New studies clarify monarch worries.

A study of Bt corn supports the contention that pollen concentrations drifting in a cornfield can kill caterpillars of monarch butterflies, but the risks diminish quickly beyond the field's edge.

Bring-your-own-pesticide corn plants, genetically engineered to make a toxin discovered in the bacterium Bacillus thuringiensis, covered some 16 million of the 80 million acres of U.S. cornfields in 1998.

Within 5 days of feeding for 48 hours on leaves dusted with 135 grains of Bt corn pollen per square centimeter, 38 percent of young monarch caterpillars died, reports Laura C. Hansen of Iowa State University in Ames. That pollen dose lies at the upper end of the range she found measuring pollen on leaves in Iowa fields, Hansen told the annual meeting of the Entomological Society of America in Atlanta this week.

Hansen's results echo a Cornell University monarch study that ignited an international furor over the potential for transgenic crops to hurt monarch butterflies (SN: 5/22/99, p. 324). One of that study's coauthors, John E. Losey, and four other researchers also provided summaries of their most recent data, presented at a meeting in November.

They found hints that monarchs in the wild may not face a serious threat. Female butterflies seem to avoid laying eggs on milkweed surrounded by corn plants, and corn pollen builds up less on upper leaves of milkweed, the preferred egg-laying site, than on lower leaves.

In the study published last spring, Losey and his Cornell colleagues dusted Bt corn pollen onto milkweed leaves in amounts that roughly reproduced the appearance of leaves around cornfields. About half their caterpillars died after 4 days of feeding.

Hansen checked some two dozen milkweed plants in or near Iowa cornfields. Plants sprouting between corn rows accumulated an average of 50 pollen grains/[cm.sup.2], but 10 meters away from the field, she found only 1 grain/[cm.sup.2].

That dovetails with a survey of milkweed in 80 Maryland cornfields, notes Galen Dively of the University of Maryland in College Park. Milkweed plants growing amid corn averaged 75 grains of pollen/[cm.sup.2], but only 15 grains/[cm.sup.2] at 6 to 10 meters beyond the field.

To test the effects of pollen-speckled milkweed, Hansen dripped a measured pollen solution onto snippets of leaves and fed them to caterpillars. The deep drop in survival that she found among young caterpillars surprises Dively, he says, because U.S. Department of Agriculture and Canadian studies last summer didn't show die-offs at pollen dustings sparser than 150 grains/[cm.sup.2].

What portion of milkweed falls within the high-pollen zone is not clear, Dively notes.

The public fuss over butterflies is obscuring more serious issues, comments Tom Turpin, a corn entomologist at Purdue University in West Lafayette, Ind. He wonders whether Bt toxins could build up and disrupt the teeming soil ecosystem. Or could reckless use of Bt corn trigger so much resistance among insects that Bt insecticide becomes useless?

In widespread Bt corn planting, "we're not saving that much pesticide," Losey argues. He says that only 3 percent of cornfields in Iowa had been sprayed for the European corn borer, the pest that Bt corn is marketed to fight. Other scientists at the meeting hotly debated whether Bt corn will cause pesticide use to shrink.

"We're taking a hard look at the risk," Losey says. "We need to take a hard look at the benefits as well."