# *Carduus pycnocephalus* 

**Italian thistle, slender thistle, shore thistle, sheep thistle
(Australia) winged thistle (New Zealand)**

# Asteraceae

The following description of *Carduus pycnocephalus* is adapted
from Munz and Keck (1973), Parsons (1973), and Goeden (1974).  

The name *Carduus* comes from the old Latin name for thistles. *Carduus
pycnocephalus*, a member of the Thistle tribe of the Composite family,
is an erect winter annual herb 3-18 dm tall. The stems are slender and
narrowly spiny-winged, especially below. The lanceolate leaves are
pinnatifid, to about 12 cm long, with spiny-tipped lobes and teeth,
white-woolly below, greenish but more or less arachnoid above. Branches
end with a cluster of one to five rose-purple subcylindric flowerheads;
the disk 1.5 cm wide. The phyllaries are not membranous- margined but
are more or less persistently floccose-tomentose. The corolla lobes are
about three times as long as the throat.

The akenes are light tan or buff, about 20- nerved, and 5-6 mm long. The
pappus is dull and 15-20 mm long. It reproduces only by its windborne
or otherwise easily disseminated pappus achenes. Heterocarpy is
well developed in *Carduus pycnocephalus* (Bendall 1974).
Hybridization between *Carduus pycnocephalus* and *Carduus* tenuiflorus
has been reported in Europe (Batra et al. 1981).

*Carduus pycnocephalus* can be distinguished from other thistle species
by its relatively small and few terminal flowerheads and narrow
phyllaries with copious tiny, firm, forward-pointing hairs, especially
on the midrib (Hitchcock and Cronquist 1973).   

*Carduus pycnocephalus* originated in western and southern Europe
but today is widespread throughout temperate parts of the world. It is
a serious pest in Australia, New Zealand, South Africa, Pakistan,
Iran, and Europe. In the U.S. it is found in only a few parts of Texas
and Arkansas but is rapidly spreading and \"out of control\" in most
of California (Dunn 1976).

Italian thistle apparently arrived in California during the 1930s
(Goeden 1974). It displaces desirable forage or cover plants, but more
commonly colonizes disturbed habitats where interspecific competition is
less intense. It is most abundant in coastal areas and occurs as a weed
of pastures, ranges, roadsides, rural areas, fallow cropland, railroad
rights-of-way, field margins, and ditchbanks (Goeden and Ricker 1978).

*Carduus pycnocephalus* reproduces only by seed. It prefers soils
of high fertility, and its seedlings establish best on bare or
disturbed sites (Wheatley 1971, Parsons 1973). If there is reasonable
ground cover during the late summer and autumn the thistle will not
invade a site, but it will come in following overgrazing or creation of
fire breaks (Parsons 1973). Drought favors a rapid increase in the
thistle population. On soils of naturally high fertility, thistle
invasion can be expected at an earlier stage than on poorer soils.
Thistles will invade basalt soils earlier than granite soils, and
granite soils before sedimentary soils (Wheatley and Collett
1981).  *Carduus pycnocephalus* commonly colonizes disturbed habitats
where interspecific competition is less intense (Goeden 1974).
However, this weed does displace more desirable forage or cover plants.
The blanketing effect of over wintering rosettes can severely reduce
the establishment of other plants, as the leaves of the rosette can
become erect in dense stands (Parsons 1973).  

Italian thistle does not reproduce vegetatively, but its seeds are well
equipped for dispersal by wind because of the large pappus
and relatively small size. The distance that seeds can be spread by
wind is not known, but it is at least several hundred meters. Seeds
are also spread when infested pastures are cut and the hay fed to
animals on clean areas. Seed dispersal by water and on animals and
machinery is less important (Parsons 1973). Ants may also play a role
in dispersing the seeds (Uphof 1942). Italian thistle seeds are
mucilaginous, unlike most other thistles. The mucilage is abundant and
adhesive enough to aid in seed dispersal (Evans et al. 1979).  Italian
thistle seeds exhibit polymorphism, with brown seeds that have less
mucilage and germinate at lower temperatures than silver seeds.  The
brown seeds do not usually dehisce from the seedheads, and this may be
important in the establishment of these seeds in the seedbed litter
(Evans et al. 1979).

