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.
Have a plant or pest question? University of Maryland Extension’s experts have answers! Send your questions and photos to Ask Extension.
Do the wasps eventually hatch out of these?????
Are they in Florida oak trees?
Thank you!