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Having cleared away the invasive vines, I could see the shrubs they were climbing on more clearly. Last year’s fruits still clung to the tops of some of the branches, telling me that the shrub was Virginia Sweetspire (Itea virginica), which makes pretty white flowers in late April or early May.
Almost simultaneously, I noticed two things I’d never seen before. The first was a thin whitish line spiraling along a branch of a shrub by the pond’s edge, and the second was a narrow groove on one of the branches of the same shrub.
I knew immediately that the white line indicated a stem-mining moth, which was something new for me. I’d looked for mining insects on Virginia Sweetspire before, and I’d never found anything like this. Just as some insects mine leaves, burrowing under the surface of a leaf and feeding on the soft layer in-between the upper and lower leaf surfaces, some insects mine stems, burrowing just under the surfaces of stems. These stem miners can create squiggly lines along the stem surface that sometimes stand out from the color of the stem. This one is likely made by caterpillars of tiny moths in the genus Marmara. Marmara trails can be long, and are sometimes reddish or whitish, like finely-crafted swirling decorations on green or red branches, even sometimes very thin branches of trees, shrubs, herbs, or vines.
The adult moths may be only 2-3 millimeters long, often with bold patterns of white on a dark brown background, or brownish speckles on whitish background. They are pretty moths, but the patterns are visible only with magnification. It appears that caterpillars of each Marmara species eat only one or a few species of plants, and many species are undescribed or are completely unknown except from trails they leave behind. One example of an adult of an unknown Marmara species seen in the preserve is below—the photo is taken from above. The moth is 2-3 millimeters long and is standing on its hind legs.
When I reported it to my colleague Charley Eiseman through iNaturalist, he indicated that a Marmara on Sweetspire had been reported once before in Louisiana in 2022. Some Marmara species are relatively common. I can find them reliably on their hosts, year after year, in similar locations. Other species are like needles in haystacks, in that I might see them once, then never again. I have a growing list of miners like that. Now I will be on the lookout for this one, and hopefully it’ll end up being more common than it appears to be.
The groove around the Sweetspire stem was more mysterious. A beetle called an Eastern Twig Girdler (Oncideres cingulata) girdles stems of hickory trees, cutting away a ring around the twig, which kills the twig. Then it lays eggs in the dying twig so its larva can feed on the decaying wood. This is not likely to be the same thing, but it isn’t clear what it is. Something else to file away until I see it again, hopefully one day with the culprit in the act…
But I also saw one leaf that looked like it was folded, and a bubble-like structure appeared where the fold was (see the photo below). This is similar to galls made by flies in the genus Contarinia. According to a gall ID website (https://www.gallformers.org/id), a few galls are known on Ironwood, but this is not one of them. So I notified a friend, Louis Nastasi, who studies gall insects, and he confirmed that this could be a new species.
The second discovery was a leaf-mining insect on Hackberry (Celtis laevigata), with tiny larvae burrowing into the leaf and eating the soft tissue in the middle layer. As discussed in post # 5, several leaf mining flies in the genus Agromyza use Hackberry as a host. Most of these are active only in the very early spring between early April and early May. This year, I found one that my colleague, Charley Eiseman has been looking for—a new species from which no adults have yet been raised. Its mines start out at the midvein of the leaves and then work outward to the leaf edge. Then the larva mines around the edge of the leaf (see the photo below). This is the first time I’ve ever seen mines of this species, although it has been seen elsewhere. Although I may not succeed in raising adults from it this year, I have a better idea of what to look for (and when to look for it) next year. This also means it might be two years before we have any adults from this species, if we are lucky, as adults emerge the following year from pupae in the leaf litter made by larvae mining the year before. I am excited that yet another potential new species is here in city preserves.
While looking for more of those Hackberry miners today, I saw that the leaf-roller leaf miner Agromyza torta (see post # 5) is also present at Horseshoe Farm Nature Preserve (see photo below). That adds another site/population for this species as well.
