
easy to obtain. Old wooden cribs should
be raised 18 inches above the ground,
using concrete, field stone, or wood for
supports. The top, walls, and door should
be covered with hardware cloth in addi-
tion to the wooden slats (fig. 9). New
wooden cribs should be built off the
ground and lined with hardware cloth.
Prefabricated cylindrical cribs made en-
tirely of sheet metal are available and are
used by many farmers, who thereby prac-
tically end their rodent troubles in this
kind of storage.

Feed Storage. Large supplies of live-
stock feed in piled sacks offer both food
and shelter for rats and mice. When feed
is stored in warehouses or barns with
open eaves, poorly fitted doors, cracked
or warped outside walls, or floors of
earth or of wood close to the ground,
there is often serious loss of grain and
damage to sacks.

Whenever possible, sacked materials
should be placed on low platforms (pal-
lets) to leave a clear space beneath. The
tiers of sacks should be in narrow piles
with lanes between, and with alleyways
between the outermost tiers and exterior
walls. Damage then will be lessened and
any rats and mice present can be con-
trolled more readily.

Rodent-proof wooden bins may be
built to contain bulk materials. The in-
teriors should be sheathed with tongue-
and-groove boards, or sheet metal. The
studs and other framing should be on
the outside. The supporting legs can be
sheathed with sheet metal to prevent ro-
dents from climbing, and the tops should
be provided with tight coverings or
screening.

Smaller stocks of feed for poultry can
be kept in large clean garbage cans cov-
ered with close fitting lids; the daily sup-
ply can be easily removed with little
spilling. Cans holding 2 or 3 sacks of
feed are usually available. If the amount
of grain given to poultry at each feeding
is closely measured so that the birds will

clean it up rapidly, little will remain in
the troughs to attract rats and mice.

Temporary stacks of newly threshed
grain left in the field sometimes are at-
tacked by rodents, and sacks may be dam-
aged. Such outdoor stacks in Australia
are protected from rodents by a tight
"fence" of solid sheet iron about 36 inches
wide, of which 6 inches is set into the soil.
The corners and the lapping between
sheets must be kept tight.

Hay in bulk or bales is difficult to pro-
tect against mice, and often is heavily
infested, sometimes also with rats. Nests
in baled hay are a common means of
carrying infestations of mice into build-
ings. No solution for excluding rodents
from hay has been found except the sheet
iron fence just described. Use of baiting
stations (see section on Poisoned Baits),
and baits other than the food present,
such as barley in a barn containing oat
hay, will often reduce the number of ro-
dents, especially if control is continued
as the stock of hay is used or gradually
moved out.

Human Food Supplies. Grocery
stores may reduce losses by keeping small
supplies of grain products, sugar, and
similar materials in metal containers. In
homes and in store rooms adjacent to
farm kitchens the same means may be
used. Covered tin cans, glass bottles, 2-
to 10-gallon crocks, and even small gar-
bage cans are useful for flour, cereals,
dried fruits, nuts, etc. Such containers
give protection from rodents and make
it easy to remove the contents without
spilling.

Cabinets and Enclosed Platforms.
The "dead" spaces between walls and
floors and built-in cabinets, platforms for
refrigerators, and similar conveniences
in homes and stores make good hiding
places for rats and mice. Often there are
entryways into these spaces where work-
men have cut holes in fitting pipes and
conduits. When such shelters are used by
rodents, it may be necessary to remove
the false floors or back partitions and stop

[17]

Fig. 10. Storage of garbage and rubbish. Above, poor; easy of access to mice, rats, and flies.
Below, good construction; doors are covered with both hardware cloth and fly screen and pro-
vided with spring hinges, thus excluding all pests. Courtesy of U. S. Public Health Service.

[18]

all entry by tacking tin sheets or hard-
ware cloth over the openings. If the space
is on the first floor, some of this work may
be done in the basement — searching with
a flashlight in dark places and closing all
entry ways.

Garbage, Boxes, and Trash. Incor-
porated cities and towns usually require
each residence or business place to pro-
vide metal garbage cans for disposing of
waste food and trash. But many house-
wives, restaurant employees, and store-
keepers are careless (fig. 10). They fail
to cover the cans tightly or they overload
them. Home and farm yards, and the
alleys behind both homes and stores often
have irregular heaps of boxes, lumber,
or trash. The open garbage cans provide
food and the piles of debris offer shelter.
The practice on many farms of throwing
all table scraps into pens for chickens or
hogs helps to support both rodents and
flies, particularly if the pens are seldom
or never cleaned up. To keep down ro-
dents, dispose of refuse promptly and
properly.

Empty boxes and stocks of lumber
should be piled compactly and neatly on
trestles or other supports that keep the
bottom 12 to 18 inches above ground.
Trash piles, brush heaps, old prunings
from fruit trees, bundles of corn stalks,
and all such litter should be burned.
Large piles of firewood, if not in separate
tiers, may serve as shelter for rats — and
in the country, are often used by ground
squirrels.

The most effective way to control
rats is to build them out and starve
them out. The houseowner, store-
keeper, or farmer should inspect his
property, keeping in mind the meth-
ods of exclusion just discussed, and
then repair his premises. In many
cases, a little work, using materials
already at hand, will result in keep-
ing all rodent pests out of buildings.

TRAPS

Trapping is the preferred method of
control in homes and office buildings, be-
cause animals killed in this way may be
easily removed, whereas poisoned rodents
may die within walls or in other inacces-
sible places and create bad odors. Almost
any trap (fig. 11) will catch some rats,
but long experience proves that the ordi-
nary spring snap trap is best and cheapest.

The many kinds of traps designed and
manufactured comprise four main types:
wooden or steel "breakback" or guillo-
tine traps, steel jaw traps, cage traps, and
automatic traps (fig. 12) . Results depend
more on the number of traps used and
on the way in which they are placed than
on the type of trap used. Contrary to
popular opinion, human odor from han-
dling traps will not keep rats or mice
away; it is not necessary to wear gloves
or to boil, wash, or smoke the traps. It is
important, however, to keep traps in good
working order and to set each trap with
a "hair trigger," so that the least touch
will release the spring.

Trapping House Mice. The small
breakback and choker-loop traps are
used where there are signs of mice : tracks,
droppings, holes, or damage. In build-
ings, place traps along the wall, with trig-
ger end toward the baseboard, or against
boxes or other objects, at intervals of 2
to 3 feet. Many traps should be used if
mice are abundant. If a mouse hole is
found, put a trap about 9 inches on either
side.

