On a recent trip, I was hiking through Big Ridge State Park in Tennessee, and I came across some remarkable patterns in maple leaves. These circular multicolored patterns resemble eyes, and they really stood out among the foliage. I figured out these patterns must be the consequence of the effect of some pathogen on the leaf, but I had never seen anything like it.
When I got back home, I did some searching, and I solved the mystery! The circular patterns are called “maple eyespot galls”, and they are caused by the larvae of flies called ocellate gall midges. These flies deposit their eggs on the underside of red maple leaves. When the eggs hatch, the larvae interact with the leaf inducing it to form a small swelling called a “gall” from which it feeds. Around the swelling, the leaf experiences changes in the concentration of its pigments. The levels of the green pigment chlorophyll decrease exposing the yellow pigments called carotenoids, and at the same time the leaf produces a red pigment called anthocyanin, which it doesn’t normally produce until autumn. After feeding on the leaf for a few weeks, the larva drops to the ground, pupates, and remains there through summer, fall, and winter to emerge as a fly in the spring, mate, and repeat the cycle.
I also wondered who studied this fly?
He was a gentleman with the amazing name of Carl-Robert Romanovich, Baron von der Osten-Sacken. He was a Russian entomologist who pursued a career in the Russian diplomatic service and lived in the United States from 1856 to 1877. During these years, and despite his diplomatic duties, Osten-Sacken found time to pursue his interest in flies and published several articles in scientific journals making important contributions to the study of American insects. Part of his work concerned the study of plant deformations caused by insects (galls), and he was the first scientist to describe the ocellate gall midge flies and the galls they produce on maple leaves in 1862. He gave these flies the scientific name of Acericecis ocellaris. The term “Aceresis” is the genus of the fly, and it has to do with the scientific name of the type of maple tree where these flies lay their eggs, Acer rubrum, the red maple tree. Ocellaris, the species of the fly, is a name derived from the Latin word “ocellus” which means “eye” and refers to the apparent eye-like shape of the spots caused by the larva on the leaves.
The things you learn from a hike in the woods!
The leaf pictures belong to the author and can only be used with permission. The photograph of Osten-Sacken from the United States Library of Congress is in the public domain.