Document ID: EPA-HQ-OPP-2006-0955-0004
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2007-01-17T05:00Z

SEQ CHAPTER \h \r 1 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

     				January 23, 2006

  SEQ CHAPTER \h \r 1 OFFICE OF

PREVENTION, PESTICIDES AND TOXIC SUBSTANCES

MEMORANDUM

SUBJECT:	Analysis of Rodenticide Bait Use 

FROM:	Angel Chiri, Entomologist/Team Leader

Biological Analysis Branch

THRU:	Arnet Jones, Chief 	

Biological Analysis Branch

TO:		Kelly Sherman, Chemical Review Manager

		Susan Lewis, Chief

		Reregistration Branch 1

Special Review and Reregistration Division 

Peer Review Panel:  11/02/05

Attached is the revised version of BEAD’s Analysis of Rodenticide Bait
Use document.  This final version incorporates comments and suggestions
on a wide range of rodenticide bait use and rodent management issues
received from stakeholders in early 2005.

Analysis of Rodenticide Bait Use

Executive Summary

This document presents an overview of the current use of nine
rodenticide baits in the United States.  Topics summarized include: the
potential impact of rodents as disease vectors on human health; the
damage caused by rodents to man-made structures and agriculture; a
description of available market information, main use sites, target
pests, and efficacy issues for these rodenticide baits; and alternative
rodent control methods.  The use profile for the nine rodenticides is
summarized in Table 1 (P.  33).

The following major conclusions are put forward in this document:

In the United States, rodents pose a significant public health risk and
cause economic damage to man-made structures and agriculture.

Because rodenticide baits are an essential component of an integrated
pest management approach to rodent control, this document concludes that
the availability of rodenticide baits is necessary for the successful
management of rodent populations. 

Since all nine rodenticide baits discussed in this document are
currently registered for the control of commensal rodents in and around
buildings, each of these can be seen as a potential alternative to each
of the others in that use site situation.

Up-to-date data pertaining to amounts used, effectiveness, and
prevalence of first generation anticoagulant resistance are not readily
available.  

I.  Introduction 

For the past several years, EPA has been assessing the risks posed by
rodenticides.   As part of its ongoing work to reassess the safety of
older pesticides, EPA issued a Reregistration Eligibility Decision (RED)
for the Rodenticide Cluster in 1998.  In that RED, EPA noted concern
about potential adverse effects to birds and nontarget mammals, and
announced a plan to further evaluate the potential ecological risks
before issuing final decisions about reregistration eligibility.  In
1999 EPA initiated a comparative ecological assessment, in which the
Agency compared and ranked nine rodenticide active ingredients in terms
of potential severity of risk.  The comparative ecological assessment
concludes that there is adverse risk to nontarget organisms from all
rodenticides, but certain compounds present more risk than others. 

The statute under which EPA regulates pesticides, the Federal
Insecticide, Fungicide, and Rodenticide Act (FIFRA), requires
consideration of benefits if adverse risks were assessed to be of
concern.  Thus, EPA must consider the benefits derived from the nine
rodenticide products being evaluated before arriving at a decision
regarding appropriate mitigation measures.  The nine rodenticides
included in the assessment are those addressed in the Rodenticide
Cluster RED (brodifacoum, bromadiolone, bromethalin, chlorophacinone,
diphacinone), as well as zinc phosphide, warfarin, difethialone, and
cholecalciferol.

The dual purpose for this document is, first, to summarize the available
benefit/usage information in order to inform any risk management
decisions, and, second, to request additional information not considered
here (See Section X. Topics for Specific Public Comments.).  EPA lacks
complete usage information on rodenticide baits, particularly in the
large homeowner market for rodenticides.  Limited information is
available concerning the professional applicator and agricultural
markets.  This document discusses the public health and other benefits
associated with rodenticide products, briefly discusses efficacy, and
concludes with a request for additional information on various aspects
of rodenticide use and benefits.  No attempt is made in this document to
quantify the societal and environmental costs and benefits resulting
from the use of rodenticide baits.  

II.  Impact of Rodents on Society

Background

Some rodents can be injurious to humans and their belongings.  Both
introduced and native species may be carriers or reservoirs for
infectious diseases.  Rodents may cause economic damage to crops;
consume and contaminate stored food supplies; disturb soil through
burrowing activities; damage houses, other types of buildings and
man-made structures; and prey on native species, including birds that
nest on oceanic islands.  It is generally estimated that commensal rats
cause between $0.5 and $1.0 billion of economic losses in the United
States annually.  This estimate is based on the assumption that there is
one commensal rat per every two people in the country, at a time when
the population of the United States numbered approximately 200 million,
and that each rat consumes or damages between $1 and $10 worth of food
and other materials, while contaminating 5 to 10 times more of it (Pratt
et al., 1976).  Commensal rats and mice have become adapted to live in
close proximity to humans, thus having relatively easy access to almost
unlimited food and shelter under certain conditions.  Commensal rodents
are introduced Old World rats and mice in the family Muridae: the brown,
sewer, or Norway rat (Rattus norvegicus), the black or roof rat (R.
rattus), and the house mouse (Mus musculus).  It is estimated that a
typical large city in the United States annually receives more than
10,000 complaints about commensal rodent problems and performs tens of
thousands of rodent control inspections and baiting services (Illinois
Department of Public Health, 2004).  Some native rodents may also
achieve a pest status comparable to commensal rodents and/or may be
especially damaging to field and orchard crops or to turf.  A high
reproductive potential and mobility allows these rodents to rapidly fill
available habitats and replace the individuals taken through available
control methods.

Rodenticide baits, along with a variety of habitat modification and
other pest management techniques, are used to reduce the damage caused
by native and introduced rodents, as well as other pest mammals. 
Rodenticide baits are especially useful for rapidly reducing rodent
numbers in cases of major infestations.  For commensal rodent control,
rodenticide baits are best used within the context of an integrated pest
management (IPM) approach that emphasizes measures such as sanitation,
exclusion, habitat modification, trapping, coordination at the community
level, public health education, and legal measures such as the
enforcement of sanitation codes. 

A.  Impact of Rodents on Public Health

In the past, commensal rodents have been the main cause of
rodent-related public health concerns in the United States.  In recent
years, however, white-footed mice and deer mice (Peromyscus spp.) have
been implicated in the transmission of  diseases such as hantavirus
pulmonary syndrome, Lyme disease, and human granulocytic ehrlichiosis.

Disease transmission

Salmonellosis - Commensal rats and mice exposed to Salmonella bacteria
in sewers or garbage may carry this pathogen in their gastrointestinal
tract.  Infected rats coming in contact with stored food, kitchenware,
or food preparation surfaces may readily contaminate them with their
droppings, which in turn could result in Salmonella food poisoning for
humans exposed to the pathogens.  Symptoms include nausea, diarrhea, and
dehydration.  This disease is rarely fatal (Blindauer, 1999).

Plague - In the Middle Ages, a plague pandemic transmitted to humans
from infected rats by the bite of the oriental rat flea (Xenopsylla
cheopis), was responsible for the death of about one third of Europe(s
population.  Plague is caused by the bacterium Yersinia pestis and
exists in three forms, systemic, pneumonic and bubonic.  Symptoms
include chills, generalized pain, and swollen lymph nodes.  Untreated,
the fatality rate could exceed 50% (Dept. Environ. Health, San Diego
Co., undated).  However, at present this disease is uncommon in humans
in the United States, and no major urban plague outbreak has occurred in
the country since 1924 (Schwarz, 2003).  Plague can be treated with
available antibiotics.  During the 1980s, human plague cases in the
United States averaged about 18 per year, with a fatality rate of one in
seven (CDC, 2003).  A plague reservoir exists in some wild rodent
populations in several Western states, principally the rock squirrel,
(Spermophilus variegatus) in the southwestern states and the California
ground squirrel (S.  beecheyi) in the Pacific states.  Townzen, et al
(1996) describe the management of a sylvatic plague epizootic in
campgrounds in California.  The primary rodent vector in that case was
the California ground squirrel.  Closing campgrounds, trapping fleas and
squirrels to index their numbers and detect plague-positive individuals,
insecticide dust for flea control, and Zinc Phosphide bait for
controlling ground squirrels were elements of the program undertaken to
combat the epizootic.  Other rodents also identified as potential
reservoirs include other ground squirrel species, prairie dogs (Cynomys
spp.), wood rats (Neotoma spp.), chipmunks, and perhaps even deer mice
and voles (CDC, 2003; Schwartz, 2003).  Although cases of plague
involving commensal rats have not been reported since 1925 (Dept.
Environ. Health, San Diego Co., undated),  there is a latent risk that
as urban areas expand, the disease could be transmitted to commensal
rats. 

Murine Typhus - This disease, caused by Rickettsia typhi, is transmitted
to humans by rat fleas when feces of infected fleas are scratched or
rubbed into a flea bite area and into the bloodstream.  Human cases of
murine typhus increased rapidly in the United States from 1916 to 1945,
with 21,572 cases being reported in 38 states from 1941 through 1945
(Andrews and Link, 1947).  In 1944, 5276 cases were reported in eight
southern States (AL, GA, FL, LA, MS, NC, TX, and TN).  In Alabama in
1932-1933, the human death rate from murine typhus was 4.5 per 1000
reported cases, but nearly one-third of the cases involving people who
were at least 65 years of ended in a fatality (Andrews and Link, 1947). 
The incidence of murine typhus cases ultimately was greatly reduced
through improved sanitation; insecticidal control of rat fleas; and
lethal rodent control by use of rodenticide baits, chiefly
anticoagulants after they became available (e.g., Beall, 1946; Wiley,
1946; Hill and Morlan, 1948; Morlan and Hines, 1951; Mohr and Smith,
1957), and fumigants.  The 10% DDT dust used to control rat fleas also
produced some debility and mortality in rats (Dent, et al 1949).  

Since disease management efforts took hold in the middle of the last
century, murine typhus fever is relatively uncommon in the United States
 Fewer than 50 cases are reported annually in the United States, mainly
in California, Texas, and Hawaii (CDC, 2002).  In some areas, such as
southern California, the opossum (Didelphis marsupialis) can be a
reservoir (Dept. Environ. Health, San Diego Co., undated).  Several
species of fleas have been implicated as vectors, including the rat flea
(X. cheopis), the cat flea (on opossums) (Ctenocephalides felis), and
the house mouse flea (Leptopsylla segnis).  Although only a few cases
are reported in Hawaii in typical years, in 2002 there were 47 cases
recorded.  Rodents suspected of being reservoirs in Hawaii include the
Norway rat, the roof rat, the Polynesian rat (Rattus exulans) and the
house mouse (CDC, 2002).   Symptoms include fever, severe headache,
general pain, and possibly a rash.  It is rarely fatal at present
(Blindauer, 1999).

Rat Bite Fever -  Rat-bite fever is a bacterial disease caused by
Streptobacillus moniliformis that can be acquired through the bite or
scratch of a rodent or the ingestion of food or water contaminated with
rat feces.  Because cases are rarely reported in the United States, the
true incidence of disease is unknown (CDC, 2003).  Symptoms include
influenza-like illness, rash, and arthritis, which may not be readily
associated with a rat bite (Blindauer, 1999).

Trichinosis - Humans become infected with the nematode (an almost
microscopic round worm) Trichina spiralis when ingesting improperly
cooked infected pork.  Nematode larvae parasitize the intestines and
muscle tissue of humans.  Pigs acquire the nematode by eating the
carcasses of infected animals or grain food or garbage contaminated with
feces from infected animals.  Several wild and domestic mammals,
including rats, cats, raccoons, and bears, may serve as reservoirs for
the pathogen, but the role of rats as a important reservoir may have
been overemphasized in the past (USDA APHIS, 1972).  According to the
CDC, trichinosis infection in humans, once a common disease, is now
relatively rare in United States  From 1991-1996, an annual average of
38 cases per year were reported (CDC, 2003).  Symptoms include nausea,
diarrhea, muscle aches, and may be fatal if the heart is involved
(Blindauer, 1999).

