I have a wild oak tree in our garden ( Kerrey, Ireland ). The tree is now...

Asked August 23, 2013, 10:11 AM EDT

I have a wild oak tree in our garden ( Kerrey, Ireland ). The tree is now about 3M high and looks very healthy, but I noticed today that all leaves have loads of round brown spots ( like insect eggs ), some as large as 5 mm diam. What is causing this? and is there any thing I could do to help the trea. I have not noticed that in previous years.

Outside United States trees and shrubs insect issues horticulture

5 Responses

Do you know what type of oak you have? Where in the canopy are the leaves most affected? Could you send a close up picture of the brown spots on the leaves?

Hi Pat.
Thanks for your response to my question about the oak tree. I have taken some photos and will try to attach it. What you will see on the back of one leave is actually the same on every leave.

look forward to your valued advise

The brown spots are galls caused by a Cynipid gall wasp Neuroterus quercusbaccarum. Galls are caused by the reaction of the oak to specialized growth hormones exuded by the larva of these wasps. The adult wasp lays its eggs in the leaf tissue and, when the larva hatches, it begins feeding on the plant. The larvae secretes a substance that causes the tissue to grow in an exaggerated fashion. This substance develops structures that are very different from the normal plant growth. The larva feeds on the cells of these modified plant parts. If you cut open one of these galls during the period when the larva is feeding, you would find small grubs of the wasp crawling around inside. The good news is most oak galls are not a significant health problem to the tree, even when infestations are fairly high. Insecticides, including systemic insecticides, have not been shown to be of value in decreasing populations of gall-making insects. The galls cause more cosmetic damage than anything else. Heavily infested leaves might fall from the tree earlier.
Cynipid gall wasp Neuroterus quercusbaccarum (Linnaeus) - See more at: http://www.forestryimages.org/brows/detail.cfm?imgnum=5378815#sthash.BUjOQWJM.dpuf
http://www.ces.ncsu.edu/depts/ent/notes/O&T/trees/note05/note05.html

Hi Thank you very much for the detailed explanation, One thing concerns me though and I am so sorry to have to trouble you again. My concern if these thousands of wasp larva are feeding on the tree are we going to have thousands of wasps in the garden later on, it is bad enough now!

Cultural methods of control may be partially effective in reducing the impact of these insects. Some fallen leaves may harbor various life stages of gall-producing pests. Therefore, it may be useful to collect and burn all infested leaves. Do not compost them.