Q: What’s this sprouting out of the leaves on the lower limbs of my wild cherry tree? I sprayed it with insecticide last night, but I don’t think it’s an insect infestation, is it?
A: They are just harmless galls, caused by an insect of some sort. Galls on plant leaves and stems are common. The galls are caused by insects, usually aphids, wasps, mites, beetles or midges. They deposit their eggs in the plant tissue, which begins to swell as the larvae hatch. A chemical irritant is released by the female at the time of egg laying or by the developing insects themselves. This chemical causes a gall to form around the eggs and, later, the larvae. Each species of gall insect has its favorite host and forms galls of a characteristic shape. Some are large and woody; others may be hollow, silky or spiny. Galls are common on oak, elm and hickory trees and on rose bushes, goldenrod and witchhazel.