Beneficial Flies

The world of insects is enormous, and it would be crazy to expect people to understand much about all of the possible insects they will see around their homes and gardens. Mother Nature also has a way of messing with our minds, by providing insects with camouflage colors or other deceptions that fool us as we attempt to identify a bug. For example, a very large family of flies has many species within it that look remarkably like bees or wasps. The reason for this, we assume, is to provide some manner of protection for the otherwise helpless and harmless fly, by making it look like an insect that is able to fight back with a nasty sting.

This family of flies is named the Syrphidae, variously given common names such as bee flies, hover flies, flower flies, or just syrphid flies. Some of the species are so amazingly similar to stinging wasps that even I – who should be able to spot such things – believed them to be wasps until days later when I bothered to count the wings, and found that they had only the single pair of wings representative of flies. (Bees and Wasps have 2 pairs of wings). This family is just one of several groups of flies that are not only harmless to people, but also are tremendously beneficial to us in our desire to rid our lives of other, more pesky kinds of critters.

When we think of “flies” we think of the filth flies, like the House fly, that lands on our food, buzzes against our window, and breeds in decaying materials that thus lend it to being a huge potential threat to our health. We don’t always know of the work going on in the background by other kinds of flies, and in this article I’d like to focus on these beneficial kinds:

  • Syrphidae – the flower flies, whose larvae feed on aphids
  • Asilidae – the robber flies, whose larvae feed on other insects in the soil and whose adults prey on other flying insects.
  • Tachinidae – the parasitic flies, who lay their eggs on caterpillars to provide a food source for their parasitic larvae.

These are not the only beneficial fly families, but they are probably the most common and noticeable ones around our landscapes and wooded habitats. Also, we really can acknowledge the beneficial role even the filth flies play in helping maintain our environment. These flies, in their feeding on dead animals and decomposing organic materials, do a fabulous job in the process of recycling material back to the soil. However, their interaction with humans and our food and homes leaves a great deal to be desired, and they truly deserve the label of “pests”.

Let’s take up this topic of beneficial flies, and begin with the rather attractive Syrphidae. Within this family there is one species with a potential dark side, and that is one called the Drone Fly. It is a great mimic of the Honeybee, and easily can be mistaken for this other stinging insect. On close inspection, though, we see the short antennae and single pair of wings characteristic of flies and not bees. These flies differ widely in the habits of the larvae, from most other species in their family. The larvae of the Drone Fly are called Rat-tailed Maggots, due to the long breathing tube at the end of their abdomen. The eggs are laid around extremely wet areas that are high in organic matter, and sometimes this site may be associated with sewage or other unwholesome situations.

The Syrphidae – Flower Flies

These flies may frighten you at first, when you see one and believe that it is an overly-curious wasp or bee. However, once you learn to recognize them you will delight in having them in your garden. Once again, flies differ from bees and wasps by having only one pair of wings, and the antennae on flies are short, composed of a large egg-shaped section with a thin hair coming off of it. These flies also differ in their flight habits, flying more like hummingbirds with rapid movements and the ability to stop and hover in one place while they check you out. The different species may mimic yellowjackets, bald-faced hornets, honeybees, bumblebees, carpenter bees, or other wasps, and since the deception is good enough to fool humans it probably does a good job of making their predators stop and take notice too – birds, frogs, or other vertebrates that eat insects.

The larvae look very much like little green caterpillars, and when you see one on your roses you may be tempted to kill it to keep your roses from being eaten. I frequently receive a huge population of aphids on a flowering plum tree in my yard, with the resulting curled leaves and messy honeydew drippings from the aphids. But, by the time I discover the aphid population I already have the Civil Defense system working, and there are hundreds of ladybug larvae and syrphid fly larvae already gorging themselves on the juicy aphids. I have even messed around with the natural processes by taking a syrphid fly larvae from a plant with no aphids, and placing it onto a thoroughly infested rose bud. The fly larva immediately got to work and within about 30 minutes there were no more live aphids on this particular bud. When opportunity knocks it appears they can be real gluttons.

So, take a closer look at that wasp you see – it could very well be one of your garden’s best friends. What looks for all the world like a hairy bumblebee might just be a Syrphid Fly.

