More info for the terms: competition, cover, density, invasive species, natural, presence, resistance, root crown, succession
Impacts: Musk thistle is a problem on range and pastures because it competes with desirable forage, and its sharp spines can limit recreation, hinder movement, deter livestock - and presumably wildlife - from grazing [11,22,55,97]. Even at low densities of musk thistle, losses in production of native and/or forage species can occur because rosettes of musk thistle can grow greater than 3 feet (1 m) in diameter [106]. A review by Rees and others [97] suggests that 1 musk thistle plant per 16 square feet (1.5 m²) can reduce forage production by 23%. Musk thistle may also threaten rare or sensitive species, such as the Mescalero thistle (Cirsium vinaceum) in New Mexico, by crowding into populations [110]. Musk thistle spreads most rapidly along roadsides, fence lines, and sparsely vegetated areas. Though plants are hardier where there is little competition, musk thistle can also grow in native and seeded ranges, irrigated pastures, and wet meadows with dense stands of graminoids [55].
Control: Musk thistle should be accurately identified before attempting any control measures, since several native species of thistles have a similar appearance. See General Botanical Characteristics for information on proper identification.
The key to successful management of musk thistle is to prevent seed production. Control data suggest that the threshold for viable seed production by biennial thistles is zero to achieve long-term population decreases, although zero seed production may not be a realistic goal. The transition from seedling to rosette may be the most precarious stage in the life cycle of musk thistle. Seedling and rosette growth stages are the most logical to target for control efforts in biennial thistles [11].
Combining control methods into an integrated management system will result in the best long-term population decreases. Desirable plant competition is important in any biennial thistle management strategy to deter establishment of thistle seedlings and the transition to the rosette growth stage. Recovery of infested areas should not be considered complete until a diverse population of desirable plants has replaced invasive biennial thistles and the thistles are a minor to nonexistent component to the plant community [11]. The requirement of light for musk thistle seed germination highlights the importance of maintaining dense pasture canopies in order to suppress thistle germination [106]. Beck [11] suggests that land managers control musk thistle diligently for 15 years or more to eradicate it, because of the long-lived seed bank. He also encourages monitoring and evaluating weed management programs to determine whether and when to repeat and/or modify control treatments.
Prevention: Prevention is the most effective method for managing invasive species, including musk thistle [11,55,72,104]. The best way to prevent or reduce musk thistle invasion is to deny it a suitable habitat. Maintaining a healthy stand of desirable vegetation will prevent or slow musk thistle invasion, since seedlings are intolerant of intense competition, especially for light [55,81,92]. Pastures that are growing rapidly, especially at the time of peak musk thistle recruitment in the fall, can suppress musk thistle germination [93], enhance competition upon seedings, prolong the rosette stage of thistle development [30], and ultimately reduce musk thistle population density and seed production [92]. Careful grazing management will help keep pastures and rangeland healthy by enhancing grass competition and deterring thistle survival from seedlings to rosettes [11]. Overgrazing by livestock and wildlife should be avoided, because survival of musk thistle rosettes is said to increase as grazing intensity increases [54,70,100], and bare spots caused by over-grazing are prime sites for musk thistle germination and establishment [11].
Preventing or dramatically reducing seed production and dispersal decreases the spread of musk thistle. It is important to clean mowers, vehicles, and other equipment after operating in an infested area. When seeding is necessary, use clean, certified weed-free seed and mulch to ensure that thistles and/or other weeds are not being sown [11]. Remove single plants and control small infestations so they do not expand, as they will furnish adjacent areas with abundant seed for infestation [55]. Control measures must be followed-up to prevent reinfestation, and monitoring programs developed to locate any new infestations [11,55].
Integrated management: The goal of any management plan should be to not only control invasive plants, but to also improve the affected community, maximizing forage quality and quantity and/or preserving ecosystem integrity, and preventing reinvasion or invasion by other invasive species, in a way that is complementary to the ecology and economics of the site [25,56]. Effective long-term control requires that invasive plants be removed and replaced by more desirable and weed-resistant plant communities [56]. Once the desired plant community has been determined, an integrated weed management strategy can be developed to direct succession toward that plant community by identifying key mechanisms and processes directing plant community dynamics (site availability, species availability, and species performance) and predicting plant community response to control measures [103]. This requires a long-term integrated management plan [11].
