I have been a fan of the cocoons of ichneumon wasps in the subfamily Campopleginae ever since I saw my first one five years ago, stuck to a chain-link fence along the Winooski River in Burlington, Vermont. They come in a variety of shapes and patterns, and to me they look like tiny Easter eggs (some say the white splotches that are common on these cocoons are meant to mimic bird droppings). Some dangle from threads, and some are able to jump around like Mexican jumping beans. The type I most commonly see is attached to pine and hemlock twigs or needles, so presumably the larva that makes it is a parasitoid of some caterpillar that feeds on conifers.
I collected the above cocoon last January, thinking maybe I could start to learn to identify the different genera of campoplegines by the cocoons they make. In April, an adult ichneumon emerged:
Unfortunately, it was dead when I found it, but at least I had a specimen I could get identified… except when I posted photos of it to BugGuide.net, Bob Carlson informed me that this was a species of Bathythrix, belonging to another subfamily (Cryptinae, which also includes the wingless Gelis that emerged from a spider egg sac I collected): it had parasitized the campoplegine parasitoid that had made the cocoon. Bob did suggest that B. triangularis has been recorded as parasitizing Phobocampe geometrae, and that this cocoon could have been made by P. geometrae. Checking the 1979 Catalog of Hymenoptera (for which Bob was the author of the Ichneumonidae section), I see that B. triangularis is not host-specific but has been recorded from another campoplegine (Hyposoter sp.), two braconids (which are also caterpillar parasitoids), and Diprion similis (Diprionidae), the introduced pine sawfly. So perhaps it is specific to Hymenoptera that have exposed cocoons, as these all do–that would make sense if females oviposit after the cocoons are formed. It’s also possible that they oviposit in caterpillars in which they detect a parasitoid wasp larva. If so, it might be that the sawfly record involved a sawfly larva (which is caterpillarlike) that had already been parasitized by another ichneumonid. I’m not sure how one would determine whether a wasp emerging from a sawfly cocoon had parasitized the sawfly larva or another wasp that had parasitized the sawfly larva. Ah, but there’s a note in the 1979 Catalog under Bathythrix that “Members of this genus oviposit into cocoons of various insects, often those of Braconidae or other Ichneumonidae,” so my first thought was right (though it still seems possible that when ovipositing in sawfly cocoons the females are really going after sawfly parasitoids).
As for the identity of the parasitized caterpillar: Taking Bob Carlson’s suggestion that the cocoon belonged to Phobocampe geometrae, it appears that this wasp is specific to geometrid moth larvae (i.e., inchworms). A number of inchworms feed on white pine, but of these only Caripeta divisata (the gray spruce looper) has been recorded as a host for P. geometrae. This species feeds on hemlock too, so it could well be the host for the wasps whose cocoons I find on pine and hemlock.