Use of native range surveys to determine the potential host range of arthropod herbivores for biological control of two related weed species, Rhamnus cathartica and Frangula alnus

The buckthorn species, Rhamnus cathartica and Frangula alnus, are shrubs and small trees of Eurasian origin that have become invasive in North America. A program was initiated in 2001 to reassess the potential for biological control of these two species taking into consideration increasing concerns over potential non-target impacts of biological control agents. The key question was whether R. cathartica and F. alnus are distantly enough related that they would not share the same arthropod complex in Europe, and, if so, which arthropod species would be less likely to use native North American buckthorns as hosts. Some 1000 insect samples collected at 99 sites in Europe indicated that the arthropod-species richness is higher on R. cathartica than on F. alnus and includes more species that are presumed to be host-specific at the species or genus level. This discrepancy supports the hypothesis that the genus Rhamnus in the temperate Old World has evolved in isolation of the genus Frangula in the Neotropics and that taxonomic isolation has an effect on species richness of specialized herbivores. The fauna was dominated by Lepidoptera (22 species), followed by Hemiptera (8 species), Diptera (4 species), Acarina (4 species) and Coleoptera (1 species). At least 12 arthropod species were found exclusively on Rhamnus, some of which may be specific to R. cathartica. Several species usually associated with Rhamnus were found rarely on F. alnus but the field host range of these species still needs to be confirmed. Only one species was found exclusively on F. alnus. The findings indicate that, with one exception, there are no species or genus-specific agents available for biological control of F. alnus at this stage. However, additional field surveys may reveal other host-specific species.