Control of rats and mice

CALIFORNIA AGRICULTURAL EXTENSION SERVICE

CIRCULAR 142

APRIL, 1948

CONTROL OF
RATS AND

TRACY I. STORER

Cooperative Extension work in Agriculture and Home Economics, College of Agriculture,

University of California, and United States Department of Agriculture cooperating.

Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914.

B. H. Crocheron, Director, California Agricultural Extension Service.

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 undis-
turbed.

Exclusion and proper sanitation are the surest and most last-
ing defenses against rats. This circular tells how to rat proof
buildings, and describes other control 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 accidents
occur.

Aid in control may be obtained from any county agricultural
commissioner or farm advisor, and in some cities and counties
from the health department.

CONTENTS

PAGE

The rat problem 3

Kinds of rats and mice 3

General features 3

Norway rat 5

Roof and black rats 5

House mouse 6

Diseases carried by rats and mice .... 6

Plague 6

Murine typhus 8

Leptospirosis 8

Food poisoning 8

Rat-bite fever 8

Trichinosis 8

Rickettsialpox 8

Damage by rats and mice 9

Evidence of rats and mice 1

Sight 1

Sound 1

Droppings 1

Runways 1

Smears 1

Holes 13

Nests 13

Urine stains 13

Methods of control 13

Exclusion 13

Foundations 15

Exterior walls 15

Openings 15

Doorways 16

Corn cribs 17

Feed storage 17

Human food supplies 19

Cabinets and enclosed platforms . 19

Garbage, boxes, and trash 19

PAGE
Methods of control {continued)

Traps 21

Trapping house mice 21

Trapping rats 21

Poisoned baits 23

Poisons 23

Baits 24

Prebaiting 24

Placing baits 25

Formulas 25

Red squill 25

Zinc phosphide 27

ANTU 28

Arsenic 28

Barium carbonate 29

Strychnine 29

Yellow phosphorous 29

Special poisons 29

Deodorants for dead rats 29

Poisonous gases 30

Calcium cyanide 30

Other gases 31

Poisonous dusts 31

Other means of control 32

Destruction of burrows 32

Flooding 32

Blocking 32

Dogs, cats, and ferrets 32

Rat virus 33

Other aspects of control 33

Legal requirements 33

Pest control operators 33

White-footed mice 33

Wood rats 35

Literature cited 37

CONTROL OF RATS AND MICE

TRACY I. STORER

Professor of Zoology and Zoologist in the Agricultural Experiment Station, Davis

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,
cabinets, 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 centuries, but
the "rat problem" is still unsolved.

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 method of control. No one fixed
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 WiM-
life Service and U. S. Public Heallh Serv-
ice. Specialists in these agenci-^s 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 meth-
ods 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-sharpening. 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 teeth (tusks).
Their check or grinding teeth have small

^ This circular replaces part of Extension Cir-
cular 79 which will not be republished. For con-
trol of field rodents, see Extension Circular 138.

[3

jjrojections used to chew food finely be-
fore swallowing it.

All of 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.

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

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

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

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. 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 coarser hair and scaly
tail.

[4]

Norway Rat

The largest of the alien rats, and the
one which does most damage, is the Nor-
way rat {Rattus norvegicus) , also known
as brown rat, house rat, wharf rat, and
sewer rat (fig. 1).

Full-grown adults are about 16 inches
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,
slaughterhouses, 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
rats take to water when necessary, and can
swim easily. r

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 can shift for themselves 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 alexandri-
nus), sometimes called Alexandrine, or
gray, rat (fig. 2) , grows to a total length
of 15 inches, with the tail measuring 8^4
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.

^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.

5]

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 as high as 5,000
feet. They inhabit homes, warehouses,
packing sheds, and feed stores, often liv-
ing in attics or upper stories, but may
also be found at ground level, especially
when Norway rats are absent. In some in-
land localities they live along stream
banks. Roof rats climb readily and travel
on the exterior of rough-surfaced build-
ings, 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
and breed are assumed to be about the
same as for the Norway rat.

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 mus-
culus) 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), and the tail is scantily
haired. The upper surface 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 when it gets cold.

