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. 13 SPOTTED KNAPWEED J. Story Montana State University, Western Agricultural Research Center, Corvallis, Montana, USA runoff and soil sedimentation (Lacey et al., 1989), and PEST STATUS OF WEED lowers plant diversity (Tyser and Key, 1988). Spot- Spotted knapweed, Centaurea maculosa Lamarck, is ted knapweed produces an allelopathic compound a purple-flowered, herbaceous, perennial weed, liv- that reduces germination of some grass species ing three to five years on average. It infests semiarid (Kelsey and Locken, 1987). range lands in the western United States and road- Geographical Distribution sides and fields in the eastern part of the country. Infested areas are dominated by the plant, reducing Spotted knapweed is native to Europe and western their grazing value and suppressing native plant com- Asia but has become widespread in parts of the munities. The plant, originally from Central Asia, has United States and Canada. The plant occurs through- been in North America for over 120 years. out the United States except for Alaska, Texas, Okla- homa, Mississippi, and Georgia (USDA, NRCS, Nature of Damage 2001). The plant is a serious invader of rangeland in Economic damage. Spotted knapweed is a serious the Rocky Mountain region. In Montana alone, the problem on rangeland, especially in the western plant infests an estimated 1.9 million ha of rangeland United States. Bucher (1984) estimated that an and pasture (Lacey, 1989). In Canada, the plant is 800,000 ha infestation in Montana was causing $4.5 abundant in British Columbia, and is common in million in annual forage losses, and that invasion of Ontario, Quebec, and the Maritimes (Watson and 13.6 million ha of vulnerable rangeland in Montana Renney, 1974). would cost cattle and sheep ranchers $155.7 million of gross revenue annually. Hirsh and Leitch (1996) reported that an 800,000 ha infestation of spotted BACKGROUND INFORMATION knapweed, in combination with two knapweeds of ON PEST PLANT minor importance in Montana (diffuse knapweed, Taxonomy Centaurea diffusa Lamarck, and Russian knapweed, Acroptilon repens [L] de Candolle) was causing $14 The taxonomy of C. maculosa has been detailed million in direct negative impacts and $28 million in by Dostal (1976) and reviewed by Müller et al. (1988) indirect effects (i.e., reduced regional economy) to and Müller (1989). Centaurea maculosa is comprised the state of Montana. Harris and Cranston (1979) of several subspecies occurring from western Asia to reported that the 30,000 ha infestation in Canada was western Europe. The C. maculosa occurring in North reducing forage production more than 88%. In the America is a short-lived perennial tetraploid (2n=36) northeastern and northcentral United States, the plant that is considered the same as C. biebersteinii de is primarily a problem of roadsides, fields, and waste Candolle subsp. biebersteinii (=C. micranthos Gmelin areas (Hoebeke, 1993); economic impact of the plant ex. Hayek), a native of western Asia. However, the in those regions has not been reported. most widely distributed C. maculosa in Europe is the Ecological damage. Spotted knapweed reduces biennial diploid (2n=18) C. maculosa spp. rhenana livestock and wildlife forage (Thompson, 1996; (Boreau) Gugler (Dostal, 1976; Müller et al., 1988; Watson and Renney, 1974), increases surface water Müller, 1989). 169 Biological Control of Invasive Plants in the Eastern United States Biology Analysis of Related Native Plants in the Eastern United States Spotted knapweed is a purple-flowered, herbaceous weed, 30 to125 cm tall, with one to 10 upright stems, The North American plants most closely related to and a stout taproot (Fig. 1). The plant is a perennial, spotted knapweed include safflower (Carthamus living an average of three to five years and frequently tinctorius L.) and possibly the two “knapweeds,” Cen- up to nine years (Boggs and Story, 1987). The flower taurea americana and Centaurea rothrockii. Recent heads, enclosed by black-tipped bracts, are borne sin- evaluations, however, suggest the latter two plants gly at the terminal ends of branches. Seed is shed should be treated as Plectocephalus americanus (Nutt.) immediately upon maturation of the seed head. The (Müller-Schärer and Schroeder, 1993). The next clos- plant reproduces solely by seed. Seed production est relatives of spotted knapweed are members of the ranges from 5,000 to 40,000 seeds/m2 (Sheley et al., tribe Cardueae, mainly Carduinae (Cirsium and 1998). Seeds can survive in the soil for eight or more Cynara [e.g., artichoke]). There are numerous years (Davis et al., 1993). Cirsium species native to North America. HISTORY OF BIOLOGICAL CONTROL EFFORTS IN THE EASTERN UNITED STATES Area of Origin of Weed The native range of the spotted knapweed (tetrap- loid) occurring in North America is eastern Europe and western Asia (Müller et al., 1989). Areas Surveyed for Natural Enemies Surveys were conducted throughout