Author
Mike Cripps
michael.cripps@lincoln.ac.nz
Lincoln University
Lincoln
Abstract
Silybum marianum (variegated thistle, milk thistle) is one of the worst weeds of hill country pasture in the east coast regions of New Zealand. A recent field survey in New Zealand found low levels of attack from generalist herbivores and minimal attack by the specialist weevil, Rhinocyllus conicus. In the native range of Europe there are at least 41 insects and 12 fungi sufficiently specialised to be considered for biocontrol. A literature search revealed some potential biocontrol agents not previously reported in past investigations for biocontrol of S. marianum. These include the specialised biotypes of the tephritid fly, Terellia fuscicornis, and the seedhead weevil, Larinus latus. Other potential agents include the stem-galling cynpid wasps, Aulacidea freesei and Phanacis zwoelferi, that are likely specific to S. marianum. The host specific smut fungus, Microbotryum silybum, is another promising biocontrol candidate. A shortlist of nine priority agents (eight insects and one fungal pathogen) was selected based on reported affinity for S. marianum, evidence for impact on the weed, and niches attacked. In the case of S. marianum, there are sufficiently specialised agents that attack several niches, including seeds, stems, and the root or root collar. Silybum marianum is also a valued medicinal herb and therefore the impact of the weed needs to be weighed against any potential benefits of the plant.
Keywords
target weed
pasture weed
variegated thistle
milk thistle
Highlights
Control of the weed is estimated to cost over one million NZD per annum
At least 41 insects and 12 fungi sufficiently specialised to be considered for biocontrol
It is recommended that Silybum marianum be considered a target for biocontrol