Milkweed Tiger Tussock Moths are a gregarious lot when young. From the time they hatch out of the egg mass until they reach what is called the third instar (the skin shedding episodes that mark the growing career of all caterpillars are called instars) they feed, shed, and poop together. They munch along from milkweed leaf to milkweed leaf as a great hairy mass consisting of several dozen individuals – leaving only the skeletal leaves in their path (see below). Most caterpillars join together like this as a defensive tactic, but such doesn’t appear to be the case here.
There really is no need for these fellows to be defensive. By ingesting the cardiac glycosides in the milkweed, they each become immune to predator attack. They can afford to flaunt their noxious nature just as their monarch relatives do. So, you could say they do it because they can. As one unit they act as a super caterpillar rolling over all that lies before them.
When they get a bit older, and head out on their own, they will flaunt themselves to the world as unedible balls of carpet. The scientific name of this Tussocked Tiger is Euchaetes egle – a name that translates from a combination of Greek and Latin meaning “well bristled shield.”
Their superb individual hairiness, even without their inner toxicity, alone acts as a predator deterrent. They are ornamented with alternating tufts of orange, black, and white hairs with a fairly tight row of bristles along the back (see here). This latter arrangement allies them with other members of the Tussock moth family who all look like bizarre toothbrushes. Personally I think a Milkweed Tiger looks more like one of those legless shaggy dogs that pace at leashes-end across the grounds at one of those fancy dog shows (without the little pink ribbon in the head hair, of course). I would suspect that these little dogs, by the way, would do well if released into the wild since they also are inedible carpet balls. Coyotes would choke on all that hair.
When I encountered this caterpillar feeding cluster they paid absolutely no attention to me as I crouched down to their level. My first few camera shots were executed without a flash and they all smiled appropriately. I used my flash for the final shots, however, and found that half the gang dropped off after the multiple explosions of light. There were but 10 or so left, along with some extremely stunted individuals who had been hiding under the layer of bristles (see below). I had, none-the-less, scattered the herd like a clap of thunder.
Those that abandoned the leaf simply rolled themselves into a defensive ball and plummeted to earth. It is likely that these skittish fellows were already at their third instar and were ready to abandon the security of the pack. As loners they will locate another milkweed, complete their growth, and emerge next spring as prim, lightly furred, gray moths.