Biological Control of European Buckthorn and Garlic Mustard

PROJECT OVERVIEW
European buckthorn and garlic mustard are non-native, invasive plant species that have rapidly spread throughout Minnesota posing serious threats to native plant communities and degrading wildlife habitat in forests and riparian areas. The two plants are considered to be the species of highest priority for development of long-term management solutions, such as biological control, which involves using natural enemies of a non-native species from its native region to control or reduce the impact of the species in the areas where they are invasive. Introducing one non-native species to control another, though, is something that must be done with care so that the introduction doesn't have unintended consequences. This appropriation is enabling the Minnesota Departments of Natural Resources and Agriculture to continue to research and evaluate biological control options for European buckthorn and garlic mustard.

OVERALL PROJECT OUTCOME AND RESULTS
European/common buckthorn (Rhamnus cathartica) and garlic mustard (Alliaria petiolata) are non-native invasive plants that severely threaten native plant communities and degrade wildlife habitat. They are widely distributed in the state and current control options, such as mechanical and chemical control, are labor and cost-intensive. They are of the highest priority for development of long-term management solutions, such as biological control. The purpose of this research was to determine 1) if there are suitable insects that can be used to reduce impacts caused by buckthorn and 2) implement introduction of insects to control garlic mustard and assess their establishment and success.

Over 30 specialized insects were identified as potential common buckthorn biocontrol. Most of these species were discarded because they lacked host-specificity. Two psyllids were host-specific, but did not cause significant damage to buckthorn and the insects were infected with the plant disease 'Candidatus Phytoplasma rhamni' (buckthorn witches' broom). A seed-feeding midge proved too difficult to work with in a research setting. After 11 years of searching for a biological control insect that is host-specific and damaging to buckthorn, we conclude that there are not promising agents at this time.

Four Ceutorhynchus weevil species are being studied as biological control agents for garlic mustard. Petitions for release were submitted to the USDA-APHIS Technical Advisory Group starting in 2008, but they have requested additional host-specificity testing over time. No biological control insects have been approved for release as of 2014. Studies conducted in the University of Minnesota Containment Facility allowed the development of efficient and consistently reliable methods to rear C. scrobicollis from garlic mustard plants. Long-term monitoring at twelve sites in Minnesota shows that garlic mustard populations can fluctuate widely from year to year. There is little garlic mustard herbivory in Minnesota. Garlic mustard cover is negatively correlated with cover of other species.

PROJECT RESULTS USE AND DISSEMINATION
Buckthorn biological control research has been disseminated in one peer reviewed journal publication, a summary report by CABI, four poster presentations, and a webpage on the DNR website: http://www.dnr.state.mn.us/invasives/terrestrialplants/woody/buckthorn/biocontrol.html.

Garlic mustard biological control research has been disseminated in one peer reviewed journal publication, a U.S. Forest service report (http://www.fs.fed.us/foresthealth/technology/pdfs/GarlicMustardBiocontrol_FHTET-2012-05.pdf), project reports, and seven conference presentations.