Sex Crazed Zombie Cicadas
Periodical cicadas in the United States are being infected by a fungal parasite – turning them into hypersexual ‘zombie’ bugs. Massospora cicadina is a fungus that specifically infects cicadas, taking over their bodies, and puppeteering them to ensure the fungal spores are spread to other host bodies.
One of many parasitic fungi, m. cicadina belongs to the order entemophthorales, meaning ‘insect destroyers.’ Fungi in this order are specialised to infect insects, taking over the host body and subtly manipulating their behaviours through chemical signals. Dr Tommy Leung (they/them), a parasitologist at UNE, has written on their blog about this species before. They explain that there’s two different types of infections, depending on how and when the cicada comes into contact with m. cicadina spores.
“[There’s] two main ways. While they’re crawling on the soil, they [might] accidentally brush up against some spores that are on the soil. Another way that they could get infected is by mating with a cicada that’s already infected.”
M. Cicadina has evolved alongside it’s chosen host – the periodical cicada – which emerges from underground every 13 to 17 years. M. cicadina spores can remain dormant in the soil for a long time, waiting for cicadas to emerge. Because fungi are stationary, m. cicadina relies on host cicadas to spread their spores, and while the spores can be spread through the wind, there’s no guarantee for the fungus that it’s going to wind up attached to a host. Sexual transmission is one way that the fungus has evolved to ensure it’s spread.
When infected by m. cicadina spores from the ground, cicadas experience a Stage 1 infection. This alters their behaviour, increasing their sexual interest and prompting male cicadas to respond to the mating calls of other male cicadas as well as searching for females. Those infected through sexual contact experience a Stage 2 infection, where they lose interest in sex, and instead fly around spreading m. cicadina spores.
Dr Leung explains that this form of transmission is an adaptation that relies on behaviours that already exist in the host.
“By initiating this kind of sexual contact, you’re actually getting the host to do your work for you. The host would be bringing the spores to other potential hosts as well, and that’s why they produce the two kinds of spores. The spores that are immediately contagious that could be sexually transmitted, and the spores that lay in the soil – and they can lay there for a very, very long time, decades even, waiting for some new generation of cicadas to come up and bump into them.”
While this infection is terrifying if you’re a cicada, there’s no chance of it jumping to humans. Human physiology and insect physiology is so different that there would be no chance of m. cicadina affecting us at all. But m. cicadina has a good thing going – it has no need to switch it up. Dr Leung tells us that the fungus has evolved to be in-tune with the life-cycle of the cicada. Any evolutionary jump to another host would require a significant advantage in doing so.
“They’re already finely tuned to go after the cicada, and aside from the periodicalness[sic] of the cicada emergences and their spores, there’s also the aspect of the biochemistry and physiology. With any parasites, they’re usually finely tuned to a particular type of physiology. If you put them into something really different, they won’t be able to survive. So there’s not really much reason for them to jump onto something that is different from a cicada.”
Thankfully, this means that humans (and other mammals) and we won’t be seeing anything from the Last of Us any time soon.
