Not that they need any help, but HBO's "The Last of Us" has made mushrooms cooler than ever, thanks to the prominent role a parasitic fungus plays in the collapse of society. The creators of the TV show and the video game series it draws on took inspiration from a real-life fungus that turns insects into zombies, which in many ways is more fascinating than fiction.
The fungus, called cordyceps, doesn't make undead zombies, but it does hijack the biology of its bug hosts, forcing the victim to do its bidding, then ultimately killing it. The fungus grows inside the expired insect corpse, consuming its tissues until bursting out and releasing more spores to start the cycle anew.
Clearly, this is a horrific way to die, but it's a fairly common tactic in the mushroom kingdom. Scattered across the globe are around 700 different species in the genus cordyceps, many of which have evolved specialized relationships with their hosts.
For years, scientists have been exploring the use of this fungi as an all-natural pesticide, providing a potent alternative to some of the more toxic chemicals typically sprayed on crops. Not only could cordyceps be a fantastic bug killer, it could help protect many agricultural industries that are currently threatened by major invasive species.
Take the cotton mealybug (Phenacoccus solenopsis) for example. It resembles more of an alien crustacean than an insect, its yellow-gray body obscured by the crusty white fuzz it envelopes over its body. Cotton mealybugs are scale insects that feed on fruits, vegetables and ornamental plants, but true to its name, it really likes to suck the sap of cotton plants. Unfortunately, it also injects toxic saliva into its meal, causing the leaves to wither away and eventually murdering the plant.
Cotton mealybugs have spread across the globe and in most places, the response from farmers is to drown their plants in chemical pesticides, which have a nasty habit of maiming or killing other non-target plants and animals. This can eventually backfire, as insects evolve resistance to common pesticides. What if there was something to kill the mealybugs that wasn't so noxious and which they couldn't possibly fight against?
Enter Cordyceps fumosorosea, a species of fungus that produces many different toxic chemicals as a way to prey on insects and arthropods. When a C. fumosorosea spore lands on an insect, it begins to produce an enzyme that dissolves the hard outer shell of the bug's body, slipping inside. Once it enters the victim, it begins slurping up nutrients until it grows tendrils throughout the insect's body, rupturing it and spewing out more spores for the next poor bug.
In a study published in January in the journal Biocontrol Science and Technology, researchers from Bahauddin Zakariya University in Multan, Pakistan found that C. fumosorosea is a highly effective pesticide against cotton mealybugs, with an 87.5% mortality rate. Other studies with different insects have reported a 100% mortality rate. But even when it didn't fully kill the hosts, it still stunted their growth and inhibited their ability to breed. The fungi seems to jack up the body temperature of its quarry, resulting in a loss of appetite and may even disrupt their ability to mount an immune defense.
Previous research has demonstrated that other insects are just as susceptible to Cordyceps fumosorosea, including diamondback moths (Plutella xylostella), a rice-shaped bug with three cream-colored diamond shapes printed on its back. Unfortunately, this moth is a big fan of the "cole crops," which includes broccoli, cabbage, cauliflower, Brussels sprouts and kale.
But C. fumosorosea dispatches the moth just as effectively, sometimes in as little as 72 hours, according to a 2021 study in the journal Insects. And other insects need also beware, including termites, red palm weevils, whiteflies and other agricultural nuisances. It's unlikely any of these pests can evolve defenses against the fungi, because it's just so versatile at controlling its hosts.
Despite its widespread use against insect pests, cordyceps did not evolve to attack plants. That means farmers can potentially spray as much of this fungus as they want on their crops without having to worry about it killing their plants or damaging other vegetation in the environment. It could be an extremely effective alternative to pesticides, which are often toxic chemicals that don't discriminate when they damage living creatures.
Using nature like this is called integrated pest management, a more comprehensive approach to managing vermin that doesn't involve toxic chemicals or genetically modifying crops to resist pathogens. There are other examples besides weaponizing mushrooms, such as blaring disruptive noises to prevent some insects from communicating.
And again, Cordyceps fumosorosea is just one of hundreds of these types of fungi. We need much more research into how effective this tactic can be using different cordyceps strains, while also ruling out any potential off-target effects. We wouldn't want to accidentally spray a field with this stuff and wipe out a rare species of butterfly, for example.
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So when can we see this fungi hitting store shelves? Some fungal pesticide products do exist, including a sort of vaccine for elm trees called DutchTrig. Scientists are actively working on taking this research out of the lab, but there are a lot of important steps to take before releasing this mushroom WMD to the public. However, some entomologists such as those at the University of Florida are experimenting with spore dispensers that look like yellow sponges, which are hung in citrus trees to dispatch Asian citrus psyllids, a really nasty bug that likes to attack oranges. So products exist, but bringing them to the market takes additional steps.
While "The Last of Us" is a fantastic, groundbreaking franchise, the mushrooms in it are purely fiction. They don't share much in common with the real world cordyceps except the name and humans, who regularly eat this fungi with no problems, don't need to worry about it hurting us. It would take much, much more complex biological warfare for such a pathogen to mind control humans, but thankfully entertainment is driving more interest in these solutions, which could improve our relationship with nature and agriculture.
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