The researchers are not completely certain why the caterpillars climb or stay aloft during daylight when they are infected. One possibility is that without the molting cue, the caterpillars simply have an urge to eat continuously and so remain in the treetops.

"Michael Grove, my former technician who initiated this study in my lab, thinks that even when the molting hormone is inactivated, the caterpillars may still be triggered to climb to molt," said Hoover. "They climb, but rather than molt, they stay where they are until they die."

To show that the egt gene is responsible for the climbing, the researchers used tall plastic bottles lined with screens for the caterpillars to climb on. The bottles contained an artificial caterpillar diet for food. The researchers tested six different virus infected groups of caterpillars and one uninfected group. Two groups were infected with different, naturally occurring virus; two groups were infected with virus that had their egt gene inactivated in different ways; and two groups had the egt gene reinserted in the viral DNA.

The two naturally occurring viruses caused the caterpillars to climb and die at the top of the container, but the caterpillars infected with virus lacking the egt gene died at the bottom of the container. The caterpillars with the restored egt gene also died at elevated positions. All the infected caterpillars exhibited the same symptoms during the initial phases of the infection, but only those infected with viruses containing egt climbed to die.

Hoover notes that this is one of the first studies to identify the gene of the parasite responsible for altering the behavior of the host animal. Many parasites manipulate their hosts, but in most cases, how this occurs in not known. Other pathogens that control host behavior in mammals include toxoplasmosis and rabies. Toxoplasmosis is a parasitic disease that mostly infects cats, but can infect other mammals. If a mouse becomes infected with toxoplasmosis, they lose their innate fear of cats and become easier for cats to catch. This benefits the infecting protozoa because it more easily spreads to its preferred host.

The rabies virus also alters behavior causing normally nocturnal animals to appear during the day and to become far more aggressive than normal.

Also working on this research from Penn State were Matthew Gardner, undergraduate honors student now at Harvard; David P. Hughes, assistant professor of entomology and biology; and James McNeil, graduate student in entomology. Also a member of the research team was James Slavicek, U.S. Department of Agriculture, Forest Service.

The U.S. Forest Service supported this work.