One hot and humid morning, a gardener works busily at weeding his azalea bed. He looks up to wipe the sweat from his face and sees a dark, skinny fly perched on an azalea stem. The gardener thinks about the lunch he plans to eat on his shaded patio in a few hours.
Just about the time he decides to swat the fly to keep it off his lunch, the fly swoops off its perch, grabs a grasshopper in midair and carries it back to his perch. The little hunter proceeds to eat a late breakfast and the gardener proceeds to reconsider his plan to kill the fly.
And well he should. This fly is a member of the family Asilidae, better known as robber flies. Robber flies and their larvae are voracious predators of a wide variety of pests: beetles, wasps, grasshoppers, leafhoppers, wasps and others. The family is an abundant one, with nearly 1,000 species in North America, 250 of which live in Texas.
Asilids are also a diverse family, which can make them hard to identify. The most commonly seen species have gray or black bodies. However, some are orange and some are black and yellow, mimicking their prey, the bumblebee. Worldwide, they range in size from 3 mm to 50 mm (about 2 inches)! They are true flies (Order: Diptera, having two wings). The characteristic that distinguishes them from other flies is a hollow space between their two large eyes.
Most family members have a long, narrow, tapering abdomen containing segments. They are “bearded,” having fine hair that covers their piercing mouthparts. The six legs are usually long, bristled, and strong for grabbing and holding prey. Adult are active in the warm summer months and they like sunny, arid hunting grounds the best.
The life cycle of the robber fly is a complete metamorphosis. An adult female lays whitish eggs in a mass that she then covers with a chalky protective covering. Eggs can be found on low plants, grasses or in crevices within soil, bark or wood.
Robber fly larvae are seldom seen, even though they can grow to 1¼ inches long. They resemble long, slender, light-colored worms with knobby projections on the six center segments of their bodies. Living in the soil or decaying wood, this immature stage consumes organic matter, white grubs, beetle pupae, grasshopper egg masses and other soft-bodied organisms.
Robber flies overwinter as larvae and pupate in the soil. As the weather warms up, the puparia migrate to the surface and emerge as adults. The entire life cycle takes at least a year.
Robber flies are among the few insects that catch their prey in mid-flight. An individual establishes a perch zone. From there, it swoops out to snatch the unsuspecting victim that is often larger than its aggressor and may even include spiders, large predatory insects, and, sometimes, other robber flies.
After injecting the hapless meal with saliva that paralyzes and digests the prey’s bodily contents, the robber fly retires back to its perch and slurps up its insect smoothie. Although humans are never on its menu, a robber fly can inflict a painful bite if mishandled.
Robber flies are not picky. The down side to their eclectic appetites is that they will dine indiscriminately on those insects we gardeners consider beneficial. The upside is that they eat harmful pests just as enthusiastically. Overall, robber flies are considered to play an important role in maintaining a healthy balance in our gardens.
Beneficials in the Garden & Landscape is an Earth-KindTM program coordinated through Extension Horticulture at Texas A&M University. Earth-Kind uses research-proven techniques to provide maximum gardening and landscape enjoyment while preserving and protecting our environment.
This web site is maintained by Master Gardener Laura Bellmore, under the direction of William M. Johnson, Ph.D., County Extension Agent-Horticulture & Master Gardener Program Coordinator.
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