These deformities are common occurrences. The answer lies in the food and protection that the plant tissues provide for the developing insect. Galls are more easily identified than the gall makers. Each species of insects makes only one type of gall, but you may find several species using the same plant. All plant parts may exhibit galls - leaves, stems, buds, flowers, twigs, and even roots.
Oaks are the most commonly attacked plants.
Over one-half of all galls produced can be found on oaks. One investigator found 75 different insects utilizing oak apple galls, some as parasites to the make, others as a convenient place to call home. The life history of each species of gall maker can be quite different, ranging from the simple to the complex. As a result, galls have received a great deal of scientific interest. A field of goldenrod will yield the easiest to find of all galls, in quantity and variety.