In March 2011, I posted about ants turned into “zombies” by a virus infection. The behavior of the infected ants is altered by the virus infection, thus the comparison to zombies in popular SciFi movies. Ants are not the only insects that can be turned into zombies by diseases. Gypsy moth caterpillars can become zombies, also.
Gypsy moth caterpillars are pests of forests that feed on the leaves of trees. Large populations can defoliate large areas of forest and eat all the leaves off a homeowner’s tree. Before molting from caterpillar to pupa, healthy gypsy moth caterpillars commonly climb down the tree trunk. However, instead of climbing down the tree, virus infected “zombie” caterpillars climb high in the tree canopy and die in the crowns of the trees. From this elevated position, gypsy moth cadavers, filled with virus, will rain down virus on other caterpillars, spreading the disease. The virus can contaminate the trees and remain active for years. Infection can spread to newly emerging caterpillars the following year.
The gypsy moth virus can be produced in large quantities (Gypchek) and is approved by EPA for use in gypsy moth control. Virus is one factor that can end gypsy moth outbreaks and suppress populations for years. Climbing to the tree crown before dying increases the virus spread.
How can a virus turn a caterpillar into a zombie? A recent study has identified a single virus gene that is responsible for the behavior. The gene (EGT) encodes an enzyme that biochemically modifies an important insect hormone, 20-hydroxyecdysone, by attaching a sugar to the hormone. In uninfected caterpillars, the 20-hydroxyecdysone triggers a cascade of neurological and behavioral events including molt initiation. It is not surprising that biochemical alteration of the behavior altering hormone, has disruptive effects on the caterpillar behavior. The EGT effect could be due to decreasing the amount of the 20-hydroxyecdysone or by unique effects of the altered hormone. The caterpillars suffer a series of “bad hormone days” drastically affecting their behavior.
The study found that caterpillars infected with a virus containing the EGT gene would climb to the tops of their container before dying. Caterpillars infected with virus that had the EGT gene removed would often die on the bottom of the container. The EGT gene is implicated in altering the caterpillar behavior and forcing the zombie caterpillar to climb to the top of a tree.
The details of the study by Hoover et al. can be found here:
Science 9 September 2011: Vol. 333 no. 6048 p. 1401