TORONTO- A new Canadian study has apparently solved a mystery involving the migration habits of one of North America's most recognizable insects and may aid conservation efforts.

Scientists have been trying to understand why monarch butterflies east of the Appalachians showed up later in the year than those found west of the mountain range, which runs roughly from Atlantic Canada to Alabama.

Researchers from the University of Guelph and Environment Canada have proved monarch butterflies migrate eastward over the Appalachians as they repopulate.

The butterflies recolonize "multigenerationally," meaning the generation that winters in Mexico flies northward, laying eggs west of the Appalachians, which then hatch into butterflies that head toward the eastern coast of the U.S.

The finding is unusual because most recolonization patterns follow a more simple south to north pattern.

"Typically these recolonizations of North America happen latitudinally so individuals move north as the season progresses and conditions get better," said Ryan Norris, a biology professor from the University of Guelph who is one of the study's authors.

"The picture of monarch migration is not as simple as we thought," Norris said.

The monarch, a delicate butterfly with distinctive black and orange wings, is already famous for its long annual migration to Mexico.

With this latest information scientists now know the insect's journey is even more spectacular than originally thought.

Weighing only a few grams, the delicate butterfly battles harsh conditions to make it to the other side of the Appalachians.

Researchers were able to make the discovery by analysing wing tissue and tracing the butterflies to their birthplaces.

Nathan Miller, a University of Guelph student who is the study's lead author, suggested the key was milkweed, a plant that monarch larvae feed on exclusively.

"(The butterfly) picks up isotope signatures from the milkweed and it stores them in its wing tissue," Miller explained.

"It provides essentially a marker that allows us to estimate where it was born, regardless of where we capture it."

Samples from the wings of 90 butterflies were collected between Maine and West Virginia and analysed.

The researchers found that 90 per cent had come from areas west of the Appalachians, with the majority from the Great Lakes region, which includes Ontario.

Aside from solving a migration mystery, the study is significant because it will allow conservation efforts to be more effective.

Norris said efforts to protect the species have so far focused on the butterfly's winter home in the Mexican highlands, where they are concentrated in about a dozen different sites.

"If you have a limited amount of resources and you put them all towards conserving monarchs say, in Mexico, that may not be the best way to do it because there may be other stages of the annual cycle that are also threatened," Norris said in a phone interview from Guelph, Ont.

"In a sense you may be wasting money by putting your resources into just one stage in the annual cycle."

The study suggests the most productive monarch populations are in the Great Lakes and the mid-western United States.

Miller, who is completing a Master degree, was one of two researchers who collected the samples. He said monarch butterflies are surprisingly hard to catch.

"They're actually pretty fast and they're very aware of people," the researcher said.

The study is already available online and will be published in print in an upcoming issue of the scientific journal Biology Letters.