Oak galls pose little short-term hazard

When something goes wrong with a tree in a homeowner’s yard, they often become concerned. A tree is a long-term investment, and they don’t want to lose it. Trees provide beauty and value to a property. If they are situated correctly, they can also provide cooling in the summer, or perhaps act as a windbreak in the winter. No, we don’t want to lose our trees.

Sometimes the problem is serious. Some diseases, such as oak wilt, can take out a tree quickly and spread to other trees.

Other times, while the disorder may look serious, it poses little short-term hazard to the tree. Most of the oak galls fall into this category.

Recently, I have had several concerned homeowners call me about these galls. Typically, it’s the horned oak gall. I’ve also seen gouty oak galls as well. These galls are usually found on small branches, and can easily be an inch or so in diameter.

Trees may start out with only a few galls, and through the years, their number increase. Sometimes branch dieback will occur when a tree is heavily infested.

Both horned oak gall and gouty oak gall are caused by tiny wasps. In early spring, wasps emerge from their overwintering sites in these woody stem galls. The females lay their eggs in the swelling buds of the host tree. Tiny galls form along the veins of new leaves. After several months, adults emerge from these galls. Females from this generation lay eggs in the trees young shoots.

The generation that hatches and feeds on these shoots are the ones that cause the large galls that we see and become concerned about. These galls may take several years to develop.

As these woody galls develop, they eventually girdle the twig, causing dieback of the twig. If these occur in sufficient numbers, it can cause decline in the tree, which could eventually kill the tree. This is a long-term process.

Recently, I checked with Dr. Chris Starbuck, MU State Woody Ornamental Specialist. Unfortunately, there really haven’t been any recent breakthroughs in treating oak galls. Current recommendations are to either do nothing, prune out the gall-infested limbs, or spray the entire tree with an approved insecticide. These last two treatments can be expensive or time consuming.

Spraying the tree with an insecticide can be difficult. The entire tree must be covered. Also, the wasps inside the galls will not be killed, since they are protected inside the gall.

Dr. Bruce Barrett, MU Extension Entomologist, outlined several approaches to chemical control of oak galls in a recent publication. The bottom line is that none of them worked very well.

I wish I had a better answer for homeowners when it comes to controlling these pests. Unfortunately, there isn’t one. Perhaps someone will come up with a good approach to control oak galls some day. If so, they’ll become extremely popular very quickly.