18 MUSK THISTLE (NODDING THISTLE) A. Gassmann1 and L.-T. Kok2 1CABI Bioscience Centre Switzerland, Delémont, Switzerland; 2Department of Entomology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA their foraging on other plants, giving musk thistle a PEST STATUS OF WEED competitive advantage. Successful biological control Musk thistle, Carduus nutans L., is an invasive weed of musk thistle (Kok and Surles, 1975) is often ac- that has become widespread in the contiguous states companied by increased growth and coverage of pas- of the United States. It is a highly competitive weed ture grasses such as fescue (Festuca arundinaria of Eurasian origin that has replaced much of the na- Schreb.) and orchard grass (Dactylis glomerata L.), tive vegetation in pastures and disturbed areas (Surles or less desirable plants such as spotted knapweed et al., 1974; Kok, 1978a,b). (Kok and Mays, 1991). Extent of losses. The rate of expansion of musk Nature of Damage thistle populations in North America has been very Economic damage. Musk thistle invades pastures, rapid since the mid-1950s, when it was first recog- rangeland, and forest lands, and areas along roadsides, nized as a weed (Dunn, 1976). A single musk thistle 2 railroad right-of-ways, waste areas, and stream banks. per 1.49 m can reduce pasture yields by 23%. In In agricultural systems, the invasive nature and pro- Canada, stands of 150,000/ha have been observed lific seed production of musk thistle result in large (Desrochers et al., 1988a). Direct losses are difficult populations of the weed, which compete with crops to quantify due to lack of long-term monitoring pro- for space, nutrients, and light. Thus, infestations may grams and data. reduce productivity of pasture and rangeland by sup- Geographical Distribution pressing growth of desirable forage plants, as well as preventing livestock from eating plants growing in Musk thistle was first reported in the United States the vicinity of thistles due to the sharp spines on their in 1953 at Harrisburg, Pennsylvania (Stuckey and stems, leaf margins and blooms (Trumble and Kok, Forsyth, 1971). In the 1970s, the musk thistle com- 1982; Desrochers et al., 1988a). In the northeastern plex (see Taxonomy for definition) has been found United States, the highest economic losses due to in at least 3,068 counties in 42 of the mainland states, musk thistle infestations occur on fertile soils formed with 12% of those counties rating their infestations over limestone. as economically severe (Dunn, 1976). Musk thistle is Ecological damage. Musk thistle generally does declared a noxious weed in some 20 states, including not pose a great threat to high-quality natural areas, Illinois, Iowa, Kansas, Kentucky, Maryland, Minne- although it has been known to invade native and re- sota, Missouri, Nebraska, North Carolina, Ohio, stored grasslands despite the presence of dense, na- Oklahoma, and Pennsylvania (USDA, NRCS, 1999). tive prairie vegetation. Musk thistle may retard natu- Thus, musk thistle extends from the east to west coast ral secondary succession processes. Because musk in both the deciduous forest and prairie biomes. It thistle is unpalatable to wildlife and livestock, selec- grows from sea level to about 2,500 m elevation. It tive grazing leads to severe degradation of native prefers moist alluvial soils but will grow in eroded meadows and grasslands as grazing animals focus uplands without difficulty. 229 Biological Control of Invasive Plants in the Eastern United States BACKGROUND INFORMATION ON PEST PLANT Taxonomy The C. nutans complex in North America has been treated either as one species with four subspecies (subsp. nutans, subsp. leiophyllus [Petrovic] Stoj. and Stef., subsp. macrolepis [Peterm.] Kazmi, and subsp. macrocephalus [Desf.] Nyman), or as three species: Carduus nutans with two subspecies (subsp. nutans and subsp. macrolepis), C. thoermeri Weinm., and C. macrocephalus Desf. (McCarty, 1978; Desrocher et Figure 1. Musk thistle rosette. (Photograph by al., 1988b). Recent work by Desrochers et al. (1988b) L.-T. Kok.) has supported the existence, in Canada, of only two closely related groups of taxa referred to as subsp. nutans and subsp. leiophyllus. Carduus thoermeri Weinm. and C. nutans subsp. leiophyllus refer to the same taxon. In North America, C. nutans ssp. mac- rocephalus has only been collected from the United States. Carduus nutans ssp. nutans is distinguished from ssp. leiophyllus by its moderate to dense pubes- cence on leaves and phyllaries, by its generally smaller head diameter (1.5 to 3.5 cm in subsp. nutans and 1.8 to 4.5 cm in subsp. leiophyllus) and by the shape of its phyllary. In subsp. nutans, the lower portion of the phyllary is more or less equal to the upper por- tion, while in subsp. Leiophyllus, the lower portion is distinctly narrower than the upper portion. The two subspecies also can be separated by their fla- vonoid compounds. Carduus nutans subsp. macro- cephalus differs from subsp. nutans by a wider head diameter and phyllaries. It also differs from subsp. leiophyllus by being pubescent on leaves and phyl- Figure 2. Musk thistle in bloom. (Photograph by L.