Three prospective agents instead of one? Cryptic diversity of the biological control agent Psylliodes chalcomera

The flea beetle, Psylliodes chalcomera, was previously permitted as a biological control agent of Carduus nutans (musk thistle) in the USA. However, during subsequent exploration for prospective biological control agents of Centaurea solstitialis (yellow starthistle), we observed individuals attacking this plant in southern Russia. We also observed this insect on Onopordum spp. (Scotch thistle), which raised the question whether this beetle is polyphagous, or whether it is comprised of biotypes, host races or cryptic species that are specialized on different host plants. We analyzed a fragment of the cox1 mitochondrial gene of adult and larval specimens collected from these three plants in Bulgaria, Italy, Russia and Turkey to study the population structure of this beetle. The results indicate at least three different groups: 1) associated with C. solstitialis in Bulgaria, Russia and Turkey, 2) associated with Onopordum spp. in Bulgaria, Russia and Turkey, and 3) associated with Carduus spp., Onopordum spp., and possibly C. solstitialis, in Bulgaria, Italy, Russia and Turkey. Adult specimens indicated a fourth group associated with Onopordum spp. and C. solstitialis in Turkey, but we did not have any larval specimens to confirm the host plant of this group. Our results suggest that three of these biotypes could be interesting to evaluate as prospective candidates for classical biological control of these three weed targets.