Zombie Ant Fungus Kills Hosts at their 'Doorstep'

A parasitic fungus, described in a new study, manipulates its ant host to die on the "doorstep" of its colony, as to ensure an endless supply of future hosts.

Ophiocordyceps camponoti-rufipedis, also known as the "zombie ant fungus," prefer this close-to-home killing in order to more effectively reproduce and transmit their infection, according to researchers at Penn State and colleagues at Brazil's Federal University of Vicosa.

The fungus controls the behavior of carpenter ant workers (Camponotus rufipes) to die with precision attached to leaves in the understory of tropical forests.

"After climbing vegetation and biting the veins or margins on the underside of leaves, infected ants die, remaining attached to the leaf postmortem, where they serve as a platform for fungal growth," lead author Raquel Loreto said in a statement.

The fungus grows a stalk that protrudes from the body of the unlucky ant victim. Infectious spores then develop in the stalk that are discharged onto the forest floor below, where they can infect foraging ants from the colony. However, ants are insects that have evolved a type of social immunity to protect them against such deadly fungi.

"Previous laboratory studies have shown that social immunity is an important feature of insect societies, especially for ants," added co-author David Hughes.

To test the theory, the researchers placed 28 ants freshly killed by the fungus inside two nests - 14 in a nest with live ants and 14 in one with no ants. They found that the fungus was not able to develop properly in any of the 28 cadavers. In the nest with live ants, nine of the 14 infected cadavers disappeared, presumably removed by the ants in an effort to thwart the disease organism.

"Ants are remarkably adept at cleaning the interior of the nest to prevent diseases," Hughes explained. "But we also found that this fungal parasite can't grow to the stage suitable for transmission inside the nest whether ants are present or not. This may be because the physical space and microclimate inside the nest don't allow the fungus to complete its development."

In a more detailed survey of these ant colonies, the researchers realized that to bypass this social immunity, the zombie fungus manipulated its ant hosts to die on the doorstep of their colonies, rather than inside them.

"What the zombie fungi essentially do is create a sniper's alley through which their future hosts must pass," Hughes said.

The fungus is described in more detail in the journal PLOS ONE.