Abstract
Thistles compete with desirable forage and grain crops like
clover and wheat and are a major pest in almost every state.
This publication focuses on two of the more troublesome thistles
(Canada and musk), with some coverage of other thistles such as
plumeless, Italian, bull, and yellow star. Since thistles are a
widespread problem, most Extension offices around the country have
guidelines on thistle control and management. This publication
focuses on the cultural, biological, organic, and least-toxic methods
available for their control.
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Table of Contents
Biology
Common along the east coast, musk thistle occurs in 45 states. (Hoback
et al., 2003) Musk thistle is a biennial—germinating in the summer
and overwintering in the rosette stage, then bolting and flowering
in the spring. It is adapted to a wide range of growing conditions
and elevations. (Hoback et al., 2003) Musk thistle is also known
as nodding thistle and is listed as a noxious weed in some states.
A single musk thistle plant can produce up to 20,000 seeds, some
of which can remain viable in the soil for seven years. (Hoback et al.,
2003) The seed has no innate seed dormancy requirement. (McCarty et al.,
1969) The seeds can germinate as soon as conditions are favorable. Eighty
percent of the wind-dispersed seeds go only 130 feet from the parent plant,
while almost none travel further than 322 feet. (Smith and Kok, 1984) Therefore,
the key is to eradicate thistle plants before they produce seed.
Seeds in the flower may mature even after the flower head has been cut,
so try to control thistles before they bloom.
Canada thistle not only grows from seeds but also from roots,
making it much more difficult to control. Young plants emerge from
underground roots to make a denser patch. Even a half–inch
piece of Canada thistle root can grow into a new plant. (Anderson
, 2001) Researcher Rich Zollinger planted one 6-inch root piece
that produced 644 feet of roots and 336 feet of above-ground shoots
in 15 months. (Anon., 1996) Canada thistles arise as male and female
plants, with only the females producing viable seed. To control
Canada thistle, you must not only take out the top growth but deplete
the root reserves to prevent regrowth. Besides Canada thistle,
other perennials include the Platte and wavyleaf thistles. Besides
musk, other biennial thistles include bull, field, Scotch, and
tall. Yellow star and Italian thistles are annuals, while plumeless
is an annual or biennial.
In pastures, thistles take advantage of bare spots to get established.
Bare spots are prevented by not overgrazing and providing adequate
fertility and lime to assure a dense stand of forage. The weakest
link in the thistle life cycle is when the seeds are germinating
and getting established. This is the most effective and least costly
point of attack for a long-term solution. Spraying adult thistle
with herbicides will kill the adult plants but will do nothing
to stop new ones from growing from seeds already in the soil.
Although thistle seeds are wind-dispersed, it is important to
avoid transporting them to new areas where they have not previously
grown. Transportation is most common with contaminated hay, equipment
that has thistle seeds on it, and flowing streams that have thistle
seeds in the water. Clean the equipment when moving it from a thistle-infested
field to a clean field, and avoid bringing contaminated hay on
to land without thistle populations. To avoid their spread, remove
or treat small thistle outbreaks on previously clean land as soon
as they become apparent.
Biological Control
Several beneficial insects have been used to reduce thistle populations.
Some have become naturalized and continue to provide control without reintroduction.
There are commercial sources of beneficial thistle-control insects.
Adult Thistle-head Weevil
Rhinocyllus conicus
Photo by Noah Poritz,
Bio-control.com
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The adult thistle-head weevil, Rhinocyllus conicus, feeds
and mates on musk, plumeless, and a few more thistles while they
are in the rosette stage. Once the plants bolt, the females lay
eggs on the flower heads and stems. When the larvae hatch, they
bore into the seed heads to feed on developing seed. This weevil
is the most widely distributed insect for thistle control in
the U.S., being found in wild populations in most states from
coast to coast where musk and plumeless thistle occur. (Poritz,
2002) These weevils can reduce thistle populations by 90
to 95% in 8 to 10 years. (Anon., 2002) Since
they also feed on native thistles, they may decrease populations
of rare thistle species too. Because of their wider-than-expected
host range, USDA APHIS has prohibited interstate movement of
the Rhinocyllus weevils.
(Poritz, 2002) They can be moved within
a state, however. These insects are not a quick fix. The best
sites for these weevils to get established have heavy musk thistle
infestation, are not currently being used for grazing, and are
where the thistles are not going to be disturbed. The weevils
must complete their life cycle in the thistles in order for the
insects to damage the plants and reproduce. These weevils typically
leave areas where cattle are present, thus they may not be a
useful management strategy for controlled grazing situations.
