Fungal Fly Fatalities
On September 17, 2012, I spotted a couple of flies that had died horribly on our windows. They had been infected by a fungus that had bloated their abdomens, and then when they died the fungus sprayed spores in all directions (you can see the spores where they hit the window glass and stuck).
The flies were glued to the window by their tongues.
Some of them had more of the fungus than others. This one looked like its abdomen was about to explode.
The fungus that killed these flies is Entomophthora muscae, the “Fly Destroyer”. It apparently infects a wide variety of flies (particularly those that resemble houseflies), and is one of a diverse selection of pathogenic fungi that infect and kill many, many different kinds of insects.
The progression of the disease starts with a fungus spore landing on a fly, and finding a chink in its exoskeleton somewhere. (Or maybe getting ingested? This would depend on whether a fly’s digestive juices kill fungus spores or not). Once inside, it infects the fly’s brain and basically makes it go insane. The fly gets the urge to find a high, exposed place, stick out its tongue, and hold it pressed against a solid surface until drying saliva makes it stick.
Meanwhile, the fungus is growing vigorously in the fly’s abdomen, and after a few days the abdomen is nothing but a mass of fungus. At which point, the fly spreads its wings to fully expose its abdomen, and dies. The fungus then erupts through the fly’s abdomen, and sprays spores in all directions, apparently with some violence. And since the fly died high up and exposed, the spores are spread far and wide.
These flies have yellow hairs on their thoraxes, and I think they are cluster flies. Since these flies come together in big aggregations to hibernate through the winter, it looks like just one of them getting infected by this fungus could easily wipe out an entire mass of flies coming together to hibernate.
There is some interest in using this fungus as a fly-control agent, as it would be highly-specific to flies without affecting other insects or animals. There are some problems with the spores having a short lifespan, though. The fungus grows best in cooler conditions, and infected flies can sometimes cure themselves by moving into hot areas.
The thing I’m wondering about, and which I’m not finding an answer to, is how the fungus gets through the winter if the spores have such a short period of viability. The only thing I can figure is that a fly that is infected just before going into hibernation might cool off enough that the fungus goes dormant inside its body. Then in the spring, as soon as the fly warms enough to wake up, the fungus would start growing again and continue its life-cycle, ultimately killing its host and spraying spores to infect the freshly-emerging flies.
 One thing to keep in mind, though, is that it isn’t really in the interest of the fungus to be too effective. If we have some horrible uber-disease that is highly contagious, has long-lived spores, and kills all the hosts it infects, then as soon as the disease wipes out all of its hosts it has signed its own death warrant. Without hosts, the disease goes, too. The best situation for the disease in the long run is to only infect enough new hosts to keep it propagating throughout the population, without actually diminishing the overall population of hosts.