Onopordum acanthium (scotch thistle)

O. acanthium, with its intermittent germination and its prickly stem and leaves at maturity, causes problems for agricultural products, poultry and other livestock farms (Hooper et al., 1970; Auld et al., 1979; Wheatley, 1981). Hooper et al. (1970) stated that infestation of O. acanthium in northern California, USA, caused annual losses to ranchers of US$25.20/ha in wet meadows, US$16.60/ha in wheatgrass stands and US$8.40/ha in downy brome (Bromus tectorum-dominated) rangelands. In Australia, O. acanthium and O. illyricum are considered to be the worst and most costly weeds of the genus Onopordum. These two thistles form problem infestations in 57% of the counties in New South Wales with infestations mainly in southern and central tablelands (Briese, 1988) and the area infested is almost 1.1 million ha (Briese et al., 1990). The median annual cost of control, including labour, was as high as $A50/ha (Briese, 1996a) and the annual cost was estimated to be $A15-20 million in 1987. They are considered to be gradually spreading and are difficult and expensive to control by herbicides, particularly as they are resistant to cheap and mild hormonal herbicides such as 2,4-D and MCPA. If farmers want to eradicate O. acanthium from their lands they need to use more potent and expensive herbicides but these can also destroy valuable pasture species (Davidson, 1990). O. acanthium can act as a living fence, limiting access to grazing and water (Hyde-Wyatt, 1968; Hooper et al., 1970; Sindel, 1991). It also causes both wool flaw and injury to animals (Auld et al., 1979).