Control with insecticides is not recommended. Common Insect Galls in Iowa Ash Flower Gall. These one-inch, brown, spherical swellings of male flowers on green ash are most obvious during the winter because they hang on the trees. Green galls present during the summer are less noticeable. Hackberry Nipple Gall.
Galls remain green all year but issue an annoying, gnat-like psyllid in the fall.
Hickory Petiole Gall. This large, globular swelling of the leaf stem is filled with a squirming mass of small aphids. The gall appears to stunt tree growth in some years. Unfortunately, there is no cure. Maple Bladder Gall.