Professor - Forages
Head - Entomology
Professor - Weed Science
Professor - Entomology
Former Professor -
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University of Arkansas, United States Department of Agriculture, and County Governments Cooperating
Musk thistle (Carduus nutans L.)
is an aggressive weed that infests
pasture and rangeland. It is native to
Europe but was introduced acciden
tally into the eastern United States
during the mid to late 1800s. Because
of its prolific seed production and lack
of natural enemies, it spread rapidly
throughout much of North America.
It is a weed of considerable economic
importance in forageproducing areas
and has been declared a noxious weed
in many states.
Musk thistle plants grow from
two to more than six feet in height
(Figure 1). Flower color varies from
purple to a deep reddishpink. Each
flower head is located at the tip of a
long stem or branch. The large flowers
commonly grow to two inches in diam
eter, causing the stems to bend or nod
over as the flower matures. Musk
thistle is also called nodding thistle.
Leaves, stems and branches of
musk thistle plants are covered with
sharp spines. The long leaves are
deeply and irregularly indented. They
have a smooth, waxy surface with a
lightcolored grayishgreen margin
and a lighter green midrib area
Musk thistle is generally classed
as a biennial, but under some environ
mental conditions it may develop as
an annual, biennial or winter annual.
Musk thistle reproduces and spreads
only by seed. A musk thistle plant
produces an average of 3,500 seeds,
but large plants can produce up to
10,000. Seeds are usually dissemi
nated by wind but can also be spread
in contaminated hay or on farm equip
ment. Although some seed may be
carried by wind currents for several
miles, most fall within 100 yards of
the site of production.
but may germinate any time moisture is sufficient.
Most seeds germinate the first year, but some
dormant musk thistle seeds can remain viable in the
soil for as long as five to seven years.
After seed germina
tion, the plant develops
a fleshy taproot with a
rosette of leaves. The
plant overwinters as a
rosette. Seed stalks are
formed in spring as the
plant starts to bolt,
followed by flowering
which normally begins
in early to midMay
(Figure 2A). Occasionally
some plants can be found
August. The plant dies
after all its seeds mature.
Figure 2A. Bolted musk
thistle beginning to flower.
Areas of Infestation
Musk thistle is commonly found along roadsides,
railroad rightsofway, fence borders, unimproved
areas and in pastures and hay meadows. Newly
established thistle rosettes are inconspicuous and
may escape notice until they bolt and bloom. Diligent
scouting should be done during fall and spring to
locate infestations. Musk thistle can be a problem in
fallplanted grains and forages but is not a serious
weed problem in crops requiring spring seedbed
preparation. Spring tillage eliminates established
thistle rosettes before they produce seed.
The economic impact of musk thistle is greatest
in pastures and rangeland. Moderate infestations of
musk thistle have been reported to reduce pasture
yields an average of 23 percent. Livestock won’t
graze around musk thistle plants or in heavily
Herbicides should be applied when the musk
thistles are in the rosette stage during fall or early
spring. Applications made after the plants begin to
flower are too late to provide adequate control. Plants
treated with herbicide after the onset of flowering
may still produce viable seed.
Detailed information on recommended herbicides
for thistle control is listed in the publications
MP44, Recommended Chemicals for Weed and Brush
tive Extension Service website at www.uaex.edu.
Mowing can reduce the amount of seed produced,
but often enough stem remains intact with the crown
to produce flowers and seed. Mowing within two days
after the terminal flower head blooms effectively
inhibits seed production and reduces some branching
of the remaining plant stems. Since thistles in a field
do not all mature uniformly, mowing will usually
need to be repeated to prevent seed production.
Mowing on poor soil may actually reduce the competi
tive effect of other plants, thus favoring musk thistle
Digging and hand pulling are very effective for
controlling light or scattered infestations of thistles.
Plants must be cut off under the rosette or crown for
effective control. If leaves or the crown bud are left
attached to the root, the plant can still regrow and
produce seed. Some landowners pile and burn any
blooming plants they have pulled or dug in an
attempt to destroy potentially viable seed.
Good forage management practices are important
in preventing serious musk thistle infestations. Over
grazing and improper soil fertility management
reduce the vigor and competitiveness of the forage,
allowing musk thistle seedlings to become estab
lished. A pasture program that makes use of soil
testing and improved grazing management can
greatly reduce the potential for thistles to
Cutting hay before the thistles produce seed
prevents onfarm and offfarm movement of seed in
the hay. Refusing to buy hay that contains musk
thistle seed can help prevent musk thistles from
becoming established on your farm.
