Friendly Flies
These flies resemble but are slightly larger than house flies. They like to land on people, animals, food, laundry and so on. These flies are most commonly found where heavy infestation by forest tent caterpillars have been observed in previous years. Residents of these areas must be thinking, “First, we were invaded by caterpillars and now flies! What is going on?”
As a matter of fact, these flies are an important natural enemy of forest tent caterpillars.
What kind of flies are they?
They belong to the flesh fly family. They are often called friendly flies (Scientific name: Sarcophaga aldrichi) because they tend to land on people without biting. Although they don’t bite, they have a habit of regurgitating where they land. Adult flies are about 6 to 12 mm long, slightly larger than house flies. Their bodies are gray with reddish eyes. The sides of their heads are hairy. There are three black stripes on their back and their abdomens are checkered. They are usually active in June and July.
What do friendly flies do to the forest tent caterpillars?
Friendly flies attack cocoons of forest tent caterpillars, and kill the cocoons. Adult flies start to emerge a few weeks before the forest tent caterpillars start to spin a cocoon. After flies mate, eggs develop into maggots inside female flies. Females lay maggots in the cocoons of the forest tent caterpillar. The maggot burrows into the pupa inside the cocoon, and kills the pupa by feeding on it. The maggot stays inside the pupa for 10 days up to more than a month, then drops to the ground to overwinter. Friendly flies that you see now will affect the caterpillar populations for the next year, not this year.
Did the DNR release the friendly flies?
No. Friendly flies are also called government flies, because some people believe that the government released the flies to control forest tent caterpillars. Despite this myth, friendly flies are native to Wisconsin. Once the population of the forest tent caterpillars increases, so does the population of friendly flies. After an outbreak of the forest tent caterpillars ends, the friendly fly population also collapses. It is all programmed by nature.
Do they transmit human diseases?
No. These flies are not believed to transmit human diseases.
What can I do to control friendly flies?
It is not easy to control friendly flies. Because they are important natural enemies of forest tent caterpillars, they don’t bite humans and they don't transmit human diseases. It is best to leave them alone. You can swat friendly flies easily, but there might be too many to deal with. Spraying the sides of buildings with synthetic pyrethroids, such as permethrin or cyfluthrin, may be effective to kill the flies and temporarily reduce fly populations.