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Master Gardener: What in the world are oak apple galls?
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Master Gardener: What in the world are oak apple galls?

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oak apple galls

The process that leads to the formation of these oak apple galls on trees is quite fascinating.

Some branches on my oak tree have these unusual growths on them. They are green and about the size of a golf ball. What in the world are these things? — DE

What you are describing is another one of nature’s curiosities, in my view. They are called oak apple galls. There are a variety of galls that grow on trees, so let’s talk about galls in general first.

Galls can be kind of hair-like structures on leaves, they can be pouch-like, and others can cause deformities on leaves or stems. There are three groups of insects that cause these galls: aphids, gall midges and gall wasps. With over 50 varieties of gall wasps in North America, there is a high likelihood that your oak apple galls were caused by the gall wasp.

The process that leads to the formation of these oak apple galls is quite fascinating. Gall-making begins when the female gall wasp injects a hormone into the bark of a tree along with her eggs.

The hormones injected into the bark affect the growth cells on the branch or leaf. Under the influence of these hormones, the leaf or branch then grows a protective cover for the eggs. Some gall wasps develop for 2 to 3 years inside the gall only to emerge, mate, lay their eggs and then die.

For some galls, this is just a deformation on the leaves. But for oak apple galls, they grow a protective shell about the size of a ping pong ball, which serves as the home for these small insects. If you find one of the oak apple galls on the ground or on an accessible branch, cut it open and look inside. It is quite fascinating since the egg sac is suspended inside of the protective shell with a hair-like structure. I know, odds are you won’t do that, but it is pretty cool. If you don’t want to cut one open, just Google oak apple galls to see what you are missing.

Fortunately, if you have any of these galls on your tree, it’s usually not necessary to take any action. You can pick up two or three of the oak apple galls, place them in a zip-close bag, then keep an eye out for a small hole where the mature insect emerged. When you see that, you know the adults are active and it would be time to spray. But since they don’t do any fatal damage to the tree, spraying would not be the recommended option.

One thing that can help reduce the gall insect population in your landscape is to remove these galls as best you can. If they are attached to leaves that glide to earth in the fall, just throw those leaves away. If you find any of the galls in your yard, provide the same courtesy by depositing these in the trash.

There is one type of gall that can be a little more problematic. These galls are caused by the gall midge.

Gall midges are a sizable group of small flies (typically less than one quarter-inch long). Gall midges lay their eggs on oak leaves and inject a toxin into the leaf, causing it to form a gall that causes the leaf’s edges to curl up for a protected home for the eggs. This in and of itself is not a problem for the oak tree, however, larvae of the gall midge are the preferred food source for another insect that wears out its welcome pretty quick: the oak leaf itch mite.

The problem with these microscopic oak leaf itch mites is that they fall from the trees onto unsuspecting hosts (you and me) as we stroll under the trees enjoying a nice respite from the summer sun. Typically, you won’t know you are playing host to the oak leaf itch mite until they bite you, and their bite can be painful. To complicate the issue, when you feel a bite, you’ll look but you won’t see anything since they are not visible to the naked eye. These bites can also be a problem for some people, who will develop red swollen bite marks.

Several years ago, we had a bit of an outbreak of these little creatures, and they were causing problems all over town. One of my friends had to mow his yard wearing sweatpants and a hoodie to keep protect himself from the bites. In the middle of an outbreak like that, it’s not unusual for up to 300,000 mites from fall from the tree each day.

If you experience an oak leaf itch mite outbreak in your yard later this year, the best advice is to stay away from the tree or wear long-sleeved shirts, long pants and a hat when working outside. That sound like fun in an Oklahoma summer, doesn’t it?

The good news is that large outbreaks like this are fairly rare in our area.

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org.

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