Centaurea stoebe subsp. micranthos (spotted knapweed)

As reproduction is mostly by seeds, the management strategy is the removal of plants before seeds are formed, and reseeding disturbed areas with competitive vegetation. Elements of an integrated vegetation management (IVM) plan include prevention, cultural, physical, chemical and biological controls (Woo et al., 2002).Prevention is an important part of spotted knapweed management. Seeds and forage should be certified weed-free. Soil should be checked for weed seeds before it is moved to new areas. Because knapweed travels with automobiles, the undercarriage of vehicles or machinery exposed to knapweed should be constantly cleaned. Many construction companies have weed control measures and can steam clean the underside of machinery (Olliff et al., 2001; Woo et al., 2002).Soil disturbance caused by water, livestock, vehicles or machinery should be minimized. Deferred or rotational grazing, water conservation, erosion control, proper fertilization, re-seeding to maintain dense, hardy grass cover, revegetation, and maintenance of competitive vegetation that can withstand weed invasion are part of prevention. Because most noxious weeds are pioneer species, a dense ground cover of desirable plants with a closed canopy will usually help prevent their establishment (Olliff et al., 2001).

Grazing by sheep and goats can suppress knapweeds. Seedheads and leaves of spotted knapweed are actually more nutritious than some native grasses (Olson and Wallander, 2001a, b). It is more palatable in late spring or early summer, and repeated grazing can reduce flower stem production, stop seed formation and gradually deplete root reserves (Popay and Field, 1996).As animals usually prefer to eat nearby grasses in lieu of knapweeds, grazing is most effective against knapweeds when the livestock is enclosed in a fenced-off, weedy area (Beck, 1994; Olson and Lacey, 1994; Whitson, 1997). However, Olson et al. (1997) found that grazing sheep for three summers reduced seeds, seedlings, rosettes and mature spotted knapweed densities, but minimally affected native grasses. Animals should graze weeds only before they flower and set seed. If this is impossible, animals should be contained for 7 to 14 days before moving them to non-infested areas to prevent spread of seeds in animal manure.Nitrogen fertilization may increase native grass competition in some cases (Sheley and Jacobs, 1997b). Unfortunately, where knapweed has already invaded, nitrogen fertilization with no other treatment may favour knapweed over natives (Prather and Callihan, 1991; Velagala et al., 1997; Sheley et al., 1998; Herron et al., 2001). For instance, Story et al. (1989) found that the addition of nitrogen to sites containing spotted knapweed, quackgrass (Agropyron repens) and crested wheatgrass (Agropyron cristatum) favoured knapweed and quackgrass but not crested wheatgrass. Also, Lindquist et al. (1996) did competition experiments between spotted knapweed, bromegrass (Bromus inermis), Idaho fescue (Festuca idahoensis) and the bluebunch wheatgrass (Pseudoroegneria spicatum). Additions of nitrogen fertilizer favoured bromegrass but did not help the native grasses compete with knapweed.The effects of fertilization may be complicated by moisture conditions. According to Spears et al. (1980), knapgrass seeds germinate best close to the surface and at soil moisture of >55%. Fertilization in dry areas may remove moisture from the soil, leading to increased competition of drought tolerant grasses such as crested wheatgrass (Berube and Myers, 1982).Soil analyses have shown that the perennial grasses that dominate climax populations on grazing lands are associated with low available nitrogen. Early colonizers such as knapweed do well in nitrogen-rich soil, but cannot compete with perennial grasses in a low nitrogen environment (Sheley et al., 1996a, b; Herron et al., 2001). To force plant succession towards the preferred climax vegetation, controlled colonization can be used. Intermediate and late colonizers are planted simultaneously. For instance, Herron et al. (2001) found that plantings of annual rye (Secale cereale) and bottlebrush squirreltail (Elymus elimoides) could deplete the soil of nutrients, especially nitrogen. As a result, late seral plants such as bluebunch wheatgrass (Pseudoroegneria spicata) will outcompete the early colonizer knapweed. Low nitrogen availability also makes knapweed more susceptible to root-feeding biological control agents (Steinger and Muller-Schärer, 1992).As knapweeds thrive in direct sunlight, shading can suppress them (Kennett et al., 1992). Shading is an especially effective method against Russian knapweed (Acroptilon repens) because it cannot tolerate dense shade. Crops such as lucerne produce dense shade under irrigation and have been used to suppress Russian knapweed in crop areas (Roche and Roche, 1991).Where knapweed is nearby, any patches of bare ground can sprout with knapweed (Lacey et al., 1990). Thus, reseeding bare areas with competitive perennial grasses is important. Crested wheatgrass (Agropyron cristatum) has been a successful competitor in dry areas of Canada (Berube and Myers, 1982). Crested wheatgrass is more competitive than Russian wildrye (Psathyrostachys juncea) or lucerne (Maxwell et al. 1992), but orchardgrass (Dactylis glomerata) and thickspike wheatgrass (Agropyron dasystachyum) are more competitive than crested wheatgrass (Larson and McInnis, 1989). Meadow fescue (Festuca pratensis) and rough fescue (Festuca altaica) are more competitive than bluebunch wheatgrass (Pseudoroegneria spicata) (Lacey et al., 1990; Steinger and Muller-Schärer, 1992).To compete successfully, intermediate wheatgrass (Elytrigia intermedia) may need much higher seeding rates (6000/m²) than currently recommended (Velagala et al., 1997; Sheley et al., 1999a). Bluebunch wheatgrass (Pseudoroegneria spicatum) is a poor competitor, as 90% reduction of spotted knapweed populations are necessary before it can successfully compete (Sheley and Jacobs, 1997a). Bromegrass (Bromus inermis) is a more aggressive competitor than wheatgrasses, especially as nitrogen levels increase (Lindquist et al., 1996).

