In late summer, monarchs in southern Canada and the U.S. northeast take flight, travelling over 5,000 kilometres to alpine forests in central Mexico.
The monarchs’ journey north is every bit as remarkable as the epic southern migration. Three or four successive generations fly to breeding grounds, lay eggs and perish. The resulting caterpillars transform into butterflies and then take on the next leg of the trip.
Monarchs arriving in Canada in late summer are often fourth- or fifth-generation descendants of the butterflies that flew south.
What may be the monarch's most striking quirk is its caterpillar’s reliance on milkweed as its sole food source, a phenomenon called “monophagy.”
Milkweed plants contain small traces of cardenolides, bitter chemicals monarchs store in their bodies to discourage predators, which associate the butterflies’ distinctive colouration with a bad taste. But relying on a single type of plant for survival is a risky strategy that has put monarchs in grave danger.
In the mid-1990s, the eastern monarch population was more than one billion. In winter 2013, the population had dropped by more than 95 per cent to 35 million, with a modest increase to 56.5 million this past winter. As University of Guelph post-doctoral research fellow Tyler Flockhart notes, a single severe storm could extinguish the monarch population.
Much of the monarch butterfly decline has been pinned on the virtual eradication of its critical food source throughout much of its migration path by profligate use of a glyphosate-based weed killer called Roundup, which corn and soybean crops have been genetically modified to tolerate.
Blanketing fields with the herbicide kills plants like milkweed. As a result, several U.S. Midwest states — the heart of monarch-breeding territory — have lost most of their native milkweed, causing monarch reproductive rates to drop by more than 80 per cent.
A recent study suggests glyphosate is merely the first of a one-two toxic punch from industrial agricultural operations.
The second is neonicotinoids, the controversial nicotine-based insecticides that have been identified as a chief culprit in the decline of honeybees, along with a host of birds, bees and butterflies.
It appears that even at one part per billion these chemicals can affect monarch caterpillar development, delivering a potential knockout blow for the imperilled insects.
The good news is that many jurisdictions are catching up with the science. Ontario’s government has proposed regulations to reduce neonic use by 80 per cent over the next couple of years. I hope this marks the turning of the toxic tide, but time is running out.
What can you do to help? A lot of milkweed must be planted over the next few years.
Planting milkweed in your backyard or balcony garden is a great start. Be sure to call your local garden centre or nursery to ensure they stock native milkweed plants this season, or make a trip to the Native Plants nursery in Pickering.
David Suzuki is the host of the CBC’s The Nature of Things and author of more than 30 books on ecology. (With files from Jode Roberts.)
