Friendly Flies: Sarcophaga aldrichi
Forest tent caterpillar parasite
The Friendly Fly or Government Fly or Flesh Fly, Sarcophaga aldrichi
These flies resemble, but are slightly larger than house flies but they belong to the family of flies known as flesh flies because they feed on the "flesh" of other insects and animals. They like to land on people, animals and food. These flies are most commonly found where heavy infestations of forest caterpillars have been observed in previous years. The public in these outbreak areas are often appalled and annoyed at the seemingly sudden appearance of huge numbers of flies. As a matter of fact, these flies are an important natural enemy of forest tent caterpillars. They are the natural response to the large populations of caterpillars they feed on.
They are not released by the DEC, nor any other government agency.
Friendly flies attack cocoons of forest tent caterpillars, and kill the cocoons. Adult flies start to show up a few weeks before the forest tent caterpillars start to spin a cocoon in June and early July. After feeding on the caterpillar in its "cocoon", the fly larva (maggot) then drops to the ground to spend the winter. Friendly flies that you see now will affect the caterpillar populations for the next year, not this year. If you had a lot of friendly flies last year, you probably have few or fewer caterpillars this year.
The DEC did NOT release these flies. Friendly flies are also called government flies, because some people believe that the government released the flies to control forest tent caterpillars. This is a persistent and inaccurate rumor as the flies are native to New York. The general public does not notice them until there is a tent caterpillar outbreak. 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. In fact, the flies are likely the main reason for the caterpillar outbreak collapse. It's all natural and part of the cycle of biological activities that regularly occur in New York's forests.
These flies do NOT have biting mouthparts and are not interested in human flesh or blood. They do however like to land on humans, probably seeking moisture and perhaps salt on the skin's surface. They are NOT believed to transmit ANY human diseases. These flies are good for us and good for the environment in that they control the caterpillars so please try to tolerate them for the two or three weeks they are abundant. They are just doing their job.
Friendly flies, Sarcophaga aldrichi, are a major factor in the collapse of forest tent caterpillar populations. During the last one to two years of an outbreak, these flies become very abundant because they use forest tent caterpillars as their food source.
Friendly flies are the most important insect parasites of forest tent caterpillars (FTC). In mid- to late-June, adult flies deposit live maggots on FTC cocoons. The maggots move into the cocoons, bore into the pupae and feed on them which kills the developing FTC. After completing their feeding, the maggots drop to the ground, form their own pupal stages and remain dormant until the next summer.
Friendly flies resemble house flies, but they are larger, slower and distinctly more bristly. Adult flies are gray in color and are 6 to l2 mm long, the sides of their faces are hairy, on each end of their two antennae is a single and branched bristle, their thoraxes have three black stripes, and their abdomens are checkered.
When friendly flies occur in large numbers, they too can be a nuisance because they drone persistently and often land on people and food. Adults are strong and active fliers and they swarm over everything (people, livestock, lights, light-colored garments and laundry hung out to dry). They don't bite but they can soil clothes, laundry and food with their regurgitations. Unlike other flies which can be shoo'd away, friendly flies must be brushed away.
Between forest tent caterpillar outbreaks, the friendly fly population collapses and they survive in low numbers by depositing their larvae on carrion, dung and various decaying materials.
Several species of other flies and wasps parasitize the eggs, larvae and pupae of the forest tent caterpillars. Predatory flies, beetles, ants, true bugs, spiders, birds, wood frogs, deer mice, skunks and toads also feed on forest tent caterpillars.