Question
I've noticed clusters of small green bumps on our maple tree's leaves this spring. What could it be?

Answer
These are galls caused by an insect.
Galls are abnormal growths of plant tissue induced by insects and other organisms. Gall-making parasites release growth-regulating chemicals as they feed, causing adjacent plant tissues to form a gall. The parasite then develops within the relative security of the gall. Several different groups of insects and one family of mites have developed the ability to induce plant galls. Galls come in an endless variety of forms. Many are strikingly colored or curiously shaped. Each gall-making species causes a gall structurally different from all others.



Controlling gall insects is difficult, and at present there are no insecticides registered for this use by homeowners. Any treatment applied after galls are already present is useless, because galls will not go away even if the parasite is killed. Fortunately, the vast majority of galls are not particularly injurious and are of no economic significance. Most plants can support a large number of galls and continue to grow normally. Pruning out heavily galled portions of a plant is sometimes feasible and may help reduce populations of the gall insects. When this is not possible, it is best to accept galls as curiosities of nature--enjoy watching their development if you are interested; simply ignore them if you are not.