Spawn of the Dead

After killing its host, the so-called zombie ant fungus grows from the cadaver and produces spores, which rain down on the forest floor to infect new hosts. Photo by David Hughes.

They aren’t quite walking dead, but the carpenter ant workers that become infected with the “zombie ant fungus” Ophiocordyceps camponoti-rufipedis are living puppets of the parasite, and are eventually forced to die within the vicinity of their colony, thus ensuring a constant supply of potential new hosts for the fungus.

In a paper published in the August issue of PLOS ONE, researchers in the college reported their findings that the parasitic fungus must kill its ant hosts outside their nest to reproduce and transmit its infection.

This fungal reproductive activity must take place outside the ant colony, in part because of the ants’ social immunity, which is collective action taken to limit disease spread, explained David Hughes, assistant professor of entomology and biology.

“Previous laboratory studies have shown that social immunity is an important feature of insect societies, especially for ants,” Hughes said. “For the first time, we found evidence of social immunity in ant societies under field conditions.”

According to Raquel Loreto, graduate student in entomology, 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.

Specifically, the fungus grows a stalk, called a stroma, which protrudes from the ant cadaver. A large round structure, known as an ascoma, forms on the stroma. Infectious spores then develop in the ascoma and are discharged onto the forest floor below, where they can infect foraging ants from the colony.

By measuring the position of manipulated ants and plotting these locations with respect to the nest, the researchers established that infected ants die on the “doorstep” of the colony.

“What the zombie fungi essentially do is create a sniper’s alley through which their future hosts must pass,” Hughes said. “The parasite doesn’t need to evolve mechanisms to overcome the effective social immunity that occurs inside the nest. At the same time, it ensures a constant supply of susceptible hosts.”

—Chuck Gill