When Noah Charney and I give a presentation about invertebrate tracks and sign, we like to spend a little time staring at the outside walls of the venue and see what we can find there. The wall at yesterday’s talk for GoodWorks in Leipers Fork, Tennessee had a lot to offer: in addition to green lacewing and stink bug eggs, which we find attached to walls regularly, we found the first mantidfly eggs we’ve seen in person; there were a number of carpenter bee holes, some of which had bees peeking out of them, others of which had what we believe were bee fly eggs laid around their entrances, and one of which had been filled with a mason wasp nest, into which a torymid wasp was preparing to lay her eggs. After the talk, we were showing these and other curiosities to a couple of the kids that attended our talk, and Noah spotted a blade of grass hanging from the alarm box, which we opened to find a grass-carrying wasp nest, complete with cocoons and the remains of the tree cricket provisions, as well as a big cluster of spider wasp nests. Then Noah called me over to take a closer look at a tiny object that was stuck to one of the pillars. We had both come across it in our explorations before the talk, but for whatever reason hadn’t felt motivated to examine it closely until now:
We both recognized this as the cocoon of a Bucculatrix species. Moths in this genus are sometimes called “ribbed cocoon-maker moths” because of the distinctive pupation cocoons spun by the larvae. However, every one of these we’ve seen before–and we see dozens any time we go for a walk–has been plain-looking, not covered with little figure-eights like this one.
The little figure-eights sent my mind back to something I had heard earlier in the week, somewhere between Vermont and Kentucky. I had been drifting in and out of consciousness while Noah was driving and listening to Annie Dillard’s Pilgrim at Tinker Creek on tape, and at one point I had awoken to hear this passage:
Some species of wasps are so “practiced” as parasites that the female will etch a figure-eight design on the egg of another insect in which she has just laid her egg, and other wasps will avoid ovipositing on those marked, already parasitized eggs.
Of course, this was a cocoon rather than an egg, but as she goes on to say:
There are over one hundred thousand species of parasitic wasps, so that, although many life histories are known, many others are still mysterious. British entomologist R. R. Askew says, “The field is wide open, the prospect inviting.”
She doesn’t mention what kind of wasps she is referring to, but I believe they are platygastrids, based on what I wrote on the last page of my chapter on eggs and egg cases:
Several platygastrids have been observed to mark each parasitized egg by using their ovipositors to scratch circles around the point of insertion, as a warning to others that the egg is already taken. Females in this family may stay with the parasitized eggs until their offspring emerge as adults.
As an aside, I happen to have found a platygastrid guarding a cluster of predatory stink bug eggs in Nashville the day before yesterday. I didn’t see any markings on the eggs, and frankly I never imagined this was something anybody would be able to see; I just included that tidbit because I thought it was interesting.
Anyway, the last sentence I quoted from Annie Dillard suggests to me that she got her information from the same place I did, R. R. Askew’s 1971 book, Parasitic Insects. I’m curious to see what the original text said that led her to describe figure-eights and me circles. In any case, it seems unlikely that this is the work of a platygastrid, since as far as I can tell these wasps all oviposit in the eggs or young larvae of other insects, not in mature larvae or pupae. But I don’t have a better explanation at this point, so I’m just going to put this out there and hope someone can enlighten me.
[Added 12/29/2012:] Norm Johnson recently wrote to ask permission to use the above photo in a paper he is about to submit on the phylogeny and evolution of the parasitic wasps of the subfamily Telenominae, which includes Trissolcus euschisti, which he told me is the wasp I photographed (“a fairly common species in the U.S., Canada, and extending south into Mexico and Central America”). I took the opportunity to ask for his reaction to this post, and he replied:
I think your description of the egg marking behavior of platygastrids is fairly accurate. I would have a couple of small quibbles. The first is the use of the word “scratch.” It’s probably only being used to evoke the right image in the mind of the reader, but I’d believe that the female wasp is depositing a chemical on the surface of the egg. I think (but it’s been a long time since I actually read it in the literature) that you can remove this mark (with water?) and then the next females in act as if the egg were not already parasitized.
Also, from an evolutionary point of view the story as usually told doesn’t make sense to me. Again my hazy recollection is that in experiments in which a second wasp came in and parasitized an egg that had already been hit, then it was the second wasp that succeeded in reproducing and the offspring of the first was killed. Kind of makes sense if in the act of oviposition the wasp is injecting a venom that disrupts the host embryo. So if the second wasp will displace the first, then why should she avoid ovipositing? My guess is that the egg marking is not done for the benefit of subsequent females, but simply helps the first one from putting a second egg (or dose of venom) into an egg that she’s already parasitized. The second one that comes along and avoids the marked eggs is simply reacting the same way she would if she had marked it herself.
Plenty of speculation: darn few facts. Sorry, but the figure 8’s are as big a mystery to me as you.