Seed germination rate in *Carduus pycnocephalus* is very high,
ranging between 83-96%. The seeds germinate at a wide variety of
constant and alternating temperatures. The greatest diurnal fluctuation
that supported optimum germination was 10 C for 16 hours and 35 C for
8 hours in each 24-hour period. Even with freezing temperatures
during the daily cold period, germination was optimum if warm-period
temperatures were from 5 to 20 C (Evans et al. 1979).  No after-ripening
is required, and seeds can germinate either rapidly or after a long
dormancy period. Seeds of Italian thistle exhibit rapid germination
(within 2 weeks) at optimum temperatures (Evans et al. 1979). Bendall
(1974) found that 85% of *Carduus pycnocephalus* seeds produce
germination inhibitors, but they are readily leached.  The length of
time the seeds can survive in the soil is not known but appears to be at
least 8 years (Parsons 1973).  Italian thistle can germinate at a
variety of soil depths. Generally it does poorly on the surface of a
bare seedbed, but on the surface of clay soils it shows 70% germination.
At a depth of 0.5-2.0 cm germination is highest, but some seeds
germinate to a depth of 8 cm (Evans et al. 1979). Seeds buried 1.3 cm
deep show the highest percentage emergence, whereas 20 to 25% of seeds
buried 5 to 10 cm deep remain dormant.  The growth of Italian thistle is
favored more by the addition of nitrogen than by phosphorus or
potassium. High pH (6.5) also favors growth (Bendall 1975).  

Italian thistle has been rapidly spreading on rangelands
previously dominated by alien annual grasses (Evans et al. 1979). This
is partly due to its germination requirements and timing. Italian
thistle germinates at temperature and moisture regimes and in
seedbed environments which would inhibit the germination of the alien
annual grass species that presently dominate California grasslands.
The seeds start to germinate in the fall with the first effective rain.
 Seedlings grow through the winter as rosettes and produce
flowering stalks in the late spring before the summer drought

Though some *Carduus* species are known to accumulate nitrates in
toxic quantities, Italian thistle has apparently not been incriminated
as a toxic weed (Goeden 1974). The primary threat of this weed is
its ability to dominate sites throughout California. Italian thistle
is a problem on Nature Conservancy property in this state and
presents additional problems on grazed pastures. It reduces the
establishment of annual grasses and reduces the value of hay and other
crops due to the blanketing effect of the overwintering rosettes, high
rate and timing of germination, and its broad range of germination
conditions.   

With the right combination of control measures, it should be possible to
eliminate Italian thistle from selected areas. Its inability to
reproduce vegetatively makes control easier, but constant monitoring
will be necessary due to its potentially long seed dormancy (to 8
years).

Monitoring is needed to determine the effectiveness of any
Italian thistle control measures. The necessity for constant monitoring
is all the more important given this weed\'s potentially long
seed dormancy period (to 8 years).   

A great deal of research has been conducted on *Carduus*
species throughout the world, but much more needs to be done on *Carduus
pycnocephalus*. Additional research needs to be conducted on
insects that can be used to control Italian thistle and on the
potential impact of these insects upon endangered native *Cirsium*
species (Kok et al. 1982). Several management techniques appear
promising, and integrated control operations involving a combination of
practices need investigation. The effects of prescribed fire on Italian
thistle also need investigation.   

Control of Italian thistle requires active management once it
becomes established in an area. Without management it cannot be
eliminated and may completely carpet the site.   

Cultivation before seed production will eventually eliminate
thistles, but only if repeated for several years. According to Wheatley
and Collett (1981), hand-hoeing is effective for small patches, but
make sure to sever the root a good 10 cm below ground level. Mowing
or slashing is not always reliable because the plant can regrow from
the base and produce seeds very quickly. Similarly, plants which are
cut close to flowering time can produce seed on the cut portion.
A significant amount of seed can be produced even if thistles
are constantly mowed at 8 cm (Tasmanian Department of Agriculture 1977).
 Slashing is more effective than mowing as it destroys the aerial
part of the plant more thoroughly (Parsons 1973).  For larger areas
where the thistles are dominant, cultivation and cropping is a
successful method of control provided a vigorous perennial pasture is
established immediately after the cropping phase.  In high fertility
situations, using a roller to compact the soil is recommended during
seedbed preparation (but not during seed sowing).  This usually forces a
massive germination of thistles that can be destroyed during cultivation
(Wheatley and Collett 1981).  

