What Are Robber Flies: Information About Robber Fly Insects

What Are Robber Flies: Information About Robber Fly Insects

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By: Kristi Waterworth

The garden is full of insects, and it can be difficult to sort out friend from foe. One garden visitor who needs a better PR department is the robber fly. Robber flies in gardens should be a welcome sight, but their bee-like appearance and aggressive nature can leave gardeners wondering, “Are robber flies dangerous?”

What are Robber Flies?

Robber flies are members of the family Asilidae and distant relatives of the common housefly. Their appearance is somewhat frightening — after all, a big, hairy, humped flying insect isn’t usually a good thing. Robber fly insects are a mixed blessing to gardeners; if they’re seriously perturbed, they can inflict a painful bite, but they also help rid the garden of harmful pests like grasshoppers, other flies, wasps, leafhoppers, white grubs and pupating beetles.

There are many different types of robber flies, ranging in size from 3/8 to 1 1/8 inches (.9-2.8 cm.) long. They may be observed hanging around on the stems of plants looking for prey or flying just above the ground. All stages of robber flies aggressively attack and eat pretty much anything they can catch, including the occasional bee, butterfly or other beneficial insect.

Where are Robber Flies Found?

Robber fly information isn’t as abundant as info about the more popular beneficials like ladybugs and lacewings. This may be because they exist in a relatively narrow climate band. Although there are over 1,000 species in the United States alone, they prefer arid, sunny habitats like deserts. A few robber fly species are found in woodland settings, but they tend to congregate along the edge of the forest, or in grasslands.

Is Robber Fly Control Necessary?

Robber flies in gardens are not considered problematic enough to require pest control, but if you want to deter them from your garden, target the soil-dwelling larvae. They often hide under wood or other objects that keep the soil moist. Bacillus thuringiensis will quickly destroy the fly larvae, but keep in mind that their removal opens your lawn up to attack from grubs and other soil pests.

Adults should not be sprayed with broad-spectrum pesticides, since this will destroy the insects that you’re presumably hoping to protect in your garden. Most gardeners tolerate this visitor, even if they do munch a few butterflies or bees. The extensive pest control they will provide in your garden and landscape far outweighs the damage they do to a few other individual beneficial insects.

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Read more about Beneficial Garden Friends

What's that Bug? The Family Asilidae Robber Flies

Gardeners often encounter unique and colorful insects in their gardens. The trick is to know which ones are friends and which ones are foes. This series of articles will help identify some of the most unusual ones and give you a peek into their lives

Robber flies are members of the Asilidae family and are found throughout the world. There are over 7,000 species and while some of them resemble each other, there are a number of them that mimic flies, bees and other insects, to let them hide in plain sight while they stalk their victims. They're quick and powerful, making them a threat to insects quite a bit larger than themselves. And it isn't unusual to see one making a meal from a large grasshopper or beetle.

Robber flies have several common names. They are also known as assassin flies and my personal favorite, the Hanging Thief. They're capable of capturing their prey on the wing, or pouncing on unsuspecting victims like little dive bombers. They inject a neurotoxin with their proboscis that renders the quarry helpless, while its insides liquefy. The robber fly then sips the resulting goo while hanging by one leg under a twig. (That's why it is called the Hanging Thief) And while they are not a threat to humans, they can deliver a painful bite if mishandled, so let them go about their business.

These aggressive little insects are quite distinctive. They have two large compound eyes and can turn their heads like a dragonfly or praying mantis. It also has a growth of hairs that resemble a moustache on its face. In fact, the proper name for these hairs is ‘mystax' which is Greek for moustache. The hairs bristle up between the eyes and are probably designed to protect its face and eyes from struggling prey. Robber Flies prefer to hunt during the hottest part of a sunny day and in the evenings they tend to roost in dense vegetation. It is believed that they have excellent eyesight and are very raptor-like in their behavior of searching for prey and stalking it from above.

Robber flies are considered a beneficial insect, but they are not very choosy as to their meals. They have been known to capture honeybees, butterflies and ladybugs right along with the grasshoppers, gnats and mosquitoes. Fortunately, they are not present in large enough numbers to do lasting harm to the population of good guys. Even robber fly larvae are predatory and can be found in leaf litter and around fallen logs devouring grubs and similar creatures.

The most prevalent robber fly in my area is Diogmites missouriensis and those are the images in this article. They tend to hang out in my herb bed when my mint and oregano are blooming, because those blooms attract a huge number of flying insects. They are wary and difficult to photograph, but every now and then one will strike a pose while waiting on his next meal.

I allow the robber flies to go about their business on my property. A healthy population of predators is the sign of a healthy ecosystem and in turn a healthy garden.

