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A monarch caterpillar that was likely killed by a virus.
A monarch caterpillar, most likely killed by a baculovirus.

Photograph courtesy Michael Grove, Science/AAAS

Brian Handwerk

for National Geographic News

Published September 8, 2011

Scientists have identified a single gene that allows a caterpillar-brainwashing virus to do its dirty work, a new study says.

The virus forces the "zombie" caterpillars to climb trees, where the invader eventually liquifies its hosts' bodies into a dripping goo. (See "Caterpillar Fungus Making Tibetan Herders Rich.")

"When gypsy moth caterpillars are healthy and happy, they go up into the trees at night to feed on leaves, and then climb back down in the morning to hide [in bark crevices or soil] from predators during the day," said study co-author Kelli Hoover, an entomologist at Penn State University.

But caterpillars infected with a baculovirus—a type of virus that infects invertebrates—are driven to the treetops and reprogrammed to stay there until they meet a doom worthy of a horror film.

"When they are infected, as they get sicker they stay up in the trees and die up there," Hoover explained.

The virus "ends up using just about all of the caterpillar to make more virus, and there are other genes in the virus that then make the caterpillar melt. So it becomes a pool of millions of virus particles that end up dropping onto the foliage below where it can infect other moths that eat those leaves."

Viruses Are Master Manipulators

Though such zombie-making viruses were previously known, their genetics have been a mystery.

So Hoover and colleagues infected gypsy moth caterpillars with half a dozen different types of baculovirus and placed the bugs in tall bottles with food on the bottom. Viruses that the scientists had determined carried a specific gene, called egt, drove caterpillars to climb to the top of the container and stay there to die.

(Read "'Zombie' Roaches Lose Free Will Due to Wasp Venom.")

Researchers then removed egt from some viruses, reinfected the caterpillars, and found that the zombie behavior stopped. When the team inserted the gene into a virus that previously lacked it, the zombie behavior returned.

"Somehow or other, using this gene, the virus is able to manipulate the behavior of the caterpillar to go to the right location in the tree to enhance transmission to new hosts. It's really amazing," Hoover said.

The gene may work by deactivating its hosts' molting hormone, according to the study, published tomorrow in the journal Science.

"That would be an advantage to the virus because it keeps the insect in a feeding state, so that they get bigger and bigger and make more and more virus."

Viruses and Moths-Natural Enemies

There are many different types of baculovirus, Hoover said, and almost all caterpillar species are infected by one or more of them.

But the virus, which is naturally occurring, doesn't greatly impact gypsy moths as a species, Hoover said. Gypsy moth populations are prone to cycles of boom and bust, so when caterpillar numbers are in check, the virus remains so as well.

When gypsy moth invasions grow, the virus may go into outbreak mode-serving as a natural control mechanism for caterpillar infestations.

"This virus probably came to North America when the caterpillars did," Hooever explained. "It's just a natural enemy of the gypsy moth."

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