Oaks are prime gall-fodder supporting over 800 different types of galls. It's why I find oaks so interesting. Three-quarters of the oak gall-makers belong to two families: the wasp family Cynipidae; and the "gall midge" family Cecidomyiidae (cecido means "gall").
Insect galls have five things in common. First, they are composed entirely of plant tissue. Second, they are produced under the direction of a living gall-maker that turns plant genes on and off at just the right times to grow the galls to house, protect, and nourish their offspring.
Third, each type of oak gall is produced by a different species of gall-maker and their individual handiwork is so unique the galls alone can be used to identify the species of the gall-maker without actually seeing the insect. Fourth, gall-maker populations tend to fluctuate widely from year-to-year with a particular gall being common one year and rare the next.
Finally, although they may be very obvious, the vast majority of insect galls cause no appreciable harm to their oak hosts. This is a good thing because there are few effective methods to manage oak galls other than to physically remove them along with the immature gall-makers.
Mighty Midge Flies
The descriptively named gnarled oak leaf gall is produced under the direction of the gall-midge, Macrodiplosis niveipila. These bizarre looking galls are currently appearing on pin oaks in southwest Ohio. The galls look like lumpy, twisted masses of leaf tissue covered in sporadic patches of short, fuzzy, white hairs. They may arise near the base of the leaf with normal leaf tissue extending beyond the gall, or the misshapen growths may be comprised of an entire leaf.
The galls tend to appear throughout the tree canopy in random collections, with several galls in close proximity to one another. However, these dramatic looking galls usually affect only a very small percentage of leaves on an entire tree. Some years, these gnarly galls dominated the reports I received of oak galls in Greater Cincinnati. Other years, they've been no-shows. Thus far, I've not received any reports about these galls, so I'm not certain how common they are this season.
Another gall-midge belonging to the same genus is responsible for a type of vein pocket gall I found on chinkapin oak. The galls grown under the direction of M. majalis appear on the upper leaf surface as slits in the veins. These slits open into bulbous pocket-like structures hanging on the underside of the leaves. The pockets house the developing maggots; however, all of those that I opened were empty signaling that the maggots either pupated or were eaten by a predator.
One of my favorite oak galls are the light green, ball-like succulent oak galls (a.k.a. roly-poly galls) produced under the direction of the gall wasp, Dryocosmus quercuspalustris. The hollow galls adorn newly expanding oak leaves and are around 1/2" in diameter. Their common name comes from the fleshy (succulent) walls of the galls.
The alternate "roly-poly" name comes from the unattached seed-like structure that rolls around inside the galls. The structure houses a single wasp larva. I like to imagine newly emerging wasps staggering around after spending time rolling around inside the galls. Probably not true, but it's an entertaining thought.
Oak-apple galls are so-named because they resemble small apples. There are over 50 species of gall-wasps that are known to produce oak-apple galls in North America and there are probably at least 10 - 15 distinct species of oak-apple gall-wasps found in Ohio. Oak-apples are constructed of leaf buds that have been hijacked by the gall-wasp. The galls house a single wasp larva located in a central seed-like chamber.
Wasp larvae have chewing mouthparts. So, what does an oak-apple gall-wasp larva eat? The inside of the gall chamber is lined with specialized cells called nutritive tissue which is constantly being replaced as it is consumed by the gall-wasp larva. Imagine lounging in a room with pizzas constantly emerging from the walls. It's a wonder they ever pupate!
Two of the most common oak-apple galls found in Ohio are at the opposite ends of the size spectrum. The so-called small oak-apple gall produced under the direction of the wasp, Andricus quercussingularis (formerly Cynips clivorum), measures around 1/2" in diameter. The appropriately named large empty oak-apple gall produced by the wasp, Amphibolips quercusinanis (syn. A. inanis), can measure almost 1 1/2" in diameter.
I came across several small oak-apple galls on Thursday decorating the leaves of a pin oak. These galls could be easily mistaken for the aforementioned succulent roly-poly galls; however, their true identity is revealed by cutting them open. The internal structure of the oak-apple gall is composed of white fibers radiating from the central chamber housing the wasp larva.
I have not yet found any large empty oak-apple galls, but I'm pretty certain they're lurking within the canopies of oaks in southwest Ohio. These galls look like the small oak apple galls on steroids. Their light green surface is covered with purplish-red bumps perhaps to complete the apple ruse by mimicking insect damage.
Both the small and large oak-apple galls turn brown shortly after the new gall-wasps emerge. Their color makes them easy to spot; however, their brittle structure means they quickly drop from their oak host.