SEATTLE, Wash. — The natural insecticide Bt, produced by GE corn and cotton, was an obvious suspect when honeybee colonies in the U.S. and other countries began to collapse in the mid-2000s.

But several studies have cleared it of the charges. Bt, which is so mild it’s allowed on organic farms, is toxic only to the larvae of certain butterflies and moths — not bees. And some places where honeybees have been in distress, like Switzerland, grow no GE crops.

But the picture isn’t as rosy when it comes to GE crops and Monarch butterflies.

A bombshell study in 1999 suggested that eating pollen from Bt corn could be fatal for Monarch caterpillars. A scientific blitz was launched, and that conclusion was largely discounted, said Margaret Mellon, senior scientist at the Union for Concerned Scientists.

But as researchers followed up, they discovered that another type of GE corn, engineered to be resistant to the weed-killer Roundup, is likely taking a toll on the beloved butterflies. Increased use of Roundup is eliminating milkweed — an essential food for young Monarchs — from the vast cornfields of the Midwest, where the species lays its eggs.

It’s still not clear how the loss of milkweed compares with the many other threats Monarchs face, including renegade logging operations in the butterflies’ winter habitat in Mexico.

— The Seattle Times