Numbers
There are more than 2,000 gall-producing insects in the United States; 1,500 are either gall gnats or gall wasps.
Size
Agent producing the gall usually very small, less than 1 mm to a few millimeters. Galls can be as large as a couple of inches.
Food
Plant tissue of the host plant.
Remarks
This is the section where images of unidentified galls are kept, hopefully temporarily.
Gall insects (and mites) are usually highly specific about what kind of plants they use, and even what part of the plant. To maximize your chances of getting a gall identified, record the plant species (include photos of the leaves, flowers, fruits, etc. if you're not sure), and if it's a leaf gall, note the position on the leaf (if it's not obvious from the photo): upper side or underside; midrib, side vein, or somewhere else. Also note whether or not the gall is detachable, the size of the gall, and anything else distinctive about it that may not be clear in the photo. With oaks in particular, which are hosts for hundreds of kinds of galls, every little detail can help to narrow down the options.
Internet References
Colo. State U.. Insect and Mite Galls.
Iowa State U.. Insect Galls on Trees and Shrubs.
U. of Florida. Insect Galls.
TAMU. Gall-making Insects and Mites.
Confessions of a Gall Hunter (article in
Natural History by Ron Russo)
