Bull, Plumeless,
Musk and Canada Thistle
Their Biology and
Management in Pastures
Jerry
Doll
Millions of acres of
grass and grass-legume pastures are utilized to support our
important dairy and beef industry. However, weeds may limit the
productiveness of pastures by competing with desirable grass and
legume species. The most troublesome weeds in Wisconsin pastures
are thistles which are especially troublesome because in
addition to lowering productivity of pastures, their spiny
nature prevents livestock from grazing near them. In fact, heavy
thistle infestations may cause large areas of pastures to be
left ungrazed.
Three species of
thistles are of greatest concern. Canada thistle (Cirsium
arvensis) and bull thistle (Cirsium vulgare) occur
throughout Wisconsin and all the North Central states. Musk
thistle (Carduus nutans) is common in the south eastern
counties plus other "hot spot" areas around the state.
Plumeless thistle (Carduus acanthoides) occurs
principally in south western and south central areas Wisconsin.
Some incorrectly call plumeless thistle by the name "Russia
thistle." This can create confusion because another plant's
official name is Russian thistle (Salsola kali).
Thistle Biology
The most common
thistles in pastures have a two-year life cycle. Bull, musk, and
plumeless thistles are biennials and only grow vegetatively
during the first year. Seeds germinate during the spring and
summer when soil moisture and temperatures are favorable. After
germination, they form a rosette ranging from 4 to 18 inches in
diameter before becoming dormant in the late fall of the first
year. Exposure to cold winter temperatures (vernalization) is
necessary to trigger these thistles to flower during the second
year.
In the spring of the
second year, bull, musk, and plumeless thistles resume
vegetative growth. In late May the musk and plumeless thistles
begin to bolt (send up a flower stalk). Bull thistles normally
bolt and flower 2 to 3 weeks after musk and plumeless thistles.
Each plant can send up several stalks and produce many flower
heads, each with many viable seeds. After flowering or with the
first frost, biennial thistles die in the second year. Biennial
thistles reproduce only by seed; therefore, successful
management programs must strive to prevent seed production.
Canada thistle is a
perennial species. Infestations can start from seed but plants
primarily regrow and spread each year from Canada thistle=s
creeping root system. The roots have adventitious buds that form
new shoots each spring and summer. Canada thistle is one of the
first pasture weeds to resume growth after winter. Buds at the
crown and on vertical stems below the soil surface also produce
shoots after mowing.
To measure how many
roots Canada thistle can produce, researchers in three states
planted either a single root segment 12 inches long or a 6-inch
diameter plug of Canada thistle plants in 4x4x8-ft above-ground
boxes filled with soil. No tillage or irrigation were done, nor
were crops planted in the boxes. Within 12 to 16 months, buds on
these roots produced an average of 174 shoots and 930 feet of
new roots, illustrating this weed's ability to spread unless
adequately managed. The weight of Canada thistle roots can
exceed 1600 lb/acre in established stands.
Canada thistle has
male and female flowers on separate plants (dioecious). In
Wisconsin we see Canada thistle patches that consist of only
female flowers and patches of females out number those of male
flowers (personal observation). Nevertheless, all female patches
seem to produce seed so male flower heads must be in the area.
Individual flower heads have about 100 florets and vigorous
stems can produce 50 to 100 flower heads, each with 80 to 90
seeds. Viable seeds are formed 8 to 10 days after flowering and
single plants can release more than 5,000 seeds. Long distance
dispersal by wind is unlikely because the seeds often remain in
the flower head while the pappus detaches and floats away.
However, seed attached to the pappus may move several to many
feet from the parent plant under the right conditions.
Thistle
Management
Cultural
Cultural weed control refers to management practices that
establish and maintain productive, weed-free pastures. One of
the most important cultural methods of pasture weed control is
rotational and controlled grazing. Overgrazing weakens the
pasture species and makes them less competitive with weeds.
Controlled and rotational grazing helps avoid weed invasions.
Pastures should be limed and fertilized periodically to keep
them healthy and vigorous. These practices help grasses and
legumes recover quickly after being grazed, thereby keeping the
upper hand over aggressive weeds.
Avoid spreading manure
contaminated with weed seeds. Do not move animals from
weed-infested pastures into weed-free pastures without a
quarantine period to allow them time to clean themselves of weed
seeds in their digestive systems. Likewise be sure to clean
mowers or choppers after leaving weed-infested pastures so that
seeds are not transported to other pastures. Keep fence rows
weed-free to prevent weeds from migrating into pastures.
Crop rotation is not
an option in permeant pastures. However, it is a valuable weed
management tool for temporary pastures. Crop rotation kills
biennial thistles because they cannot tolerate tillage or crop
competition. Rotating temporary pastures infested with musk or
plumeless thistles to small grains, corn, or soybeans will
control these thistles. Rotation alone will not affect Canada
thistle but does allow you to use a management strategy you
could not use in pasture: planting a glyphosate resistant crop
of corn or soybeans and applying Roundup when Canada thistles
are in the bud to early flower growth stage.
Biological - Musk
thistle The rapid spread
of musk thistle in North America is due in part to the lack of
natural enemies. Scientists found a seed-eating weevil (Rhinocyllus
conicus, the musk thistle weevil) in Italy, the original
home of musk thistle, that offered the best potential for
controlling musk thistle in North America. After extensive tests
to prove that the weevil would only feed on thistles, it was
imported and released in several states in 1969.
The musk thistle
weevil has one generation per year and overwinters as an adult.
In the spring, adults move from their overwintering sites to
musk thistles before and during the bolting stage. Females lay
100 to 150 eggs either singly or in clusters of 4 to 5 on the
underside of developing flower heads. The eggs hatch 6 to 8 days
later and the larvae burrow into the seed producing tissues of
the flower head. Larval feeding can last 25 to 30 days after
which larvae create individual cells in which they pupate.