Leptospirosis - This disease can be contracted by exposure to urine from
infected commensal rats and mice.  The causative organism, the
spirochete bacterium Leptospira icterohaemorrhagicae, infect humans,
dogs, and other domestic and wild animals by entering through mucous
membranes or through cuts and scratches of the skin.  Humans can become
infected by handling contaminated items, from rodent bites, or from
exposure to contaminated water.  Symptoms due to infection may range
from those associated with the common cold to kidney damage and liver
failure (Blindauer, 1999). 

Rickettsial Pox - This disease, caused by the obligate intracellular
bacterium Rickettsia akari, can be transmitted to humans by the house
mouse mite.  Infection produces a chicken pox-like rash which is rarely
fatal (Blindauer, 1999).  

Tropical Rat Mite - In southern California and in the southern states,
the tropical rat (Ornithonyssus bacoti) mite is a common ectoparasite of
the roof rat.  In rat-infested houses or buildings, this mite may also
bite humans, causing irritation and dermatitis (Ebeling, 1975).       

Hantavirus - A localized outbreak of hantavirus pulmonary syndrome
resulting in several deaths occurred in 1993 in the area shared by
Arizona, New Mexico, Colorado, and Utah, known as the Four Corners.  The
main virus host in this outbreak was the deer mouse (Peromyscus
maniculatus).  Since then, isolated cases have been reported in
Louisiana, Florida, and New York involving the rice rat (Oryzomys
palustris), the cotton rat (Sigmodon hispidus), and the white-footed
mouse (Peromyscus leucopus) as reservoir hosts of several hantaviruses. 
The virus can be transmitted to humans through contact with accumulated
urine and droppings from infected mice when tiny droplets of the
material are stirred and become airborne (CDC, 1999).  People may
breathe in contaminated dust while cleaning or working in an infected
area.  Human exposure can also occur by introduction of the virus
through the eyes, by ingesting contaminated food, or when bitten by an
infected rodent.  Symptoms include (fever, muscle ache, cough, rapid
progression into severe lung disease( and often death (NPCA, 1994).
Deaths occur due to rapid filling of lungs with bodily fluids (Larson
and Morgenthaler, 1994).  According to Boren and Valdez (2003), the
mortality rate for humans is approximately 43% for the U.S. and 51% for
New Mexico, while Mills et al. (2002) report that as of June, 2002, a
total of 318 cases had been identified in 31 states, with a fatality
rate of 37%.  Hantaviruses do not cause illness in host rodents.

Other Diseases - Additional diseases known to be transmitted directly or
indirectly by commensal and wild rodents in the United States include
the following: lymphocytic choriomeningitis, toxoplasmosis, Colorado
tick fever, Rocky Mountain spotted fever, Lyme disease, relapsing fever,
babesiosis, western equine encephalitis, California encephalitis, human
granulocytic ehrlilchiosis, and cutaneous leishmaniasis, and Yersinia
pseudotuberculosis (Blindauer, 1999).

Bites – Based on data collected from many city health departments,
Scott (1965) and Clinton (1969) independently estimated that in the
United States there were approximately 10 rat bite incidents per 100,000
city dwellers per year, or a total of 14,000 bite incidents per year for
the then estimated 140 million city dwellers in the U.S.  Most of the
bites occurred where people lived under crowded conditions, in
substandard housing, in areas with poor environmental sanitation, or in
neighborhoods where rat-infested property was being eliminated.  This
estimate may still hold true for certain urban areas.    

B.  Impact of Rodents on Domestic Animal Health 

Commensal rodents can also transmit diseases to domestic animals, such
as leptospirosis and tapeworms to dogs and cats; brucellosis and
foot-and-mouth disease to cattle; hog cholera and trichinosis to hogs;
and salmonella, erysipelas, and fowl pox to poultry.

C.  Impact of Rodents on Agriculture

Preharvest

	Rodents such as voles, deer mice, Old World rats, ground squirrels, and
gophers, may at times become agricultural pests by feeding on crops or
by damaging farm structures and equipment by gnawing or through their
burrowing activities.      

	Some species of voles (Microtus spp.) can damage or kill young trees
and shrubs by feeding on the inner bark layer of the trunks near the
base, both above and below ground.  When voles gnaw completely around
the trunk (girdling), the flow of water and nutrients is disrupted and
the tree or shrub is killed.  This type of damage is often more
prevalent in fall and winter months, when their preferred food plants
are not available (Fisher and Hygnstrom, 1997).  Tree crops affected
include avocado, apple, citrus, cherry, olive, and tree nuts.  Voles
also feed on flower bulbs, nursery stock, turf and other landscape
plantings; on vegetables, including carrots, turnips, sweet potato,
artichokes, celery, sugar beets, potatoes, tomato, and Brussel sprouts;
and on various field and forage crops such as alfalfa, timothy hay,
clover (Fisher and Hygnstrom, 1997; Salmon and Gorenzel, 2002; Montana
Dept. Agr., 2001a).  Lowe and Hudson (2004) report that in the state of
Washington, yield and quality losses in timothy hay due to uncontrolled
vole damage may cause a loss in net revenue of approximately 50%.  In a
letter submitted to EPA’s docket, Kiess (2004) informs the Agency that
in California, where over one million acres of alfalfa are grown, a vole
infestation in 1999 caused a yield and price reduction to this crop of
11%, resulting in a $126 million loss to the industry. 

 

Deer mice (Peromyscus spp.) can cause economic damage to Douglas fir and
Ponderosa reforestation activities in the West and Northwest logging
areas because of their ability to locate and collect broadcasted pine
seeds, as well as naturally dispersed seeds.  Deer mice can also dig up
planted seeds, including melon and alfalfa seeds.  High populations of
deer mice have caused damage to almond, avocado, citrus, pomegranate,
and sugar beets (Silberhorn, et al., 2003).

Several species of pocket gophers (Geomyidae) feed on tubers and plants
with succulent tap roots.  Crops damaged include alfalfa, pastures,
conifer plantations, row crops, and flower and vegetable gardens. 
Pocket gophers are a major problem wherever they occur in irrigated
alfalfa.  In Montana, for instance, yield losses in alfalfa may be up to
20-40%.  In addition, pocket gopher mounds interfere with harvesting
operations and damage harvest equipment.  If gopher populations are
moderate to dense, their mounds may cover 10-20% of the soil surface. 
It is estimated that the average pocket gopher can move 2,000 to 3,000
pounds of soil each year. (Montana Department of Agriculture, 2001b).  

	In Central Oregon, over 100,000 acres of hay are at risk of damage from
voles, pocket gophers, ground squirrels, and woodchucks (Central Oregon
Hay Growers’ Association, 2004).   

	Ground squirrels can inflict significant damage to agriculture.  In
California, ground squirrels cause damage to a wide variety of field and
orchard crops, including grain, fruits and nuts (almonds, apples,
apricots, grapes, peaches, pistachios, prunes, oranges, strawberries,
and walnuts), vegetables, and field crops (sugar beets, alfalfa, and
cotton) (Silberhorn et al., 2003)      

For instance, Schramm and Bullard (2004a) estimate that, uncontrolled,
ground squirrels in California could theoretically cause a $7.5 million
annual loss to the pistachio industry.  This estimate assumes that: a)
one ground squirrel consumes approximately 50 pounds of pistachio nuts
per year; b) at infestation levels, ground squirrels average 20 per
acre; c) 5,000 acres of pistachio orchards are affected; and d) the
value of pistachios is $1.50 per pound.  Ground squirrels and pocket
gophers may cause extensive damage to irrigation systems by chewing
through hoses and water lines; their tunnels and burrows can create
hazardous conditions for machinery used to harvest pistachios; they
cause direct damage by eating the pistachio nuts; and wounds inflicted
upon tree trunks and roots can open avenues of infection by pathogenic
fungi (Aspergillus spp.), which can destroy the trees.  

	In California, the main pest of carrots is the California ground
squirrel, while minor rodent pests include the black tailed jackrabbit
and pocket gophers.  Large ground squirrel populations can denude 5-10
acres of carrot seedling tops.  Since carrot cost of production is
estimated at $2,000 per acre, the economic loss under such circumstances
could be considerable (Schramm and Bullard, 2004b).   Marsh (1998)
estimates that the California ground squirrel may cause between $12 and
$16 million in crop losses annually to California agriculture.  When
locally abundant, the burrowing activities of ground squirrels and
prairie dogs in agricultural areas and pasturelands can also cause
damage to farm equipment, pose a risk to livestock, and contribute to
soil erosion.  

      

In agricultural areas, commensal rats and mice can be significant field
pests of crops such as sugarcane, citrus, and Macadamia nuts.  They can
also damage agricultural equipment, including farm machinery, 
irrigation pipes and hoses.

Post-harvest

Rodents often eat, damage, or contaminate (with urine, droppings, and
hair) tons of stored food items, including grain, flour, cereals, sugar,
vegetables, fruit, nuts, meat, animal feed, pet food, and any existing
kind of stored edible material (Ebeling, 1975).  The most damaging
species in the U.S. are the commensal rodents and several species of
voles.  According to Hopf et al. (1976), cited by Brooks and Fiedler
(1999), stored food losses to rodents in developing countries commonly
range from 1% to 10%, but occasionally can be as high as 50%.  Rodents
typically contaminate far more stored grain than they consume.  During a
year, a single mouse will deposit several thousand pellets (droppings)
and about one pint of urine (Hygnstrom, 1995). The amount of rodent
filth in stored grain is regulated by FDA and USDA: two or more rodent
pellets or equivalent quantity of other animal filth per 1000 grams of
grain reduces its quality to (U.S. Sample Grade,( which can only be used
for livestock feed (Brown, 1994; Hygnstrom, 1995).  While the numbers of
rodent droppings found at any one time cannot be correlated with rodent
population density, the rate of dropping accumulation can provide a
relative census tool (Frantz and Davis, 1991).  The appearance of  fresh
droppings is indicative of an active infestation.  Although damage to
stored food by rodents is not nearly as high in the U.S. as in
developing countries, some losses do occur.  The rodent damage to stored
grain resulting from feeding and contamination was estimated at $8.4
million per year in Nebraska alone (Hygnstrom, 1995).  Such levels occur
although efforts to mitigate rodent damage commonly are employed in the
United States 

D.  Structural/Industrial Rodent Damage

Rodents( incisors never stop growing and are regularly worn down by
their own self-wearing action and by gnawing hard materials (Frantz and
Davis, 1991).  In houses and buildings, commensal rodents can gnaw
through gas pipes, electric wiring and its insulation, and building
insulation, thereby creating a fire risk.  They can also damage
electronic and computer equipment.  White-footed and deer mice often
enter cabins and other buildings, where they may build nests and raise
their young, causing damage to furniture, clothing, books, paper files,
and other belongings (Timm and Howard, 1994).  

Ground squirrels and chipmunks may, when abundant, cause structural
damage by burrowing under foundations, patios, retention walls, and
other structures.  Estimated annual damage by the California ground
squirrel to structures such as levees, earthen dams, and roadbeds may
amount to $8-12 million.  The combined California ground squirrel damage
to structures and agriculture (see above) represents estimated annual
losses of $20-28 million in California (Marsh, 1998).   