Robber Flies

Asilidae (robber flies) in the order Diptera (flies)

Robber flies are a diverse group of predatory flies that typically perch in an open area, swiveling their heads to look around, then flitting out to chase insect prey. Most have a long, tapered abdomen, a rather humpbacked appearance, and spiny legs, and typically rest with wings folded down the back. The face often appears bearded, and the mouthparts are a pointy knifelike proboscis for injecting saliva into prey. Between the two large compound eyes, the head is depressed, with three tiny eyes (ocelli) in that little valley. Many make a distinctive buzz or rattle in flight.

Larvae are grublike and live in or on soil, among decaying materials, or in rotting wood.

Similar species: Some robber flies are so long and slender they resemble damselflies. Many other robber flies have black and yellow striped patterns and make loud buzzing sounds as they fly these are bee and wasp mimics. Some robber flies in the genus Laphria look almost exactly like bumble bees, complete with yellow fuzzy patches on their rather pudgy bodies. But true flies always have the same characteristics: only one pair of wings (bees and wasps have two pairs) the second pair of “wings” on true flies are reduced to tiny knobby structures called halteres (which function like gyroscopes, assisting flight). Also, flies usually have huge eyes and short antennae.

Body length (not including appendages): most are ½ to ¾ inch varies with species some reach 1¼ inches.

Common Name: Robber Fly

Scientific Name: Order Diptera, family Asilidae, many species

Identification: Adults have a large head, prominent eyes and probosis, 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.

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.

Horse Flies

The name horse fly can apply to any species in the family Tabanidae, including deer flies, or the name can be more specifically applied to flies in the genus Tabaninae.

Tabaninae horse flies are known for their size, and the painful bite inflicted by females.

Horseflies often live near water environments, where their larvae feed and grow on the local insects and small fish. Thinking habitat suggests that North American horsefly populations increase in the Southeast and decrease in the Southwest.

Adults can grow over an inch in length, and females feed on blood from mammals, including humans, a practice that places them into the insect pest category.

For humans at least, the use of over the counter insect repellents containing deet (N-diethyl-meta-toluamide) is usually sufficient to deter them during their season. (Note: deet is a strong chemical and it should only be applied according to the directions given on the container.)

A couple dozen different horse fly species inhabit forests and fields across North America. Apart from size, many horse fly species can be identified by their colorful eyes.

Eye configuration can also help determine gender. The eyes of male horseflies are set close together, female horse fly eyes have a space between them. The picture at the top of the page shows a female horse fly.

The picture at the top of this section shows a female black horse fly.

Wild Side: Robber flies

From robust to tiny, these furry-legged fellows can be hard to find.

Among my favorite groups of insects are the robber flies, the predatory members of the family Asilidae. Ranging in length from a half-inch or so to well over a full inch, these formidable hunters typically ambush flying prey from a perch, piercing their victim with a stout beak and injecting saliva that both paralyzes the prey and contains enzymes that digest it. Lacking chewing mouthparts like all flies, robber flies literally drink their meals, sucking up broken-down tissues through the same beak they used to kill their prey.

Although it’s a good-size family, with some 7,500 species worldwide and more than 1,000 in North America, Asilidae does not seem especially well represented on the Vineyard. My personal photo archive contains only about 20 species, and very little other information currently exists that could expand that number. But it’s a safe assumption that I’ve missed some — perhaps many — species that occur here, and so I keep a sharp eye out for these agile aerialists.

I hit the perfecta on June 21, finding two new species within a few feet and a few seconds of each other as I worked the scrub oak barrens of Correllus State Forest. Both species turned out to be unusual ones, uncommon, or at least infrequently observed, and with very little known about the specifics of their natural histories. Conveniently, these two species fall close to the extremes of robber fly size, and in their appearance, they illustrate the range of characteristics this family can show.

Let’s start with the big one, an inch-long, massive bumblebee mimic that proved to be called Laphria champlainii (it is scarce enough that it has no common name). While we have robber flies with longer bodies on the Vineyard, we only have one species I’m aware of that approaches this beast in sheer bulk and intimidating appearance. In addition to its size, L. champlainii (like most members of its large genus) is prodigiously hairy: Black or yellow fuzz covers its entire body. The resemblance to a stinging bee presumably helps deter attacks by other predators, and may also help this species bushwhack bumblebees, which may well figure among its menu items.

I mistook this insect for Laphria grossa, a similar species I’ve documented previously on the Vineyard. I did note that this individual turned up in a much drier setting than did the small number of L. grossa that I’ve found. So I was not entirely surprised, when I posted photos of this insect in a Facebook robber fly group, to learn that I botched the ID. The dry habitat proved to be a meaningful clue: L. champlainii associates strongly with scrub oak barrens wherever it occurs. There are also subtle differences in appearance between this species and L. grossa: The first segment of the abdomen is yellow in champlainii, black in grossa, and the latter species has a constriction at the base of the abdomen that champlainii lacks. Such is the level of detail on which insect ID depends!

