New Research Suggests Bt-Corn Not Harmful To Monarch Butterfly

Field research conducted by Prof. Mark Sears, chair of the University of Guelph's Department of Environmental Biology and chair of the Canadian Corn Pest Coalition, shows pollen from Bt corn -- Bacillus thuringiensis, a naturally-occurring soil-borne bacterium which selectively targets specific groups of insects -- is not found in high enough doses on most milkweed plants (the food plant of the caterpillars) to hurt Monarch butterfly larvae.

Although Bt is harmless to humans and other animals, a U.S. study claimed pollen from Bt-corn damaged Monarch butterfly larvae. The study sparked a media frenzy and public concern about genetically modified foods.

Sears says the U.S. study didn't demonstrate to what extent its preliminary findings applied to field situations. The study was completed in a lab, and the dosage of pollen used wasn't reported.

"The actual threat to the Monarch butterfly can only be determined by assessing the dosage that affects the larvae and their degree of exposure to Bt-corn pollen in the field," says Sears.

Sears is leading a two-year project to determine the ecological impacts of Bt-corn pollen on selected non-target butterfly species, including the Monarch. So far, studies indicate that Bt- corn is not as big a threat as environmentalists and the news media had anticipated.

"Outside of corn fields, you probably wouldn't find concentrated dosages of pollen because wind and rain removes it from the surface of the milkweed leaves," says Sears. Sears's study focused on Bt pollen and how far it travels. He examined milkweed stands in corn fields, at their edges, then at distances of five, 10 , 25, 50 and 100 metres away. He found that within the fields, approximately 150 pollen grains/cm2 were found on milkweed leaves. At the field edges, 80 to 100 grains/cm2 were found, and at five metres, only one grain/cm2 was found. He then compared these findings to values obtained from a "dose-response assay" — from which data of increasing doses are plotted against increased mortality rates — to determine dosages with negative effects on Monarch butterfly larvae.

Results show that 135 grains/cm2 — the lowest dosage he has tested on milkweed leaves so far and similar to that found on milkweed leaves in the field — had no greater effect on Monarch larvae than when they were fed non-Bt pollen.

"Our findings are consistent with other studies across North America," says Sears. "Bt- corn has always shown to be harmless to both humans and animals, and we now know it isn't a major threat to the Monarch butterfly."

Sears is being assisted by research associate Diane Stanley-Horn and research technician Heather Mattila from the University of Guelph, along with seed industry representatives and corn growers. His research is sponsored by the Canadian Food Inspection Agency and Environment Canada.