Common Name: Robber Fly
Scientific Name: Order Diptera, family Asilidae, many species
Size: Adult—1/2" to 3/4"
Identification: Adults have a large head, prominent eyes and proboscis, a bristly humped thorax, long legs, and a thin tapering abdomen. Slender bodies, but some resemble bumblebees. Most are gray, brown, or black. Some mimic bees and wasps.
Biology and life cycle: Adult females lay small cream-colored eggs on plants, on rotten wood, or in the soil. Small, clean, segmented, cylindrical larvae have a distinctive head. Pupae are spiny and not enclosed in a puparium. They pupate in the soil. Larvae overwinter in the soil. One generation a year. They are encouraged to be present by flowering plants. Complete metamorphosis.
Habitat: Many vegetable and ornamental crops. Biodiverse gardens and wooded areas. Larvae are found mostly in decaying organic matter under litter.
Feeding habits: Adults prey on small to medium insects. Larvae live in the soil and feed on small soil-borne insects, including grubs, root maggots, beetle pupae, and grasshopper eggs—basically anything they can catch. They attack and eat butterflies, wasps, bees, horseflies, winged ants, grasshoppers, leafhoppers, beetles, and other flies. They will eat beneficials, although rarely. The troublesome plant feeders are slower and easier to catch. They suck their prey dry with hypodermic-like mouthparts.
Economic Importance: Help to control troublesome insects, especially flies, beetle grubs, and mosquitoes.
Natural Control: Birds.
Organic Control: None needed.
Insight: The common name of this ferocious insect comes from its habit of pouncing on prey. The larvae of robber flies are also predaceous. They live in decaying organic matter and attack other insect larvae, especially beetle grubs. Adults can give a painful bite.