Viral Gene Propagates Through Altered Host Behavior

By Robert J. Korpella

First posted on 09-14-2011

For decades, scientists observed gypsy moth caterpillars that were sick with a virus, do something they would normally never do in daylight. The larvae mustered the last of their strength to climb onto a leaf as high in the tree as they could before succumbing to the disease. The question was why?

An article published in the journal, Science, says that a specific viral gene is enough to alter gypsy moth behavior in order to present the best means of spreading the virus while killing the host. This discovery includes genetic evidence of a parasite’s ability to influence host behavior, which scientists refer to as an extended phenotype.

A normal, healthy gypsy moth caterpillar typically feeds at night, hiding in the crevices of the tree’s bark or descending the tree and into the soil as means of avoiding predators. But the behavior altering virus is at a distinct advantage when gypsy moths climb onto a high leaf to die. The caterpillar’s body liquefies after death, and it releases millions of virus particles into the air, contaminating other gypsy moth caterpillars in that tree as well as in neighboring trees. That release would not occur as effectively if the caterpillar held tight inside the tree’s bark.

This isn’t the only incidence in nature of pathogens influencing host behavior. An ant fungus turns the infected insects into the ant version of the living dead. The diseased creatures position themselves on the tree canopy, the ideal place for a release of fungal spores. The ants even hold tight to leaves with their strong mandibles so the wind doesn’t carry away their weak bodies.

“Who knew that a virus could change the behavior of its host?” said U.S. forest Service biologist Dr. Jim Slavicek, a member of the research team. “Maybe this is why we go to work when we have a cold.”