Grazing by sheep, goats, and horses can be effective in
controlling thistles, but cattle are of little value (Parsons 1973).
Bendall (1974) describes a grazing control method that has proven
successful in Australia: thistle-infested areas are closed to grazing in
the fall when seedlings appear. They are left ungrazed until the
pasture has reached a height of 10-15 cm (about 6 to 10 weeks). The
areas are then heavily grazed with sheep at more than twice the normal
stocking rate. Sheep selectively graze the tender thistles and will
kill 90-95% of the weeds. Only 2-3 weeks should be required for control.
 For this method to be successful, the autumn grazing break is necessary
so that vigorous growth of other plants is allowed to occur, forcing the
thistles to grow tall and tender. Continuous grazing significantly
reduces thistle numbers but is not as effective as the use of an autumn
break (Bendall 1973).  

Biological control techniques for *Carduus* species have
been extensively studied. *Carduus pycnocephalus* was one of the first
weeds selected for biological control study by the USDA (Schroeder
1980).  The search for its natural enemies has included Italy, Greece,
Iran, and Pakistan, as well as southern California (Baloch et al.
1971, Baloch and Kahn 1973, Goeden 1974). According to Goeden
(1974), Italian thistle serves as an alternative food-plant or breeding
host to a diversity of phytophagous insects, most of which are
euryphagous, ectophagous, sap- or foliage-feeding species.   In southern
Europe, more than 80 species are associated with Italian thistle; about
one third of them are stenophagous and restricted to host plants
belonging to the tribe Cynareae.   In southern Europe, all major parts
of Italian thistle plants are damaged by one or more insect species,
whereas in southern California the thistles are relatively free of
insect damage. In California, more than 40 species of indigenous or
introduced phytophagous insects have adopted this alien weed as an
alternate food-plant, at least 15 of which also find it a suitable
reproductive host. Unfortunately, half of the identified species of
insects found feeding on Italian thistle in southern California are also
pests of cultivated plants, thus not good choices for biocontrol.  

Only three insect species appear to hold promise as biological
control agents in California (Goeden 1974). These species are
*Psylliodes chalcomera, Rhinocyllus conicus,* and *Ceutorhynchuys
trimaculatus*. All three species are unknown as artichoke or safflower
pests, apparently only reproduce on Cadruinae, cause injury to vital
plant parts at a critical growth stage of their host-plant (and thus
appear capable of influencing the reproductive potential of Italian
thistle), and occur over a relatively wide geographic area.

However, according to Charles Turner (1985) of the USDA Biocontrol Lab
in Albany, California, it is possible that these insects may also prey
on several of the endangered native thistles in the genus *Cirsium*.
Because of this concern, their use has been somewhat limited.  

*Psylliodes chalcomera* was a fairly consistent associate of
Italian thistle in its vegetative and early reproductive stages
throughout central and southern Italy. The adults are foliage feeders,
but more importantly the larvae mine crowns of rosettes and tips of
expanding and expanded shoots, blasting the latter, and thus reduce
the production of flowerheads. This species has been studied in depth
by USDA entomologists as a natural enemy of musk thistle, but
certainly should also be considered for importation into California for
Italian thistle control\" (Goeden 1974).  

Larvae of *Rhinocyllus conicus* feed within the flowerheads of
Italian thistle, mining the receptacle and destroying the developing
achenes (Goeden 1974, 1978). This weevil mainly reproduces on
certain thistles belonging to the *Carduus*-*Silybum*-*Cirsium* complex.
It survives best at low temperatures with short photoperiods (Kok 1979).
 *Rhinocyllus conicus* was first introduced into Canada in 1968 for
the biological control of musk thistle (*Carduus* *nutans* L.) and
plumeless thistle (*Carduus* *acanthoides* L.) (Harris and Zwolfer
1971).   It was imported into California in 1969 for the biological
control of milk thistle (*Silybum* *marianum* Gaertn.) (Hawkes et al.
1972) and in 1973 to control Italian thistle in southern California
(Goeden and Ricker 1978). After their introduction to See Canyon in
southern California, the weevil destroyed 90% of the achenes and
infested 91% of the capitula; however, the population of Italian thistle
did not decline (Goeden and Ricker 1978). The weevil was released
on infestations of Italian and slender-flower thistles at 16 sites in
11 counties in northern California during 1975-77 with establishment
at most of the sites (Hawkes et al. 1978).  

