Herbicide-Resistant Crops-Another GE Threat to Monarchs?

Excerpt from:
THE GENE EXCHANGE
A Public Voice on Biotechnology and Agriculture, June-July-August 1999
Published by Union of Concerned Scientists

Like engineered Bt corn, herbicide-resistant crops may take a toll on monarch butterfly populations, but for different reasons, according to Dr. O.R. Taylor, a University of Kansas entomologist. Taylor, who also coordinates a monarch conservation network called Monarch Watch, was interviewed in the April Environmental Review. He explained that the rapid adoption of engineered herbicide-resistant corn and soybeans may seriously decrease the milkweed population in fields, which, in turn, may lead to a decline in monarchs, as milkweed is the sole food source for the butterfly caterpillars.

Traditional weed-control regimes are not as great a threat because large numbers of milkweeds typically survive the herbicides applied to corn and soybeans. This is one of the problems that Monsanto and other companies set out to solve by engineering crops to resist herbicides, like glyphosate (Roundup). With the new crops, farmers can use broad-spectrum weed killers to rid fields of milkweed and other recalcitrant weeds which sprout after the crop has started to grow. Without engineered resistance, glyphosate can only be used to kill weeds before planting. As more and more herbicide-resistant crops replace traditional ones-nearly half the soybean crop is expected to be herbicide-resistant this year-milkweed stands could well decline, followed by a decrease in the monarch population.

This possibility takes on a more ominous dimension in light of new information on monarch breeding. The Midwest is an important breeding ground for the adult monarchs that migrate south every year to spend the winter roosting in forests in Mexico where they await a signal to begin their annual northern migration. Canadian scientists have recently found that a substantial portion, roughly 50 percent, of the monarchs that overwinter in Mexico originate from a relatively small area between Nebraska and Ohio-the heart of US corn and soybean production-and the site of rapid adoption of herbicide-resistant crops.

Sources:

B. Hartzler, "Monarch butterflies and herbicide resistant crops," Iowa State University, www.weeds.iastate.edu/weednews/monarchs.htm;
"Monarch butterflies may be threatened in their North American range," Environmental Review 6(4):1-9, April 1999;
"Monarchs and their roots," Science 283:171, January 8, 1999;
L. Wassenaar and K. Hobson, "Natal origins of migratory monarch butterflies at wintering colonies in Mexico: new isotopic evidence," Proceedings National Academy of Sciences 95:15436-39, December 1998;
Monarch Watch at www.monarchwatch.org.