So-called galls on goldenrod (Solidago spp.) leaves produced in association with the midge fly Asteromyia carbonifer (family Cecidomyiidae) appear as white, circular structures on both the upper and lower leaf surfaces. As the galls mature, they develop a faint black ring near the outer edge making the galls look target-like.
The scientific name, Asteromyia carbonifera, is descriptive. The prefix “Astero-“ refers to the aster family, Asteraceae. The suffix “-myia” is from the Greek for “fly.” The midge flies belonging to this genus target members of the aster family. The specific epithet, “carbonifera,” refers to the coal-like fungal stromata (singular "stroma") found within the galls.
The leaf galls were originally described as being the result of infections by a fungus in the genus, Rhystisma. However, as with many things in nature, there's more to this story than meets the eye.
What Lies Beneath
Fungi are in fact involved with producing the gall structures. These structures and their interactive relationships between a triad involving a midge fly, fungi, and parasitoid wasps have been the focus of some fascinating research highlighted in “Selected References” below.
The primary fungus found in the galls is Botryosphaeria dothidea (previously identified as Sclerotium asteris). The midge fly and the fungus have an obligate mutualistic symbiotic relationship meaning that the two live for the mutual benefit of one another and the relationship is so strong that without the fungus, the flies could not develop on goldenrod, and vice versa.
Indeed, the female flies carry spores of the fungus in specialized structures (mycangium) in their terminal abdominal segment. When the flies insert eggs into the goldenrod leaf tissue, they also inoculate the plant with the fungus.
Most gall-making midge flies that belong to the family Cecidomyiidae directly orchestrate the growth of their associated galls. However, it appears that the growth of the galls on goldenrod is directed by B. dothidea as well as perhaps other fungi.
A paper published this year showed that the midge fly larval (maggot) microbiome included a rich diversity of fungi belonging to the genera Alternaria, Aureobasidium, Filobasidium, Fusarium, and Tilletiopsis. The larvae carried 29% of the fungi that appear to play a functional role in perhaps initiating gall development and/or the composition of the mycelia found in the gall structure. The exact role of these fungi is not well understood.
However, the B. dothidea fungus appears to be the central character in the fungal tableau. The mycelium of this fungus surrounds the midge fly maggots and the maggots feast on a fungal buffet. This is why this midge fly, as well as others, are sometimes called Ambrosia Gall Midges.
Eventually, a layer of the fungal mycelium differentiates to form a black, tough, dense vegetative structure called a stroma. The black, carbon-like stroma may be revealed by carefully peeling away the surface of the galls on the lower leaf surface.
The stroma adds rigidity to the gall structure, and it also helps to protect the midge maggots and pupae from the depredations of the parasitoid wasp Torymus capitae. The wasp probes the gall with its ovipositor seeking to lay an egg in the immature flies; however, their ovipositors cannot easily penetrate the fungal stroma.
The midge-fungus is a symbiotic relationship that keeps on giving. The midge fly has multiple generations per season with each new generation being entirely dependent upon the fungus, and vice versa.
A “Plant Gall,” a Fungal Infection, or Both?
As noted above, the unusual structures on goldenrod leaves produced in association with the midge fly A. carbonifera were originally believed to be the result of infections by a fungus in the genus Rhytisma. The misidentification is understandable given how similar the fungal stromata on goldenrod look like the large, black stromata produced by members of the Rhytisma genus on maples to produce the disease called Maple Tar Spot.
Of course, this begs the question. Are the structures on goldenrod truly a plant gall, or nothing more than fungal infections that are taken advantage of by the midge fly?
The British Plant Gall Society defines a gall as “an abnormal growth produced by a plant or other host under the influence of another organism. It involves the enlargement and/or proliferation of host cells, and provides both shelter and food or nutrients for the invading organism.” The unusual pinecone-like galls produced on willows by the Willow Pinecone Gall Midge, Rabdophaga strobiloides (family Cecidomyiidae) checks all of the boxes.
In contrast, the structures on goldenrod leaves are poles apart other than housing and providing food for A. carbonifera. How much does the midge fly actually influence the development of the structures other than carrying and inoculating the leaves with the fungus, B. dothidea? Is the so-called “gall” just the outward expression of a fungal infection, or is the midge fly influencing fungal growth perhaps by exuding chemicals that somehow directs the development and direction of the infection? I believe whether the structures are actually “galls” remains an open question.