“Got one!” said the robber fly, carting off its prey. “Oh no!” cries the beekeeper as another worker bites the dust. As if honey bees and their keepers don’t have enough to worry about, we have scads of robber flies all over the continent from sea to sea.
Although you may not be familiar with robber flies, the worldwide species count now stands at about 7500, which is divided into 14 subfamilies. Roughly 850 of those species are found in North America alone, denizens of understory and gardens, fields, and roadsides.
Large and hairy with humped backs and aggressive behavior, they don’t seem very friendly. But in fact, they are important control agents that prey on grubs, leafhoppers, grasshoppers, wasps, and other flies. They are not picky eaters, however, so they also feast on the occasional bee, butterfly, or dragonfly. If they can catch it, they will eat it.
Also known as assassin flies, all robber flies are in the family Asilidae. They are true flies and come in a variety of formats, depending on where they live and the particular arthropods they prefer. Some mimic bees in appearance—a disguise that helps keep them safe from predators—and some are just plain creepy looking.
If you have bee hives, you will often see the flies lurking in the flowering bushes near the apiary, especially on a warm and sunny afternoon. Why go hunting when the prey will come to you?
All the necessary equipment
Robber flies are well equipped to catch flying insects and can actually snatch a snack right out of the air. Their spiny legs allow them to grasp tightly, and bristly faces protect them from prey that might try to bite back. Like most flies, they have short antennae and three small ocelli between their compound eyes.
Once a robber fly embraces its prey, it pierces the body with a long proboscis and injects paralyzing saliva. The saliva also contains enzymes that begin to digest the victim’s innards. Once the prey becomes kind of soupy on the inside, the robber fly slurps it up through the proboscis.
Robber flies are okay
Robber flies are a part of a healthy ecosystem and beekeepers shouldn’t fret needlessly. Although they take a bee now and then, the threat they pose is nothing compared to bee-specific pests like varroa mites or diseases like foulbrood.
The awesome photo below was sent to me by Ohio beekeeper Erik Luthy. I’ve seen many photos of robber flies with insects, but I’ve seldom seen one with a honey bee. Sorry about the bee, Erik, but thanks for sharing. It’s a great photo.
Honey Bee Suite