Researchers at Pennsylvania State University have identified the cause of an abnormal virus-spreading behavior in the caterpillars of gypsy moths that has caused them to become characterized as "zombie caterpillars."
A century ago, observers became aware of their atypical behavior. Some gypsy moth caterpillars would climb to the top foliage of trees during the day. Migration up and down trees is dependent on light intensity and population density.
When the population is sparse, caterpillars display their typical form of migration. They rest and feed on foliage at the middle of trees during the day to protect themselves from the sun's rays. At night, they climb to the tops of trees and feed there while temperatures are cool. If the population is dense, however, they feed continuously at various levels during both day and night.
Usually, caterpillars go through a number of cycles of molting to shed their exoskeletons, allowing them to grow in size. After each molt, the caterpillars increase in size before they turn into pupae, the stage preceding adulthood. As a result, there is no gradual change in size. Males typically go through five molts, females go through six. The ‘zombie caterpillars' do not continue their molting cycle when they reach the tops of trees during the day.
Instead, the caterpillars quickly die at their lofty perches due to overexposure to heat. They undergo a natural decomposition characteristic of normal caterpillars in which their exoskeletons disintegrate and their bodies liquefy, dripping down to lower parts of the tree.
When this behavior was first noticed, it was believed to be caused by a virus. Not until recently was the virus identified by Kelli Hoover at Penn State. The virus belongs to a large family of baculoviruses; these viruses are species-specific, affecting over 600 species of invertebrates, most commonly moths.
While plants and mammals cannot be affected by baculoviruses, some act as carriers, such as the plants that gypsy moth caterpillars feed on. The virus completely hijacks the host's body, quickly infecting all of its somatic cells, or cells not involved in reproduction.
Hoover found that a single gene of the virus was the cause of the caterpillars' erratic behavior. The egt gene is responsible for the production of an enzyme that prevents proper molting in the host.
Under normal circumstances, eating is not the cause of caterpillar growth during the molting cycle. Instead, caterpillars shed their previous skin and simply emerge larger than they were before. When caterpillars are infected by the virus, they are compelled to eat during the molting cycle, causing a large increase in body mass. As a result, more cells are available for infection by the virus.
After death, the liquefied remnants of the infected caterpillar begin dripping down onto other caterpillars and infecting them. It is through this means of transmission that the caterpillars are "zombies," because their infected cells can infect unsuspecting caterpillars.
Birds also facilitate in the spreading of the virus; before eating the caterpillars, birds bash them against the tree trunk to remove the caterpillars' hair. Slow-motion video evidence shows that this practice also allows liquefied caterpillar guts to fall down into the lower parts of trees onto other healthy caterpillars.
To prove the cause of this behavior, Hoover and her associates synthesized a new strain of the virus in which the egt gene was not expressed by a genetic deletion, resulting in a ‘knockout,' or a complete loss of a gene from a species' DNA.
Caterpillars purposefully infected with this virus do not exhibit the abnormal migratory patterns of their affected counterparts and remain in the lower foliage during the day. Their molting cycle also remains normal, and when they die, they are unable to affect others and carry the virus to new hosts.
Despite the horror that the idea of zombie caterpillars invokes, there may very well be a useful application of these baculoviruses. Gypsy moth caterpillars can become pests based on their population size. By manipulating the virus that causes their zombie-like behavior, it could be possible to diminish their ability to be agricultural pests.
