Musk thistle [Carduus
nutans L.][CRUNU][CDFA: A] Photographs
Map of Distribution Biocontrol
Giant plumeless thistle or Plumeless thistle [C. acanthoides L.][CRUAC][CDFA:
A] Photographs Map
Italian thistle [C.
pycnocephalus L.][CRUPY][CDFA: C][CalEPPC: B] Photographs
Slenderflowered thistle [C. tenuiflorus Curtis][CRUTE][CDFA: C]. Photographs
GENERAL DESCRIPTION:Erect thistles with prickly winged stems and leaves. Plants
exist as basal rosettes until flowering shoots develop at maturity.
Refer to the table Carduus thistles for a quick comparison of distinguishing characteristics. Also
see Comparison of spiny-leaved thistles.
The thistle head weevil (Rhinocyllus conicus), an introduced biocontrol
agent, attacks Carduus species and several other thistles, including
some native thistles (Cirsium spp.). Control of Carduus thistle
infestations by the weevil varies by species and regionally from excellent
to poor. The weevil is established in California and much of the northwestern
and north central U.S. The fungus musk thistle rust (Puccinia carduorum)
has recently been found in California and may soon be state approved as a
biocontrol agent to help control musk thistle.
SEEDLINGS:musk thistle: Cotyledons nearly sessile, oblong, with tips often
squared, 7.5-15 mm long, 2.5-6 mm wide. Cotyledon veins are white
and broad. First 2 true leaves appear opposite. Subsequent leaves
are alternate and form a basal rosette. Leaves are pale
green, waxy, oval to elliptic, shallowly lobed, and irregularly
prickly toothed. Hairs are sometimes scattered on the upper
surface and the main veins of the lower surface. Seedling descriptions
are unavailable for plumeless, Italian, and slenderflowered thistles,
but given the similarity of mature plants, seedlings of these
species probably closely resemble those of musk thistle.
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branched near the top. Basal leaves elliptic to lanceolate, pinnately
lobed, and with prickly-toothed margins. Stem leaves
alternate, reduced, with bases that extend down the stem forming
spiny wings (decurrent).
and UNDERGROUND STRUCTURES:Taproots long, thick, fleshy, occasionally branched,
capable of penetrating the soil to depths of 40 cm or more.
of deeply lobed, purple to pink, (rarely white) disk flowers. Phyllaries
spine-tipped,overlapping in several rows. Receptacles flat, densely
covered with cream-colored bristles interspersed among the disk
flowers. Insects pollinated.
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and SEEDS:Achenes elliptic, curved, slightly compressed, sometimes
slightly 4 to 5 sided in cross section, smooth, glossy, golden
to brown. Pappus bristles numerous, cream-colored, fine,
minutely barbed (with magnification), united at the base to
form a ring and deciduous as a unit.
CHARACTERISTICS:Foliage is killed by hard frost, but plants remain
intact for an extended period after death. The persistent spiny
character of the foliage helps to distinguishes plants.
colonize disturbed open sites, roadsides, pastures, annual grasslands,
and waste areas.
time, populations of musk and giant plumeless thistle
are limited to specific regions. Italian and slenderflowered
thistle are widely distributed.
by seed. Seeds fall near the parent
plant or disperse by wind, water, birds, small mammals, and human
ADDITIONAL ECOLOGICAL ASPECTS:Musk
thistle seeds appear to possess allelopathic qualities. They
can inhibit germination and radicle growth in other pasture species,
but stimulate or have no affect on other seeds of their own species.
This suggests that the allelopathic potential of musk thistle
seeds may be an evolved mechanism to encourage its own establishment.
Emerged musk thistle plants can also weaken other pasture
species by an allelopathic interaction at the early bolting stage,
when the larger rosette leaves are decomposing and releasing soluble
inhibitors, and at the stage when bolting plants are dying and
releasing insoluble inhibitors. No specific chemicals have been
identified. Although musk thistle is sometimes associated
with fertile soils, it is more likely to increase in situations
of declining fertility. Furthermore, it has the potential to induce
long-term decline of soil nitrogen input. This appears to be related
to its allelopathic activity. Decomposing rosette leaves have
a strong potential to inhibit white clover (Trifolium repens)
nitrogen fixation. Thus, dense musk thistle stands create
conditions that favor their proliferation and are unsuitable for
white clover establishment, persistence, and ultimately nitrogen
FAVORING/DISCOURAGING SURVIVAL:Thistles compete poorly with healthy established
grasses and other vegetation. Disturbances such as fire, overgrazing,
or trampling create prime sites for thistle colonization.
SPECIES:Canada thistle [Cirsium
arvense (L.) Scop.][CIRAR], bull thistle [Cirsium
vulgare (Savi) Ten][CIRVU], and Scotch thistle [Onopordum
acanthium L. ssp. acanthium] may be confused with Carduus
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Good range and pasture management techniques, including grazing, cutting and
forage production, can reduce weed establishment and impact. This includes
using certified seed, clean hay, bedding, and equipment, avoiding overgrazing
and poor fertilization, keeping vehicles and grazing animals out of infested
Mechanical: Mowing can help reduce seed production but mowing alone
will not eliminate an infestation. Early mowing is ineffective for control
of musk thistle. The optimum mowing timing is 2 to 4 days after initial flowering.
Mowing 3 ft tall musk thistle plants to a 6 in stubble will prevent seed production,
but thistles quickly recover from remaining buds near the base. Tillage can
also be used to control musk thistle. However, this technique is not always
practical in non-crop areas.