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. They seldom bur-
row in the ground, but often make com-
pact nests of cloth, sacking, or other
materials 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
parasites carry several important
human diseases (Hull, 1941). This
is a most important reason for con-
trol of rats and mice.

Plague

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

[6]

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.

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 3Vi inches.

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.

[7]

Murine Typhus

This endemic typhus is a New World
disease of rats and man carried by rat
fleas. It is related to the louse-borne ty-
phus of Europe, and to Rocky Mountain
spotted fever, but is not as deadly. Human
cases of murine typhus have increased
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 disease. Dogs also are
susceptible, and one or more epidemics
have occurred among city dogs in Cali-
fornia.

Food Poisoning

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

Rat-bite Fever

This is another bacterial disease of
rats transmissible to man. Several cases
have been reported in the United States
in recent years.

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 infected animals. When flesh
of one infected animal is eaten by another
of the susceptible animals, the larvae
taken in with the flesh become mature,
breed, and produce many new larvae. If
the latter are abundant, the poisonous
substances they secrete lead to illness or
death of the new host. Rats aid in spread-
ing this disease among pigs, and man be-
comes infected by eating pork or bear
meat which has been imperfectly cooked
or cured and therefore contains many
living trichina larvae.

Rickettsialpox

This disease of house mice is trans-
mitted by a tiny blood-sucking mite, and
was first recognized in 1946, when about
80 human cases occurred in two housing
centers in New York City.

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

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

8

human beings, and may carry disease.
The tropical rat mite (Liponyssus hacoti)
is so small that it can just be seen by the
unaided eye. It is common on rats in Cal-
ifornia, 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 com-
pared with the amount of damage
these animals do.

No one knows how many rats there are,
but it has often been assumed that the
rat population equals the human popula-
tion. In the United States, then, there
would be 140,000,000 rats. The average
adult Norway rat probably eats about an
ounce of food a day. If the rats lived en-
tirely on wheat, they would need a daily
food supply of 4,375 tons, or 145,833
bushels! The annual levy would be more
than 53 million bushels, and, with wheat
at $1.00 per bushel, the annual bill for
supporting the rat population would be
$53,229,000. Of course, a higher wheat
price means a higher toll. Rats consume
much that is unfit for human use, but they
often take high-priced foods, and damage
more than they eat. Hence this estimate
based on wheat gives a rough idea of the
cost of these rodents.

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
disposed 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 sides of
other meats may be eaten and gnawed
extensively. House mice sometimes live
continuously 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 the motor, which then must
be rewound.

[9]

fc -^^^s

\ ".

•^m

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.

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 burrows, nests, dam-
age to food or property, gnawings,
and disappearance of bait.

Sight

Rats and mice try to avoid being seen.
One may have quite an infestation with-
out seeing a single live rodent. Gunder-
son (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 present;
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 numbers are
present, rats and mice may usually 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). Un-
disturbed droppings last for a long time;
hence only those in places often swept
are useful to indicate recent activity, and

whether few or many rodents are present.
A Norway rat (captive) 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 1/16 by 1/4 inch.

Runways

Paths regularly used by numbers of
rats may become beaten runways on the
ground or over piles of refuse, and
smoothed routes over board surfaces in-
doors. In 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 be-
lieved 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 for finding tracks in
dark corners.

Smears

Grease, soot, and dirt from the belly
hairs will leave smears on well-travelled
runways. These can be best 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 o <^ O shaped marks between the
joists (fig. 5). Smears are most evident
as dark marks on whitewash or non-

[11]

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 w w v^ v^ -shaped markings.
Courtesy of U. S. Public Health Service.

^.^.„, bolt holes x,rv.

entrance

nest vS

■,»^. I ,\t^. . \'j^. ,\\t . .'.I < . ,\\i, . ,\t^, . ,\'. I .'.',• . .\\' . .'.■ . . .v . .

