First, a quick update on the hackberry galls: Mike Palmer has found a bunch of them in Oklahoma, and in fact they may already all have been abandoned there. Some of them have holes near the base, and others have the beginnings of holes in the same place, clearly made from the outside, as if something was trying to get in but gave up. We’re hanging onto them just in case there are some agromyzid larvae left inside, but in any case, I now have a picture of one to reinforce the search image for those of you who are keeping an eye out for them:
The withering leaves beyond the galls may be the easiest way to spot them. For anyone who finds some to collect, I would recommend cutting the twigs several inches below the gall and putting them in a resealable plastic bag right away to retain moisture. The larvae seem to develop pretty quickly, so I would say go ahead and collect any you find, rather than waiting to make sure they are mature.
Here is the underside of the same gall, showing the presumed larval exit hole:
Anyway, the third and final(?) thing I’d like you to keep an eye out for this spring is something that has been bugging me ever since I got going on this leafminer business five years ago. During my first visit to Nantucket in September 2011, I got a ride over to the little island of Tuckernuck to spend a few hours looking for insect signs there. One of the leaf mines I found there was this contorted, narrow, linear one on arrowwood (Viburnum dentatum):
Over the following winter, as I began reviewing all the published information on North American leafminers, it became clear that this wasn’t a mine that anybody knew about. So when I returned to Nantucket in early August of 2012, I made a point of looking for more examples, and I found them to be abundant there. I noticed that they always ended up at the midrib near the base of the leaf, and on close inspection I could see that the mines continued down the petiole.
As you can see in the above photo, the mine continues for a short distance in the petiole and then goes deeper in the plant tissue, where it can’t be followed anymore. Some mines were visible for longer than others, and after examining several it was clear that this species follows the petiole down into the twig, where it continues to mine deep in the bark. So I cut several sprigs of arrowwood as described above for the hackberry galls—several inches below the mined leaf, with the hope that the mining larvae were still inside. Over the next few weeks, I found three different types of larvae in the bag. There were these yellowish ones, about 1.35 mm long…
…these whitish ones, which were even smaller…
…and a single one like this, about 1.5 mm long:
Eventually I realized that the yellowish ones were larvae of Sackenomyia commota (Cecidomyiidae), the midge responsible for these blisterlike galls on arrowwood leaves:
The whitish ones may have been larvae of another, inquiline midge species, or possibly of a parasitoid wasp. The third larva, however, bore a striking resemblance to a Marmara larva I’d seen illustrated in a paper. Marmara is a genus of mostly bark-mining moths in the family Gracillariidae, subfamily Gracillariinae. Since I knew Dave Wagner and Don Davis are working on this group, I asked them for their opinion. They both agreed that it looked like Marmara, but neither of them was aware of a Marmara on Viburnum—the only gracillariid recorded from that host is Phyllonorycter viburnella, which is said to be rare even where arrowwood is common, but like many normally rare things, it is common on Nantucket:
About the Marmara, Dave commented: “If the Viburnum feeder proves to be localized to Nantucket and Tuckernuck Island—a biogeographically bizarre distribution—it would not totally surprise me. Out West there are many Marmara that exhibit such localized distributions.”
The Marmara larva I had photographed was nowhere near mature, and having seen Marmara bark mines on other hosts like ash and pine, I knew they can be very long—it was no wonder the sprigs I had collected hadn’t been big enough for the larvae to complete their development. Marmara species typically have a year-long life cycle that involves overwintering in the bark, then continuing to feed in the spring and transforming to an adult around June.
In June of 2013, I returned to Nantucket and found that there were in fact bark mines visible on some arrowwood plants, though mostly for short stretches; it was clear that most of the feeding trail was too deep to see, and following it to its end would be impossible.
Some Marmara species conveniently spin their cocoons at the ends of their mines, but others exit the mine and wander off before pupating, and there was no way to know what this one did. I searched the plants that had visible bark mines, but I didn’t see any signs of cocoons, so I assumed this was a species that wanders off.
The following month, I was doing fieldwork in southeastern Massachusetts (the same job that brought us Orchestomerus eisemani and the scuttle flies I wrote about a few weeks ago) and I spotted some more of the leaf mines—the first examples I’d found off the islands. I collected some more sprigs, knowing I wouldn’t be able to raise larvae to adults, but hoping that once again one would pop out of the bottom of a twig, and this time I’d preserve it for DNA barcoding, to see if it could be matched with some adult Marmara specimen. None ever materialized, but two of these did:
Recognizing from the horn at the back end that they were some kind of sphinx moth (Sphingidae) caterpillar, and knowing that the host was Viburnum, it was easy to identify them (using Dave Wagner’s guide to eastern caterpillars) as immature hummingbird moths (Hemaris thysbe). I’d had no idea what hummingbird moth caterpillars looked like before then.
