The findings could help explain the recent massive decline in the North American monarch population.

Led by U of G integrative biologist Dr. Ryan Norris, two studies examined effects on monarch caterpillars raised on milkweed treated with the insecticide clothianidin, which is coated on soy and corn seeds and taken up into the foliage.

One study found poorer survival rates among caterpillars eating treated milkweed, although egg-laying survivors appeared unaffected.

The other study found that larvae raised on treated milkweed were much smaller and lighter than those feeding on untreated milkweed.

Most of the corn and soy grown in North America comes from neonicotinoid-coated seeds, and much of the milkweed that monarchs feed on grows in agricultural areas.

Even more worrisome for Norris is that monarchs may prefer milkweed grown in pesticide-treated soil. Monarchs lay eggs only on milkweed plants. The butterflies laid more eggs on treated milkweed than on untreated plants, although researchers aren’t sure why.

Whatever the reason, the neonic-grown milkweed then becomes an “ecological trap” for the butterflies, Norris said.

“Neonicotinoids have been shown to have both lethal and sublethal effects in other invertebrate and pollinator species, so we were not surprised to see that monarch larvae also seemed to be affected.”

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University of Guelph researchers have pinpointed the North American birthplaces of migratory monarch butterflies that overwinter in Mexico, vital information that will help conserve the dwindling species.

The researchers analyzed “chemical fingerprints” in the wings of butterflies collected as far back as the mid-1970s to learn where monarchs migrate within North America each autumn.

The largest percentage of monarchs migrated to Mexico from the American Midwest, but the biologists were surprised to find the insects’ origins were spread fairly evenly throughout Canada and the United States.

“We expected the vast majority of monarch butterflies to be found in the Midwestern states,” says Tyler Flockhart, lead author and Liber Ero Postdoctoral Fellow at U of G. “However, just 38 per cent come from that part of the U.S.A. If we just focus conservation activities on this area, this research shows we will be missing a large number of butterflies born elsewhere in North America.”

Monarch numbers have dropped significantly in recent years, likely due partly to the eradication of milkweed, which began in the mid-1990s. Monarchs feed on milkweed and lay their eggs on the plants.

Analyzing more than 1,000 samples, the research team looked at chemical isotope signatures showing where the butterflies were born in the previous summer and fall.

They found that 12 per cent of the insects were born in the northwestern U.S. and Canadian Prairies, 17 per cent in the north-central States and Ontario, 15 per cent in the northeastern U.S. and the Maritimes, 11 per cent in the south-central U.S. and eight per cent in the southeastern States.

Co-author and integrative biology professor Ryan Norris says the study shows monarch conservation efforts must begin immediately throughout North America. He calls for better collection and analysis of butterflies in their Mexican overwintering grounds to monitor the effects of conservation efforts.

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