Rolled oats is almost always effective
as bait (cheese is not!). Bread crumbs,
oatmeal, or other cereals may be tried.
Meat or fish baits are seldom necessary.
A little loose bait should be placed under
the trigger, to catch mice skillful at rob-
bing traps. The "set" of the trigger should
be delicate. A string or light wire from
the trap to some nearby object will pre-
vent a prowling cat from carrying the
trapped mouse away, and will also keep
a mouse caught by the tail or foot from
crawling into hiding.

[19]

•V*

Fig. 11. Traps for rats and mice. A, wooden snap or breakback rat trap; the same type in
smaller size is used for mice; B, steel jump trap size 0 (the leather padded jaws are not necessary);
C, Schuyler trap (made in both large and small sizes); D, all steel quickset mouse trap; E, 4-hole
choker-loop mouse trap; F, wire cage trap, 2 sizes available.

[20]

Trapping Rats. Rats are considered
somewhat "wiser" than mice, yet many
are taken with wooden snap traps or steel
traps. Trapping is most useful in homes,
offices, and generally in places where
there are few rats. If there is a heavy in-
festation, other control methods should
be used first, and traps placed to kill those
remaining.

The wooden-based spring trap is gen-
erally used, but some trappers employ
the No. 0 steel trap. If traps are set and
baited at once, some rats will be caught.
However, better results will be had if the
traps are placed and baited, but left unset
for 3 to 5 nights, then baited and set. This
practice overcomes the rats' fear of a new
object.

Traps should be placed along runs or

where there is other evidence of rats,
spaced at intervals of 6 feet or more, with
the trigger ends close to the wall. Each
trap should be fastened to some nearby
object by a cord or light wire about 2
feet long.

In places where regular rat runs are
found, baited traps are usually set 2 or 3
feet off the trails. If no rats are taken, then
traps, baited or unbaked, are set with the
trigger directly across the runway.

When rats are using overhead pipes
or beams, traps can be placed or fastened
on the runs with the trigger across the
line of travel. The best method to support
such traps is to drill a hole in the trap
bottom near the trigger, then drive a
finish nail into the beam or twist a wire
around the pipe, leaving a short upstand-

Fig. 12. Placing traps and antu dust for rat control. The steel trap (unbaited) is set with its
trigger pan on the same level as the jaws, the spring is turned at a slight angle with the trap
axis, the trap chain is nailed to some solid object, and the trap is placed where rats have been
running, as indicated by black smears on the pipes. One wooden trap, baited with cut apple, is
set near the run on a flat surface used by rats, as shown by the droppings. Other kinds of bait
may be used. A thick layer of white dust containing antu has been sprinkled on the edges of
timbers traveled by rats; any Norway rat that evades the traps but walks through the dust is
likely to be poisoned. It is usual practice either to set traps or put down dust patches but not to
combine these two methods at one site. Courtesy of U. S. Public Health Service.

[21]

ing end of wire. The hole in the trap is put
over the nail or wire end. A soft wire from
the other end of the trap is fastened to
some object below the level of the run-
way. When sprung, the trap and rat will
bounce off the support and hang from
the wire, leaving the runway free for other
rats to find other traps similarly placed.

When a trap is placed on the ground,
the base may be countersunk in the soil
so that only the trigger projects above
the surface. Indoors or out, for especially
wary rats, it is often helpful to cover a
trap with sawdust, fine soil, or other cam-
ouflage that does not interfere with the
trigger or action.

To increase the chance of catching any
rat that crosses a trap, some workers add
a trigger card, a piece of corrugated card-
board, about 2 x 2% inches, forced over
the trigger to increase its effective sur-
face.

Rats that ignore an exposed trap will
often be caught if a board or box is placed
to shield the trap, leaving a space for
entry and for the trap spring to operate.

Rat baits may be of almost any human
food. In many places these rodents de-
velop special preferences for the kinds of
food most readily available. In general
the Norway rat seems to prefer protein
and fatty foods, whereas the roof and
black rats seem to have greater liking for
cereals and fruits.

Sometimes a trial with several kinds
of baits will indicate what the local rats
prefer. Rolled oats and other cereals,
bread crumbs and corn meal are common
baits; ground meat, bacon, fish, nut
meats, apples, and raisins also are used.
When bait is finely divided, some should
be scattered under the trigger as well as
on top. Solid materials should be pressed
onto or tied to the trigger. If the material
spoils or dries, baits not taken should be
replaced the next day, but dry cereals can
remain longer.

Traps should be examined at least once
each day, preferably in the morning.
Dead rats should be removed promptly

and buried at least 2 feet deep, burned in
an incinerator, or placed in a tight gar-
bage can for early removal. Since fleas
and mites will leave dead rats and may get
onto people, the trapper should handle
dead rats as little as possible, and should
wear gloves.

Cage and box traps that capture one
rat at a setting are provided with 1 or 2
drop doors. Some have an overhead trig-
ger on which the bait is fastened and the
door is released when a rat works at the
bait. Others have a treadle in the floor on
which the rat steps to drop the door. The
"automatic" traps are intended to catch
more than one rat. When one enters, a
balanced door closes behind the animal,
preventing its escape. As the rat moves
into another compartment the outer door
reopens. Several types of electric or
"electronic" traps have been offered for
sale or lease. None of these special traps
have proved more efficient than a good
supply of snap traps, and all are much
more expensive.

POISONED BAITS

Poisons. The chief method of routine
control by all government agencies, com-
mercial pest control operators, and many
others is use of poisoned food or baits.
The principal poisons are red squill, zinc
phosphide, antu, and warfarin, although
arsenic, barium carbonate, phosphorus,
and strychnine have been used. Many
other substances have been tried with
little or no success. Thallium sulfate and
Compound 1080 are both effective
poisons, but they are too dangerous to
be used by the general public.

All but one of the materials named are
"stomach poisons"; when sufficient doses
are eaten they are absorbed and kill by
toxic action. The exception, warfarin,
must be eaten in small amounts over sev-
eral days, when it prevents blood clotting
and death results from internal hemor-
rhages.

Many types of commercial rat and
mouse poisons are on sale. In California

[22]

Except for red squill, all these
poisons are dangerous to man, pets,
and domestic animals. Stocks of
poison and poisoned bait and all
equipment used with them must be
labeled POISON and must be se-
curely locked up when not in use.
Care also must be used in handling
and placing poisoned baits and in
the disposal of unused baits and of
poisoned animals.

all poisons and poisonous baits (economic
poisons) for pest control and for sale to
the public must first be submitted to the
State Department of Agriculture (Calif.
Agric. Code, 1945, Sees. 1061-1079) , and
only those which are satisfactory are reg-
istered for sale within the state. The
printed label on each package must in-
clude the name and percentage of the
active ingredient (s), a statement about
the poison, and information about anti-
dotes. The Federal Insecticide, Fungi-
cide, and Rodenticide Act of June 25,
1947, provides that all rodent poisons in-
tended for interstate commerce must be
similarly registered and labelled. Licens-
ing does not guarantee that a material will
always provide effective control, but it
eliminates the poorer rodent poisons in-
cluding some advertised in magazines for
direct sale to users.

