Biological Control of Musk Thistle: A Reassessment

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Musk thistle and nodding thistle are names commonly used to describe Old World thistles in the Carduus nutans species group (family Asteraceae). These include Carduus macrocephalus Desfontaines (big-headed thistle), C. nutans ssp. leiophyllus [Petrovic]) Stoj.and Stef. (nodding thistle) a.k.a. C. thoermeri, and C. nutans L. (musk thistle proper). According to (1986), C. macrocephalus is a subspecies of C. nutans along with C. nutans ssp. leiophyllus. Other classifications exist (Kazmi 1964, McCarty 1982, Desrochers et al. 1988). In this chapter, the term “musk thistle” will be used generically to refer to any and all of the names used in the various classification schemes. Musk thistle became established in North America sometime during the mid-1800s (Stuckey and Forsythe 1971). Introduction appears to have been accidental, possibly as seed contaminants of imported grain. By the mid-1900s, musk thistle had proliferated throughout much of the eastern and north central parts of United States (Dunn 1976) and into parts of southern Canada (Harris and Zwolfer 1971). In 1981, this weed occupied an estimated 730,000 hectares in 40 of the United States (Batra et al. 1981). In Kansas, C. nutans occurred on more than 400 hectares in 75 of the 105 counties (Robinson 1976). Musk thistle’s weed status derives from a combination of structural, developmental and ecological characteristics. The presence of sharp spines on stems, leaf margins and floral bracts of plants, and the fact that plants may reach two meters or more in height, represent a nuisance in urban and suburban landscapes. Encounters with stands of musk thistle by livestock also are reported to interrupt grazing (Batra 1978, Trumble and Kok 1982, Rees 1992). Economic losses also have resulted from the thistle’s competition with forage crops, particularly in overgrazed or disturbed pastures and rangeland (Trumble and Kok 1982, Reece and Wilson 1983, Moore et al. 1989, Fick 1995, Thompson et al. 1995). Musk thistle has been categorized as a noxious weed in many states, where control is mandated by law.