This is it, people. The once-in-a-200-year event where two broods of periodical cicadas rise up from the ground is upon us.
Across swathes of the US, billions of the winged insects are emerging from the ground for a mating bonanza, but for an unlucky few a mysterious pathogen is taking over, turning them into flying “saltshakers of death”.
Locking your doors and hiding under the covers yet? Well before you do, hear this: while zombie insects sound like the plot of a low-budget 70s horror movie, they’re totally harmless to people, plants and pets. So feel free to marvel – maybe from a distance – as one of the most powerful forces of nature takes over.
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Why are cicadas turning into zombies?
Cicada species are ubiquitous all over the planet, but the ones emerging in droves across the US at the moment are what’s known as ‘periodical cicadas’, meaning they spend years underground as nymphs before emerging to mate as adults.
Right now, two so-called ‘broods’ – one of which emerges every 13 years and another every 17 – are syncing up in some eastern areas of the US. Those living in areas where this is occurring will notice they’re accompanied by a cacophony of noise, often louder than traffic.
The current takeover is likely to last until the end of June, when most cicadas will die, leaving their eggs behind to hatch and head underground for another 13 or 17 years.
“One of the big reasons for this periodical emergence is that it prevents predators from developing specialised ways of preying on them,” Dr Jonathan Larson, an entomologist at the University of Kentucky, tells BBC Science Focus.
The only exception, Larson says, is a fungal pathogen known as Massospora cicadina. “It plays the long game, likely lying dormant and not causing any symptoms until the cicadas come above ground as an adult,” he says.
Once in the surface world, infected cicadas’ usually hollow bodies are filled with white fungal spores. The abdomen breaks and peels apart, revealing what Larson describes as a “chalky white plug”.
Once exposed, the spores are free to infect more cicadas…
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How the fungus takes over
Imagine if a third of your body was filled with a hostile pathogenic fungus. You might not feel up to very much, and for good reason. Infected cicadas, however, are able to go about their usual business. In fact, the fungus even changes their behaviour in some pretty pivotal ways.
“They are able to carry out normal behaviours as if they were uninfected, and this is what allows them to keep on passing it around,” Larson explains.
What is normal behaviour for a cicada? After 17 (or 13) years underground, cicadas only have one thing on their mind – finding a mate. Male cicadas attract females by shaking their abdomen, which is what makes these eruptions so noisy. Infected males do this too, bringing in unsuspecting females to meet their doom.
But the fungus has another trick up its sleeve.
“Female cicadas flick their wings in response to the loud, boisterous singing from the males,” Larson explains. “But in the case where males have been infected with the fungus, they start flicking their wings too, duping other males into thinking they’re mating with a female.”
Just how the cicadas can keep going with so much of their bodies chock full of spores remains a mystery, but scientists have found a few clues in recent years.
For instance, a 2019 study published in the journal Fungal Ecology found the fungus was producing a type of amphetamine, like those used as powerful human stimulants, in the fungal plugs. Could this be what’s keeping the bugs going as the Massospora cicadina takes hold?
Larson explains that we don’t know exactly what the psychoactive substances are doing to the cicada, but that the insects must be being spurred on somehow. “This is the real zombification part; they’re performing tasks that they’re supposed to be doing and are alive, but they’re just sort of going through the motions. The lights are on but no one's home,” he says.
Should we be worried about the zombies?
Firstly, the proportion of insects infected is pretty small – less than five per cent. Still, five per cent of billions, possibly trillions, of insects is a sizable sum of zombies to have buzzing around.
Thankfully, this fungus is pretty specialised to cicadas – the ‘cicadina’ in ‘Massospora cicadina’ is a bit of a giveaway – so you needn’t worry about getting infected yourself.
Larson also points out that “infections tend to be very localised, so you have hot spots but it’s not common everywhere”.
It’s definitely something to be aware of if you’re planning on eating any, though. Yes, a lot of people out there eat cicadas – here’s a recipe for spicy popcorn cicadas, in case you’re interested – but you really don’t want to be eating the infected kind.
“If you are catching the cicadas, the infected ones would be demonstrably and noticeably different,” Larson says. “If parts of it are falling off and you see this chalky mass, toss that one and don’t eat it.” Who’d have guessed that eating insects infected with a pathogenic zombie fungus is a bad idea? Other than everyone, that is.
About our expert
Jonathan Larson is an extension entomologist and assistant professor at the University of Kentucky, providing insect expertise for people dealing with issues in urban landscapes, turfgrasses, nursery crops, greenhouses, and households. He is also co-host of the informative insect podcast, Arthro-pod.
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