The Asilidae – Robber Flies

In this case you probably will not notice the larvae of these flies, as for the most part they are feeding in the soil or within rotting logs, on beetle larvae or pupae or perhaps on the egg cases of grasshoppers. It is the adults that can be seen as they fly from branch to branch in search of the best vantage point for spotting their prey. They are some of our largest flies, with some species growing almost to one and a half inches long. One species in the eastern half of the United States is dubbed the “Nebraska Bee Killer”. It is about 1.2 inches long, and along with many other flying insects it also may dine on honeybees, so on occasion robber flies do pick off other beneficial insects. I suppose there really was no way for it to know.

A close look at Robber Flies will reveal two traits that help you to discover its behavior as a predator, and one which flies out and picks off its food while in flight. The first will be its front legs, which are similar in their shape to those of praying mantids. They are held up and at the ready and obviously are meant for grasping and holding. These are referred to as “raptorial” legs, and you see them on many other predaceous insects as well. The second feature is their eyes, which are so large they occupy nearly the entire head, much like the eyes of another familiar group of predators – the dragonflies – do. Having such large eyes enables the Robber Fly to spot potential food and movement from all directions.

If you are observant while walking along wooded trails or in fields you begin to see these flies on a regular basis. Often they may be carrying a captured goody in their legs as they fly, on their way to some carefully chosen spot where they can feed safely and privately. Often they will be perched on the ends of twigs, watching and waiting for some fast food to fly past them, at which point they fly off the perch and quickly grab their prey. It has been said by some investigators that they even play an important role in keeping grasshopper numbers down in rangeland.

Like the Syrphid flies, the robber flies often are excellent mimics, in their appearance, of large bees, wasps, or biting flies. I once watched as what I thought was a huge Horse Fly flew past me and then landed on the end of a bare branch. However, the way it landed and the position it then assumed rung a little bell in my mind, and I realized that its behavior was characteristic of the robber flies and not horse flies. I captured it for inspection, and sure enough, it was a huge robber fly. The color and shape of a horse fly might afford it protection from certain predators of its own, that could believe from past experience that this food might bite back.

The Tachinidae – The Parasitic Flies

Our third family is another extremely beneficial group, but the adults look very much like many of the annoying flies or even some of the seriously dangerous flies, such as Bot Flies, which lay their eggs in the flesh of humans and animals. The adults of these flies are often very large and noticeable, sometimes colored orange or with red stripes, and profusely covered with long, stiff hairs. It is these hairs all over their bodies that gives you your best clue as to the identity of them. When they accidentally enter your home they may head right to the lights or the window, attracted to the brightness, and begin flying around and back and forth with their loud, buzzing noise.

Some species may even be mistaken for stinging insects, but in reality the Tachinid flies are harmless to people but deadly to caterpillars. Some kinds even have larvae that are parasites of other bugs, such as beetle larvae, grasshoppers, wasp larvae, stinkbugs, and even sowbugs, but it is the caterpillars where they excel. In fact, they do such a thorough job that they can become a bit annoying to a moth collector like myself. On several occasions I have found caterpillars of moths that I wished to raise through to the adult moth, and the larvae proceeded nicely through their last couple of stages, transformed to the pupa, and as I patiently waited for the magic moment when a beautiful moth would appear I received, instead, a dozen large flies.

It seems that prior to my discovery of the caterpillar in its natural setting a Tachinid fly had already found it, and had laid a batch of eggs on the small caterpillar. Soon after they hatch from these eggs the fly larvae burrow inside their living food item and feed within it, even as the caterpillar is largely unaffected by the presence of these aliens eating it from the inside. The fly larvae eventually complete their larva stages, shed their outer skin to reveal the pupa stage, and within a few more days emerge through the skin of the caterpillar as fully developed adult flies. At this point the caterpillar will die from the invasion.

As gruesome as this description is, this family of flies is extremely important to people and our effort to grow our food and maintain our landscape and forests. Some species of Tachinid flies are bred for this purpose, and some species have been imported to this country as potential “bio-controls” for damaging insect larvae. There can be a dark side to this, as on at least one occasion the imported flies took to laying their eggs on the larvae of our large, native silk moths too, and the result in the last decade of the 20th century was a noticeable decline in the numbers of these beautiful moths.

So, a fly is not just a fly, but within the major groups of our insects we will find a dazzling array of lifestyles and feeding preferences. While we definitely need to eliminate the filth flies from our urban environments we also need to recognize the beneficial flies, and encourage their presence and enjoy their activities.