In: Van Driesche, R., et al., 2002, Biological Control of Invasive Plants in the Eastern United States, USDA Forest Service Publication FHTET-2002-04, 413 p.
Pest Status of Weed
Nature of Damage
Like many other Carduus species, slenderflower thistle, Carduus tenuiflorus Curtis, is associated with pastures, disturbed areas, and vacant lots. Invasion is favored by annual burning of pastures. The thistle protects forage from grazing and is a competitive weed in improved pastures.
Slenderflower thistle occurs in Pennsylvania, New Jersey, and Texas but the most serious infestations occur in California, Oregon, and Washington (USDA, NRCS, 1999). The closely related species, Italian thistle, Carduus pycnocephalus L., is known from New York, Alabama, and South Carolina in the eastern United States.
Background Information On The Pest Plant
Carduus tenuiflorus is very similar to C. pycnocephalus, and the two species are sometimes treated together. Flowering stems are single or multiple from the base, branched, strongly ribbed, and slightly woolly. Spiny wings are continuous on stems to the base of the flower heads, but are discontinuous on C. pycnocephalus. The flower heads of C. tenuiflorus occur in clusters of five to 20, whereas those of C. pycnocephalus are in smaller clusters. The slender flower heads are less than 2 cm long, and lack stalks. Rosette and stem leaves are deeply lobed with numerous spines along the margin.
Carduus tenuiflorus is a winter annual, sometimes a biennial. Plants can grow from 0.3 to 2.0 m tall. It prefers soils of moderate to high fertility, in areas with moderate rainfalls.
Analysis of Related Native Plants in the Eastern United States See this section in the chapter on musk thistle.
History of Biological Control Efforts in the Eastern United States
As pointed out by Dunn (1978), the oldest document relating to biological control of Carduus thistle was a USDA note from 1956 regarding the abundance of C. pycnocephalus and C. tenuiflorus in California. The program against this species began in 1959, with the establishment of the USDA overseas laboratory in Rome, Italy. Thistle insect surveys by USDA staff in Italy initially focused on C. pycnocephalus but later were extended to C. tenuiflorus and Carduus nutans L. During the surveys on the latter two species, it was found that musk thistle supported a larger complex of insects than the other Carduus species, and work was subsequently concentrated on musk thistle. Slenderflower thistle also was included in the survey of European thistles carried out by the Commonwealth Institute of Biological Control (now CABI Bioscience) in the 1960s and funded by the Canada Department of Agriculture (Zwölfer, 1965). Major surveys for natural enemies of C. pycnocephalus were conducted also by Goeden (1974) in central and southern Italy, and in Greece during 1971 and 72.
Area of Origin of Weed
The native range of slenderflower thistle is western and southern Europe and the Mediterranean area, extending northward to Scandinavia.
Areas Surveyed for Natural Enemies
Areas surveyed included southern England, France, Austria, Germany, Italy, the northern part of former Yugoslavia, and Greece (Zwölfer, 1965; Goeden, 1974; Dunn, 1978).
Natural Enemies Found
Most of the C. tenuiflorus and C. pycnocephalus populations sampled by Zwölfer (1965) were in western and southern France, respectively. Altogether, some 15 oligophagous insect species were recorded on C. tenuiflorus and C. pycnocephalus in Europe (see Table 1 in the chapter on musk thistle). Although concern about the invasiveness of slenderflower thistles was the reason for the initiation of the Carduus biological control program in North America, attention soon was redirected to musk thistle. No biological control agent was specifically targeted for slenderflower thistle. Populations of the seed-feeding weevil R. conicus (from C. pycnocephalus in Italy) and the root-crown fly C. corydon have been released against C. tenuiflorus and C. pycnocephalus in the United States. The host range and biology of the two species are described in the chapter on musk thistle.
Host Range Tests and Results
See the chapter on musk thistle.
Releases Made (from Julien and Griffiths, 1999)
Rhinocyllus conicus. Releases of this seed-feeding weevil originating from Italy were made on C. tenuiflorus in 1973 in California and Oregon only. Releases have been made on C. pycnocephalus as well.
Cheilosia corydon. This thistle rosette fly from Italy was released in 1990 in Maryland and New Jersey, as well as in Montana and Oregon. The fly also has been released on C. pycnocephalus in Oregon.
Biology and Ecology of Key Natural Enemies
See the chapter on musk thistle.
Evaluation of Project Outcomes
Establishment and Spread of Agents (from Rees et al., 1996; Julien and Griffiths, 1999)
Rhinocyllus conicus. This weevil (Fig. 1) has become established and contributed to the control of slenderflower thistle in Oregon, especially in unburned areas.
Cheilosia corydon. Establishment of this fly has not been confirmed.
Puccinia carduorum. This rust (Fig. 2) has been accidentally introduced in North America. It is recorded on C. tenuiflorus in California and Oregon. Puccinia carduorum is native to the Mediterranean area but also is reported from Bulgaria and Romania. The fungus was imported from Turkey by the USDA for host range tests at the Foreign Diseases–Weed Research Laboratory in Frederick, Maryland (Politis and Bruckart, 1986). It also was tested and released for musk thistle control in 1992 (Baudoin et al., 1993). (For details, see the chapter on musk thistle). The disease appears first as tiny yellow specks. In several days, rust pustules containing thousands of spores become visible (Figs. 3, 4).
Baudoin, A. B. A. M., R. G. Abad, L.-T. Kok, and W. L. Bruckart. 1993. Field evaluation of Puccinia
Dunn, P. H. 1978. History of the biological control of musk thistle in North America and studies with the
Goeden, R. D. 1974. Comparative survey of the phytophagous insect fauna of Italian thistle (Carduus
Julien, M. H. and M. W. Griffiths (eds.). 1999. Biological Control of Weeds. A World Catalogue of
Politis, D. J. and W. L. Bruckart. 1986. Infection of musk thistle by Puccinia carduorum influenced by
Rees, N. E., P. C. Quimby, Jr., G. L. Piper, E. M. Coombs, C. E. Turner, N. R. Spencer, and L. V.
USDA, NRCS. 1999. U.S. Department of Agriculture, Natural Resources Conservation Service) PLANTS
Zwölfer, H. 1965. A list of phytophagous insects attacking wild Cynareae species in Europe. Progress
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(Tech. Coordinators). 2003. Invasive Plants of the Eastern U.S.:
Identification and Control. USDA Forest Service - Forest Health
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