Most often, a single control method is not enough to sufficiently control an invasive plant, but there are many possible combinations of methods that can achieve the desired objectives. Methods selected for removal or control of musk thistle on a specific site will be determined by land use objectives, desired plant community, extent and nature of the infestation(s), environmental factors (nontarget vegetation, soil types, climatic conditions, important water resources), economics, and effectiveness and limitations of available control techniques [98].
Herbicide applications in spring followed by dormant seeding of competitive perennial grasses in the fall is an example of an effective management system for biennial thistles in the western United States. Similarly, integrating herbicides and biological control agents is likely to be more effective than using biological control insects alone [11] (see "Biological control" below, for more information). For information on integrated weed management without herbicides, see the BIRC website.
Some examples of combined approaches and considerations are presented within the following sections. Managers are encouraged to use combinations of control techniques in a manner that is appropriate to the site objectives, desired plant community, available resources, and timing of application.
Physical/mechanical: Any mechanical or physical method that severs the root below the soil surface will kill musk thistle. However, it is essential to revegetate the site with desirable plants, particularly competitive grasses, to compete with biennial thistles that may reinvade from seeds left in the soil. Mechanical methods are not always practical on rangeland and natural areas, but can be effective in improved pastures and roadsides. Tillage, hoeing, and hand pulling may provide effective control of musk thistle, providing these operations are done before the reproductive growth stage to prevent seed production [11].
Hand pulling is a common and effective practice for musk thistle infestations of low density or as a follow-up operation after broad-scale treatment of dense or large infestations. Plants must be severed 2 to 4 inches (5-10 cm) below the surface to prevent resprouting [33,92,108]. This may be done with a sharpened shovel or other implement. Plants may be less likely to regrow if allowed to bolt before severing the root crown [108]. Any musk thistle flowerheads must be removed and burned or otherwise destroyed to eliminate seeding [33,55], since florets are sometimes capable of maturing and producing viable seed on severed plants [92]. Hand chopping at ground level just before flowering eliminated musk thistle from a 350-acre pasture on the Konza Prairie [108]. Hand pulling of musk thistle reduced the density of infestations following fires in Mesa Verde National Park [34].
Mowing or slashing can improve the appearance of thistle-infested pastures [92] and affectively reduce the population size, but proper timing is critical. Plants cut before the appearance of the terminal flower bud are likely to regrow and produce viable seed. Viable seeds can be also be produced from heads severed later than 4 days after anthesis [76]. The greatest reduction in seed production is when musk thistle plants are mowed just before flowering [55,108]. Mowing within 2 days of the 1st terminal flowerheads in a plant population showing anthesis eliminated the production of germinable seed from all mowed stalks [76]. If cut after plants have flowerheads, they should be burned or otherwise destroyed so the seeds will not mature [55]. A single mowing will not control a musk thistle infestation, because infestations often consist of plants of variable age, and stands therefore have nonuniform development and flowering [11,76]. For this reason it is necessary to mow several times each year to effectively minimize seed production [11,72,92]. Combining mowing with chemical control may improve results [72].
Musk thistle rarely occurs in croplands suggesting that it is intolerant of repeated cultivation procedures. Evidence suggests that musk thistle seed longevity is prolonged by burial; however, shallow tillage may help control musk thistle by promoting more rapid depletion of seed stocks and killing seedlings [92]. Tillage is not appropriate in wildlands and rangelands since it can damage important desirable species, increase erosion, alter soil structure, and expose the soil for rapid reinfestation by musk thistle and other invasive species [70].
Fire: See Fire as a control agent for information on this topic.
Biological: Biological control of invasive species has a long history, and there are many important considerations to be made before the implementation of a biological control program. The reader is referred to other sources (e.g. [98,129]) and the Weed Control Methods Handbook [117] for background information on biological control. Additionally, Cornell University, Texas A & M University, and NAPIS websites offer information on biological control.
