B-71

NUTRIA

Dwight J. LeBlanc

State Director

USDA-APHIS-

Animal Damage Control

Port Allen, Louisiana 70767

Fig. 1. Nutria (Myocastor coypus)

Damage Prevention and

Control Methods

Exclusion

Protect small areas with partially

buried fences.

Wire tubes can be used to protect

baldcypress or other seedlings but

are expensive and difficult to use.

Use sheet metal shields to prevent

gnawing on wooden and styrofoam

structures and trees near aquatic

habitat.

Install bulkheads to deter burrowing

into banks.

Cultural Methods and Habitat

Modification

Improve drainage to destroy travel

lanes.

Manage vegetation to eliminate food

and cover.

Contou r stream banks to control

burrowing.

Plant baldcypress seedlings in the fall

to minimize losses.

Restrict farming, building

construction, and other “high risk”

activities to up land sites away from

water to prevent damage.

Manipulate water levels to stress

nutria p opulations.

Frightening

Ineffective.

Repellents

None are registered. None are

effective.

Toxicants

Zinc phosphide on carrot or sweet

potato baits.

Fumigants

None are registered. None are

effective.

Trapping

Commercial harvest by trappers.

Double longsp ring traps, Nos. 11 and

2, as preferred by trappers and

wildlife damage control specialists.

Body-gripping traps, for example,

Conibear® Nos. 160-2 and 220-2,

and locking snares are most

effective when set in trails, den

entrances, or culverts.

Live traps should be used when

leghold and body-gripping traps

cannot be set.

Long-handled dip nets can be used to

catch unwary nutria.

Shooting

Effective when environmental

conditions force nutria into the

open. Night hunting is illegal in

many states.

Other Methods

Available control techniques may not

be applicable to all damage

situations. In these cases, safe and

effective methods must be tailored

to specific problems.

PREVENTION AND CONTROL OF WILDLIFE DAMAGE — 1994

Coop erat ive Extension Division

Institute of Agriculture and Natural Resources

University of Nebraska - Lincoln

United States Department of Agriculture

Animal and Plant Health Inspection Service

Animal Damage Control

Great Plains Agricultural Council

Wildlife Committee

B-72

Identification

The nutria (Myocasto r coypus, Fig. 1) is

a large, dark-colored, semiaquatic

rodent that is native to southern South

America. At first glance, a casual

observer may misidentify a nutria as

either a beaver (Castor canadensis) or a

muskrat (Ondatra zibethicus), especially

when it is swimming. This superficial

resemblance ends when a more

detailed study of the animal is made.

Other names used for the nu tria

include coypu, nutria-rat, South

American beaver, Argentine beaver,

and swamp beaver.

Nutria are members of the family

Myocastoridae. They have short legs

and a robust, highly arched body that is

approximately 24 inches (61 cm) long.

Their round tail is from 13 to 16 inches (33

to 41 cm) long and scantily haired. Males

are slightly larger than females; the aver-

age weight for each is about 12 pounds

(5.4 kg). Males and females may grow to

20 pounds (9.1 kg) and 18 pounds (8.2

kg), respectively.

The dense grayish underfur is overlaid

by long, glossy guard hairs that vary in

color from dark brown to yellowish

brown. The forepaws have four well-

developed and clawed toes and one

vestigial toe. Four of the five clawed

toes on the hind foot are intercon-

nected by webbing; the fifth outer toe

is free. The hind legs are much larger

than the forelegs. When moving on

land, a nutria may drag its chest and

appear to hunch its back. Like beavers,

nutria have large incisors that are yel-

low-orange to orange-red on their

outer surfaces.

In addition to having webbed hind

feet, nutria have several other adapta-

tions to a semiaquatic life. The eyes,

ears, and nostrils of nutria are set high

on their heads. Additionally, the nos-

trils and mouth have valves that seal

out water while swimming, diving, or

feeding underwater. The mammae or

teats of the female are located high on

the sides, which allows the young to

suckle while in the water. When pur-

sued, nutria can swim long distances

under water and see well enough to

evade capture.

three Canadian provinces in North

America since their introduction.

