The friendly fly (Sarcophaga aldrichi), also know as the flesh fly or government fly, is a native New York species that is an important natural enemy of forest tent caterpillars. These flies resemble house flies but are slightly larger. They belong to the family of flies known as flesh flies because they feed on the "flesh" of other insects and animals, but do not have biting mouth parts or pose any threat to humans. They like to land on people, animals and food.
A Natural Predator
These flies are most commonly found where heavy infestations of forest caterpillars have been observed in previous years. Huge numbers of these flies in outbreak areas can seem sudden, and typically last only a few weeks. Outbreaks of these flies are a natural response to the large populations of caterpillars on which they feed.
Friendly flies attack and kill cocoons of forest tent caterpillars. 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 will probably have few or fewer caterpillars this year.
Harmless to Humans
The DEC does 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. Their populations are not usually noticed 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. This is a natural 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.
