Improving the evaluation process of Cosmobaris scolopacea, a prospective biocontrol agent of Salsola tragus, using a molecular approach.
Russian thistle, Salsola tragus (sensu lato), (Chenopodiaceae) is a weed native to Central Asia that was accidentally introduced in the U.S. in the early 1870s with seeds imported from Russia. Due to the dramatic impacts of its invasiveness on ecology and human activities, the weed has been targeted for classical biological control since the 1970s. Two biocontrol agents have been introduced but they are not providing sufficient control. Further foreign exploration in the Mediterranean Region and Central Asia led to the discovery of several prospective new biological control agents, including Cosmobaris scolopacea (Coleoptera: Curculionidae). The larvae of this univoltine weevil are known to feed and pupate within the stems of several Chenopodiaceae including beet, Swiss chard, Chenopodium album, Halimione portulacoides, and Russian thistle. We wondered whether this nominal weevil species had cryptic host-specific populations that would be useful as biological control agents. A previous phylogeography study revealed a strong geographic structure of the mitochondrial CO1 genetic diversity with two divergent lineages in the native range, with one being restricted to Sicily and potentially associated only with S. tragus ssp. kali. The high CO1 divergence between the two lineages allowed defining lineages-specific amplification of the CO1 region. This approach revealed three morphocryptic sublineages of Sicilian origin. Two-way choice oviposition experiments conducted in 2011 confirmed the clear preference of one Sicilian sublineage for S. tragus ssp. kali, with occasional oviposition on Bassia hyssopifolia, Chenopodium album and Suaeda taxifolia (all Chenopodiaceae). This affordable molecular approach has provided an effective method to reveal cryptic diversity in this little explored taxon and to investigate accurately the specificity of this Sicilian sublineage. The next steps in this project are to characterize the lineage boundaries, which will require complementary insights from nuclear genes, and to examine further factors associated with pre- and postzygotic barriers.