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Tachinid Fly vs Monarch: 9 Things to Know (Eggs, Larvae)

The tachinid fly is one of many beneficial insect predators in your garden. But they are also harmful to monarchs. But if you are raising monarch caterpillars, the tachinid fly is an enemy.

Tachinid flies lay their eggs on monarch eggs and caterpillars. Once the egg is laid, that monarch is as good as dead. Over the next couple of weeks, the tachinid egg will hatch. And the maggot will eat the caterpillar from the inside.

Eventually, the tachinid larva will emerge to further develop in the soil. And the monarch will die.

tachinid fly

Learn more about the monarch’s life cycle.

Has Your Caterpillar Been Attacked by Tachinid Flies?

If you’re collecting monarch larvae outdoors, it’s hard to tell if they have been attacked by tachinid flies.

The best thing to do is to gather monarch eggs when you see the mama oviposit (lay eggs) onto the milkweed.

You can determine if your pupae are parasitized (meaning, your caterpillar was parasitized and is now an ‘infected pupae’).

How Does Tachinid Fly Paratize Monarch Eggs?

The tachinid species that parasitizes the butterfly injects their egg into the butterfly egg or caterpillar. This is an important detail. The fly doesn’t lay its eggs on milkweed leaves but inside the eggs or larvae. Methods vary across tachinid flies and host species. More on this below.

The eggs then incubate and grow inside the host monarch.

Don’t worry about looking for tachinid fly eggs on your milkweed leaves. There won’t be any on the leaves.

Here’s how the process of tachinid flies parasitizing caterpillars works. I should mention that the video might be best watched away from meals.

Watch this tachinid fly maggot emerge from a monarch caterpillar.

What to Do if You Discover an Infected Monarch?

If you leave the pupa as is, taking no action, then the tachinid fly pupae will hatch. The monarch is already dead or will soon be.

The best thing to do is to destroy the monarch pupa before the tachinid fly pupa can come out and hatch.

Once infected, there is no way to prevent the inevitable death of the monarch. All that can be done is to stop the tachinid fly from hatching and killing more monarchs.

What Do Parasitized Monarch Larvae Look Like?

What signs will you see in parasitized larvae (caterpillars)? Watch for brown discoloration. They often look like rust spots on the chrysalis.

You might also see where it appears that the pupa has been injured, with an opening but probably no visible liquid.

How to Identify Adult Tachinid Flies

It is surprisingly easy to recognize an adult tachinid fly. The best way to identify them is to look at their abdomens for hair. If you see a hairy ‘booty,’ then chances are it is a tachinid fly!

Tachinid flies also have large compound eyes and some species have red eyes.

Tachinid flies can also be identified by their flight pattern. They don’t fly erratically or quickly like houseflies or horseflies. Rather, they move slowly and will wait for the right moment to lay their eggs onto an unsuspecting caterpillar (or egg).

The tachinid will lie in wait and sneak up on the monarch caterpillar while it is eating and oviposit its eggs usually, behind the caterpillar’s head before flying off. Because they are not fast-moving flies, killing them in your butterfly garden shouldn’t be too difficult.

Just be on the lookout for a hairy fly and get ready to squish them or zap them!

tachinid fly monarch

Are all Tachinid Flies the Same?

There are more than 1300 tachinid fly species in North America. And more than 10,000 species worldwide. Their appearance varies greatly among these many species.

There is also great variance in their method of reproduction.

Many tachinid fly species that lay eggs deposit them directly on or in the body of their host. However, some simply deposit their eggs on the host’s food plant and leave it up to the larvae to find suitable victims. Many tachinid flies do not lay eggs. Instead, they deposit young larvae on, in or near their hosts. The young larvae feed their way into their hosts, where they chew on the gut wall. Usually, a single larva develops inside an individual host insect.

Washington State University

This article is part of our Monarch butterfly series.

Your Turn

Have your monarch caterpillars been affected by tachinid flies? How do you combat them? Have a fact or story to share? Please share in the comments!

Shirley Weismann

Saturday 10th of September 2022

I have been losing hundreds of Monarch caterpillars for 3 years to an unknown problem. I have experience all of the other diseases of the caterpillar and butterfly through the years (over 20) so I know it isn't any of them. My cats hatched from eggs started out eating normaly until their 2nd to 4th instar and them slowed down eating and growing and eventually starved to death. My latest theory is that the Tachinid fly layed its egg in the Monarch egg and the larva killed the cat but that would be before the fly larva matured which wouldn't make sense. I disected the caterpillar and there wasn't anything to see and the caterpillar was perfect but deflated and turned a rust color within a short time. Am I on the right track?