E.  Rodent Predation on Island Species

Introduced commensal rats contribute to the endangerment and extinction
of island plants and animals.  When commensal rats or other rodents
become established on oceanic islands, they typically prey on the eggs,
chicks, and sometimes the adults of ground- and hole-nesting birds,
reptiles, and other types of organisms.  The victims often include rare
and endangered species.  Often there are few or no native predators on
such islands, and the resident birds have not evolved defense mechanisms
which enable them to cope with introduced predators such as rats.  If
uncontrolled, this type of predation can lead to the local extinction of
the affected bird populations (e.g., National Park Service, 2000). 
Brodifacoum baits have often been used to eradicate rats from islands. 
Cholecalciferol and diphacinone baits have been tested as potential
alternatives to brodifacoum use in oceanic islands (Donlan et al. 
2003).           

F.  Other Types of Rodent Damage 

Rodents also cause economic losses to poultry operations by consuming
feed, by causing structural damage to poultry facilities, and by
vectoring pathogens, such as Salmonella enteriditis.  New York poultry
farmers, for instance, consider rodents, along with flies, as serious
pest problems that cause an economic loss to the poultry industry
(Harrington et al. 1998).  Pocket gophers often damage lawns, golf
courses, parks, and other noncrop areas through their burrowing
activities.  In Colorado alone, about three million acres of private
lands are reported to be damaged by prairie dogs and other rodents
(State of Colorado Department of Agriculture, undated).  Muskrats,
nutria, and ground squirrels may damage levees, ditch banks, and
culverts in agricultural areas. 

III.  Rodenticide Use in Rodent Control

Rodenticides are used, sometimes as part of community rodent-control
programs, in situations where major commensal rodent infestations must
be reduced rapidly.  Rodenticide baits are also used to control rodents
that have entered homes and other buildings or to eliminate rodents that
remain after buildings have been rodent-proofed (CDC 1999).  The impact
of coordinated rodenticide bait use can be significant in urban areas. 
For instance, during 1969-1981 the Center for Disease Control and
Prevention (CDC)'s Urban Rat Control Program funded rat control projects
in over 100 communities in 31 states.  As a result, by 1980 there were
36,000 rat-free city blocks in the communities covered by the program,
with another 20,000 blocks in the process of becoming rat-free.  CDC
estimates that by 1980 seven million people lived in areas  that were
made rat-free through this program (Blindauer, 1999).

The use of rodenticide baits is one of several techniques available for
controlling noxious rodents.  There is limited information publicly
available on the use of and market for rodent control chemicals in
residential, commercial, and agricultural settings.  EPA proprietary
data that are available for residential and commercial use of
rodenticide baits provide limited information on two broad markets for
rodent control: the homeowner and the professional or pest control
operator (PCO) markets.  The private residential ((homeowner() market
includes products purchased by people to control rodents at home.  The
professional applicator market includes products purchased to control
rodents in both residential and commercial settings.

The data available for agricultural rodenticide use are limited to
general on-farm use and some specific field uses.  A description of each
market follows.

A.  Homeowner Market for Rodent Control

Based on EPA proprietary data, the annual expenditures by homeowners on
rodent control products are estimated to equal more than $90 million. 
The largest regional homeowner market is estimated to be the Southern
United States (an estimated 35%-40% of the total market), followed by
the Midwestern, Northeastern and Western United States.  An estimated
90% of the market for the control of rodents is in the form of dry bait
rodenticides, with other methods, such as glue boards, spring traps, and
gas cartridges for burrowing rodents, making up the remaining 10% of the
market.

D-Con( brand baits are the most widely used rodenticide product in the
homeowner market.  The most widely used active ingredient in d-Con(
products is, at present, brodifacoum.  Chlorophacinone, diphacinone, and
other active ingredients also are available on the homeowner market. 

B.  Professional (Pest Control Operator) Market for Rodent Control in
Residential and Commercial Settings

The estimated expenditures on rodent control products used by PCOs are
more than $15 million annually (EPA proprietary data).  The largest
regional market for professional rodent control is the Southern United
States (40% to 45% of the total market), followed by the Midwestern,
Northeastern and Western United States.  An estimated 75% to 80% of the
professional market is rodenticide products, with the remaining 20% to
25% of the market including other methods of rodent control, such as
glue boards and traps.  An estimated 60% to 65% of professional rodent
control is for house mice, 35% to 40% is for commensal rats, and 1% to
2% is for other rodents (e.g., pocket gophers, ground squirrels, etc.). 
An estimated 60% of the PCO work to control rodents is residential and
40% is commercial.

As much as 80% of the rodenticide sales to professional applicators are
of products containing the active ingredients bromadiolone and
brodifacoum.  We estimate from available information that bromadiolone
comprises 50% and brodifacoum 30% of the total professional market for
rodenticides.  The primary brand names for products containing
bromadiolone are Contrac(  and Maki(.  For brodifacoum, the primary
brand names are Talon( , Final(  and WeatherBlok(.  Other rodenticide
baits with estimated use by professional applicators include
difethialone, diphacinone, chlorophacinone, zinc phosphide, bromethalin
and cholecalciferol.

C.  Rodent Control in Agriculture

EPA proprietary and USDA National Agricultural Statistics Service (NASS)
data (1999) on rodent control in agriculture provide estimates of the
use of rodenticide baits only.  There is no information readily
available on the extent of use of other methods, such as glue boards and
traps, although it is assumed that these methods are used as well in
agricultural settings.  Based on the extent of use of these other
methods in residential and commercial settings by homeowners and PCOs,
it is assumed that the use of other methods in agricultural settings
ranges from 10% to 25% of the total market for rodent control in these
areas, with rodenticide baits accounting for the rest of the market.

	According to the California Farm Bureau Federation (2005), which
represents 37,000 farmers and ranchers in that state, zinc phosphide,
chlorophacinone, and diphacinone are the primary rodenticides used in
the state for controlling squirrels, deer mice, meadow mice, and other
field rodents in cropland and rangeland.  

 

According to EPA proprietary and USDA NASS data, the primary rodenticide
used in the field for rodent control is zinc phosphide.  No other
rodenticide appears to be used in significant amounts in agricultural
fields.  An estimate of the total amount of zinc phosphide used is not
available, however, the data suggest that approximately 30% of total
zinc phosphide use is in agricultural fields.  Zinc phosphide is also
used in noncrop areas (see below), in residential areas and on turf.  Of
the 30% of total zinc phosphide used in agriculture, an estimated 10% is
in sugar beet fields, 10% is in grain fields (e.g., wheat, barley, oats,
etc.), and 10% is on rangeland (U.S. EPA, 1998).  The regional
distribution of this use is not known.  

The largest proportion of on-farm zinc phosphide use is in general farm
areas.  An estimated 40% of total zinc phosphide use is in and around
farm buildings and structures, roads, ditches, and other noncrop areas
(e.g., drainage ditches, irrigation canals, riding areas, feedlots,
fence rows, etc.).  In addition to zinc phosphide, the available data
(USDA-NASS, 1999) estimates (but does not quantify) the use of
brodifacoum, bromadiolone, bromethalin, cholecalcifoerol, difethialone,
diphacinone, and warfarin in and around building structures; and for
brodifacoum, bromadiolone and warfarin in roads, ditches and other
non-cropland areas.  Brodifacoum, bromethalin, diphacinone, and warfarin
are also used in grain storage facilities for rodent control.

IV.  Summary of Rodent Management Methods 

A.  Management of Commensal Rodents in Urban and Suburban Settings  

In addition to the use of poison baits, there are several control
methods available and  recommended for managing commensal rodent
populations.  These approaches aim at permanently denying them of access
to food, water, and shelter.  Denying rodents access to food, water, and
shelter is called (sanitation( (Bjornson and Wright, 1956).  Denying
rodents access to existing buildings is called  (exclusion( (Frantz and
Davis, 1991).  Designing structures to deny or deter rodent entry are
called "proofing" and "stoppage," respectively (Scott, 1982).  Other
types of habitat modification that may reduce the attractiveness of an
area to rodents include changes to landscaping such as replacement of
ground cover, such as English ivy with grass.  To be successful in
large, infested urban areas, these measures must be implemented on a
community-wide basis.  Most practical rodent control manuals, handbooks,
and related literature include a section on nonchemical rodent
management measures.  Comprehensive reviews of this subject can be found
in Frantz (1988) and Frantz and Davis (1991).   

 

Access to indoor and outdoor food sources can be limited by using
rodentproof refuse containers, keeping garbage containers and bins
tightly sealed, increasing the frequency of garbage pickup, rapidly and
completely cleaning food spills, ensuring that no food or water remains
exposed overnight at home, storing dried foods in rodentproof containers
or rodentproof pantries, and retrieving seeds spilled from bird feeders
on a daily basis.  Eliminating non-toxic food sources also may make
rodents more inclined to accept toxic baits.

Nesting and hiding places can be limited by thinning the planting
density or by removing all the plant ground cover that offers protection
to rodents near buildings, using plants that do not provide rodent
harborage or attract rodents, removing or mowing tall vegetation growing
in vacant lots; removing wood, construction debris, or any other
materials stacked against buildings; cleaning up and reducing clutter in
basements and other rarely-used rooms. 

Young commensal rats can squeeze through openings (higher or wider than
( inch( (Baker, et al, 1994).  House mice can access openings about (
inch by 1( inch (Frantz, 1988) or larger than ( inch (Baker, et al,
1994).  To prevent rodents from gaining entry, buildings can be
(rat-proofed( by sealing probable points of entry with heavy wire mesh
or other appropriate materials.  Potential points of entry may include
air vents; openings around water and sewer pipes, electric lights,
telephone wires, and TV cables; cracks around windows; doors and door
frames, especially between lower edge and floor; holes in floors, walls,
and ceilings; and any other type of opening that permits access to
rodents.  Trimming tree branches that overhang or touch homes or
buildings keep roof rats, squirrels, and deer mice from using them to
get access to homes.

Snap traps can be used to control rodents under low infestation levels,
or where  rodenticide use is too hazardous, or where the odor of dead
rodents in inaccessible places would not be acceptable.  Using snap
traps correctly requires a good understanding of rodent habits.  Glue
boards are sticky material that trap rodents upon contact and work best
with mice and immature rats.  Glue traps are considered to be less
humane than snap traps (Frantz and Padula, 1983).  Naphthalene and
paradichlorobenzene (PDB) may discourage rodents from entering an
enclosed space that is not frequented by humans.

B.  Management of Rodents in Agriculture

Rodenticide baits are widely used to control pest rodents in
agricultural fields, rangeland, and pastureland.  Bait can be
broadcasted or placed selectively in specific locations.  Although the
use of poison baits is, in most cases, the fastest and most economical
way of reducing pest rodent populations in these areas, several
non-chemical rodent management methods exist that can be used in
conjunction with available chemical control tools, in what amounts to an
IPM approach.  Some of these methods are summarized below.

Habitat modification:  Keeping the orchard floor closely-mowed and
maintaining weed-free strips under trees eliminates cover and reduces
the chances of build-up of vole populations in orchards.  Voles and
other small rodents need alternative food sources and protection from
predators if their populations are to grow.  Depriving them from these
resources is an effective way of  inhibiting vole population growth. 
Because voles tend to avoid open spaces, the installation of a 4-foot
diameter circle around the base of young trees and vines is recommended.
 Similarly, keeping weed-free strips around fields, gardens, and other
areas to be protected tend to discourage voles from crossing the buffer
to get to the food source (Salmon and Gorenzel, 2002).

Exclusion: Placing guards made of materials such as galvanized hardware
cloth or woven wire tree around tree trunks can prevent tree girdling by
voles.  Voles can also be discouraged from entering small flower beds or
gardens by installing woven wire or hardware cloth fences around them.