(Hartzler, 1995)
See:
www.co.stevens.wa.us/weedboard/htm_bio/rhinocyllus%20conicus.htm for
more information on Rhinocyllus weevils.
Back to top
The thistle rosette weevil, Trichosirocalus horridus, feeds
on musk thistle during the rosette stage, killing first-year
rosettes and stopping the growth of older plants. (Hoback
et al., 2003) The thistle defoliating beetle, Cassida
rubiginosa, feeds
on leaves of Canada, musk, and plumeless thistles. Adult beetles
lay an average of 800 eggs per female, and both the adults and
larvae feed on thistle foliage throughout the growing season.
Read more on Cassida rubiginosa at: http://res2.agr.ca/lethbridge/weedbio/agents/acasrub_e.htm.
The thistle-stem gall fly, Urophora cardui, attacks
the Canada thistle stem, boring in and causing the plant to form
a gall. The gall lowers normal plant function and reproduction,
causing the plants not to flower in some cases. The severity
of impact on Canada thistle depends on whether the galls are
on the main shoots or lateral shoots, and whether the insects
attack the thistle early or late in the season. Larger galls
on young main stems reduce plant height by more than half. (Harris,
2003) To learn more about the stem gall fly, see: http://res2.agr.ca/lethbridge/weedbio/agents/aurocard_e.htm.
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Thistle Rosette Weevil,
Trichosirocalus horridus |
Thistle
Defoliating Beetle, Cassida
rubiginosa |
Thistle-Stem Gall Fly, Urophora
cardui |
Photos
by Noah Poritz, Bio-control.com
Used with permission |
A flower weevil, Larinus planus, also attacks Canada
thistle and reduces seed production much as the musk thistle-head
weevil does. These larvae feed on the flowers, and the adults
consume foliage. Being an accidental introduction, Larinus weevils
are no longer permitted for interstate transport by USDA APHIS,
but occur in the wild on the eastern and western seaboards of
the U.S. (Poritz, 2002) To learn more about Larinus planus, see: http://res2.agr.ca/lethbridge/weedbio/agents/alaripla_e.htm.
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Flower Weevil,
Larinus
planus |
Stem-Mining Weevil,
Ceutorhynchus
litura |
Photos
by Noah Poritz, Bio-control.com
Used with permission |
A stem-mining weevil, Ceutorhynchus litura, feeds
on young Canada thistle plants. As the thistle plant bolts, the
larvae continue to mine through the stem, eventually causing
exit holes where they leave the stem near the ground. The larvae
pupate in the soil, emerging in late summer to overwinter as
adults. To learn more about Ceutorhynchus litura, see: http://res2.agr.ca/lethbridge/weedbio/agents/aceutlit_e.htm.
Starthistle is attacked by the yellow starthistle bud weevil, Bangasternus
orientalis, and the yellow starthistle hairy weevil, Eustenopus
villosus. Larvae and adult bud weevils feed on the flowers,
thus reducing seed production. The hairy weevil adults feed
on the outside of the flowers, while the larvae feed inside
the flowers. As with the bud weevil, the hairy weevil also
reduces seed production. More information on yellow starthistle
is available from: http://wric.ucdavis.edu/yst/manage/management6.html#insects.
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Yellow Starthistle
Hairy Weevil, Eustenopus villosus |
Yellow Starthistle
Bud Weevil, Bangasternus orientalis |
Photos
by Noah Poritz, Bio-control.com
Used with permission |
Even though research has shown these beneficial thistle insects
to be effective in reducing a stand of thistle, they are often
slow to get established and would not be of much help in freeing
fields of thistle in the first year. On the other hand, once
they are established, and if they are not killed off by pesticides,
they can provide long-term biological control of thistle. In
general it seems apparent that some thistle plants will have
to remain in order for these beneficial insects to retain their
populations. In situations where all the thistles in an area
are being hand cut or spot sprayed, and thus not allowed to complete
their life cycle, most of the above mentioned biocontrol insects
would not sustain themselves. These biocontrol insects seem most
appropriate in remote inaccessible pastures and on range land
where mowing or treating with herbicides is not practical. A
combination of competitive forages, wise grazing management,
and biocontrol provided by these insects can virtually eliminate
thistle by suppressing and stressing the thistle plants out of
existence. Biological Control of Weeds, Inc. (see References),
a Montana-based distributor of beneficial insects, sells several
of the beneficial insects mentioned above that attack musk thistle,
Canada thistle, and yellow starthistle.
High-intensity, short-duration rotational grazing reduces thistle
populations by promoting a dense, competitive stand of forages.
