The monarch butterfly is one of the glories - and marvels - of the natural world. The brilliant orange-and-black monarch migrates by the millions from the northern United States and Canada to central Mexico, a trip of thousands of miles that takes the butterflies four generations to complete.

But the monarchs' numbers are dwindling to their lowest recorded level. Their decline is attributed to a number of factors, including drought, bad weather, pesticides, but mostly to the loss of habitat - and the milkweed that is the central sustaining element of their existence.

The butterfly uses milkweed for food and as a place to hatch its eggs. As caterpillar food it produces a chemical that makes the monarchs unpalatable to predators. But milkweed, which once grew rampant in the Midwest and Great Plains is being eliminated as grassland is increasingly used for corn and soybeans. The New York Times reported this week that genetically modified varieties of crops are being planted that are resistant to herbicides, allowing their increased usage - to the detriment of milkweed.

But a campaign is under way to save the migrating monarchs by saving the milkweed. The Times cites the work of Monarch Joint Venture, a cooperative effort of university scientists, non-profits and government agencies that is leading the way in research and conservation to increase the monarchs' flagging numbers.

Meanwhile, Monarch Watch has enlisted volunteers to plant milkweed in backyards and other way stations along the route of migration. The National Resources Defense Council has funded its efforts to supply milkweed plants to schools. Utilities are being encouraged to plant milkweed along power line easements.

It's a real grassroots effort - to restore a lowly weed that sustains the aptly named monarch butterfly. Given the sharp decline of the monarchs' wide-ranging habitat, it could be a race against time.