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If you spy a fly buzzing around your garden and sipping nectar from your plants, you should probably give him or her a tiny high five. You owe a surprisingly big thank you to that little guy. If it’s lapping nectar from a flower, there’s a very good chance that fly is a tachinid fly, which is the world’s largest and most important group of parasitoidal flies. Yes, that means that tiny fly is a big-time helper to you and your garden. Let me introduce the two of you – I’m sure you’ll be best buds before you know it.

What is a tachinid fly?

I used the word “parasitoidal” in the paragraph above, so I guess I should start by telling you what that means, just in case you don’t already know. If you’re familiar with the term “parasite” then you’ll be a quick study. Parasites are organisms that live off of another organism, whom we call its “host.” There are tens of thousands of different parasites in this world, some animal, some plant, and some fungal. In the animal kingdom, examples of human parasites would be ticks or lice or tapeworms (ack!). Those fleas your dog had last summer are parasites, too. A parasite leaves its host alive. A parasitoid, on the other hand, is a lot like a parasite except that it brings eventual death to its host (***insert sinister fly laugh here).

Yep, that’s right. That little fly you just high fived in your garden is a natural born killer. Except its host isn’t human. Depending on exactly which species of tachinid fly you came across, its host could be a geranium budworm, a corn ear worm, a stink bug, a squash bug, a Japanese beetle, or any number of other common garden pests.

Tachinid flies fall squarely into the category of beneficial insects when it comes to the role they play in our gardens. But it isn’t the adult fly that’s the harbinger of death. Instead, it’s the larval fly. The baby fly, if you will. But before I share the fascinatingly gory details on how that works, I want to tell you what tachinid flies look like so you’ll know exactly who to high five.

What does a tachinid fly look like?

There are over 1300 different species of tachinid flies in North America alone. Worldwide, there are at least 10,000. There’s a huge diversity of physical appearances among all of those species. Adult tachinid flies measure anywhere from 1/3″ to 3/4″ long. Their coloration, body shape, and texture vary greatly too.

Some tachinid fly adults are grey and fuzzy and look almost exactly like a housefly. Others are iridescent blue/green like a blow fly. There are chubby and red tachinid flies, and species that are slender and black. Some are covered with bristly hairs while others are smooth. Which is all to say that each species has its own unique appearance. But, one easy way to tell them apart from houseflies is that adult tachinid flies drink nectar and houseflies generally do not (they much prefer carrion and poop and picnic food!). If you see a fly on a flower sponging up nectar, there’s a very good chance you’re looking at a tachinid fly.

Tachinid fly lifecycle

An important place to start when it comes to understanding the tachinid fly lifecycle is the knowledge that each species of tachinid fly can only use either a single species of insect as its host or a closely related group of host insects. They are highly specialized parasitoids. In other words, a species of tachinid fly that uses a squash bug as its host will probably not also be able to lay eggs on a tomato hornworm. Some species are more specialized than others for sure, but they’ve all coevolved with a specific host (or set of hosts, as the case may be). This is why having a diversity of tachinid fly species in the garden is a very good thing! It also means that tachinid flies WILL NOT lay eggs on humans or our pets, so no worries about that!

I promised you the gory details of how these fly friends help us gardeners, so here goes. Most female tachinid flies lay their eggs on the bodies of host insects. They’re easy to spy on the backs of their hosts (see photos below). The female fly simply lands on its host and sticks eggs to it – singly or in small groups. The egg hatches a few days later, and the tiny fly larva burrows down into the host and starts feeding on it. Host insects continue to live with the larval fly growing inside. In some cases, the larva doesn’t reach maturity and kill the host until the host reaches adulthood, but death always comes to the host – that’s what a parasitoid is, after all.

A few other species of tachinid flies lay eggs on plants that are being eaten by their host insect. When the host insect takes a bite of the leaf, they also ingest the egg. You can guess what happens from there.

How do tachinid fly larva turn into adult flies?

Once the fly larva reaches maturity, it’s ready to pupate into an adult fly. Sometimes this happens within the dead body of its host, but most of the time pupation occurs only after the larval fly (called a maggot – I know, gross!) emerges from its now-dead host. It wriggles off or burrows down into the soil to form a pupal case (cocoon) and morph into an adult, just as a caterpillar changes to a butterfly. The adult fly pops the top off its cocoon and flies off to start another generation of garden helpers.

What kind of garden pests to tachinid flies help us manage?

As you know now, there are over 10,000 species of tachinid flies in the world, which means of course that there are a huge number of host insects, too. Some of the most common host insects include:

Corn ear worms

Tobacco budworms

Cutworms

Japanese beetles

Mexican bean beetles

Colorado potato beetles

Caterpillars of many kinds (including cabbage worms, gypsy moths, hornworms, cabbage loopers, tent caterpillars, and many more — see note below about tachinids and butterfly caterpillars)

Sawfly larvae

Four-lined plant bugs

Harlequin bugs

Lygus bugs

Leaf-footed bugs

Squash bugs

Cucumber beetles

Earwigs

And lots more!

Tachinid flies and butterfly caterpillars

As good as they are for the garden, tachinids have gotten a bad reputation among folks who raise monarch caterpillars and other butterflies. Yes, tachinid flies will lay eggs on butterfly caterpillars if they are a host insect for that species. They’re not evil or awful for doing so. They’re just doing what they evolved to do, and they are an important part of the ecosystem. They deserve to be here just as much as the butterfly does. Just because tachinid flies aren’t a pretty covergirl of the insect world, doesn’t mean they don’t have a valuable role to play. Yes, it’s disappointing to raise a monarch caterpillar only to see the chrysalis turn into brown mush instead of turning into a beautiful butterfly, but if you’ve ever seen a National Geographic wildlife special, you know that’s how nature works. Plant more milkweed to encourage a greater population of monarchs.

If you find a chrysalis that’s turned to brown mush, instead of cursing the tachinid fly responsible for it, think about how amazing it is that a momma fly laid an egg on a teeny tiny caterpillar. And how amazing it is that that caterpillar continued to grow right along with the fly larva housed inside its body. Soon you’ll see the larval fly drop out of the butterfly chrysalis, form a pupal case, and then emerge as an adult. Really, it’s a transformation that’s just as amazing and miraculous as the butterfly’s.

How to encourage tachinid flies in your garden

All adult tachinid flies require nectar, but they don’t sip this sugary goodness from just any flower. Their mouthparts are like sponges, not straws, so skip the deep, tubular flowers. Opt instead for tiny flowers with shallow, exposed nectaries. Members of the carrot family are particularly good, including fennel, dill, parsley, cilantro, and angelica. The daisy family is another great choice for supporting tachinid flies. Plants like feverfew, boltonia, chamomile, Shasta daisies, asters, yarrow, heliopsis, and coreopsis are great picks.

Provide tachinid flies with a diversity of nectar-rich flowers and they’ll be more than happy to help you out in the garden. And all they ask in return is for you to eliminate pesticides so there will be plenty of host insects around for their egg-laying needs… oh, and they’d also be appreciative of the occasional high five.

For more about beneficial insects in the garden, pick up a copy of my book, Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control (2014, Timber Press, winner of the 2015 American Horticultural Society’s Book Award) or my book Good Bug Bad Bug (St. Lynn’s Press, 2011 2nd ed).

You can also read more about beneficial insects in these articles:

5 surprising facts about ladybugs

The black and yellow garden spider

The best plants for beneficial insects

Build a pollinator palace

How to help our native bees

Common garden bees and how to ID them

Have you ever come across a tachinid fly in your garden? Did you know what it was? Share your experience in the comment section below.

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