Since we have ten fingers and ten toes, I suppose it’s worth mentioning that it was ten years ago today that Tracks & Sign of Insects and Other Invertebrates was first published. People are often surprised when I tell them that Noah and I didn’t know much about the subject matter when we started writing it, and that it didn’t take years and years to finish. We set out to write it precisely because we didn’t know: we wanted a book that would solve those nagging natural history mysteries, like what are those shiny red discs we always find on the undersides of rocks, and what makes those feathery patterns in the algal film on birch bark, and what’s the deal with the flattened, dead flies we keep seeing stuck to the undersides of leaves?
Mark Elbroch had suggested we give ourselves a year to put this book together, but given the vastness of the topic and how little we knew about it, I decided on a year and a half. In the fall of 2007 I began scouring the literature in earnest; throughout 2008 we took thousands of photos, traveling throughout the US to photograph things that couldn’t be found at home in New England; and the next winter was spent compiling and writing. I ended up getting a one-month extension on the deadline for submitting the manuscript—I think from March 1 to April 1, 2009—and even so, I remember we were still working on finishing the introduction the day before the new deadline (including Noah running out into the woods behind his house to take the pictures of white pine trees that appear on pages vi and ix).
It was a good thing we had a firm deadline, because we could easily have spent the rest of our lives adding and revising. As it was, I did keep slipping new tidbits in for a while longer until the editor cut me off. I still remember the last thing I tried to slip in that didn’t make the cut; it was my discovery that a beetle named Chalepus walshii was responsible for those distinctive white rectangles I kept seeing on Canada bluejoint and other marsh grasses while conducting botanical fieldwork:
The answers to other riddles have remained elusive. In May 2011, I wrote here about a Bucculatrix cocoon Noah and I found in Tennessee that was curiously marked with little figure-eights.
As you can read in that post, I speculated that these markings might have been the work of a parasitoid wasp. This seems far-fetched to me, but I still don’t have a better explanation. Annette Braun never mentioned any such markings in her 1963 revision of the genus Bucculatrix, and I have never found another cocoon like this among the hundreds or thousands of Bucculatrix cocoons I’ve encountered over the past decade.
Or rather, I had never until October 10 of last year, when I went for a run down the road from my house and stopped to inspect a heart-leaved aster (Symphyotrichum cordifolium). John van der Linden had told me about a fly he found in Iowa that sneakily mines the petioles of heart-leaved asters, hardly leaving any discernible evidence besides its puparium; I wanted to see if I could find it in Massachusetts and rear adults to learn its identity. Curled in one of the petioles of the aster I stopped to examine was a Bucculatrix cocoon covered with little “8”s.
Were these “8”s inscribed in the surface or just a result of the Bucculatrix larva using two different colors of silk to spin its cocoon? I couldn’t tell for sure—the latter seemed almost plausible with this new cocoon, but looking back at the one from Tennessee, the “8”s are just too rounded and irregular; I think it has to have been done from outside. Naturally I collected the cocoon to see what would emerge.
Two weeks later I was doing some fieldwork in Northampton and I spotted two more cocoons decorated with little “8”s, this time attached to pine needles.
The same sprig of pine needles also had a similar cocoon without the “8”s:
Both the aster and the pine were directly below red oak trees, and I believe oak was the actual larval host of the Bucculatrix larvae that spun these cocoons. Many species of Bucculatrix start out life mining in oak leaves, then complete their development feeding on the surface of these leaves. When mature, they drop down on strands of silk and may wander some distance before finding a suitable place to spin a cocoon. There is a Bucculatrix cocoon just outside my second-floor bathroom window, 100 feet from any tree. Maybe it’s one of the aster-feeding species and didn’t have to walk that far, or maybe it blew there on a very windy day. The point is, one shouldn’t assume that the plant a cocoon is found on is the caterpillar’s actual food plant; my hope would be that the moth that emerges from the cocoon would be a species whose larval host is already known. If the “8”s indicate parasitism, though, this will make it very tricky to learn the moth’s exact identity—unless a moth emerges from that one unmarked cocoon and can be assumed to be the same species as the others.
Well, a few days ago this wingless wasp emerged from the cocoon on the aster petiole:
I believe it is an ichneumonid in the genus Gelis. Whereas many parasitoid wasps that emerge from their hosts’ cocoons arise from eggs that are laid when the host is still a larva or even an egg, Gelis species oviposit in their hosts’ cocoons. So I suppose if it’s really a wasp scribbling little “8”s on these Bucculatrix cocoons, Gelis would be a good candidate for the culprit. If the two cocoons on the pine needles have the same fate, that will be good supporting evidence. I’m still open to other explanations, but have none to offer at the moment.
This is the (now somewhat moldy) cocoon with the exit hole chewed by the emerging adult wasp. The “8”s are fainter now but still discernible.