Twenty-five years after: post-introduction association of Mecinus janthinus s.l. with invasive host toadflaxes Linaria vulgaris and Linaria dalmatica in North America
Toševski, I., Sing, S.E., De Clerck-Floate, R., McClay, A., Weaver, D.K., Schwarzländer, M., Krstić, O., Jović, J., Gassmann, A. (2018). Twenty-five years after: post-introduction association of Mecinus janthinus s.l. with invasive host toadflaxes Linaria vulgaris and Linaria dalmatica in North America. Annals of Applied Biology, [online] 173(1), 16-34. http://dx.doi.org/10.1111/aab.12430
Plain language summary
Yellow toadflax and Dalmatian toadflax are perennial herbaceous plants originating from Europe that have long been regarded as invasive weeds of agriculture and natural ecosystems in North America. A biological control program was initiated in Canada and the USA in the 1960s to find and test toadflax-feeding insects from Europe to be used in the control of these weeds. The first effective insect (‘biocontrol agent’) for toadflax control was released in North America in the 1990s; a type of beetle (weevil) that feeds within toadflax stems. This insect subsequently was believed to have established on both toadflax species in Canada and the USA, but demonstrated greater success in controlling Dalmatian toadflax than yellow toadflax. Inconsistencies in the successful establishment of the weevil in North America, plus results of a recent study on the weevil’s genetics and evolutionary origins in Europe, led us to suspect that there may have been issues with its identification during early introductions and use of the agent. The current study sought to determine what has been released and established on the two toadflax species in North America. We first collected the toadflax weevils from European locations where they were originally obtained for our program in the 1990s (Switzerland, Germany, Macedonia), and also from where they have established on Dalmatian and yellow toadflax in North America. Genetic analyses conducted on these weevils revealed that instead of us releasing one weevil species to control two invasive toadflax species, we instead have been using two different weevil species. These two species are indiscernible visually, but can be distinguished through genetic analysis and by the fact that one species is primarily associated with yellow toadflax, and the other with Dalmatian toadflax. This information will inform more effective strategies in toadflax management going forward.
Linaria vulgaris, common or yellow toadflax, and Linaria dalmatica, Dalmatian toadflax (Plantaginaceae), are Eurasian perennial forbs invasive throughout temperate North America. These Linaria species have been the targets of classical biological control programmes in Canada and the USA since the 1960s. The first effective toadflax biological control agent, the stem-mining weevil Mecinus janthinus (Coleoptera: Curculionidae) was introduced from Europe in the 1990s. This weevil has become established on L. dalmatica and L. vulgaris in both countries, although it has shown greater success in controlling the former toadflax species. Genetic and ecological studies of native range M. janthinus populations revealed that weevils previously identified as a single species in fact include two cryptic species, now recognised as M. janthinus, associated with yellow toadflax, and the recently confirmed species Mecinus janthiniformis, associated with Dalmatian toadflax. The results of a comprehensive study characterising haplotype identities, distributions and frequencies within M. janthinus s.l. native range source populations were compared to those populations currently established in the USA and Canada. The presence of both Mecinus species in North America was confirmed, and revealed with a few exceptions a high and consistent level of host fidelity throughout the adopted and native ranges. Genetic analysis based on mitochondrial cytochrome oxidase subunit II gene (mtCOII) defined the origin and records the subsequent North American establishment, by haplotype, of the European founder populations of M. janthinus (northern Switzerland and southern Germany) and M. janthiniformis (southern Macedonia), and provided population genetic indices for the studied populations. This analysis together with existing North American shipment receipt, release and rearing records elucidates probable redistribution routes and sources of both weevil species from initially released and established adopted range populations.