Wild Ideas: There be tigers in my milkweed   

In early August, as the summer was maturing, the green fields and gardens of Rappahannock County took on the warm tones of summer — mostly yellow and oranges, set off by black and white.

As I was walking my dog past a small patch of butterfly milkweed in one of my gardens at dawn one day in the first week of August, I noticed a mass of mostly black insects crawling around in the patch. The two affected plants had apparently finished blooming for the year, or at least paused their blooming, but I was hoping to get some of their seed pods to plant in other areas.

Unlike its more-aggressive cousin, common milkweed, butterfly milkweed spreads only by seed, not rhizome. I could see the third plant in the group still had some of the bright-orange blooms and no bugs . . . yet. It wasn’t light enough to see which insect species was defoliating the plants, so I decided to come back in a couple of hours, when the light would be better, to take a closer look.

I got busy on a deadline when I got back to my house, totally forgetting about the bugs in the milkweed until late afternoon. At that point, the little patch of milkweed was already in shade but I still hoped to get enough photos to help in identifying the tiny insects. When I arrived at the patch, the insects were eating the last of the foliage on the remaining milkweed plant. They formed a fast-moving eating machine, devouring the foliage and remaining blooms at an amazing speed.

In inspecting the quivering mass, I realized they were caterpillars —hairy ones, with tufts of orange and white hairs sticking out among shorter black ones, like little unkept tigers. In checking my favorite caterpillar guide, Princeton’s “Caterpillars of Eastern North America,” I discovered they were the larvae of a species of tussock moth, Euchaetes egle, commonly known, aptly, as the milkweed tiger moth, or as the milkweed tussock moth.

By Pam Owen
Telling head (the end pointing down) from tail can be a challenge on the milkweed tiger moth caterpillar.

I’d never seen caterpillars feeding in such a frenzy. They were moving so fast and were so small (about a half inch) that I feared I wouldn’t be able to get a shot of them, especially as the light continued to fade. I fired away, trying to make a record of them before they were done. Within a few minutes, most had finished the rest of the milkweed foliage and disappeared among the neighboring thicket of aster and goldenrod. Only a few stragglers were weaving around at the ends of the bare plants looking for more food. They soon gave up and disappeared, too.

The only other milkweed on the property is another small patch of butterfly milkweed in my herb garden, about 10 feet from the patch the caterpillars had just destroyed, I wondered if such tiny little animals would find their way there over a mowed lawn but haven’t seen any sign they have.

Since I live in an area that is mostly forested, I had only a slim hope of attracting monarchs to my milkweed when I planted it. Although milkweeds are the monarch’s sole host, it generally prefers species other than butterfly milkweed, with common milkweed a favorite. I’d tried adding seeds of that species of milkweed to a nearby meadow garden last fall, but none came up this year.

Tussock moth and the monarch larvae may not really compete for milkweed as a host, as the author of the Princeton guide, David L. Wagner notes: “In my experience, the two insects commonly affect different sorts of plants. I associate monarchs with young, vigorously growing shoots, while milkweed tussock moths are content to eat older foliage, sometimes that which has already started to yellow.”

I figured the milkweed patch had at least helped host one lepidopteran species, although the fate of the caterpillars, in two early instars (growth stages), is still not clear. Despite the fact that tussock moths can be voracious, I love to watch them. Their distinctive tufts of hair vary in color from species to species. Although the hair of some can cause an allergic reaction in some people, it always makes me think of Yorkshire terriers. And as with Yorkies, it’s often hard to tell one furry end from the other.

The adult of this tussock moth species, as is often the case, is much less unimpressive than the caterpillar. About an inch and a half long, it has mousy gray wings but does retain a hint of the tiger: an orange body with black marks.

This moth ranges widely throughout the eastern United States and eastern Canada. Its only hosts appear to be milkweed (Asclepias) species and plants in a related genus, dogbane (Apocynum), both of which carry a toxin that a lot of other wildlife can’t tolerate. Although there are more than 100 species of both on the planet, here where I live only there are just the two small patches I planted. This doesn’t bode well for the little gang of caterpillars that demolished my butterfly milkweed. Unless they find my other patch, they’ll likely starve or become prey for the many predators here.

