A parasitic fungus kills ants "on their doorstep."
The fungus must be outdoors in order to reproduce and spread to other hosts, so it kills the ant it manipulates the ant it infected to die right outside of the colony, Penn State reported.
Ophiocordyceps camponoti-rufipedis, or the "zombie ant fungus," controls the behavior of carpenter ants to die 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," said study lead author Raquel Loreto, doctoral candidate in entomology, Penn State's College of Agricultural Sciences.
The fungus then grows a stalk, called a "stroma," which protrudes from the ant's dead body. It develops a large round structure called an ascoma, which releases infectious spores. The stalk must grow outside of the any community, or else social immunity will occur and the hosts will work to limit the infection.
"Previous laboratory studies have shown that social immunity is an important feature of insect societies, especially for ants," said study co-author David Hughes, assistant professor of entomology and biology, Penn State.
"For the first time, we found evidence of social immunity in ant societies under field conditions."
The team tested social immunity by placing 28 ants that had recently succumbed to the fungus in two live nests, one with the live ants and the other without. They found the fungus was not able to develop properly in the nest with live ants.
"Ants are remarkably adept at cleaning the interior of the nest to prevent diseases," Hughes said. "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."
The team also recorded the prevalence of the fungus of ant colonies in Mata do Paraíso, Brazil. They discovered 17 nests within an area of 52,623 square yards that harbored the infection. By measuring the position of the manipulated ants at these locations they found the ants "die on their doorstep."
"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."
The findings were published Aug. 18 in PLOS ONE.
