“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.
Aggressive behavior
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.
Rusty
Honey Bee Suite
Another great update on Yet Another Bug.
I noticed there’s a time stamp among the hot links after the “related” buttons. Is that new? Or have I been oblivious? Either way, I’m glad to see it here, and on other older stories that I checked. Much easier for new-ish readers to figure out whether your advice/commentary is from 2011 or just last month!
gap,
The date has always been there, from day one, nearly ten years.
Interesting as always. But even though I am convinced that all creatures have a purpose, this fly included, I cannot get myself to like it. Poor bee 🙁
About two weeks ago, I came into contact with one of these creatures and did not have a clue as to what it was. I was working one of my outyards and one of these flew upon the dash of my truck with a captured bee and commenced to relieve her of her body fluids. I took a picture with my phone and displayed it to all my neighbor friends trying to identify just what this creature was. Everyone I asked said they had never seen anything like it before and to my surprise there it was in your article.
Thank You,
John Hammond
John,
Excellent. Happy to solve a mystery.
After reading this article, I think I saw another species of a Robber Fly. We surprised this fly and he dropped the caterpillar on the trail, went in the brush and actually flattened itself to the ground. Then when it thought the coast was clear, it came back out, grabbed the caterpillar and took off across the trail. Please see the attached picture with the little caterpillar victim. This was taken in Mendocino CA.
I’m new at all this ….two months in……dealing with robber bees currently….how I ended up here. It seems like everything wants to kill my hive. I’m trying……I’m really trying.
I haven’t seen any bees at my house this summer. I have, however, noticed an unusual amount of robber flies. I walked outside earlier to check my lantana for bees and didn’t see any, but I saw more than one robber fly sitting on blooms. I have plenty of plants with blooms that should be attracting the bees by now.
I am concerned that the robber flies are part of my problem. I do not use insecticides. My mosquito population is thriving.
Saw a bee in my pool and scooped it out. It staggered for a while and than out of the corner of my eye I saw it flip and it was cut in half and a small black and yellow striped thin winged bug was eating it and had it gone in less than 10 minutes. Smaller than a paper wasp but bigger than a sweat bee. What could it have been? It flew away with the final mouth full.
Chris,
I have no idea because so many insects eat insects. Bees are enjoyed by many, including wasps and robber flies and others as well. It’s always interesting to see it, though, like you just did.