About one-third of these states still

have viable populations that are stable

or increasing in number. Some of the

populations are economically impor-

tant to the fur industry. Adverse cli-

matic conditions, particularly extreme

cold, are probably the main factors

limiting range expansion of nutria in

North America. Nutria populations in

the United States are most dense along

the Gulf Coast of Louisiana and Texas

(Fig. 2).

Habitat

Nutria adapt to a wide variety of envi-

ronmental conditions and persist in

areas previously claimed to be unsuit-

able. In the United States, farm ponds

and other freshwater impoundments,

drainage canals with spoil banks,

rivers and bayous, freshwater and

brackish marshes, swamp s, and com-

binations of various wetland types can

provide a home to nutria. Nutria habi-

tat, in general, is the semiaquatic

environment that occurs at the bound-

ary between land and permanent

water. This zone usually has an abun-

dance of emergent aquatic vegetation,

small trees, and/or shrubs and may be

interspersed with small clumps and

hillocks of high ground. In the United

States, all significant nutria popula-

tions are in coastal areas, and fresh-

water marshes are the preferred

habitat.

Food Habits

Nutria are almost entirely herbivorous

and eat animal material (mostly

insects) incidentally, when they feed

on plants. Freshwater mussels and

crustaceans are occasionally eaten in

some parts of their range. Nutria are

opportunistic feeders and eat approxi-

mately 25% of their body weight daily.

They prefer several small meals to one

large meal.

The succulent, basal portions of plants

are preferred as food, but nutria also

eat entire plants or several different

parts of a plant. Roots, rhizomes, and

tubers are especially important during

winter. Important food plants in the

Fig. 2. Range o f the nutria introduced in North

America.

Range

The original range of nutria was south

of the equator in temperate South

America. This species has been intro-

duced into other areas, primarily for

fur farming, and feral populations can

now be found in North America,

Europe, the Soviet Union, the Middle

East, Africa, and Japan. M. c. bonarien-

sis was the primary subspecies of nu-

tria introduced into the United States.

Fur ranchers, hoping to exploit new

markets, imported nutria into Califor-

nia, Washington, Oregon, Michigan,

New Mexico, Louisiana, Ohio, and

Utah between 1899 and 1940. Many of

the nu tria from these ranches were

freed into the wild when the busi-

nesses failed in the late 1940s. State

and federal agencies and individuals

translocated nutria into Alabama,

Arkansas, Georgia, Kentucky, Mary-

land, Mississippi, Oklahoma, Louisi-

ana, and Texas, with the intent that

nutria would control undesirable veg-

etation and enhance trapping opportu-

nities. Nutria were also sold as “weed

cutters” to an ignorant public through-

out the Southeast. A hurricane in the

late 1940s aided dispersal by scattering

nutria over wide areas of coastal

southwest Louisiana and southeast

Texas.

Accidental and intentional releases

have led to the establishment of wide-

spread and localized populations of

nutria in various wetlands throughout

the United States. Feral animals have

been reported in at least 40 states and

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United States include cordgrasses

(Spartina spp.), bulrushes (Scirpus

spp.), spikerushes (Eleocharis spp.),

chafflower (Alternanthera spp.), pick-

erelweeds (Pontederia spp.), cattails

(Typha spp.), arrowheads (Sagittaria

spp.), and flatsedges (Cyperus spp.).

During winter, the bark of trees such

as black willow (Salix nigra) and bald-

cypress (Taxodium distichum) may be

eaten. Nutria also eat crops and lawn

grasses found adjacent to aquatic

habitat.

Because of their dexterous forepaws,

nutria can excavate soil and handle

very small food items. Food is eaten in

the water; on feeding platforms con-

structed from cut vegetation; at float-

ing stations supported by logs,

decaying mats of vegetation, or other

debris; in shallow water; or on land. In

some areas, the tops of muskrat houses

and beaver lodges may also be used as

feeding platforms.

General Biology,

Reproduction, and

Behavior

General Biology

In the wild, most nutria probably live

less than 3 years; captive animals,

however, may live 15 to 20 years. Pre-

dation, disease and parasitism, water

level fluctuations, habitat quality, high-

way traffic, and weather extremes af-

fect mortality. Annual mortality of

nutria is between 60% and 80%.

