Backyard Gardener: Explaining oak gall

Hello Mid-Ohio Valley farmers and gardeners! Although we got off to a slow start to the growing season, farmers around the area are harvesting hay and gardeners have planted plenty of tomatoes, peppers and sweet corn. Last week many of our nurseries and home garden centers were sold out of tomatoes and bell peppers.

However, if you enjoy hot peppers there are still plenty of transplants out there. I would recommend jalapeno or pablano peppers but it you really love the heat, go for the habanero. As for the ghost pepper, you are on your own. Some advice. When you harvest hot peppers, wear gloves.

If you have noticed some strange golf ball size woody growths on your oak tree, some even with horns coming out of it, your tree may have oak gall. Gouty and horned oak gall are abnormal growths of woody tissue found on the twigs and branches of certain species of oak trees (Quercus spp.) including the red, black, pin, scarlet, and shingle oak species.

I have seen an increase in inquiries about galls on the branches of oak trees, especially pin oaks. In a typical situation, a few galls are observed at first and there is steady increase on an individual tree over a period of years until the tree begins to show branch dieback. There are two types of galls, including horned and gouty, caused by two closely related species of tiny wasp.

These unusual deformities are caused by plant growth-regulating chemicals produced by the gouty oak wasp (C. quercuspunctata ) and the horned oak wasp (Callirhytis cornigera). Wasp larvae release chemicals from their bodies which force the stem to create the swollen galls. They are unsightly, but are not usually directly lethal to the tree. Tissue does become disorganized within the gall structure, which disrupts vascular flow. The portion of the stem beyond the gall is often starved of water and dies. This doesn’t kill trees, but canopy dieback may be severe and the stress can cause the tree to begin to decline.

The life cycle of the tiny wasp that causes these galls is very complex. They have what is called an ‘alternating generations’ biology. In the early spring the wasps emerge from the woody stem galls and mated females deposit eggs in young oak twigs. The next spring small swellings (galls) develop on the twigs and enlarge over the next two or three years.

The galls provide protection, food, and shelter for the developing larvae. When the larvae reach maturity, the galls develop small spines or horns. An adult wasp emerges from each horn and another life cycle of wasps begins. The developing wasps in these stem galls may take up to three years to complete their development. Gall-forming wasps usually overwinter as adults in protected places away from the host tree.

As the woody stem galls increase in size they eventually ‘girdle’ the twig, stopping the movement of water and nutrients. In most cases damage from these types of galls range from light to moderate branch dieback. However, under severe infestations the galls bring about the decline of the tree as they repeatedly girdle new growth. Over time this may be fatal to the tree.

Unfortunately, controlling horned oak gall is very difficult. On young trees with only a few galls, they can be pruned off and destroyed. On older trees, the best advice is to water and fertilize to keep trees healthy. Another effective method to avoid heavy galling is to remove trees that have become heavily selected by the wasps.

Current options for the gall wasps are to either do nothing, prune out gall infested limbs, or treat with an insecticide. Chemical control is not recommended. Treating existing stem galls with insecticides is not very effective because insecticide uptake is substantially reduced in woody tissue compared to leaf tissue.

Companies market services to inject or otherwise treat trees for oak gall, but university research shows this has no major benefit. Neither foliar sprays nor systemic insecticide injections showed any reduction in the number of new woody galls, nor did they appear to kill the insects already inside the galls. Contact me at the Wood County WVU Extension Office 304-424-1960 with questions. Good Luck and Happy Gardening!


Question of the Week: My asparagus plants grow into tall fern looking plants every summer. Can I mow them off?

No, do not mow the ferns off. They are feeding energy into the roots for next year’s harvest. An asparagus plant is composed of ferns, crown and a root system. The ferns are not true leaves but are stems that will capture and transfer energy to the crown. The crown is a collection of rhizomes or modified roots and lateral roots which will initiate new ferns.

Spears which are not harvested develop into a large fern which manufacture and store energy in the crown. The spears (what we harvest to eat) are really just immature ferns. Fern should never be pruned or cut back. The asparagus fern is the “factory” that supplies energy to the crown and storage roots for next year’s crop. After the first frost, mow to your heart’s content.


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