The friendly fly

Photo of a man's hand with 3 flies on it.

The friendly fly: A native insect that signals the end of an FTC outbreak.

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.

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.

The flies are native to Minnesota and have never been imported. With each FTC outbreak the rumor always starts that the DNR or someone imported the flies. Past years rumors have said that we imported them from Korea. This year's rumor is that we imported them from Australia. A rumor heard yesterday was that the DNR imported the flies from Australia and so many flies developed that we imported the Asian ladybugs to eat the flies. Reminds me of the kids song "There was an old lady who swallowed a spider to eat the fly. But I don't know why she swallowed the fly. I guess she'll die" It keeps getting worse until she finally swallows a horse and does die. So anyway before this rumor gets any worse, the fly was not imported and we didn't import the ladybug to eat the fly either.

To some people, the plague of ?friendly flies? at the end of a FTC outbreak is worse than the 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, unlike other flies which can be shaken off. Fortunately, they don?t bite.

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.

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.