Garlic Mustard: Negative impact on hardwood trees, seedlings, mycorrhiza & rare West Virginia White butterflies

A Perspective on the Ecological Ramifications Of Garlic Mustard
by Donald A. Sutherland - Biodiversity Section of the OMNR

“The garlic mustard/sugar maple relationship would give Norway maple an additional competitive advantage, particularly in the ravines of Toronto and other riparian areas of southern Ontario where garlic mustard is abundant. Garlic mustard could reduce or eliminate the soil mycorrhizae necessary for the survival of sugar maple seedlings and thereby give a competitive advantage to Norway maple. I have no idea whether Norway maple seedlings are similarly dependent on soil mycorrhizae for their survival, but both exotic, Norway maple and garlic mustard have presumably evolved together. Norway maple produces abundant seed with high viability rates and is tolerant of air pollution such that it has already a competitive advantage over the native Sugar Maple in urban and near-urban areas. In more native forest situations, a loss of sugar maple could in the long term have grave implications for forest song birds which are dependent on the many sugar maple-obligate invertebrate species which feed on the maple. Many forest lepidopteran species, for example Bruce's Spanworm, are largely dependent on sugar maple and the loss of these species (and particularly their larvae) could considerably reduce available food for forest birds and their nestlings.

Garlic mustard is already posing a serious problem for one of the province's SAR butterfly species. The West Virginia White, currently a special concern species in Ontario, is a relatively widespread but uncommon forest butterfly throughout its eastern North American range. The larval host of the West Virginia White is a native member of the mustard family, called toothwort. Increasingly, West Virginia White is being observed nectaring on the closely-related, early-flowering garlic mustard; however, not only is garlic mustard a common source of nectar for the butterfly, but the West Virginia White is preferentially laying its eggs on the exotic mustard. The eggs hatch, but the mustard is evidently unpalatable to the maturing larvae and the butterfly larvae don't survive beyond their first instar - thus garlic mustard is effectively an ecological cul-de-sac for the West Virginia White. Never abundant, the West Virginia White has recently been placed among the ranks of globally rare taxa by NatureServe, largely thanks to problems with garlic mustard.

There is often the perception that some exotic species are benign. Exotic species are never benign they only appear so because their impacts haven’t yet been recognized.”

Please also see:

• Invasive Plant Suppresses the Growth of Native Tree Seedlings by Disrupting Below ground Mutualisms. Stinson KA, Campbell SA, Powell JR, Wolfe BE, Callaway RM, et al. (2006) PLoS Biol 4(5): e140
• Invasive species harms native hardwoods by killing soil fungus - Steve Bradt, Harvard University, Apr 26/06 “Find is first to identify specific mechanism by which invasive plants harm native species”
• Garlic Mustard photos & information: Global Invasive Species Database – ISSG – Garlic Mustard

• For more information please see other Garlic Mustard, mycorrhiza or invasive plant resources posted on this blog.