SIDE VIEW

entrance ^

Il^ bqit hqies^5^

'■^:-S'iM

nest $

:<%<f:-yyi:':

H

;v*-V\vV-.v'viif

ii^'^^-^ \^ -^:^*^^^^V^^^

^[^iii^^^"*"^

WirSW^'i

^6"—

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.
(Excavated by Rocci G. Pisano.)

12

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 newspapers or
soft earth, and later noting which are re-
opened, is an easy way to recognize those
being used.

Nests

The nests of Norway rats 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 burrows) , 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 even in office desks. When living in
fields, house mice nest in burrows.

Urine Stains

These are not usually easy to find, al-
though 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 fluores-
cent, but identification is possible by
trained inspectors.

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 temporary relief.

EXCLUSION

Exclusion of rats and mice means
essentially only two things: leaving
no opening more than Ys inch in
width into a building, or other
enclosure, and protecting all food
supplies from these animals (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 :

I) Concrete floors and exterior cur-
tain 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 1/3- or 1/4-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.

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

[13]

a small plumbing job, can give rodents a
chance to reinfest the building. Crates,
baled goods, and other packaged ma-
terials entering the building should be
examined for rodents or young in nests.

Foundations

Rats burrow readily under concrete
slab floors laid on the ground. Any in-
terior openings in such floors, as around
sumps, may be used as entrances. Rats
also tunnel under the foundations of ex-
terior walls, even when the foundations
extend 24 inches below the ground sur-
face. To prevent such burrowing, a thin
curtain wall, L-shaped in section, may be
poured around the foundation or slab. It
need be only 2 or 3 inches thick. The foot
of the L should project outward to dis-
courage 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 covered, cracks battened
tightly, and the bottom boards are nailed
down securely. The lower edge should be
sheathed with strips of light weight (28
gauge) galvanized iron sheeting.

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, as well 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 lower-
most inside boards to place stops of one
kind or another as shown in figure 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 antenna 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 foot-
hold on outside drains or rain ducts. Rats
may cut fly screen, particularly near the
ground or roof, although 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
must be screened completely with hard-
ware cloth of 1/3- or 1/4-inch mesh. This
includes all attic ventilators and louvers,
tops of air shafts, elevator 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, extending several inches be-
yond the edges of the opening,
and nailed to the wall (fig. 7,G).

[15]

3) At entrances to steam tunnels, air
ducts, or other places where ven-
tilation is necessary and where
workmen must enter from time to
time, close-fitting but removable
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 clear-
ance 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, grocery
and feed stores, and granaries or feed
rooms should be sheathed along the bot-
tom and up each side, for 6 to 12 inches,
with a 3-inch width of galvanized sheet
iron, securely nailed (fig. 8). An even
better practice is to form U -shaped chan-
nels 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

Fig. 8.— Permanent rat-proofing for windows and doors. Left, four folded channels of galvan-
ized sheet iron are 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. An enlarged detail of the channel is shown below. Right, three rectangular
channels are bent from galvanized sheet iron to fit closely around the bottom and lower edges
of a door and nailed or screwed into place. These channels add strength to the door, they prevent
damage to the lower edge, and they will stay in position much better than fiat strips of sheet
metal tacked to one side of the door.

[16]

space between the door and wall. Often
strips of wood covered with metal can be
fitted to the doors or casings to keep them
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-

rats and mice. Space below the floor gives
shelter for nesting, and the open con-
struction above makes food easy to ob-
tain. Old wooden cribs should be raised
18 inches above the ground, using con-
crete, field stone, or wood for supports.
The top, walls, and door should be cov-
ered with hardware cloth in addition to

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 between
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.

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

the wooden slats (fig. 9). New wooden
cribs should be built off the ground and
lined with hardware cloth. Prefabricated
cribs made entirely of sheet metal are
available and are used by many farmers,
who thereby practically end their rodent
troubles in this kind of storage.

Feed Storage

Large supplies of livestock feed in piled
sacks offer both food and shelter for rats
and mice. When feed is stored in ware-
houses 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

[17]

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

[18

should be stored in narrow piles with
lanes between, and an alleyway between
the outermost tiers and exterior walls.
Damage then will be lessened and any
rats and mice present can be controlled
more readily.

