Wednesday, October 14, 2009

Alien Invasions. Our Native Butterflies and the Intruders that Threaten Them

The invasive cabbage white Pieris rapae, a pest of cabbage and related plants.

When we see a white butterfly it is most likely an invasive European species, called a cabbage white. Its beauty often blinds us to the fact that it is an unwelcome intruder which behaves as a pest. Growers of cabbages, radishes and other plants in the mustard family hate the damage that its caterpillars do to plants. You may have seen that same damage if you grow ornamental cabbages in your garden. The cabbage white or Pieris rapae, is a sturdy fellow that does well in many habitats and keeps spreading throughout the continent.

West Virginia White (Pieris virginiensis), beleaguered by non-native plants and butterflies.

The North American native species of white butterflies are well behaved, feeding in a number of native plants, such as toothwort, watercress and other members of the mustard family. They are no threat to native nor cultivated plants as their numbers are not overwhelming and a good balance has been reached between butterflies and food plants.

There are four native species; they all look very similar but you can tell them apart by subtle differences. The lovely green veined butterfly or mustard white butterfly (Pieris oleracea) lives in woodlands across northern North America and in the Rockies and Appalachian mountains. The West Virginia white (Pieris virginiensis) is found in the Appalachian and Great Lakes regions. These butterflies face several threats to their survival; in addition to the more familiar one, loss of habitat, there is a more serious and insidious one due to the presence of an invasive plant, garlic mustard.

Garlic mustard (Alliaria petiolata)

Garlic mustard (Alliaria petiolata) is very familiar to most of us; an introduced invasive species originally from Europe and Asia; it was brought to America as food, although nowadays nobody uses it for this purpose. It promptly escaped cultivation and now is most commonly found in areas disturbed by humans. It can be found almost anywhere in the eastern United States, especially in the spring, growing vigorously in sun or shade, blooming and spreading its seeds. It belongs to the mustard family and this is the undoing of our native white butterflies. Females can't tell the difference between this plant and others of the same family and so they lay their eggs on this plant. Little do they know that garlic mustard is toxic to their babies; their larvae don't do well and die as a result.

One of the reasons for the successful spread of garlic mustard is that when brought to North America, it left behind most of its enemies, about 63 species of insects, including several species of butterflies. It is ironic that the one that tries to take advantage of it is being poisoned and killed by it.

You can learn more about the West Virginia White butterfly and conservation efforts in
Conservation Assessment for the West Virginia White (Pieris virginiensis Edwards) and garlic mustard and methods of control in Missouri Department of Conservation website.

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