Predators of nutria include hu mans

(through regulated harvest), alligators

(Alligator mississippiensis), garfish

(Lepisosteus spp.), bald eagles (Haliae-

etus leucocephalus), and other birds of

prey, turtles, snakes suc h as the cotton-

mouth (Agkistrodon piscivorus), and

several carnivorous mammals.

Nutria densities vary greatly. In Loui-

siana, autumn densities of about 18

animals per acre (44/ha) have been

found in floating freshwater marshes.

In Oregon, summer densities in fresh-

water marshes may be 56 animals per

acre (138/ha). Sex ratios range from

0.6 to 1.6 males per female.

In summer, nutria live on the ground

in dense vegetation, but at other times

of the year they use burrows. Burrows

may be those abandoned by other ani-

mals such as armadillos (Dasypus

novemcinctus), beavers, and muskrats,

or they may be dug by nutria. Under-

ground burrows are u sed by individu-

als or multigenerational family groups.

Burrow entrances are usually located

in the vegetated banks of natural and

human-made waterways, especially

those having a slope greater than 45

o

.

Burrows range from a simple, short

tunnel with one entrance to complex

systems with several tunnels and

entrances at different levels. Tunnels

are usually 4 to 6 feet (1.2 to 1.8 m)

long; however, lengths of up to 150

feet (46 m) have been recorded. Com-

partments within the tunnel system

are used for resting, feeding, escape

from predators and the weather, and

other activities. These vary in size,

from small ledges that are only 1 foot

(0.3 m) across to large family chambers

that measure 3 feet (0.9 m) across. The

floors of these chambers are above the

water line and may be covered with

plant debris discarded during feeding

and shaped into crude nests.

In addition to using land nests and

burrows, nutria often build flattened

circular platforms of vegetation in

shallow water. Constructed of coarse

emergent vegetation, these platforms

are used for feeding, loafing, groom-

ing, birthing, and escape, and are often

misidentified as muskrat houses. Ini-

tially, platforms may be relatively low

and inconspicuous; however, as vege-

tation accumulates, some may attain a

height of 3 feet (0.9 m).

Reproduction

Nutria breed in all seasons throughout

most of their range, and sexually

active individuals are present every

month of the year. Reproductive peaks

occur in late winter, early summer,

and mid-autumn, and may be regu-

lated by prevailing weather conditions.

Under optimal conditions, nutria reach

sexual maturity at 4 months of age.

Female nutria are polyestrous, and

nonpregnant females cycle into estrus

(“heat”) every 2 to 4 weeks. Estrous is

maintained for 1 to 4 days in most

females. Sexually mature males can

breed at any time because sperm is

produced throughout the year.

The gestation period for nutria ranges

from 130 to 132 days. A postpartum

estrus occurs within 48 hours after

birth and most females probably breed

again during that time.

Litters average 4 to 5 young, with a

range of 1 to 13. Litter sizes are gener-

ally smaller during winter, in

suboptimal habitats, and for young

females. Females often abort or assimi-

late embryos in response to adverse

environmental conditions.

Young are precocial and are born

fully furred and active. They weigh

approximately 8 ounces (227 g) at birth

and can swim and eat vegetation

shortly thereafter. Young normally

suckle for 7 to 8 weeks until they are

weaned.

Behavior

Nutria tend to be crepuscular and noc-

turnal, with the start and end of activ-

ity periods coinciding with sunset and

sunrise, respectively. Peak activity

occu rs near midnight. When food is

abundant, nutria rest and groom dur-

ing the day and feed at night. When

food is limited, daytime feeding

increases, especially in wetlands free

from frequent disturbance.

Nutria generally occupy a small area

throughout their lives. In Louisiana,

the home range of nutria is about 32

acres (13 ha). Daily cruising distances

for most nutria are less than 600 feet

(183 m), although some individuals

may travel much farther. Nutria move

most in winter, due to an increased

demand for food. Adults usually move

farther than young. S easonal migra-

tions of nutria may also occur. Nutria

living in some agricultural areas move

in from marshes and swamps when

crops are planted and leave after the

crops are harvested.

Nutria have relatively poor eyesight

and sense danger primarily by hear-

ing. They occasionally test the air for

scent. Although they appear to be

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clumsy on land, they can move with

surprising speed when disturbed.