Pupation last 1 to 2 weeks and then in July adults emerge from
the seed heads and begin seeking overwintering sites.
Once the weevil
infestation is sufficiently high, most seeds are destroyed. As
many as 40 larvae per head have been found and each one destroys
approximately ten developing musk thistles seeds. After several
years of effective and widespread larval feeding, the thistle
population is reduced because musk thistle reproduces only by
seeds. Successful biocontrol programs exist in Montana,
Virginia, Missouri, Nebraska, and other states.
These weevils were
released in southeastern Wisconsin in the mid 1970's. They have
survived and moved several miles from the release site. However,
little evidence of reduced musk thistle populations is noticed.
Part of the reason is that the weevils attack primarily the
"first generation" of flower heads. Additional flowers
arise from secondary branches and flower 10 to 20 days later.
The weevils are no longer laying eggs when these flowers appear
and they proceed to produce seed. Secondly, no attempt to spread
the weevils to new sites has been made because the Department of
Natural Resources is concerned that the insect might attack the
dune thistle (Cirsium pitcherii), a threatened species,
found on the shores of Lake Michigan. While further releases are
not permitted, the weevils already present should continue
spreading and may reduce the musk thistle population somewhat.
Unfortunately, this
weevil is much less effective on plumeless thistle and no other
biocontrol organisms are close to being released for this, the
most serious of our biennial thistles. Biological control alone
will never eradicate a thistle infestation, but a successful
program should help reduce thistle populations, especially in
undisturbed, remote, or inaccessible areas.
Most releases of
biocontrol agents are based on the assumption that the food
chain is simple and unbranched. In such situations, the weevil
would have a very high specificity for the target weed, in this
case musk thistle. However, the Rhinocyllus conicus weevil
has attacked five native thistles species in national parks and
preserves. Seed production of the desired Platte thistle in
Nebraska was reduced up to 86% by the weevil. Weevil populations
increased sharply in all sampled sites 1992 and 1996. This
illustrates that every control strategy has both benefits and
risks.
Biological - Canada
thistle A natural
organism, Pseudomonas syringae pv tagetis, infects
Canada thistle in non-disturbed sites like pastures and
roadsides. Once infected, the organism produces a toxin that
inhibits chloroplast formation and plants appear cream colored
to yellowish white. This weakens plants and minimizes flowering
and seed formation. Canada thistle populations infected plants
often diminish and on occasion completely disappear over a
period of years.
Bacterial pathogens
have not been practical biocontrol agents of weeds because
plants need to be wounded to initiate infection, but the
University of Minnesota developed a new inoculation technique
that greatly increases the infection rate. The use of "superwetter"
oganosilicon surfactants seems to achieve infection via the
stomata and inoculation before plant reached the three to four
leave stage (plants 3 to 4 inches tall) gave severely bleached
and stunted thistles that compete poorly with other plants.
Infected plants usually fail to flower and often die.
A commercial
formulation of the bacterium was field tested in many states in
1995 but failed to induce the typical symptoms. Effort continue
to find ways to make the spray application of the bacterium
successful in the field. Mean while, Mother nature continues to
spread the bacterium and this can only help weaken the
competitive ability of Canada thistle.
Mechanical Persistent
and timely mowing, clipping or hand weeding can greatly reduce
biennial thistle infestations but will have minimal effect on
Canada thistle. Biennial thistles should be cut as close to the
ground as practical each time they begin to flower. Field
observations indicate that bull thistles have less regrowth
capability after clipping than musk or plumeless thistles. This
may explain why bull thistles are not increasing in seriousness,
while musk and plumeless thistles are spreading into new areas
and becoming more abundant in already infested pastures.
Nevertheless, repeated, timely mowings are beneficial because
they reduce seed production. To prevent seed production for musk
and plumeless thistle, plants must be cut at the soil surface;
higher cutting allows crown buds to resprout and flower.
Chemical Even
with the best cultural and mechanical efforts, thistles may
become established in pastures. Fortunately, effective and
economical herbicides are available to control thistles in
pastures. Timeliness of application is the key to success and
this depends wholly on the plant's life cycle.
Biennial thistles
must be treated when plants are in the rosette stage. After
bolting, they become very tolerant of most herbicides. A
treatment of 2,4-D amine or ester at 2 qt/acre in early May or
mid-September gives very effective control (Table
1). In the
spring and early fall, biennial thistles are in the rosette
stage and are very easily controlled by any treatment applied.
Once thistles start to bolt (late May) or flower (mid June),
dicamba (Clarity or Banvel) alone or the combination of 2,4-D
and dicamba (sold commercially as Weedmaster) are more effective
than 2,4-D alone. The ester formulation of 2,4-D should be used
unless crops sensitive to vapor drift are in close proximity to
the application site. The data in Table 1 illustrate the
importance of application timing to control plumeless thistle;
similar results would be expected for musk and bull thistle.
Treatment timing is much different for perennials than
biennials. Effective herbicides are systemic (they move from the
foliage into the roots) and this movement is much greater when
plants are in the bud to early flower growth stages than at
earlier stages. Recommended herbicides include dicamba (Clarity
and Banvel), clopyralid (Stinger), metsulfuron (Ally), and
triclopyr plus 2,4-D (Crossbow). A single application of any
product will reduce but not eliminate Canada thistle
infestations. Stinger provides the best long-term suppression
but it is also the most expensive product. All herbicides that
kill Canada thistle also kill forage legumes. Glyphosate gives
excellent Canada thistle control but kills all treated
vegetation; so if glyphosate is used, treated areas will need to
be reseeded.
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