Good plants and bad plants. Sometimes it is difficult to know the difference just by looking. This is true for butterflies and three plants in the mustard family that grow on lands managed by the Bureau of Land Management-Eastern States Milwaukee Field Office. “Bad plant” garlic mustard (Alliaria petiolata) is a Eurasian herb that is aggressively invading forests throughout portions of the United States and southern Canada. Botanical surveys conducted during 2006 on the Milwaukee River Public Domain Islands in Wisconsin revealed an extensive presence of garlic mustard, and methods to control the populations of this invasive exotic plant are currently underway.

The “impostor” host plant fools the pollinators.

In southeastern Wisconsin, garlic mustard displaces “good plants” such as the cut-leaved toothwort (Cardamine concatenata) and broad-leaved toothwort (Cardamine diphylla) — two native mustard plants that are important hosts for the West Virginia White butterfly (Pieris virginiensis). This rare pollinator is listed in Wisconsin's State Wildlife Action Plan as a “Species of Greatest Conservation Need.”

The West Virginia White is restricted to well-shaded, rich, mesic woodlands, and the pollinator’s entire lifecycle is closely tied to host plants.  Native toothwort plant species not only offer a source of nectar for adult West Virginia White butterflies but also provide crucial habitat for developing butterfly larvae. Unfortunately, females of the West Virginia White are chemically attracted to garlic mustard and deposit their eggs on the nonnative plant.  When this happens, larvae are unable to hatch or are poisoned when they try to eat the garlic mustard, adding to the decline of the butterfly species.

BLM, Partners Restore Habitat for Native Host Plant

With the help of our partners, including the USDA Forest Service, Wisconsin Bureau of Endangered Resources, Waukesha County Parks, University of Wisconsin - Milwaukee and the Chicago Botanic Garden, the BLM-MFO staff has initiated a project to germinate and propagate cut-leaved toothwort as a native replacement for garlic mustard on the Milwaukee River Public Domain Islands.  On May 25, 2007, BLM-MFO staff members Jill Hapner, June Wendlandt and Chris Carusona collected cut-leaved toothwort seeds at a local State Natural Area in Waukesha County, Wisconsin. Plans are underway for official transfer of additional toothwort seed material from the Plant Conservation Alliance Seeds of Success program. Selected toothwort plants will be maintained as seed sources by the UWM Field Station, while other plants will be transplanted to the Public Domain islands following garlic mustard removal.  BLM-MFO will closely monitor the plantings of toothwort as part of a long-term adaptive management plan.

Future Efforts will Conserve Native Plant, Pollinator Relationships 

This project permits the BLM-MFO and multiple partners to develop native plant materials, replace invasive species on Public Domain Land and restore native host plants for a rare pollinator species. Garlic mustard invasion is a widespread concern and investigations such as this will improve the scientific basis of methods to manage the exotic species.  By focusing on important native plant and animal relationships through experimental research, long-term monitoring, and evaluation, we can contribute to the development of a framework for improving management of invasive plants using native replacement strategies. This article highlights the Department of the Interior’rs role in and commitment to conserving native plant habitats for pollinators.

For additional information, contact Jill Hapner, STEP Natural Resource Specialist

Bureau of Land Management-Eastern States at (414) 297-4434; Jill_Hapner@blm.gov