Charixena iridoxa (Astelia zig-zag moth)
Superfamily: Yponomeutoidea (Moths)
Family: Plutellidae (Diamond back moths)
Scientific name: Charixena iridoxa
Common name: Astelia zig-zag moth. Astelia leaf miner
This is a native moth seen December and January flying during the day. The caterpillars live in the heart of the astelia nervosa plant. It is described as “a most beautiful moth, metallic purple-bronze with pale lemon-yellow markings,
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.
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.
More details at http://rsnz.natlib.govt.nz/volume/rsnz_55/rsnz_55_00_003280.html
Leaf miner is a term used to describe the larvae of many different species of insect which live in and eat the leaf tissue of plants. The vast majority of leaf-mining insects are moths (Lepidoptera), sawflies (Symphyta) and flies (Diptera), though some beetles and wasps also exhibit this behavior.
Like Woodboring beetles, leaf miners are protected from many predators and plant defenses by feeding within the tissues of the leaves themselves, selectively eating only the layers that have the least amount of cellulose. When attacking Quercus robur (English oak), they also selectively feed on tissues containing lower levels of tannin, a deterrent chemical produced in great abundance by the tree.
The precise pattern formed by the feeding tunnel is very often diagnostic for which kind of insect is responsible, sometimes even to genus level. The mine often contains frass, or droppings, and the pattern of frass deposition, mine shape and host plant identity are useful to determine the species of leaf miner. A few mining insects utilise other parts of a plant, such as the surface of a fruit.
Some patterns of leaf variegation are part of a defence strategy employed by plants to deceive adult leaf miners into thinking that the leaf has already been predated.