It’s hard to feel much compassion for a common housefly. But even the most insect-phobic person might muster up a bit of empathy for any fly falling victim to a fungus capable of rendering it a necrophiliac and then killing it.
Entomophthora muscae, also called the “zombie fly fungus,” is a parasitic fungus that survives by infecting houseflies, and it’s right here in Maine attacking flies in houses and gardens around the state.
As parasites go, the zombie fungus is particularly horrifying, with a life cycle that seems more at home in a Stephen King novella than a scientific research article. A recent study out of Denmark shows the fungus actually takes control of a female fly’s behavior while consuming it from the inside out. It then uses the corpse to attract and infect male houseflies.
According to the study, once the fungus infects a female fly with its spores, it spreads and feeds on her body from the inside. After about six days of feasting on the living fly, the fungus takes over its behavior and forces it to climb to the highest available location where the female finally dies, leaving only a hollowed-out corpse.
Then things really get gruesome.
According to Dr. Seanna Annis, associate professor of mycology and plant pathology specialist with University of Maine Cooperative Extension, the spores position the fly before it’s even dead.
“They direct the fly to this high location and to put its head down, abdomen sticking up in the air and the wings spread out,” Annis said. “Then it dies in that position.”
After it dies, the fungus cracks the corpse open, breaking out and shooting off its spores.
“The idea is the spores will fly high and far enough that a new fly will come in contact with them,” Annis said.
But the fungus has another trick up its sleeve.
Once the host fly is dead, the fungus starts to release a special chemical signal aimed at male flies.
“The chemical signals act as pheromones that bewitch male flies and cause an incredible urge for them to mate with lifeless female carcasses,” explains Henrik H. De Fine Licht, an associate professor at the University of Copenhagen’s Department of Environment and Plant Sciences and one of the Denmark study’s authors. “Our observations suggest that this is a very deliberate strategy for the fungus [and] it is a true master of manipulation — and this is incredibly fascinating.”
In this chemical-induced zombie necrophilia, male flies copulate with dead females, and the fungal spores are showered onto the males, who will later suffer the same gruesome fate as the dead female. It also means the fungi’s spores are successfully spread after it flies away.
That, according to Debbie Eustis-Grandy, biology instructor at the Maine School of Science and Mathematics in Limestone, is the ultimate goal.
“Spore dispersal is one of the ways fungi reproduce,” Eustis-Grandy said. “For really all organisms [reproduction] is the key goal.”
Most fungi disperse their spores via air currents, but Eustis-Grandy said others have taken a different approach by co-evolving with a second, entirely different species to get the job done. Such is the case of Entomophthora muscae.
“They have evolved side by side,” she said. “Subtle changes in one result in subtle changes in the other.”
Another example of this are the parasites that can cause toxoplasmosis in pregnant human females. It’s found in cat feces and is the reason pregnant women are strongly discouraged from being around soiled cat litter.
“The parasite requires two hosts, and one is mice,” Eustis-Grandy said. “When it infects the mice it makes them stupid, so they are eaiser for cats to catch, and that’s important because cats are the final host for this parasite.”
In the case of the zombie files, Licht’s research also showed that the longer the female fly is dead, the more seductive it becomes because the number of fungal spores increases with time.
In the future, according to Licht, the chemical found in the fungus could be used to create a biological pesticide.
In the meantime, there is no indication that Entomophthora muscae is at all harmful to humans or capable of turning them into zombies. It’s just one of the many fungal spores humans breathe in and out every day.
“You often see ‘zombie flies’ high up on a windowsill or ceiling,” Annis said. “But you are never going to get a zombie person because of it.”