© Janet
Davis
In my home in Southern
Ontario, late summer is prime time for the lovely Monarch butterfly (Danaus
plexippus). Come mid- to
late-September, the orange and black beauties make their way by the hundreds of
thousands to the northern shores of Lake Ontario and 
Lake Erie where they’ll feast on
nectar from garden flowers, wild goldenrod and asters until they’re ready to
make their long flight across these immense waters.
Getting across the lake is no sure thing, however. I’ve seen many Monarchs washed up along with seagull feathers and driftwood, victims of an inhospitable wind, a sudden rain, or simply fatigue.
In my garden, I’ve enjoyed watching Monarch butterflies visit the shrub I consider, hands-down, the best magnet for attracting them: Butterfly bush (Buddleia) ‘White Profusion’, shown at left. Over the years, I’ve tried other cultivars including “Royal Red”, “Black Knight” and the compact ‘Nanho Blue’ and although they lured their share, they paled in comparison to the white. And as much as I love the vibrant rose color of the plump flower panicles of ‘Pink Delight’, I’ve never seen a butterfly on it. But ‘White Profusion’ could be a Monarch convention, with the beautiful jeweled beauties fluttering above it and hanging suspended from the long, sweet-scented panicles.
Saskatoon scientists Dr. Len Wassenaar of the National Water Research Institute and Dr. Keith Hobson of the Canadian Wildlife Service made headlines last winter when they published a remarkable paper describing their research measuring deuterium (heavy water) isotopes found in the wing membranes of Monarch butterflies. Deuterium occurs naturally in the earth’s atmosphere and, therefore, in rainfall, but concentrations vary consistently according to regional temperatures, something that allows scientists to accurately pinpoint where in the world it originated. When the deuterium-laden rainfall enters groundwater, it finds its way into plant roots, including wildflowers like milkweed, the larval food of the Monarch.
The Monarch butterfly undergoes a
four-stage metamorphosis from egg to larval caterpillar to pupa (chrysalis) to
adult butterfly. The adult female lays her eggs on the leaves of various types
of milkweed plants, either common milkweed (Asclepias syriaca), swamp milkweed
(A. incarnata), orange-flowered butterfly milkweed (A. tuberosum) or tropical
scarlet milkweed (Asclepias curassavica). When the larval caterpillar emerges,
it immediately begins munching the milkweed leaves. Since no other plant
species can sustain the larvae, survival of the Monarch butterfly is
inextricably linked to milkweed.
When a Monarch from Toronto successfully crosses Lake Ontario in late summer, it makes an almost miraculous trip south to spend the winter suspended from the branch of an oyamel fir tree in one of 13 wintering colonies in Mexico. (Western Monarchs winter on the Monterey Peninsula.) It was in Mexico where Hobson and Wassenaar measured the deuterium in the wings of 1,600 dead butterflies under the trees and discovered that there was a perfect correlation between the isotope content and the regions where the butterflies started out.
But the scientists also raised an alarm about the imminent loss of milkweed in the U.S. Midwest. Seems farmers there are successfully eradicating it from their fields by using the herbicide Roundup™ on corn and soybean crops genetically engineered to survive the spraying (they’re called “Roundup™-Ready”). And without sufficient milkweed, the Monarchs originating in this agricultural belt, which represent half of the entire population, are at risk.
Adapted from a column that appeared originally in the Toronto Sun