Biological control, the practice of using natural
enemies, can reduce musk thistle populations and
reduce the spread of musk thistles. Two species of
weevils that attack musk thistle have become estab
lished in Arkansas. These weevils are the flower head
weevil (Rhinocyllus conicus Froelich) and the rosette
weevil (Trichosirocalus horridus Panzar). These
natural enemies, native to Europe, were studied
extensively to ensure they would not damage
some species of native thistles.
Establishment of these weevils in Arkansas has
been through both natural dispersal and releases of
weevils collected from established populations in
Missouri. Established populations of both weevil
species have been found in 18 counties in central and
The musk thistle weevils offer the benefit of
reducing musk thistle populations in areas where no
control measures are being made and can contribute
significantly to longterm control efforts in areas
where musk thistle populations are high. Control of
thistles by the weevils is a slow but effective process.
Missouri research has shown that the weevils can
reduce the number of thistles by 50 to 95 percent over
a six to tenyear period.
Flower Head Weevil
Musk thistle flower head weevils overwinter as
adults. The adults are slender and brown with scat
tered golden spots on the wing covers (Figure 3). They
inch long and have a short, broad snout.
In early spring, the adults emerge from overwintering
sites and seek out musk thistle rosettes. Adults feed
on leaves of the plants but do little damage. Females
then mate and begin laying eggs when the plants
start to bolt and bloom. Eggs are deposited on the
bracts of the flowers. Each egg is covered with a
secretion of chewed plant material, giving the eggs an
easily noticeable brown, scalelike appearance
(Figure 4). Each female lays an average of 100 eggs
during its lifetime.
The eggs hatch in six to eight days. The larvae
tunnel into the thistle flower where they feed on the
developing seeds (Figure 5). Some flower heads turn
brown prematurely due to the damage caused by the
larvae feeding in the flower or in the stem just below
the flower (Figure 6).
Larvae feed for about 25 to 30 days then begin
pupation. Pupation lasts another 8 to 14 days. The
pupa rests in an excavated cell in the flower, where it
transforms into an adult. The adults emerge in July
and seek overwintering sites under new musk thistle
rosettes, ground litter or wooded areas, where they
will remain dormant until the following spring.
Flower head weevils usually produce only one
generation per year.
Figure 3. Adult musk thistle flower head weevil.
Figure 4. Flower head weevil eggs covered with
Figure 5. Musk thistle flower infested with weevil larvae.
Figure 6. Flowers infested with weevil larvae turn
Rosette weevils are slightly smaller than the
flower head weevil with a shorter and more rounded
body (Figure 7). This weevil is about
has a narrow snout. It also undergoes one generation
per year. Adult weevils emerge from summer
dormancy in early October. They feed on the under
side of rosette leaves by puncturing leaf tissue.
Females lay eggs during the fall and on warm days
during the winter. These same adults overwinter and
resume egg laying the following spring, up until
about May 1. The eggs are usually laid in the midrib
on the underside of rosette leaves or placed directly
in the rosette crown (Figure 8). In late spring some
egg laying occurs in secondary buds.
Emerging larvae burrow their way into the crown
of the plant where they feed, causing damage to the
growing point of the plant. Larvae complete their
development then leave the rosette and pupate in
Over time, rosette weevils have the potential to
provide greater thistle control than the flower head
weevil since the feeding damage by the larvae can
kill a rosette outright or weaken the plant so it
produces fewer flower heads and, thus, less seed.
However, the damage caused by the rosette weevil is
complementary to that caused by the flower head
weevil in controlling musk thistles because the
weevils are not in competition with each other.
Prevention of a musk thistle infestation is easier
than eradicating a population that has become well
established. New infestations of musk thistle or
invading scattered plants on a farm should be eradi
cated before seed production occurs. A combination of
methods previously described provides more effective
control than reliance on any single method.
The life cycle of the musk thistle weevils in
relation to the seasonal development of musk thistle
is shown in Figure 9. The chart shows an integrated
control program using proper timing of mechanical
and chemical control practices that encourages weevil
populations for effective thistle control.
Figure 9 shows that if only the flower head weevil
is established, rosettes can be sprayed with herbicide
between midMarch and late April, thistles can be
mowed in midJuly after the weevil completes its
life cycle, and new rosettes can be sprayed from
September to midOctober. If both the flower head
weevil and the rosette weevil are present, herbicide
applications should be limited to the fall. Spraying
thistle rosettes in the spring will not only kill the
thistles but will also kill the rosette weevil larvae
feeding in the crowns of the thistle plants. This inte
grated approach allows maximum benefit from the
weevils, yet allows other methods to be used for more
effective thistle control.
