Zombie Caterpillars Lurch Through Forest Canopy, Infecting Their Brethren

gypsy moth caterpillar "face"

The zombie caterpillar apocalypse has begun!   A recent study published in Science by Dr. Kelli Hoover and her research group at Penn State showed that a virus infecting gypsy moth caterpillars  causes them to become like the living dead.  Infected caterpillars crawl to the treetops where they die and their putrefying corpses shower virus particles over the terrified survivors.

An infected caterpillar hangs decomposing from a branch

Gypsy moth caterpillars are normally nocturnal and only forage in the the tree tops at night when they can avoid predators.   During the day they return to their hiding places  in the forest understory.  However, caterpillars infected with the virus climb to the top of the tree during the day and feed continuously.  They also stop normal development toward adulthood but become ravenously hungry,  growing larger and larger and providing the virus with more flesh to feed on.  Eventually, they die and their corpses liquefy and drip infectious material over the remaining caterpillars.

The coolest part?  The behavior is caused by a single gene in the virus.    This baculovirus has a gene that codes for an enzyme called  EGT, which  inactivates caterpillar molting hormone.  Caterpillars have to molt to grow and develop, so when they become  infected,  caterpillars are stuck in perpetual childhood, eating, growing bigger but never developing to adulthood.  The EGT enzyme also causes the climbing behavior, because without the urge to molt, caterpillars are driven to eat without stopping, and so they feed continuously day and night without ever coming down to rest.

Dr. Hoover and her team showed that EGT was causing the climbing behavior with a simple experiment.  They took some natural  strains of the virus with the EGT gene and two strains where they had artificially  inactivated the EGT gene.  Then they took Gypsy moth caterpillars, infected some with each virus  and placed them in soda bottles with holes.    The caterpillars with the natural virus climbed to the top of the soda bottle and died, just like they did in the wild.  Those that received the virus with the disabled gene died on the bottom of their cages.  In the wild, caterpillars that die in the understory or forest floor wouldn’t be able to infect many others, so the EGT gene is very beneficial for the virus becaus it allows it to spread rapidly.  Lots of parasites use this kind of mind control to force their hosts to spread them , but this is an exciting result, because it’s one of the first times science has shown that a single gene in a parasite can alter host behavior.

Of course, no zombie story is complete without some heavy-handed social commentary.  The gypsy moth is an invasive species introduced to North America in the 1860s and it spread quickly through America’s hardwood forests.   It’s since become a huge pest, and it’s boom and bust population growth cycles can cause massive defoliation in forests.  Scientists are actually using simlar viruses to control gyspy moth populations in areas that are too sensitive to use pesticides.  So the virus may actually provide forestry researchers with a new tool to control the gypsy moth menace.   As always, it’s human nature not the zombie hoard we should truly fear.


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