Cultural: Prescribed burning will remove dense stands of mature thistles
and create a good environment for subterranean clover to germinate and grow.
However, burning may not completely control plants still in the basal rosette
stage. Thistle establishment is less likely if desirable vegetation remains
dense throughout the year. Many thistle problems occur when range or pastures
are overgrazed in summer and early fall, or when conditions, such as drought
stress or poor fertility leave bare soil. Targeted grazing of thistle with
goats and other farm livestock provides a useful technique to control thistle.
Cattle and sheep prefer the vegetative tissues of musk thistle. In contrast,
goats virtually ignored the leaves of musk thistle, but relish the flowers.
Even in the presence of palatable subclover and grass pasture, goats seemed
to prefer musk thistle flower heads. Thus, goats will drastically reduced
average seed production per plant. Seeds ingested by goats and other ruminants
are nearly all digested and are not spread within the feces. The use of goats
and other livestock can represent an important management technique and can
be effective in a long-term integrated approach for the control of musk thistle.
Musk thistle germination is greatest on bare ground. Control of musk thistle
is maximized when range or pasture cover is dense during the weed seedling
emergence period. It has been reported that allelochemicals released by some
pasture and range species could be partially responsible for inhibition of
seedling emergence and growth. Grasses inhibit seedling emergence and subsequent
growth and survival of rosettes to a significantly greater level than legumes.
Once seeds have germinated, dense grass and legume cover provides shading
of developing rosettes and suppresses the growth of musk thistle. Perennial
grasses are more effective than annual grasses or legumes.
Biological: Three insects have become established for the control of
musk thistle; thistle head weevil (Rhinocyllus conicus), thistle crown weevil
(Trichosirocalus horridus), and thistle crown fly (Cheilosia corydon). Rhinocyllus
conicus was the first species released in the United States for control of
musk thistle. It has one generation per year. It lays its eggs on bud bracts
and the larvae infest the seed head or stem. The larvae feed on the seeds
and are more destructive than other insect stages. The thistle head weevil
infests a number of host genera in the thistle tribe, including species of
Carduus, Cirsium, Onopordum, and Silybum. It has proven to be a very effective
control agent on musk thistle. Trichosirocalus horridus also has one generation
per year. Its larvae feed on the growing tip of the thistle rosette and the
adults may also slightly defoliate plants. Like Rhinocyllus conicus, it can
attack other thistle species, including plumeless thistle, Italian thistle,
Canada thistle, bull thistle, and Scotch thistle. Suppression of musk thistle
is only slight thus far, and requires other biocontrol agents to be present.
Cheilosia corydon is a fly that also produces one generation per year. Its
larvae damage the leaves, stems, and crown of musk thistle, Italian thistle,
and plumeless thistle. This organism can lower total seed production and can
kill the plant when it infests roots. It was only released in 1990, so little
information is available on its effectiveness.
Chemical: Few herbicides provide effective preemergence control of
musk thistle in rangelands and pastures. Chlorsulfuron has both pre- and postemergence
activity. Preemergence application with chlorosulfuron (0.75 - 1.5 oz ai/A)
in the fall are not very effective for control of seedlings or mature plants.
However, treatment with chlorsulfuron (0.37 - 0.75 oz ai/A) in early bloom
stage reduced seed production by over 99%. Several postemergence herbicides
will control musk thistle. Typically, spring treatments give better control
than fall herbicide applications, as many new seedlings which emerge after
a fall treatment will escape injury. Dicamba, 2,4-D, clopyralid, MCPA, glyphosate
and combinations of these compounds provide excellent control with a spring
application, and somewhat less control with a fall treatment. The table below
lists the herbicides, rates, and timing that provide effective control of
musk thistle. Picloram (0.25 lb ae/A) also gives excellent control of both
musk thistle seedlings and mature plants following a spring or fall treatment.
However, this compound is not registered for use in California.
Table 1. Herbicide recommendations for musk thistle control.
Combinations of 2,4-D with either clopyralid or glyphosate can also be effective.
Rope wick applications of glyphosate and 2,4-D shows good activity on musk
thistle up to 2 ft tall. However, only bolted plants are controlled with this
treatment. Those plants that have not yet elongated above the forage canopy
will not be contacted by the wick. It is important to recognize that there
are grazing and cutting restrictions for many of these postemergence herbicides.
This restriction period is provided on the herbicide label. In New Zealand,
MCPA resistant musk thistle developed in dairy and sheep pastures following
continuous use of the herbicide. This population was found to be cross resistant
to 2,4-D, MCPA, and MCPB, but not resistant to dicamba, clopyralid or picloram.
Even without the use of 2,4-D, MCPA or MCPB it is unlikely that the proportion
of resistant individuals will decrease over time, as these plants are no less
competitive than the susceptible individuals within the population. In addition
to musk thistle, a population of Italian thistle is also suspected of having
developed resistance to the phenoxy herbicides in New Zealand.
Integrated Weed Management: Using an integrated approach, the application
of sublethal applications of phenoxy herbicides such as MCPA, 2,4-D amine
and 2,4-DB amine in conjunction with heavy stocking rates of grazing livestock
(sheep) has been a long accepted control method in Australia. Integrating
late herbicide application (bolting or bud stages) and infestation with the
musk thistle seed head weevils can provide excellent control and reduced herbicide
use and costs.
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