Milkweed and the Monarch Migration

By Nicole Czorny

Travelling over 4,000 kilometres from Mexico to Canada, the monarch butterfly is as graceful as it is determined. Yet despite the amazing odds and miles they are up against, human interference has become the monarchs biggest obstacle.

Every year monarchs leave their overwintering sites in Mexico and head North towards Canada and the United States. The typical monarch can live up to four weeks, so it takes 2-3 generations to complete the trip. The final super generation lives up to eight months and flies the entire trip back to Mexico all on its own. It is still unknown exactly what guides monarchs on this incredible journey.

As they make their way North, they rely on milkweed (common milkweed [Asclepias syriaca], butterfly milkweed [Asclepias tuberosa] and swamp milkweed [Asclepias incarnata]) for the survival of their species. Females lay an average 400 tiny yellow eggs on the underside of the milkweeds leaf, and once hatched, milkweed is the only food the larvae and striped yellow, black and white monarch caterpillars eat.

Containing toxic and bitter “milk” like steroids called cardenolides, monarchs store the milkweeds toxins and emerge as butterflies poisonous and tasting awful to predators, with their yellow and black markings being a clear warning sign to its predators.

An adulte monarch feeding on an New England aster
A adult monarch feeds on a pollinator friendly New England aster.

In 2013, an estimated 60-million monarchs were counted at over wintering sites, occupying the smallest area since population monitoring began in 1993. There are several factors putting the North American monarch migration at risk, one crucial component being the decline of milkweed host plants.

Where has all the milkweed gone?

It is believed milkweed originated in Mexico, and as North America got warmer, the milkweedand monarchsmade their way North. But with urban sprawl and declining green spaces along this important corridor, milkweed population is on the decline. 

Loss of Habitat

  • Seen as an annoyance and a weed, milkweed has been aggressively treated with pesticides and eradicated from fields and roadsides across North America, affecting monarchs successful reproduction cycle
  • Currently under review, Ontario’s Ministry of Agriculture and Food’s (OMAF’s) Weed Control Act lists milkweed as a noxious weed, leaving agricultural landowners responsible by law for its removal

Big Agriculture

  • Monarch Watch estimates more than 60 million hectares of milkweed has been destroyed by herbicide resistant crops in the United States
  • Once covered with milkweed and other pollinator friendly flowers and native plants, North America’s grasslands have been taken over by GMO corn and soybean fields
  • Resistant to Monsanto’s infamous Roundup weed killer, these fields are sprayed with the harmful chemical, eliminating all other plants that monarchs and other wildlife rely on

Climate Change

  • More precipitation and warmer temperatures have monarchs arriving North before the milkweed is ready, with some plants still in the ground
  • This leaves females with no milkweed to lay their eggs on, pushing back the migration cycle

Dog-strangling Vine (as horrible as it sounds!)

  • Rampant in the Don Valley, dog-strangling vine is an extremely invasive species from the milkweed family that takes over fields and forests, literally strangling out native plant species and ruining habitats for butterflies and other pollinators like the birds and the bees
  • Monarchs can mistakingly lay their eggs on the vine and they do not survive without the proper nutrition the milkweed gives the larvae
  • Good news—the Ontario government is reviewing the Weed Control Act, proposing milkweed be taken off the noxious weed list, and dog-strangling vine be added!

Take it to the streetsand your backyard!

Don’t fret, there are things you can do to keep your backyard bursting with butterflies.

By digging in and getting a little dirty, you can keep the North American monarch migration alive, and do you part to keep butterflies and other pollinators visiting Ontario and gracing your garden for generations!


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