As I go through my Leafminers of North America e-book and update each chapter for the (now nearly complete) second edition, I’ve been putting together a spreadsheet of mystery leaf mines that need further investigation. There are now over 700 rows in that spreadsheet, and new mysteries continue to be added faster than old ones are solved.
Today Julia and I drove 4.5 hours (round trip) to explore a site in western Vermont with the hope of investigating one of these mysteries—a nepticulid moth that so far is known only from a few empty mines scattered across eastern North America. I first became aware of this species several years ago when Erik van Nieukerken told me that, while perusing the pressed leaf mines that are preserved in the Canadian National Collection, he had seen mines of an unknown nepticulid on dogwood leaves from Ontario. Later, while working either on the dogwood key in my book or on my first paper on agromyzid flies with Owen Lonsdale, I discovered that I had photographed one of these mines in western Iowa in September 2012 and had filed it under Phytomyza agromyzina. P. agromyzina is a leafminer of dogwoods that is common across North America as well as in Europe and Asia; here is a typical example on a leaf of alternate dogwood (Cornus alternifolia):
Here is the mystery moth mine from Iowa, found on roughleaf dogwood (C. drummondii):
Both are simple linear mines, but whereas the frass trail of the agromyzid fly alternates from side to side (this is most evident toward the end of the mine), the nepticulid moth deposits its frass in a central line. This is because the fly larva lies on its side while it feeds, periodically rolling over onto the other side, while the moth larva lies on its back or on its belly.
The central frass line is more clearly visible in this fresh (but apparently aborted) mine I photographed on flowering dogwood (C. florida) in southeastern Ohio in early August of 2016:
The third time I found one of these mines was a few months later, in October 2016, at the site in western Vermont we visited today:
This example was on gray/panicled dogwood (C. racemosa). Although the frass pretty well fills the width of the mine and it’s not easy to see that it forms a central line, on close inspection it is made up of closely spaced zigzagging arcs, which are characteristic of Nepticulidae.
And I have never seen another of these mines since, including today. (The trip was still worthwhile, since we got to explore a beautiful place as well as add two species to the list of moths known from Vermont and collect a couple of mines of another mystery moth.) Nor has anyone else, as far as I know. Just to be sure of this, this evening I reviewed the ~100 iNaturalist observations of Phytomyza agromyzina that I hadn’t already looked at. As a result of this exercise (which involved weeding out a number of observations that didn’t show P. agromyina, or even leaf mines or dogwood in some cases), there are now 550 verified observations of P. agromyzina on iNaturalist, and I have one more mystery leaf mine on dogwood to wonder about, thanks to this leaf that Jeff Clark photographed in Virginia last October:
This mine has a central frass line as in the nepticulid, but the silvery-whitish color indicates that it was formed entirely in the leaf’s epidermal cells, as opposed to in the mesophyll. Also, the frass is in a continuous line rather than composed of tiny particles, indicating that the larva was consuming only the liquid contents of the cells. This type of mine is characteristic of moths in the family Gracillariidae, and this is the first I’ve heard of a gracillariid making a long, linear mine in dogwood leaves (there is an unknown Marmara species that makes linear mines in dogwood stems, but that’s another story). This looks very much like a Phyllocnistis mine, but without seeing that it ends in a silken pupal chamber, I can’t be sure that that’s what it is*. Here’s a link to Jeff’s original observation, which includes the full-resolution photo as well as a second photo showing another leaf with the same type of mine.
So if there are any dogwoods near you, I’d greatly appreciate it if you could keep an eye out for either of these mystery mines, and please collect any you find! Even mines with dead larvae inside would be tremendously valuable, since it may be possible to match them to adult specimens through DNA barcoding. And if you find any occupied agromyzid mines, those would be worth collecting for rearing too: I recently reared some adults from these mines that Owen Lonsdale identified as Phytomyza notopleuralis, which probably should be synonymized with P. agromyzina, but more specimens are needed to demonstrate that there is no clear line between the two species. For further details about that, see:
Eiseman, Charles S., Owen Lonsdale, John van der Linden, Tracy S. Feldman, and Michael W. Palmer. 2021. Thirteen new species of Agromyzidae (Diptera) from the United States, with new host and distribution records for 32 additional species. Zootaxa 4931(1): 1–68.
* Edit, 8/21/2022: Natalia Kirichenko just reminded me about this paper, published four years ago, which discusses four species of dogwood-mining Phyllocnistis species in Northeast Asia, describing three of them as new. Could the Virginia species be one of them? All four have publicly available DNA barcodes, so that question would be easy to answer if someone can collect a mine with a larva or pupa inside…