-T. Kok.) laries, and by having phyllaries that have the lower portion more or less equal to the upper portion. Hy- winter annual. It is 20 to 200 cm tall, with a long, bridization between C. nutans and Carduus fleshy taproot. The taproot is large, corky, and hol- acanthoides L. also has been reported (Warwick et low near the surface of the ground. One or more al., 1990). Presumably, the distribution of subsp. highly branched stems grow from a common nutans in the United States is similar to its distribu- rootcrown. Musk thistle grows in all soil textures, tion in Canada, where it is mainly distributed in the although the soils must be well drained. Leaves are eastern part of the country, while only subsp. dark green with light green midribs with a white leiophyllus and subsp. macrocephalus are present in margin (Fig. 1). The plant blooms in May and June. the Great Plains (McGregor, 1986). The showy flowers (Fig. 2) are terminal, large, soli- Biology tary, and nodding (slightly bent). They are deep rose to violet or purple in color. The seeds are straw col- The biology of musk thistle has been reviewed by ored and do not have a light requirement for germi- Desrochers et al. (1988a). Carduus nutans L. is a her- nation, but are affected by temperature. Higher ger- baceous biennial though occasionally it becomes a mination rates occur at temperatures between 20 and 230 Musk Thistle (Nodding Thistle) 28 °C. Musk thistle does not appear to have any spe- species in the subtribe Carduinae. The history of bio- cific climatic requirements other than a cool period logical control of thistles was reviewed by Dunn of vernalization, a minimum of 40 days below 10 °C (1978) and by Schroeder (1980). for flowering. It does not reproduce vegetatively and is propagated by seeds dispersed primarily by wind. Area of Origin of Weed Most seeds are deposited within 50 m of the release The genus Carduus is native to the Eastern Hemi- point and less than 1% are blown farther than 100 m sphere, where its distribution extends over Europe, (Smith and Kok, 1984). Up to 11,000 achenes may be central Asia, and East Africa. Franco (1976) recog- produced per individual with as many as 1,500 seeds nized 48 species in Flora Europaea. Several taxa have per flower head. Seed viability remains high for more been reported in North America and separated into than ten years. three groups: the slender-flowered thistles (Carduus Analysis of Related Native Plants in the Eastern tenuiflorus Curt. and Carduus pycnocephalus L.), the United States small-flowered thistles (Carduus acanthoides L. and Carduus crispus L.), and the large-flowered thistle There are no native North American species in the (Carduus nutans s.l.). Southern Europe is considered genus Carduus. Carduus nutans belongs to the tribe to be the center of origin for Carduus because of the Cardueae (family Asteraceae) which is largely an Old many endemic Carduus species found there. World group. The tribe is further divided into four subtribes (Echinopsidinae, Carlininae, Carduinae, Areas Surveyed for Natural Enemies and Centaureinae) including some 13 genera in North Areas surveyed included southern England, France, America (Bremer, 1994; USDA, NRCS, 1999). From Austria, Germany, Italy and the northern part of the these, only three contain native species – Centaurea former Yugoslavia (Zwölfer, 1965; Boldt and (two species, subtribe Centaureinae), Saussurea (seven Campobasso, 1978). Other surveys have been car- species, subtribe Carduinae, but the position of the ried out in Pakistan, Iran, and Japan (Schroeder, genus in the tribe remains uncertain), and Cirsium 1980). (subtribe Carduinae). The genus Cirsium includes about 100 native species, of which 21 species occur Natural Enemies Found in the eastern United States. One of these, Cirsium pitcheri (Torr. ex Eat.) Torr. and Gray, is listed as Some 130 insect species have been recorded on C. threatened under the Endangered Species Act. This nutans s.l. in Europe (Zwölfer, 1965; Boldt and species occurs in sand dunes along the shores of the Campobasso, 1978). In Italy alone, 109 species from Great Lakes in Illinois, Indiana, Michigan, Wiscon- six orders and 33 families fed or reproduced on musk sin, and Ontario. thistle. Some 25 species were reported to be broadly oligophagous on plants in the subtribe Carduinae (Table 1), and only very few were considered to have a host range restricted to plants in the genera Carduus, HISTORY OF BIOLOGICAL CONTROL Cirsium, and Silybum, or to be monophagous. Since EFFORTS IN THE EASTERN there was no concern about non-target impact on na- UNITED STATES tive thistles in the earliest phase of the program, oli- gophagy on several thistle species in the genera Musk thistle was among the first 19 weeds selected Carduus, Cirsium, and Silybum was considered as an for biological control when the USDA overseas labo- advantage and only those species recorded as eco- ratory was established at Rome, Italy in 1959.