Baits. Rolled oats and other cereals,
corn meal, bread crumbs, and diced bread
are the commonest baits. In England dry
stale bread soaked in water until mushy
is used successfully. Wheat soaked over-
night in water and then drained has
served with some poisons. Addition of
mineral oil, corn oil, molasses, peanut
butter, or tomato puree to bread or cereal
has been advised by the U. S. Fish and
Wildlife Service (Silver and Garlough,
1941).

Trials on the relative attractiveness of
different baits for Norway rats in the
northeastern states showed preferences

in decreasing order to be : raw meat, raw
fish, rolled oats, whole wheat, corn meal,
bread crumbs, canned fish, canned meat,
cooked cereals, cheese, meat scrap,
powdered milk, fish meal, fresh vege-
tables, cooked vegetables, and fresh fruits
(O'Connor, et al., 1935) . Rats in different
localities and in places where one or an-
other kind of food is most available often
definitely prefer particular foods. Several
kinds of bait should be tried to determine
what the rats like best. Wet baits must be
prepared and used fresh each day. Meat
or fish in dry or hot places will soon gloss
over and spoil.

Prebaiting. For many years it has
been common practice simply to put
down baits in places frequented by rats,
leave them for one or more nights, and
then remove the uneaten baits. Some rats
were killed, but results were usually un-
satisfactory. A recent major improve-
ment is the use of prebaiting (Bartlett,
et al., 1946). Small piles of clean un-
poisoned baits are placed at selected sites
or in special bait containers for several
nights, the supply being replenished
whenever rats take the material. The pre-
bait is then replaced by poisoned food of
the same kind, which is left 1 to 3 nights,
and then the remainder removed.

Prebaiting takes account of an impor-
tant feature in rat behavior — a suspicious
fear and avoidance of new objects, even
a new food. If clean prebait is offered,
they will, after one or more nights, over-
come their fear and usually accept the
new food. Also, when sites are prebaited
for several nights in succession, increas-
ing numbers of rats will come to feed at
these places. Then when poisoned bait is
substituted, a larger number will be
killed.

A convenient program is to prebait on
the first, third, and fifth days, replace
with poisoned bait on the sixth day, and
remove all remaining poison and dead
rats on the eighth day. If acceptance of
the prebait is poor, the prebaiting period
should be extended for several more days.

23]

The largest take of poisoned bait will be made, on the basis of % ounce (10

on the first night it is exposed; a little grams) per rat, since the animals eat a

may be taken the second night, but prac- smaller amount of poisoned food. A week

tically none thereafter. or two after poisoning, an estimate of

Prebaiting may provide a crude those remaining can be made by using a

method of estimating the number of rats new bait (unpoisoned) and determining

using bait stations. A given weight of the amount removed,

prebait — say 2 ounces, which may be Placing Baits. Both prebait and

measured by bulk with a suitable spoon — poisoned bait should be carried in a can

is put out at each station. On succeeding or bag and placed with a long-handled

days, if much is eaten, the station is re- spoon. Baits may be put directly into rat

filled, doubling the amount each time (4, burrows, in rat-infested spaces under

8, 16 ounces, etc.), and this practice is buildings, or in crevices about founda-

continued until the amount taken "levels tions or walls. In buildings, baits may be

off," when a maximum number of rats placed directly on floors, but should not

will be eating at the station. be set out on boxes, packages, or sacks

Assuming each animal eats about 1 containing human food or animal feed,
ounce (25 to 30 grams), one may esti- To avoid spilling and waste, baits some-
mate the number of rats. Then after times are placed on small sheets of paper
poisoned bait has been substituted, a or in shallow paper or metal cups. The
guess as to the number killed can be baits should be put down near walls, be-

Fig. 13. Containers for safe exposure of baits used to poison rats.

A. Leaning board. Provides tunnel under which rats may eat; if space beneath is narrow, large
cats cannot reach the bait; also may be placed over sodium fluosilicate dust (p. 31) to increase
chance of rats running through the dust, and prevent them from scattering the material.

B. Box shelter. Narrow box, 2 sides removed and ends cut to make 3 x 3-inch opening; will
shelter either bait or trap.

C. Sheet-metal hood. Useful outdoors to protect bait from rain. Board about 8x14 inches with
half cylinder of sheet metal (14 x 18 inches, or sides of two No. 10 cans); small can 3 or 4 inches
in diameter by 1 inch high holds the bait; used by Navy in Pacific Ocean areas (T. B. Murray,
U. S. Naval Med. Bui., 46: 1318, 1946).

D. Bait box. Of %-inch wood, open at both ends with door in middle of top on leather hinge;
used on garbage dump; too small for cats to enter (Coogan, Public Works, 75: 25, Sept., 1944).

E. Bait box. Of wood; about 12 inches on each side, but dimensions may be varied; entrances
about 3x3 inches; diagonal cleat on inside floor forms bait enclosure; slat at back for nailing on
wall; not easy to remove prebait (J. Silver and F. E. Garlough, 1941, p. 15).

F. Metal bait box. Made of 28-gauge galvanized iron; rear door hinged at bottom to insert bait
and clean; about 12 inches long and 8 inches on side (W. S. Mangold and George King).

G. Drain tile or sewer pipe. Useful on garbage dumps; also may be used where poultry are
present, if open end is faced within 3 inches of a wall; pipe is 3 inches in diameter and 24 inches or
more in length; one end plugged with wood or concrete; closed end may be partly buried in gar-
bage dump or embankment; bait should be placed well inside by use of long spoon (A. D. Middle-
ton, Rat control on farms. Great Britain Ministry of Fisheries and Agriculture, 1945).

H. P3 (protected poison point). Special box of wood, %- to 16-inch thick (or of galvanized sheet
iron); removable cover (not shown) fits over top; box when placed against a wall forms a tunnel
without floor; rat must travel in 3 directions to reach bait inside on bottom, below small ledge; large
enough for several rats to feed at one time; poultry or pets cannot reach into bait compartment; lid
may be fastened by inside spring to prevent opening by children, and box may be bolted to floor
or wall; bait not easily spilled if box is overturned, yet prebait can be removed easily (British
Patent, No. 541,844; designed by Charles Elton and others, Bureau of Animal Population, Oxford,
England; see also S. A. Barnett, et al., Rats and Mice, Great Britain Ministry of Food, pp. 13-17,
1946).