Flooding: Periodic flooding and soil tillage practices discourage
gophers and other burrowing rodents from becoming established in
croplands (Montana Dept. Agr., 2001b).

Sanitation: Promptly eliminating fallen fruit in orchards is one way of
reducing a preferred food source for rodents such as voles, especially
when poisoned baits are to be used.         

Trapping: On a small-scale basis, voles, white-footed mice, squirrels,
and other rodents can be trapped using one of several traps available in
the market.  Trapping is also used to identify rodents and monitor their
populations.  

Fumigants: Ignitable gas cartridges and aluminum phosphide tablets can
be used to fumigate ground squirrel and gopher burrows.  These materials
are placed in burrows, which are then sealed with soil.  Efficacy of
fumigants is reduced in dry, porous soil, since much of the gas escapes
to the surface.  Gophers also routinely plug sections of their burrows,
thus providing a barrier that prevent the gas from dispersing throughout
the system.

Predation:  In the field, rodents are a major food source for a wide
range of predators such as hawks, owls, and other birds of prey; coyotes
and foxes; weasels, badgers, and other mustelids; and snakes.  Life
expectancy for the smaller rodents seldom exceeds one year.  Thus, where
predators are present, predation contributes to the natural mortality of
rodents, although by itself, predation is seldom sufficient to keep
rodents below damaging levels.         

Integrated Pest Management: As for other pest organisms, applying a
combination of control methods suitable for a given target rodent
species, the degree of infestation, and the site affected, rather than
relying on a single pest control approach, is generally a more effective
way to manage rodent pests.

V.  Rodenticide Bait Profiles

Rodenticide baits are used extensively to manage rodents that feed on,
contaminate, or cause various types of damage to a wide range of crops
and farm infrastructure and equipment.  There are currently six
anticoagulant rodenticides registered for use in the United States.  
These are: warfarin, chlorophacinone, diphacinone, brodifacoum,
bromadiolone, and difethialone.  The first three, often described as
(first-generation anticoagulants,( generally require multiple feedings
over several days to cause the death of a target rodent.  Anticoagulants
inhibit the formation of prothrombin, a key protein in the blood
clotting process, thus leading to capillary damage, internal bleeding,
and eventually to death.  

Resistance to first generation anticoagulant rodenticides in commensal
rodents is known to be widespread in United States.  Norway rat
populations in which resistance exceeded 5%, as indicated by samples
submitted for testing, has been documented for 16 U.S. cities (Frantz
and Padula, 1980).  However, it may be possible to counteract this type
of resistance with baiting schemes that alternate periods of baiting
with warfarin, or other first generation anticoagulants, with periods of
no anticoagulant exposure (Frantz and Padula, 1980; Frantz and Madigan,
1998). 

 

Brodifacoum, bromadiolone, and difethialone are considered to be
(second-generation anticoagulants.(  The expression (second-generation
anticoagulant( was coined to describe anticoagulant compounds that were
believed to be effective against rodents that are resistant to the
first-generation anticoagulants (Marsh, et al, 1980).  Second-generation
anticoagulants sometimes are also called (single-feeding anticoagulants(
because rodents exposed to them may ingest a lethal quantity of the
poison in a single night(s feeding.  As the toxic symptoms of
anticoagulant rodenticides do not take full effect for several days,
however, rodents exposed to second-generation anticoagulant baits may
continue to feed relatively normally on them for 3 or 4 days and
accumulate a (super-lethal dose.(  

Anticoagulant rodenticide baits are used primarily to control commensal
rats and mice and are mixed with grain products and other ingredients to
make ready-to-use bait in meal, pelleted, or wax-block form.  Meal and
pelleted anticoagulant baits are intended for use in dry areas in and
around buildings.  Pelleted and meal formulations may be marketed in
small plastic or paper bags (placepacks) which contain them at the site
of placement until an animal breaks into the bag to feed on bait. 
Wax-block baits may be used where meal and pelleted baits are used and
also may be used in sewers if the label of the specific product provides
for such use.  All placements of baits used to control commensal rodents
in structural situations must be in tamper-resistant bait or stations if
such placements otherwise would be accessible to children under six
years-of-age, as well as to domestic animals and/or nontarget wildlife. 
Some commercial bait stations are designed especially to hold
paraffinized bait blocks. 

This assessment also considers rodenticides that are not anticoagulants.
 These compounds are zinc phosphide, cholecalciferol, and bromethalin. 
Zinc phosphide is an acute poison that may kill a target rodent as the
result of a single bout of feeding.  Zinc phosphide has a garlic-like
smell that may attract some rodents and, reportedly, repel many
nontarget species.  Zinc phosphide(s natural emetic action may, in some
cases, serve to protect some nontarget species from toxicosis after they
have eaten bait containing this active ingredient (Cornell University,
2001).  Exposure of target rodents to untreated grain (prebaiting) is
generally recommended to increase bait acceptance and is required for
some use patterns.  Bromethalin induces anorexia after a toxic, and
usually fatal, amount has been consumed.  Death ensues within one to
several days after consumption.  Cholecalciferol may require several
feedings for death to occur. 

Although all nine rodenticides briefly discussed above are registered
for commensal rodent control, only zinc phosphide, chlorophacinone,
diphacinone, and warfarin are registered for control of small native
mammals, including (field( rodents such as voles, deer mice, ground
squirrels, pocket gophers, prairie dogs, and kangaroo rats; jack
rabbits; and moles.  Target species claims vary according to chemical
and label.  Depending on application directions and use restrictions,
baits used to control native rodents in agricultural and nonagricultural
lands can be broadcasted or applied by hand in selected areas.    

A.  Warfarin and its Sodium Salt

Warfarin, the first anticoagulant rodenticide to be developed, was first
registered in 1950.  Warfarin was named  after the foundation that
developed it, the Wisconsin Alumni Research Foundation (WARF) (Link,
1959).  EPA issued a Registration Standard for Warfarin in 1981 (EPA,
1981) and issued the Warfarin RED in 1991.  Activities associated with
those reregistration efforts brought the Warfarin products that remained
registered up to the standards of the times and led to the cancellation
of many other Warfarin products.  Although resistance to warfarin and
other first generation anticoagulants has been detected in rats and mice
in Europe since the late 1950s, current reliable estimates of its
prevalence in the United States are not available.

Warfarin is formulated predominantly as dry (meal, pelleted, or
paraffinized) baits.  All but one of these baits are 0.025% active
ingredient, with the exception being a 0.054% warfarin bait registered
only for control of house mice.  Other registered end-use products
containing warfarin include 0.3% and 0.5% active ingredient concentrates
for preparing baits.  The sole remaining registered formulation of
sodium salt of warfarin (0.54% active ingredient) is designed to be
diluted with water to make liquid baits. 

Warfarin is federally registered mainly for control of Norway rats, roof
rats, and house mice in and around homes, agricultural buildings, and
commercial and industrial sites, including food and feed handling
establishments.  Warfarin is also registered for deer mouse and
white-footed mouse control in and around homes, agricultural and
industrial buildings and similar structure; in parks, woodlots, yards
and lots surrounding residential buildings and noncrop areas near
agricultural buildings.  Finally, there is a recent warfarin
registration for the control of several species of moles (eastern,
starnose, hairy-tailed, coast, broad-footed, Townsend) on lawns, turf
areas, golf courses, and other non-food grassy areas.  The warfarin bait
registered for mole control must be applied directly into their
underground tunnels.

B.  Chlorophacinone

Chlorophacinone is a first-generation anticoagulant that was first
registered in 1971.  Chlorophacinone may kill some rodents after a
single night(s feeding, but multiple feedings are needed in most cases. 
Death in rodents occurs within three to ten days.

Chlorophacinone is formulated as tracking powder, loose-grain bait,
paraffinized pellets, bait in ready-to-use placepacks, and paraffin
blocks.  Bait formulations contain 0.005% active ingredient.  Baits are
applied as often as needed for controlling commensal rodents.  Most
field uses have a limited number of applications.  Both restricted use
and unclassified products are registered.

Most of the unclassified products are ready-to-use baits registered for
structural use to control commensal rodents.  Application rates for
placepacks and paraffinized blocks are similar to those for loose baits,
but application amounts typically are expressed in terms of the numbers
of placepacks, blocks, or pieces of blocks to deploy at each placement
location.  There also is one 0.005% chlorophacinone bait registered to
control pocket gophers and another registered to control moles.  These
unclassified products must be applied directly into the underground
burrow systems of pocket gophers and moles, respectively.

The restricted-use products containing chlorophacinone include 0.2% a.i.
tracking powders that are federally registered for indoor use to control
commensal rodents and some 0.005% and 0.01% a.i. baits registered under
Section 24(c) of FIFRA to control various types of field rodents.  At
the time of product reregistration, all chlorophacinone products
registered for above-ground uses to control field rodents will be
classified as Restricted Use Pesticides. 

Chlorophacinone is federally registered for control of commensal rodents
in and around homes; industrial, commercial, or agricultural buildings
and structures; and inside sewers.  There also are 24(c) (special local
needs( (SLN) registrations authorizing use of chlorophacinone baits in
19 states for control of numerous small mammals in a variety of
agricultural and non-agricultural field sites.  Chlorophacinone is
registered for control of eastern moles (Scalopus aquaticus), star-nosed
moles (Condylura cristata), or moles (Scapanus spp.) in lawns, golf
courses, and other turf areas; and pocket gophers in lawns, golf
courses, range land, and noncrop areas.  It is also registered to
control various types of voles (Microtus spp.) in dormant and
non-bearing orchards; California voles in artichoke fields; and ground
squirrels around farm buildings, in orchards, alfalfa, rangelands,
pastures, and noncrop areas.  Other target mammals claimed on SLN labels
for chlorophacinone baits are deer mice (Peromyscus maniculatus), wood
rats (Neotoma), chipmunks (Eutamias spp.), prairie dogs (Cynomys spp.),
muskrats (Ondatra zibethica), nutria (Myocastor coypus), and jackrabbits
(Lepus spp.).  Field use sites listed on labels include orchards,
forests, groves, nurseries, tree plantations, inside transport vehicles,
commercial transportation facilities, and food processing, handling and
storage areas and facilities.

C.  Diphacinone and its Sodium Salt 

Background Information - Diphacinone is a first-generation anticoagulant
rodenticide first registered in 1960.  As with chlorophacinone,
diphacinone may kill some rodents after a single night(s feeding, but
multiple feedings will occur and may be needed to cause death. 

Diphacinone is federally registered for control of commensal rodents in
and around homes, industrial and agricultural buildings and similar
man-made structures; in sewers; in wet or damp areas including river
banks, irrigation ditches, gullies, railroad tracks, loading areas,
along fences, under granaries, garbage dumps, and landfills. 
Diphacinone is also federally registered for control of pocket gophers
(Thomomys bottae, T.  talpoides, T.  mazama, T.  townsendi, T. 
bulbivorus, T.  monticola, and Geomys bursarius) in rangeland, grain
fields, forage crops, hay and alfalfa crops, vegetable crops, forest,
nurseries, and noncrop areas, including parks and around homes.    