When frightened, nutria head for the

nearest water, dive in with a splash,

and either swim underwater to protec-

tive cover or stay submerged near the

bottom for several minutes. When cor-

nered or captured, nutria are aggres-

sive and can inflict serious injury to

pets and humans by biting and

scratching.

Damage and Damage

Identification

Kinds of Damage

Nutria damage has been observed

throughout their range. Most damage

is from feeding or burrowing. In the

United States, most damage occurs

along the Gulf Coast of Louisiana and

Texas. The numerous natural and

human-made waterways that traverse

this area are used extensively for travel

by nutria.

Burrowing is the most commonly

reported damage caused by nutria.

Nutria are notorious in Louisiana and

Texas for undermining and breaking

through water-retaining levees in

flooded fields used to produce rice

and crawfish. Additionally, nutria bur-

rows sometimes weaken flood control

levees that protect low-lying areas. In

some cases, tunneling in these levees is

so extensive that water will flow

unobstructed from one side to the

other, necessitating their complete

reconstruction.

Nutria sometimes burrow into the

styrofoam flotation under boat docks

and wharves, causing these structures

to lean and sink. They may burrow

under buildings, which may lead to

uneven settling or failure of the foun-

dations. Burrows c an weaken road-

beds, stream banks, dams, and dikes,

which may collapse when the soil is

saturated by rain or high water or

when subjected to the weight of heavy

objects on the surface (such as vehicles,

farm machinery, or grazing livestock).

Rain and wave action can wash out

and enlarge collapsed burrows and

comp ound the damage.

Nutria depredation on crops is well

documented. In the United States, sug-

arcane and rice are the primary crops

damaged by nutria. Grazing on rice

plants can significantly reduce yields,

and damage can be locally severe. Sug-

arcane stalks are often gnawed or cut

during the growing season. Often only

the basal internodes of cut plants are

eaten. Other crops that have been

damaged include corn, milo (grain sor-

ghum), sugar and table beets, alfalfa,

wheat, barley, oats, peanuts, various

melons, and a variety of vegetables

from home gardens and truck farms.

Nutria girdle fruit, nut, and shade

trees and ornamental shrubs. They

also dig up lawns and golf courses

when feeding on the tender roots and

shoots of sod grasses. Gnawing dam-

age to wooden structures is common.

Nutria also gnaw on styrofoam floats

used to mark the location of traps in

commercial crawfish ponds.

At high densities and under certain

adverse environmental conditions, for-

aging nutria can significantly impact

natural plant communities. In Louisi-

ana, nutria often feed on seedling

baldcypress and can cause the com-

plete failure of planted or naturally-

regenerated stands. Overutilization of

emergent marsh plants can damage

stands of desirable vegetation used by

other wildlife species and aggravate

coastal erosion problems by destroying

vegetation that holds marsh soils

together. Nutria are fond of grassy

arrowhead (Sagittaria platyphylla)

tubers and may destroy stands propa-

gated as food for waterfowl in artificial

impoundments.

Nutria can be infected with several

pathogens and parasites that can be

transmitted to humans, livestock, and

pets. The role of nutria, however, in

the spread of diseases such as equine

encephalomyelitis, leptospirosis, hem-

orrhagic septicemia (Pasteurellosis),

paratyphoid, and salmonellosis is not

well documented. They may also host

a number of parasites, including the

nematodes and blood flukes that cause

“swimmer’s-itch” or “nutria-itch”

(Strongyloides myopotami and

Schistosoma mansoni), the protozoan re-

sponsible for giardiasis (Giardia

lamblia), tapeworms (Taenia spp.), and

common liver flukes (Fasciola hepatica).

The threat of disease may be an impor-

tant consideration in some situations,

such as when livestock drink from wa-

ter contaminated by nutria feces and

urine.

Damage Identification

The ranges of nutria, beavers, and

muskrats overlap in many areas and

damage caused by each may be similar

in appearance. Therefore, careful

examination of sign left at the damage

site is nec essary to identify the respon-

sible species.

On-site observations of animals and

their burrows are the best indicators of

the presence of nutria. Crawl outs,

slides, trails, and the exposed

entrances to burrows often have tracks

that can be used to identify the species.

The hind foot, which is about 5 inches

(13 cm) long, has four webbed toes

and a free outer toe. A drag mark left

by the tail may be evident between the

footprints (Fig. 3).

