Potential for biocontrol of Silybum marianum (variegated thistle): 2. Literature search and prioritisation of biocontrol agents

Silybum marianum (L.) Gaertner (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 (Frölich). In the native range of Europe there are at least 41 insects and 12 fungi sufficiently specialised to be considered for biocontrol. The literature search revealed some potential biocontrol agents not known during the historical biocontrol surveys for S. marianum. These include the specialised biotypes of the tephritid fly, Terellia fuscicornis Loew, and the seedhead weevil, Larinus latus Herbst. The literature search also revealed the discovery of the stem-galling cynpid wasps, Aulacidea freesei Nieves-Aldrey and Phanacis zwoelferi Nieves-Aldrey, that are likely specific to S. marianum. The discovery of the smut fungus, Microbotryum silybum Vanky & Berner, host specific to Silybum species, is another promising biocontrol candidate. A shortlist of nine priority agents (eight insects and one fungal pathogen) were 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. Testing for complementary agents that attack different niches is likely to improve the chance of successful biocontrol. It is recommended that S. marianum be considered a priority target for classical biocontrol in New Zealand.