Chapter 11 Sustainable Control of Spotted Knapweed (Centaurea stoebe) D.G. Knochel and T.R. Seastedt Abstract Spotted knapweed is native to Eastern Europe, with a locally scarce but widespread distribution from the Mediterranean to the eastern region of Russia. The plant is one of over a dozen Centaurea species that were accidentally introduced into North America and now is found in over 1 million ha of rangeland in the USA and Canada. Land managers spend millions of dollars annually in an attempt to control spotted knapweed and recover lost forage production, and meanwhile the plant perse- veres as a detriment to native biodiversity and soil stability. These ecological concerns have motivated intense scientific inquiry in an attempt to understand the important fac- tors explaining the unusual dominance of this species. Substantial uncertainty remains about cause–effect relationships of plant dominance, and sustainable methods to control the plant remain largely unidentified or controversial. Here, we attempt to resolve some of the controversies surrounding spotted knapweed’s ability to dominate invaded com- munities, and focus on what we believe is a sustainable approach to the management of this species in grasslands, rangelands, and forests. Application of both cultural and bio- logical control tools, particularly the concurrent use of foliage, seed, and root feeding insects, is believed sufficient to decrease densities of spotted knapweed in most areas to levels where the species is no longer a significant ecological or economic concern. Keywords Biological control • Biological invasions • Centaurea stoebe L. ssp micranthos • Centaurea maculosa • Knapweed • Sustainable management 11.1 Introduction Knapweeds and yellow starthistle, plants belonging to the genus Centaurea and the closely related genus Acroptilon, are members of the Asteraceae that were acciden- tally introduced into North America from Eurasia over a century ago. These species D.G. Knochel () and T.R. Seastedt Department of Ecology and Evolutionary Biology and INSTAAR, an Earth Systems Institute, University of Colorado, Boulder, CO 80309, USA [email protected] Inderjit (ed.), Management of Invasive Weeds, 211 © Springer Science + Business Media B.V. 2009 212 D.G. Knochel and T.R. Seastedt have been identified as major problems for rangelands and forests of the western USA and Canada, occupying millions of hectares and causing millions of dollars of control costs and forage production losses (Duncan et al. 2004; Smith 2004). These species have also been identified as a threat to native biodiversity (Ortega et al. 2006). Sustainable, cost-effective management strategies to mitigate and diminish the negative effects of these knapweeds represent a priority activity desired by a broad spectrum of society, including ranchers, public land managers, and conserva- tion biologists. Smith et al. (2001) listed 495 research publications on various aspects of knap- weed ecology, with most of the research focused at perhaps the most aggressive and widespread of these species, spotted knapweed (C. stoebe micranthos, also identi- fied as C. maculosa and C. biebersteinii; see Ochsmann 2001; Hufbauer and Sforza 2008). A comprehensive review on management of spotted knapweed was provided in the late 1990s (Sheley et al. 1999); however, approximately 170 research articles on spotted knapweed have been published since then (Web of Science search by the authors, August 2007). In spite of this effort, our scientific understanding about cause–effect relationships of plant dominance has only slightly improved, and the important factors explaining the unusual dominance of this species remain largely unidentified or controversial. Sustainable management techniques are clearly desired, as Centaurea ranked as the most commonly cited noxious weed genus on government lists in the USA and Canada (Skinner et al 2000). Here, we define sus- tainable control as the process of using cost-effective management tools that cause a long-term reduction of the target weed to lower densities at which plants persist yet are no longer of ecological nor of economic concern. Ideally, such a sustainable management effort would employ both direct (reduction of plant fecundity and fit- ness via top–down controls by pathogens and herbivores) and indirect (bottom–up reduction in the available resources for the target plant via cultural methods that enhance native plant competition) methods that require minimal management inputs and reduce susceptibility to reinvasion. Here, we attempt to resolve some of the controversies surrounding spotted knapweed’s ability to dominate invaded com- munities, and focus on what we believe is a sustainable approach to the manage- ment of spotted knapweed in grasslands, rangelands, and forests. 