Ulcer on the cob.
The peritrophic matrix in a caterpillar larva gut is perforated after eating both the Mir1-CP enzyme and a transgenic toxin (bottom). At top, the gut of a larva eating the transgenic toxin alone.

Courtesy of Dawn Luthe

How to Handle a Hungry Caterpillar?

Take a bite out of a living mammal and it bleeds. But take a bite out of some plants, and they send poison to the wound. Now, scientists have discovered the biological mechanism of one such defense in corn that thwarts even the hungriest of insects: the caterpillar.

For 20 years, W. Paul Williams--a U.S. Department of Agriculture Agricultural Research Service geneticist at Mississippi State University in the town of Mississippi State--has been crossing exotic strains of insect-resistant corn from the Caribbean island of Antigua. His colleague Dawn Luthe, a plant biologist at Pennsylvania State University in State College, wanted to understand the mechanism behind the natural insect resistance. She found that when a caterpillar chomps on these varieties of corn, a previously unknown enzyme called Mir1-CP starts to accumulate at the site of the wound.

Further experiments revealed that the enzyme makes a part of a caterpillar's gut, called the peritrophic matrix, permeable, inhibiting digestion. As a result, the caterpillar larva remains small and more vulnerable to predators. The damaged gut also allows microorganisms and parasites to pass from the caterpillar's food into the insect's body and kill it, Luthe reported yesterday at the annual meeting of the American Chemical Society in Atlanta, Georgia.

Once Luthe and colleagues identified the mechanism behind the corn's resistance, they wanted to see how this natural defense compared with transgenic toxin. To do this, they fed caterpillar larvae pure bacillus thuringiensis toxin. The Mir1-CP enzyme caused up to 4-times more morbidity than did the toxin. However, it took longer for those fed the enzyme to die because of how the enzyme works. Lastly, the team fed caterpillar larvae both the Mir1-CP enzyme and the bacillus thuringiensis toxin. Luthe says that this combination was much more deadly than either one was alone. "When you mix two mechanisms of attacking a caterpillar, it's unlikely that they will develop a resistance to both mechanisms," she says.

Even though the Mir1-CP enzyme doesn't kill caterpillars outright, Gregg Howe, a plant biologist at Michigan State University in East Lansing, says "simply slowing down the growth of the insect larvae would provide opportunities for predators to come in and reduce those populations pretty quickly." And because this is a natural defense rather than a genetically modified one, he says, it may "get around the transgenic technology that has been objected to in Europe and in other places."

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