Since I started paying attention to invertebrate tracks and signs, I have become especially interested in leaf mines. A leaf mine is the trail left by an insect larva–it could be a moth, fly, beetle, or sawfly–feeding between the epidermal layers of a leaf. In most cases the larva completes its development within a single leaf, and it may either metamorphose within the leaf or exit to do so underground or attached to some object above ground. Leafminers tend to be very host-specific, feeding on just a particular species or genus of plant. Each species has its own particular style of path that it follows within the leaf, and its own particular way of distributing its droppings. Therefore it is often possible to identify a leaf mine to species based on the host plant and the characteristics of the mine. I think these are fun puzzles to solve, and I am fascinated by the specificity of these insects’ food habits and behaviors.
John Pearson of Iowa, a botanist, has also developed an interest in leaf mines. On a recent trip to New Zealand, he photographed some distinctive zigzagging ones while backpacking though high-elevation beech (Nothofagus) forest in mountains along the Routeburn Track in Fjordland National Park. John sent me the photo below to see if I could figure out what made it. He noted that the plant was Astelia fragrans, a member of the Lily family.
With a little help from Google I was able to figure out that this is the work of Charixena iridoxa, a moth in the family Plutellidae. There are 18 species in this family in North America, and I’m not aware of any that are leafminers. As far as I know the most common species is the diamondback moth (Plutella xylostella), which is introduced from Europe and feeds on various mustards. It makes a distinctive open-mesh cocoon, though not as fancy as the one made by Cyana meyricki. Speaking of which, C. iridoxa was first described in 1916 by Edward Meyrick, for whom I assume C. meyricki was named. Anyway, I haven’t been able to find an image of an adult C. iridoxa, but it is described as “a most beautiful moth, metallic purple-bronze with pale lemon-yellow markings.”*
The mines always begin at the leaf tips, widening toward the base in correspondence with the larva’s growth. I was puzzled by the way the mine is torn open in several places along the leaf margin. Morris Watt’s 1924 description of the mines makes clear what is going on here–these are not like typical leaf mines, which are made in fully formed leaves:
All mining is carried on in the bulb of the plant at or just below the surface of the ground; and as the leaves grow the gallery is stretched and elongated, and mostly loses resemblance to a typical mine, since the extremely thin outer cuticle is torn and in most places lost, excepting in the most recent portion of the gallery. The zigzag formation of the mine is necessitated by the situation of the larva in the bulb, and its extent is dependent on the rate of leaf-growth: during fast growth the successive angles will be large, while slow growth will cause the transverse portions of the gallery to be almost parallel to one another. Occasionally one will find a length of mine fairly straight for an inch or so, parallel to the long axis of the leaf and most usually close against the midrib; the reason for this may be found on careful search of this portion of the gallery—a cast skin adhering to the wall shows that a moult has taken place here, and while the larva was laying up for the purpose the leaf grew sufficiently to allow it later to mine normally parallel to the long axis till again arrested in the bulb and forced once more to zigzag.
He goes on to explain some other unusual features resulting from the mines being made within the bulb:
Frequently in old leaves the mines may be found to terminate abruptly, or several inches may be missing; examination of the plant will reveal the continuation of the mine, or the missing portion, on some other leaf, and further examination will show that both leaves, at the time of the change, had been in close apposition to one another in the bulb, the larva having mined from one into the other, and perhaps later back again. One may find not a mine, but only a very faint and slight impression of one, on the surface of an otherwise sound leaf; this is due to the pressure caused by the larva mining in the leaf next against it while in the bulb. Never more than one larva will be found to be mining in one half of a leaf, but both halves of the same leaf may be mined by separate larvae. In such cases there is, as one would expect, a direct parallelism in the course of the mines.
* Watt, Morris N. 1924. The Leaf-mining Insects of New Zealand: Part 4 — Charixena iridoxa Meyr., Apatetris melanombra Meyr., Philocryptica polypodii Watt (Lepidoptera). Trans. N.Z. Inst. 55:327-340. [Available here.]