[24]

side piles of boxes or sacks, and in any
other places where there is evidence of
rats. It is important to distribute enough
baits so that all rats will have access to
one or more.

Several kinds of special containers
have been designed (figs. 13, 14) for
safer exposure of poisoned baits. Most
of these are built so that only rats and
mice may enter. If one of the more com-
plex types is used, the containers must
be placed, prebaited, and left for some
days, even for 2 weeks, until rats enter
freely. Then a regular schedule of pre-
baiting may be started.

Be sure to keep poisoned material
away from children, pets, and poul-
try.

If the first poisoning is successful, it
need not be followed by another for about
3 months, depending on the amount of
new "sign" of rats that appears. When
the number remaining is small, trapping,
with a new bait, may be tried.

Should one campaign fail to kill
enough rats, a follow-up may be started
about 2 weeks later. A new bait and new
poison must be used, because any rats
made ill but not killed in the first attempt
are likely to be "prejudiced" against the
bait and poison first used. After a good
cleanup, rat proofing is in order.

Poisoned animals and uneaten bait
must be burned, or buried at least 2 feet
deep. They should not be put into gar-
bage, since garbage often serves as feed
for hogs, which might thereby be poi-
soned.

The real measure of success is not the
number of rats found dead after a poison
campaign, but the number that escape.
For good results, 90 per cent or more
must be killed; otherwise the young in
nests and those developing in surviving
females will soon repopulate the area.

Formulas. Some poisoned baits may
be bought already prepared, but if these
are used, prebaiting is not possible.

Poisons are sold by dealers in chemicals
and in garden and orchard supplies, and
by some drug stores. Bait materials can
be purchased from groceries and feed
stores. To use the prebaiting method it
is necessary to buy the bait and poison
separately and prepare the poison mix-
ture according to one of the following
formulas.

Red Squill, This rat poison is ob-
tained from an onion-like plant ( Urginea
maritima) native to lands about the
Mediterranean Sea. At present there are
some experimental plantings in this coun-
try. The large (5 to 6 pound) bulbs are
sliced, dried, and ground to a fine reddish
powder. This powder* keeps well when
stored in a tightly-capped can or bottle.

Of all rat poisons, red squill has been
the safest for use by the general public,
but not always the most efficient. Experi-
ments show that cats, dogs, chickens,
pigeons, and hogs usually will not eat
squill preparations, or will quickly vomit
any they do eat. Poisoned baits, however,
should not be put where children or pets
can find them.

Formula 1

Bait (cereal, bread crumbs, fish, or ground
meat, etc.) 9 parts by weight

Red squill, powdered 1 part by weight
To measure dry materials by weight,

use 1 level tablespoon of red squill to 1

measuring cup of bait. Sift the dry powder

over the bait, turning and mixing until

the poison is well distributed.

Commercially prepared red squill baits
are available as a dry mix of poison and
bait, either loose or twisted in small
squares of paper as "torpedos." Other
squill preparations are baked as small
biscuits or wafers. Such products often

* Different batches of squill bulbs yield
powder varying widely in toxicity. For effective
control, rats should be killed by 500 to 600
milligrams of red squill per kilogram of live
rats as determined by laboratory test (bio-
assay). Most or all red squill* powder now sold
in the United States meets this specification,
since squill of lower toxicity is fortified until of
proper strength.

[26]

r '

Fig. 14. Poison station for house mice (using
drinking fountain for baby chicks). Poisoned ma-
terial in the jar flows into open-topped metal
base where mice can feed. The jar should be
labeled POISON. This device should be used
only in a place that can be securely locked to
prevent entry by children, unauthorized per-
sons, or pets, and where the odor of dead mice
will not be objectionable.

are issued by city health departments to
citizens for rat control about homes;
but prebaiting then is impossible.

A liquid red squill extract is also avail-
able, but generally it seems less efficient
than the powder. One pint of liquid serves
to moisten % pound of bait (meal, or
stale bread cut in V^-inch cubes).

Zinc Phosphide. This chemical (Zn3
P2) is a heavy dark gray powder with a
faint odor of phosphorus resulting from
the slow release of phosphine (PH3).
Both the powder and gas are serious
poisons. Zinc phosphide should be
weighed, mixed, and handled only out-
of-doors or in a well-ventilated room,
and workers should wear gloves of leather
or rubber when mixing or distributing
poison baits.

Bait

Zinc phosphide,
powdered

Formula 2
96 to 100 parts by weight

1 part by weight

Corn oil, mineral oil, or glycerin, at 12
to 24 fluid ounces per 100 pounds of bait,
is often added to any dry cereal to make
the poison adhere. The bait and poison
are first mixed dry until the zinc phos-
phide becomes an evenly distributed coat-
ing. Then the oil, well warmed, is added
slowly and the mixture is stirred again.
Baits may be cereal or bread crumbs
alone or may include some fresh fish,
horse meat, or hamburger. Ground apple
or carrot are sometimes used. Stale bread,
without an oil, but mixed with an equal
amount of water, has been used with 2%
to 5 per cent zinc phosphide in England.

Antu. The chemical known as antu
(alphanapthyl-thiourea) is of use in con-
trol of Norway rats. It is a fine bluish-
gray powder that keeps well when dry
and is insoluble in water. It may be mixed
with wet or dry foods (cereals, bread,
etc.) and adheres well when dusted from
a sifter on cut moist baits of apple, sweet
potato, cantaloupe, or water melon and
on ground meat, ground fish, or chicken
heads.

Formula 3

Bait
Antu

100 parts by weight
2 or 3 parts by weight

This substance is quite poisonous to
dogs, cats, and pigs; when used care is
needed to keep these animals from the
poisoned baits. Antu will not seriously
poison poultry or man. For rodents other
than the Norway rat it is far less toxic
and other poisons should be used for con-
trol of those animals (Richter, 1945;
Ward, 1946).

Warfarin. New in materials for ro-
dent control is warfarin (Compound 42)
which is licensed for manufacture and
sale by the Wisconsin Alumni Research
Foundation. Unlike the usual poisons,
this substance produces fatal internal

[27]

bleeding (hemorrhage) when eaten in
sufficient amount over several days. Addi-
tional differences are that rats take baits
containing warfarin without hesitation
and will continue to do so until death
overcomes them after several days. No
prebaiting is necessary. (Hayes and
Gaines, 1950.)

