Periodically I pause to check the three great, old maples in our front yard for interesting goings-on. I’m not looking for anything particular. An interesting bug, perhaps. Some fungus maybe. Whatever’s happening on the trunks at the time. Sometimes there’s nothing at all that catches my eye. But about a month ago I spotted these. There were three of them, two on the biggest maple, and one on the third. I’ve continued to check on them, from time to time, as I do my maple trunk surveys; they’re still there, and I expect will be for the winter.
It was pretty obvious to me from the outset what they were: those hairy, oval masses couldn’t be anything but moth cocoons. But what was the hard foamy mass on the outside? I had a suspicion, but I took some photos and came inside to confirm.
Sure enough, the answer was on page 71 of the fabulous Tracks & Sign of Insects. The cocoon belonged to a tussock moth. This group of species lays their eggs on the outsides of their cocoons – because the females are flightless, and so don’t travel far.
That answered what. But what about who?
I noticed as I was examining them that inside the hairy cocoons were the hard shells of pupal cases. I couldn’t really tell if there was still a moth in the case or not, so I reluctantly peeled one off the tree to see (I don’t like to disturb things usually, if I can help it). As I flipped it over, I noticed two things. The first was that the case was empty. I hadn’t yet gone in to look up the book at the point that I was examining the cocoon, but if, as I suspected, the white mass was eggs, then it wasn’t a great surprise that the case was empty.
But the other thing that was obvious was the three white spots on the back of the case, remnants of the outfit the individual wore as a caterpillar. This quickly answered the who: this was the cocoon, and egg mass, of a White-marked Tussock Moth (Orgyia leucostigma). Caterpillars of this species are striped lengthwise with yellow and black, bearing three long, dark tufts at the front and rear, and four short, thick white tufts along the back. Very distinctive!
The species overwinters at the egg stage (which makes sense if they were laying them in the fall). Caterpillars are about as generalist as they get. Caterpillars of Eastern North America lists as possible host plants “apple, birch, black locust, cherry, elm, hackberry, hickory, oak, rose, willow…fir, hemlock, larch, spruce and other conifers.” They’re fairly common, so you might well find a cocoon or two if you try checking out some old, ridgy trees.
Here’re a few photos of the species, including one of a female in the act of laying eggs (how cool!).