Bt corn variety OK for black swallowtails

The first published field study of Bt corn and butterflies—and the first check for any Bt-pollen risks to black swallowtails—finds no harm from a common corn variety, say researchers of the University of Illinois at Urbana-Champaign.

Bt corn makes its own pesticide, thanks to genetic material borrowed from the soil bacterium Bacillus thuringiensis. The pattern of pollen wafting from Bt corn variety Pioneer 34R07 showed no relationship to the pattern of deaths among black swallowtail caterpillars on nearby wild plants, report C. Lydia Wraight, May R. Berenbaum, and their Illinois colleagues. Their paper will appear in the Proceedings of the National Academy of Sciences.

Pioneer 34R07 also seemed benign in lab tests, the researchers report, even at concentrations of 10,000 pollen grains per square centimeter. That’s 40 times as dense as the heaviest dusting researchers observed in the field.

The black swallowtail caterpillars are not immune to Bt toxins, explains Berenbaum. After 3 days of dining on the same high concentration of pollen from another Bt corn, Novartis Max 454, some 80 percent of the caterpillars died. Both varieties target European corn borers.

The lesson of the research, Berenbaum says, is that by selecting different genetic versions, “it may be possible to manage nontarget effects.”

Because no one has published quantitative studies of lethal doses, Berenbaum declines to say whether black swallowtails are more or less susceptible to Bt than are monarchs.

Bt looked like a great start for a smart pesticide, with low risks to humans and other mammals. Different Bt strains tended to zap rather focused clusters of related insects, targeting groups of moths but sparing bees, for example. The method’s white-hat reputation was muddied last May by a Cornell University study showing that in the laboratory, nearly half of young monarch caterpillars died after 4 days of eating leaves dusted with Bt pollen (SN: 5/22/99, p. 324: https://www.sciencenews.org/sn_arc99/5_22_99/fob1.htm). Ever since, researchers have been scrambling to find out what happens outside the laboratory.

A workshop last November and presentations at the Entomological Society of America’s annual meeting last December released early results of field studies of Bt effects. Overall, they looked encouraging for the method, but none has been published.

Berenbaum and her colleagues tested black swallowtails because they, like monarchs, favor plants abundant beside crop fields and fencerows. Also, Berenbaum has devoted 25 years to studying the species. Support for the project came from a university fund for environmental research.

The Bt corn the researchers studied gets its pest-killing power from a genetic construct called Monsanto event 810. The researchers arrayed the caterpillars’ food plants, wild parsnips, at five distances from the field, from 0.5 to 7 meters. They then tucked young caterpillars among the leaves of each plant and set out glass slides smeared with Vaseline to monitor how much pollen landed.

Plenty of caterpillars died, as expected for a field-side jungle harboring spiders and other dangers. However, the deaths didn’t taper off with the pollen density or distance from the field. Hence, the researchers say, pollen wasn’t an issue.

“It’s an excellent study,” says John E. Losey of Cornell, a coauthor of the monarch study that ignited the recent fuss. He welcomes the data on a second species but cautions that pesticide susceptibility arises in complex patterns. “It’s too early to make sweeping generalizations,” he says.

Linda S. Rayor, another Cornell coauthor of the monarch paper, greeted the swallowtail study as much-needed ecological research. Rayor’s subsequent work on Bt corn, not yet published, supports the idea that “there’s a big difference in varieties.”

Demanding a pesticide that affects only the targeted villains may be a hopeless quest, Berenbaum muses. “The plain fact of the matter is that growing food has nontarget effects—plowing has nontarget effects,” she says. “Our challenge is to minimize them.”