This substance is marketed under va-
rious trade names (all packages show the
active ingredient as warfarin). As of-
fered for sale warfarin is available in
two forms. One is a 1 :200 (5%) mixture
with flour or other powder that is to be
combined with bait at 1:19 ratio by
weight so the final mixture contains war-
farin in a strength of 1:4000 (about 100
milligrams per pound). With dry baits
(rolled oats, etc.) mineral oil at one quart
per 15 or 20 pounds of bait may be added
so that the powder will adhere and be
spread uniformly. The other form is
ready-to-use bait with warfarin in 1 : 4000
ratio (0.025 per cent).

The final mixture can be exposed like
other baits in small piles or in bait boxes
(Fig. 13). As taken by rats the supply
must be replenished so that it will be
available for the animals to eat as often
as they desire. Bait spots should be so
maintained for 10 days to 2 weeks or
longer. In trials with warfarin it is com-
mon experience that after rats begin
feeding there is a large initial take of
bait for several days, then lesser amounts
are removed as rats die off, and finally
small quantities are eaten irregularly as
rats from outside enter the area baited.

The only serious hazard in using
warfarin is to domestic cats and dogs
(two ounces of bait per day for 3 or 4
days would probably kill an 11 pound
dog) ; like rats, these pets are killed by
feeding on the baits for several days. A
single meal, even of some size, is usually
not fatal. Cats may die after feeding sev-
eral days on poisoned rats.

There is litle danger to human beings
from warfarin. If a child or person eats
some of the prepared bait, he should be

induced to vomit and a physician called
at once. Treatment includes transfusion
with whole blood and giving Vitamin K
preparations by mouth or intravenously.
Arsenic. The heavy white powder
commonly called arsenic (also known as
arsenic trioxide, arsenious acid) has
often been used for rat control. It is now
less in favor in the United States than in
England and elsewhere. The arsenic must
be of very fine particle size (6 to 9 mi-
crons) for good results, since the larger
particles are less toxic. It should be well
refined because impurities make arsenic
less acceptable to rats. The laws of some
states and countries require that arsenic
be colored blue or green to distinguish it
from flour, soda, and other harmless or
edible white powders, but this does not
affect its toxicity.

Formula 4

Bait

Arsenic trioxide

90 parts by weight
10 parts by weight

Arsenic has been used with various dry
and moist baits. In preparing and dis-
tributing baits, workers should be care-
ful to keep arsenic off the skin, which is
irritated by the chemical.

Barium Carbonate. Formerly this
heavy, fine white powder was used com-
monly but it has been largely discarded
in favor of more dependable poisons.
While cheap, it is not always easy to buy.
Formula 5

Bait

Barium carbonate

4 parts by weight
1 part by weight

Corn meal, rolled oats, bread, and
ground meat or fish have served as baits.
Dry materials are first mixed with the
barium carbonate, then moistened. Bread
baits may be moistened with milk;
"sloppy" baits may be more effective in
hot weather. Fresh foods should have the
dry barium carbonate sifted over them,
and should be stirred until evenly coated
or mixed with the poison.

Strychnine. Both alkaloid and sulfate
strychnine have been tried for rat con-
trol, but results are often unsatisfactory,

28]

since some rats will not take baits with
this poison and others eat but little.
House mice will often take strychnine-
coated wheat, and this bait has been used
especially for mice in outbuildings. In
residences, trapping mice is better prac-
tice.

Yellow Phosphorus. The basis of
many commercial rat and mouse poisons
is yellow phosphorus which is often sold
in a sirup to be spread on baits of bread
and other foods. Some of these prepara-
tions are fairly effective.

Phosphorus has a distinctive, garlic-
like odor, and is luminous in the dark,
but these features do not prevent rats
from eating phosphorus baits. There is a
fire hazard from phosphorus probably
only when the particles are larger than of
microscopic (colloidal) size.

No directions for making phosphorus
preparations are given here because the
process is too complicated. Children and
even adults sometimes eat phosphorus-
poisoned sirup by mistake, with serious
or fatal results. Hence this rodent poison
is more of a hazard in homes than other
types.

Special Poisons, Two other rodent
poisons are not available to the general
public because of the great hazards they
involve. Baits poisoned with these chem-
icals have no distinctive appearance,
taste, or smell. Both are toxic to rodents,
other wild animals, domestic livestock,
and man.

Thallium sulfate (thallous sulfate,
T12S04) is a "heavy metal" poison ob-
tained from smelters. It has been used by
government agencies for control of both
rats and field rodents. Thallium poisoned
grain has several times improperly come
into the hands of persons who, not know-
ing it was poisoned, used it for food. A
number of illnesses and several deaths re-
sulted. Sale or possession of thallium for
rodent control in California is restricted
by law to federal, state, county, and mu-
nicipal officers and employees (Calif.
Agric. Code, 1945, Sec. 1066.5).

Since 1945, a new wartime discovery,
"1080" (sodium fluoroacetate), has been
used by military and government agencies
for rodent control (Kalmbach, 1945;
Ward, 1946). It is very effective, but ex-
tremely dangerous. Pet dogs and cats
may be poisoned by eating rats or mice
killed by "1080." Several human deaths,
particularly among children, have oc-
curred with "1080" in rat control. Like
thallium, its sale is restricted by law.

These poisons can only be used by
properly trained persons and in places
where there is little chance of secondary
poisoning.

There is no certain antidote for either
of these poisons. Thallium is a slow-
acting poison, but "1080" works so rap-
idly that it has been impossible to save
experimentally poisoned animals even
with prompt first aid under the best of
laboratory conditions.

DEODORANTS FOR
DEAD RATS

Manufacturers of some commercial
rat poisons have claimed that their prep-
arations will "embalm the dead rats," or
cause them to have no odor, or that rats,
when poisoned, will leave a building to
find drinking water. None of these claims
has been proved.

Rats dying within a wall often produce
persistent offensive odors. Powdered ac-
tivated charcoal blown into the place is
reported to be the most effective deodor-
ant. Other substances recommended are
chloride of lime and solutions of lead
nitrate, lead acetate, or formaldehyde.

POISONOUS GASES

Many rats in burrows or hidden in
ships, buildings, and other enclosures
can be killed by use of poisonous gases.

Calcium Cyanide. This compound,
Ca(CN) 2, is the commonest material used
for gassing. In the presence of moisture
(water in the air or soil of rat burrows) ,
this chemical forms hydrocyanic acid gas

[29]

Any careful person may use poi-
sonous gas out-of-doors, but within
buildings gas is more dangerous, and
should be used only by a trained
official or licensed pest control oper-
ator wearing a suitable gas mask.
All of the gases used are poisonous
to man and domestic animals. Take
care to avoid accidents.

