In my ongoing survey of gallmakers and leafminers on the island of Nantucket, I have been trying where possible to verify the identities of these insects by actually rearing them to adults. And so in May of 2012, I collected a bunch of black and scrub oak leaves (Quercus velutina and Q. ilicifolia) with midge galls that I was pretty sure were caused by Macrodiplosis niveipila based on Ray Gagné’s 1989 book, The Plant-feeding Gall Midges of North America. They consisted of swellings along veins, with fuzz-lined longitudinal slits on the upper leaf surface to allow the mature larvae to exit.
Within a few days, the little white larvae came squirming out of the galls, and when I moved them to jars of soil they quickly burrowed down.
The following April, several adults emerged (about 3 mm long).
I sent them to Ray Gagné (along with assorted other gall midges), and he had this to say about them:
The Macrodiplosis females you reared from oaks cannot be identified further. What further identification would require is an exclusive and at least three-year project collecting galls from all the oaks east of the Mississippi, retaining some larvae and rearing adults from each kind of gall and host. Basing Macrodiplosis identifications on anything less than that would just be guesswork.
So we’ll stick with “Macrodiplosis sp.” for now. But some other things emerged from the same jars of soil, within a few days of the midges, and I had better luck getting them identified, although it took me a while to find someone who could do it. Two of them were platygastrid wasps, which I sent to Peter Neerup Buhl in Denmark.
This one, about 2 mm long, is Metaclisis floridana. It has never been reared before, so there’s no telling how host-specific it might be.
This one, about 1 mm long, is a Synopeas species. Because I just had this one male specimen, Peter was unable to place it to species with confidence, but it might be S. pubescens, which likewise has never been reared.
As far as is known, all platygastrids in the subfamily Platygastrinae are parasitoids of gall midges, so it was not surprising to find these tiny wasps coming out of the jars. It was surprising when this zippy little scuttle fly (Phoridae) appeared (about 2 mm long):
In my ignorance, I had thought of scuttle flies as mostly generalist scavengers, but it turns out the larvae of this genus (Megaselia) have extremely diverse habits: in addition to scavengers, there are specialized predators, parasites, and even herbivores. Two have previously been reared from galls: M. submarginalis from the boxelder gall midge Contarinia negundinis, and M. chainensis from the elm cockscomb gall aphid Colopha ulmicola*. In both of those cases the Megaselia larvae are specialized predators, devouring several aphids or midge larvae before completing development.
I sent the fly off to scuttle fly specialist Brian Brown at the Los Angeles Natural History Museum, and he reported that it didn’t key out to any of the described North American species. A year later, he informed me that Emily Hartop, who has been focusing on the genus Megaselia, had taken it through every other key in the world and determined that it is a new species. So Emily and I named it Megaselia nantucketensis, in a paper that was theoretically published in October but just today showed up online**.
There are plenty of Megaselia species left to be found. Earlier this year, Emily described 30 new species that were caught at 30 sites right in Los Angeles, inspiring articles in the New Yorker, the Washington Post, and elsewhere. You can read Emily’s own account of their discovery here. What makes M. nantucketensis unusual among these discoveries is that we have some idea of what it does for a living, although there is certainly more left to learn there too.
* Robinson, W. H. and B. V. Brown. 1993. Life history and immature stages of two species of Megaselia (Diptera: Phoridae) predatory on gall-inhabiting insects. Proceedings of the Entomological Society of Washington 95(3):404–411.
** Eiseman, Charles S. and Emily A. Hartop. 2015. A new species of Megaselia Rondani (Diptera: Phoridae) reared from a Macrodiplosis Kieffer (Diptera: Cecidomyiidae) gall on black oak. Proceedings of the Entomological Society of Washington 117(4):463-466.