Friendly fly: Forest tent caterpillar parasite

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.