“My oak tree suddenly has large green growths on it – what’s wrong with it?”
Galls on oaks are a common sight in Central Texas. The good news is that they’re unlikely to harm your tree. This is especially good news because controlling them is difficult and potentially harmful to your tree’s ecosystem.
The gall pictured above is called “Oak Apple Gall” and is caused by a wasp – a cynipid wasp to be exact. The adult cynipid wasp is very small, similar in size to fruit fly and is not harmful to humans in any way.
“So, it’s caused by a wasp, but why?”
In short, the gall protects the developing larva. The female cynipid wasp lays her eggs in a developing leaf. The developing larva secretes hormones that cause the oak’s leaf to grow into a gall rather than the typical leaf structure. This gall provides food and shelter for the developing larva. Eventually the larva will emerge as an adult and the gall will typically fall off the tree.
“Should I do something about it?”
Not really. If it really bothers you because you think it is unsightly, you can trim them off the tree; but in general, you should learn to ignore it or appreciate its own strange beauty. In the right light, the galls will appear almost translucent.
“Okay, where should I go to learn more?”
As always, our friends at Agrilife Extension have good resources for us:
https://insects.tamu.edu/extension/publications/epubs/e-397.cfm
http://agrilife.org/etg/2008/04/16/oak_apple_gall_a_curiosity/