Musk thistle weevils are not produced
commercially but may be collected from established
populations and released at new sites. Flower head
weevils can be collected in early to midMay. Rosette
weevils are most readily found in late May to mid
June but are more difficult to collect than the flower
head weevil. Studies show springreleased adult
weevils are 80 times more effective in colonizing
musk thistle than adult weevils collected and
released in July. Because of this, attempts to
establish populations of weevils by moving flowers or
plants infested with weevil larvae are generally
Weevil Life Cycle
new adult overwinters
A minimum of 500 flower head weevils or
to help ensure establishment of either weevil species.
Weevils can be sprinkled over the leaves and blooms
of musk thistle plants, then planttoplant movement
of the weevils provides adequate dispersal. Written
records of the release and a photograph of the site
will help document the establishment and effect of
the weevils over time.
Simple equipment can be used for collecting
weevils. Weevils can be collected in a canvas insect
sweep net, plastic trash bag or large plastic bucket.
A large pan or plastic dish pan will serve to deposit
weevils in for sorting. Other items include leather
gloves, a threefootlong dowel or stick, small card
board boxes or onepint ice cream cartons for each
500 weevils collected, large ice chest and ice packs
warm days. It is best to collect flower head weevils
when plants have bolted one to two feet. Rosette
weevils will be more numerous after the plants begin
to bloom. Because of some overlap of emergence and
egg laying of flower head and rosette weevils, both
species are sometimes collected at the same time.
To collect the weevils, bend the bolting portion of
a musk thistle plant into the canvas sweep net while
wearing leather gloves. Rap on the plant several
times with the dowel rod. This will cause the weevils
to feign death and drop into the sweep net.
After netting 50 to 100 weevils, dump the sweep
net contents into the plastic wash basin for sorting.
Keep the basin in the shade to prevent its surface
from heating up and causing the weevils to fly off.
Adult weevils can be stored and transported in
lots of up to 500 in small cardboard cartons. A thistle
bud or bloom should be included in the carton, and
the lid should be sealed tightly to prevent escape.
Plastic cartons should not be used because they allow
moisture to build up, increasing mortality. Cardboard
cartons of weevils can be stored for up to a week in
an insulated chest if kept dry and cool (but not
frozen) with ice or ice packs. Release weevils at the
new site as soon as possible after collection to enable
them to deposit most of their eggs at the release site
rather than in the carton.
Suggestions for Successful
Studies show establishment success of musk
thistle weevils is improved by following these
1. The area should not be mowed or sprayed.
(Rightsofway and unimproved fields
2. Areas with heavy infestations work best (at
least 1,000 musk thistle plants).
3. All the weevils should be placed in the same
area (five to ten per plant).
4. Release weevils away from livestock.
5. Five to seven years may be required before
weevil populations are high enough to
provide significant thistle control.
• Musk thistle plants reproduce only by seed.
Mature plants die after seed is produced.
• Herbicides are most effective if applied
during spring or fall to musk thistles in the
• Rosette weevil larvae feed in the thistle
crown and weaken or kill the plant.
• Flower head weevil larvae feed in the flower
and reduce the number of seed produced.
• Mowing is most effective when done within
two days after the terminal flower blooms.
• Good pasture management practices reduce
establishment of musk thistles.
• Integrated control, using a combination of
biological, chemical, mechanical and cultural
control methods, is the most effective
program for reducing infestations of
The authors wish to thank Doug Ladner, USDAAnimal and Plant Health Inspection Service; David Blackburn, Arkansas State Plant
Board; Joe Williams and Larry White of the Arkansas Soil and Water Conservation Commission; Glen Sutton, Natural Resources
Conservation Service; and Tom Riley, Cooperative Extension Service, University of Arkansas, for serving on the steering committee and
for their contributions to this project.
Acknowledgment is given to Dr. Ben Puttler, Extension Assistant Professor of Entomology, University of Missouri, for providing
information and technical review of this publication.
Printed by University of Arkansas Cooperative Extension Service Printing Services.
professor weed science, with the University of Arkansas System
Division of Agriculture are located in Little Rock. DR. GUS LORENZ,
associate department head entomology, with the University of
Arkansas System Division of Agriculture is located in Lonoke.
DR. DON STEINKRAUS, professor entomology, is located in the
Department of Entomology at the University of Arkansas, Fayetteville.
of Arkansas, Fayetteville.
Issued in furtherance of Cooperative Extension work, Acts of May 8
and June 30, 1914, in cooperation with the U.S. Department of
Agriculture, Director, Cooperative Extension Service, University of
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ture offers all its Extension and Research programs and services
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