Robber Flies

Robber flies assassinate their prey in the air, injecting them with paralyzing neurotoxins and enzymes that liquify their preys’ insides. Also known as assassin flies, robber flies (Asilidae) aggressively hunt other insects. They often look more like bees or dragonflies to the casual observer. And they are in your garden, for better or worse.

'Choerades fimbriata' robber fly (Kurt Kulac) CC BY-SA 2.5

Robber fly description
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There are over 7,500 different species of robber fly, ranging in size from 0.2 inches up to 2 inches long. All of them are sturdy, bristled flies with a distinct mustache, made of of bristles (setae), called a mystax. Robber flies, like many other insects, have three simple eyes (ocelli), in a depression on top of their head, between two very large compound eyes. [Simple eyes, like ours, have only one lens, while compound eyes have many lenses.] Robber flies generally have robust legs with spines, and short, segmented antennae. Most species of robber fly have long, skinny bodies with stinger-like ovipositors, while others look more like bumblebees. Robber flies will inflict a painful bite with their proboscis (tubular mouth part), if they feel threatened.

Leptogastrinae (Fritz Geller-Grimm) CC BY-SA 2.5

Robber fly life cycle

There is surprisingly little known about the private lives of robber flies. They seem to prefer dry, sunny, open environments. Each robber fly can live for up to 3 years. Eggs can be translucent (hyaline) or pigmented, spherical or oval, depending on the species. Yellowish or white larvae, with tapered bodies and a dark head, are believed to live in leaf mold, rotting wood, and in the soil. There are four larval stages, or instars. Scientists are only now learning about the feeding behavior of robber fly larvae. It’s pretty strange. For one thing, the first instar does not eat insects; it probably eats dead things. The second instar eats beetle larva secretions. [Don't ask.] Later instars are actual predators. Robber fly pupa are naked and have leg stubs, so they can move around, similar to hornworm larvae.

Robber fly prey

Robber flies are generalists. That means they will chase after and kill pretty much anything that flies by, beneficial or not. That is why I said, for better or worse.” Robber flies will kill many garden pests, but they will also kill beneficial insects. Some of these prey insects are pretty substantial in size, compared to a robber fly. But, adult robber flies are excellent fliers. You can usually hear them coming, but for their prey, it’s already too late. Hiding in ambush, a robber fly spots its target, gives chase, and grabs ahold with its tarsi (front legs) before injecting a chemical cocktail that paralyzes the prey and liquifies its insides. The most common victims of robber fly attack include:

• ants, bees, wasps, sawflies (Hymenoptera)
• butterflies, moths (Lepidoptera)
• beetles (Coleoptera)
• flies, crane flies, horseflies (Diptera)
• grasshoppers, crickets, katydids (Orthoptera)
• leafhoppers, stinkbugs, aphids (Hemiptera)

They have also been recorded feeding on:

• dragonflies, damselflies (Odonata)
• lacewings, antlions (Neuroptera)
• spiders (Arachnida)
• termites, cockroaches (Blattodea)
• thrips (Thysanoptera)

And, hummingbirds. It’s rare, but it has happened. You can see some excellent video of robber fly behavior at Mike Blair Outdoors.

Have you seen any robber flies in your garden? Do you consider them to be Good Guys or Bad Guys?