On one or two of the Nantucket arrowwood plants, I had seen the Marmara mines going within a few inches of the ground before disappearing again. I mentioned this to Dave, and he thought it was likely that this species overwinters in the bark of the roots. This is not unheard of; the mine of M. basidendroca on green ash (Fraxinus pennsylvanica) extends up to 7 cm below ground before resurfacing for pupation and emergence of the adult.
It looked like drastic measures would be required to learn the identity of this moth. So in the summer of 2014, Julia and I returned to Nantucket and tied pink flagging on several arrowwood plants that had leaf mines. Unfortunately, the mines weren’t nearly as common that year as they had been in 2012. Julia returned in December and dug up and potted the flagged plants. Meanwhile, I had found a few examples of the mines in the Berkshires of western Massachusetts—on wild raisin (Viburnum nudum) and hobblebush (V. lantanoides) as well as on arrowwood—and I had potted a couple of those too. Julia sewed some bags from transparent fabric to surround the potted plants and trap any emerging insects. We kept the plants in our shed all winter.
Last spring, we brought all the bagged plants into our kitchen and checked them several times a day for any signs of life.
On April 2, this 6-mm sawfly emerged—a male, given its fancy antennae. Dave Smith identified it as a species of Monoctenus (Diprionidae), which would have fed as a larva on juniper foliage, then burrowed into the soil among the arrowwood roots to overwinter and pupate.
On April 13, this 2-mm wasp emerged. I thought it might be a parasitoid of the Marmara, but it turned out to be an Encyrtus (Encyrtidae), which must have emerged from a scale insect on the bark of the arrowwood.
Toward the end of April, a moth finally emerged—but it was a tortricid, not a Marmara.
Specifically, this 8-mm moth was Zomaria interruptolineana, whose larva is a leaftier on blueberry and huckleberry, both of which are abundant on Nantucket. Coincidentally, I’d photographed this same moth species on an arrowwood leaf on June 10, 2013, at the same location where Julia dug up that plant.
But alas, no Marmara adults ever emerged.
Last July, on our way to Colorado we stopped at a rest area along I-70 in Illinois. The border of the picnic area was lined with forest, so we walked along it to see what leaf mines we could find. There were a couple of arrowwood plants, and we found a few of the Marmara mines. Then, much to my surprise, we found two Marmara cocoons! The species that pupate at the end of their mines make their cocoons under a little bark flap, like so:
Since this species is sneaky and its mines mostly aren’t visible on the bark surface, it was just dumb luck that we noticed these. Where the bark has been removed in the above photo, you can see a few dark lines, which are the frass trail from the larva’s mine. To give a sense of the scale, as well as a view of the white silken cocoon beneath the bark flap, here’s a shot with my finger holding the bark flap back:
Of course, both cocoons were already empty, so we had no hope of getting adults from them, but now we know there’s no need to dig up the plants to rear them; we just need to search more carefully for these bark flaps in the spring. These were several feet off the ground, so apparently the larva doesn’t always mine all the way to the roots—or if so, it may mine all the way back to the height of its original leaf mine.
A closer look at the cocoon under the bark flap reveals that it is decorated at either end with “frothy bubbles.”
These “bubbles” are expelled through the walls of the cocoon, for unknown reasons, by the larvae of certain genera of Gracillariinae. Interestingly, the Marmara species that spin their cocoons under bark flaps at the end of their mines aren’t supposed to make these bubbles, so it seems pretty clear that this is a new species rather than a described one using a previously undocumented host plant.
Here is a photo of the cocoon of another gracillariid that shows the “bubbles” more clearly:
When backlit, you can see that they are faceted like little jewels:
Anyway, if you have any kind of viburnum near you, please keep an eye out for these bark flaps this spring. Of course, it would be easier to know which plants to look at if you had flagged plants that had leaf mines on them last year, but maybe you’ll luck out as we did in Illinois. For whatever reason, I’ve never found the mines in the central part of Massachusetts where I live, even though I’ve found many examples in the extreme western and eastern parts of the state.
And if you don’t have viburnum near you, I’m sure there are many more Marmara species left to discover. In fact, I’m pretty sure the number left to discover is larger than the number that already have names, based on bark mines I’ve found on host plants from which Marmara adults have never been reared. When the mines are fresh (and when it’s not a sneaky species that mines too deep in the bark to see), the larva can be seen through the bark epidermis, as in this example on Ceanothus sanguineus from Oregon (also not a known host for any barkminer):
Not all bark mines are caused by Marmara species; they can also be the work of other moths such as Zimmermannia, a subgenus of Ectoedemia (Nepticulidae). Or possibly something else. I’m including bark mines in the keys in my leafminer book, because let’s face it, no one is ever going to write a whole book about bark mines.