Bt corn poses little threat to Monarch butterfly

In the litany of environmentalist charges against Bt corn -- the genetically engineered strain undergoing field tests near Gen. Santos City -- the Monarch butterfly has become a metaphor for all insects that, to their supposed peril, either feed on Bt corn or on plants dusted with Bt corn pollen.

In protesting the field tests last summer, local environmentalists echoed the warning that the toxin produced by Bt corn can kill beneficial and innocent insects.

The toxin they referred to is a protein generated by a gene taken from Bacillus thurigiensis, a common soil bacterium, and inserted in the corn genome through gene engineering techniques. True, the toxin kills insects, but here the metaphor breaks down. The toxin is deadly against only the Lepidoptera family of moths and butterflies, including two of the worst corn pests against which it is targeted -- the European and Asian corn borer.

The Asian corn borer is considered the most prevalent pest in Southern Mindanao. Losses due to corn borer infestation have ranged from 30 to 80 percent in many fields.

Last year, three US scientists at Cornell University discovered in laboratory experiments that pollen from Bt corn could, under certain circumstances, kill larvae of the Monarch butterfly, another member of the Lepidoptera family. They reported this outcome to the world in the journal Nature, but stressed the preliminary nature of their work. "This study is just the first step, we need to do more research and then objectively weigh the risks versus the benefits of this new technology," they said upon publication. "We can't forget that Bt corn and other transgenic crops have a huge potential for reducing pesticide use and increasing yields."

Despite the scientists' self-acknowledged limits of their study, anti-biotechnology activists seized the results and broadcast them worldwide. The story quickly became central to the argument against Bt corn. After all, everybody knows and loves the beautiful Monarch butterfly, and the idea of its becoming an endangered species is unthinkable.

The truth regarding the Bt corn threat to Monarchs is actually far more complicated. Consider the laboratory test itself. The larvae ingested an amount of toxin far beyond what they would normally encounter in the field. One scientist who reviewed the results of the experiment drew an analogy. If one eats 100 pounds of salted popcorn at a movie and died, it might be reported that it was death due to popcorn, he said. But realistically, eating that much-salted popcorn simply does not happen.

In the real world, the favorite food of Monarch butterfly larvae is not corn, but milkweed, which is usually found on cornfield perimeters. The larvae ingest the toxin by eating leaves of milkweed that have been dusted with corn pollen. So the question arises: how far does Bt corn pollen spread and in what density?

Last summer, several scientists made follow-up studies of the Cornell experiment. Hansen and Obrycki examined Bt-corn pollen dispersal on milkweed plants within, and adjacent to, field corn. As one might expect, pollen levels were highest on the few milkweeds inside a cornfield that survived customary herbicidal spraying. Surprisingly, they found the larvae mortality rate on these heavily dusted milkweeds to be only 16 percent.

Pollen does drift beyond a cornfield's perimeter, however, but how far? John Sears, a scientist at the University of Guelph in Ontario, Canada, and chairman of the Ontario Cornborer Coalition, reported that virtually all corn pollen grains travel less than 10 meters from the field and 90 percent less than five meters. He also determined that it takes at least 500 grains of pollen per square centimeter of milkweed leaf to sicken caterpillars. But after three days of accumulation during pollination season, Sears found this concentration was barely attained on nearby milkweed leaves.

A scientist from Iowa State University reported that "88 percent of milkweed within one meter of a corn field would fall below the level where they could hurt the caterpillars and 100 percent of the milkweed just two meters from a Bt field would be Monarch-safe."

This further testing thus shows that the notion of a cloud of death dealing Bt corn pollen drifting down on innocent and beneficial insects in adjacent fields is highly exaggerated. In fact, one scientist suggested that chemical drift from spraying insecticides poses a significantly higher risk to the Monarch population than incidental Bt corn pollen.

During the coming summer, US scientists plan further studies on basic Monarch biology and ecology. But based on the data gleaned from last summer's tests, the risk to the Monarch butterfly is "extremely remote."