Droppings may be found floating in

the water, along trails, or at feeding

sites. These are dark green to almost

black in color, cylindrical, and approxi-

mately 2 inches (5 cm) long and 1/2

inch (1.3 cm) in diameter. Addition-

ally, each dropping usually has deep,

parallel grooves along its entire length

(Fig. 4).

Trees girdled by nutria often have no

tooth marks, and bark may be peeled

from the trunk. The crowns of seedling

trees are usually clipped (similar to

rabbit [Sylvilagus spp.] damage) and

discarded along with other woody

portions of the plant.

In rice fields, damage caused by

nutria, muskrats, and Norway rats

(Rattus norvegicus) can be confused.

Nutria and muskrats damage rice

plants by clipping stems at the water

line in flooded fields; Norway rats

reportedly clip stems above the surface

of the water (E. A. Wilson, personal

communication).

B-75

Legal Status

Nutria are protected as furbearers in

some states or localities because they

are economically important. Permits

may be necessary to control animals

that are damaging property. In other

areas, nutria have no legal protection

and can be taken at any time by any

legal means. Consequently, citizens

experiencing problems with nutria

should be fam iliar with local wildlife

laws and regulations. Complex prob-

lems shou ld be handled by profes-

sional wildlife damage control

specialists who have the necessary

permits and expertise to do the job

correctly. Your state wildlife agency

can provide the names of qualified

wildlife damage control specialists and

information on pertinent laws and

regulations.

Damage Prevention and

Control Methods

Preventive measures should be used

whenever possible, especially in areas

where damage is prevalent. When con-

trol is warranted, all available tech-

niques should be considered before a

control p lan is implemented. The ob-

jective of control is to use only those

techniques that will stop or alleviate

anticipated or ongoing damage or

reduce it to tolerable levels. In most

cases, successful control will depend

on integrating a number of different

techniques and methods.

Timing and location of control activi-

ties are important factors governing

the success or failure of any control

project. Control in sugarcane, for

example, is best applied during the

growing season, after damage has

started. At this time, nutria in affected

areas are relatively stationary and con-

centrated in drainages adjacent to

fields. Conversely, efforts to protect

rice field levees or the shorelines of

southern lakes and ponds should be

initiated during the winter when ani-

mals are mobile and concentrated in

major ditches and other large bodies of

water.

Fig. 4. Nutria dropping in relatio n to a 2-inch (5.1-cm) camera lens cover. Note longitudinal grooves

along the length of the dropping.

Fig. 3. Nutria tracks. Note unwebbed outer toe

on the hind foo t and the tail drag mark between

the tracks. The adult hind foot is approximately

5 inches (12.7 cm) long.

B-76

Nutria are best controlled where they

are causing damage or where they are

most active. Baiting is sometimes used

to concentrate nutria in specific loca-

tions where they can be controlled

more easily. After the main concentra-

tions of nu tria are removed, control

efforts should be directed at removing

wary individuals.

Exclusion

Fences, walls, and other structures can

reduce nutria damage, but high costs

usually limit their use. As a general

rule, barriers are too expensive to be

used to control damage to agricultural

crops. Low fences (about 4 feet [1.2 m])

with an apron buried at least 6 inches

(15 cm) have been used effectively to

exclude nutria from home gardens and

lawns. Sheet metal shields can be used

to prevent gnawing damage to

wooden and styrofoam structures and

trees. Barriers constructed of sheet

metal can be expensive to erect and

unsightly.

Protect baldcypress and other seed-

lings with hardware cloth tubes

around individual plants or wire mesh

fencing around the perimeter of a

stand. Extensive use of these is neither

practical nor cost-effective. Plastic

seedling protectors are not effec tive in

controlling damage to baldcypress

seedlings because nutria can chew

through them.

Sheet piling, bulkheads, and riprap can

effectively protect stream banks from

burrowing nutria. Installation requires

heavy equipment and is expensive.

Use is usually restricted to industrial

or commercial applications.

Cultural Methods and Habitat

Modification

Land that is well-drained and free of

dense, weedy vegetation is generally

unattractive to nutria. Use of other

good farming practices, such as preci-

sion land leveling and weed manage-

ment, can minimize nu tria damage in

agricultural areas.