Rodent proof wooden bins may be
built to contain bulk materials. The in-
terior 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 top needs
tight covering 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 such control is con-
tinued as the stock of hay is used or grad-
ually moved out.

Human Food Supplies

Grocery stores may reduce losses by
keeping small supplies of grain products,
sugar, and similar materials in metal con-
tainers. In homes and in store rooms ad-
jacent to farm kitchens the same means
may be used. Covered tin cans, glass
bottles, 2- to 10-gallon crocks, and even
small garbage cans are useful for flour,
cereals, dried fruits, nuts, etc. Such con-
tainers give protection from rodents and
make it easy to remove the contents with-
out 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
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 and closing all entry ways.

Garbage, Boxes, and Trash

Incorporated cities and towns usually
require each residence or business place
to provide metal garbage cans for dispos-
ing of waste food and trash. But many
housewives, restaurant employees, and
storekeepers are careless (fig. 10). They
fail to cover the cans tightly or they over-
load 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

[19]

.">^?*^<?^-

&

U.m,

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 neces-
sary); 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

chickens or hogs helps to support both
rodents and flies, particularly if the pens
are seldom or never cleaned up. To keep
down rodents, 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 give shelter for rats— and in the
country, also for 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 methods
of exclusion just discussed, and then
repair his premises. In many cases, a
little work, using materials already at
hand, will result in keeping all rodent
pests out of buildings.

TRAPS

Trapping is the preferred method
of control in homes and office
buildings, because 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 will catch some
rats, but long experience proves
that the ordinary 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
handling 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 arid choker-loop
traps are used where there are signs of
mice: tracks, droppings, holes, or dam-
age. In buildings, place traps along the
wall, with trigger end toward the base-
board, 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 are 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.

Trapping Rats

Rats are considered somewhat "wiser"
than mice, yet many are taken with
wooden snap traps or steel traps. Trap-
ping is most useful in homes, offices, and
generally in places where there are few
rats. If there is a heavy infestation, 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

[21]

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 unbaited, are set with the
trigger directly across the runway.

When rats are using overhead pipes
or beams, traps are braced 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-
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 plaped on the ground,
the base may be countersunk in the soil
so that only the trigger projects above
the surface. Indoors or out, for especiaTy
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 anv
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
them to enter 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,
corn meal, and bread crumbs may find
favor; ground meat, bacon, fish, apples,
and raisins also are used. When any finely
divided bait is used, some should be scat-
tered 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 re-
placed 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
floor 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
has proved more efficient than a good
supply of snap traps, and all are much
more expensive.

[22]

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.

POISONED BAITS

Poisons

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

Except for red squill, all these poi-
sons 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 securely 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.

Many types of commercial rat and
mouse poisons are on sale. In California,
all poisons and poisoned baits (economic
poisons) for pest control and for sale to
the public must first be submitted for
registration to the State Department of
Agriculture (Calif. Agric. Code, 1945,
Sees. 1061-1079), and only those which
are satisfactory are registered for sale
within the state. The printed label on each
package must include the name and per-
centage of active ingredients, a statement

[23

about the poison, and information about
antidotes. The Federal Insecticide, Fung-
icide, 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 some ineffective rodent poisons
formerly sold, such as those sometimes
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 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 vegetables, cooked vegetables,
and fresh fruits (O'Conner et at, 1935).
Rats in different localities and in places
where one or another kind of food is most
available often definitely prefer particu-
lar 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 the general
practice simply to put down baits in places
frequented by rats, leave them for one
or more nights, and then remove the un-

eaten baits. Some rats were killed, but
results were usually unsatisfactory. A re-
cent major improvement is the use of
prebaiting (Bartlett, et al, 1946). Small
piles of clean unpoisoned baits are placed
at selected sites or in special bait con-
tainers for several nights, the supply be-
ing replenished whenever rats take the
material. The prebait 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 days. The
largest take of poisoned bait will be on the
first night it is exposed; a little may be
taken the second night, but practically
none thereafter.

