CHAMPAIGN, Ill. Pollen from a Bt corn variety carrying a now-phased-out genetically inserted pesticide known as event 176 dramatically reduced growth rates among black swallowtail caterpillars in University of Illinois field tests, researchers report.
Because rainfall repeatedly reduced pollen concentrations during the summer 2000 test period, the results “must be considered conservative,” the scientists said.
The findings and those of five other related projects done elsewhere, and which also targeted Bt corn, appear online this week in the Proceedings of the National Academy of Sciences. The papers are being released ahead of publication at the request of the U.S. Environmental Protection Agency.
“The results of this study suggest that pollen from Bt corn varieties engineered with the 176 event may have sublethal effects on black swallowtails (Papilio polyxenes) feeding on host plants situated outside of cornfields,” the authors wrote. Researchers also attempted to study the effects on monarch butterflies (Danaus plexippus), but a high death rate was believed to be more likely the result of predation than proximity to pollen.
Bt corn refers to genetically modified varieties that resist the European corn borer (Ostrinia nubilalis). Bt is short for Bacillus thuringiensis, a soil organism that produces toxic proteins that kills the borers, which cause $1 billion in crop damage annually in the United States. Scientists can control when and in what part of the plant the toxin is produced by combining gene sequences with specific promoters. Successful transformations of corn with the genetically engineered sequences are called “events.”
The UI team, led by entomologists May R. Berenbaum and Arthur R. Zangerl, planted Novartis Max 454 Bt corn, which contains Novartis event 176, in a 30-by-30 meter tract northeast of the UI campus in late May 2000.
This variety of Bt corn is known to produce pollen with higher concentrations of the pesticide than other varieties and has been linked to a reduced survival of monarchs at concentrations naturally occurring in and near cornfields. (Registration of event 176 expires this year and renewal is not expected, the authors noted. Less than 1 percent of U.S. corn acreage in 2000 contained event 176.)
Researchers put 20 potted parsnip plants and 25 potted milkweeds at intervals ranging from one-half meter to 7 meters from the corn when it began shedding pollen in late July. Pollen levels were monitored. Black swallowtails were released to feed on the parsnip and the monarchs on the milkweed, which attracted more predators.
The disappearance of monarch larvae was rapid over the next six days, but the disappearance was not affected by proximity to the crop, Zangerl said. The death rate was lower among black swallowtails and again unaffected by proximity. However, he said, the growth rate of the swallowtails varied dramatically; larvae 7 meters from the corn were three times as large as the larvae located one-half meter away from the corn.
UI researchers last year had reported high death rates of black swallowtail larvae fed high concentrations of event 176-containing corn pollen in laboratory tests. The new study shows that much lower levels can cause significant mortality in the laboratory.
The earlier study also noted that a widely used Bt corn variety containing Monsanto event 810 had no adverse affect on black swallowtails living in weeds near cornfields.
“Results of this new study tell us again that careful event selection by producers is advisable,” Berenbaum said. “Our findings also suggest that much more research is needed on each new Bt variety to make sure non-target species will not be adversely affected once it is planted in the field.”
Co-authors are Berenbaum, Zangerl, graduate students Duane McKenna and Mark Carrol, and undergraduates C. Lydia Wraight, Peter Ficarello and Rita Warner. The UI Foundation and Center for Advanced Study funded the research.