Draining and Grading. Any drain-

age that holds water can be used by

nutria as a travel route or home site.

Consequently, eliminate standing

water in drainages to reduce their

attractiveness to nutria. This may be

extremely difficult or impossible to

accomplish in low-lying areas near

coastal marshes and permanent bodies

of water. Higher sites, such as those

used for growing sugarcane and other

crops, are better suited for this type of

management.

On poorly drained soils, contour small

ditches to eliminate low spots and sills

and enhance rapid drainage. Use pre-

cision leveling on well-drained soils to

eliminate small ditches that are occa-

sionally used by nutria.

Grading and bulldozing can destroy

active burrows in the banks of steep-

sided ditches and waterways. In addi-

tion, contour bank slopes at less than

45

o

to discourage new burrowing.

Sculpting rice field levees to make

them gently sloping is similarly effec-

tive. Continued deep plowing of land

undermined by nutria can destroy

shallow burrow systems and discour-

age new burrowing activity.

Vegetation Control. Eliminate

brush, trees, thickets, and weeds from

fence lines and turn rows that are adja-

cent to ditches, drainages, waterways,

and other wetlands to discourage nu-

tria. Burn or remove cleared vegeta-

tion from the site. Brush piles left on

the ground or in low sp ots can become

ideal summer homes for nutria.

Water Level Manipulation. Many

low-lying areas along the Gulf Coast

are protected by flood control levees

and pumps that can be used to manip-

ulate water levels. By dropping water

levels during the summer, stressful

drought conditions that cause nutria to

concentrate in the remaining aquatic

habitat can be simulated, thus increas-

ing competition for food and space,

exposure to predators, and emigration

to other suitable habitat. Raising water

levels in winter will force nutria out of

their burrows and expose them to the

additional stresses of cold weather.

Water level manipulation is expensive

to implement and has not yet been

proven to be effective. Nevertheless,

this method should be considered

when a comprehensive nutria control

program is being developed.

Other Cultural Methods. Alternate

field and garden sites should be con-

sidered in areas where nutria damage

has oc curred on a regular basis. New

fields, gardens, and slab-on-grade

buildings should be located as far as

possible from drainages, waterways,

and other water bodies where nutria

live.

Late-p lanted baldcypress seedlings are

less susceptible to damage by nutria

than those planted in the spring. For

this reason, plant unprotected seed-

lings in the early fall when alternative

natural foods are readily available.

Frightening

Nutria are wary creatures and will try

to escape when threatened. Loud

noises, high pressure water sprays,

and other typ es of harassment have

been used to scare nutria from lawns

and golf courses. The success of this

type of control is usually short-lived

and problem animals soon return.

Consequently, frightening as a control

technique is neither practical nor

effective.

Repellents

No chemical repellents for nutria are

currently registered. Other rodent

repellents (such as Thiram) may repel

nutria, but their effectiveness has not

been determined. Use of these without

the proper state and federal pesticide

registrations is illegal.

Toxicants

Zinc Phosphide. Zinc phospide is the

only toxicant that is registered for con-

trolling nutria. Zinc p hosphide is a

Restricted Use Pesticide that can only

be purchased and applied by certified

pesticide applicators or individuals

under their direct supervision. It is a

grayish-black powder with a heavy

garlic-like smell and is widely used for

controlling a variety of rodents. When

used properly, zinc p hosphide p oses

little hazard to nontarget species,

humans, pets, or livestock.

Zinc p hosphide is highly toxic to wild-

life and humans, so all precautions and

instructions on the product label

B-77

should be carefully reviewed, under-

stood, and followed precisely. Use an

approved respirator and wear elbow-

length rubber gloves when handling

this chemical to prevent accidental

poisoning. Mix and store baits treated

with zinc phosp hide only in well-

ventilated areas to reduce exposing

humans to c hemical fumes and dust.

When possible, mix zinc phosphide at

the baiting site to avoid having to store

and transport treated baits. Never

transport mixed bait or open zinc

phosphide containers in the cab of any

vehicle. Store unused zinc phosphide

in a dry place in its original watertight

container because moisture causes it to

deteriorate. Immediately wash off any

zinc phosphide that gets on the skin.

