Published in the Contra Costa Times on December 9, 2006
Question: What is a gall? My oak tree is covered with large balls and the leaves had weird growths on them called galls. How do they get there?
Answer: A plant gall is a type of unusual growth formed completely by plant tissues. Galls come in a large variety of shapes, sizes and colors, and are a response to some kind of attack, by insect, nematode or disease organisms.
Plants create unique galls in response to each species, encapsulating the intruder and isolating it from the rest of the plant.
The large galls you see on your valley oak are called apple galls, caused by a tiny cynipid wasp, Andricus quercuscalifornicus, laying her eggs at the base of a bud. The oak responds to the larvae by forming a gall, which serves as a nursery, providing protected space with ample food. Months later, the adults emerge by chewing holes to the outside, where they mate and start the process over again. The cynipid wasps are much too tiny to sting you.
It is clear what the wasps get out of this deal, with the oak providing a cozy habitat for the larvae, but it is less obvious what the trees receive. Perhaps keeping the larvae isolated and unable to penetrate into the tree's tissues is enough. On the other hand, since oak trees do not produce colorful blossoms, the interesting and colorful galls may be the horticultural equivalent of jewelry.
There is no way to discourage the galls from forming, so your best approach is to appreciate them and the fascinating co-evolutionary relationship they portray. If you would like to see some photos of the wonderful and weird world of galls, check out CalPhotos at calphotos.berkeley.edu/.
Bethallyn Black is director of the Contra Costa Master Gardeners, a program associated with the University of California Cooperative Extension and supported by Contra Costa County. Reach her at bblack@ucdavis.edu, or 925-646-6586. General questions for the Master Gardener should be sent to mgcontracosta@ucdavis.edu.