Monarch numbers increasing in droves

Jim McCormac

On Sept. 15, 2013, my Nature column covered monarch butterflies. The message was gloomy. Following is an excerpt:

“Far fewer Monarchs are making this journey (to Mexican wintering grounds). Only 20 years ago, their ranks blanketed 45 acres of Mexican fir forest. Last winter, the butterflies occupied less than three acres. Many people have commented on their absence this fall. Some authorities estimate that Monarch numbers declined by 60 percent over the past two years.”

Then, monarchs appeared to be in dire straits, and a prime factor in the butterfly’s decline was the loss of milkweed. Monarchs require these plants as hosts — milkweeds are the only flora that the caterpillars can consume.

There are 13 native milkweed species in Ohio, and monarchs probably use them all. But the two species that do most of the heavy lifting are common milkweed and swamp milkweed. The latter grows in damp soil, and as we’ve lost about 90 percent of our state’s presettlement wetlands, swamp milkweed has also declined.

The aptly named common milkweed is by far our most numerous monarch fuel, and it will grow in nearly any dry, open habitat. Roadside mowing, excessive herbicide use, and the proliferation of increasingly sterile agricultural landscapes has greatly decreased this plant.