Gnarly-looking leaf galls are appearing on pin oaks (Quercus palustris) in southwest Ohio. The unsightly, lumpy leaf growths are produced by a gnat-like gall-midge (Macrodiplosis niveipila, family Cecidomyiidae) and I’ve commonly called the growths Gnarled Oak Leaf Midge Galls. Not very creative, but descriptive.
The galls appear as irregular, brain-like twisted masses of leaf tissue that may arise near the base of the leaf with normal leaf tissue extending beyond the gall, or the misshapen growth may appear to envelop an entire leaf. The affected leaf tissue is often thickened and darker green compared to normal tissue, and covered in sporadic patches of short, fuzzy, white hairs.
The gnarled oak leaf midge galls tend to appear throughout the tree canopy in random collections, with several galls close to one another. The galls are outward evidence of a remarkable relationship that has evolved between the gall-maker and its host. The same type of interconnection also applies to other gall-making groups such as gall-wasps (family Cynipidae).
The female gall midges initiate gall formation when they deposit an egg into undifferentiated (meristematic) leaf bud tissue. Along with the egg, they also inject chemicals that are an exact molecular match to plant hormones, or the chemicals are plant hormone analogs meaning they act like plant hormones. The chemicals are also exuded by the eggs and in some cases the resulting gall-maker larvae.
The chemicals act on the meristematic bud tissue to direct gall growth. In other words, cells that would have become normal leaf tissue are hijacked to form a structure that both feeds and protects the immature gall-maker.
The meristematic cells are like teenagers; they don’t know what they’re going to be until they grow up. So, leaf galls can only be made to develop from meristematic tissue; they can’t develop from differentiated plant tissue. For example, once a nascent leaf parenchyma cell becomes a full-fledged normal leaf parenchyma cell, it can’t be formed into a gall cell. Thus, leaf gall formation tends to occur in the spring.
A final gall-point is that arthropod gall-makers tend to be very host-specific. This is commonsensical given that gall growth and development require a tightly choreographic dance between the gall-maker and its host. For example, the gnarled oak leaf midge galls appear to be confined to pin oaks. I’ve never seen them on any other oak within the red oak group.
Although the gnarled oak leaf midge galls may present a dramatic appearance, as with the vast majority of oak leaf galls, they have little to no impact on the overall health of their host tree. Usually, only a very small percentage of the leaves on an entire tree are affected, so controls are not needed. Galls may be pruned from small trees; however, make sure to follow proper pruning practices to improve rather than impair the long-term shape of the tree.
Simply leaving the galls alone is also justified given that populations of the midge fly gall-maker are highly sporadic from year to year. The galls may not reappear next season, or even many years later, so there's little chance of an accumulated impact on tree health.