(HCN). Both calcium cyanide and the
gas are deadly poisons for animals, in-
sects, and man. In parts of the arid West
and Southwest during the dry season
there may not be enough moisture for
successful use of cyanide, and cracks in
the ground may permit quick escape of
the gas, which is lighter than air.

Calcium cyanide is available both as a
dust (Cyanodust A) and in granular
form (G Fumigant). The dust is applied

Fig. 15. Use of pump with calcium cyanide
dust to gas rat burrows. Courtesy of U. S. Public
Health Service.

with a special pump (fig. 15). Air is
forced through a glass jar which contains
the powder, and the dust-laden air passes
through a hose into the burrow. The tip
of the hose is placed 10 to 12 inches in-
side the burrow, the entrance is closed
with earth, and several strokes are made
with the pump. If dust is seen coming out
of hidden "bolt holes" (fig. 6) or through
cracks in the ground, such openings
should be covered with earth. The dust
produces a high concentration of gas in
the burrow, but only for a short time.

Corn cribs may be temporarily rid of
rats by use of a dust pump. There is no
danger in using corn from fumigated
cribs after several days of damp air, since
the poisonous gas is generated rather
quickly and the residue is only lime.

When using granular cyanide, place 1
tablespoon (one ounce) of the material 6
to 8 inches inside the burrow with a long-
handled spoon, and block the entrance
tightly, taking care that the cyanide is
not covered. If there is moisture within
the burrow, a strong concentration of
cyanide gas will develop just inside the
blocked entrance, where it will kill any
rat trying to dig out. In damp weather
the cyanide may be placed during the
afternoon. When the temperature is low,
the material should be put into burrows
during the morning to give time for the
gas to form. This method is not effective
on garbage dumps, under piles of trash,
or in places where the ground is cracked,
as the gas will leak away too rapidly.

Cyanide readily dissolves in water; it
can be used when the air is damp, but
not during a rain or when the ground is
wet. The supply can should be covered
tightly except when cyanide is being re-
moved. The chemical should always be
kept off the operator's hands and clothing.

Other Gases. No other gas is so
widely used as cyanide for rat control.
Carbon disulfide (CS2) applied with a
pump or on waste balls, as for control of
ground squirrels, can be used against
rats. Fluid carbon disulfide burns readily,

30]

and the vapor is highly explosive. It
should not be used where there is any fire
hazard from open flames, cigarettes, elec-
tric sparks, or other sources. Details on
the use of carbon disulfide are given in
Calif. Agr. Ext. Cir. 138, "Control of
Field Rodents in California."

Sulfur dioxide (S02), produced by
burning sulfur in air and forcing the
fumes into burrows, is of some value, but
now is seldom used.

Methyl bromide (CH,Br) is a highly
volatile fumigant occasionally used for
burrows, especially where killing fleas
for disease prevention is necessary, but
it requires special equipment for use.

Carbon monoxide (CO) from an auto-
mobile exhaust may be forced through a
hose into rat burrows. The pressure from
the engine drives the gas into all parts of
a burrow. Unlike some other gases, car-
bon monoxide will remain in a burrow
for a considerable time, not being ab-
sorbed in the soil or soil water; hence it
has a long period of effectiveness. This
gas may be used for burrows under
cement-floored farm buildings where
cyanide would be dangerous to livestock.

POISONOUS DUSTS

If sodium fluosilicate (Na2SiF6) is
spread on floors where rats and mice run,
some of the dust gets on their hair and
feet, proves irritating, and is licked off
and swallowed (Mackie, et al., 1934).
The rodents die in 3 to 6 days. This chem-
ical contains fluorine and must not be
used where it might contaminate food.
The dust should not be stirred up; and
under no circumstances should kernels
of cereals on floors dusted with fluosili-
cate be swept up and used as food for
man or animals.

The law states that no food product
may carry a residue of fluorine in excess
of 0.019 grain per pound. Products found
to contain more may be seized, and the
owners prosecuted.

In some places sodium fluosilicate may
be put down in a narrow strip on the floor

Cyanide in any form must be han-
dled with extreme care. It should not
be used indoors, or for burrows un-
der or near a building where people,
livestock, or poultry are present.
Control should not be attempted
during a strong wind. In opening
cyanide cans, loading a pump, and
using the pump or spoon, the opera-
tor should stand to windward and
avoid exposure to dust or fumes.

close to the walls in a room or building.
Then boards may be leaned along the
floor and against the wall to prevent the
dust from being scattered to nearby ob-
jects. Rats and mice usually run along
the bases of walls, and tend to go beneath
boards so placed.

Antu, up to 20 per cent, when mixed in
flour, pyrophyllite, or talc, may be dusted
heavily (% inch or thicker) on rat runs
and entrances to burrows for control of
Norway rats. It has little value for roof
or black rats. This is not a major means
of control, but has been used to kill a few
rats that escape poison, traps, or gas.

In areas where murine typhus is a haz-
ard, DDT dust (5% to 10%) is placed
on runways to catch on the feet and fur of
passing rats and kill many of their fleas.
When the number of fleas, which carry
endemic typhus to man, is reduced, there
is less danger of spreading the disease
when the rats are killed off.

Heavy dusting with DDT is reported
to reduce or eliminate house mice in
places where this material can be used
with safety. It is less effective for rats.

Any of these dusts can be applied with
a sifter can. When much dusting is to be
done, however, it is helpful to have a
large (1 gallon) can with fly screen sol-
dered on one end and a carrying handle
on the side. For dusting rat runs on
beams and pipes, a small sifter can
fastened at right angles to the end of a
3- or 4-foot stick is convenient.

[31]

OTHER MEANS OF CONTROL

Destruction of Burrows. When rat
burrows are numerous in fields or poul-
try pens, the first efforts at control should
be by methods already described. Then
the area should be plowed to a depth of
18 inches with a subsoiler or "chisel" to
destroy most of the burrows. With fewer
safety retreats, it will then be easier to
keep down the number of rats.

Burrows in yards or about buildings
can be collapsed by use of a pick, crow-
bar, or shovel.

Flooding. Rats may sometimes be
killed by flooding their burrows, espe-
cially on poultry or pigeon farms. Rat
burrows are shorter than those of field
rodents, and the animals may escape the
water; results therefore are not always
satisfactory.

Blocking. In some places it is possible
to close all rat entrances to a building
and then kill most or all of the rats inside.
This practice is called blocking. A careful
preliminary search is made and all but
the one or two entrances in most active
use are blocked securely with sheets of
tin, large rocks, or stout boxes filled with
earth. On the night selected for opera-
tions, the remaining entrances are quietly
blocked about an hour after dark. Then
two or more persons, equipped with
strong flashlights, enter and kill the rats
with clubs. Boxes and other objects are
moved when necessary to expose all the
rats. Rats may even be prebaited to con-
centrate in a building, so that a larger
number can be destroyed. Two or three
dozen rats have been reported killed at
one time in a poultry house or similar
structure.

