Strange Growths on Plants

While walking home the other day I saw some strange spots on a maple leaf and became very curious. A week later someone brought in another strange growth, this time on an oak leaf. So I looked it up and found that these growths are called galls. Galls can be attributed to bacteria, fungi, insects, mites, or nematodes that have laid their eggs on the plant or where their spores have germinated. Galls are the home for the developing young, which generally do not harm the host plant. The plants defend themselves from this invasion by growing specialized cells around the area where the eggs or spores are, thus forming a gall. Insects are responsible for most of the galls that we see and of the 2,000 insect producing galls, 1,500 of them are wasps and gnats (aka midges). The pictures below are an example of both a wasp and a midge gall. 

The gall on the left is an oak apple gall formed by a cynipid wasp. On the right is the maple eyespot gall formed by a midge (Acericecis ocellaris).

Here is an inside view of the oak apple gall. The dark brown ball in the center is where the cynipid wasp larva is developing. When it is an adult it will emerge from the gall and leave a small exit hole behind.