Because it is so rarely observed, little is known about the specific habits of L. champlainii. Like other members of the genus, champlainii probably spends its larval stage in decaying wood or under bark, feeding on wood-boring or tunneling larvae or beetles. As an adult, it probably rests on the tip of a scrub oak leaf (like my individual was doing), waiting for flying prey to pass by. Some sources opine that bees and wasps are favored prey of Laphria in general but most of the photos I’ve found of this genus with prey shows them eating flies of various kinds. In all likelihood, they are generalists, taking whatever happens to pass by. Such powerful predators can probably handle nearly any arthropod prey, from soft-bodied leafhoppers up to well-armored beetles.

At the other extreme was the tiny Taracticus octopunctatus (again, too poorly known to have acquired a common name, but “octopunctatus” refers to a set of white crescents on the abdomen). A bit under a centimeter long (sources give 8 mm as the average length), Taracticus is as delicate as Laphria is robust, and as hairless as Laphria is fuzzy. Only the shape and position of the eyes and the presence of stout bristles on the lower legs tipped me off to the fact that this insect was a robber fly at all. But it’s a beautiful creature, especially its eyes, with flashing, iridescent highlights.

Taracticus seems to occur sparingly across much of the Northeastern U.S., roughly from Kansas to Maine. I have no idea whether it is truly uncommon, or simply overlooked because of its small size. Again, little has been published on its habitats, but the subfamily it belongs to is noted for its penchant for preying on bees. They would surely have to be small ones, given the diminutive scale of Taracticus.

The range of variation found among robber flies is a source of endless fascination for me, even as my ignorance about this group is a source of frustration. Encountering two new species in a single outing makes me think I still have lots of work to do in order to fully account for the Vineyard members of this lethal family.

Robber Flies

Profiles of Pollinators & Backyard Wildlife

Robber flies are predatory flies. Several different genera and species can be found in Kentucky. As predators, they serve an important role in our ecosystems, even the ecosystems we create in our gardens and backyards. Although they will catch and eat pollinators, robber flies also catch and eat a wide variety of other insects. Photo credit: Shannon Trimboli, all rights reserved

Our gardens and yards can hold rich ecosystems with both predators and prey. One group of those predators are the robber flies. Robber flies are in the family Asilidae and can be found throughout the most of the world. Several different types of robber flies can be found in Kentucky. They are most common in open areas, which makes fields of wildflowers and pollinator gardens great places to observe them.

Robber flies can live between 1 and 3 years. Most of that time is as a larva in the soil. The robber fly will pupate in the soil and go through complete metamorphosis before emerging as a fully formed adult. Adult robber flies are predatory flies that can catch prey larger than themselves in mid-air. Insects such as bees, other flies, and butterflies are their primary prey. However, they will also occasionally prey on spiders and hummingbirds. After catching its prey, the robber fly will inject the prey with a combination of enzymes that immobilize and begin liquefying the prey. The robber fly will then find some place to land and will use its proboscis to suck out the liquefied insides, much like a spider eats its prey.

The easiest way to identify a robber fly is to look at its eyes. Robber flies have big fly-eyes but the eyes don’t touch and there’s nothing but air between them at the top. These predatory flies also tend to be fairly large, with the body often an inch long or longer some of the species can look pretty intimidating. The ones I see, or at least recognize most often, are the large ones with long abdomens. When I see them out of the corner of my eye, they sometimes remind me of a weird looking dragonfly or wasp until I take a closer look. However, other species of robber flies are smaller, lack the long abdomen, and are bee mimics. If I’ve seen those species, I didn’t recognize them as robber flies.

It can be tempting to vilify robber flies because they do eat our pollinators. However, overall, they are considered beneficial insects because they also eat other things besides just our pollinators. A healthy, fully functioning ecosystem should have both predator and prey species, and it should be able to easily sustain both types of populations. So, the next time you are out enjoying your pollinator garden or a patch of wildflowers, take a few minutes to look for and appreciate the predators who may also be visiting that area. Seeing a good mix of both predators and prey is an indication that you are creating a healthy ecosystem for pollinators and other wildlife.

Shannon Trimboli is a beekeeper, farmer, wildlife biologist, and author. She owns Grassy Roads Farm and Busy Bee Nursery & Consulting. Busy Bee Nursery & Consulting specializes in plants and habitat consulting services for honey bees, native pollinators, and wildlife conservation. In 2018, her first book, Plants Honey Bees Use in the Ohio and Tennessee Valleys, was published. Shannon also writes a weekly blog called Kentucky Pollinators and Backyard Wildlife. The blog features profiles of pollinators and wildlife, tips for attracting pollinators and wildlife, highlights of different plants for pollinators and wildlife, and life on the farm and nursery. You can sign up to have her blog emailed to you.

Watch the video: Robber Fly


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