Past studies have shown that zinc

phosphide can kill over 95% of the

nutria present along waterways when

applied to fresh baits at a 0.75% (7,500

ppm) rate. Today, the use of zinc phos-

phide at this concentration is illegal.

Federal and state registrations, how-

ever, allow lower rates to be used. For

example, the label held by USDA-

APHIS-ADC (EPA Reg. No. 56228-9)

allows for a maximum 0.67% (6,700

ppm) treatment rate. At this rate,

approximately 94 pounds (42.7 kg) of

bait can be treated with 1 pound (0.4

kg) of 63.2% zinc phosphide concen-

trate.

Where to Bait. The best places to

bait nutria are in waterways, ponds,

and ditches where permanent standing

water and recent nutria sign are found.

Baiting in these areas increases effi-

ciency and reduces the likelihood that

nontarget animals will be affected.

Small chunks of unpeeled carrots,

sweet potatoes, watermelon rind, and

apples can be used as bait.

The best baiting stations for large

waterways are floating rafts spaced

1/4 to 1/2 mile (0.4 to 0.8 km) apart

throughout the damaged area. In

ponds, use one raft per 3 acres (1.2 ha).

Rafts measuring 4 feet (1.2 m) square

or 4 x 8 feet (1.2 x 2.4 m) are easily

made from sheets of 3/8- to 3/4-inch

(1.0- to 1.9- cm) exterior plywood and

3-inch (7.6-cm) styrofoam flotation.

Install a thin wooden strip around the

perimeter of the raft’s surface to keep

bait from rolling into the water. The

raft should float 1 to 4 inches (2.5 to

10.2 cm) above the surface and should

be anchored to the bottom with a

heavy weight or tied to the shore

(Fig. 5).

In small ditches or areas where nutria

densities are low, use 6-inch (15.2-cm)

square floating bait boards made of

wood and styrofoam, in lieu of rafts

(Fig. 5). These can be maintained in

place with a long slender anchoring

pole made of bamboo, reed, or other

suitable material that is placed through

a hole in the center of the platform.

This allows the board to move up and

down as water levels change. Attach

baits to small nails driven into the sur-

face of the platform. Bait boards

should be spaced 50 to 100 feet (15.2 to

30.5 m) apart in areas where nutria are

active.

Other natural sites surrounded by wa-

ter can also be baited for nutria. Small

islands, exposed tree stumps, floating

logs, and feeding platforms are excel-

lent baiting sites. Avoid placing baits

on muskrat houses and beaver lodges.

Baits can be attached to trees, stumps,

or other structures with small nails

and should be kept out of the water.

Baiting on the ground should only be

used when water sites are unsuitable

or lacking. Grou nd baiting is justified

and effective when eliminating the last

few nutria in a local p opulation. Use

care when ground baiting because

baits may be accessible to nontarget

animals and hu mans. Place grou nd

baits near sites of nutria activity, such

as trails and entrances to burrows.

Prebaiting. Prebaiting is a crucial

step when using zinc phosphide

because it leads to nutria feeding at

specific sites on specific types of food

(such as the baits; carrots or sweet p o-

tatoes are preferred). Nutria tend to be

communal feeders, and if one nutria

finds a new feeding spot, other nutria

in the area will also begin feeding

there.

To prebait, lightly coat small (approxi-

mately 2-inch [5.1-cm] long) chunks of

untreated bait with corn oil. Place the

bait at each baiting station in late after-

noon, and leave it overnight. Use no

more than 10 pounds (4.5 kg) of bait

per raft, 4 pieces of bait per baiting

board, or 2 to 5 pieces at other sites at

one time. Prebaiting shou ld continue

at least 2 successive nights after nutria

begin feeding at a baiting site. Large

Fig. 5. Examples of a 4-foot (1.2-m) square raft (left) and a 6-inch (15.2-cm) square baiting bo ard,

which are used to concentrate nutria for shoo ting, trapping, or poisoning. These baiting platforms

are constructed of plywood and styrofoam and baited with sweet potatoes.

B-78

(more than 1 week) gaps in the

prebaiting sequence necessitate that

the process be started over.

Observations of prebaited sites will

help you decide how the control pro-

gram should proceed. If nontarget ani-

mals are feeding at these sites (as

determined by sign or actual observa-

tions of animals), then prebaiting

should start over at another loc