Prebaiting may provide a crude
method of estimating the number of rats
using bait stations. A given weight of
prebait— say 2 ounces, which may be
measured by bulk with a suitable spoon-
is put out at each station. On succeeding
days, if much is eaten, the station is re-
filled, doubling the amount each time
(4, 8, 16 ounces, etc.), and this practice
is continued until the amount taken
"levels off," when a maximum number of
rats will be eating at the station.

Assuming each animal eats about 1
ounce (25 to 30 grams), one may esti-
mate the number of rats. Then after

[24]

poisoned bait has been substituted, a
guess as to the number killed can be
made, on the basis of 1/3 ounce (10
grams) per rat, since the animals eat a
smaller amount of poisoned food. A week
or two after poisoning, an estimate of
those remaining can be made by using a
new bait (unpoisoned) and determining
the amount removed.

Placing Baits

Both prebait and poisoned bait should
be carried in a can or bag and placed with
a long-handled spoon. Baits may be put
directly into rat burrows, in rat-infested
spaces under buildings, or in crevices
about foundations or walls. In buildings,
baits may be placed directly on floors, but
should not be set out on boxes, packages,
or sacks containing human food or ani-
mal feed. To avoid spilling and waste,
baits sometimes are placed on small sheets
of paper or in shallow paper or metal
cups. The baits should be put down near
walls, beside piles of boxes or sacks, and
in any other places where there is evi-
dence 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 complex types is
used, the containers must be placed, pre-
baited, and left for some days, even for 2
weeks, until rats enter freely. Then a regu-
lar schedule of prebaiting may be started.

Be sure to keep poisoned material
away from children, pets, and poultry.

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 campaign may
be started about 2 weeks later. A new

bait and new poison must be used, be-
cause any rats made ill but not killed by
the first attempt are likely to be "preju-
diced" against the bait and poison first
used. After a good cleanup, rat proofing
is in order.

Dead animals and uneaten bait
should be burned, or buried at least
2 feet deep. They should not be put
into garbage^ since garbage often
serves as feed for hogs, which would
thereby be poisoned.

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
treated.

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 mixture according to
one of the following formulas.

Red Squill

This rat poison is obtained from an
onion-like plant {Urginea maritima) na-
tive to lands about the Mediterranean Sea.
At present there are some experimental
plantings in this country. 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.

^ 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 (bioassay) . 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.

[25

Fig. 13.— For legend, see facing page.

Of all rat poisons, red squill is the
safest for use by the general public, but
is 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
are issued by city health departments to
citizens for rat control about homes; sat-
isfactory prebaiting is not possible with
these.

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 V2-iiich cubes) .

Zinc Phosphide

This chemical (ZngPg) is a heavy dark
gray powder with a faint odor of phos-
phorus 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 leather,
or rubber, gloves when mixing or dis-
tributing poison baits.

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 8 x 14 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 M-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 (/. 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
garbage dump or embankment; bait should be placed well inside by use of long spoon (A. D.
Middleton, Rat control on farms. Great Britain Ministry of Fisheries and Agriculture, 1945).

H. P3 {protected poison point). Special box of wood, Ys- to i^-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 com-
partment; 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; design by Charles Elton and others. Bureau of Animal Popu-
lation, Oxford, England; see also S. A. Barnett, et al. Rats and Mice, Great Britain Ministry of
Food, pp. 13-17, 1946).

[27]

ANTU

Fig. 14.— Poison station lor house mice (using
drinking fount for baby chicks). Poisoned
grain or seeds in the inverted jar fiow into the
open-topped metal base, where mice can feed.
The jar should be labelled POISON. This
device should be used only in an attic or store-
room securely locked to prevent entry by chil-
dren, unauthorized persons, or animal pets,
and where the odor of dead mice will not be
objectionable.

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.

This new poison (alphanaphthyl-
thiourea), has proved useful for control
of Norway rats. It is much less toxic for
black or roof rats, house mice, or field
rodents, and other poisons should be used
for those animals. ANTU will not seri-
ously poison poultry and man, but may
kill cats, dogs, and pigs, and should be
handled carefully (Richter, 1945; Ward,
1946).

