Musk thistle control introduction

A FORAGE FACTS Publication of the KANSAS FORAGE TASK FORCE

Kansas State University Agricultural Experiment Station and Cooperative Extension Service

MUSK THISTLE CONTROL

INTRODUCTION

Musk thistle is an introduced invasive broadleaf weed native to Europe, Siberia, Asia Minor and North Af-

rica. It was first introduced into the United States in 1852 in Pennsylvania. Musk thistle now occurs throughout

most of the United States and is found in nearly all 105 Kansas counties. Musk thistle was first reported in Kansas

in 1932 and was declared a noxious weed in 1963. It can be found growing on roadsides, railroad right-of-ways,

building sites, vacant lots, range and pastureland, alfalfa fields and in wheat fallow.

Musk thistle is primarily a biennial or winter annual, relying on seed to reproduce. Seedlings may emerge

any time during the growing season with optimum germination occurring in moist soils with temperatures be-

tween 59 to 86˚F. The plant spends about 90 percent of its lifecycle as a rosette. The rosettes can be identified by

the distinct light green midrib. Leaves are coarsely lobed, spiny, hairless and often have a silver-gray leaf margin.

The stem elongates (bolts) in late April and May and the plants can exceed 6 feet in height. Flowering generally

starts in May and may last several weeks. Musk thistle flower heads are large and have a “powder puff” shape in

comparison to the “shaving brush” heads of many other thistles. Musk thistle flowers generally are rose-purple in

color. Seed dispersal occurs 7 to 10 days after blooming. A single plant is capable of producing in excess of 10,000

seeds. Seeds can remain viable in the soil for a decade or longer.

CONTROL OPTIONS

The goal of any control practice should be to pre-

vent seed production. The first line of defense against

musk thistle invasion on range and pasture is good

grazing management. Use stocking rates designed to

avoid overgrazing, that maintain a competitive cover

and prevent bare ground. Prescribed burning in the

late spring just as the  grasses are starting growth will

not directly kill musk thistle, but does stimulate the

warm season grasses that help prevent musk thistle

from becoming established. Cool-season pastures

should also be managed to maintain vigorous competi-

tive stands. Proper stocking rates, proper season of use

for grazing or haying, and maintenance of soil fertility

will generally reduce the likelihood of musk thistle in-

vasion in cool-season grass stands.

MECHANICAL

Scattered plants of musk thistle can be effectively

controlled by hand cutting and digging. Cut through

the taproot at least 2 inches below the crown to prevent

resprouting. Individual flowering heads can be re-

moved but new heads will develop in the leaf axils un-

less the entire plant is dug and allowed to dry out.

Flowering heads that are removed should be placed in

a tight container and either buried or allowed to rot to

prevent possible seed germination.

Mowing can be an effective control of musk

thistle if done when the plants are in the late-bloom

stage. Repeated mowing is generally necessary to

eliminate seed production and control musk thistle. Re-

search at Kansas State University has shown that a

single mowing at the late bud stage only killed 11 per-

cent of the musk thistle plants. A second mowing, 4

weeks later, increased control to 79 percent.

CHEMICAL

A number of herbicides are labeled and effective

for control of musk thistle. These include 2,4-D,

FOR AGE FACTS

Publication Series

Research and Extension

dicamba, picloram and metsulfuron methyl. All chemi-

cals must be applied according to label directions. Spe-

cific recommendations can be found in the annual issue

of Chemical Weed Control for Field Crops, Pastures, Range-

land and Noncropland available from Kansas State

University.

Chemical control of musk thistle declines after

the plant bolts and begins to flower. Treatment of musk

thistle rosettes in the fall or spring with recommended

herbicides typically results in 90 to 100 percent control.

Herbicide treatment after musk thistle bolts and begins

to flower may control some plants but will probably

not completely eliminate seed production.

Musk thistle control with herbicides is more ef-

fective when the plant is actively growing under condi-

tions of good soil moisture and favorable air tempera-

tures (70 to 90˚F). Air temperatures should exceed 50˚F

when herbicides are applied in the fall. A 6-hour rain-

free period after application is sufficient to ensure ad-

equate absorption of the herbicide.

BIOLOGICAL

The head weevil, Rhinocyllus conicus, can provide

biological control of musk thistle. The head weevil was

first released in Kansas in 1973 in Riley County with

widespread release of the organism by 1982. The head

weevil is now commonly found in musk thistle stands

throughout most of northeast and north central Kansas.

The adult weevil lays its eggs on the bracts of the

flower buds. The eggs hatch in 6 to 9 days and the lar-

vae feed on the seed-producing tissue. Adults emerge

starting in mid-July. The head weevil overwinters as an

adult seeking protection under litter, at the base of

plants and in wooded areas.

The rosette weevil, Trichosirocalus horridus, was

first imported and released in Kansas in 1978. The

adults begin laying eggs in the fall within the midrib

on the undersides of the leaves on musk thistle ro-

settes. As larvae hatch, they begin feeding within the

midrib and migrate toward the center of the rosette.

Continual feeding by the larvae causes a blackened ne-

crotic center on the thistle plant. This feeding may

eventually kill the plant but often results in shorter,

multistemmed plants the next spring.

Biological control is a long-term approach to

musk thistle control. Typically, it takes 6 to 10 years af-

ter weevil release to see a significant impact on the

thistle population. Any plan relying on the use of the

musk thistle head or rosette weevil needs to be ap-

proved by the Kansas Department of Agriculture. Con-

tact your local county weed director for assistance in

developing a plan for biological control of musk thistle.

INTEGRATED PEST MANAGEMENT

Mechanical, chemical and biological options exist

for control of musk thistle. The approach to use de-

pends on the severity and location of the infestation. A

single approach will work in many cases but long-term

control may be enhanced by integration of methods.

Control options can be integrated in time or space.

Stands with head weevils can be sprayed in the fall or

early spring when thistles are in the rosette stage.

Mowing can be used after the primary heads have

started to die and the adult weevils are emerging. Bio-

logical control might be used exclusively in remote or

environmentally sensitive areas with large thistle

populations. Herbicides or mechanical methods could

be used in adjacent areas to prevent the spread of musk

thistle, allowing time for the weevils to establish and

for suppression to occur.

OTHER PUBLICATIONS

Chemical Weed Control for Field Crops, Pastures,

Rangeland and Noncropland (Report of Progress issued

annually)

Musk Thistle Identification and Control (L-231

revised)

Biological Control of Musk Thistle in Kansas (L-873)

Kansas Department of Agriculture. Musk Thistle Of-

ficial Control Program (http://www.ink.org/public/

kda/phealth/phprot/mthistle.html)

Kansas State University Agricultural Experiment Station and Cooperative Extension Service

It is the policy of Kansas State University Agricultural Experiment Station and Cooperative Extension Service that all persons shall have equal opportunit y and

access to its educational programs, services, activities, and materials without regard to race, color, religion, national origin, sex, age or disabilit y. Kansas State

Universit y is an equal opportunity organization. Issued in furtherance of Cooperative Extension Work, Acts of May 8 and June 30, 1914, as amended. Kansas

State University, County Extension Councils, Extension Districts, and United States Department of Agriculture Cooperating, Marc A. Johnson, Director.

Contact: Walter H. Fick

Associate Professor

Range Management

Telephone: [phone]

FAX: [phone]

E-mail: [e-mail]