COLUMN: Do you have galls?

Michele Olson
Jones County Naturalist

     Have you ever wondered, what in the world those strange bumps were on the leaves from the trees in your backyard?  If you have then maybe you’ve got galls!

     A plant gall is simply an abnormal growth of plant tissue. Galls can be formed from a mechanical injury or in reaction to insects, fungi, mites, nematodes, bacteria, or viruses. Galls can be found on leaves, stems, twigs, roots, or flowers. When initiated by an insect they often provide a safe home for an insect larvae or nymph to feed and grow safely, hidden away from view. In Iowa there are many different types of galls that can be discovered when you really stop to take a closer look.

     A commonly observed gall in the autumn, as leaves fall from the trees, is the hackberry psyllid gall. These strange, raised galls shaped like an elongated pencil eraser can be quite numerous on hackberry leaves. The tiny, often overlooked, adult cicada looking psyllids leave the galls in the fall and overwinter unseen under the bark of trees. Once spring arrives, eggs are laid within the buds of the hackberry tree where larvae will feed on the inside of the gall the leaves create. Here they are safely hidden from predators as they grow throughout the summer and pupate into their adult form emerging as the next generation in the autumn.

     Maple bladder galls are another frequently observed gall. The undersides of some silver maple leaves will be packed with these small irregularly shaped wart-like red and black colored bumps.  Adult maple bladder gall mites will overwinter under the bark of the tree emerging in spring to lay eggs on new leaf growth.    Maple spindle galls are also created by tiny mites.  These galls are aptly named for their long skinny spindle like shapes.

     Another fun gall to find is the oak apple gall. These are rather large round green galls resembling a very small apple. Petite, specialized female wasps lay an egg with a gall inducing chemical into the plant tissue of a new leaf bud. Wasp larvae live within the gall in a center chamber and feed as they mature. When cut open, these galls reveal thin fibers radiating from the central larval chamber to the internal shell of the gall creating a fascinating growth chamber. It has been reported there are over 50 species of gall wasps known to produce oak apple galls in North America. If you find one totally hollow with a loose larvae or pupa rolling about, you most likely have a succulent or roly-poly oak wasp gall.

     I’m sure if you explore the trees and leaves of the plants near your home you will make numerous gall discoveries. I think it’s safe to say we all have galls. Get out and discover yours today!

     A wonderful free resource to learn about Iowa’s common galls and see images of each one is the ISU Extension publication – Insect Galls on Trees and Shrubs which can be downloaded at   Want more?   Visit a county park near you to discover more galls



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