Please enjoy this guest post by the Ancilla College Director of Library Services, Cassaundra Bash. I was very pleased when I learned that Cass was an insect enthusiast, and I’ve learned a lot in conversations with her. She graciously offered to write up a post to share with you all.
Have you ever wandered past a big stand of milkweed, thinking how peaceful and quiet it is, with perhaps only a monarch butterfly or two flitting over it? Ever thought that perhaps the only thing out there munching away like hermits are monarch caterpillars? Monarchs are synonymous with milkweed in most people’s minds, but there is another common caterpillar that relies heavily on the plant. They may not have the stateliness of a monarch, but the milkweed tiger moth, also known as the milkweed tussock moth, makes up for it with another trait we humans find endearing.
Alright, it’s technically hair, and some people can be allergic to the hairs. (I myself seem to be sensitive to it, but only after they shed it to make their cocoons, meaning that handling the crawling caterpillar is safe for me, but I wear gloves handling the cocoons or risk itchy, irritated fingers and palms.) And like any insect that partakes of a milkweed meal, these caterpillars, like monarch caterpillars, have learned to incorporate the milkweed’s poisons into their own defenses. Like monarchs, any animal that eats a bitter-tasting milkweed tussock moth is likely to feel ill afterwards—and they learn to leave these caterpillars alone. And if taste isn’t enough of a deterrent, these caterpillars bear some of the typical insect “danger” colors: black, orange, red, and yellow are all colors that insects use to warn other animals that they are dangerous in one way or another. The black and orange of a monarch adult is so successful that the monarch mimic, the adult viceroy, “borrows” the color scheme and general pattern to fool birds into thinking it’s a monarch. (Viceroy caterpillars mimic bird droppings, as some other caterpillar species do—but that’s a topic for another post.) The black and orange mixed in with the white, so similar to cat lovers that I’ve heard some people call these caterpillars “calico”, is just a continuation of the warning coloration.
Ironically, the moth isn’t all that colorful and, aside from an orange body, the wings are pretty much a soft grey or brown (depending on who you ask and perhaps on color variations in the population—mine tend towards the grey), but then that makes sense—they’re nocturnal, and their main predators are bats, which aren’t known for being sight hunters. But these moths have, as adults, one trick that they’ve developed that works as well for the adults against bats as the coloration warning works against birds when they’re caterpillars. The moths have the ability to create a clicking pattern that the bats can hear and, after eating a distasteful tussock moth adult, will learn as a future warning against eating more of this species.
While monarchs lay their eggs singly, preferring to spread out their offspring across an entire field of milkweed, the milkweed tussock moth lays her eggs in clusters on a few plants. This means that an entire milkweed plant may have dozens of tiny furry caterpillars, all eating and growing together. This may provide some protection from predators and parasitic species that might want to pick out an individual among the crowd; it’s a lot harder to do that when the caterpillars are bunched together, even before they get their warning colors (very young ones are cream-colored, as seen below). Eventually, as they get closer to the time to make their cocoons, they will start to separate and spread out, but by then, they’ll have gotten their distinctive warning colors.
You may be worried about competition between monarchs and milkweed tussock moths, but fear not—neither one is aggressive towards the other and I’ve raised both monarchs and milkweed tussock moths in the same tank. While competition over a single plant means that the small herd of tussock moths will crowd out an individual monarch, both species, as caterpillars, can and will move to a fresh plant if necessary. As long as there is enough food, the caterpillars will feed around each other, and in fact some scientists have pointed out that it seems as if the tussock moth prefers the older, tougher leaves on the bottom while the monarch prefers the tenderer leaves on top. But that’s another reason why it’s so important to have large stands of milkweed; both species are native and deserve the opportunity to flourish and thrive, and without milkweed, it won’t happen for either.
I would be remiss if I didn’t admit that there is a third, less common (at least here in Indiana) moth caterpillar that relies on milkweed: the unexpected cycnia. I kid you not; its name really is unexpected cycnia. Most years, I never see them, but every so often, in late summer or early fall, I’ll find their caterpillars, usually on a special species of milkweed that monarchs and tussock moths use as a last resort because all the other milkweed species have turned yellow and dropped their leaves. This milkweed species is called the whorled milkweed, and it looks like a small, spindly pine tree that saps like a milkweed if damaged. Unlike other milkweed, this species sends out tough, shallow roots like strawberry runners, popping up a stem every so often. And since it grows tall and narrow, I let mine come up anywhere it likes in my pollinator garden, because it doesn’t compete much with any of the other plants for sun or water, and it’s extremely drought-tolerant, which is why monarchs and tussock moths fall back on it in times of need. Like the tussock moth family, the cycnias also have fur, but not so much that it completely covers the orange warning color of their bodies.
In addition to these three species, I’ve also found giant leopard moths and banded woolly bears feeding on milkweed—though not exclusively or even commonly, as they tend to be general feeders of various other native plants including plantain, violets, honeysuckle, dandelions, stinging nettle, and many others. In addition to caterpillars, at least two types of beetles also eat milkweed exclusively, and milkweed is a favorite of the appropriately-named milkweed aphid, which in turn draws ants that “milk” the aphids for the sweet honeydew they produce, ladybugs that eat aphids, spiders, pollinating bees, and parasitic wasps and flies that look for hosts for their young. While a stand of milkweed may look like a calm and lonely spot, it’s really quite the bustling metropolis of the insect world.