A dense stand of grass minimizes thistle’s ability to get
established from seed. The young seedlings have great difficulty
emerging from below a thick cover of grass or a dense mat of
dead grass on the ground. Additionally, sheep and cows will nibble
at thistles in the rosette stage. Goats eat thistles more frequently,
especially the flowers, which eliminates seed production. Goats
can be run with cattle to generate additional income and control
weeds. Horses, donkeys, and llamas will also eat Canada thistle
flowers.
(Anderson, 2001)
Back to top
Mechanical Control
Musk and other annual or biennial thistles reproduce only by
seed. Tilling, hoeing, or hand pulling should be done before
flowering. Cut the plant below the ground or as close to the
ground as possible to prevent regrowth. Cutting or mowing is
more effective later in the season when the stem core is hollow,
but before flowering. At that time, the plant is least likely
to regrow. If cut in the rosette stage, they will regrow easily.
Mowing can wait until two days before blooming to prevent seed
production. (Anderson, 2001) Mowing or hand cutting only four
days after flowering will allow some seeds to mature. Plants
cut after the flowers open should have the flowers removed. Put
the flowers in a tight container and bury or otherwise destroy
them. Timing a mow can be difficult since thistles don’t
all bloom at the same time. It’s important to reestablish
desirable forage or crop plants with adequate fertility soon
after thistle is controlled, to provide competition to future
thistle plants. Appropriate grazing management that eliminates
bare ground and produces a dense stand of forage goes a long
way toward good thistle control.
Canada thistle, being a perennial, requires repeated removal
of the top growth to weaken or kill the plants. Farmer recommendations
for controlling Canada thistle include repeated mowing and various
tillage operations. North Dakota farmer Terry Jacobson (Bowman,
1997) recommends using a “Noble” wide-blade sweep
plow set with three 7-inch sweeps when thistle plants are in
the purple-bud stage. This plow undercuts surface weeds. A coulter
runs in front of each sweep to prevent clogging. Continue to
till every 21 days until frost. Reportedly, this produces about
95% eradication of Canada thistle. In integrated methods, spottreatment
with a systemic herbicide for these perennials could be an option.
Integrated Methods
California researchers (Thomsen et al., 1996) tested mowing
in combination with controlled sheep grazing and subterranean
clover plantings for starthistle control. The researchers theorized
that the subclover would help fill the niche vacated by the yellow
starthistle populations. They chose an undeveloped pasture dominated
by starthistle and annual grasses. They compared subclover seeding
+ grazing + two mowings, grazing + mowing without subclover,
and an untreated area without grazing, subclover, or mowing.
Thistle seed production was 130 times higher where only one mowing
was done, and 1,720 times higher where nothing was done, as compared
to the area that had been grazed and mowed twice. Thistle flower-head
density was 0.2 per square meter for mowing+grazing, 43 per square
meter for one mowing, and 420 per square meter for no treatment.
This study shows that excellent yellow starthistle control can
be achieved with competition from desirable plants, such as subclover,
combined with mowing and rotational grazing.
Researchers at South Dakota State University (Tonneson, 1988)
tested 56 combinations of crops, cultivations, and sprayings
for their ability to control Canada thistle. Six of them gave
75 to 90% control. They are as follows:
- Cultivate every three
weeks during good growing periods such as June or July, using
a field cultivator or a blade implement or a one-way disc;
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Spray grain crops in the five-leaf stage with 2,4-D Amine at ¾ pound
per acre to reduce stands by 30%, then cultivate three or four
times after harvest;
- Follow small grain harvest by plowing,
then spray thistles that emerge before frost;
- Spray thistles
after small-grain harvest and plow the field in October—this
plan has the hazard of leaving the soil bare over the winter;
- Prepare the soil with a sweep cultivator or blade before
drilling soybeans, forage sorghum, buckwheat, sudangrass,
or millet during late June. Following harvest, plow and cultivate
once or twice;
- Cultivate with a blade or sweep cultivator every three
weeks from June 1 to August 15, then seed alfalfa or a perennial
grass mixed with alfalfa.
In a two-year Virginia study (Kok et al., 1986)
researchers investigated the effect of tall fescue in combination
with two thistle weevil species (Trichosirocalus horridus and Rhonocyllus
conicus). They found that fescue competition had a higher
impact on thistle than either of the weevil species. Musk thistle
seeds had low germination rates when planted in a one-year old
stand of tall fescue. Seeds that did manage to germinate in the
fescue stand grew poorly and never reached their reproductive
stage.
