If you had forest tent caterpillars in your area during the last two summers, then it's likely that you're experiencing a 'friendly fly' outbreak this year. Friendly flies are the most important parasite of forest tent caterpillar (FTC) and when they become abundant, they signal the end of the FTC outbreak. They'll be here from early June to late-July.
To some people, the plague of 'friendly flies' at the end of a FTC outbreak is worse than the FTC outbreak itself. When friendly flies occur in large numbers, they can be a nuisance because they drone persistently and often land on people. They're probably called 'friendly' because you need to brush them off you, unlike other flies which can be shoo'd away. Fortunately, they don't bite.
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. They are native to the Lake States and, contrary to popular rumors, were never imported to our area.
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 moth. 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.
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, bears and toads also feed on forest tent caterpillars.