That's right, the life cycle of this weird caterpillar gets even weirder. When it nears the end of the line for the caterpillar stage,the caterpillar bores a chamber into solid wood. We probably found this one not too long after it began digging, and at this point its chamber is deep enough to fit half its body in. The hole is being drilled into the untreated pine of a boardwalk railing.
Rachel, whose office is in nearby Fayetteville, went back two days later to see what the tunnel looked like. It was done, and expertly sealed. While the entrance looks to be solidly plugged with wood, the cap is actually a thin veneer of silk produced by the caterpillar. Remember, the moth must somehow escape the tunnel late next spring, and apparently it can punch its way through this silk operculum. The silken cover disguises the pupatorium entrance well, and few if any creatures would probably pay it any mind.
David Wagner, in his ground-breaking book Caterpillars of Eastern North America, notes that Harris's Three-spot caterpillars consume a great many species of common woody plants. Yet the caterpillars/moths seem rare. In spite of years of searching, it wasn't until this year that I finally found this species. Many caterpillar hunters that I know have never seen one. I think the creation of its pupal chamber may be the species' Achilles heel. Spending two nights making the chamber leaves the caterpillar quite vulnerable to predators, and it seems likely that many would be picked off at this stage. However, if the caterpillar successfully completes the burrow and seals itself in, it is probably as safe as a caterpillar can be.