Europe and western Asia for natural enemies. Natural Enemies Found Schroeder (1985) listed 38 arthropod species that were Figure 1. Spotted knapweed, Centaurea maculosa Lamarck. (Photo by Jim Story.) known to be associated with spotted knapweed in Eurasia. Of these, 12 species were screened and re- The life history of the plant has been described leased in North America against the plant (Table 1). by Watson and Renney (1974). Seed germination oc- Host Range Tests and Results curs in the fall or early spring, depending upon mois- ture availability. Seedlings develop into rosettes; The number of plants included in the test plant list plants that have overwintered as rosettes usually pro- for each of the 12 insect species varied, but averaged duce floral stems the following summer. Stem elon- around 45 test plant species per insect. Most of the gation occurs in June followed by flowering in July plant species used were from the family Asteraceae, and seed dispersal in August. but representative species from one or more other Spotted knapweed is adapted to a range of habi- families also were often tested. Particular emphasis tats and soil types, but is especially well suited to was placed on plants in the Asteraceae tribe Cardueae relatively dry sites (Watson and Renney, 1974). In which includes the genus Centaurea. The test plant Europe, the plant is most aggressive in the forest list for Larinus minutus is presented in Table 2 (Jor- steppe but can form dense stands in more moist ar- dan, 1995) because it is fairly representative of the eas on well-drained soils including gravel, and in drier plants tested on all 12 insect species. The only plants sites where summer precipitation is supplemented by of economic importance in North America included runoff (Sheley et al., 1998). in the tests were Carthamus tinctorius L. (safflower), 170 Spotted Knapweed Table 1. Insects Released in the United States for Biological Control of Spotted Knapweed State/Date of Known Estab. Plant Part Where Date of First Release in Scientific Name Insect Type in Eastern U.S. Attacked Collected U.S. Release Eastern U.S. and Canada and Canada Urophora affinis Fly Flower head France, Austria 1973 IN 1997 Frauenfeld (Tephritidae) MD 1983 NY 1983 x VA 1986 x MN 1990 x WI 1991 x MI 1994 x Quebec 1979 x Ontario 1970 Urophora Fly Flower head Former USSR 1980 Quebec 1979 x quadrifasciata (Tephritidae) MD 1983 x (Meigen) NY 1983 VA 1986 x MN 1990 x WI 1991 x MI 1994 x IN 1997 x Terellia virens Fly Flower head Austria, Switzer. 1992 MN 1994 (Loew) (Tephritidae) Chaetorellia Fly Flower head Austria,Switzer. 1992 MN 1996 acrolophi White (Tephritidae) and Marquardt Metzneria Moth Flower head Switzer. 1980 MN 1991 paucipunctella (Gelechiidae) VA 1986 x Zeller Agapeta zoegana Moth Root Austria, Hungary 1984 IN 1996 L. (Cochylidae) MN 1991 x WI 1991 Pterolonche Moth Root Hungary 1988 - inspersa (Pterolonchida- Staudinger e) Pelochrista Moth Root Austria, Hungary 1984 - medullana (Tortricidae) (Staudinger) Cyphocleonus Weevil Root Austria, 1988 IN 1996 achates (Curculionidae) Romania MN 1994 (Fahraeus) Bangasternus Weevil Flower head Greece 1990 MN 1992 fausti Reitter (Curculionidae) Larinus obtusus Weevil Flower head Romania, 1992 MN 1995 Gyllenhal (Curculionidae) Serbia Larinus minutus Weevil Flower head Greece, 1991 IN 1996 x Gyllenhal (Curculionidae) Romania MN 1994 x Helianthus annuus L., (common sunflower), and slightly on artichoke. The feeding by C. achates was Cynara scolymus L. (globe artichoke). None of the not of concern, however, because no eggs were laid insects oviposited or fed on any of these three plants on the plant. In general, attack by all of the insects except for Cyphocleonus achates adults which fed was restricted to the genus Centaurea, and usually to 171 Biological Control of Invasive Plants in the Eastern United States Table 2. Test plant list used for Larinus minutus FAMILY: ASTERACEAE Subtribe: Carduinae Tribe: Cardueae Arctium lappa L. Subtribe: Centaureinae Carduus acanthoides L. Genus: Centaurea C. nutans L. Subgenus: Acrolophus Cirsium arvense (L.) Scop. Centaurea arenaria Bieb. C. crassicaule (Greene) Jeps. C. cineraria L. C. creticum C. diffusa Lamarck Europe C. undulatum (Nutt.) Spreng. C. diffusa USA Cynara scolymus L. C. friderici Vis. Galactites tomentosa C. maculosa Lamarck Europe Onopordum acanthium L. C. maculosa USA Silybum marianum (L.) Gaertn. C. micranthos S. G. Gmelin Tribe: Anthemideae C. paniculata L. Achillea millefolium L. C. vallesiaca (D. C.) Jordan Anthemis tinctoris L. Subgenus Calcitrapa Artemisia absinthium L. C. calcitrapa L. Chrysanthemum leucanthemum L. C. iberica Trev. Sprengel Tribe: Astereae Subgenus: Cartholepis Aster novi-belgii L. C. macrocephala Muss. Solidago canadensis L. Subgenus: Centaurea Tribe: Calenduleae C. ruthenica Lamarck Calendula officinalis L. Subgenus: Cyanus Tribe: Carlineae C. cyanus L. Carlina vulgaris L. C. montana L. Tribe: Cichorieae Subgenus: Jacea Cichorium intybus L. C. jacea L.