Gypsy moth caterpillars infected with baculovirus forfeit safety and stay in the treetops during the day because a virus gene manipulates their hormones to eat continuously and forego molting, according to entomologists. The caterpillars die where they climb and infect other gypsy moth caterpillars.

"Normally, gypsy moth caterpillars are active at night," said Kelli Hoover, professor of entomology, Penn State. "They hide during the day in the soil or bark crevices protected from birds. They climb up the foliage at night to feed."

Researchers have long known that gypsy moth caterpillars, like nearly all caterpillars, have baculoviruses that infect them and that a gene in the virus, egt, blocks molting in the caterpillar, keeping it in a feeding state. These viruses use most of the tissue of their hosts to reproduce and almost always kill their host.

"Baculoviruses have been known to induce climbing behavior in their caterpillar hosts for over 100 years," the researchers report in today's (Sept 9) issue of Science. "Until recently, determining the evolutionary basis for these altered behaviors has proven difficult in the absence of a mechanistic explanation."

The infection was labeled tree top disease, but 100 years ago, researchers could not look at either the virus' genetic material or the metabolic pathways in the caterpillar. Hoover and her team looked into the mechanism by which the gypsy moth baculovirus manipulates the behavior of the caterpillars.

They identified a specific viral gene, egt, that codes for an enzyme, EGT -- UDP-glycosyltransferase -- that inactivates the hormone that triggers molting. Male gypsy moth caterpillars molt five times during their lives, while females molt six times before they pupate and emerge as moths. But infected caterpillars do not molt again once levels of EGT become high enough. EGT induces the caterpillar to climb to the treetops, hang onto the leaf or bark with their prolegs and die. Then, they liquefy and rain viral particles over the leaves for other caterpillars to ingest and become infected. Older caterpillars are induced to die on the bark next to their fellow gypsy moths that pupate and emerge to walk over the dead cadavers, picking up virus that can be transmitted to the next generation during egg laying

"One of the best ways to control complex behavior is to manipulate hormones," said Hoover.

Genes that influence hormones are perfect targets to change behaviors. The viral gene egt blocks molting by inactivating the molting hormone ecdysone, keeping the insect in a feeding state.

"It is good for the virus because if host spends 24 hours not feeding while they prepare to molt, this is time that the host is not getting bigger to maximize the host's biomass to make into more virus," said Hoover. "In this case we've found that that the gene also somehow induces the caterpillars to go to just the right location to enhance transmission of the virus to new hosts."