What Are All Those Flies in My Fall Garden?

If you’ve noticed an unusual number of large, bristly flies hanging around your flowers this fall, take a closer look before reaching for the fly swatter. They may be tachinid flies, and they are some of the good guys in the garden.

Tachinid flies are a large and diverse family of beneficial insects. More than 1,000 species are found in North America. Depending on the species, they can look a bit like an ordinary housefly—gray, brown or black—but many are noticeably bristly. Some are surprisingly large and robust.

 What makes them beneficial?

It’s what happens during their larval stage that makes tachinid flies important to gardeners.

Female tachinids use other insects as hosts for their young. Some lay eggs directly on a host, while others have different strategies for getting their larvae inside. Once there, the developing fly larva feeds within the host and eventually kills it.

Their hosts can include caterpillars, beetle larvae, grasshoppers, sawflies and true bugs. That means tachinids can help naturally suppress a number of insects we consider garden pests.

The adult flies are much more peaceful. They visit flowers for nectar and can also contribute to pollination. University of Idaho recommends flowering plants as a way to support tachinid flies and other beneficial insects.

Why am I seeing so many in fall?

You may barely notice tachinid flies earlier in the season and then suddenly find them everywhere in late summer or fall. There are many different tachinid species, with different life cycles and hosts, so their numbers and timing can vary considerably.

Late-season gardens can also make the adults much easier to notice. As the number of blooming plants dwindles, remaining flowers become busy gathering places for insects looking for nectar. A successful generation of tachinids developing from insects earlier in the growing season can also mean more adults appearing later.

So a sudden gathering on fall flowers doesn't necessarily mean something is wrong. It may simply mean your garden has provided both the insect hosts and the floral resources these beneficial flies need.

Let them stay awhile

If tachinid flies are visiting your garden, there’s generally nothing you need to do. Keep some flowers blooming as late into the season as possible and avoid unnecessary broad-spectrum insecticides that can kill beneficial insects along with pests.

They may not be as charming as a butterfly or as recognizable as a lady beetle, but tachinid flies are quietly doing some valuable work.

Sometimes a garden full of flies is actually a very good sign.

https://hort.extension.wisc.edu/articles/tachinid-flies/

https://agsci.colostate.edu/agbio/ipm-pests/tachinid-flies/

https://treefruit.wsu.edu/crop-protection/opm/tachinids/