*Ceutorhynchus trimaculatus* occurs in Europe and northern Africa
(Bolt et al. 1980). \"Larvae thought to be *Ceutorhynchus trimaculatus*
were recovered from mines in crowns of Italian thistle rosettes in
central Italy, though only adults were positively identified as
fairly consistent associates of this plant. This weevil has been studied
in depth as a candidate biological control agent by USDA
entomologists and may be usefully employed against Italian thistle and
other *Carduus* species in North America in the near future\" (Goeden
1974).  

The fungal rust *Puccinia cardui-pycnocephali* Sydow, is known to
occur only on the genus *Carduus* (Batra et al. 1981). Repeated
greenhouse inoculations of the rust on growing rosettes significantly
reduced the growth of Italian thistle but not the number of florets
(Oliveri 1984). In field situations, the added hardship of intra-
and interspecific competition may cause a greater effect on
thistle populations. Optimum conditions for rust infection (18 to 20 C,
90 to 100% relative humidity) are likely to occur in autumn, in regions
with a Mediterranean climate. This period also corresponds to
the germination and vegetative growth periods of Italian thistle
(Oliveri 1984).

Two other rusts, *Puccinia centaureae* D.C. and Puccinia galatica Sydow
are also reported to attack *Carduus pycnocephalus*, but their impact
has not been researched (Batra et al. 1981).  

The use of herbicides to control Italian thistle may not be appropriate
on natural areas such as Nature Conservancy preserves.  Near streams or
lakes particular cautions should be taken when using herbicides. Prior
to using any herbicide, check with the County Agricultural Commissioner
to determine which chemicals are legal to use in a given situation. The
labels also give more precise information on mixing and safety
precautions. A Certified Pest Control Applicator should be hired for
large jobs or those requiring nonselective herbicides.  

A variety of herbicides have been used on Italian thistle, but they give
only temporary control (Wheatley and Collett 1981).  Picloram (Tordon):
Dr. Jim McHenry (1985) of the University of California, Davis,
recommends picloram to control Italian thistle on Nature Conservancy
lands. Since it is a nonselective herbicide, it must be carefully
applied. It is most effective when applied in February or March at 1/8
to 1/16 lb acid equivalent per acre.  Picloram kills half the test
animals (LD50) at 8000 mg/kg body weight and is considered to be of
\"relatively no hazard.\" One drawback is its long persistence in the
soil, up to 18 months.  

2,4-D is a phenoxy-type herbicide used for broadleaf weed control that
works as a selective hormone or growth regulator. 2,4-D does not affect
grasses. It is foliar absorbed and translocated, making it effective in
destroying the roots. 2,4-D is available in ester, amine, emulsifiable
acid, and low volatile ester formulations.  The chemical is noncorrosive
and is generally considered nonharmful to wildlife. 2,4-D is lethal to
50% of tested animals (LD50) at 300-1000 mg/kg body weight and is
classified as being of \"little\" to \"some\" hazard. It persists in the
soil for 1-4 weeks.   2,4-D has been applied to Italian thistle with
limited success (Taylor 1977). 2,4-D ester should be applied when the
thistles have a central stock height of no greater than 0.25 m (Wheatley
and Collett 1981).  Application should be at the rate of 1-1.5 lbs/100
gallons of water with 1 quart of surfactant/100 gallons. Surfactants
affect the surface property of the spray by lowering surface tension to
increase the herbicide\'s effectiveness.  2,4-D can be used in
combination with biological control measures to control *Carduus*.
Several recent studies (Kok 1980, Trumble and Kok 1980a, 1980b) have
shown that the weevil *Rhinocyllus conicus* is not adversely affected by
field applications of 2,4-D.  

There are several petroleum oils used for weed control. The herbicidal
use of oils depends on their chemical and physical properties. Most
contact oils evaporate slowly and owe their plant toxicity to their high
content of aromatic compounds. Spraying oil on thistle will be effective
only if entire plants are coated.  