Controlled burns can potentially help control knapweeds. However, knapweeds are not very flammable, resulting in low temperature fires and patchy, discontinuous burns. Even when the surface area of the plant is killed, knapweed can often resprout from root reserves. Fires might actually be detrimental to knapweed control, creating disturbances favourable to knapweed establishment and growth (Sheley and Roche, 1982; Whitson, 1997; Sheley et al., 1998).Flaming with a propane torch can kill young knapweed seedlings or rosettes. The flame sears the plant, causing cells to rupture and the plant to dehydrate and die. Flaming is done on green plants; not on dead foliage. Plants can be seared at any time before flowering. Flaming is useful for clearing small areas (Whitson, 1997).Pulling or digging out knapweeds is best done in the spring during the rosette or early bud stage, before flowers appear, and when soils are moist. As roots can extend a few feet into the soil, the entire taproot should be pulled out, otherwise the plant may resprout (Payton et al., 1986; Sheley et al., 1999b).Uprooted plants should be disposed of properly. They can be piled and burned, buried in a landfill, buried in a deep, covered pit, or hot composted to kill the seeds. The first and second years of a hand-pulling programme will involve intensive removal because first year rosettes will then mature. During the third and fourth years, knapweed numbers decrease and pulling efforts are noticeably easier (Payton et al., 1986).Mowing decreases flower and seed production, and in the long-term it may effect knapweed densities. Timing is critical. Rinella et al. (2001) found that best results for spotted knapweed were obtained with one mowing in August, when the plants were in the flowering stage.

In general, herbicides should only be used as a last resort, because they are expensive and do not provide lasting control. They can also cause negative impacts on water quality, and select for resistant weeds that may be worse than the original problem (Powles and Shaner, 2001). The persistent herbicides clopyralid and picloram, which are often recommended for knapweed control, are not metabolized by grazing animals or destroyed by composting (Fay et al., 1991; Bezdicek et al., 2001; Houck and Burkhart, 2001). If herbicides are used, they should be applied before seeds are produced, and should be combined with a revegetation programme (Woo et al., 2002).

