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Now a story about a very hungry caterpillar and a very clever virus. Each year, gypsy moth caterpillars damage a million acres of forest in the U.S. But the damage would be greater if it weren't for a virus that can infect these caterpillars and cause them to engage in reckless, even suicidal behavior. NPR's Jon Hamilton reports on new research that appears to explain how this virus seizes control of its unwitting host.

JON HAMILTON: Gypsy moth caterpillars are an invasive species with a simple survival strategy. They feed on tree leaves at night, when birds and squirrels can't see them. Then, during the day, they climb down and hide. But Kelli Hoover, an entomologist at Penn State, says caterpillars abandon that sensible strategy when they're infected with something called a baculovirus.

Ms. KELLI HOOVER (Pennsylvania State University Professor): As they get sick they climb up to elevated positions and stay there and die.

HAMILTON: And Hoover says it's a gruesome death.

Ms. HOOVER: The inside of the caterpillar gets pretty much converted to millions and millions of virus particles and then there are other enzymes that cause the exoskeleton to melt. And that liquefies the caterpillar and then it can rain virus down on the leaves below.

HAMILTON: Which is how the baculovirus infects lots of other caterpillars. The virus is so effective that the government actually sprays it on trees to help control gypsy moth outbreaks.

But scientists wanted to know how the virus makes caterpillars lose their minds. Hoover and a team of researchers suspected that the virus was using a special gene, a gene that could control caterpillars' feeding behavior. To find out, the scientists infected some caterpillars with virus that carries the normal version of this gene and some with virus carrying an inactivated version. Then they put the caterpillars in tall plastic containers lined with screen.

Ms. HOOVER: Every time the caterpillars were infected with the normal gene, they would die at an elevated position in the container, and if the gene was knocked out, they didn't.

HAMILTON: Hoover says that makes sense because one thing this gene does is interfere with a system that tells the caterpillar when to stop eating. When that system breaks down, she says...

Ms. HOOVER: It stays in a feeding state. And to feed, you need to be up in the tree.

HAMILTON: Other scientists say the finding reveals just how clever a pathogen can be.

Mr. JIM SLAVICEK (U.S. Forest Service): Who knew that a virus would be able to manipulate the behavior of its host?

HAMILTON: Jim Slavicek is a research biologist with the U.S. Forest Service who contributed to the new study.

Slavicek says knowing precisely how baculovirus overwhelms the gypsy moth could help scientists develop more potent strains of the virus. It could also help them determine when in the gypsy moth's life cycle it is most vulnerable to infection. And he says all that could help bring down the cost of spraying with baculovirus. Right now, he says, land managers often use cheaper methods, such as insecticides, or a deadly fungus.

Mr. SLAVICEK: The advantage of the virus is that it is specific for gypsy moth larvae, and so it will impact no other animal, insect, plant in the treatment zone.

HAMILTON: Slavicek says gypsy moth outbreaks are less severe than they were a couple of decades ago. But the pest remains a major threat that can leave a forest bare in a matter of weeks. He says during an outbreak there are so many caterpillars that their remains make some roads dangerously slick. And if you drive on those roads at night, he says...

Mr. SLAVICEK: Millions of moths will fly to the car and it can be so dense that it's like a snowstorm. You can't see what's in front of you. So you've got to pull off to the side of the road, turn off your lights, wait for a few minutes and then drive on.

HAMILTON: The new research appears in the journal Science.

Jon Hamilton, NPR News.

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