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Least-toxic Herbicide Control
Several herbicides are labeled for thistle control. Information
on these and their rates should be available through your local
Extension office. Using a low-toxicity herbicide for spot spraying
should suffice as a sustainable practice in situations where
thistle density is low.
U.S. Department of Agriculture Researchers in Maryland tested
various strengths of vinegar on Canada thistle. (Daniels, 2003)
They found that either a 5% or 10% solution of vinegar burned
off the top growth of Canada thistle. The plants, however regrew
from the roots. Acetic acid in vinegar kills plant tissue by
dissolving the cell membrane, which causes the plant to dry out.
Vinegar works best when used in the sun. A word of caution, however:
vinegar in concentrations greater than 5% acetic acid may be
hazardous—burning the skin or damaging the eyes—and
should be handled with care. Additionally, vinegar is not registered
with EPA for use as herbicide. (Byczynski, 2003)
Several vinegar-based herbicides have been developed, including
St. Gabriel Labs’ BurnOut Weed and Grass Killer concentrate
and Nature’s Glory Weed and Grass Killer concentrate. These
two come as 25% concentrates with instructions to dilute them
to 6.25% and use on non-crop, right-of-ways, and industrial lands.
(Byczynski, 2003) More dilute products that come ready-to-use
include Fast-Acting BurnOut RTU, Nature’s Glory Weed and
Grass Killer RTU, and Greenergy’s Blackberry and Brush
Block, with acetic acid concentrations from 6.25% to 7%. The
Greenergy product lists acetic acid as an inert ingredient and
citric acid (at 7% concentration) as the active ingredient.
SummerSet Products sells a vinegar-based herbicide called Alldown
Green Chemistry Herbicide that has been approved for use in organic
farming by the Organic Materials Review Institute (OMRI). Alldown
contains vinegar, garlic, and citric acid.
Nature’s Glory Weed and Grass Killer RTU and Fast Acting
Burn Out RTU are registered with EPA. Greenergy’s Blackberry
and Brush Block and Alldown Green Chemistry herbicide have a
25(b), “minimum risk pesticide,” exempt status with
EPA, and so do not have EPA registration numbers. The Greenergy
product lists acetic acid as an inert ingredient and citric acid
(at 7% concentration) as the active ingredient. If an herbicide
has an EPA registration number, it has been approved for sale “at
the Federal level.” However, companies must still register
their products with the individual states to sell them there.
Soil Conditions and Pasture Management
Some farmers report that thistles grow where soil calcium levels
are low, iron is high, and phosphorus is low or complexed. (Anderson
, 2001) Thistles seem to prefer soils high in anaerobic bacteria,
where residue decay is poor, or the soil is compacted. Virginia
farmer, writer, and speaker Joel Salatin offers his experiences
with thistles in his book Salad Bar Beef.
He discusses the poor soil quality on his farm when the family
first moved onto the place in 1961. The pastures were nothing
but dewberries, briars, other weeds, and broomsedge. The farm
grew so many thistles that it looked like a snowstorm when they
baled hay. After several years of controlled grazing and applying
compost, Salatin’s
pasture is lush with red clover, white clover, thick grasses,
and a healthy smattering of forbs. Fewer than a dozen thistle
plants can be found on the whole farm. (Salatin,
1995) Salatin speaks highly about the power of natural succession,
using animals and plants to progress to the next higher level
of plant community—in
this case, one with few thistles. He moves cattle to new pasture
paddocks every 24 hours, allocating the correct amount of forage
they will need each day in a given paddock. Read more about rotational
grazing in the ATTRA publications Rotational
Grazing,
Matching
Livestock and Forage Resources in Controlled Grazing, and
Sustainable
Pasture Management.
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References
Anderson, Lee. 2001. Keeping thistles at bay. ACRES USA. February.
p. 26.
Anon. 2002. Understanding life cycle in helpful in managing
musk thistle.
High Plains Journal. September 9. p. 7-B.
Anon. 1996. Why Canada thistle’s so tough to kill.
Soybean
Digest. April. p. 16.
Biological Control of Weeds, Inc.
1418 Maple Drive
Bozeman, MT 59715
800-334-9363 (toll-free)
406-586-5111
406-586-1679 FAX
bugs@bio-control.com
www.bio-control.com
Bowman, Greg (ed.). 1997. Steel in the Field; a farmer’s
guide to weed management tools. Sustainable Agriculture Network
Handbook Series, Book 2. National Agricultural Library, Beltsville,
MD. 128 p.
Byczynski, Lynn. 2003. The vinegar brouhaha.
IPM Practitioner. September/October. p. 7-8.
