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

Abstract

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