Are there good flies?

By Bonnie Dailey - Delaware Soil & Water Conservation District

Just a few days ago I was taking my dogs out and I noticed something buzzing through my lovely patch of sweet marjoram. The sound was coming from a very unattractive, bristly, fat creature that looked suspiciously like a housefly on steroids, but it was acting like a bee. I took a photo, and after some detective work, we were able to identify the homely looking thing as a fly from the Tachinidae family. This is the largest and most important group of parasitic flies with more than 1,300 species in North America. We knew there were beneficial insects, including many parasitoid wasps, but we had no idea that there were so many good flies!

The adults usually feed on aphid honeydew, nectar, and pollen, which is why I spotted one on the pink flowers in the sweet marjoram. They like umbelliferous plants such as carrot, dill, and other herbs, along with composite flowers such as asters and rudbeckia, and other flowering plants. Most of us probably never gave these flies a second glance when we have seen them in our flower beds and vegetable gardens.

While tachinid flies are valuable as pollinators, they also work hard on our behalf as predators, controlling pest insects that plague us. Adult tachinid flies use several means to ensure their young will have access to food as they grow. Most are endoparasites, meaning the developing larvae (commonly referred to as maggots) feed within their hosts. This can occur in several different ways. One method is for the adult females to lay their eggs on the leaves that the pests will consume; when the pests consume the leaves, they also ingest the eggs. These eggs hatch inside the victim. Other tachinids have a piercing ovipositor (egg laying organ) and can insert their eggs or larvae directly into the pest. Yet, a third method is for the adult female tachinid to affix the eggs to the outside of the prey, and upon hatching, the larvae bore into the body of the host. In all of these scenarios, the developing larvae, also called maggots, consume the prey as they grow. Goodbye and good riddance to cabbage looper caterpillars, European corn borer, Colorado potato beetle, squash bugs, hornworms, Japanese beetles, and more!

Tachinid flies are one of several parasitoid insects, who eventually kill the host upon which they feed. According to the University of Maryland Extension, the easiest way to determine if your garden has some of these insects is to look for symptoms in the host insects, such as:

• Change in color. Hosts often change color because normal development has been disrupted, the immature parasitoid developing inside is visible through the host’s skin or the host is dead.

• Parasitized eggs may also become much darker.

• Look for eggs, larvae, or cocoons attached to the outside of the host’s body.

• Mummified bodies of the parasitized hosts may be found attached to leaves.

• Exit holes left by the emerging new parasitoids may be seen in the bodies of dead hosts. These holes are usually round and relatively smooth-edged.

• Hosts that are parasitized may change their behavior. They may move from their preferred plants, stop feeding altogether, or barely move about.

Tachinid flies come in many colors, shapes and sizes. You can see a few at More pictures can be seen at

September is just around the corner which means it’s time for the 2019 Delaware County Fair, our annual fish fingerling sale, and the annual milkweed pod collection. To learn more, check out and follow us on Facebook and Instagram.

By Bonnie Dailey

Delaware Soil & Water Conservation District

Bonnie Dailey is deputy director of the Delaware Soil & Water Conservation District. For information, go to

Bonnie Dailey is deputy director of the Delaware Soil & Water Conservation District. For information, go to