Most Bt corn pollen falls in or near cornfields and before most monarch butterfly caterpillars emerge.

That's the word from University of Nebraska Entomologist John Foster and graduate student Pete Clark, whose research supports pollen distance studies elsewhere. This summer and last, they tracked corn pollen shed and monarch activity surrounding five Bt cornfields in eastern Nebraska. They measured corn pollen on milkweed at different distances from fields. Genetically modified Bt corn produces toxins fatal to European corn borers and other caterpillars.

First-year results showed most pollen fell within 5.5 yards of the field, with highest pollen counts within the first yard, Foster said. None was found on milkweed more than 44 yards from fields. Beyond 5.5 yards, researchers found no pollen counts above 3.2 grains per square inch on milkweed leaves. Studies elsewhere using certain Bt corn hybrids showed pollen densities less than 24 grains per square inch on milkweed didn't affect monarch caterpillars. That's about seven times higher than the pollen levels the IANR researchers measured.

Milkweed often grows near Nebraska cornfields, Foster said. However, the study found little threat to monarchs because 95 percent of the corn pollen shed occurs before monarch caterpillars develop. Researchers found no dead monarch caterpillars on any milkweed.

"I can tell my granddaughter without reservation, 'the monarchs will be here tomorrow,'" Foster said.

Foster and Clark launched field studies after a Cornell University laboratory experiment suggested Bt corn pollen could pose a significant threat to monarch caterpillars. That study raised fears Bt corn pollen might blow onto nearby milkweed plants.

A consortium including industry and the USDA helped fund this research.

– Molly Klocksin