A monarch butterfly feeds on silky red milkweed. Monarch larvae feed on the plant during their long migration.A monarch butterfly feeds on silky red milkweed. Monarch larvae feed on the plant during their long migration. (Steve Smedley,The Pantagraph/Associated Press)

Monarch butterflies take a more dramatic route home during migration than was previously believed, Canadian researchers say.

Monarchs travel between 1,900 and 4,500 kilometres from start to end, according to the World Wildlife Federation. They migrate from Canada to Mexico and return.

But certain butterflies — from the Great Lakes and the Midwest — journey over the Appalachian Mountains and breed on the U.S. eastern seaboard for a few months before going home, according to a study conducted by researchers at the University of Guelph in conjunction with Environment Canada. It was formerly believed that the butterflies came north directly from the Gulf coast.

"It solves the long-standing mystery of why monarchs always show up later on the east coast compared to the interior," said Ryan Norris, a University of Guelph professor in the department of integrative biology, in a release. "Importantly, it means that the viability of east coast populations is highly dependent upon productivity on the other side of the mountains."

"Ours is the first proof of longitudinal migration," Miller said.

The scientists collected 90 butterfly samples at 17 sites between Maine and Virginia, in June and July 2008 as well as 180 samples of milkweed, which monarch larvae feed on during their migration. Using hydrogen and carbon isotope dating, they determined the location where the monarchs were born, and how old they were by examining when they consumed the milkweed.

The scientists found that 88 per cent of the monarchs were born in the Midwest and Great Lakes areas. "This means that the recolonization of the east coast is by second-generation monarchs that hatched around the Great Lakes and then migrated eastward over the Appalachians," Miller said.

The findings mean conservation authorities should shift their efforts to the Great Lakes.

The study is published July 14 in the journal Biology Letters.