Twenty-two states currently have at least one 24(c) special local needs
diphacinone registration for field uses.  Uses of these products include
control of  voles, mainly Microtus spp., in dormant bearing and/or
non-bearing tree fruit orchards in New Hampshire, Ohio, Pennsylvania,
Virginia, Washington, West Virginia; control of voles in orchards,
Christmas tree farms, commercial nurseries, and tree plantations in
North Carolina; control of ground squirrels (Spermophilus spp.) and
muskrats in noncrop areas, control of jack rabbits (Lepus californicus)
in noncrop areas bordering agricultural fields and at airports, control
of California voles  (Microtus californicus) and montane voles (M.
montanus) in orchards and groves, and control of wood rats (Neotoma
spp.) in/around cabins and plantations of citrus trees and conifers in
California; control of ground squirrels (Spermophilus spp.) in levee or
ditch banks, around farm buildings, along fence lines, in orchards, in
or near crops, and in noncrop areas in Washington (State); control of
voles around small grain crops in Washington and Idaho; control of
cotton rats (Sigmodon hispidus), rice rats (Oryzomys palustris) and
Florida water rats (Neofiber alleni) in noncrop areas adjacent to sugar
cane and sweet corn fields in Florida; control of mongoose in Hawaii and
the Virgin Islands for protection of numerous species of ground nesting
birds, including endangered bird species; control of rats and mice
around the perimeter of planted fields and nurseries in Florida; control
of commensal rats in forests, offshore islands and other noncrop outdoor
areas in Hawaii and the Virgin Islands; control of commensal rats for
conservation purposes on islands in the Alaska Maritime National
Wildlife Refuge; and control of deer mice, jackrabbits, chipmunks,
muskrats, woodrats, voles.

The only remaining registered products containing sodium salt of
diphacinone as the active ingredient (at 0.106%) are registered only for
mixing liquid baits to be used to control commensal rodents in and
around homes, industrial and agricultural buildings, and similar
man-made structures.

D. Brodifacoum 

Brodifacoum, first registered in 1979, is a second-generation
anticoagulant and the most widely used by homeowners in urban areas at
present.  A single night(s feeding may be sufficient to kill a target
rodent within a three to ten days, but rodents often feed and behave
normally for 2-3 days after their first exposure to the bait.     

   

Brodifacoum is formulated as meal baits, paraffinized pellets, and
paraffin bait blocks.  All end use formulations contain 0.005% active
ingredient.  Baits may be applied as often as necessary.  All registered
brodifacoum products are unclassified.

Brodifacoum is currently registered for use only against commensal rats
and mice in and around homes, agricultural buildings, commercial
buildings, public buildings, and industrial buildings, inside transport
vehicles, in and around related port or terminal buildings, in alleys,
and in sewers.

At times, Brodifacoum has been used under (18 emergency exemptions from
the requirements of FIFRA in Federal conservation programs to control
rodents on oceanic islands.  However, Brodifacoum does not have a
Section 3 registration for that purpose. 

E.  Bromadiolone

Bromadiolone is a second-generation anticoagulant which first was
registered in 1980.  Bromadiolone is formulated as meal baits,
paraffinized pellets, and paraffin bait blocks.  All formulations
contain 0.005% a.i.  Some pelleted baits and meal baits are sold in
placepacks.

Bromadiolone is registered for control of commensal rats and mice in and
around buildings, inside transport vehicles, in alleys, and in sewers in
urban areas.  It is limited to indoor use in homes and agricultural
buildings in non-urban areas. 

F.  Difethialone

Difethialone is a second generation anticoagulant rodenticide that was
first registered in 1995.  Difethialone is registered for the control of
Norway rats, roof rats, and house mice in and around homes and
industrial, commercial, and public buildings in urban areas; in
transport and cargo vehicles (ships, trains, aircraft) and in and around
related port or terminal buildings.  In non-urban areas, difethialone
may only be used inside of homes and agricultural buildings.

G.  Zinc Phosphide

Zinc phosphide was registered in the United States in 1947, the first
year of the Federal pesticide registration program under FIFRA.  Prior
to that time, zinc phosphide was used as a rodenticide in the United
States and was used in that capacity in Italy as early as 1911.  Once
ingested, zinc phosphide reacts with moisture in the gastrointestinal
tract to liberate phosphine gas, which is the lethal agent. 

Zinc phosphide is the rodenticide most commonly used in agriculture.  It
is available in ready-to-use dry baits (whole-grains and pellets) and in
the form of a dry concentrate to be used by applicators to prepare toxic
baits.  A 10% zinc phosphide tracking powder is registered to control
house mice.  All zinc phosphide end-use products are classified as
Restricted Use Pesticides except those that containing 2%
active-ingredient or less and are limited to use in and around buildings
or to manual, subterranean applications to the burrow systems of pocket
gophers or moles.

Depending upon the specific use pattern and label, zinc phosphide baits
may be applied manually, by ground or aerial bait broadcasting
equipment, or by use of other specialized equipment such as trail
builders or burrow builders.  As zinc phosphide baits often are not
readily accepted by target rodents, labels often advise that
bait-acceptance tests be conducted prior to baiting operations.  
Depending upon the specific use pattern, labels may recommend or require
 prebaiting the entire infested area with rodenticide-free grain of the
type to be used in the toxic bait to accustom target rodents to feeding
on that particular grain.

Zinc phosphide is registered for control of commensal rats and mice,
white-footed mice (Peromyscus leucopus), and voles in and around homes
and industrial, commercial, agricultural, and public buildings.  It is
also federally registered for use against a wide range of small mammals,
mainly rodents, such as the following: meadow voles in alfalfa and
timothy hayfields; muskrats and nutria in areas where they damage
levees, irrigation ditch banks, or water impoundments, or where they
might feed on rice, soybean, milo, corn, or damage turf; pocket gophers
in rangeland and Christmas tree plantations; pocket gophers and moles in
grain fields, forage crops, hay and alfalfa crops, and vegetable crops,
forest areas, parks, nurseries, lawns, golf courses, homes, and other
noncrop areas; voles, white-footed and deer mice, and jumping mice
(Zapus spp.) in grape vineyards; voles and white-footed mice in berry
production areas; voles in sugar beets; voles and deer mice in noncrop
areas, including right-of-ways, lawns, parks, nurseries, and golf
courses; voles and white-footed mice in pastures, ornamentals, orchards,
vineyards, rangelands, forests, reforestation areas, lawns, golf
courses, parks, nurseries, and highway medians; roof rats in macadamia
nut orchards; deer mice and voles in orchards, groves, nursery stock
(ornamental and non-bearing fruit trees), and conifer and hardwood
seeding plantations; prairie dogs and ground squirrels in rangeland and
reforestation seedlings and plantings; kangaroo rats (Dipodomys spp.) in
rangeland, pastures, and noncrop areas; rats (Polynesian, Norway, roof,
rice, Florida water, cotton) in sugarcane fields; California ground
squirrel and voles in noncrop rights-of-way; voles; ground squirrels in
dormant orchards or vineyards, ornamentals, cemeteries, golf courses,
nurseries, canals and ditch banks, rangelands, pastures, lawn and turf
grasses, and along fence rows; prairie dogs on rangelands, pastures, and
reforestation seedlings and plantings in the western United States;
woodrats in rangelands, pastures, noncrop right-of-ways, dormant
orchards, tree farms, and rural agricultural buildings.

H.  Cholecalciferol

Cholecalciferol, also known as Vitamin D3, was first registered in 1984.
 Cholecalciferol triggers the mobilization of calcium from the bone
matrix into blood plasma, resulting in death from hypercalcemia in 3-4
days after ingestion of a lethal dose.  Cholecalciferol is not an
anticoagulant.  Cholecalciferol is formulated into baits containing
0.075% active ingredient.

Cholecalciferol is registered for the control of Norway rats, roof rats,
and house mice in and around homes; industrial and commercial buildings;
and similar man-made structures, inside transport and cargo vehicles
(ships, trains, aircraft), and in related port or terminal buildings.  

I.  Bromethalin

Bromethalin is a diphenylamine rodenticide that acts, after one or more
feedings, by blocking nerve impulse transmission, causing paralysis of
the central nervous system and respiratory arrest in 2-4 days. 
Bromethalin was initially registered in 1982.   

Bromethalin is formulated into various types of rodenticide baits,
including paraffinized blocks, meal baits, and pelleted baits.  Most
formulated products contain 0.01% active ingredient.  Some bromethalin
baits are sold in placepacks.  Two products consist of nine 0.01%
bromethalin bait in a ready-to-use bait station.  These bait stations
have not been shown to be tamper-resistant.

Bromethalin is registered for control of commensal rats and mice in and
around homes, industrial and agricultural buildings, and similar
man-made structures; in alleys, in transport vehicles (ships, trains,
and aircraft), and in and around related port or terminal buildings.

VI.  Efficacy Considerations

The rodenticide active ingredients discussed in this document were
subjected to the product performance (efficacy) data requirements that
were in effect at the time that they were registered.  Those
requirements have changed over time, with the most significant
developments being the amendments to the Federal Insecticide, Fungicide
and Rodenticide Act (FIFRA) passed 1972 and 1978, the development of the
Product Performance portion (Subdivision G) of the Pesticide Assessment
Guidelines (Schneider and Hitch, 1982), and the promulgation of the
regulations published in 40 CFR (158.640.

The 1972 amendments to FIFRA required EPA to formalize its data and
testing requirements through publication of relevant guidelines,
instituted data compensation requirements, and required EPA to (register
and reclassify pesticides registered( under FIFRA prior to the effective
date of (regulations promulgated( to provide for (registration and
reclassification of pesticides under the provisions of( FIFRA, as
amended.  This (registration and reclassification( activity later came
to be known as (reregistration(.

The 1978 amendments authorized conditional registration of pesticide
products, expanded on the subject of reregistration, and granted
discretionary authority to the EPA administrator to (waive data
requirements pertaining to efficacy.(  EPA thus initiated a policy of
waiving efficacy data requirements for all pesticidal claims except
those pertaining to control of pests considered to be of significance to
public health.  The efficacy waiver policy was expanded in 1982 to
include essentially all pesticidal claims.  The policy soon was changed
back to one of routinely requiring efficacy data to support claims for
controlling pests of significance to public health and emphasizing that
the waiver policy only applied to the efficacy data requirement. 
Registrants were expected to test pesticide products to verify that they
were efficacious.  The efficacy data submission requirements and
policies are summarized in 40 CFR (158.640.

Of the claims being made for the rodenticides discussed in this
document, the following rodent types have been considered to be of
significance to public health: Norway rats, roof rats, house mice, deer
or white footed mice (Peromyscus spp., except for endangered
subspecies), ground squirrels (Spermophilus spp.), and prairie dogs
(Cynomys spp., except for the endangered Utah prairie dog, C.
parvidens).  Applicable efficacy data must be submitted or cited to
support claims for controlling these species.  Distinctions are not
drawn between urban and rural, domestic and industrial, or structural
and agricultural use sites because rodent-vectored diseases may be
spread to humans in any of these locations.  There is a public health
aspect to virtually all efforts to control these species.

As indicated in 40 CFR (158.640, EPA reserves the right to require that
efficacy data be submitted (on a case-by-case basis for any pesticide
product.(  For the rodenticides discussed here, that authority has been
considered in a few cases in which the claims being evaluated seemed
unlikely for the product formulation involved.

Four of the nine rodenticides discussed in this document were registered
prior to passage of the 1972 amendments.  In order of the date of
initial registration, those compounds are: zinc phosphide, warfarin,
diphacinone, and chlorophacinone.  In addition to the efficacy data that
may have been required for registration, baits containing these active
ingredients -- especially zinc phosphide and warfarin (  were tested
rather extensively for effectiveness against commensal rodents under
actual use conditions by university, municipal, and public health
personnel (e.g., Emlen and Stokes, 1947; Hayes and Gaines, 1950). 
Additional uses of these four compounds that were accepted under Section
3 of FIFRA after passage of the 1972 amendments were subject to
prevailing data requirements and, if efficacy data submission
requirements applied, were thoroughly researched (Tietjen, 1976;
Tietjen, 1979; Tietjen and Matschke, 1981).

