Tuesday, August 07, 2007

Ephemeral

The West Virginia white (Pieris virginiensis) is a native butterfly, a small and delicate creature that flits through in the forests in early spring. I have never seen the species, which is known from my state, but not my county. The West Virginia white is rare throughout its range; populations are often small and isolated. Like many butterfly species, only more so, the West Virginia white is a habitat specialist. There are only two known host plants, both species of toothwort (Dentaria diphylla and Dentaria laciniata).

Toothworts are what we plant nerds call spring ephemerals: they emerge early in the season, bloom, set seed, and senesce in a few short weeks or months. The West Virginia whites emerge from their pupae early in the spring and feed on nectar from various spring wildflowers, laying their eggs on toothworts. When the larvae hatch, they eat their host plant, and they have to hurry. Toothworts generally fade completely before the end of June, leaving nary a trace above the ground.

To further complicate matters, the West Virginia white is an obligate forest dweller. It will not cross openings of any kind. Trails and small roads do not present a impediment, as long as the canopy remain closed, but even a typical small two-lane road is an insurmountable barrier. With such specific habitat requirements, the West Virginia white is obviously quite vulnerable to forest fragmentation. Another threat comes from invasive species.

Dentaria diphylla

Garlic mustard (Alliolaria petiolata) is of particular danger, and not just because it chokes out toothwort, although it most certainly does. Garlic mustard is in the same family, the Brassicaceae, as toothwort, and the plants are apparently closely enough related so that female West Virginia whites will oviposit on garlic mustard plants. The trouble is, when the larvae hatch and begin feeding, they all die. Garlic mustard is toxic to West Virginia white larvae.

However, garlic mustard was a host plant for a closely related butterfly species, the cabbage white (Pieris rapae), back in its native range, for the cabbage white is also non-native. With the spread of garlic mustard throughout our northeastern forests, the cabbage white has become more and more adventive, moving into the limited territory of the West Virginia white. This brings another problem, in the form of a parasitoid wasp. See, the cabbage white is not as limited in host plants as the West Virginia white.

The cabbage white feeds on all manner of cole crops: brussel sprouts, broccoli, cauliflower, collards, kale, kohlrabi, mustard, turnips, watercress, and of course, cabbage. All of these important food crops are in the same family as the native toothworts. Anyway, a parasitoid wasp (Cotesia glomerata) was introduced as a biological control agent for the cabbage white. As the cabbage white moves out of the fields and into the forest, the parasitoid wasp follows. Unfortunately for the West Virginia white, the parisitoid wasp kills native whites as well.

Sources: NatureServe Explorer, New York Natural Heritage Program, and Cornell University.

3 comments:

Mary Stebbins Taitt said...

This is a VERY SAD STORY and I am glad you blogged about it. I hope someone other than me cares about it and wish there was something reasonable that could be done.




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Nadine said...

Fascinating. And not the kind of thing most people ever think about.

Lupo Grigio said...

But important--VERY important. Who knows what else we're doing to earth, besides killing off babies in Iraq with bombs?