ANTU is a fine bluish-gray powder,
insoluble in water, that keeps well when
stored dry. It may be mixed with wet or
dry foods, and adheres well when dusted
from a sifter on cut moist baits of apple,
sweet potato, cantaloupe, watermelon, or
on ground meat, ground fish, or chicken
heads.

Formula 3

Bait
ANTU

100 parts by weight
2 or 3 parts by weight

Arsenic

This heavy white powder (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 90 parts by weight

Arsenic trioxide 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.

[28]

Barium Carbonate

This heavy, fine white powder has been
largely discarded in favor of more recent
discoveries. It is cheap but not always
easy to buy.

Formula 5
Bait 4 parts by weight

Barium carbonate 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 control, but results
are often unsatisfactory, 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 practice.

Yellow Phosphorus

This is the basis of many commercial
rat and mouse poisons, and is often sold
in a sirup to be spread on baits of bread
and other foods. Some of these prepara-
tions seem quite 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 avail-
able to the general public because of the
great hazards they involve. Baits poisoned
with these chemicals have no distinctive
appearance, taste, or smell. Both are toxic
to rodents, other wild animals, domestic
livestock, and man.

Thallium sulfate (thallous sulfate,
TI2SO4) 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."

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

It is often claimed by manufacturers
of commercial rat poisons that their prep-

29

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. A
commercial product, Hyamine 1622, is
reported to reduce such odors.

POISONOUS GASES

Many rats in burrows or hidden in
ships, buildings, and other en-
closures can be killed by use of
poisonous gases. Any careful per-
son may use this method out-of-
doors, but within buildings gas is
more dangerous, and should be
used only by a trained official or
licensed pest control operator
wearing a suitable gas mask. All
of the gases used are poisonous to
man and domestic animals. Take
care to avoid accidents.

Calcium Cyanide

This compound, Ca(CN)o, is the com-
monest material used for gassing. In the
presence of moisture (water in the air or
soil of rat burrows) , this chemical forms
hydrocyanic acid gas (HCN). Both cal-
cium cyanide and the gas are deadly
poisons for animals, insects, and man. In
parts of the arid West and Southwest dur-
ing 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
with a special pump (fig. 15). Air is

forced through a glass jar which contains
the dust, 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
pump produces a high concentration of
cyanide 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 consists
only of 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 in any form must be handled
carefully. The material should not be used
indoors, or for burrows under or near a
building where people, livestock, or poul-
try are present. Control should not be at-
tempted during a strong wind. In opening
cyanide cans, loading a pump, and using
the pump or spoon, the operator should
stand to the windward and should avoid
exposure to dust or fumes. Cyanide read-
ily dissolves in water; it can be used when
the air is damp, but not during a rain or

[30]

when the ground is wet. The supply can
should be covered tightly except when
cyanide is being removed. The chemical
should always be kept off the operator's
hands and clothing.

Other Gases

No other gas is so widely used as cya-
nide 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, and the vapor is
highly explosive. It should not be used
where there is any fire hazard from open
flames, cigarettes, electric sparks, or other
sources. Details on the use of carbon di-
sulfide are given in California Agricul-
tural Extension Circular 138, "Control of
Field Rodents in California."

Sulfur dioxide (SOg), 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 forces the gas into all parts of
a burrow. Unlike some other gases, car-
bon monoxide remains in a burrow some
time, not being absorbed 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 build-
ings where cyanide would be dangerous
to livestock.

POISONOUS DUSTS

If sodium fluosilicate (NaoSiFg) is
spread on floors where rats and mice run,
some of the dust gets on to their hair and
feet, proves irritating, and is licked off
and swallowed (Mackie, et al, 1934) . The

f4.

'i^^'^F'

Fig. 15.— -Use of pump with calcium cyanide
dust to gas rat burrows. Courtesy of U. S. Pub-
lic Health Service.

rodents die in 3 to 6 days. This chemical
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 fluosilicate 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 by govern-
ment authorities, and the owners may be
prosecuted.

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

31

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 practice is not
a major means of control, but has been
used to dispose of a few rats that escape
death by 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.

Any of these dusts may 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.

