Conclusions from 11 years of buckthorn biocontrol research

Background/Question/Methods

Rhamnus cathartica (common buckthorn) and Frangula alnus (glossy buckthorn) (Rhamnaceae) are both shrubs and small trees of Eurasian origin which have become invasive in North America.  In 2001, a new research program to develop biological control for buckthorns was initiated.  Candidate biological control agents would be monospecific to R. cathartica or F. alnus or their host ranges restricted to a few non-native species in either the Rhamnus or Frangula genera. 

Results/Conclusions

Initial surveys and research found that there were no host-specific potential biocontrol agents for F. alnus.  Over 30 specialized insects were identified as potential candidates for biocontrol of R. cathartica.  Most of these species were discarded because they lacked host-specificity.  Two psyllids were promising in terms of host-specificity, but did not cause significant damage to buckthorn and the insects were infected with the plant disease ‘Candidatus Phytoplasma rhamni’ (buckthorn witches’ broom).  Due to the limited research on this disease and since it is not known to be in the United States, there is low potential that the psyllids would be approved for release in the US.  A final potential biocontrol insect, a seed-feeding midge, proved too difficult to work with in a research setting.  It was not possible to obtain adult fruiting trees of native North American Rhamnus species for testing in Switzerland.  After 11 years of searching for a biological control insect that is host-specific and damaging to buckthorn, we conclude that we do not have any promising agents at this time.