To date, 13 beneficial insect species from Europe have been released in the USA and Canada as biocontrol agents of spotted and diffuse knapweeds. The biocontrol complex includes four moths, four weevils, one beetle and four flies. Eleven species have established, and at least seven are well established and spreading (Julien and Griffiths, 1998; Smith, 2001). Larval forms do most of the damage and attack either the roots or seedheads (Muller-Schärer and Schroeder, 1993; Rees et al., 1996; Smith, 2001). Knapweed densities are lower in areas where biocontrols have been released (Clark et al., 2001a). So far, the most successful biocontrol agents are the root moth Agapeta zoegana, the root weevil Cyphocleonus achates, and the seedhead flies Urophora affinis and U. quadrifasciata (Story and Piper, 2001). The 13 beneficial insects that have been released in the USA and Canada as biocontrol agents of spotted and diffuse knapweeds are:Agapeta zoegana, a small yellow root-boring moth works best where knapweeds are abundant but not yet a monoculture (Rees et al., 1996; Lang, 1997). The moth may preferentially attack older plants and is more successful in combination with competitive plantings of grasses (Story et al., 2000).Metzneria paucipunctella, a seedhead moth was released to control spotted knapweed. By 1988 it spread onto diffuse knapweed. Larvae can destroy up to 90% of the seeds. However, low temperatures restrict the geographical range (Rosenthal et al., 1991; Rees et al., 1996; Good et al., 1997; Lang, 1997). Two other moths, Pterolonche inspersa and Pelochrista medullana were released but did not establish (Rees et al., 1996).The four weevil and one beetle species released for biocontrol of knapweed include three that attack seedheads and two that attack roots. Bangasternus fausti, a seedhead weevil, attacks diffuse, spotted and squarrose knapweeds, and to a lesser extent, purple and yellow starthistle. Larvae feed inside seedheads, destroying up to 100% of its contents. B. fausti requires undisturbed release sites with dry summers (Rees et al., 1996; Lang, 1997).Larinus minutus and L. obtusus are seedhead weevils. L. minutus prefers diffuse knapweed, and L. obtusus prefers spotted knapweed (Story and Piper, 2001). Adults are strong flyers and can easily disperse to new patches. The larvae feed on flowers and seeds, reducing seed production by up to 100% in infested seedheads. The weevils require dry areas with some bare ground, such as the outer edges of knapweed patches (Jordan, 1995; Lang et al., 1996; Rees et al., 1996; Lang,1997; Kashefi and Sobhian, 1998).Cyphocleonus achates, a root-boring weevil, prefers spotted knapweed, but will attack diffuse knapweed (Story and Piper, 2001). The weevil does not fly. Adults feed on knapweed rosettes. Larvae destroy the interior of the taproot and increase the plants susceptibility to pathogen attack. The insect requires high soil temperatures (Rees et al., 1996; Story et al., 1996, 1997; Lang, 1997). Weevil numbers are highest when knapweed cover is 30-70%. Largest establishment rates in Montana, Idaho and Washington occurred at about 900-1500 m (Clark et al., 2001b).Sphenoptera jugoslavica, a root borer, is a flat, blue-black, copper-coloured beetle that attacks roots of diffuse knapweed. The larvae feed on taproots, causing gall formation near the root crown. Although larval root feeding can kill rosettes, mature plants survive (Rees et al., 1996; Lang, 1997; Lang et al., 1998).Urophora affinis and U. quadrifasciata, two seedhead flies, attack both spotted and diffuse knapweeds. Eggs are laid between floral bracts and hatching larvae induce gall formation on the seedhead, causing new flowers to abort. Together, the seedhead flies can cause up to a 95% reduction in seed production. Both species prefer open areas with full sun (Maddox, 1979; Rosenthal et al., 1991; Rees et al., 1996; Lang, 1997; Lang et al., 1997). Although galls are eaten by deer mice, extremely cold temperature is the most important factor affecting overwintering survival of the flies (Nowierski et al., 2000; Pearson et al., 2000).Chaetorellia acrolophi, a seedhead fly, feeds on both diffuse and spotted knapweed. Larvae feed on the flower buds, reducing seed production. However, the effects of larval feeding on diffuse knapweed are unclear (Rees et al., 1996; Lang, 1997). Terellia virens, a seedhead fly, primarily attacks spotted knapweed. It does not form galls and has two generations a year (Rees et al., 1996; Lang, 1997).A number of pathogens attack knapweeds. Maculosin, a host-specific toxin produced by the black leaf blight fungus Alternaria alternata can destroy two-thirds of spotted knapweed foliage; however, younger leaves and buds are not affected and plants are able to resprout (Bobylev et al., 1996). Sclerotinia sclerotiorum, a common soil fungus with a broad host range, is effective in killing juvenile spotted knapweed (Rosenthal et al., 1991; Jacobs et al., 1996).Fusarium avenaceum [Gibberella avenacea], a stem blight fungus, causes stunted growth, yellowing, and stem decay on spotted knapweed. Originally isolated from diseased knapweeds in Montana, a strain of the fungus can prevent seedling germination and plant growth (Czembor and Strobel, 1997).

Combination treatments of fertilizer and herbicides have given inconsistent results. Sheley and Roche (1982) found that knapweed suppression and grass growth was greater at sites receiving both treatments. Hubbard (1975) got similar results in dry areas, but not where rainfall was greater. Sheley and Jacobs (1997b) found that the addition of nitrogen fertilizer had no effect on the growth of knapweed or native grasses in areas treated with the herbicide picloram for knapweed suppression.Bounty programmes, where rewards are offered for plant removal, have been successful in Montana as part of IPM programmes (Lacey et al., 1988).