The other 5 rodenticides considered here (brodifacoum, bromadiolone,
bromethalin, cholecalciferol, and difethialone) were not registered
until after passage of the 1978 amendments. These 5 compounds were
extensively tested for efficacy according to procedures similar to those
described in Subdivision G of the Pesticide Assessment Guidelines with
respect to claims for controlling commensal rodents.  Such testing
entailed laboratory assessments of toxicity to target species,
laboratory (choice( feeding trials to assess palatability and lethality
of candidate bait formulations, and regional field testing.  Although
Bromethalin was initially registered in 1982, during the time of the
expanded efficacy waiver, its laboratory and field testing regimens were
largely completed before that policy went into effect.

Research has shown that seemingly small changes to bait composition can
have drastic adverse effects upon palatability to commensal rodents. 
Therefore, specific bait formulations claimed to control commensal
rodents are required to be screened for efficacy.  Most commensal
rodenticide bait formulations registered since the 1972 amendments have
been screened for effectiveness against Norway rats and house mice, if
both species are claimed on product labels.  A few such products
registered during the period of the expanded efficacy waiver (mid 1982
to early 1984) were not screened for efficacy prior to registration, but
will be if they are to be reregistered.  Warfarin baits registered prior
to the 1972 amendment were screened for efficacy prior to their being
reregistered in the early 1990's.  Registered commensal rodenticide
baits must be screened for efficacy again if they are reformulated.

Cholecalciferol and difethialone were registered after November 1, 1984
and, therefore, are not subject to reregistration requirements.  Prior
to registration, however, these active ingredients were thoroughly
tested for efficacy against commensal rodents in laboratory and field
trials.  All registered commensal rodenticide bait formulations
containing these compounds have been screened for efficacy.

For registered baits products containing the compounds currently
undergoing registration (zinc phosphide, diphacinone, chlorophacinone,
brodifacoum, bromadiolone, and difethialone), new efficacy data
generally will be needed for reregistration only if their registrants:
(1) seek to modify existing bait formulations; (2) desire to make claims
of effectiveness which go beyond simple claims for controlling the
target species claimed on the products( labels, or (3) have not yet
supported some claims for which submission of efficacy data is required.

Although the methods for screening rodenticide baits for efficacy in
laboratory tests vary somewhat according to the type of product and the
nature of the claim being investigated, the product performance criteria
are similar across the nine compounds being considered in this document.
 In controlled laboratory tests with fresh (dry( or liquid baits claimed
to control commensal rodents, at least 90% of the rodents exposed to the
rodenticide must die for the bait to be considered to be efficacious. 
For multiple-feeding anticoagulants (e.g., warfarin, diphacinone, and
chlorophacinone), there also is a requirement that fresh baits be
accepted as 33% or more of total intake in a choice test involving EPA(s
standard challenge diet for dry baits or water for liquid baits. 
Slightly lower criteria (80% mortality and 25% acceptance) apply when
experimentally (weathered( anticoagulant baits are evaluated for
suitability for use in wet or damp areas (e.g., sewers) or are claimed
to be (weather-resistant.( 

The performance criteria in field tests are similar across active
ingredients used in rodenticide baits.  EPA accepts field efficacy
studies of rodenticide baits if appropriate procedures were employed in
the research and if rodent activity indices indicate that at least 70%
control of the baited population was obtained.  All the rodenticide
compounds discussed here have been tested according to essentially
similar criteria and are considered by EPA to be useful for controlling
commensal rodents.

Three of the nine compounds under discussion are registered for use to
control (field( rodents of public health significance (e.g., ground
squirrels, prairie dogs, Peromyscus spp.).  These compounds are: zinc
phosphide, diphacinone, and chlorophacinone.  Zinc phosphide bait
products labeled for controlling these species and other field rodents
(e.g., Microtus spp. voles; Geomys and Thomomys spp. pocket gophers;
non-endangered kangaroo rats, Dipodomys spp. etc.) typically are
registered under Section 3 of FIFRA.  Most diphacinone and
chlorophacinone products labeled for field rodent uses are available
only in certain States under (special local needs( registrations issued
according to the provisions of Section 24(c) of FIFRA.  Reregistration
of field uses for these three compounds will entail submitting or citing
relevant data, including efficacy data regarding claims for controlling
pests that are of significance to public-health claims.  Some
information of that nature that has been reviewed to date is tacitly
considered in this assessment.  New efficacy studies may be needed if
there are any public-health claims that are not fully supported by
existing data.

Although rodenticide compounds registered for similar uses have been
required to meet essentially the same performance criteria, the field
tests that have been reported to EPA to fulfill efficacy data
requirements for these chemicals were run at different sites, at
different times, and often by different personnel.  That these and other
factors may affect study outcomes should be kept in mind when comparing
results from such tests.

Relatively few field efficacy trials have been conducted specifically to
compare the performance of various rodenticide baits under United States
field conditions.  Generally, such trials were conducted under
conditions in which control of the target species had proven to be
difficult.  For example, Tickes, et al (1982) ran comparative efficacy
trials with valley pocket gophers using three compounds that are
registered for that purpose, but reported that none of the six bait
formulations tested appeared to control that organism well.  Some of the
comparative studies that have been reported have primarily involved use
of compounds which are not included in this evaluation and/or are no
longer are registered (e.g., Albert and Record, 1979; Emlen and Stokes,
1947).

Ashton, et al (1983) conducted comparative efficacy trials for several
currently registered rodenticide baits containing warfarin, bromethalin,
brodifacoum, or bromadiolone.  Baits  were evaluated for efficacy
against house mice and Norway rats.  As those trials were performed at
sites where resistance to warfarin (see below) had been detected, the
test results obtained for warfarin were likely lower than typical
results would have been with populations of non-warfarin resistant
rodents.  Frantz and Madigan (1998) report that a significant number of
rats that had previously been found to be resistant to warfarin,
according to WHO testing protocol, died after been exposed again to
warfarin a second time, suggesting that warfarin may prove to be
effective against resistant rats by modifying the baiting schedules.    

VII.  Factors Affecting Rodenticide Bait Selection   

Although the nine rodenticide compounds considered here have been
required to meet similar criteria in efficacy trials, several additional
factors must be considered when assessing the degree to which compounds
registered for similar uses may serve as alternatives to one another.  
These factors are identified and briefly discussed below.

Registered Uses - Rodenticide baits may only be used at the use sites
that are indicated on their labels and may not be applied by application
methods that are prohibited by label text (e.g., (Do not broadcast
bait(, (Apply this product only by the methods prescribed on this
label(, (This product must be applied directly into pocket gophers(
underground burrow systems(, etc.).  While all of the nine rodenticides
considered here are registered for controlling commensal rodents, zinc
phosphide is the only one that has been accepted under Section 3 of
FIFRA for broad usage to control field rodents and is the only one for
which tolerances have been issued to allow for food and feed uses. 
These circumstances leave users with no toxic-bait alternative to zinc
phosphide for many uses.  In certain states, baits containing
Chlorophacinone or Diphacinone are available under Section 24(c),
(special local needs,( registrations for control of field rodents.

Classification - Not all rodenticide baits are available to all
prospective users.  Products labeled as (Restricted Use Pesticides( may
only be purchased by certified applicators and may only be used by such
applicators or by persons under their direct supervision.  Such products
include zinc phosphide baits registered for above-ground uses to control
field rodents.  All above-ground uses of any of the other eight
compounds to control field rodents will likely be classified as
restricted use pesticides at the time of reregistration. 

Speed of Kill - How fast a compound kills a given target species may
affect the choice of the toxicant.  Of the nine rodenticides considered,
zinc phosphide and bromethalin come closest to being true (acute(
rodenticides, with individual rodents generally feeding on the bait for
only one day and dying within one or two days after feeding occurs. 
With cholecalciferol baits, significant feeding usually occurs on more
than one day with death ensuing several days later.  

Anticoagulants generally take at least four days from the onset of
feeding until rodents begin to die.  It may take two weeks or more for
some individuals to die.  Although second-generation anticoagulants also
have been touted as (single-feeding( anticoagulants, that expression is
misleading.  It has been shown that although the amount of bait
containing bromadiolone, brodifacoum, or difethialone eaten in one
24-hour period can be sufficient to cause the deaths of rodents, the
animals generally continue to feed normally for two or three more days. 
Thus, free-ranging rodents exposed to a palatable second-generation
anticoagulant bait would be expected to eat nearly as much of it as they
would if exposed to a palatable first-generation anticoagulant bait,
thereby accumulating a (super-lethal dose.(  

Perceived risks to nontarget species - The extent of the applicator(s
knowledge and concern about nontarget effects would likely play a part
in the selection of a toxicant.  If, for example, bait applications are
to be primarily used in indoor locations to which children or nontarget
animals, such as pets and livestock, do not have access, an applicator
might feel more comfortable in using a second-generation anticoagulant
than if outdoor placements were needed in areas frequented by birds or
nontarget mammals.  

Resistance - Resistance to first-generation anticoagulants has been
detected in commensal rodent populations in many places throughout the
world including some localities in the United States (Frantz and Padula,
1980; Frantz and Madigan, 1998).  Efficacy data submitted to the Agency
for brodifacoum indicate that baits containing that second-generation
anticoagulant at 0.005% active ingredient are likely to be effective
against Norway rats and house mice of United States origin that are
resistant to warfarin and other first generation anticoagulants.  Data
submitted to the Agency suggest that bromadiolone baits at 0.005% a.i.
are likely to control warfarin-resistant Norway rats of United States
origin and that 0.0025% difethialone baits that are registered for
controlling house mice are likely to control warfarin-resistant house
mice of United States origin.  The expression (warfarin-resistant( is
used here because the test subjects ultimately killed by the
second-generation anticoagulants mentioned initially were screened for
resistance to warfarin.  Resistance to other rodenticides has been
recorded in other parts of the world as well.  For instance, in Velen,
Germany, 20% of rats trapped on livestock farms were found to be
resistant to bromadiolone, according to a blood-clotting response (BCR)
test (Endepols et al., 2003).

Some success in controlling United States rodent populations resistant
to warfarin has been reported in field trials involving
second-generation anticoagulants and bromethalin (Ashton, et al, 1983). 
Anticoagulant-resistant rodents likely would be susceptible to any
rodenticide that is not an anticoagulant.  Resistant rats may also
become again susceptible to warfarin if the baiting schedule is adjusted
cyclically so that each warfarin baiting is followed by a period of at
least 30 days, during which exposure to anticoagulants baits is
suspended (Frantz and Padula, 1980; Frantz and Madigan, 1998). 
Alternating anticoagulant use with use of alternative control methods,
including a toxicant with a different mode of action, would be expected
to lessen the selective pressure favoring anticoagulant-resistant
individuals.  Frantz and Madigan (1998) suggested that a non-chemical
strategy should be considered in areas where resistant rodents exist and
that habitat modifications, especially limiting alternate food sources,
might enhance the effectiveness of cyclical applications of
first-generation anticoagulants in such areas by increasing the amount
of bait consumed.  Frantz and Madigan (1998) and Baroch (2004) present
data which suggest that, if used cyclically and in conjunction with
sanitation, Warfarin itself may eventually control resistant
populations.

