Parasitic Flies (Tachinid Flies)

Tachinid fly visiting a mint flower
Scientific Name
About 1,350 species in North America north of Mexico
Tachinidae (parasitic flies) in the order Diptera (flies)

Tachinid flies are one of the largest families of flies. They are parasitic flies whose larvae are parasitoids of other insects or arthropods. A parasitoid is a parasite that ultimately kills its host. Some tachinid species are limited to only certain types of insects as hosts, while others may be able to grow up on a wide variety of host species.

Many tachinid flies look a lot like chunky house flies or blow or bottle flies. Others, less noticed, are smaller and look like little wasps or bees. Many are very bristly. A key identifier is the presence of a subscutellum, a flaplike plate that is positioned below and on either side of the scutellum. In flies, the scutellum is a shieldlike plate attached to the back end of the thorax, something like a spoiler on the back end of a car.

Identifying the various genera and species can often be quite difficult. Specialists scrutinize details of bristle numbers, placement, and size; facial characteristics; and antennae shape.

Similar species: House flies are most likely to be confused with most of the tachinids people notice; the subscutella and many spiny bristles are good clues to a tachinid's identity. Note that there are many insects whose larvae are parasitoids on other insects, including ichneumon flies, braconid wasps, and more.

Other Common Names

Adult length: about ¼–½ inch; varies with species.

Where To Find

Statewide. Different species may have different habitats and distributions.

Most people notice adult tachinid flies as they are visiting flowers, which means they are usually seen in gardens, yards, and grassy open areas like prairies, wetlands, and old fields. The tiny larvae are rarely seen, although if you look closely, you may see the eggs on the surfaces of various insects.

Tachinid adults are commonly seen as they take nectar from flowers. If you look at photos of tachinid adults online, you'll see most of them have been photographed on a variety of pretty flowers.

Larvae are parasitoids of other insects or arthropods. A parasitoid is a parasite that ultimately kills its host. Some tachinid species are limited to only certain types of insects as hosts, while others may be able to grow up on a wide variety of host species. In some species of tachinids, the mother deposits eggs directly on, or injects them into, a suitable host insect. If you look closely at insects, you may see the tiny eggs of tachinids on the bodies of their hosts. The larvae hatch from the eggs and burrow into their hosts.

In other groups, however, the eggs are scattered in a place where host species are likely to be present. In some of these cases, the eggs, scattered on foliage, are extremely tiny and are ingested by the unwitting host as it chews the leaves, and the larva gets inside its host that way. In other cases where eggs are scattered, the newly hatched larvae are weird-looking, flattened things called planidia, which must wait for, or even locate and attach themselves to a suitable host, or perish. There are amazing adaptations that allow the larvae to accomplish this feat. In some tachinid species, the mothers deposit larvae instead of eggs.

Moth caterpillars are the food for many species, but nearly all groups of insects have a tachinid fly to specialize in them.

Life Cycle

Mother tachinid flies deposit eggs of larvae in places where they can find suitable hosts in which to develop. A variety of deposition behaviors occur, and the mother's egg-laying anatomy, egg types, and larval forms can vary in amazing ways. See "Food" above for details of larval development, which is a big part of the life cycle of these flies. Once inside its host, the larval tachinid eats all but the critical organs, which keeps its host alive as the young tachinid continues to eat. The larva saves the vital organs for last, when the larva is getting ready to pupate and is done with the host.

The larvae pupates and emerges as a winged, sexually mature adult. The host insect, if not already dead, dies soon after.

Humans have employed tachinid flies as agents of biological control for invasive or pest species. About 16 species have been purposefully introduced to North America for this reason. In at least one case, this strategy backfired, which is a lesson in humility and hindsight: Compsilura concinnata was brought to our continent from Europe in 1906 because it parasitizes the invasive gypsy moth, which can destroy forests. It injects its larvae into the caterpillars' bodies, and the larvae eat the caterpillars from the inside, killing them. Unfortunately, the tachinid also parasitizes our native giant silk moths, which it turns out are more vulnerable to the tachinid than the gypsy moth. Many of these spectacular native giant silk moth species are now declining.

Tachinids play a huge role in naturally controlling populations of their prey insects, many of which are problematic for people as plant pests. For example, the tachinid Winthemia quadripustulata can destroy 50 to 100 percent of an armyworm population. A feather-legged tachinid, Trichopoda pennipes, can destroy similarly large percentages of populations of true bugs, including squash bugs.

Some tachinids, however, harm species of insects that humans like to have around, making the tachinids the pests. For example, there is a tachinid species that parasitizes silkworms in Asia, to the dismay of people in the silk-making industry. Certain tachinids parasitize bees, which are important pollinators, and ground beetles, which are themselves important predators of pest insects.

Globally, more than 10,000 species of tachinid flies have been described scientifically, and this may represent only about half the species that actually exist. Within the true fly order (Diptera), there is only one family with more species than the tachinids, the Tipulidae (large crane flies), where more than 15,000 species have been described.

Adult tachinids, as they visit flowers for nectar, play a role in pollination, which may be overlooked since they are often active on flowers at times when most other pollinators are not active, such as early in mornings when temperatures are relatively low.

It's hard to estimate the cumulative impact that tachinids have on the populations of other insects. By attacking their hosts early in their development, they destroy them before they have a chance to mature and reproduce, greatly reducing exponential increases in their host species.

Tachinid populations often display a pattern of peaks and lows, following the populations of their host species. A peak in numbers of their host species leads, in subsequent years, to peak populations of the tachinids. The large numbers of tachinids depress the populations of their hosts, and the so-called crash of those populations lead to a subsequent crash in the tachinids. The population of the host species starts to rebound, and the beat goes on. Population dynamics is an important and useful branch of biology.

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About Land Invertebrates in Missouri
Invertebrates are animals without backbones, including earthworms, slugs, snails, and arthropods. Arthropods—invertebrates with “jointed legs” — are a group of invertebrates that includes crayfish, shrimp, millipedes, centipedes, mites, spiders, and insects. There may be as many as 10 million species of insects alive on earth today, and they probably constitute more than 90 percent all animal species.