11.2 Life History Information on Spotted Knapweed Spotted knapweed is native to Eastern Europe, with a locally scarce but widespread distribution from the Mediterranean to the eastern region of Russia (Hufbauer and Sforza 2008). The plant is one of over a dozen Centaurea species that were acci- dentally introduced into the North America and now is found in 45 states within the USA. It occupies over 1 million ha of rangeland in the western USA and occurs in over 60,000 ha of Canada (Story 1992). Spotted knapweed is a C3 perennial forb with a central taproot that grows as a rosette the first year and forms between one and ten flowering stems per year throughout its lifespan (Story et al. 2001). A review by 11 Sustainable Control of Spotted Knapweed (Centaurea stoebe) 213 Sheley et al. (1999) indicated that the plant may live up to 9 years, and is capable of producing 5,000–40,000 seeds per m2 per year. Schirman (1981) reported that spot- ted knapweed produced an average of 24–33 seeds per flower head during a 4-year interval, and over the same time period estimated seed production at 11,300–29,600 seeds per m2. The large seed bank that this species can create within the soil is also persistent, with 25% of seed viable after 8 years (Davis et al. 1993). Given such persistence, any management activities used to reduce densities of this weed must be sustained for a substantial period of time to prevent reestablishment from the seed bank. The plant has some nutritional value to wildlife, but tends to be an inferior food source for most generalist native species as well as to cattle. While sheep consume this species as a forage crop, sociological and economic issues have not favored the use of sheep in control activities or as a logical alternative to grazing by cattle (Alper 2004). 11.3 Dominance of Knapweed Species As previously mentioned, the mechanism(s) that allow Centaurea spp to achieve a high degree of dominance in many plant communities remain poorly identified and controversial. The ability of top–down controls (herbivores) to suppress this species has been debated for over a decade (e.g., Müller-Schärer and Schoeder 1993; Callaway and Ridenour 2004), and the competitive responses of this species across gradients of plant competition and soil resources (e.g., Maron and Marler 2007), and effects on those resources (e.g., Hook et al. 2004) have likewise been a major research topic. Allelopathy has been identified as a potentially significant competi- tive mechanism. Callaway and Ridenour (2004) and Callaway et al. (2005) argued that the relatively high levels of allelopathic compounds produced by the knap- weeds and released upon a plant community that had not evolved tolerance to these chemicals could explain the dominance of invasive Centaurea species in North America. Alternatively, a reduction in soil pathogens and/or enhanced positive feedbacks from microflora, including mycorrhizae, encountered in the new envi- ronments could also explain this dominance (e.g., Mitchell and Power 2003; Mitchell et al. 2006). The possibility also exists that both allelopathy and positive feedbacks from microbial communities contribute to the dominance and abundance of this species. Additional mechanisms and alternative hypotheses explaining Centaurea abun- dance have been proposed. Gerlach and Rice (2003) indicated that C. solstitialis was successful as an invader due to its abilities to persist within a community and exploit resource opportunities, a characteristic also shared by diffuse knapweed (LeJeune et al. 2006; Seastedt and Suding, 2007). Suding et al. (2004) demon- strated that rosettes of diffuse knapweed were strong competitors under ambient nutrient conditions, but were less competitive under lower nutrient conditions that may have characterized North American grasslands until recently. Further, the 214 D.G. Knochel and T.R. Seastedt superior ability of knapweeds to access soil water beyond the reach of native plants was suggested to allow greater photosynthetic-related advantages (Hill et al. 2006). Elsewhere, experiments on soil biota have tested the collective benefits provided by mycorrhizae and the absence of soil pathogens in the success of invasive species of Centaurea. Mycorrhizae fungi provided a competitive advantage to spotted knap- weed (Marler et al 1999; Callaway et al. 2004), but in another study were not bene- ficial to plants losing tissue due to simulated herbivory (Walling & Zabinski 2006). However, Callaway et al. (2001) demonstrated that mycorrhizal interactions with C. melitensis allowed the plant to exhibit compensation to grazing damage. Somewhat surprisingly, low to intermediate levels of herbivory can apparently benefit knapweeds (Callaway et al. 1999;