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If you see a dead fly on the windowsill, surrounded by tiny white spores, it’s a death trap. The insect has been invaded by a fungus that takes over its brain, directs the fly to find the highest place it can go, and causes the fly to die.
From there, the fungus releases its spores into the air to infect as many healthy flies as possible. Even weirder: The male tries to mate with the dead female, which is swollen by the fungus.
Now, a study has revealed that the fungus creates a “love potion” by releasing chemicals that lure flies to increase their chances of infecting them.
Carolyn Elya, a molecular biologist and postdoctoral researcher at Harvard University who studies fungi (but was not involved in the study), said: “The new study shows another way that fungi migrate to new hosts. They have taken a big step forward.”
Before the new study, some scientists had observed male flies trying to mate with the carcasses of females that had died from the fungus. Entomophthora muscae. It appeared that this contact could help the fungus spread, but it was not clear at the time whether the fungus would attract males.
Henrik de Fine Licht, an ecologist at the University of Copenhagen, and Andreas Naundrup Hansen, a graduate student, tested whether the fungus was luring healthy males to dead females by making them feel like mate or not.
First, Hansen infects female flies with the fungus, and as soon as they die, he places them one by one in a petri dish. Each time, he added a healthy male to the plate and watched to see if it approached the dead female, how long she stayed nearby, and whether she tried to mate. He also performed control experiments that included uninfected females that he killed by freezing.
As a result, the males were five times more likely to have access to mating when the female died of the fungus. Sometimes the mating is so intense that a cloud of spores is produced, but even simple contacts are enough to infect a healthy male.
In another experiment, healthy males could choose between two dead females in the same petri dish, one infected and one uninfected. The males will mate more often (than if no female is infected), but they cannot distinguish between the two females. Hansen suspects the fungus emits some kind of mating signal. I said: “It’s almost like an aphrodisiac that can take a male’s sexual behavior to a supernormal level.”
Hansen then tested whether the male was indeed attracted to the fungal spores. He placed four male flies in a small chamber containing two opaque petri dishes. Inside each petri dish were two pieces of fly paper, one covered with fungal spores and the other without. In 43 trials, all four flies landed on the paper with fungal spores. The other fly-catcher only caught all four flies in 17 trials.
Matthew Kasson of West Virginia University, who specializes in insecticidal fungi, says: “It’s really a great study.”
The team suspects the strong smell of the fungus – a grassy, slightly sweet smell – is part of the attraction. By placing an electrode on the antennae of a fly, Hansen showed that the fungus stimulated an electrical current in the brain. To find out what chemicals the fungus secretes, he extracted compounds from dead flies with a solvent. Working with chemical ecologists at the Swedish University of Agricultural Sciences, the team found that flies infected with the fungus contained more chemicals than healthy flies, the presence and abundance of several species. This changes according to the time the fly is infected.
Certain chemicals, called methyl-branched alkanes, have previously been known to induce male house flies to mate.
The researchers were unable to identify the specific chemical attractant of the fungus, but they say if it can be isolated and produced, it could be useful as a bait for trapping house flies. But in the meantime, the researchers say they’re astounded by the fungus’ ability to manipulate its host. “I was really impressed and amazed at the level of adaptation it presented,” de Fine Licht said.
Fungal attraction can be detected indoors or outdoors. “If people are interested in this, my advice is to stop and — I wouldn’t say smell — but stop and watch the flies.”
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