The Rise of the Zombie Ants

 

A Chestnut Carpenter Ant (Camponotus castaneus)
infected with the Cordyceps fungus Ophiocordyceps unilateralis.
Rowan County, North Carolina. 

Picture if you will, a parasite invades the body, begins to replicate, and takes control of the brain and behavior of the host.  Then, after consuming its victim from inside, the parasite bursts out of the body, spreading spores to infect more unfortunate hosts.  This scenario from a science fiction horror film plays out on miniature scale right in our forests.  The parasite is a fungus called Cordyceps, and it infects insects including ants. 

After watching a David Attenborough nature documentary on the Cordyceps-ant interaction, a student at Catawba College in Salisbury, North Carolina discovered infected ants in the college’s Ecological Preserve.  Dr. Carmony Hartwig, a biologist at Catawba College, led me to a site where the fungus had been killing ants.  These areas are called graveyards and dead ants decorate the trees with their bodies.    

 

A Chestnut Carpenter Ant with its jaw's clamped
onto a limb and the first sign of O. unilateralis 
emerging from behind the head. 
Rowan County, North Carolina. 

An ant becomes infected when a Cordyceps spore attaches to its exoskeleton.  The fungus spore germinates and a cylinder of cells, a hypha grows out.  It secretes enzymes that dissolve a tiny hole in the ant’s exoskeleton, and the fungal filament enters the body. Inside the ant the fungus consumes the internal organs of the ant and grows around the brain.  The Cordyceps then releases chemicals that control the ant’s behavior.  The infected ants are now walking dead insects controlled by the fungus.  They are zombie ants.   

 

Chestnut Carpenter Ant with an actively growing
O. unilateralis stroma.
Rowan County, North Carolina. 

Chestnut Carpenter Ant with a stroma bearing
the spore producing perithecium.
Rowan County, North Carolina. 

The fungus directs the infected ant to crawl up a tree and out onto a thin branch.  When the ant reaches a height of 4-7 feet it stops crawling, and clamps its jaws in a death grip to the bottom of a stem.  Then a fungal stalk, the stroma, begins to grow from just behind the doomed ant’s head.  The stroma produces a reproductive structure, the perithecium, that makes spores that are released into the air.  The spores fall to the ground and can infect other ants.  By making the infected ants climb onto a tree branch. the Cordyceps spores can disperse longer distances than if the ant stayed on the forest floor. 

 

A healthy Chestnut Carpenter Ant.
Rowan County, North Carolina. 

The genus Cordyceps has recently undergone a revision and was split into three genera. The name Cordyceps is still used as a general term for these fungi but the ones I observed are probably Ophiocordyceps unilateralis. The infected ants with their jaws clamped to the tree branches are Camponotus castaneus, the Chestnut Carpenter Ant. 

There are more than 200 species off Ophiocordyceps and they can infect many species of insects and arachnids.  This drama of ant and fungus has been playing out for at least 99 million years.  Scientists in China recently found an ant preserved in amber with an Ophiocordyceps growing from it.  

A popular video game and subsequent TV series, The Last of Us is based on a Cordyceps infection that turns humans into zombies.  It is downright macabre seeing zombie ants in the trees a half mile from my house. 

A Cordyceps infected ant with a double stroma and two perithecia.
Rowan County, North Carolina.