Marsh Marigold Menagerie

In the spring of 2013, I wrote about the four-toed salamander surveys I was conducting at the time, which involved crawling around in swamps all over northwestern Massachusetts. On June 1, the last day of the survey, Julia tagged along, and one of the swamps we visited had a large population of marsh marigold (Ranunculaceae: Caltha palustris). The plants were probably all fruiting by that point, but this is what they would have looked like a month or so earlier:

We noticed that many of the plants had brown markings that appeared to be leaf mines, radiating from the point where the petiole attached.

 A little investigating revealed that the brown streaks continued down the petiole, and in some cases into the stem.

Julia split open a few of the petioles and found elongate, black-headed larvae feeding inside.

I recognized them as being some sort of “lower” fly (Nematocera), which among the leafminers I knew of would indicate some sort of midge (Chironomidae). No other leaf-mining fly larvae have well-defined head capsules like this.

We stuffed a bunch of the mined leaves and petioles into a bag to see what would come out. The first adult appeared six days later.

It was not a midge, but some sort of dark-winged fungus gnat (Sciaridae). I hadn’t seen any references to leaf-mining sciarids in North America, but a little online search revealed that a few are known in Europe. As luck would have it, no one in the US studies sciarids, but I found someone who was willing to take a crack at identifying my specimens, so I sent them off once they had finished emerging. In the meantime, a number of other insects had appeared in the bag.

In addition to the thirteen sciarids, there were eight of these frit flies (Chloropidae)…

…twelve of these scuttle flies (Phoridae)…

…three of these weird little ichneumon wasps…

…and this fruit fly (Drosophilidae).

All of these insects had apparently emerged from the tunnels excavated by the sciarid larvae in the plant tissue. Their larvae were parasitoids or predators of the sciarid larvae, or else they were scavengers, feeding on the dead plant tissue or possibly the waste products of the sciarid larvae. There had also been some beetle larvae feeding on the surface of the leaves:

Curious to see what they turned out to be, I hadn’t picked them off when we were collecting the mined leaves. After two weeks, they crawled away from the leaves and pupated…

…and I ended up with five of these adult beetles:

The beetles proved to be the easiest to identify. They were Prasocuris boreella (Chrysomelidae), which had been collected once before from marsh marigold, but apparently no one had ever found the larvae and reared them before.

I sent the other insects to various specialists, but I hadn’t heard back from anyone yet by the next spring, and the person with the sciarids didn’t respond when I inquired about them. So when I encountered more of the mines while doing wetland plant work in June 2014, I collected them to see if I could rear some more. I collected marsh marigold samples from two sites, and succeeded in rearing another seven or eight sciarids from each. I also ended up with another fruit fly, eleven more frit flies (five from one site and six from the other), and this braconid wasp:

I asked Ray Gagné, the gall midge specialist, if he knew of anyone who studies dark-winged fungus gnats, and he put me in touch with Frank Menzel, a German, who referred me to Kai Heller, another German, who was happy to help. So off to Germany some of the new flies went, and before long Kai had identified them as Zygoneura calthae, a species that had been described from Finland in 1960 and was not known to occur in North America. The collection data for the type specimen included “Larven und Puppen im Blattstiel von Caltha palustris“: larvae and pupae in the petiole of marsh marigold. Nothing seems to have been published about the species since then.

I wanted to write something up for publication about the discovery of this species in North America, but first I needed to round up identifications of all the associated insects, which took another year or so.

Terry Wheeler and Julia Mlynarek reported back that my three sets of frit flies all represented the same undescribed species of Elachiptera. Julia is working on a revision of this genus, so she will give this fly a name sooner or later. My impression is that it is not a species either Terry or Julia had seen before, but maybe some will turn up in museums by the time the revision is complete. Most Elachiptera species seem to have larvae that feed on decomposing plant tissue damaged by other insects. This one seems to have a special relationship with marsh marigold, if not Zygoneura.

Brian Brown and Emily Hartop identified the scuttle flies as Megaselia limburgensis. Interestingly, this species has been reared in Europe in association with an agromyzid fly that bores in marsh marigold petioles. Hering* stated that “strange mine off-shoots” from the petiole were constructed in the leaf blade, and he wasn’t sure whether these were made by the agromyzid or the scuttle fly. These sound suspiciously like the leaf mines of Zygoneura, but apparently he didn’t find any associated sciarids.

The ichneumon wasps now reside in the British Museum of Natural History, where Gavin Broad identified them to the genus Neurateles. Two described species are known to occur in North America, but they more likely belong to one of several undescribed species. Neurateles is in the subfamily Orthocentrinae, which seems to be mostly made up of fungus gnat parasitoids.

The fruit fly, Rick Lapoint informed me, was Scaptomyza pallida, a very common, cosmopolitan species that feeds on decomposing plant tissue.

Joseph Fortier identified the braconid wasp as a species of Ephedrus (Aphidiinae), indicating that it must have emerged from some unseen, mummified aphid on the plant surface rather than having anything to do with the sciarid mines.

Having collected all of this information, I was now ready to write my paper. This past February, I wrote to Kai Heller with a quick question regarding where Zygoneura calthae had been found besides the original collection site in Finland. When he responded, he mentioned that DNA barcoding had shown my specimens “to be different genetically from the European ones by more than 6%. Additonally they look a bit different. It may be possible, that two different species or subspecies are present.”

I thanked him for this bit of information, and remarked that it agreed with my suspicion that this is a native species rather than a recent introduction (which should closely match a European population). He wrote back and said that he had rechecked the DNA results, and in fact the difference was nearly 10%: unquestionably a new species. So I put off submitting the paper for a little longer, as Kai wrote up a description of the species and Björn Rulik wrote up the DNA analysis. I decided to name it Zygoneura calthella, which seemed fitting for a species that’s pretty much the same as Z. calthae but a little smaller. There are also some subtle differences in the male genitalia between the two.

Our paper** describing Zygoneura calthella, along with a review of insects known to feed on marsh marigold, was published at the beginning of November.

* Hering, Erich Martin. 1951. Biology of the Leaf Miners. Junk, the Hague, Netherlands.

** Eiseman, Charles S., Kai Heller, and Björn Rulik. 2016. A new leaf-mining dark-winged fungus gnat (Diptera: Sciaridae), with notes on other insect associates of marsh marigold (Ranunculaceae: Caltha palustris). Proceedings of the Entomological Society of Washington 118(4):519-532.