Neonic-treated milkweed an ‘ecological trap’ for monarchs

A commonly used neonicotinoid pesticide may harm monarch butterflies, University of Guelph research has revealed.

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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