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. 17 CANADA THISTLE A. S. McClay Alberta Research Council, Vegreville, Alberta, Canada PEST STATUS OF WEED Canada thistle, Cirsium arvense (L.) Scop. (Fig. 1), is a vigorous, competitive weed that occurs in a wide range of habitats and is difficult to control due to its ability to regrow from its extensive, deep creeping root system (Nadeau and Vanden Born, 1989). Figure 2. Canada thistle [Cirsium arvense (L.) Scopoli] infestation in canola (Brassica rapa L.). (Photograph by A. S. McClay.) state weed control legislation in Delaware, Illinois, Indiana, Iowa, Kansas, Kentucky, Maryland, Min- nesota, Missouri, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, South Dakota, and Wisconsin (USDA, NRCS, 1999). Ecological damage. Canada thistle can be an invasive species in some natural communities, includ- ing prairie potholes and wet or wet-mesic grasslands in the Great Plains and sedge meadows in the upper Midwest (Nuzzo, 1997). It usually is a problem in Figure 1. Canada thistle [Cirsium disturbed areas and moister sites. Canada thistle was arvense (L.) Scopoli]. among the most prevalent weeds on Conservation (Photograph by L. M. Dietz.) Reserve Program (CRP) land in Minnesota, occur- ring in 65 to 75% of CRP fields throughout the state. Nature of Damage Canada thistle ground cover in these fields frequently Economic damage. Canada thistle causes extensive reached 50 to 75%, giving rise to concern about seed crop yield losses through competition and, perhaps, dispersal into neighboring agricultural land (Jewett allelopathy (Stachon and Zimdahl, 1980) (Fig. 2). The et al., 1996). It was ranked as “urgent” for control in prickly mature foliage also is thought to reduce pro- a review of exotic plants at Pipestone National Monu- ductivity of pastures by deterring livestock from graz- ment, Minnesota (Hiebert and Stubbendieck, 1993). ing. Canada thistle is the species most frequently de- Extent of losses. A density of 20 Canada thistle clared noxious under state or provincial weed con- shoots per m2 caused estimated yield losses of 34% trol legislation in the United States and Canada (Skin- in barley (O’Sullivan et al., 1982), 26% in canola ner et al., 2000). It is listed as a noxious weed under (O’Sullivan et al., 1985), 36% in winter wheat 217 Biological Control of Invasive Plants in the Eastern United States (McLennan et al., 1991), and 48% in alfalfa seed Analysis of Related Native Plants in the Eastern (Moyer et al., 1991). Densities of Canada thistle in United States field infestations can reach 173 shoots per m2 (Donald and Khan, 1996). The genus Cirsium is a large one, with 92 native spe- cies in North America, of which 20 occur in the U.S. Geographical Distribution states that fall wholly or in part east of the 100th me- ridian (USDA, NRCS, 1999). One of these, Cirsium Canada thistle occurs in all eastern U.S. states south pitcheri (Torrey) Torrey and Gray, is listed as threat- to Kansas, Arkansas, Tennessee, and North Carolina, ened under the Endangered Species Act. This species but it is sparsely distributed south of latitude 37° N occurs in sand dunes along the shores of the Great (USDA, NRCS, 1999). The main areas of occurrence Lakes in Illinois, Indiana, Michigan, Wisconsin, and are the northeastern, mid-Atlantic, Great Lakes, and Ontario. Phylogenetic studies of North American northern Great Plains states. In a survey in Mary- and Eurasian Cirsium species are needed to elucidate land, Canada thistle was found in about 17% of suit- relationships among species in the genus and to pro- able sites in the eastern and central part of the state, vide a basis for planning host-specificity tests and but only 10% of sites further west (Tipping, 1992). interpreting resulting data. Studies have been initi- ated using the external transcribed spacer (ETS) re- gion of ribosomal DNA to develop a phylogeny for BACKGROUND INFORMATION North American and selected Eurasian Cirsium spe- ON PEST PLANT cies (D. Kelch, pers. comm.). Taxonomy Canada thistle is a member of the genus Cirsium, HISTORY OF BIOLOGICAL CONTROL subtribe Carduinae, tribe Cardueae, and family EFFORTS IN THE EASTERN Asteraceae (Bremer, 1994). It differs from most other Cirsium species by its dioecious flowers, and from UNITED STATES most native North American members of the genus Canada thistle was among the first 19 weed species by its extensive creeping roots and small, numerous selected as targets for biological control when the flower heads borne on branched stems. Several vari- USDA Rome Laboratory was established in 1959 eties have been described based on variations in leaf (Schroeder, 1980). However, most host specificity shape and degree of spininess. testing of agents for Canada thistle was conducted Biology from 1961 to 1984 by staff of Agriculture Canada or by the International Institute of Biological Control The biology of Canada thistle was extensively re- (now CABI Bioscience) working with Canadian