Genetically Modified Corn Not A Threat To Swallowtail Butterfly Larvae

The study, published online June 6 by the Proceedings of the National Academy of Sciences, differs from a May 1999 Nature report by Cornell University researchers, who, citing laboratory tests, said that genetically modified Bt corn slowed the growth and caused 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 a Pioneer variety containing 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 was carefully monitored and measured at a variety of locations ranging from 1/2 meter to 7 meters from the cornfield.

"Many of the 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 attributed to predation by spiders, carnivorous insects and other environmental factors. 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, researchers exposed more caterpillars to Bt corn pollen from plants in the field, as well as pollen from non-modified but genetically similar corn plants. The toxin from the same Bt corn again had no adverse effect, nor did pollen from non-modified corn. Pollen from another transformed variety, Novartis Max 454, however, was fatal to the caterpillars. Antibody assays of the Max 454 showed that it had 40 times as much toxin 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." The report, she added, suggests risks to non-target organisms might be possible by the choice of Bt variety.

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.