Gardening: Tiny wasps are causing galls on oak tree

Horned oak gall are caused by wasps. The gall eventually causes a branch to drop to the ground. If the horns are gone, so is the wasp.
Horned oak gall are caused by wasps. The gall eventually causes a branch to drop to the ground. If the horns are gone, so is the wasp.

Q. I came across a big tree, probably an oak at the Strafford Subdivision in Fairview Heights, which bears many acorns and a single fruit that looks like a drupe. How is this possible? I have attached pictures?

— C.C. of Fairview Heights

A. The photo you sent does look like a drupe, but on closer inspection, I found a development stage of a Horned Oak Gall (Callirhytis cornigera). This is caused by a tiny wasp-type insect. A female will lay eggs on the developing leaves early in the spring. The eggs hatch out and form little hairy growths on the veins of the leaf with small larvae living inside sucking sap from the veins.

Later in the year, males and females emerge and mate. The females lay eggs on the smaller one-year growth twigs. In the spring of the next year, the galls begin to form drupe-looking galls. You have cut some of them open and you found what looked like a seed but this is an encasement over the insect. These twig galls develop over a two year period and will form projections from the outside tissue that resemble horns.

Near the end of the gall cycle, the branch usually breaks off from the tree and falls to the ground. The female wasp will emerge from this gall in the spring and the life cycle starts all over again. When you find the gall on the branch on the ground, you can examine it and see if the horns are missing, which means the female wasp has already emerged.

This type of gall-forming insect usually doesn’t kill the tree but it can destroy a good full symmetrical oak appearance.

Clean-up of the fallen branches on the ground is about the only method of destroying the life cycle as the wasps are very small, and you have a very small window of spraying insecticide to kill them. Once the insects hatch out, they grow into the leaves or into the bark on the small stems.

Q. I have several oak trees that have leaves turning reddish brown and that appear to be dying. I also have a mature dogwood on which all the top leaves (top one-third) are wilting and brown leaves are forming. Am I losing my trees?

— G.L. of Belleville

A. Leaf scorch occurs when the leaf overheats. This summer has been unusual with all the rain that has fallen and how cool the air temperatures have been. Recently, the rain slowed down, the air temperatures rose and the leaves became overheated.

With all the rain that we have had how does this happen, you ask? When the soil receives too much rain, poorly drained soils do not allow enough oxygen, a gas, to get into the soil. The root hairs begin to die without the oxygen and not enough water can be sent upward to the leaves. As a result, the leaves cannot cool themselves through transpiration (similar to sweating), and the leaf tissue dies. If the tree does not carry on enough photosynthesis, there will not be enough energy stored for next year’s growth. A large number of branch buds die as well.

You will probably need to fertilize the tree after it has gone dormant in winter so the remaining roots can absorb enough nutrients to help the tree’s vigor for the coming year.

Charles Giedeman is a local contributing writer. Send your gardening questions to Lifestyle Editor Patrick Kuhl, Belleville News-Democrat, P.O. Box 427, 120 S. Illinois St., Belleville, IL 62222-0427, or email them to

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