Parasitic Flies

Because flies are often such terribly annoying pests it may be too easy for us to decide that all flies, therefore, are pests. The fact is, though, that a great many kinds of flies are extremely beneficial to have around, and do not qualify at all as “filth” flies or blood feeding flies. We have another BugInfo article covering many of the beneficial flies in our category of “Shoo Fly Shoo”, and I hope you’ll read that one as well. But, correcting the evil image of flies as “all bad” is important enough to add this second article on those flies that are parasites – they seek out other insects as a food source for their own larvae, and in this way serve to lower populations of some insects in our landscape or forests that might otherwise be pretty damaging. These kinds of flies may, at times, be found buzzing around inside your home, and can be very annoying at that time. But, as we learn to distinguish the good from the bad we recognize that the proper course of action, for these beneficial insects, is to try to capture them alive and put them back outside where they belong.

The Parasitic Flies – the Tachinidae

One of the most important and most visible of these parasitic flies is the family called Tachinidae, commonly referred to simply as the Tachinid Flies or Parasitic Flies. There are well over 13,000 different kinds of these flies known around the world, and over 1,300 different species just in North America. This makes them the second largest family of flies in the U.S., and thus you are very likely to see them on a regular basis. Some of the species can be pretty frightening in appearance, especially when all you know is that it’s some sort of fly and therefore probably evil. In the southern states we have one enormous species that can be nearly an inch long, with a bright orange body and thick, long, bristly hairs sticking out all over it. Most species are fairly large, and possibly very noisy as they fly, so our instinct of course would be to kill them when we see them. This is one major challenge we have in good environmental stewardship – to learn to tell the good from the bad in our landscapes, and to attempt to preserve the good guys when they are present.

Tachinid flies are such effective parasites that this is one group which we humans have used to our advantage, breeding certain species in large numbers and releasing them as biological control agents. They commonly parasitize moth and butterfly larvae, and since caterpillars are one group of insects that potentially will damage our food crops or our landscape trees and shrubs, when a non-toxic alternative such as flies can be used effectively it is an excellent option. Raise and release programs have been used for many agricultural crops around the country. This is not without its pitfalls though, for the flies are not necessarily selective in what they attack, and native, desirable species of moths or butterflies are just as likely to be dined on as are the pesty moth species. In my various entomological endeavors I have occasionally found large silk moth cocoons, and brought them home in the hopes of raising them through to see the adult moth. Too often, I’m afraid, all I got for my months of waiting was a jar full of Tachinid flies that had found the caterpillar before it pupated, and what I had retrieved from the plant was just a rookery of fly pupae inside the moth’s silk cocoon.

It’s even possible that some biological control programs using Tachinid flies have had the unfortunate side effect of some environmental damage. In the eastern U.S., over the past 20 years or so, biologists believe they have seen a great decline in the numbers of the large silk moths, such as the Cecropia Moth. Some recent studies suggest that the decline may be due in great part to the introduction of a new Tachinid fly species for the purpose of a bio-control program. The intended fly food was an agricultural pest insect, but the Tachinids also found the large silk moth to be perfectly acceptable as well, and the damage was done. This points out the need for caution when implementing a pest control program with the use of living organisms. There often is a strong influence on regulators to disallow the use of pesticides and to stick with biological controls, but obviously even bio-controls can have a negative side if we don’t study their potential well enough in advance. This may be particularly important when we face yet another exotic invader to this country, and the effort is then made to find those parasites and predators that kept it under control in its native environment. Introducing one exotic invader to control another exotic invader must be done cautiously.

The adult fly of Tachinids themselves feed on nectar and other sugary liquids, for these flies are capable of ingesting only liquid materials. In nature this may often be the sugary blobs of sap that we call “honeydew”, the drippings from aphids or scale insects that are feeding on plants. You may see this honeydew as the sticky, shiny material on plants and leaves, or even on sidewalks or your car windshield if you parked under a tree infested with aphids. The female fly, once she has developed her batch of eggs, then seeks out insects to place her eggs onto, and it may be grasshoppers, caterpillars, spiders, spider egg cases, or the larvae of many other kinds of bugs. She places a small batch of eggs directly onto that insect, and when the eggs hatch the new fly larvae burrow right on in and begin feeding on their prey from the inside, but slowly to allow their host to stay alive long enough for the flies to complete their development. Some species make the journey into the host insect by having the eggs laid onto plant tissue, and when a plant-feeding bug such as a caterpillar munches on that leaf it ingests the fly eggs along with the green tissues that it meant to take in.