OTHER MEANS OF
CONTROL

Destruction of Burrows

When rat burrows are numerous in
fields or poultry 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 hold 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 flood-
ing their burrows, especially on poultry
or pigeon farms. Rat burrows are shorter

than those of field rodents, and the ani-
mals may escape the water; results there-
fore 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 pre-
liminary 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 block-
ing, or when stored goods are being
moved in a room or warehouse. The dog
will catch rats that escape into corners or
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

32

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," contain-
ing 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 bacteriologists in the
United States have shown that such cul-
tures spread on baits give far poorer re-
sults than the manufacturers claim. The
bacteria used belong to the food-poisoning
group (Salmonella) and may contami-
nate 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).

OTHER ASPECTS OF
CONTROL

Legal Requirements

In many communities, rat control is
dealt with in certain 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 a 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 requirements
for proper rat proofing of buildings, both
new construction and alterations. In prac-
tice, many details of construction and
maintenance are ignored, because owners,

contractors, and workmen are seldom
"rat conscious." If rodent-borne disease
flares up in a community, a wave of pub-
lic interest in rat proofing and a program
of rat riddance may arise, but unfortu-
nately it soon declines as the threat of im-
mediate danger passes.

Pest Control Operators

When the owner of a residence or busi-
ness establishment does not want to at-
tempt rat control himself, he may engage
a trained pest control operator for the
work. The latter will do a premises as a
single job for a specified charge, or will
agree to do periodic inspection and con-
trol as needed under a contract for a stip-
ulated monthly or annual fee. The degree
of efficiency will vary with the individual
operators or companies.

Several states now require that any per-
son engaging for hire to do pest control
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.

WHITE-FOOTED MICE

The commonest rodents in Cali-
fornia, outside cities and culti-
vated areas, are the small "deer
mice" or white-footed mice (genus
Peromyscus). There are several
kinds in various parts of the state,
but one or another is present in
every sort of wild environment
from the humid coast belt to the

[33

driest desert areas, and from sea
level to the high mountains. Ex-
clusion and trapping are the best
methods for control of these mice.

The head-and-body length of adults is
3 to 5 inches, and the tail is 21/^ to 5 2/3
inches long in different species. The eyes
are large and black, the ears thin and

mice,

often invaded by white-footed
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.

The owner of a cabin must make special
effort to build out these small rodents.

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 conspicu-
ous, 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.

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 plant materials.

About homes, cabins, and barns where
no house mice {Mus musculus) are pres-
ent, they will come indoors and become
household pests. Mountain cabins that
are closed after the summer vacation are

The detailed directions for exclusion of
house rats and mice (pp. 13-21) 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.

[34

The chimney should be capped tightly
with hardware cloth of 1/3- or 1/4-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 1/3 inch. Ir-
regular crevices may be wedged with
folded strips of fly screen. Pipes entering

Strychnine-coated grain sometimes will
serve for control, but may be refused by
the mice. If the owner cannot control
these mice by the methods described, he
should get help from some government
agency (p. 3).

Dead mice in mountain cabins should
be carefully handled to avoid the chance
of live fleas or ticks getting on to a per-

Fig. 17.— The wood rat, or pack rat (Neotoma). Its head-and-body length is 7 to 8
inches and the tail 6I/2 to IVi 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 blunt, and
the tail is hairy.

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 make good bait,
but other foods should be tried if neces-
sary. 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.

son's clothes or skin. Gloves should be
worn, and the clothing sprayed with a
pyrethrum spray immediately after pick-
ing 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
buildings outside cities and towns.
Exclusion and trapping are the
best means of control.

The brown-footed wood rat {Neotoma
juscipes, fig. 17) of the foothills and
lower mountains, has a head-and-body

[35

length of 7 to 8 inches, and the tail is 6^^
to 7Y2 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 sup-
plies in buildings or in outdoor camps.
Wood rats often remove small bright ob-
jects, 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 cinerea) 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
similar places proof against wood rats.
No double walls or other spaces should be
left to provide nesting places for these
and other rodents. Means just described
to protect food and bedding from white-
footed mice will serve also against wood
rats. Rock piles near cabins or crevices
in foundations are other common shelters
for these rodents.