A fourth surprise came in the form of a strange caterpillar on Black Cherry (Prunus serotina). These caterpillars are in a group called the “case-bearers” or Coleophora moths, that make a case for themselves out of silk, sometimes pieces of vegetation, and sometimes insect poop. They wander around inside the cases, and either chew the plant leaves or mine the leaves from the safety of their silk case. The one I found was a leaf chewer, not a miner, and it is likely to be Coleophora sacramenta, a new state record (see photo below).
Spring often feels like a time of rediscovery for me—seeing “old friends” return after some time away (or time in dormancy) and seeing the tumult of life return to the forests and fields. This spring it has been nice to find discovery mixed in with rediscovery. Here’s to new and old friends. Happy Spring!
Marmara mines on the trunk of Wax Myrtle (Morella cerifera)
Almost exactly four years ago, I was working on plant surveys along the coast for the State of North Carolina, when I found these mines on Wax Myrtle in a coastal swamp on the outer banks. I had found mines of many different species of Marmara moths before—their larvae often overwinter in the trunks, then finish feeding in the late winter or early spring. They make a semi-circular cut at the end of their mine, in a thin, flexible layer of bark near the surface (often on younger branches). Then they use silk to make that thin layer buckle, creating a pocket of air underneath called a bark flap (see photo below). Silk can do this because it shortens as it dries, so if they stick their silk to the bark surface, then as the silk shortens, this bends the bark flap. Larvae crawl into the bark flap and spin a cocoon, making strange tiny silk-wrapped spheres called “frothy bubbles” and fixing them on the outside of the cocoon (see photo below). No one knows what purpose these balls serve. Wagner et al. (2000) guessed that they protect the cocoons in some way from parasitoids that can eat insect prey from the inside out (after which their prey dies)—the bubbles create a physical barrier. When the cocoon is finished, the larva pupates inside the cocoon, and the adult emerges a couple of weeks later.
Bark flap on Wax Myrtle--you can see the semi-circular cut in the bark on the right, and the bulge or pocket where the cocoon might be in the center of the branch.
Marmara cocoon on Wax Myrtle. You can see the frothy bubbles on the white silk surface of the cocoon.
Mamara moths are sometimes leaf miners, often stem miners, and are sometimes both. Evidence suggests that many Marmara species are host-specific, meaning that many species probably feed on one or a few plant species only. Although it is necessary to raise an adult to identify species, they are often ridiculously difficult to raise to adulthood. This is because they are inside tree branches or trunks of only certain plants, and are at a stage when they can be collected for only a short time—they die if they are collected too early. Thus, raising adults requires that I find their cocoons. If they pupate in bark flaps, I can cut those bark flaps with cocoons inside them off of the branches with razor blades (see photo below—the bark flap came off of this one but had no cocoon in it yet). If they wander off of their host plants to spin cocoons, I would need to catch larvae before they exit the mines and hope they exit and spin cocoons in the container.
Bark flap cut from Wax Myrtle. The flap itself fell off, leaving the exposed bark below. There was no cocoon in the bark flap, so the larva was likely still in the layers underneath.
I figured I hadn’t much hope of raising the Marmara on Wax Myrtle from coastal plants. I don’t get to the coast very often, so it would be more of a gamble to find them at the right time. However, this coastal native plant has been introduced to, or has colonized, much of the Piedmont in North Carolina as well. In Raleigh and throughout the Piedmont, Wax Myrtle acts as an invasive species, moving into native habitats and taking up spaces that natives had filled previously. I found Wax Myrtles growing at Durant Nature Preserve along our lakes and was happy to find the Marmara mines on several of those plants. I have spent the last 2-3 winters observing these miners, and have found out that they do in fact make bark flaps (whew!). Through trial and error (mostly error), I was able to triangulate their pupation timing.
This year I was ecstatic that two 2-3 mm-long adults emerged from bark flaps I’d collected from Wax Myrtle trees (see photo below). Apparently, leaf-mining Marmara had been found on this host once in Texas, but no stem miners had been documented on this host before (Eiseman et al. 2017). With adults in hand, we now have hope of identifying the species or describing it if it is new. That may still take a while, as it involves dissection, comparison with other known species, writing, drawing, and everything that goes into publication. It still helps to take a long view. We may solve this mystery in the next few years. Other mysteries will take much longer to solve than this one.
