On pages 219-220 of Tracks & Sign of Insects, I described the cocoon of Neurobathra strigifinitella (Gracillariidae) according to the account of Heinrich and DeGryse (1915)*. I then included a photo of a mysterious cocoon found on the underside of a sugar maple leaf in Nashville, Tennessee, which seemed to be somewhat similar but lacked the pearl-like globules that decorate the cocoon of N. strigifinitella.
Well, last weekend I collected some N. strigifinitella larvae mining oak leaves on Nantucket, and last night I got to see my first cocoon of this species. It is now clear that the mystery cocoon is nothing like it, nor is it like the cocoon of any other gracillariid moth I have reared.
This one is less than 8 mm long, not 14 mm as stated by Heinrich and DeGryse. Noah and I found four examples of the mystery cocoons on that sugar maple, ranging from 8 to 12 mm across.
The example below was overlapping a Phyllonorycter (Gracillariidae) leaf mine and had a thrips running around inside its outer wall. Neither had anything to do with the maker of the cocoon. Curiously, there doesn’t seem to be any entrance hole in the cocoon, and it seems like the thrips may have gotten trapped inside it after chewing its way out of the leaf mine.
I haven’t found any of these cocoons since that day six years ago (July 4, 2008), but I sure would like to know what they are. It may or may not have been significant that they were on sugar maple leaves. Unfortunately the larvae aren’t visible enough in the photos to even say to what order they belong. I’m starting to wonder whether they might be some obscure type of neuropteran (lacewing or something similar), since those tend to make some variety of double cocoon (e.g this dustywing).
* Heinrich, Carl and J. J. DeGryse. 1915. On Acrocercops strigifinitella Clemens. Proceedings of the Entomological Society of Washington 17:6-23.