Bt Corn Variety Study Shows No Adverse Effect On Black Swallowtail Caterpillars

The study -- published online Tuesday (June 6) by the Proceedings of the National Academy of Science -- differs sharply from a May 1999 Nature report in which Cornell University researchers, citing laboratory tests, reported that genetically modified Bt corn slowed the growth and led to early deaths of Monarch caterpillars.

Black swallowtail larvae, the UI researchers noted, are just as likely as are monarch caterpillars to encounter corn pollen in the field during a key developmental time between late June and mid-August. "Yet under actual field conditions, no mortality directly or indirectly attributable to ingestion of endotoxin-containing corn pollen could be detected," they wrote.

In field tests, researchers grew Pioneer variety 34R07, which contains Monsanto event 810 -- a particular genetic configuration of corn carrying the gene that encodes the Bacillus thuringiesis toxin fatal to European corn borers, which ravage corn crops in some parts of North America. Pollen from the Bt corn was carefully monitored and measured at a variety of locations ranging from 1/2 meter to 7 meters from the cornfield.

"We found that many caterpillars died, but not, as far as we could tell, due to anything connected to the corn or the corn pollen," said May Berenbaum, head of the UI entomology department. "There was no correlation between mortality and distance from the cornfield or between mortality and pollen load."

Some of the deaths were directly attributed to predation by spiders, carnivorous insects and other environmental factors, the researchers said. Black swallowtail females can lay up to 800 eggs during their two-week lifetime; overall life expectancies for caterpillars in the field invariably are low.

"We also measured the weights of the surviving caterpillars, and we found no negative pattern suggesting a problem in their growth and development," said UI entomologist Arthur R. Zangerl.

In the laboratory, the researchers exposed more caterpillars to Bt corn pollen from the plants in the field, as well as pollen from non-modified but genetically similar corn plants. The endotoxin from the same Bt corn again had no effect on survivorship of the caterpillars, nor did the pollen from non-modified corn. Pollen from another transformed variety, Novartis Max 454, however, did cause mortality in the laboratory. Antibody assays of the Novartis event showed that it contained 40 times as much endotoxin on average than did the 810 variety.

"This is not the green light for all forms of genetically modified organisms," Berenbaum said. "In this study, we examined only one GMO event -- just one genotype of Bt corn -- in the field. This report does, however, suggest that there are ways to reduce the risk to non-target organisms at the very least by event selection."

The UI Environmental Council, a campus organization devoted to environmental research, education and service, funded the study with a grant to C. Lydia Wraight, an undergraduate entomology student who worked with Zangerl, Berenbaum and graduate student Mark Carroll.