We always get questions about weird things that grow on leaves. Most of these leaf abnormalities are very normal galls - green balloon-like growths that look like cherry tomatoes attached to a leaf; brown bubbles on oaks; leaves that look like they are blistering. As weird as these all are, these are all examples of galls.
No, galls aren’t diseases. A gall is a deformity in a plant that’s caused by another organism, like a fungus, bacteria or a mite, but many of the most conspicuous ones are insect-caused. There are wasps and flies [including midges] and aphids and a few other insects that cause these galls.
The insect has somehow re-programmed the plant—and in many cases it’s not really understood what is going on physiologically. In some cases it’s just a physical disturbance that’s causing the plant to change its growth, and in some cases it’s chemical, and in some cases I believe it’s actually genetic engineering by the insects themselves.
So the plant is in some way redirected to make this growth that surrounds the developing egg and larva of the insect. And it provides all of the food and shelter that the insect needs as it develops, and then the adult insect emerges when it’s mature.
So now that we know what exactly a gall is, what can we do about it when these weirdos appear on our tress?
A gall is really a sophisticated relationship between the plant and the gall-maker. It like the two have an agreement that the insect is just going to damage what might be just one part of one leaf, and the rest of the leaf is free to photosynthesize and go about its business.
Gall-makers are about the least-damaging insect herbivores—they’re not munching the plant down to nothing.
Although, galls can form on other plant parts, not just on the leaves, they are still rather non-threating to the plant. It is not uncommon to see galls on fruits, roots, flowers, leaves—all parts of plants. All are very host-specific and very location-specific, so a particular insect will typically be on one genus if not one species of plant, and on one part of that plant.
Oak trees in particular seem to get really gall-y. There are actually several hundred species of gall wasps that go after oak trees. Most of them are bi-modal galls, which means they make one form of gall in the spring and another form in the fall. For example, a ping pong sized gall and then then a fluffy one, or a white one, or one that looks like a dense rosette of leaves.
There's another similar predator out there - the mine. Let's leaves the mines for the next post. Check back to read about mines - what they are and how they effect plant life.