Fly in the Ointment: Host-Specificity Challenges for a Candidate Agent for the Biological Control of Saffron Thistle in Australia.

In classical biological control of weeds, the risk posed by a candidate agent to close relatives of the target weed in the intended area of release is a key criterion (i.e., candidate agents that demonstrate a high degree of host specificity). In this study, we investigated if the rosette crown-feeding fly Hennig (Diptera: Anthomyiidae) could meet this criterion and thus be considered a good candidate to control saffron thistle L. (Asteraceae: Cardueae) in Australia. Previous studies indicated that is specific to and did not infest the closely related crop, safflower ( L.). However, more recent field observations made in Greece reported that infested safflower in cultivated fields. To determine if is safe for release as a biocontrol agent, we re-examined the host range of by performing new host-specificity testing combined with field surveys carried out in the south of France during two consecutive years. We also investigated the species identity of the flies by comparing DNA sequences ( barcode region) of specimens collected in France from and L. with those from Greece collected from and Lam. Our analyses confirmed that French and Greek samples identified as belonged to the same species, while a second group of Greek samples matched , indicating two distinct species. Our results also demonstrated that has a wider host range than previously suggested. Laboratory testing indicated that , and are suitable for the development of . Field surveys also revealed that is part of the host range of the fly. Based on the results reported here, may have the potential to control both the target weed, and but it may also be a threat to safflower, . Further investigations to assess under what conditions attacks may help clarify the level of risk to Australian growers.