 Herbicides can be applied uniformly over an area (for
large infestations) or by spot spraying only the individual plants.
Dr. McHenry recommends using a flat-fan nozzle (Spraying Systems Co.
#8003 or 8004 nozzle tip) rather than the cone nozzles available on
most garden sprayers. Cone sprayers produce greater atomization of
the chemicals and increase the chance of drift into unwanted
areas. Spraying should be done on calm days with dry plants (dew or rain
will dilute the herbicide, reducing its effectiveness). When
spraying large areas, a horizontal boom (6-8 feet long) made from
aluminum tubing will improve herbicide coverage.  

*Carduus pycnocephalus* is present on both the Ring Mountain and Jepson
Prairie preserves in California, with Ring Mountain having the most
significant infestation. Although other thistle species present greater
problems on these preserves, Italian thistle control has been included
as an adjunct to other control efforts. Former preserve manager Greg
Wolley (1986) has used both hand pulling and cutting to destroy the
plants at Ring Mountain Preserve, CA. This has proven most effective
during the spring and early summer.  Contact: Larry Serpa, Area Manager,
3152 Paradise Drive, Room 101, Tiburon, CA 94920 (415) 435-6465    

*Carduus pycnocephalus*, a vigorous annual thistle, arrived in
California during the 1930s and has since become a serious weed problem.
It occurs in a variety of disturbed habitats and germinates rapidly and
in large numbers. Hand pulling, cultivation, and grazing are all
effective control measures.  Biological control agents, particularly the
weevil *Rhinocyllus conicus* and the rust *Puccinia
cardui-pycnocephali,* show considerable promise in controlling Italian
thistle. Chemicals such as Picloram and 2,4-D may be of some use in
controlling the weed, but an integrated management program involving a
combination of techniques will prove most effective.   

CITED REFERENCES

Baloch, G.M., A.G. Khan, and M. Mushtaque.  1971. Biological control of
*Carduus* spp. (Compositae: I. Insects associated with these in West
Pakistan). Commonwealth Institute of Biological Control, Technical
Bulletin 14:51-58.  

Baloch, G.M. and A.G. Khan. 1973. Biological control of
*Carduus* species. II. Phenology, biology and host-specificity of
*Terellia serratulae* L. (Diptera: Trypetidae). Commonwealth Institute
of Biological Control, Technical Bulletin 16:11-22.  

Batra, S.W.T., J.R. Coulson, P.H. Dunn, and P.E. Boldt. 1981.  Insects
and fungi associated with *Carduus* thistles (Compositae).
USDA Technical Bulletin No. 1616. 100 pp.  

Bendall, G.M. 1973. The control of slender thistle, *Carduus
pycnocephalus* L. and *Carduus* *tenuiflorus* Curt. (Compositae),
in pasture by grazing management. Australian J. Agricultural
Research 24:831-837.  

Bendall, G.M. 1974. Slender thistles in pasture: control by
grazing management. Tasmanian J. of Agricultural Research 45:62-63.  

Bendall, G.M. 1975. Some aspects of the biology, ecology and control of
slender thistle, *Carduus pycnocephalus* L. and C. *tenuiflorus*
Curt. (Compositae), in Tasmania. J. Austr. Institute of
Agricultural Science 41:52-53.  

Bolt, P.E., G. Campobasso, and E. Colonnelli. 1980.
Palearctic distribution and host plants of *Ceutorhynchus trimaculatus*
and *Trichosirocalus horridus* (Coleoptera: Curculionidae). Annals of
the Entomological Society of America 73:694-698.  

Dunn, P.H. 1976. Distribution of *Carduus* *nutans*, C. *acanthoides*,
*C. pycnocephalus* and C. *crispus*, in the United States. Weed
Science 24:518-524.  

Evans, R.A., J.A. Young, and R. Hawkes. 1979.
Germination characteristics of Italian thistle (*Carduus pycnocephalus*)
and slenderflower thistle (*Carduus* *tenuiflorus*). Weed Science
27:327-332.  

Goeden, R.D. 1974. Comparative survey of the phytophagous insect faunas
of Italian thistle, *Carduus pycnocephalus* in southern California and
southern Europe relative to biological weed control.  Environmental
Entomology 3:464-474.  

