How to Stop Galls on Oak Leaves

Q. I am inquiring about wasp galls on the underside of oak leaves. I am in the commercial tree business. Any control measures that have been found to minimize the wasp from laying the eggs or stop the oak from forming the fuzz ball? As I recall the fuzz ball is reaction from the oak defending itself. Any information would be greatly appreciated.

Oak leaf gall

A. Plant leaf galls can be caused by fungi, bacteria and insects like aphids, moths, midges, wasps as well as mites. You can identify what caused the gall by its shape and color and the host plant. In this particular case, if we are seeing the same picture in our minds, you are right, it is caused by egg laying of a tiny wasp.

Poplar flower gall caused by tiny mites

Many times it is only a specific insect or fungus that inhabits a specific species of oak or plant. If you had a picture we could take it further. I remember seeing some leaf galls on native oaks in the spring mountain range here. I assume they were caused by wasps as well.

Galls produced on plants are unique to the organism that caused it. Sometimes it is caused by feeding damage, infection or reproduction. The pairing of an insect or fungus with a specific species or type of oak in this instance is frequently unique.

oak gall causing leaf drop

A guy who is done a lot of work in this area and in our part of the world is Ron Russo, a naturalist. Here is a page taken from Natural History Magazine published in 2009.

Plant galls click here

I have been telling people to appreciate nature and live with it. Unless it's really causing problems like defoliation I would leave it alone. I guess if I had to treat it I would probably use something like imidacloprid as a soil drench when new leaves are beginning to form.

The problem in treating for this insect is that we don't know it's lifecycle. So we don't know when it is laying its eggs to put down a protective spray. I feel that a soil drench will give the plant longer protection and is less of a problem to the environment.

But I would really resist using this insecticide unless I was forced to use it by the client. Environmental concerns surrounding this product, in my opinion, don't justify its use unless it's under extreme conditions.

And even if you do use an insecticide like this the old leaves will still have the galls and it will only protect new growth.

I will post your question and my response on my blog next week and you can direct anyone with questions to that spot.