We received a couple questions this season from curious residents of Montgomery County who found “fruits” that look like small plums. These objects had dropped from oak trees.

While these fruit-like items do look similar to plums, they are not edible at all! These are acorn plum galls, which are caused by a type of wasp in the family Cynipidae.

Galls are abnormal growths formed by a plant, like a tumor, in response to an organism such as an insect, fungus, or bacterium. In the case of acorn plum galls, a tiny cynipid wasp lays an egg on an acorn cap of an oak tree. As the larva hatches and develops, a secretion from the larva causes the tree to respond by making these abnormal growths that protect the larva. Each gall has a developing wasp larva in the center. The galls appear fleshly throughout, like a plum or apple. When they are mature, the galls drop from the tree. There may be many galls on a single oak tree.

Acorn plum galls are harmless to trees and are more of an aesthetic issue or just a nuisance if a large number of them fall from a tree and land on sidewalks, decks, or roads.

Learn more about different types of galls you might find on plants in your landscape.

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