In order to persist in insect societies, a parasitic fungus that turns ants into zombies manipulates its host to die outside of the colony, within the vicinity of foraging trails, a new study has found. This ensures a constant stream of susceptible hosts while also avoiding defense strategies employed by the colonies. The study has been published in PLOS ONE.
Ant-infecting fungi of the genus Ophiocordyceps are renowned for their remarkable ability to manipulate host behavior in order to achieve reproductive success. After infecting a poor unsuspecting ant, these parasites manipulate the host into migrating to a spot optimal for fungal development. The zombie ant then performs a so-called “death grip” where it bites onto a specific part of vegetation, such as the vein of a leaf, before being killed by the fungus.
After the host dies, the fungus grows a stalk (stroma) that tears through the corpse. A round structure then forms on this stalk, from which pathogenic spores are released onto the forest floor, creating an infectious “killing field.”
Insect societies have evolved defense mechanisms to reduce disease pressure, which is called social immunity. This involves mounting collective actions to prevent and control the spread of diseases, for example cleaning the nest. Consequently, the parasite needs to evolve strategies to avoid this social immunity in order to ensure reproductive success. While previous studies had investigated this dynamic host-parasite relationship in the lab, until now no one had looked at social immunity in ant societies in the field.
For the present study, Penn State scientists chose to investigate Ophiocordyceps camponoti-rufipedis, which infects carpenter ant (Camponotus rufipes) workers. They placed 28 ants recently killed by the fungus inside two nests- half in a nest containing live ants and half in a nest with no ants. Interestingly, they found that the fungus couldn’t develop to transmission stage in any of the ant corpses, suggesting the microclimate within the nest somehow prevents parasite development. Furthermore, 9 of the corpses were removed from the nest containing live ants, which was likely a tactic to prevent disease spread.
Next, the researchers investigated parasite pressure in the study area, which was near the Mato do Paraíso research station in southeast Brazil. They made 3D maps of both the position of the dead ants and the foraging trails that worker ants were taking around the colonies, 100% of which were found to be infected in the study area. From this information, the researchers were able to determine that the infected ants die on the “doorstep” of the nest.
“What the zombie fungi essentially do is create a sniper’s alley through which their future hosts must pass,” study author David Hughes said in a news-release. “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.”
Despite the fact that the researchers recorded 100% parasite prevalence in the colonies studied, the infection rate was found to be low. Furthermore, none of the colonies collapsed due to infection, but none recovered either. According to Hughes, this suggests that the parasite represents a chronic disease in the colonies that, as in humans, can be controlled but not cured.