There be tigers in my milkweed slideshow:

© 2017 Pam Owen

On the hunt for caterpillars

A few days after I found the milkweed tiger moth caterpillars,, I visited Robin Williams to look for caterpillars in her butterfly garden and among other host plants she’s planted or conserved on her property in Slate Mills. She’d seen some that sounded interesting, including a saddleback, which is known for its amazing looks but even more for its amazing sting, which comes from chemicals on its barbed hair). However, we only found a some more larvae of the milkweed tiger moth munching away on Robin’s diverse milkweed species, along with a few mystery larvae.

In my research later, I could only ID one of the latter — a black-waved flannel moth caterpillar that, in the early instar we found it, looked like a wispy chunk of hair pulled from Santa Claus’ beard. I originally thought it was a yellow bear caterpillar, the larva of the Virginian tiger moth, whose color can range from light brown or grey to yellow or white. But the wispiness was more typical of the flannel moth in question. The other two larvae appear to be from sawflies.

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Pam Owen
About Pam Owen 278 Articles
Writer, editor, photographer, and passionate nature conservationist living in Rappahannock County, in the Blue Ridge Mountains of Virginia. Two favorite quotes: By E.O. Wilson, who coined the term "biodiversity," "Nature holds the key to our aesthetic, intellectual, cognitive and even spiritual satisfaction”; by Douglas Adams, “I love deadlines. I love the whooshing sound they make as they pass by.”

7 Comments

  1. I have just discovered Tussocks one my one and only milkweed that I planted for the monarchs. I’m afraid at the rate they are devouring the plant, there will not be enough left for the monarch’s to finish feeding on. Is is safe to move the tussocks somewhere else ?

    • Monarchs will lay their eggs on milkweed, and the caterpillars will feed on it in spring and summer. But this time of the year, the monarchs should be finished reproducing and be starting to migrate. I’ve seen my first coming through already. As I mentioned in the column, the monarch caterpillars seem to prefer young milkweed, whereas the tussock moth caterpillars will feed on milkweed later, when it’s tougher. If you have adult monarchs still around from this summer, or passing through on their way south, they will nectar on many plants, and milkweed, which should be past bloom in many locations (including where I live), is not a big attraction for them. Living in the forest, I never saw monarchs until the past couple of years, when I planted a few more flowering native plants. Now one lone monarch will stop to nectar this time of year, but they are just as happy on my nonnative Autumn Joy sedum flowers, one of the few plants to provide nectar here into the fall.

    • In answer to your question, the milkweed tussock moth caterpillars only feed on milkweed, as far as I could discern, so moving them to a nonmilkweed wouldn’t work. I worried that, with so little milkweed in my little gardens, that the caterpillars that demolished the foliage on them would not have sufficient options to continue to develop into adults. They took off as soon as the foliage was gone. Although more appeared on another small patch, they were obviously from a different batch of eggs, although perhaps from the same parent, since the patch is only a few feet from the original one.

      • I went to look at how the milkweed was holding up under the pressure of so many caterpillars and tussock moths only to find all the caterpillars gone and only 2 or 3 tussock moths left. They had so few leaves left…but where did the caterpillars go to overnight? I will be planting more milkweed next year to insure that there is sufficient food for all. Thank you for your quick response.

        • Mine took off, too. I would imagine they’re looking for more milkweed, but there isn’t any more up here, so they may not make it. Hope the birds at least enjoy them. We have yellow-billed cuckoos here, which eat hairy caterpillars.

  2. This is an amazing post. I love the slide show. Thank you for taking the time to care. Tussocks are a mystery to me as well a natural nuisance. I raise my monarchs so it is frustrating to see the plants destroyed. I tend to leave them if the season for them to come to Kentucky is not at peak – but alas, I have been known to move them to another area when they start threatening in the late Spring and Fall. There are enough problems for those little caterpillars to deal with as it is and I have to have decent milkweed to feed them.

    • Thanks for taking the time to read it. By this time of year, if you don’t have monarch caterpillars on the milkweed, you probably won’t, since the monarchs have already started their migration here where I live in Virginia, and I imagine the same is true in Kentucky. They’ll nectar on a wide variety of plants, as you probably know, and the milkweed should be pretty much done flowering, so the milkweed should have already done its job in hosting the Monarch’s larvae. My butterfly milkweed (not particularly favored by monarchs) had the foliage stripped by the tussock caterpillars, but the plant then started to come back, until my someone trimming on the property whacked it to the ground. Some other milkweeds, including the common milkweed, or preferred by the monarchs, and those species spread by rhizome, so any damage at this point in the year should not affect their ability to host monarchs next spring and summer.

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