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 Have you ever noticed little fuzzy patches on the undersides of oak leaves or what appear to be hard marble-sized balls on their stems? People frequently bring these novelties to the Louisiana Nature Center and are always surprised when told they are animal in origin, yet produced by the plant.
The source of marble galls on our live oak trees illustrates the uniqueness of their story. A tiny gall wasp deposits her egg in a bud or stem. The tree, reacting to the egg as foreign tissue, produces a layer of inactive cells which isolate the "intruder" from the living portions of the plant. The egg becomes a larvae, feeds on the gall tissue, and eventually drills its way through the gall to begin its adult life.
There are many types of gall-inducing organisms and most are associated with a specific host plant and produce a distinctive gall. Tiny jumping plant lice produce cone-shaped galls on hackberry leaves. Certain types of aphids may induce the development of a variety of galls on grape, elm, and poplar leaves as well as their roots. Willows sometime have a pine cone-like gall which is the result of gall midge (a small fly) activity and the wooly oak galls come from gall wasps.
 Among the most distinctive gall bearing plants in the New Orleans area are the golden rods which have two types, both of which appear as thickened stems. One is caused by a gall fly and is identified by its spherical shape and thick walls. The other is elongate with thin walls and is produced by a gall moth. Both of these, as are most of our local galls, are most abundant from mid-summer through fall and winter.
Galls have not been shown to be harmful to plants. Remember, they are the result of additional plant growth and they are isolated from the plant's vital tissues. If you have never noticed galls on plants, take the time to examine leaves and stems of local trees.
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