Behavior-modifying parasites are getting more press as of late, with reports of zombie ants and Toxoplamsa-infected rats that become sexually attracted to cats. But it's not just organisms that manipulate their hosts; there's at least one behavior-modifying virus. Just as the parasitic organisms do, baculoviruses change their host's behavior for their own benefit, ensuring their propagation. When infected with a baculovirus, European gypsy moth caterpillars behave in a way that healthy gypsy moth caterpillar never would.

A healthy gypsy moth spends its days hiding in bark crevices or climbing down the tree to the soil during daylight, to avoid predation from birds. They only venture back out onto the leaves under the safety of night. A gypsy moth infected with a baculovirus, however, behaves quite differently. The aptly named "tree top disease" makes the infected gypsy moth caterpillar climb to the top of a tree to die, liquefy, and release millions of infective virus particles.

Once the host is liquified, virus dispersal is then facilitated by rainfall. Scientists have been watching infected moth larvae behave this way for a century, but have only recently identified the mechanism behind the behavior. A team of researchers, led by Dr. Kelli Hoover of Pennsylvania State University, and included other researchers from Pennsylvania State University, Harvard Medical School, and the US Forest Service, have now identified the viral gene behind tree top disease.

The team hypothesized that tree top disease in induced by the expression of a baculovirus gene known as egt. To test this theory, researchers tested six modified viruses for their impact on insect climbing behavior. They placed individual moth larvae in tall plastic bottles that contained food at the bottom and a fiberglass screen for the larvae to climb on. Viruses with the egt gene caused larvae to climb to the top of the container and stay there to die. Deleting the egt gene eliminated this behavior, while reinsertion of the gene restored the climbing behavior once more.

The egt gene encodes a protein that inactivates a hormone used by the caterpillars, so it's easy to see how it could have global effects on behavior. The results offer a genetic explanation for what's known as an extended phenotype, which encompasses the direct effects of a gene as well as its influences on the survival chance of that gene.

Science, 2011, DOI: 10.1126/science.1209199