Update on garlic mustard biological control

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Garlic mustard is a biennial invasive weed native to Europe that is now widely distributed across North America from Georgia to Alaska. In a recent Landscape Alert (Vol. 22, No.2, April 13, 2007), Rebecca Finneran described garlic mustard’s two year life cycle and recommended control strategies for homeowners and land managers.

Ongoing research has shown that garlic mustard seeds can remain viable in the soil for at least ten years. Thus, long term management by hand is impossible for all but the smallest infestations. A search for suitable biological control agents for garlic mustard was initiated in 1998 by MSU collaborators at Cornell University, the University of Minnesota, and CABI Bioscience in Switzerland. Efforts are now focused on four beetles (weevils) in the genus Ceutorhynchus, whose larvae target multiple stages in garlic mustard’s life cycle.

Larvae of these species mine inside the root crowns of overwintering rosettes (Ceutorhynchus scrobicollis), mine in garlic mustard stems and leaf petioles (C.  alliariae, and C. roberti), or feed on developing garlic mustard seeds (C.  constrictus). Adults of all four species feed on garlic mustard foliage, which further reduces seed production. Population modeling studies by MSU researchers Adam Davis (now with USDA ARS in Illinois), Doug Landis and Doug Schemske suggest that releasing the root crown miner either alone or in combination with one of the stem-mining species is likely to be the most effective strategy for the greatest number of invaded sites. Current work in our lab is focusing on refining these models with new data we are collecting from Michigan and Illinois populations of garlic mustard.

Before weed biological control agents can be released, researchers must evaluate the breadth of their feeding preferences to demonstrate that they won’t harm non-target plant species. This host specificity testing has been completed for the root crown miner, and a proposal to conduct test releases is currently being prepared for the federal Technical Advisory Group, which oversees weed biocontrol. Testing is continuing on the other three candidate agents in Switzerland, and in a quarantine facility at the University of Minnesota. This means that biocontrol agents will not be available in Michigan for at least another one to two years, if they are approved. Until then, keep pulling garlic mustard to stop it from spreading.

For more information on garlic mustard invasions and control in Michigan,  please visit http://www.ipm.msu.edu/invasive_species/garlic_mustard and check back for updates throughout the year.

More thoughts on managing garlic mustard

The goal of management is to prevent garlic mustard from producing new seeds. Most first year seedlings and rosettes will die naturally, so management efforts should be focused on second year plants. Second year plants are easy to distinguish from first year rosettes in late spring once their stems elongate in May and they begin producing white flowers. First year rosettes will not have long stems or flowers and lay close to the ground. In summary, small infestations can be managed with vigilant pulling of second year plants every year prior to seed production or with careful application of glyphosate (e.g. Roundup) during the late fall or winter when most native species are dormant.

We now know that viable seeds can develop on any garlic mustard plants that have already flowered, even after the plants have been pulled. For this reason, we no longer recommend burying pulled plants. Plants should be bagged in plastic, tied up, and removed.

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