Dogs, Cats, and Ferrets. A rat
terrier often is helpful in blocking, or
when stored goods are being moved in a

room or warehouse. The dog will catch
rats that avoid human efforts.

Some domestic cats are good mousers,
and cats are kept about barns or ware-
houses to help hold down the population
of house mice. Cats seldom kill large rats,
but when a property has been largely
cleared of rats by other means, cats may
capture incoming migrants. On the whole,
however, they are of minor value.

Ferrets sometimes are trained to pur-
sue rats in their burrows and force the
rodents out so that they may be shot.
Sometimes the ferret kills and eats the
rat, then remains below ground, and the
owner must dig to recover his pet. Ferret-
ing is a sport, not a serious means of rat
control. In California, possession of fer-
rets is illegal save for research and testing
of certain biologies used in public health
work (Calif. Admin. Code, Title 14, Sec.
700, 1945; Chap. 76, Calif. Statutes,
1933).

Rat Virus. Several kinds of "rat
viruses," containing cultures of bacteria
presumed to spread disease through a
population of rats, have been exploited.
Their use is commoner in continental
Europe than elsewhere. Unbiased studies
by rodent-control officials and bacteri-
ologists in the United States have shown
that such cultures spread on baits give
far poorer results than the manufacturers
claim. The bacteria used belong to the
food-poisoning group (Salmonella) and
may contaminate human food. In such
institutions as hospitals, use of bacterial
cultures for rat control has been followed
by outbreaks of food poisoning, both in
this country and in Europe. Their sale or
exposure for sale has been prohibited in
California since 1926 (Calif. Admin.
Code, Title 17, Sec. 7925, 1945).

In order that the information in our publications may be more intelligible it is sometimes necessary
to use trade names of products or equipment rather than complicated descriptive or chemical iden-
tifications. In so doing it is unavoidable in some cases that similar products which are on the market
under other trade names may not be cited. No endorsement of named products is intended nor is
criticism implied of similar products which are not mentioned.

[32]

OTHER ASPECTS OF CONTROL

Legal Requirements. In many com-
munities, rat control is dealt with in cer-
tain ordinances or in the public health
laws, such as the California Health and
Safety Code (Division 3, Sees. 1800-
1813, 1945). The burden for control is
placed on the owner or occupier of the
premises, and failure to carry on proper
control is declared a misdemeanor that
may be made the subject of court action.

In increasing degree, many cities are
also prescribing the detailed require-
ments for proper rat proofing of build-
ings, both new construction and alter-
ations. In practice, many details of con-
struction and maintenance are ignored,
because owners, contractors, and work-
men are seldom "rat conscious." If
rodent-borne disease flares up in a com-
munity, a wave of public interest in rat
proofing and a program of rat riddance
may arise, but unfortunately it soon de-
clines as the threat of immediate danger
passes.

Pest Control Operators. When the
owner of a residence or business estab-
lishment does not want to attempt rat
control himself, he may engage a trained
pest control operator for the work. The
latter will do the premises as a single job
for a specified charge, or will agree to do
periodic inspection and control as needed
under a contract for a stipulated monthly
or annual fee. The degree of efficiency
will vary with the individual operators or
companies.

Several states now require that any
person engaging for hire to do pest con-
trol must first pass an examination and
be licensed. California provides for two
types: structural pest control operators,
who work in and about buildings; and
agricultural pest control operators, who
deal with pests of crops and farm lands.
(See Calif. Structural Pest Control Act,
Sept. 15, 1945, a part of the Business and
Professions Code, Div. 3, Chapt. 14, Sec.
8500-8677; and Calif. Agric. Code, Sec.

150). State and national organizations
of the operators hold schools and confer-
ences for improving the technical and
practical qualifications of their members.

NATIVE MICE AND RATS

White-footed Mice. The commonest
rodents in California, outside cities and
cultivated areas, are the small "deer
mice" or white-footed mice (genus Pero-
myscus) . There are several kinds in vari-
ous parts of the state, but one or another
is present in every sort of wild environ-
ment from the humid coast belt to the
driest desert areas, and from sea level to
the high mountains. These mice often in-
vade buildings. Exclusion and trapping
are the best methods for their control.

The head-and-body length of adults is
3 to 5 inches, and the tail is 2% to 5%
inches long in different species. The eyes
are large and black, the ears thin and
prominent; the upper surface of the body
is pale brown to gray, and the under parts
and feet white (fig. 16).

White-footed mice are strictly night-
time foragers, but they are active through-
out the year. They make no runways.
They hide and nest in cavities of old
stumps or downed logs, in rock piles, and
probably in burrows. From 2 to 6 young
are produced in a litter, and several
broods may be borne during the year by
one female. In nature these mice feed on
a wide variety of plants and insects.

About homes, cabins, and barns where
no house mice (Mus musculus) are pres-
ent, white-footed mice will come indoors
and become household pests. Mountain
cabins that are closed after the summer
vacation are often invaded by these mice,
which may cut clothing or bedding to
obtain nest material, and eat any human
food not in mouse-proof containers. They
are particularly undesirable because in
some parts of California they are infected
with plague.

[33]

The owner of a cabin must make spe-
cial effort to build out these small rodents.
The detailed directions for exclusion of
house rats and mice (pp. 13-19) should
be adapted as necessary to make cabins
rodent proof.

Old and loosely built structures, and
those of "rustic" design with bark slabs
and other rough or irregular materials
will need special attention. Buildings of
natural stone combined with wood afford
many irregular crannies where mice may
hide, especially after parts of the struc-
ture have settled. Foundations of loose
stones not solidly cemented, or of partly
rotted logs, give shelter to the mice.

Besides openings on the first floor, the
roof, upper windows, and chimney top
are paths of entry to mice in winter when
snow blankets most or all of a building.
The chimney should be capped tightly
with hardware cloth of %- or %-inch
mesh. Loose shingles or shakes must be
fastened down, and all crevices between
boards covered with battens, well nailed.
Sheet tin or hardware cloth serves to

cover cracks or holes wider than % inch.
Irregular crevices may be wedged with
folded strips of fly screen. Pipes entering
walls or floors should be surrounded by
metal collars (fig. 7).

Bedding, mattresses, and clothing not
in use in summer, or stored during the
winter, are safe only in a tight closet
lined with sheet metal or hardware cloth.
All foods should be in tightly covered
containers of metal, glass, or crockery,
or in cabinets fully lined with metal.

