Sunday, August 30, 2015

A strange gall, and an unexpected occupant

A lush snarl of Green-headed Coneflower, Rudbeckia laciniata, colors the Junglebrook Marsh at Malabar Farm in Richland County, Ohio. I was up this way last Friday and Saturday to help with the 7th annual Flora-Quest, which was based out of Mohican State Park. We had a great time, and the event was superbly organized courtesy of Cheryl Harner, Paula Harper, and everyone else who was involved.

Saturday's main activity was field trips at legendary Malabar Farm. I was stationed at a small wetland known as Junglebrook, along with some expert naturalists such as Lisa Rainsong, Judy Semroc, Mark Dilley, and Larry Rosche. Various F-Q groups were shunted our way all day long, and we'd lead them about looking for interesting things.

Junglebrook Marsh may be small, but it is exceptionally diverse. Seeps keep the place wet and boggy, and spawn a rich array of plant life. The wetland may be best known for its population of Turtlehead, Chelone glabra, which host a population of Baltimore Checkerspot butterflies.

We focused heavily on the flora, as after all this was a botanical event. The bright flowers of the aforementioned Green-headed Coneflower always drew comment. The plants' robust stature and colorful blossoms could not be missed.

We quickly noticed that an especially conspicuous gall was infesting many of the coneflowers. It formed large, softish lumps near the terminal parts of the plants, usually where the flowers occurred. Oftentimes, as in the above photo, flowers attempted to emerge from the tumorous masses, but were stunted and aberrant.

It was an easy guess that some sort of insect was causing these galls, as various flies, wasps, moths and other bugs routinely cause large galls in plant tissues. But none of us had a clue as to exactly who the culprit was. I recall seeing such galls before on Green-headed Coneflower, but not often enough to recall specifics of where and when.

Enter The Google. Judy Semroc had her iPhone along, and had a connection. A few smartly chosen keywords in Google, and we had an answer. The galls were caused by a midge known as Asphondylia rudbeckiaeconspicua, a name much longer than the little fly that it denotes. I'm not sure what the genus name means, but the specific epithet is clear: rudbeckia = genus of the midge's host plant; conspicua = conspicuous.

Now that we knew the name of the gall-making critter, the next step was obvious. Cut one open. Judy whipped out a pocket knife and deftly carved apart one of the galls. And, unsurprisingly, there was a midge grub ensconced in a chamber deep within the gall. Judy's knifework work was superb, I'll say. She managed to expose the grub and its chamber without making mash of it.

What was not expected (although perhaps it shouldn't be surprising) was another occupant of the gall. As I attempted to make images of the grub above with my macro lens, I was startled to see a tiny insect clamber out of one of the inner chambers of the gall. While I got one or two OK images of this other bug in the field, it was really too small for my 100mm macro. So, I pocketed five galls, and took them home for more detailed autopsy, and images with Canon's incredible mega-macro lens, the strange MP-E 65mm. The photos that follow were taken with that lens, coupled to the Canon 5D Mark III with illumination via Canon's MT-24 Twin-Lite flash system.

And here's the other gall inhabitant - some sort of truly elfin parasitoid wasp. This thing is only about one to two (1-2) millimeters in length, and would scarcely be noticed with the unaided eye. It presumably is preying on the rightful gall inhabitant - the midge grub. If things work as they usually do in the parasitoid world, the wasp somehow lays its eggs on or near the midge eggs or grubs, presumably when the gall is in its infancy. When the wasp grubs hatch, they commence feeding on the midge grub. The midge grub, presumably, feeds on its host plant's tissues.

I think we only dissected one gall in the field, and it held at least one wasp. Of the five that I brought home, wasps were in four galls. It would appear that the incidence of parasitism, at least in this midge gall colony, is high.

One of the tiny wasps (Superfamily Chalcidoidea, I suspect), stands by what might be one of the wasps's cocoons. It was nestled in one of the passages that I presume was made by a midge grub as it tunneled through the gall. The apparent cocoon seemed to be cottony, as have been other parasitoid wasp cocoons that I've seen.

A closer look at the cocoon, if that's what it is.

I also found a few pupal wasps. They were dislodged during my excavation efforts, and I'm not exactly sure of their locations within the gall, but they had to have been somewhere within the midge feeding tunnels. It would appear that this pupa is not long from transformation to adult, and I found another that looked similar but was actually starting to flex and move.

Another look at the mystery wasp. I would be most appreciative if someone out there could cast a more informed light on this situation. It was tough enough finding any substantive information on the gall midge, and nothing that I've come across mentions anything about parasitoids, wasps or otherwise. It would be interesting to know the wasp's name (if it has one), and more about the life cycle. Such as how it invades the host midge gall, and how the adults exit. If mature wasps are in the gall at this time of year, how do they overwinter? Questions, questions.

If one is willing to look at Nature with a broadly sweeping eye, the little mysteries just keep on coming.


Cathy said...

What a fascinating look into the jungle world of parasitic wasps and their hosts.
Perhaps a retired entomologist - Frank Kurczewski could help you with some facts. He's asked to use a picture of a spider wasp I'd taken in a monograph he and a colleague are creating: " . . . reproduce one of your two photos under Dipogon papago in a monograph on host records of spider wasps that G. B. Edwards, the jumping spider specialist, and I are publishing? "
Worth a try.

Lisa Rainsong said...

I am so glad you studied these so closely and took such detailed photos. Thank you for explaining the incredible complexity of these relationships.

Mommer1 said...

wow! excellent teamwork produces who knew? and what next?