the thing that came from the tree

June 28, 2007

in Field work,Insects

There are pros and cons to being the coordinator for a big field project. The upside is that I get to spend a lot of time in the field myself. The downside is that it is often in places that volunteers don’t want to cover. Sometimes it turns out to be interesting; it’s how I ended up exploring Detroit’s urban prairie. This week I got stuck doing areas in some of Detroit’s older blue-collar suburbs, miles and miles of nothing but endless residential streets of small homes, bordered by commercial streets of stores and strip malls. Actually, the urban prairie areas in the city have more habitat.

I previewed my destination on Google Earth, and therefore sought out a lone green spot in a sea of homes: a triangle of trees wedged up hard against the interstate. It was a park, with a little picnic shelter, an expanse of unkept lawn, and a little woodlot. I picked up a few decent birds, such as Cooper’s Hawk, Red-eyed Vireo, and Indigo Bunting, although I could barely hear myself think with the freeway noise. Soon, I was more interested in the insect life.

All around the trunks of the oaks, dead ashes, and maples in the lawn area were dozens of moths. I realized what they were before I netted one: gypsy moths. Most seemed to be males. I wandered about a bit, looking for females, larvae, or egg cases.  At the base of one living oak, I noticed a lot of nickel-sized holes (there are over two dozen in the photo).

Gypsy moth larvae are defoliators, so they had nothing to do with the holes in the tree. The emerald ash borers that killed the ashes are minute creatures that create exit holes barely wider than a pencil lead. The thick bark around many of the holes was flaked away, perhaps indicating an exit of some force. I leaned in for a closer look.

Carpenter_moth_holeIn the nearest hole, I could see something sticking out.  Brown
and papery, it appeared to be an empty pupae. I grabbed a corner and began extracting it, going slowly and carefully so it wouldn’t tear.

I was astonished that it just seemed to keep coming and coming. Finally, I was holding a pretty revolting-looking pupae a whopping 6 cm long. It was molded mostly in the shape of an abdomen, which looked like a moth abdomen —  it reminded me of a sphinx moth. The head area was split open but did not resemble a moth pupae much to me, as the wing impressions were absent (or so reduced that I could not see them without my reading glasses!). I extracted another one and explored a little more before I left. I found not other trees with holes, or any moths or other insects large enough to be responsible for this damage or to be former tenants of the pupae.

Carpenter_moth_empty_pupaePart of what I enjoy about photographing insects is the challenge of identifying them. Frankly, I had never heard of a humungous wood-boring moth. Impatient to know what I’d found, I called my buddy Mark, the coolest entomologist ever. Although his specialty is wasps and dragonflies, he was only stumped for a minute, and spewed out a Latin name which meant nothing to me. We both reached for a reference, which confirmed his guess. I’d found evidence of Prionoxystus robiniae, the Carpenterworm Moth (Hodges#2693).

These moths look a bit like sphinx moths, but are unrelated. Instead, they are in the same family which brings us the worm found in the bottom of a bottle of mescal or tequila. The larvae of the Carpenterworm Moth take three years to fully develop, apparently smell like goats, and can sometimes be seen peeping out of their holes in summer when they are getting ready to undergo metamorphosis. I think, given the photo I found of a mature larvae, that would have probably grossed me out a bit.  I would have been hard pressed to distinguish this from a big, disgusting beetle grub.

CarpenterwormlarvaI’ve tramped through a lot of woodlots and tend to be quite observant, yet this is the first time I’ve encountered this insect, or the remains of one. Although this area is full of older homes (1940s to 1960s), there really aren’t a lot of mature trees, except in this woodlot. It must have one of the only concentrations of food for the gypsy moths and the Prionoxystus for miles. 

I’ll try to do a follow up visit in a week or so.

Larva photo by James Solomon, USDA Forest Service,


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