The Bombyliidae – the Bee Flies

So, this is a very important family of flies when it comes to keeping down the insects we consider The Enemy. Another fly family is a really curious one, and this is one called the Bee Flies – the family Bombyliidae. This is another large family of common flies, and I will bet that most of us have seen them while taking our walks in landscapes or along natural trails. There are over 750 species of Bee Flies in North America, many being large and conspicuous. The adults often will be close mimics of bees, with hairy bodies and mannerisms that might tell us we are looking at some sort of stinging insect. However, bee flies cannot bite or sting, and the adult flies survive on nectar and pollen, in some instances acting as important pollinators of crops and other plants. They often have amazingly long feeding tubes, called a “proboscis”, that they can extend down into flowers to sip the nectar. They have an extremely rapid flight, and as they zero in on a nectar source they hover in place much like a hummingbird. These are beautiful insects, and if you are lucky enough to see one it should be enjoyed.

The larvae of bee flies eat other insect larvae, feeding on caterpillars, beetle larvae, wasp and fly larvae, and just about any other larvae or even insect egg masses they can find. They are effective killers of white grubs that damage your lawn as well as locusts and grasshoppers that feed on your garden plants. While their general feeding habits may also cause them to dine on the occasional beneficial insect larvae, their consumption of so many undesirable insects truly rates them the label of “Beneficial”.

The Dolichopodidae – Long-legged flies

A third family of highly beneficial flies is called the Dolichopodidae, or the “long-legged flies”, a name given due to the relatively long, slender legs of these small flies. I find them commonly in and around my own garden, and the species in my area often are a shiny, metallic green or bluish color. They have very slender bodies and are very easy to tell apart from the filth flies we call “blow flies”. In our other article on beneficial flies we discussed the big predators called Robber Flies, which like to hang out on the ends of branches so they can see what potential food materials are flying by them. The Dolichopodids act much the same, preying on other insects, but of course they will pick on much smaller food items due to their much smaller size and strength.

The larvae of the Dolichopodids are not well studied, but those that have been are known to be predators as well, feeding on the larvae of other insects they find under loose bark, in decaying vegetation, or in other hidden places. The difference between a “predator” and a “parasite” might not mean much to the animal being eaten, but generally speaking a predator consumes the whole prey food, generally in one sitting and very quickly. Think of a praying mantid as a predator. A parasite, on the other hand, feeds on its host very slowly, often leaving it alive for a long period so it can complete its own life cycle. It may not even kill the host at all, finding it better to leave that food source available for future meals. Think of a flea as a parasite.

The Tabanidae – horse and deer flies

One last family of flies to mention is one with a Love-Hate relationship, and this is the family Tabanidae, or the horse and deer flies. Anyone living in the damp forests of the north-central states can appreciate the misery these flies bring to us, as the female flies, just like mosquitoes, feed on blood. We can call them “horse” or “deer” flies, but human blood is just as acceptable, and in my frequent vacations to northern Wisconsin to visit relatives I have been tormented by them on many occasions. They have a pair of mandibles that act much like scissors, slashing the skin to cause the blood to flow, at which point they lap it up with their mouths. The male flies feed on nectar and other sweet liquids, but the females are biters. The larvae of the Tabanids are another group of predators, feeding on whatever insect larvae they come across in damp soils, in rotting logs, under debris on the soil, or even along the edges of ponds. So, we can thank them for their work as larvae, but wish they’d go away as adults.

We’ll pretty much end it with these four families of flies, but the message is that Nature is a diverse and potentially confusing collection of organisms, and the more we learn about it the better chance we have of managing it correctly. Flies of some kinds are the causes of the vast majority of human misery, spreading a great many kinds of diseases that kill millions of people around the world each year. However, the kinds that we have discussed are just a few of the many flies that do not cause these problems, but instead should be welcome residents of our landscapes and forests. With over one million kinds of insects currently described as species around the world, and possible five times that many actually out there, it becomes very, very difficult for you to be able to identify them when you see them. However, we should make that effort before we reach for the fly swatter.