Conditioned Food Aversion (Bait Shyness) - Animals that become ill
after ingesting a new food with a novel taste often avoid that food
taste subsequently.  When the new food is a toxic rodenticide bait,
rodents obtaining sublethal oral doses may avoid both the bait base
material and the toxicant in the bait base in the future, perhaps for
the rest of their lives.  Such acquired bait shyness can greatly
diminish the effectiveness of rodent control programs and is most
pronounced with quick-acting poisons that have salient flavors.  Most
such compounds no longer are registered in the United States.  Among the
rodenticides considered here, bait shyness is often a significant
problem with zinc phosphide.  With small target rodents such as voles
(Microtus spp.), bait shyness to zinc phosphide can be mitigated
appreciably by formulating baits to an active ingredient strength such
that a single ingested bait particle is fatal to the rodent (Eadie,
1950).  For larger species, prebaiting target mammals by exposing them
to untreated bait grain several days before the toxic bait is applied
may result in good control (Tietjen, 1976).

Due, at least in part, to the delayed toxic action of anticoagulants and
cholecalciferol, bait shyness does not develop with these rodenticides. 
Bromethalin produces anorexia as an early symptom of exposure, but
animals that recover do not seem to avoid the same bait subsequently.

Weather - Changes in ambient weather conditions can alter the behavior
of rodents and may effect the amount of time that some species spend
foraging above ground.  Wet weather can also affect exposed baits
directly by changing their integrity (especially with some types of
pelleted baits) and by altering their toxic properties.  Wet zinc
phosphide baits (prematurely( liberate phosphine gas, reducing the
toxicity of each particle and affecting palatability.

Structural Use Considerations - All rodenticide baits under discussion
currently are registered for use to control commensal rodents in and
around buildings.  Consequently, each of these compounds potentially is
a viable alternative to each of the others in that use situation, except
perhaps at sites where a high incidence of resistance to
first-generation anticoagulants has been detected.  Absent resistance
problems, selection among active ingredients by professional applicators
is likely to be based upon past experience, safety, cost, and possibly
product promotions from manufacturers and/or distributors.  In
structural situations, most incidents for primary exposure are reported
for young children (under 6 years of age) and for nontarget species,
such as dogs, cats, and livestock, although primary exposures of
wildlife may also occur. By following label directions regarding bait
stations and proper use of tamper-resistant bait stations, professional
users can minimize incidents of primary exposures to nontarget species. 
As secondary exposures are not affected by the use of bait stations and
may occur even if all bait placements are located indoors, professional
users may consider the likelihood of secondary exposures of avian and
mammalian predators and scavengers when selecting among rodenticides.

A large proportion of rodenticide use by non-professional applicators
(i.e., homeowners) occurs in structural situations, chiefly in and
around their homes.  Selection among rodenticide products by consumers
depends to a large extent on local availability.  The labels on
rodenticide baits that consumers purchase are similar to the labels on
baits used by professional applicators, but tamper-resistant bait
stations are seldom offered at locations where consumers buy
rodenticides.  Some ready-to-use bait station products (bait stations
loaded with bait) are offered for sale at retail outlets, but none of
those stations has been demonstrated to be tamper-resistant. 
Ready-to-use bait stations that are made of cardboard are not
tamper-resistant.

Field Uses - Most field uses of rodenticide bait are, or are soon to
be, limited to labels of products that are classified as "Restricted Use
Pesticides".  Consequently, most applications to control field rodents
or commensal rodents in field situations are to be made by certified
applicators or by those under their direct supervision.  Such
applicators are expected to have access to training materials and
equipment appropriate for the types of applications that they intend to
make.  Broadly, such types of applications include: above-ground
treatments made manually or by aerial or ground broadcast equipment or
by trail-builders; and below-ground applications made by hand, by
hand-operated probes, or by machine-drawn burrow builders.  Manual,
above-ground treatments include "spot" placements of small amounts of
bait in discrete locations, scattering bait over small areas near active
burrows or runways, and placing larger amounts of bait in bait stations.

All above-ground placements are available to nontarget species to some
extent.  For spot, scattering, and trail-builder applications, some
mitigation of primary nontarget exposure is realized by confining
treatments to places where target species are likely to be the first
animals to encounter baits.  Bait stations used in the control of field
rodents often are designed to exclude nontarget animals that are larger
than the target species.  In some cases, special designs have been
developed which exploit behavioral traits and limitations of species of
small nontarget mammals that are to be protected and allow control of
larger rodents with little risk to those smaller species (e.g., Erickson
, et al, 1990; Whisson, 1998)

With broadcast applications, spatial dilution of bait and its settling
into vegetation may make it somewhat more likely to be encountered by
the olfactory-guided foraging strategies of rodents than by visually
guided granivorous birds.

For all above ground uses, proper timing of application in terms of
season and time of day increases the likelihood of uptake by target
species and thereby limits the amount of bait remaining for nontarget
species. 

Currently, for zinc phosphide, and following product reregistration for
chlorophacinone and diphacinone, the field rodenticide uses available to
nonprofessional applicators will be limited to manual, below-ground
applications to control moles and/or pocket gophers.  If label
directions for such products are followed, primary nontarget exposures
to such baits would be limited to species that entered the burrow
systems of the target species.  The likelihood that consumers would
follow such directions may be greater for the mole and pocket gopher
baits than for baits used to control commensal rodents because it is not
necessary to obtain additional equipment to treat mole and gopher
burrows and because there is little hope for successful control of moles
and pocket gophers unless bait is applied directly to their burrow
systems.

Quarantine Uses - When there is a need to ensure that no target pests
survive, the selection of a rodenticide or combination of rodenticides
is likely to favor the most highly toxic compounds.  Whether there is a
need to control a vector or an invasive rodent, a short-term increase in
local risk factors might be offset by the long-term benefits of
arresting the disease or preventing the invasive species from
establishing itself.

Conservation Uses - If an invasive rodent species has become
established, rodenticide baits may be incorporated in management or
eradication programs intended to benefit native species.  Such programs
may be feasible on islands of small to moderate size to protect colonies
of ground- and hole-nesting birds.  If eradication is judged to be
feasible, one season(s use of a highly toxic rodenticide could eliminate
the invasive rodent population to the subsequent benefit of the native
species.  Applying baits after migratory species have left the area may
simultaneously promote acceptance of bait by food-stressed rodents while
minimizing adverse impacts on native species.  Some adverse impacts of
island baiting programs on native species have been documented (e.g.,
Howald, et al, 1999).  Recognizing the potential adverse impacts of
baiting programs and planning appropriate mitigation actions are
essential steps in designing strategies for managing or eradicating
rodents for conservation purposes (e.g., National Park Service, 2000).  

VIII.  Alternative Methods of Rodent Control

A.  Chemical Alternatives

Fumigants (gas cartridges, aluminum phosphide, magnesium phosphide,
Vargon, acrolein, all available as Special Local Need registrations
only]) -  Some types of rodents can be controlled by application of
fumigant products to their burrow systems.  Such applications
potentially will kill target species and any other animals in the
burrows.  However, burrow treatments with some fumigants have been less
than fully effective for organisms such as pocket gophers and moles,
which have complicated burrow systems and may also have behavioral
defenses against certain types of fumigants.  Apparently for these
reasons, gas cartridges do not appear to be effective against pocket
gophers (e.g., Matschke, et al 1995).

The usefulness of burrow fumigants is further limited by prohibitions on
treatments of burrows that are close to buildings and the need for soil
moisture to be adequate to retain toxic gases.  Especially under dry
conditions, using gas cartridges may present a fire hazard.  Aluminum
phosphide, magnesium phosphide, and acrolein are Restricted Use
Pesticides for which applicator certification and specialized training
are required.  Special equipment is needed to apply acrolein and Vargon
to burrows.  Burrow fumigations are time-consuming and labor-intensive,
with repeat treatments often being needed.  Consequently, burrow
fumigations may be too costly to conduct except when controlling small
infestations, "mopping up" survivors from a toxic baiting program, or
protecting extremely valuable resources.

Structural fumigation aimed at controlling insect pests may also control
exposed rodents. Benefits realized in this regard usually are in
addition to the invertebrate control which was the primary treatment
objective.

Repellents (e.g., Dr. T(s., Ro(Pel, Naphthalene) - There are few
products registered in the United States as rodent repellents.  Such
products generally are claimed to be area repellents or feeding
deterrents.  Evidence supporting either type of claim is limited. 
Denatonium compounds have been claimed to act as feeding deterrents to
certain types of rodents as well as other mammals.  At lower
concentrations (e.g., 0.001%), however, denatonium benzoate is used in
rodenticide baits and is claimed not to deter feeding by commensal
rodents (Kaukeinen and Buckle, 1992).

Area repellents containing materials such as naphthalene or
paradichlorobenzene are irritating to humans and are not appropriate for
use in occupied buildings unless the products themselves are in sealed
containers, in which case they would be useless as rodent repellents.

B.  Non-Chemical Alternatives	

Exclusion and Rodentproofing - Rodent-resistant and rodentproof
modifications to existing or new constructions (collectively called
(stoppage() can greatly reduce the likelihood that rodents will enter
buildings and are an important element of successful structural rodent
management operations.  However, not all building designs allow for
complete rodent proofing, considering that young commensal rats can
enter holes (" (13 mm) in diameter and house mice can enter even
narrower openings (Frantz, 1988; Baker, et al, 1994).  Even with
conventional rodent proofing, rats can occasionally enter buildings by
coming up through the plumbing that drains toilets.  Business and farm
operations which require doors to be kept open for extended periods of
time, especially at night, cannot be completely rodentproofed. 
Exclusion is useful as a first approach, or as an accompanying IPM
measure to existing infestations when rodenticides must also be applied.
 Exclusion reduces the ability of commensal rodents to survive because
their food, water, and shelter resources are diminished or eliminated.  

Habitat Modification - Properties can be made less attractive to
commensal rodents by rather simple procedures such as improving
sanitation, thereby reducing sources of harborage, food, and water
(Timm, 1994a; 1994b).  These approaches are helpful in deterring
colonization by rodents and, as directed on existing labels for
commensal rodenticides.  These interventions should accompany or precede
baiting or trapping programs.    

Trapping - Trapping is useful for verifying the species responsible for
a rodent infestation and may be sufficient to control small
infestations, especially at the time of colonization (e.g., late summer
invasions of homes by house mice).  For larger infestations, rodent
removal by trapping alone can be an extremely labor-intensive and costly
(Boddicker, 1983).  However, some management programs for commensal
rats, such as the IPM program implemented by the San Francisco
Department of the Environment, have found trapping to be a viable and
often preferable rodent management technique (Blumenfeld, 2004).  Where
toxic baits cannot be used for reasons of safety, trapping may be the
only removal method that can be implemented.  Lethal and non-lethal
traps of various types may be used to control various types of rodents
and moles.  Traps may be especially useful for controlling small
infestations of house mice, Peromyscus spp. mice, pocket gophers,
woodchucks, and Norway rats.  Appropriate trap placement is important to
success, and trap prebaiting may enhance the rate of capture of
neophobic species (e.g., Timm, 1994a; 1994b).    

	The use of lethal traps may not always be  acceptable to everyone, as
the following comment submitted to the Agency by Thill (2004) indicates:
 “In many instances residents cannot tolerate trapping, cannot afford
private pest control services, and for one reason or another, (are) not
able to do certain exclusion work, or other associated rodent control
work.  In many instances the foul odor of a decaying rat is much more
tolerable than the handling of a dead rat in a trap.  The use of
rodenticide can be a very valuable tool in these cases, but still needs
to be used in conjunction with other control methods in order to achieve
adequate control.”       

	However, the City of San Francisco’s IPM project has found that for
some infestations, such as in restaurants, quick control is essential
for public health reasons, and this can be achieved by trapping. 
Furthermore, in addition to a quick reduction of the rat population,
using traps avoids the problem of dying or dead rats in public areas or
inside walls where decomposing bodies will cause long-lasting putrid
odors (Blumenfeld, 2004).

