Research led by the University of Guelph and Environment Canada has revealed a new twist in the life story of monarch butterflies. Scientists using stable isotope measurements studied butterflies collected along the eastern United States coast to better understand their natal origins. It was believed that coastal butterflies migrate north directly from the Gulf of Mexico.

Unexpectedly, 88% originated in the mid-west and Great Lakes regions. Of the 90 insects sampled, just 12% originated from parents that migrated directly from the Gulf coast. This provides new evidence of a second chapter in their migration story: A majority of monarchs fly north to breed inland in spring and a second generation then undertakes a west-to-east migration to the coast.

The mystery of why east coast monarchs arrive later in spring than inland monarchs is solved. Stable hydrogen and carbon isotope analysis revealed the place of birth of the insects and now we know that east coast monarch populations rely on healthy habitat in the mid-west and the Great Lakes.

Conservation efforts can be specifically targeted to benefit wildlife populations when their movements and habits are closely understood. For the monarch, healthy habitat includes milkweed, the sole food source of monarch larvae.

The paper is available on the website of the science journal, Biology Letters:

Nathan G. Miller, Leonard I. Wassenaar, Keith A. Hobson, and D. Ryan Norris. Monarch butterflies cross the Appalachians from the west to recolonize the east coast of North America. Biol Lett 2010 : rsbl.2010.0525v1-rsbl20100525.