Monarch butterflies, corn pollen coexist in cornfields

September 21, 2001

MINNEAPOLIS / ST. PAUL--Milkweeds growing in cornfields sometimes support monarch butterfly larvae at the same time the corn is shedding its pollen, according to a survey of cornfields in the Midwest, Maryland and Ontario. The overlap implies that monarchs feeding next to corn genetically engineered to contain the insecticide known as Bacillus thuringiensis (Bt) toxin could be exposed to the toxin. The study, led by University of Minnesota ecologist Karen Oberhauser, is published on the Proceedings of the National Academy of Sciences Web site,, along with other papers dealing with monarchs and Bt toxin. The series of papers address the issue of whether "Bt corn" is harmful to monarchs in the field as well as in the laboratory.

"We still don't have the data to come to the conclusion that the risks are negligible," said Oberhauser, a research associate in the department of ecology, evolution and behavior.

The researchers examined milkweed and monarch densities in various habitats--cornfields, cornfield edges, other agricultural fields and nonagricultural fields--in the butterfly's breeding range during summer 2000. The study area comprised 20 plots in four regions: Minnesota and Wisconsin, Iowa, Maryland, and Ontario. Plots were in cornfields, land adjacent to cornfields or nonagricultural land, all containing milkweeds. Except for one plot each in Maryland and Iowa, cornfields contained nonBt corn. Every week, researchers examined several hundred milkweed plants and noted the number of monarch larvae and, in cornfields, whether pollen was being shed.

In Minnesota and Wisconsin, corn pollen was shed during a time--mid-July through the first week in August--when the highest numbers of monarch larvae were in cornfields. In Iowa, pollen was shed during the first half of July, before the peak in monarch larvae.

"The situation in Iowa is less risky for monarchs," said Oberhauser. "The farther north the cornfield, the later the pollen was shed and the greater the chance that monarch larve will be exposed to it." When the researchers calculated the percent of monarch larvae in the fields when pollen was shed, Ontario led the way with 62 percent, followed by Minnesota-Wisconsin (40 percent), Maryland (20 percent) and Iowa (15 percent). They also estimated that in general, 73 times as many monarchs come from cornfields as from nonagricultural land in Minnesota-Wisconsin. In Iowa, the figure was 80 times as many, with both corn and soybean fields contributing large numbers of the butterflies.

"One of the most important conclusions of this study is that agricultural habitat is important to monarchs," said Oberhauser. "If an event that hurts monarchs or their host plant, milkweeds, should occur in cornfields, it could have population-wide impacts."

Oberhauser cautioned, however, that studies by other researchers have suggested that Bt toxin is not harmful to monarchs at levels found in cornfields. But many of those studies occurred in laboratories and looked at pollen only. In cornfields, plants shed part of the anthers (the structures that produce pollen), and anthers tend to have higher concentrations of Bt than pollen does. Even the studies done in the field may have underestimated the amount of time that monarchs were exposed to toxic corn tissue. While monarch larvae don't avoid pollen on milkweed leaves, it is not known whether they also eat anther material, Oberhauser said.

"I think we still don't know the effects of long-term exposure to Bt," she said. "Some lab studies may not have exposed them long enough to tell." And, even if Bt doesn't kill monarchs at levels found in cornfields, "We don't know how Bt affects factors like monarch reproduction, flight ability and size," Oberhauser said.

Oberhauser's colleagues in the study were from the University of Guelph, Ontario; the University of Maryland; Iowa State University; the Southwest Purdue Agricultural Program, Indiana; the U.S. Department of Agriculture (USDA); and the University of Minnesota. Financial support came from the USDA, the Canadian Food Inspection Agency; Environment Canada and the Ontario Ministry of Agriculture, Food, and Rural Affairs.

For the last seven years, Oberhauser has directed the Monarchs in the Classroom project. This fall, more than 30,000 monarch eggs and larvae were given to hundreds of Minnesota schools to be nurtured by children. When adult butterflies emerge from their chrysalises, the children will release them for the species' annual migration to Mexico.
Karen Oberhauser, Department of Ecology, Evolution and Behavior, 612-624-8706

University of Minnesota

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