Milkweed Tiger Moth Caterpillars
(Euchaetes egle)
(Photo: Merlot - Aug. 18, 2012)
While weed whipping on Sec. 8 last Saturday, I kept seeing these caterpillars on the plants, mostly a single caterpillar, but sometimes two or three. Then I got to this almost totally eaten plant and a larger group of caterpillars. Upon further research, these are the caterpillars of the Milkweed Tiger (or Tussock) Moth. This moth lays its eggs in clusters and the larvae are "gregarious" which is why large groups of caterpillars may be seen on a single plant. They primarily eat milkweed and dogbane (Apocynum cannabinum), a milkweed look-a-like.
The black and orange coloring of the caterpillars is a signal to birds and other predators that they are 'toxic' and eating them will cause illness and 'tufts' also advertise a bad taste. Another caterpillar with black and orange coloring is the Monarch Butterfly caterpillar. Both the Milkweed Tiger Moth and Monarch Butterfly caterpillars feed on milkweed and ingest a substance from the plant called cardiac glycosides* which makes them toxic - a predator may make a mistake and eat one caterpillar, but after getting ill, it likely won't go back for "seconds" at the caterpillar buffet. This toxic affect carries through to the adult phase of the moth and butterfly.
As adults, the Monarch again advertises its toxicity with bright orange and black coloring. The adult Milkweed Tiger Moth has very drab large gray wings although its body is orange with black spots, a specific warning to predators. Unfortunately for the moth, bats can withstand a larger amount of cardiac glycosides and will eat them and at night, are not warned away by the orange and black abdominal coloring. To keep bats away, the moth makes an ultrasonic clicking sound with its tymbal (sound-making) organs.
Whatever plants (milkweed or dogbane) the caterpillars were on Saturday, there was a lot of it along the trail between the Humpback Picnic Area sign (concrete post) and Battery Cliffs. Next summer, if you're hiking this section, keep an eye out for these colorful and interesting caterpillars.
*Cardiac glycosides are found in other plants as well, such as foxglove, common oleander, yellow oleander, and lily of the valley. The Cane (Bufo) Toad also produces cardiac glycosides.
Links for sources of information and more pictures/detail on the Milkweed Tiger Moth:
- Great close-up caterpillar pictures and a good adult moth picture. Detailed & labeled drawing of the caterpillar:
- A blog with pictures of the phases of the larvae, a pupae picture and more general information:
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