Jim Monaco of Sag Harbor sent me a photo of a very striking black-white-and-orange-colored caterpillar he found on his backyard milkweed plants. Actually, it wasn’t a singular caterpillar but a dense mass of over forty caterpillars so tightly bunched together that it was a challenge to count them.I recognized the caterpillar in the photos but knew nothing about it. It is the larval stage of the milkweed tussock moth (Euchaetes egle), and not surprisingly shares several characteristics with the more familiar monarch caterpillar. The host plant for egg laying and larval development is one of several species of milkweed (Asclepias spp.) and it stores the toxins (cardiac glycosides) found in milkweed leaves in its tissues as an anti-predator strategy. As with the monarch larvae, its striking color pattern makes it very visible and is a warning sign to potential caterpillar predators: “eat at your own risk.”
The monarch larvae develop into equally striking—and toxic—butterflies. And, as with the monarch caterpillar, butterfly consumers ‘learn’ to avoid the highly visible, diurnal, winged adults only after trying to ingest one. But, I wondered, how does this strategy carry over to a small, very non-descript, gray-colored moth that is active at night?
The anti-predator strategy of developing into a colorful moth to advertise their toxicity wouldn’t work to deter the main predators of adult milkweed tussock moths: bats. Being blind and relying on echolocation to locate prey, bats wouldn’t fall for a color scheme to deter them, and the moths would have to develop a different trick. They did: sound.
They use their tymbal organs to create an ultrasonic click that is detected by bats. As with the blue jay that first tries swallowing a monarch butterfly, the bat that first tries eating the “clicking” moth finds itself biting into a very noxious, distasteful prey that it just can’t swallow.
After mating in early summer, the female lays a cluster of eggs on the undersides of milkweed and dogbane leaves. The tiny larvae are gray-colored during their first few instars, and feed in groups (unlike the monarch which lays a single egg and whose larva are usually solo). In addition to the toxic compounds found in its leaves to deter herbivores, milkweeds have a milky latex “sap” in their leaf veins and plant stems that many browsers find unpalatable. The milkweed tussock larvae avoid the sap by eating around the leaf veins, eventually consuming the entire leaf and leaving a plant with just spikes (leaf midribs) attached to the stem.
Monaco took the photo of the nearly fully-grown caterpillars in early August. By month’s end they left the plants to form a cocoon and overwinter in their pupal stage.