Goeden, R.D. 1978. Initial analyses of *Rhinocyllus conicus* (Froelich)
(Col. Curculionidae) as an introduced natural enemy of milk thistle
(*Silybum* *marianum* (L.) Gaertner) and Italian thistle (*Carduus
pycnocephalus* L.) in southern California. Pp. 39-50 in K.E.
Frick, Biological control of thistles in the genus *Carduus* in the
United States.  

Goeden, R.D. and D.W. Ricker. 1978. Establishment of *Rhinocyllus
conicus*  (Col. Corculionidae) on Italian thistle in
southern California. Environmental Entomology 7:787-789.  

Harris, P. and H. Zwolfer. 1971. *Carduus* *acanthoides* L.,
welted thistle, and C. *nutans* L., nodding thistle (Compositae). Pp.
76-79 in Biological control programmes against insects and weeds in
Canada 1959-68. Tech. Commun. Commonw. Inst. Biological Control 4.  

Hawkes, R.B., L.A. Andres, and P.H. Dunn. 1972. Seed weevil released to
control milk thistle. California Agriculture 26:14.  

Hawkes, R.B., L.A. Andres, P.H. Dunn, and D.M. Maddox. 1978.  Biological
control of problem weeds in non-cropland areas. Pp. 71-75 in Proc. 30th
Anniversary California Weed Conference.  

Hitchcock, C.L. and A. Cronquist. 1973. Flora of the Pacific Northwest.
Univ. of Washington Press, Seattle. 730 pp.  

Kok, L.T. 1979. Influence of temperature and photoperiod on
the mortality of diapausing *Rhinocyllus conicus*
(Coleoptera: Curculionidae). Annals of the Entomological Society of
America 72:206-208.  

Kok, L.T. 1980. Compatibility of *Rhinocyllus conicus,* 
*Trichosirocalus horridus* and 2,4-D for *Carduus* thistle control.
Proc. V Int. Symp. Biol. Contr. Weeds, Brisbane, Australia:441-445.  

Kok, L.T., T.J. McAvoy, G.R. Johnson, and P.H. Dunn. 1982. Further tests
on C*eutorhynchu*s *trimaculatus* R. as a candidate for the biological
control of *Carduus* thistles. Crop Protection 1:67-74.  

McHenry, J. 1985. University of California, Davis.
Personal communication.   

Munz, P.A. and D.D. Keck. 1973. A California Flora and Supplement.
 University of California Press, Berkeley. 1905 pp. 

Oliveri, I. 1984. Effect of PUCCINIA CARDUI-PYCNOCEPHALI on
slender thistles (*Carduus pycnocephalus* and C. TENUIFLORUS). Weed
Science 32:508-510.  

Parsons, W.T. 1973. Noxious weeds of Victoria. Inkata Press,
Ltd., Melbourne, Australia. 300 pp.  

Schroeder, D. 1980. The biological control of thistles. Biocontrol News
and Information 1:9-26.  Tasmanian Department of Agriculture. 1977.
Annual Report 1976-77.  Tasmania, Government Printer. No. 64. 88 pp

Taylor, R.L. 1977. Control of winged and slender winged thistles.  Proc.
30th New Zealand Weed and Pest Control Conference: 38-41.  

Trumble, J.T. and L.T. Kok. 1980a. Impact of 2,4-D on CEUTHORHYNCHIDIUS
HORRIDUS (Coleoptera: Curculionidae) and their compatibility for
integrated control of *Carduus* thistles. Weed Research 20:73-75.  

Trumble, J.T. and L.T. Kok. 1980b. Integration of thistle-head weevil
and herbicide for *Carduus* thistle control. Protection
Ecology 2:57-64.  

Turner, C. 1985. USDA Biocontrol Lab in Albany, California.  Personal
communication.   

Uphof, J.C.T. 1942. Ecological relations of plants with ants
and termites. Botanical Review 8:563-598.  

Wheatley, W.M. 1971. Thistles\--prickly problem of pasture improvement.
Agricultural Gazette of New South Wales 82:258-259.  

Wheatley, W.M. and I.J. Collett. 1981. Winning the thistle war.
 Agricultural Gazette of New South Wales 92:25-28.  

Wolley, G. 1986. Former Preserve Manager, The Nature Conservancy, Ring
Mountain Preserve, CA.

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