The friendly fly
A major parasite of the forest tent caterpillar is the friendly fly, Sarcophaga aldrichi. It is gray to black in color and is nearly 1/2 inch long. Its face has hairy sides, its thorax has three black stripes and its abdomen is checkered. As its name implies, the friendly fly is does not bite. Unfortunately, it tends to cling to people's skin and clothing and land on picnic tables and foods.
Adults are not the parasites, as they only feed on various sugar-containing materials such as nectar, sap, fruit juices, and aphid honeydew. Maggots (larvae) are the parasites of forest tent caterpillars. Maggots are deposited on forest tent caterpillar cocoons where they penetrate the silk, chew into the prepupae or pupae, feed on caterpillar tissues and eventually cause their death.
Several other species of smaller flies and certain wasps feed directly on or parasitize eggs, caterpillars, or pupae of the forest tent caterpillar. Predatory beetles, ants, true bugs, spiders, birds, and large and small animals, as well as viral and fungal disease organisms also serve as biocontrols.