Fly in the Ointment: Host-Specificity Challenges for Botanophila turcica, 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 <i>Botanophila turcica</i> Hennig (Diptera: Anthomyiidae) could meet this criterion and thus be considered a good candidate to control saffron thistle <i>Carthamus lanatus</i> L. (Asteraceae: Cardueae) in Australia. Previous studies indicated that <i>B. turcica</i> is specific to <i>Ca. lanatus</i> and did not infest the closely related crop, safflower (<i>Carthamus tinctorius</i> L.). However, more recent field observations made in Greece reported that <i>B. turcica</i> infested safflower in cultivated fields. To determine if <i>B. turcica</i> is safe for release as a biocontrol agent, we re-examined the host range of <i>B. turcica</i> 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 (<i>COI</i> barcode region) of specimens collected in France from <i>Ca. lanatus</i> and <i>Centaurea solstitialis</i> L. with those from Greece collected from <i>Ce. solstitialis</i> and <i>Centaurea diffusa</i> Lam. Our <i>COI</i> analyses confirmed that French and Greek samples identified as <i>B. turcica</i> belonged to the same species, while a second group of Greek samples matched <i>B. brunneilinea</i>, indicating two distinct species. Our results also demonstrated that <i>B. turcica</i> has a wider host range than previously suggested. Laboratory testing indicated that <i>Ca. lanatus</i>, <i>Ca. tinctorius,</i> and <i>Ce. solstitialis</i> are suitable for the development of <i>B. turcica</i>. Field surveys also revealed that <i>Ce. diffusa</i> is part of the host range of the fly. Based on the results reported here, <i>B. turcica</i> may have the potential to control both the target weed, <i>Ca. lanatus,</i> and <i>Ce. Solstitialis,</i> but it may also be a threat to safflower, <i>Ca. tinctorius</i>. Further investigations to assess under what conditions <i>B. turcica</i> attacks <i>Ca. tinctorius</i> may help clarify the level of risk to Australian growers.