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Killer Corn
May 09, 1999

The difficulties scientists can encounter with creating new kinds of plants via genetic engineering was evident this week when scientists at Cornell University found that toxic pollen from a widely planted genetically engineered corn can kill Monarch butterflies.

The hybrid, known as Bt corn, is genetically-engineered to carry and express a gene for the bacterium, Bacillus thuringiensis. This gene instructs the corn to produce a protein that is toxic to a major pest: the European corn borer (ECB). The ECB larvae die after feeding on toxin-containing leaves and stem of the Bt corn. Bt and Bt corn is safe for humans and other mammals.

John Losey , one of the authors of the Cornell University study, explains. "I think what is unique about our study is we’re showing this form is mobile, it can get on other plants, once it gets on other plants, other insects can eat it and we really have to look at those other insects and see if they are going to be affected."

Monarch Mortality

The Bt toxin is also expressed in the corn’s pollen. The milkweed, the exclusive food of Monarch caterpillars, tends to grow in areas surrounding cornfields. Should the wind carry the pollen outside of the cornfield, the pollen will dust the surfaces of nearby plants, such as the milkweed. Inadvertently, untargeted species such as Monarchs may consume the dust coating and also be poisoned.

The Cornell scientists fed milkweed leaves that had been dusted with the Bt corn pollen to Monarch caterpillars. After four days, the total length of the study, nearly half the population died. Those caterpillars that remained alive ate less and grew to a smaller size. The researchers believe that if the experiment had continued for several more days, all the Monarchs would have died. All the control animals, those that lived on plants dusted with normal corn pollen or no pollen, were healthy. The paper was published in the 20 May 1999 issue of the journal Nature.

As Losey explains, Monarchs might not be the only species affected. "Monarchs could be just a sort of bell, the canary in the mine, in a sense. There are a lot of other (species) out there on other plants and what we’d like to do is study what other plants occur around cornfields and what other butterflies and moths feed on those plants. And then we could start to see what other insects this is going to impact."

From Lab to Field

In response, Randy Krotz, a spokesman for the Monsanto Company , which owns the patent for the hybrid, said it was too early to draw any conclusions about Monarchs in the wild from lab studies. He said that Monsanto was planning field studies to determine if the threat against the Monarch butterflies in the wild is real.

image: Nebraska Game & Parks Commission

The issue isn’t whether the pollen is toxic to butterflies, but, rather, whether sufficient corn pollen can be carried by the wind out of the cornfield onto other plants. Industry scientists believe that the pollen does not travel far from the cornfields, while other researchers believe it may. No one knows for sure.

John Obrycki and Laura Hansen of Iowa State University recently presented unpublished research that measured amounts of Bt corn pollen on milkweed leaves within and around cornfields, but they took no measurements beyond three meters. Obrycki said that the majority of the pollen is found within three meters, based on previous studies, but that other studies cited in Losey’s Nature paper demonstrate wind-dispersal of corn pollen as far as 60 meters from the source.

Risks vs. Benefits

Last year, U.S. farmers planted more than seven million acres of the Bt corn crop and at least 18 different Bt-engineered crops have been approved for field testing in the US. There are several good reasons to consider further development of transgenic crops.

Around 7 percent, or $1.2 billion, of the world’s 560 million tons of corn harvest is lost just to ECB. Doyle Karr, a spokesman for Pioneer Hyrbids International points out that by increasing yield, the food needs of an increasing population can be met without having to put increasing amounts of fragile ground into crop production.

Monsanto’s Randy Krotz , claims the product also benefits the environment by reducing pesticide use. "Prior to the introduction of Bt corn, farmers controlled the corn borer with insecticides acutely toxic to Monarchs and other desirable nontarget insects. Using broad spectrum non-target insecticides," Krotz says. "Bt corn reduces the harm to nontarget species and to the environment."

But Obrycki says that while Bt corn does increase yields, it does not decrease the use of pesticides, but rather, introduces a new toxin into the mix. "Data I find for 1995 states that 2.6 percent of acres in IA were treated with broad-spectrum insecticides against the corn borer," says Obrycki. "If [Bt corn was] replacing insecticides it would be a benefit that would make sense—if we were using a lot of insecticides against the corn borer, but we are not. Not in Iowa."

Corn borer
image: Kansas State University

Recently, other researchers have been investigating concerns that the corn borer may develop resistance to Bt corn. Resistance to Bt is believed to be a recessive trait in most pests, therefore many scientists think that Bt hybrids will be effective for a long time. Bt pesticides have been available for over 40 years and they remain effective (their limitation is that they cannot penetrate into the stock, where pests like the corn borer do their damage).

But new research by Randall Higgins and his colleagues at Kansas State University is raising some questions in this arena as well. In their laboratory study (Science, 7 May 1999), they developed a lineage of the ECB that is resistant to the effects of the Bt toxin. The Bt toxins they used were not identical to those expressed by the transgenic plants. But if these results turn out to be similar to those obtained under field conditions, then the usefulness of the Bt corn may be significantly diminished.

John Losey believes academia and industry agree on the goal of continuing to develop transgenic crops, while minimizing their environmental impacts. "We need to look at the big picture here," Losey says. "Pollen from the Bt corn could represent a serious risk to populations of monarchs and other butterflies. And we can’t forget that transgenic crops have a huge potential for reducing pesticide use and increasing yields. The study is just the first step. We need to do more research and then objectively weigh the risks versus the benefits of this new technology."

"These are our tigers"
image: Nebraska Game and Parks Commission

Monarchs need milkweed to produce a chemical that protects them from predators. When birds eat a Monarch, they become very sick and learn to never eat a Monarch again. The Viceroy butterfly, which does taste good to birds, patterns itself like the Monarch, and in doing so, has taught us an example of mimicry as a survival adaptation.

The Monarch butterflies are migratory, like birds and whales. They fly over three thousand miles twice a year, sometimes returning to the exact same trees. The largest population of Monarchs winter in Mexico, where their habitat is seriously threatened. The summers find over 50 percent of the population originates within the Corn Belt of the United States.

Monarchs capture the minds and hearts of children, young and old, with their colorful beauty and carefree flutter. Monarch Watch, an organization of researchers, teachers, students, and volunteers, has been tagging and tracking the migration pattern of Monarchs, which are not officially protected, but are declining in number. As this year’s crop of Bt corn begins to mature, the effect on the Monarchs will be closely monitored (and you can help).

Cornell’s Linda Rayor found her own results "really frightening. . . I don’t know what the extent of the problem is, but it is clear that we have potentially toxic Bt corn pollen that’s blowing in the wind."

Elsewhere on the web

Corn Controversy (STN2)

Killer Corn II (STN2)

Bugs Bite Back

The companies involved with the development of Bt corn are Monsanto, Novartis, and Pioneer Hybrids International.

Monarch Watch has a lot of material online about Monarch Butterflies. You can also volunteer to help.

The news release from Cornell
The news release from Kansas State University
John Obrycki’s home page
Iowa State European Corn Borer

by Rogene M. Eichler West

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