Killing of white-footed mice in build-
ings is best done with mouse traps, using
a number of traps and persisting until
all are caught. Rolled oats makes good
bait, but other foods should be tried if
necessary. Some owners leave a number
of traps baited and set when closing their
buildings for the winter. Dusting the
floor with napthalene flakes or sulfur is
reported to reduce mouse damage and
DDT dust in heavy layers may also be
used. Strychnine-coated grain sometimes
will serve for control, but may be refused
by the mice. If the owner cannot control

\ <*.

&£?

Fig. 16. The white-footed mouse, or deer mouse (Peromyscus). Its head-and-body length is 3 to 5
inches and the tail about the same. The large black eyes are conspicuous, the ears are large, the
upper surface of the body is usually pale brown (bluish gray in young), and the feet and under
parts are white.

[34]

Fig. 17. The wood rat, or pack rat (Neotoma). Its head-and-body length is 7 to 8 inches and the
tail 6V2 to 7V2 inches. The fur is soft, the upper surface is warm brown (bluish gray in young), the
ears are large and scantily haired, the nose is bluni, and the tail is hairy.

these mice by the methods described, he
should get help from some government
agency (p. 4).

Dead mice in mountain cabins should
be handled carefully to avoid the chance
of live fleas or ticks getting on a person's
clothes or skin. Gloves should be worn,
and the clothing sprayed with a pyreth-
rum spray immediately after picking up
any mice. The dead rodents should be
completely burned, out-of-doors, or
buried to a depth of 2 feet.

Wood Rats. The native wood rats,
commonly known as "pack rats" or
"trade rats," are sometimes a nuisance in
homes, summer cabins, and other build-
ings outside cities and towns. Exclusion
and trapping are the best means of con-
trol.

The brown-footed wood rat {Neotoma
juscipes, fig. 17) of the foothills and
lower mountains, has a head-and-body
length of 7 to 8 inches, and the tail is 6V2
to 7% inches long. The nose is blunt, the
medium-sized ears are slightly haired,
and the body coat is warm brown on the
upper surface. The coat is softer than that

of alien house rats, and the tail is mod-
erately haired.

This wood rat builds large conical
nests of sticks and forest litter, either on
the ground or on horizontal branches in
oaks and other trees and, if given a
chance, will nest in buildings. These ro-
dents are active throughout the year,
mainly at night. They climb readily, and
eat a wide variety of plant materials.
They may also be attracted to human
food supplies in buildings or in outdoor
camps. Wood rats often remove small
bright objects, such as spoons, forks,
knives, or mirrors, sometimes leaving
sticks or other items "in trade."

In the higher mountains there is a
larger gray, bushy-tailed wood rat (Neo-
toma drier ea) that makes a nest of piles
of rough sticks in rock slides. It also in-
vades cabins for food or shelter. Since
wood rats at times may carry diseases
transmissible to man, they should be ex-
cluded from homes and buildings.

The directions for excluding alien rats
and white-footed mice apply to making
foothill cabins, mountain resorts, and

[35]

similar places proof against wood rats, are not readily attracted by other grain

No double walls or other spaces should be baits, and are not easily poisoned with

left to provide nesting places for these strychnine.

and other rodents. Means just described For safety in respect to disease, besides

to protect food and bedding from white- clearing buildings of these rodents, those

footed mice will serve also against wood in nearby nests, logs, or rock slides

rats. Rock piles near cabins or crevices should be trapped. Dead wood rats should

in foundations are other common shelters be handled with the precautions stated

for these rodents. for white-footed mice. When a building

Wood rats are less wary than alien rats, is closed for the winter, several baited rat

and may be caught in wooden-based traps should be placed to capture any

spring rat traps baited with rolled oats, wood rats that gain entrance during the

peanut butter, raisins, or prunes. They owner's absence.

LITERATURE CITED

Bartlett, S. A., and others

1946. Infestation control: rats and mice. Min. of Food, London. iv + 1-36. 41 figs.
Beck, M. Dorothy, and Alwine Van Allen

1947. Typhus fever in California, 1916-1945, inclusive. Amer. Jour. Hyg. 45:335-54, 1 fig.
California, State of

1945. California Agr. Code. California State Printing Office, Sacramento. 476 p.
Gunderson, Harold

1943. Clean out the rats! Farm Science Reporter (Iowa State College) 4(1) :10-12.
Hall, E. R.

1927. An outbreak of house mice in Kern County, California. Univ. California Publ. Zobl. 30:189-
203.

Hayes, W. J., Jr., and T. B. Gaines

1950. Control of Norway rats with residual warfarin. U. S. Public Health Reports. 65:1537-
1555, 2 figs.
Hull, T. G.

1947. Diseases transmitted from animals to man. 3d ed. Charles C. Thomas, Springfield, 111.
xvi + 571 p. Illus.

Kalmrach, E. R.

1945. "Ten-eighty," a war-produced rodenticide. Science 102:232-33.
Mackie, D. B., W. C. Jacobsen and W. B. Carter

1934. Rodent control with sodium fluosilicate by a new method. California Dept. Agr. Monthly
Bui. 23:192-96.

O'Connor, M. G., R. E. Buck and C. R. Fellers

1935. Properties, toxicity and palatability of red squill and powder baits to rats. Indus, and
Engin. Chem. 27:1377-80.

Piper, S. E.

1928. The mouse infestation of Buena Vista Lake basin . . . 1926 . . . 1927. California Dept. Agr.
Monthly Bui. 17:538-60.

Pisano, R. G., and T. I. Storer

1948. Burrows and feeding of the Norway rat. Jour, of Mammal. 29:374-83, 2 pis., 3 figs.
Richter, C. P.

1945. The development and use of alpha-naphthyl thiourea (ANTU) as a rat poison. Amer. Med.
Assoc. Jour. 129:927-31.

Silver, James, and F. E. Garlough

1941. Rat control. U. S. Dept. Interior, Fish and Wildlife Service. Conserv. Bui. 8:1-27. 10 figs.
Storer, T. I.

1931. A minor plague of house mice at Davis, California, in 1926. Jour, of Mammal. 12:316-17.

1949. Control of field rodents in California. California Agr. Ext. Circ. 138 (revised) :l-50. 25 figs.
Ward, J. C.

1946. Rodent control with 1080, ANTU, and other war-developed toxic agents. Amer. Jour. Pub.
Health 36:1427-31.

35m-3,'52(9060)L.B.

Co operative Extension work in Agriculture anil Home Economics, College of Agriculture, University of California, and United Slates Department of Agriculture CO-
operating. Distributed in furtherance of the Acts of Congress of May X, and June 30, 1914. J. Earl Coke, Director, California Agricultural Extension Service.