Biological control of musk thistle has had substantial success in at least part of its range. Several agents have been considered and tested for musk thistle control, and those in the following table have been introduced in North America:
Biological control agent Mode of action Areas established References thistlehead weevil (Rhinocyllus conicus) larvae eat seed-producing tissue well established in most northwestern and northern plains states; GA, TN, TX, VA [20,49,61,97,99] thistle crown weevil (Trichosirocalus horridus) larvae feed on the growing points of thistle rosettes and developing shoots CO, KS, MO, MT, NC, OR, VA, WA, WY [20,61,79,97,99] thistle crown fly (Cheilosa corydon) larvae burrow into shoots where feeding causes a disruption of the plant's water and nutrient transport, decreasing flowering and seed production MD [97,99] musk thistle rust (Puccinia carduorum) attacks leaves, stems and bract leaves throughout the season; reducing seed set and seed quality DE, GA, IN, KT, MD, OH, SC, TN, VA [97]
Rhinocyllus conicus was introduced from Europe to Montana and Virginia in 1969. It does not destroy 100% of musk thistle seeds, and decreases in seed production are highly variable, ranging from 10 to 78%. It can be extremely effective by itself in those areas where the plant and insect life cycles are synchronized. A review by DeLoach [20] indicates 90 to 99% reduction of in musk thistle stands in Montana and Virginia and 80 to 90% reduction in Missouri. Rhinocyllus conicus will use Carduus, Cirsium, Silybum, and Onopordum genera as hosts but prefer the Carduus nutans group [11,97]. Several strains of R. conicus have been identified and they vary in their utilization of various thistle species. At least 1 of these strains does attack some native Cirsium species [20,71,97], and reviews by Beck [11] and Wilson and McCaffrey [129] indicate that it is known to attack native and rare thistles. Therefore, before releasing insects in a new area containing native Cirsium species, investigate whether any of the local species may be attacked [97]. A detailed discussion of the biology of R. conicus is given by Harris and Shorthouse [49].
Trichosirocalus horridus was introduced to the U.S. in 1974. This weevil uses thistles of the subtribe Carduinae, including musk thistle, plumeless thistle (Carduus acanthoides), Italian thistle (C. pycnocephalus), Canada thistle (Cirsium arvense), bull thistle (C. vulgare), and Scotch thistle (O. acanthium). Reports of suppression vary from slight to great. Trichosirocalus horridus is more effective when used in conjunction with R. conicus [97]. In areas of Missouri where R. conicus and T. horridus have been present for over 15 years, an 80 to 90% reduction in thistle population has occurred [108].
Integrated management systems may be developed utilizing knowledge of thistle and weevil life cycles, to encourage survival of the weevils over the long-term, while reducing the size or limiting the spread of bull thistle infestations in the short term. There is evidence that both Rhinocyllus conicus [116] and Trichosirocalus horridus [62] are somewhat tolerant of certain phenoxy herbicides, although more research is suggested. It may be useful, in the meantime, to create spatial or temporal separation of herbicide use and biological control. For example, insects can be released in the center of a biennial thistle infestation, and the borders sprayed to prevent further weed spread. Or, herbicides can be applied at a time when direct exposure of insects to herbicides will be minimized, such as when insects are pupating [11].
Cattle and domestic sheep and goats sometimes consume musk thistle flower and seedheads, and it has been suggested that domestic goats can help to minimize the presence of both bull [131] and musk [92] thistles. There is concern regarding whether cattle and domestic goats may facilitate musk thistle seed dispersal by eating seedheads. However domestic goats may prefer early flowering stages [92].
Chemical: Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but are rarely a complete or long-term solution to weed management [16]. Herbicides are more effective on large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. Control with herbicides is temporary, as it does not change conditions that allow infestations to occur [133]. See the Weed Control Methods Handbook for considerations on the use of herbicides in natural areas and detailed information on specific chemicals. Chemical control of musk thistle is reviewed by Beck [11].
