Master Gardener: Mites and insects cause most galls on trees and plants

If you find oak apple galls in your yard, there's typically no need for concern because, outside of a severe infestation, these galls don't do any damage to the tree.

‘ I have some weird, seemingly hollow, green, round balls falling from my oak tree. What in the world are these things?" — S.Y.

You are describing an interesting growth that can occur on a variety of plants. In this case, these are called oak apple galls, likely because of their resemblance to an apple. But let's talk about galls in general first.

Galls are the result of an interesting association between a plant host and a gall maker. Gall makers include insects, mites, bacterium, fungus and nematodes, but most are caused by mites and insects.

The galls are formed when the female insect injects a chemical into the plant along with her egg. The eggs themselves can also be the ones secreting the gall-inducing chemicals, but either way, these chemicals mimic plant growth hormones, which cause the galls to grow.

Different insects cause different types of galls to grow. Some can be kind of hairy, or they can be a pouch. Some cause growths at the edges of the leaves, and others cause these larger growths called oak apple galls.

There are three groups of insects that can cause galls: aphids, gall midges and gall wasps. In North America, we have over 50 varieties of gall wasps, but the oak apple galls are caused by the gall wasp.

When the female gall wasp injects an egg along with the hormone into the plant, it causes the host plant to grow a protective cover for the egg. Some gall wasps grow inside this gall for two to three years before emerging, mating, laying their eggs and then dying.

Oak apple galls grow a round protective covering about the size of a table tennis ball, which then serves as the home for this developing insect. If you find one lying on the ground, you can open it up to see an egg sack suspended inside the protective shell, held in place by a hair-like structure.

If you find these in your yard, there's typically no need for concern because, outside of a severe infestation, these galls don't do any damage to the tree. If you do have a large outcropping of oak apple galls, you can get a few, place them in a zip-lock bag and watch for when the insect hatches. This would tell you that the adults were now active in the tree and now would be the time to contact someone to spray your tree. However, since they don't do real damage to the tree, spraying would be an unnecessary option.

Instead, when you see any of these oak apple galls on the ground, just throw them in the trash and know you have put a little bit of a dent into their population. There is one type of gall insect called the gall midge that can be problematic: not to the plant, but to us.

Gall midges are a type of small fly (less than 1/4 inch of length). These midges lay their eggs on oak leaves while injecting a hormone into the leaf, causing it to grow the gall. Under the influence of the hormone, the leaf's edges curl up along with some additional growth that turns into a protected home for the eggs.

The challenge for us is that these gall midge larvae are a preferred food source of another insect that can cause us issues: the oak leaf itch mite.

These microscopic insects can be found up in the trees feeding on the gall midge larvae. The problem is oak leaf itch mites can drop from the trees onto an unsuspecting host (you and me) as we are out and about under the tree canopy. Bites tend to be on the head, neck, and arms because these are the areas that are first exposed to the dropping mites.

Since they are very small and hard to see, you typically won't know anything is up until you feel the bite and then attempt to smack or swat something away that you can't see. Some people can be allergic to the bites, which can then cause a red swollen bite mark.

A few years back, we had an outbreak of these tiny little insects that were causing problems all over town. I would feel their bites occasionally, but I have a friend who took to wearing sweatpants and a hoodie when mowing the yard to protect himself from the biting insects. You might be wondering how big of an issue this really was. Well, in the midst of a population surge of these mites, it would not be unusual for 300,000 mites per day to fall from a tree. That's a lot of potential biters falling from the sky.

Life cycles would have these mites present in greatest numbers during late summer. So, if you start to feel like you are being bitten but don't see an insect, you might want to start wearing long-sleeved shirts, long pants and a hat when working outside. That's not typically how I approach summer clothing. But the good news is that large outbreaks like that one are fairly rare. See you in the garden!

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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 St. or emailing us at [email protected].

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