Dorothy is a Master Gardener, former newspaper reporter, and the author of several books. Michael is a landscape/nature photographer in NM.
Ant Infected With "Zombie" Fungus
I can't for the life of me recall a time that I ever felt sorry for an ant. I just want them to get away from me or die, with either scenario being acceptable. But I have just recently been introduced to a type of "zombie" fungus called Ophiocordyceps unilateralis sensu lato that only affects ants in the Camponotini tribe. I still don't want them anywhere near me or mine but the way this fungus has to complete its life cycle is pretty creepy indeed.
The fungus was discovered by a British naturalist Alfred Russel Wallace in 1859 and considered to be an entomopathogen or insect-pathogenesis method of infection that is found primarily in tropical forest ecosystems, although some infected ants have been seen in the United States as well.
Only Select Ants Are Infected
But this parasitic fungus that can manipulate the ants doesn't affect just any old ant. Instead, the microorganism is apparently able to recognize the brains of different ant species and releases its body-controlling chemical only when inside its preferred hosts, which includes only one certain tribe of ants, although there are over a thousand species of ants included within the Camponotini tribe. Our own research has shown that most, if not all, infected ants have been carpenter ants.
Once an ant has been affected, the fungus penetrates the cuticle and begins to enslave the insect and cause it to begin some strange behavior, including climbing upwards to a very high point where the deadly spores, when released, will have the most impact below. Once high enough, the infected ant bites down and grips the stem with its mandible, anchoring it in place. Death comes to the insect as the fungus begins to infect its entire body, and eventually, the parasite’s large stalk bursts through the back of the ant’s head. Once the parasite is done growing, the spores burst from the tip, scattering along the floor of the jungle or forest as the cycle begins again with ants that are nearby.
In the End, the Fungus Wins
The fungus secretes tissue-specific metabolites and causes changes in the gene expression of the host insect as well as atrophy in mandible muscles. That altered behavior, although obvious, leaves researchers scratching their heads wondering how the fungus is able to coordinate the effects in order to manipulate the behavior of the infected ant.
David P. Hughes, Ph.D., Associate Professor of Entomology and Biology at Penn State University, stated in an interview with nationalgeographic.com that he considers the infected insects to be chimeras: part ant and part fungus. It's apparent that as the parasite's life cycle continues there is more fungus and less ant and in the end, there is only the fungus.
Host Ant Directed to Bite Down, Then Die
The Cycle Leading to Death
Certain death comes to any ant that is "selected" for infection. The fungi need ants to complete their life cycle so when an ant comes across fungal spores while foraging, the fungus begins to infect the insect spreading quickly throughout its entire body. It takes about three to nine days for the infected ants to become completely zombified.
The fungus slowly fills the ant's body and head, causing muscles to wither away and muscle fibers to spread apart. The infected ant's central nervous system is hijacked by the chemicals that are released and the ant is manipulated by this mind-controlling virus to climb up vegetation to higher ground. It becomes a hapless drone of sorts and is then directed to clamp down on a twig or possibly a leaf before it dies. After the fungus has killed the ant, a spore-releasing stalk emerges out of the back of the victim's head infecting more ants on the ground.
Researchers at Penn State have discovered that the fungus kills at solar noon when the sun is at its strongest, speculating that sunlight might be needed for the synchronization of the final stage of the infection. They have also reported that the fungus manages to complete its entire life cycle without infecting the host ant's brain, discussed more at length below.
Convulsions Cause Ants to Fall From the Trees
All infected ants don't die up in trees. Where normal ants would rarely deviate from a trail along a tree, ants infected with this fungus wander aimlessly, often suffering convulsions that cause them to fall from the tree. On the ground, the ants remain a cool, moist area of vegetation above the soil and under the main forest canopy, an area that provides optimum conditions under which the fungus can reproduce.
In a few days, the fungus begins to direct the ant to clamp down on a leaf and cause fibers to detach within the muscles responsible for opening the ant's mandibles resulting in a type of lockjaw effect. The ant is then unable to let go of the leaf and a stable place for the fungus to grow is created. A deadly poison is released and the host dies.
The fungus begins to grow a stroma through the top of the ant's head and the stroma releases its spores to infect another unsuspecting insect.
New Study Sheds New Light
According to a study done by Penn State University, an ant's brain is not infected by the Ophiocordyceps unilateralis sensu lato parasite that takes over its body. Instead, it surrounds and invades muscle fibers throughout the insect's body, and fungal cells form a 3-D network that can allow them to collectively control the behavior of its victim. It appears, according to researchers, as if the parasite controls the behavior of the infected host peripherally.
The fungus somehow manipulates an ant, tweaking its muscles while leaving the brain intact, leading researchers to believe that the brain is preserved because the parasite needs it in order to direct the host to an area where other ants can be infected. The parasite itself is unable to get inside an ant colony because the microclimate there does not foster its growth.
The Emergence of a Fungus-Killing Fungus
This is where the story of the zombie ants takes an interesting turn. It seems there is actually another fungus out there that effectively chemically castrates the Ophiocordyceps unilateralis sensu lato fungus. Hughes stated in an interview with nationalgeographic.com that some forests are virtual graveyards filled with the cadavers of infected ants. He began to wonder how the lucky ants escaped the parasite and began to explore the cause of their good luck.
He found out that the majority of spores were taken "out of the game" by another fungus and that only 6.5 percent of zombie-ant fungus specimens could produce spores, which limits the spread of the parasite covering the original fungus. The second fungus referred to as a hyperparasite effectively prevents the original fungus from ejecting its spores by growing over the ant corpse and the emerging fungus stalk.
Scientists have also observed small bugs laying their eggs in the infected ant corpse, allowing their larvae to eat the fungus.
- https://www.wired.com/2014/08/zombie-ant-fungus-in-the-us/ (Retrieved from website 7/8/2018)
- https://news.psu.edu/story/492948/2017/11/07/research/zombie-ant-brains-left-intact-fungal-parasite (Retrieved from website 7/10/2018)
- https://news.nationalgeographic.com/news/2011/05/110511-zombies-ants-fungus-infection-spores-bite-noon-animals-science/ (Retrieved from website 7/11/2018)
- https://www.tes.com/lessons/aBRr4byypj8ngg/zombie-ants (Retrieved from website 7/6/2018)
- https://www.nature.com/news/fungus-that-controls-zombie-ants-has-own-fungal-stalker-1.11787 (Retrieved from website 7/12/2018)
© 2018 Mike and Dorothy McKenney
Mike and Dorothy McKenney (author) from United States on October 29, 2018:
I know what you mean. Kinda creepy, eh? Happy Halloween to you and yours!
Lorelei Cohen from Canada on October 29, 2018:
OMG even the zombie ant image is enough to make me want to disinfect my back yard. Wow this was information on the insect world that certainly will leave me looking much differently at these small creatures. Seems funny to discover this article two days before Halloween. Have a happy Halloween.