Clopyralid, dicamba, MCPA, picloram, 2,4-D, metsulfuron, and chlorsulfuron and some combinations thereof will all kill bull and musk thistles. Herbicide choice and rates are influenced by growth stage, stand density, and environmental conditions (e.g., drought or cold temperatures) [11]. Auxin herbicides are effective only when applied during periods of active growth of seedlings or rosettes, usually during spring or fall [11,22,84,96]. Fall is a good time to control biennial thistles with herbicides because all live plants will be seedlings or rosettes, though cold weather may decrease the effectiveness of some chemicals [11]. Popay and Medd [92] and Medd and Lovett [82] suggest that spring application of herbicides may be more effective since musk thistle seedlings may still emerge in the spring after fall treatments. Herbicide tolerance of musk thistle increases after bolting [33,76]. Spraying thistles after they have bolted may reduce, but not eliminate, viable seed production. Because of the large numbers of seeds produced by musk thistle, reduction of flowering and seed production is unlikely to lead to an overall reduction in levels of infestation. Since seeds accumulate in soil, even effective herbicide treatment may have only a short term effect on musk thistle populations [92]. Clorsulfuron and metsulfuron are effective when applied in the bolting and early bud stages and may, therefore, be compatible with biocontrol agents [11].
Combinations of herbicides and nitrogen fertilizer were tried in Nebraska, with results suggesting that the added nitrogen favored the thistle over the perennial grasses [96]. Similarly, Austin and others [6] determined in a greenhouse experiment that yield of musk thistle increased in response to increasing nutrient concentration.
Musk thistle was found to be resistant to 2,4-D in New Zealand and may also be cross-resistant to other synthetic auxin herbicides [47]. When herbicides are used for musk thistle control, managers should consider an herbicide rotation to prevent development of resistance [73]. See weedscience.org for more information on herbicide resistance.
Cultural: No matter what method is used to kill weeds, reestablishment of competitive, desirable plant cover is imperative for long-term control. Reseeding with competitive, adapted species is often necessary in areas without a residual understory of desirable plants [98]. Vegetative suppression is applicable both before weed establishment and after weed control.
Bull and musk thistle germination and establishment are favored in open areas and by disturbance [11]. Vigorously growing grass competes with musk thistle, and fewer thistles occur in pastures where grazing is deferred. However, musk thistle can also become a problem in pasture or rangeland that is in good condition [9]. In New Zealand, pasture species and legume swards inhibited seedling shoot and root growth of both bull and musk thistles. Rate of musk thistle seedling emergence was also negatively correlated with pasture cover density (r=-0.666; p<0.05), presumably as a consequence of alteration of light quality by the pasture [124]. On Konza Prairie, musk thistle was present in heavily grazed spots, and burning favored establishment of musk thistle seedlings in areas where grazing had reduced vigor of prairie grasses. However, where the native prairie plants were vigorous, musk thistle was crowded out, whether burned or not [54]. A field plot study in Virginia indicated that musk thistle growth was seriously reduced by competition with tall fescue (Festuca arundinacea), more than infection by T. horridus, and R. conicus. Musk thistles planted simultaneously with tall fescue produced flowering plants but with reduced size and number of seeds per plant. When musk thistle was planted in 1-year-old tall fescue, germination was low and seeds that did germinate did not reach reproductive age. Because of its extensive root system and its ability to effectively compete for nutrients, seeding tall fescue where it is well adapted may be a practical method of musk thistle control in pastures, waste areas, and roadsides [63]. Tall fescue is not appropriate in drier areas [70]. Tall fescue is also an exotic, invasive species and is not, therefore, appropriate for natural areas.
Re-establishment of desirable vegetation after controlling existing populations of musk thistle will usually be necessary for successful control. Choice of species to sow will depend upon climate, location, and management objectives. The Natural Resource Conservation Service and land grant universities are sources of information about appropriate species for a particular purpose and location [11]. Reece and Wilson [96] express the importance of controlling weeds for more than 1 year to provide grasses with a competitive advantage that will help grasses recover fully. Following musk thistle population explosion after fire at Mesa Verde National Park, planting native perennial grasses slowed but did not stop the spread of musk thistle. Aerial seeding was the most effective treatment for preventing the spread of musk thistle when compared with hand pulling and biocontrol [34]. Pasture competition decreases germination and increases mortality of musk thistle seedlings, and slows growth of survivors. Encouraging early, rapid growth of pasture in fall can help reduce numbers of established musk thistle seedlings [30]. Management that allows grasses to grow taller in spring can also shade musk thistle seedlings, thus decreasing musk thistle establishment and growth [11].