viewed by Moore (1975), Donald (1994), and Nuzzo funding. The agents released in the United States have (1997). It is a perennial herb with an extensive creep- been those that became available as a result of the ing root system that can give rise to new shoots from Canadian program, the results of which were re- adventitious root buds. The stems usually die back viewed by Schroeder (1980), Peschken (1984a), and over winter and new shoots are produced each spring McClay et al. (2001). Most releases in the eastern from old stem bases or root buds. Canada thistle is United States were made by USDA, ARS staff at the almost perfectly dioecious and can produce abundant Beltsville Agricultural Research Center; some stud- seeds, which are dispersed by wind (Lloyd and Myall, ies also were carried out by staff of the Maryland 1976). It is a long-day plant, requiring a photoperiod Department of Agriculture. Biological control of of at least 14 to 16 hours (depending on ecotype) for Canada thistle in New Zealand has been reviewed by flowering to be induced (Hunter and Smith, 1972). It Jessep (1989). occurs in a wide range of habitats and soil types. 218 Canada Thistle Area of Origin of Weed ruficauda (F.) (=Orellia ruficauda F.) (Diptera: Canada thistle is native to Europe, parts of North Tephritidae) is distributed across Canada, and pre- Africa, and Asia south to Afghanistan, Iran and Pa- sumably also occurs widely in the eastern United kistan, and east to China. Its exact center of origin States. A survey showed it to be present in South within the native range is not known, although it is Dakota (R. Moehringer, S. Dakota Dept. of Agricul- suggested by Moore (1975) to be in southeastern ture, pers. comm.), and specimens are known from Europe and the eastern Mediterranean area. Michigan. The root-feeding weevil Cleonis pigra (Scopoli) (Coleoptera: Curculionidae) occurs in New Areas Surveyed for Natural Enemies York, Pennsylvania, Michigan, Indiana, Ontario, and Extensive surveys of natural enemies of Canada Quebec (O’Brien and Wibmer, 1982; Anderson, 1987; thistle and other Cardueae species in western Europe C. W. O’Brien, per. comm.). The rust Puccinia were carried out starting in 1959. Other surveys have punctiformis (Strauss) Röhling is widespread in North been carried out in Japan, Iran, and northern Paki- America. stan (Schroeder, 1980), and in China (Wan and Har- A phytopathogenic bacterium, Pseudomonas ris, unpub. data). Further surveys in southern Rus- syringae pv. tagetis (Hellmers 1955) Young, Dye and sia, central Asia, and China are currently under way Wilkie 1978, causing apical chlorosis, has been iso- (Gassmann, unpub. data). In addition to surveys spe- lated from Canada thistle. Field tests of applications cifically carried out for biocontrol purposes, the gen- of this bacterium in a commercial corn field resulted eral European entomological literature contains much in 57% mortality of Canada thistle as well as damage information on insects associated with Canada thistle to several other weedy Asteraceae species. A surfac- (e.g., Redfern, 1983; Stary, 1986; Volkl, 1989; Freese, tant is required to allow penetration of the Canada 1994; Berestetsky, 1997; Frenzel et al., 2000). The thistle cuticle by the bacterium. Further work on for- phytophagous insects associated with Canada thistle mulation of this agent is under way (Johnson et al., in Poland are listed by Winiarska (1986). 1996). The bacterium occurs in Maryland (P. Tipping, pers. comm.). Natural Enemies Found One species which was introduced as a biologi- Surveys by Zwölfer (1965a) in Europe found 78 spe- cal control agent for Carduus species, the seed-head cies of phytophagous insects feeding on Canada weevil Rhinocyllus conicus (Frölich) (Coleoptera: thistle. Of these, six are reportedly monophagous, five Curculionidae), also is recorded attacking Canada are found on Canada thistle and a few related spe- thistle (Rees, 1977; Youssef and Evans, 1994). This cies, 26 are oligophagous on plants in the same species is widespread in the eastern United States, and subtribe, and the remaining 42 are less specific and has been found attacking Canada thistle in Maryland of no interest for biological control (Schroeder, 1980). (P. Tipping, pers. comm.). A number of European insects and pathogens Host Range Tests and Results attacking Canada thistle have been accidentally in- troduced into North America, and some of these have In the earlier part of the period 1961 to 1984, host been studied as potential biological control agents. specificity testing for agents attacking Canada thistle The leaf-feeding tortoise beetle Cassida rubiginosa was focused on assessing potential risks to economic Müller (Coleoptera: Chrysomelidae) occurs widely species of Cardueae, of which the two most impor- in the eastern United States, south to Virginia and tant are safflower (Carthamus tinctorius L.) and globe west to southern Michigan and Ohio, and in Canada artichoke (Cynara scolymus L.).