Wood rats are less wary than alien rats,
and may be caught in wooden-based
spring rat traps baited with rolled oats,
peanut butter, raisins, or prunes. They
are not readily attracted by other grain
baits, and are not easily poisoned with
strychnine.

For safety in respect to disease, besides
clearing buildings of these rodents, those
in nearby nests, logs, or rock slides should
be trapped. Dead wood rats should be
handled with the precautions stated for
white-footed mice. When a building is
closed for the winter, several baited rat
traps should be placed to capture any
wood rats that gain entrance during the
owner's absence.

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.

36

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. Raymond

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

Hull, Thomas G.

1941. Diseases transmitted from animals to man. 2d ed. Charles C. Thomas, Springfield,
111. 403 p. Illus.

Kalmbach, 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, Stanley E.

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

RicHTER, Curt 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, Tracy I.

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

316-17.
1947. Control of field rodents in California. California Agr. Ext. Circ. 138:1-51. 23 figs.

Ward, Justus C.

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

35m-4,'48(A7854)

[37

DOYOU HAVE QUESTIONS

about crops . . . soils . . . fertilizers
irrigation . . . or special problems
affecting your own farm operation q

YOUR FARM ADVISOR CAN GET THE BEST INFORMATION

He is ready to talk over your farming
problems with you and help you solve
them. There is no charge for this service.

You will find the Form Advisor for your district in the following list:

Alameda County:

Post Office Bldg., Hayward
Butte County:

Federal Bldg., Oroville
Colusa County:

Federal Bldg., Colusa
Contra Costa County:

Cowell
El Dorado County:

Post Office Bldg., Placerville
Fresno County:

Room 20, Federal Bldg., Fresno
Humboldt County:

Post Office Bldg., Eureka
Imperial County:

Court House, El Centro
Kern County:

2610 M Street, Bakersfield
Kings County:

131 E. 8th St., Hanford
Lake County:

Kelseyville
Lassen County:

Memorial Bldg., Susanville
Los Angeles County:

908 N. Spring St., Los Angeles 12
Madera County:

Post Office Bldg., Madera
Marin County:

Post Office Bldg., San Rafael

Mendocino County:

362 North State St., Ukiah

Merced County:

County Adobe Bldg.,

Court House Square, Merced
Modoc County:

Court House, Alturas
Monterey County:

Court House, Salinas
Napa County:

Post Office Bldg., Napa
Nevada County:

Memorial Bldg., Grass Valley
Orange County:

1104 West 8th St., Santa Ana
Placer County:

135 Court Street, Auburn
Plumas County:

Court House, Quincy
Riverside County:

Post Office Bldg., Riverside
Sacramento County:

315 Federal Bldg., Sacramento 2

San Benito County:

Court House, Hollister

San Bernardino County:

Federal Bldg., San Bernardino

San Diego County:

404 U. S. Customs Bldg.,
San Diego 1

San Joaquin County:

145 S. American St., Stockton 7

San Luis Obispo County:

Post Office Bldg., San Luis Obispo

San Mateo County:

Half Moon Bay
Santa Barbara County:

Federal Bldg., Santa Barbara
Santa Clara County:

201 Post Office Bldg., San Jose 18
Santa Cruz County:

Court House Annex, Santa Cruz
Shasta County:

County Office Bldg., Redding
Siskiyou County:

Court House, Yreka
Solano County:

County Library Bldg., Fairfield
Sonoma County:

Court House, Santa Rosa
Stanislaus County:

Federal Bldg., Modesto
Sutter County:

Post Office Bldg., Yuba City
Tehama County:

Federal Bldg., Red Bluff
Trinity County:

County Office Bldg., Weaverville
Tulare County:

Post Office Bldg., Visalia
Tuolumne County:

Memorial Bldg., Sonora
Ventura County:

52 N. California St., Ventura
Yolo County:

Court House, Woodland
Yuba County:

Federal Bldg., Marysville

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UNIVERSITY OF CALIFORNIA . BERKELEY