Daniels, C.H. 2003. What’s cooking with vinegar recommendations.
Acetic Acid as Herbicide. North Dakota Pesticide Quarterly. Vol.
21, No. 1. January. 8 p.
Greenergy Inc. (maker of Blackberry and Brush Block)
P.O. Box
6669
Brookings, OR 97415
Greenergy@Earthlink.net
www.greenergyinc.com
Harris, P. 2003. Classical biological control of weeds established
biocontrol agent,
Urphora cardui (L.). Stem-gall fly.
Agricluture and Agri-Food, Canada. 5 p.
Available at http://res2.agr.ca/lethbridge/weedbio/agents/aurocard_e.htm.
Hartzler, Robert G. 1995. Evaluation of a biological control
agent for musk thistle.
Leopold Center Progress Report. Vol.
4. p. 77-80
Hoback, W. Wyatt, Kerri M. Skinner, and Leon G. Higley. 2003.
Exotic
Species Curriculum for Agricultural Problem-solving Education.
Kearney,
NE : University of Nebraska . www.unk.edu/acad/biology/hoback/escape/muskthistle_facts.html
Kok, Lok T., T.J. McAvoy, and W.T. Mays. 1986.
Impact of tall
fescue grass and Carduus thistle weevils on the growth
and development of musk thistle. Weed Science. Vol. 34. p. 966-971.
McCarty, M.K. et al. 1969. Germination and Early Seedling Development
of Musk and Plumeless Thistle. Nebraska Agricultural Experiment
Station Resource Bulletin No. 229.
Nature’s Glory Weed and Grass Killer
866-298-2229
www.naturesglory.com
Poritz, Noah. 2002. Personal communication.
Salatin, Joel. 1995. Salad Bar Beef. Polyface, Inc. Swoope,
Virginia. 368 p.
Smith, L.M., II, and L.T. Kok. 1984. Dispersal of musk thistle
seeds.
Weed Science. Vol. 32. p. 120–125.
St. Gabriel Labs – makes BurnOut herbicide
800-801-0061
(toll free)
www.milkyspore.com/burnout.htm
SummerSet Products
584 Kennebec Dr., Eagan, MN 55122
651-452-0599
651-452-4041 FAX
www.sumerset.com
Thomsen, Craig D., William A. Williams, William Olkowski, and
Dave W. Pratt. 1996. Grazing, mowing and clover plantings control
yellow starthistle.
The IPM Practitioner. February. p. 1-4.
Tonneson, Lon. 1988. Fifty ways to win the war against thistles.
The
Farmer. January 2. p. 14.
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Web Resources
Exotic Species Curriculum for Agricultural Problem-solving
Education
By W. Wyatt Hoback, Kerri M. Skinner, and Leon
G. Higley
University of Nebraska, Kearney, NE
www.unk.edu/acad/biology/hoback/escape/muskthistle_facts.html
An
educational tool about musk thistle.
Managing Canada Thistle
Colorado
State University Extension Service www.colostate.edu/Depts/CoopExt/Adams/weed/canada_thistle_mgt.html
Managing Musk and Scotch Thistle
Colorado
State University Extension Service www.colostate.edu/Depts/CoopExt/Adams/weed/msthistle_mgt.htm
Canada Thistle
http://dnr.metrokc.gov/wlr/lands/weeds/thisc.htm
Bull Thistle
http://dnr.metrokc.gov/wlr/LANDS/Weeds/thisb.htm
Musk Thistle Identification and Control
Kansas
State University
www.oznet.ksu.edu/library/crpsl2/sections/l231d.pdf [PDF
204 kb] Download
Acrobat Reader
Canada Thistle
Montana Weed Control Association www.mtweed.org/Identification/Canada_Thistle/canada_thistle.html
Scotch Thistle
http://dnr.metrokc.gov/wlr/lands/Weeds/Scotthis.htm
Yellow Starthistle
IPM Practitioner’s
Association, Eugene, OR
http://members.efn.org/~ipmpa/Noxystar.html
Canada Thistle
by K.G. Beck
www.ext.colostate.edu/pubs/natres/03108.html
Fact Sheet 3.108.
Integrated Management of Musk Thistle
Oklahoma
State University Extension Service
www.ento.okstate.edu/ipm/weeds/muskthistle.html
Management of Canada Thistle
www.co.larimer.co.us/depts/pubwor/weeds/management/MgtCanadaThistle.htm
By Preston Sullivan
NCAT Agriculture Specialist
©NCAT 2004
Edited by Paul Williams
HTML Formatting by J. English
CT156
Slot 199
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