I have a pretty much endless mental list of natural history mysteries that I would like to follow up on at some point, but there are several that, for various reasons, are in the front of my mind right now. A few of these are things that should be looked for in the spring and that I think it’s unlikely I will have a chance to look for this year. So I thought I would put them out there, one at a time, for those of you who like to have a mission when you go out exploring. As an extra incentive, if anyone is able to find larvae or pupae of any of the insects I have in mind, I promise to drop whatever I’m doing at the moment and make a page on this website detailing my methods for rearing leafminers, gallmakers, and other insects.
First up is Cecidomyiaceltis deserta, a fly that causes twig galls on hackberry (Celtis occidentalis). There are some photos of the galls here. If we know the fly’s name and what it does for a living, you ask, what is the mystery? Well, C. deserta was described in 1897 by William H. Patton, an amateur entomologist in Connecticut, in a short paper in which he proposed a system for naming galls whose causers were unknown*. The system involved combining the name of the insect group with the name of the host plant, and so from Cecidomyiidae (gall midges) and Celtis (hackberry) he coined the genus name Cecidomyia-celtis. The description of C. deserta consists of just two sentences:
Galls are hollow, elongate swellings of young twigs, from which emerge, about the first of June, single Cecidomyian flies from a perforation near the base. Length of gall one half inch to one inch.
That sort of thing doesn’t fly anymore, but according to the International Code of Zoological Nomenclature, species names based on the work of an animal are valid if they were proposed before 1931. In the same 1897 paper, Patton named five other hackberry galls that had been described, but not named, in a previous publication. He placed all of these in the genus Cecidomyiaceltis.
All six of the species names Patton proposed for hackberry gallmakers are still considered valid. The problem is that in the spring of 1982, Ray Gagné determined that Cecidomyiaceltis deserta is actually an agromyzid fly (agromyzids are commonly known as “leafminer flies,” but some have other habits including gallmaking). He noticed the galls forming in the Washington, DC area in the second week of April, and the larvae exited at the bases of the swollen twigs in the first week of May. Unfortunately he was unable to rear adults, but the larvae formed puparia, and agromyzid puparia are easily distinguished from the naked pupae of gall midges. Since Patton apparently did not preserve any insect specimens associated with his galls, we can only speculate as to whether he saw the agromyzid larvae emerging and mistook them for gall midge larvae, or possibly only saw the exit holes and assumed that “Cecidomyian flies” had emerged. Because C. deserta was the first described species of Cecidomyiaceltis, it is the type species of that genus, and Gagné had to create a new genus, Celticecis, for Patton’s other five species, which really were gall midges. As I discussed in this post, there are now 21 recognized species in Celticecis***. Two of Patton’s species, C. spiniformis and C. capsularis, are featured in that post.
The upshot of all this is that someone needs to rear adults of Cecidomyiaceltis deserta to find out what they look like, and then we can determine what genus they actually belong to. Maybe they will turn out to be related to this fly, which I believe to be Agromyza varifrons—it emerged yesterday from a hackberry leaf mine I found at Marcie and Mike O’Connor’s place in Wisconsin last July:
There isn’t any native hackberry growing anywhere near me, which is why I’m putting out this call for help. In addition to the east coast records mentioned above, these galls have also been found in Kansas (where they were believed to be the work of some unknown moth), so it is probable that these galls could be found anywhere that hackberry is common. Who’s up for the challenge?
* Patton, W.H. 1897. A principle to observe in naming galls: two new gall-making Diptera. The Canadian Entomologist 29: 247–248. [online here]
** Gagné, R. J. 1983. Celticecis (Diptera: Cecidomyiidae), a new genus for gall makers on hackberries, Celtis spp. (Ulmaceae). Proceedings of the Entomological Society of Washington 85: 435–438 [online here].
*** Gagné, R. J. and J. C. Moser. 2013. The North American gall midges (Diptera: Cecidomyiidae) of hackberries (Cannabaceae: Celtis spp.). Memoirs of the American Entomological Society 49: 1–103. [online here].