Potential for biocontrol of Silybum marianum (variegated thistle): 1. Survey of natural enemies in New Zealand

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. As part of an initial investigation into the potential for biocontrol, a survey of the phytophagous insects associated with S. marianum in New Zealand was carried out at 16 populations from 29 November to 16 December 2021. Phytophagous associations were determined by collecting insects on the plant and by seedhead dissections. The species collected were grouped into three categories: ‘specialists’, ‘generalists’, and ‘tourists’ that comprised 27.4%, 66.3%, and 6.3% of the total number of specimens collected, respectively. The specialist group was almost entirely comprised of the seedhead weevil, Rhinocyllus conicus (Frölich). While adults of R. conicus were commonly collected on variegated thistle, only 6.5% of seedheads contained larvae of the weevil, indicating that S. marianum is not a common developmental host. Generalist species commonly collected on S. marianum included: Nysius caledoniae Distant, Rhypodes clavicornis (F.), Closterotomus norwegicus (Gmelin) and Nezara virdula (L.). Overall, the field surveys document low levels of attack from generalist herbivores and minimal attack by the specialist weevil, R. conicus. Vacant niches are present on the weed in New Zealand, providing a strong case for introducing specialised biocontrol agents.