 

Lethal traps may pose a risk to small nontarget animals, and can cause
injury to applicators and other on-site personnel.  Placing lethal traps
within locked rooms or in the rodent areas of properly applied
tamper-resistant bait stations or other suitable objects can mitigate
the risks posed to some nontarget species. 

Live-trapping requires users to deal with captured animals.  This
circumstance often allows for the release of captured nontarget animals
but presents to the trapper the problem of deciding the fate of target
animals and may increase the opportunity for transfer of certain disease
to humans (e.g., CDC, 2004).  With repeating-type mouse traps, multiple
house mice may be caught in a single trap (Timm, 1994a).  With traps,
there is some danger of targeted animals becoming trap-shy through their
own experiences (i.e., near misses). 

Electrical or electronic rodent traps have been marketed at times. 
These traps generally have been able to trap rodents, with some traps
also killing them.  Due to high unit costs, it is doubtful that such
traps have had much usage.

Shooting - Some control of diurnal rodents such as prairie dogs and
ground squirrels can be effected through shooting.  The method has been
described as selective, but not very efficient for controlling colonial
species (e.g., Hyngstrom and Virchow, 1994; Marsh, 1994).  For
woodchucks, shooting can be used to manage local populations
(Bollengier, 1994).  However, shooting can only be employed where and
when State laws and local ordinances permit the activity.  Prohibitions
on hunting near buildings or in parks and the (game( status of the pest
are some of the factors affected by laws and ordinances.

Pest Control Devices -  Under FIFRA, pest control devices are regulated
to some extent, but there is no requirement that the products themselves
be registered before they are marketed.  Some pest control devices
marketed for rodent control are discussed below.

	Glue boards - Glue boards are sticky traps which have been marketed in
ready-to-use condition or with the glue offered separately, enabling the
user to apply it to a suitable surface for capturing rodents.  The
issues with glue boards are much like those discussed above for traps,
except for logistical and humaneness problems since the glue restrains
but does not kill the rodent, often leading to prolonged struggling in
awkward positions (Frantz and Padula, 1983). Humans may be injured when
picking up glue boards holding rodents that only appear to be dead. 
Neophobic responses to glue boards have been reported for house mice
(Corrigan, 1998).

	Burrow collapsers - Products that collapse rodent burrow systems are
relatively new.  One type of such a device ignites a mixture of propane
and oxygen in underground burrow systems, causing an explosion which
reportedly kills organisms living in the burrow.  Compression and the
collapsing of the tunnels are believed to effect rodent control, with
the leveled burrow system subsequently being less inviting to immigrant
rodents than the intact system would have been.  The utility of this
type of product would be limited to burrowing species.

Issues of safety to humans, property, environment are largely unstudied
for this type of product.  Appropriate personal protective equipment
(PPE) should be worn when using a burrow collapsing device.  Nontarget
animals living in treated burrow systems also would be expected to be
killed.  The abrupt shifting of soil along the course of the treated
burrow system could cause damage to nearby buildings, roads, fencing,
utility poles, or other structures.

	Electromagnetic Devices - Since the mid 1970's, various devices have
been claimed to be able to control various types of pests, including
rodents, through creation of electromagnetic fields.  EPA collected and
evaluated many such units that were available in the 1970's and
concluded that the principle of controlling pests via low levels of
electromagnetism is flawed (EPA, 1979).  Consequently, electromagnetic
devices are considered to be unlikely to exert rodent control effects on
their own; and claims that they enhance the effectiveness of
conventional rodent control techniques such as traps and toxic baits are
regarded as highly unlikely.

	Ultrasonic devices - Although many types of rodents can perceive
ultrasonic stimuli and communicate via ultrasonic vocalizations, tests
of ultrasound generators as rodent control agents have suggested little
promise for them.  Even though rodents may appear to perceive and may
initially react to ultrasonic noise, they seem to adapt to it. 
Ultrasound rapidly attenuates over distance, is extremely directional,
and is (shadowed( by objects.  Consequently, commensal rodents appear to
be able to coexist with ultrasonic signals by staying largely behind and
under cover, which is typical behavior for these animals.  EPA studied
units available in the 1970's and early 1980s and concluded that they
were not likely to be of significant value in rodent control.

C.  Integrated Pest Management

	Since first articulated by Stern et al. (1959), as “integrated
control,” there have been many attempts to redefine the integrated
pest management (IPM) concept.  As it applies to urban pests, including
rodents, Frantz (1996) defines IPM as “the coordinated use of pest and
environmental information with available pest suppression methods to
prevent unacceptable levels of pest damage by the most economical means
and with the least possible hazard to people, property, and the
environment.”  As currently practiced, rodent IPM could be described
as the use of complementary non-chemical and chemical rodent control
techniques in a safe, cost effective, and least environmentally
disruptive manner, to prevent rodents from adversely affecting the
welfare and interests of humans.  Frantz and Davis (1991) provide a
comprehensive review of the principles and application of the IPM
approach to rodent control, drawing from observations and experiences in
the U.S. and several Asian countries.     

Management of rodent pests, whether field or commensal species, is best
achieved through a combination of viable methods (i.e., IPM) , with the
approaches selected being adapted to conditions prevailing at the site
when the program is initiated.  For example, Timm (1994) presents a flow
chart outlining the factors and control options that an applicator might
consider in dealing with an infestation of house mice.  If there are no
rodents present, rodentproofing and habitat modification should be
implemented to increase the likelihood that rodents will not take up
residence on the premises.  Planning to incorporate rodent stoppage into
building designs, seeing to it that stoppage is effected during
construction, and diligently maintaining stoppage and sanitation on the
premises may prevent rodent infestations from occurring at a site
(Frantz and Davis, 1991).

	Several New York City agencies are currently implementing coordinated
rat control programs that include lot cleaning, cracking down on illegal
garbage dumping, making regular inspections, and exterminating rats. 
The New York Department of Parks, for instance, uses an IPM approach,
emphasizing prevention and exclusion methods that include frequent
garbage collection, use of rodent-resistant closed containers, pruning
of overgrown areas to reduce outside shelter, and masonry work to reduce
rodent harborage inside structures.  Rat populations are monitored to
determine if the use of rodenticide baits is needed.  This initiative
was built on an IPM pilot project funded by the CDC for implementation
in a 48-block portion of Bushwick, Brooklyn, which had a long history of
rodent infestations (Frieden, 2003).

	The Contra Costa Mosquito and Vector Control District implements an IPM
program for rodents that involves community education, interagency
cooperation, and baiting programs (Thill, 2004).  This program’s
community awareness and education component uses a wide range of
outreach and information dissemination approaches to educate the
community residents about appropriate rodent management techniques. 
This component is complemented by two baiting program, one of which
targets Norway rats in sewer systems and the other commensal rodents in
creeks, canals, waterfronts, parks, and other public areas.  A non-toxic
bait is often used to detect rat activity and determine where to best
use toxic baits.

	The City of San Francisco currently implements an IPM program for rats,
based on an IPM ordinance adopted in 1996 that commits the City to
selecting the least hazardous, effective rodent management methods in
City properties (Blumenfeld, 2004).  This program emphasizes the use of
non-chemical methods, such as trapping, while still using rodenticide
baits as needed.  The program has found that for some infestations, such
as in restaurants, quick control is essential for public health reasons,
and this can be achieved by trapping, whereas baiting is better suited
for use in certain locations, such as sewers.  

IX.  Matrix of Use Sites 

	

Table 1. summarizes the use patterns of the nine rodenticides discussed
here.  The names of the rodenticides are listed on the first column. 
The second column shows a common classification scheme for these
chemicals based on mode of action and developmental history.  The next
two columns indicate that all nine rodenticides are registered for
commensal rodent control, whereas only four of these, warfarin,
chlorophacinone, diphacinone, and zinc phosphide are registered for
non-commensal rodent control.  Of  these, zinc phosphide has by far the
most   uses in terms of number of target species and use sites.  The
fifth column shows the ranking of the four rodenticides that pose the
greatest overall risk to birds and nontarget mammals, based on EPA(s
2004 ecological risk assessment for the nine rodenticides discussed in
this document.  The sixth column includes some of the limited usage data
publicly available at present.  The seventh and last column attempts to
categorize common use patterns in relation to the areas where these
rodenticides are most commonly used.      

Table 1.  Use Matrix for Currently Registered Rodenticide Baits 

Rodenticide Baits	

Active Ingredient 

Classification 	

Used Against Pest Mammals	

Potential Overall Risk to Birds and Nontarget Mammals4	

Usage5	

Land Use Category

(Use is Mainly in these Areas)

Commensal Rats and Mice	

Mainly Native Rodents2

	

Warfarin1	

1st Generation Anticoagulant	

X	

X	

	

No Estimate Available	

Urban / Suburban

Chlorophacinone	

1st Generation Anticoagulant	

X	

X 

(Sect. 24c)

(RUP3)	

	

No Estimate Available	

Urban / Suburban / Rural

Diphacinone	

1st Generation Anticoagulant	

X	

X

 (Sect. 24c) (RUP3)	

High (4th)	

No Estimate Available	

Urban / Suburban / Rural

Brodifacoum	

2nd Generation Anticoagulant	

X	

	

High (1st)	

Most used homeowner rodenticide.  Products have an estimated 30% of PCO
market	

Urban / Suburban

Bromadiolone	

2nd Generation Anticoagulant	

X	

	

	

Products have an estimated 50% of PCO market	

Urban / Suburban

Difethialone	

2nd Generation Anticoagulant	

X	

	

High (3rd)	

No estimate Available	

Urban / Suburban

Zinc phosphide	

Non-Anticoagulant	

X	

X 

(Sect. 3)  (RUP3)	

High (2nd)	

Most used rodenticide  in agricultural fields	

Rural / Agriculture

Cholecalciferol	

Non-Anticoagulant	

X	

	

	

No estimate Available	

Urban / Suburban

Bromethalin	

Non-Anticoagulant	

X	

	

	

No estimate Available	

Urban / Suburban

1 Resistance to this a.i. has been reported in some United States
commensal rat populations. 

2Also jackrabbits, moles, mongoose, and nutria (an introduced rodent)  

3Restricted Use, except for underground baiting of pocket gophers and
moles

4Based on Erickson and Urban (2004)

5There is a limited amount of data available on the usage of
rodenticides in the three primary rodenticide markets (homeowner, PCO
(residential and commercial), and agriculture).  Not Available means
that there is no estimate available for the usage of the rodenticide. 
Not Available does not mean that the rodenticide is not used.  Annual
value of homeowner market for rodent control estimated at more than $90
million annually.  The estimated annual value of the market for rodent
control products used by pest control operators (PCOs) is more than $15
million.     

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XI.  List of Contributors

Angel Chiri, U.S. EPA, OPP, Biological and Economic Analysis Division

William Jacobs, U.S. EPA, OPP, Registration Division

Jonathan Becker, U.S. EPA, OPP, Biological and Economic Analysis
Division

Tim Kiely, U.S. EPA, OPP, Biological and Economic Analysis Division 

Kelly White, U.S. EPA, OPP, Special Review and Reregistration Division

Laura Parsons, U.S. EPA, OPP, Special Review and Reregistration Division

William Gross, U.S. EPA, OPP, Biological and Economic Analysis Division

1 In addition to brodifacoum, bromadiolone, bromethalin,
chlorophacinone, diphacinone, the 1998 Rodenticide Cluster RED also
presented EPA(s reregistration decision for pival and its sodium salt,
which were not supported and, therefore, determined to be ineligible for
re-registration.

 The U.S. EPA does not normally classify products for general use;
products that are not restricted remain unclassified. See 40 CFR
152.160(a).

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