Gardening Harmful insects

The Wrong Caterpillar on My Milkweeds

What to do when you plan for a monarch caterpillar
and a stranger shows up in its place?

Question

I let milkweed grow freely in my yard. And I saw several monarch butterflies earlier this summer, so I’m just thrilled with the results. These last few days, I realize that I have lots of small caterpillars eating the leaves of my milkweeds. But they don’t look right. Are these monarch butterfly caterpillars?

And if they aren’t, what should I do? Will they harm the milkweeds?

Even if they are baby monarchs, will these late ones have time to become butterflies?

Martha Lemieux

Answer

No, these are not monarch butterfly caterpillars (Danaus plexippus), but caterpillars of another butterfly, the milkweed tussock moth (Euchaetes egle).

Monarch butterfly caterpillar
This is the real monarch butterfly caterpillar. It’s totally hairless and therefore really quite different from the very fuzzy milkweed tussock moth caterpiller! Photo: Judy Gallagher, Wikimedia Commons

You see, milkweeds (Asplecias spp.) are hosts to more than one species of predatory insect. The monarch butterfly, the insect world’s famous long-distance migrator, gets all the publicity, but there are other butterflies, plus aphids, plant bugs and other insects, that feed on milkweeds. None of them have the star appeal of the big, colorful, well-traveled monarch, though.

At least milkweed tussock moths, unlike monarch butterflies, are not limited to only one genus, milkweeds (Asplepias spp.). They also feed on plants in the related genus, dogbane (Apocynum spp.).

Adults Not That Colorful

The milkweed tussock moth is fairly mundane moth species, found throughout Eastern North America from Mexico to Canada.

Adut milkweed tussock moth.
The adult keeps its brightly-colored abdomen largely hidden under brown wings during the day. Photo: Patrick Coin, Wikimedia Commons

Yes, it has a orange and black abdomen, orange and black and yellow and black being color combinations often used by insects to warn their predators that they are poisonous or otherwise dangerous. (Most insects that feed on milkweeds pick up toxic compounds known as cardiac glycosides from feeding on the poisonous milkweed and become poisonous themselves.)

However, bright coloration is not too useful to a nocturnal moth like the milkweed tussock one. After all, black is essentially invisible in the dark and orange is scarcely any better. And the brown wings it hides its abdomen under have no particular attraction.

However, the tussock moth does warn its main enemy, night-flying bats, of its toxicity. It does so by actually giving off a warning click that apparently says, “You may not be able to see that I am orange and black, but I certainly taste orange and black!

The Color is in The Caterpillars

Third instar milkweed tussock moth caterpillars covered with cofolrful hair.
Tussock moth caterpillars are gregarious, at least in the first instars, and appear in large numbers. Later generations separate and go it on their own. Photo: Judy Gallagher, Flickr

Milkweed tussock caterpillars, being out and about in the daylight, are colorful little critters, covered in tufts of bright orange, black and white hairs, at least in the later instars. And that’s what you’ve been seeing.

Lots of Time

If the monarch butterfly has to hurry along rapidly in the fall to make it to Mexico before winter, the milkweed tussock caterpillars is in no hurry. It doesn’t migrate. In fact, it feeds right through September. Then it pupates underground for the winter. In the spring, with the return of warm weather and renewed milkweed growth, it emerges as a butterfly, ready to lay eggs for the new season of caterpillars. There is only one generation per year in the North, two in the South.

It certainly seems like a simple plan!

What Should You Do?

FIrst instar milkweed tussock moth caterpillars.
The first two instars of milkweed tussock moth caterpillars are still fairly colorless; the other generations are much more spectacular! Photo: Jacy Lucier. Wikimedia Commons

Since the milkweed tussock moth prefers the oldest milkweed shoots and monarch butterflies, the younger ones, they’re not necessarily such serious competitors. Even so, tussock moths are also native insects. They should enjoy the same rights to milkweeds as the colorful monarchs. I mean, isn’t species diversity is an essential part of a healthy ecosystem?

So, I suggest … not doing anything!

The idea of planting milkweeds to help monarch butterflies to recover is in order to help restore an endangered ecosystem and keep monarchs thriving. Milkweeds are a key species and were being removed on a large scale to allow the development of the huge monocultures so typical of modern farms, so offering them a place in our gardens is certainly a help. And when you plant a native plant, let it grow on its own, and then its other natural enemies show up…

Well, what could be more natural than that?

Tussock moths pupa.
Tussock moths pupate underground over the winter. Photo: Polinizador. Wikimedia Commons

The caterpillars will eat some milkweed leaves, as the monarch would have done, but not enough to kill the milkweed plant. Mama milkweed, a perennial, will readily grow back the following year and will be preyed upon by whatever insects come her way. Most of the time, each species will get its fair share of milkweed to feed on and the milkweed plant will produce more than enough green leaves to be able to carry on.

So, just let Mother Nature do her thing. She’s worked on this plan for tens of thousands of years and ought to know what she’s doing!

But I Can’t Stop Myself!

If you can’t stop yourself from coming to the aid of your milkweed plants and removing any tussock moths you see, don’t feel too guilty about it.

The milkweed tussock moth is a common moth throughout its range and is not considered endangered. But just don’t poison the entire environment in trying to remove it! So, don’t succumb to the temptation of using toxic sprays and thus killing all insects, good or bad.

Hand knocking caterpillillars into bucket of soapy water.
If you really can’t stand baby milkweed tussock caterpillars, just knock some of them off into a tub of soapy water.. Ill.: WDRfree & Kindping.com, montage:: Laidbackgardener.blog

All you have to do is to knock or drop the little critters you don’t want into a bucket of soapy water and that will put an end to them. Do wear gloves if you have to handle tussock moth caterpillars, though, as their hairs can be irritating.

Let live and let live!

You’ve read that in this blog before and this certainly won’t be the last time.

Do try to learn that Mother Nature really does know best!

Garden writer and blogger, author of more than 60 gardening books, the laidback gardener, Larry Hodgson, lives and gardens in Quebec City, Canada. The Laidback Gardener blog offers more than 2,500 articles to passionate home gardeners, always with the goal of demystifying gardening and making it easier for even novice gardeners. If you have a gardening question, enter it in Search: the answer is probably already there!

3 comments on “The Wrong Caterpillar on My Milkweeds

  1. Marcia L. Barr

    This was a delight–thank you!

  2. Michael Foster

    Nice read, thx. My garden outlet sold annual milkweed, I thought that was odd. The Monarchs devoured it! I planned milkweed to keep them off everything thing else, like fennel. I need to find a perennial source. I do miss seeing the caterpillars!

    • If you are seeing what looks like a monarch caterpillar on fennel(and dill, parsley etc.) the yellow swallowtail butterfly caterpillar mimics the colour of the monarch caterpillar in later instar stages. Lucky you. Do not move them!!

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