Carduus nutans (nodding thistle)

As C. nutans is a biennial found mainly in pastures, a key to controlling it in such systems is to utilize its dormancy when covered by green vegetation, preventing it from germinating (Phung and Popay, 1981). Techniques to utilize this include sowing pastures in dry areas with drought-tolerant cultivars and avoiding overgrazing of pastures during dry times of the year by reducing stock numbers and growing supplementary crops (Harrington, 1996). Although sheep and cattle avoid grazing C. nutans plants, goats will graze them especially as seed-heads are forming (Holst and Allan, 1996). C. nutans is still spreading to new areas and increasing its density in presently infested areas. Declaring the species noxious and thus applying regulatory pressures to reduce its incidence is a common strategy for controlling the species (Skinner et al., 2000). Prohibiting the presence of any C. nutans seed within certified pasture and crop seed is a common strategy to reduce further spread of the species (Popay and Medd, 1990).

Hand grubbing is commonly practised for infestations of low density or as a follow-up operation after herbicide treatment of denser or larger infestations (Popay and Medd, 1990). Grubbing hoes must cut the plants 5-10 cm below ground level to prevent resprouting from dormant axillary buds. Mowing the plant during flowering can greatly reduce seed production, though a single mowing is seldom sufficient due to the wide differences in the maturity of plants in a natural population (McCarty and Hatting, 1975).

Good control of C. nutans can be obtained using herbicides such as 2,4-D, MCPA, clopyralid, dicamba and picloram, though the favoured option in clover-based pastures is to use the less effective MCPB while seedlings are young and thus susceptible to this herbicide (Popay and Medd, 1990). MCPA or 2,4-D will successfully control larger rosettes without damaging the clover substantially, but application must occur before rosettes become too large, and plants need to be actively growing (Popay et al., 1989). Repeated application of phenoxy herbicides to C. nutans populations over several decades has led to resistant ecotypes developing in some parts of New Zealand (Harrington, 1990).

Of the organisms listed in the Natural Enemies section, some have been introduced widely throughout the world to control C. nutans whereas others have been only tentatively released thus far and several have been deemed unsuitable for use.Rhinocyllus conicus has been released in Canada, USA, New Zealand and Australia since the 1970s and is well established in all of these countries (Popay and Medd, 1990). Although R. conicus has been recorded as being successful against C. nutans in parts of the USA, it has generally been less successful in New Zealand where it has been estimated that 69% reduction in seed production is required to reduce C. nutans populations, more than is usually achieved by the weevil (Shea and Kelly, 1998).Trichosirocalus mortadelo is probably restricted to C. nutans, and was introduced from Germany to Canada and from there to New Zealand and then Australia, and from Italy to the USA. However, it remains a possibility that the original introductions for C. nutans also included T. horridus and examination of voucher specimens is required to clarify the issue (Julien and Griffiths, 1998). Cartwright and Kok (1985) found the response of C. nutans in the USA was dependent on plant size and growing conditions, though infested plants consistently produced more stems and larger crowns than uninfested individuals due to the destruction of apical dominance. However, small and medium infested plants were shorter and produced fewer seeds and heads than uninfested plants. In Australia, Woodburn (1997) found T. horridus killed some plants and reduced rosette growth of survivors by 50%. The reproductive potential of the attacked plants, measured as seeds per plant, was reduced by 67%.The third insect to be used quite extensively as a biocontrol agent is Urophora solstitialis (Julien and Griffiths, 1998). It has been used in Australia and New Zealand to destroy seeds produced late in the season which are not controlled by R. conicus. However, there have been some problems with competition between the two agents (Woodburn and Briese, 1996). There have been limited releases of Cheilosia corydon (=C. grossa), Psylliodes chalcomera and Puccinia carduorum in USA, but the impact of these species cannot yet be ascertained (Gassmann and Kok, 2002).Research has been underway for a number of years in New Zealand to develop a mycoherbicide for C. nutans and several other thistle species using the fungus Sclerotinia sclerotiorum (Lib.) de Bary (Bourdot and Harvey, 1996), but a commercial product has still not been released from this work. Although there has been limited success to date in controlling C. nutans with biological control agents, it is generally agreed that they are an important component in any integrated weed control programme for this weed rather than as a sole control method (Woodburn and Briese, 1996).

As discussed by several authors, the most effective strategy for controlling C. nutans is to use a combination of regulatory, cultural, physical, biological and chemical techniques (e.g. Harrington, 1996; Huwer et al., 2002).