Flower Flies, Hover Flies or Syrphid Flies - Family Syrphidae
The flower flies are by far the most important family of predatory flies. The adults are not predaceous, but the larvae of many species are. Syrphid larvae tend to prey primarily on aphids, mealybugs, scale insects and related Homoptera. The adults, which have a yellow and black striped abdomen and look superficially like a small bee, feed on the nectar and pollen from flowers. The adults seek colonies of aphids and lay their eggs nearby. The headless, legless, maggot-like larvae tend to be pale yellow to light green in color. The large species get to be slightly bigger than 1/4 inch long when fully grown. They often pupate right where they finish feeding, forming a tan-colored, drop-shaped puparium. The life cycle, from egg to adult, usually takes two to four weeks. Several generations occur each year. Although normally thought of as aphid predators, many syrphids probably have a wider host range. In Europe, one species is known to commonly feed on small caterpillars. In Wisconsin, one very common species, generally considered an aphid predator, is also an important predator of cranberry tipworm, which is the larva of a small midge that attacks the cranberry plant.

Gall Midges - Family Cecidomyiidae
As the name implies, the larvae of most gall midges feed within plant tissue, creating abnormal plant growths called galls. However, the larvae of a large number of species are predaceous and some are even reported as parasitoids. These insects are very tiny, usually only two to three millimeters in length. Probably the most common prey are aphids and spider mites, followed by scale insects, then other small prey such as whiteflies and thrips. Many eat the eggs of insects or mites. The adults, which are very tiny, fragile midges, locate colonies of appropriate prey and lay their eggs in these locations. Because the small, maggot-like larvae are incapable of moving considerable distances, there usually has to be a fair population of prey present before the adults will lay eggs. These insects can frequently be seen where there are colonies of aphids or outbreaks of spider mites; they are usually considered important natural enemies of these pests. In a recent survey of the predators of aphids in an organic apple orchard in Wisconsin, gall midges were found to be the most numerous predators, especially during the summer months. At least one species, Aphidoletes aphidimyza, is commercially available, although rather expensive to use.

There are many additional species of Diptera that are predatory. In some cases, all members of a family may share this trait; in other cases, predation is considered somewhat out of the ordinary when looking at the majority of species in a family. In most cases, these flies have little impact on agricultural pest control. However, a few of the more common or interesting groups are mentioned here.

The larvae of some mosquitoes (family Culicidae) are highly predaceous and frequently feed on the larvae of other mosquito species that may be of more importance as health pests or nuisances. There have been some cases of successful introduction of predaceous, non-pest species into areas to reduce the populations of serious mosquito pests.

The longlegged flies (family Dolichopodidae) are small but beautiful flies, brightly colored with metallic green, blue or copper. They are particularly abundant near swamps, streams, woodlands and in meadows. The adults are predaceous on small insects, and some can be found feeding on aphids. The larvae of many are predaceous, living in moist habitats such as decaying logs or moist organic soil. The larvae of the genus Medetera live under bark and feed on bark beetles.

Robber flies (family Asilidae) are large insects and often quite numerous, although they tend to occur more commonly in the southern states. Many are stout bodied and very hairy; some resemble bumblebees. Adults are predaceous and attack a variety of insects including wasps, bees, dragonflies, grasshoppers and whatever else they can capture, usually in flight. Larvae live primarily in soil or decaying wood and prey on the larvae of other insects.

Marsh flies (family Sciomyzidae) are small to medium-sized insects, often yellowish or brownish; many have spotted or patterned wings. They are common along the banks of streams and ponds, in marshes, swamps and woods. Larvae feed on snails, snail eggs and slugs.

By: Dan Mahr University of Wisconsin Madison
From: Midwest Biological Control News, Vol. V. No.4.
Published in Hort Impact/September 1998 Vol. 98-9 p.7

Diptera images can be found at this USDA site or this NCSU site.

This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

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