You may have noticed what appears to be “apples” clustered on the branches of your oak tree and wondered if it was some sort of fruit. Even though it looks very much like an apple and feels like one if you are able to reach one, please, DO NOT eat it!
So if they are not “apples” what are they?
They are what is known as galls and are the result of a reaction between tiny insects and the tree. Galls are swollen abnormalities that are created in response to insects laying eggs in the tree’s tissue. The insect responsible for the gall pictured here is the California oak apple gall wasp.
Wasp? Does it sting?
No, they do not sting humans or animals. They are tiny, (2-3 mm) dark wasps known as cynipids. These wasps are the most common oak gall makers.
So how do these wasps create the “oak apples”?
The gall wasp does not make the “apple” in your tree, the oak does it all on its own. But the gall wasp gets the process started when the female wasp lays her eggs on an oak twig. It’s sort of like getting a mosquito bite for trees. Just like your body reacts to a mosquito bite and creates a bump where the mosquito bit you, the oak tree creates a gall in reaction to the wasp’s egg laying. Timing is crucial, the gall wasp waits until the tree is in a specific stage during the growing season that is conducive to egg laying and development of the gall. Gall wasps can lay eggs in spring, throughout summer, and into fall.
What is the purpose of these galls?
Galls are basically an incubator for developing wasp larvae. Galls are entirely made of plant material. Fluids deposited with the wasp egg stimulate the tree’s cells to rapidly multiply. Once the egg hatches inside the gall, it produces additional substances that maintain the gall. The larvae feed off the gall until it transforms into an adult. At this stage, the gall wasp drills a tiny hole to exit the gall. So instead of making a safe home for its offspring, the gall wasp tricks the oak tree into making it for her.
What happens if I cut open a gall?
Good question and one I asked myself. After plucking a gall off an oak tree I used a knife to slice it open. The results are pictured here. You can see the tiny eggs of the gall wasp.
What happens to galls once the new adult wasp exits the gall?
Apple galls start off in colors of yellow, red, and green and turn tan-brown as development continues. Galls will eventually brown and die once the adult insect emerges. Other insects may inhabit the discarded gall. The old galls can remain attached to the tree or fall to the ground.
Do birds or animals eat galls?
Yes. Native birds have learned to peck away the tips of the galls formed by the oak gall wasps and eat the juicy larvae inside, which are rich in protein. This helps them survive the crucial early spring period when other food is scarce. Squirrels are known to chew on galls too.
It’s a bug-eat-bug world
Not only do animals such as birds and squirrels eat the young wasp larvae but other bugs take advantage of the wasp’s home. They will invade the gall, kill off the wasp larvae and deposit their own eggs into the gall.
Are galls harmful to my oak tree?
No, not really, even though they are parasitic and use the tree’s resources for their own benefit without returning anything to the tree. There are some galls that result in small twig dieback, however, the impact is minimal and not enough to be fatal to the tree.
Do oaks produce oak “apples” every year?
Yes, but the quantity of “apples” can vary from year to year.
Is there anything I can do to get rid of gall wasps?
Raking up leaves and removing old, discarded galls can help limit the number of insects the following season.
Are oaks the only trees that have galls?
No. Galls are not only common on oaks but on willow, poplar, rose, and many other plants grown in California. There are lots of insects and their relatives, mites, that cause curious swelling not only on trees but plants as well. And these swellings are all known as galls.
Is the California oak apple gall wasp the only wasp to inhabit my oak?
No. There are over 200 species of gall wasps that are known to cause galls on the leaves, flowers, buds, acorns, and twigs of oaks. And each species produces a differently-shaped gall. It’s not uncommon to see a half dozen different species of galls on one leaf at the same time. Stay tuned, and we will discuss other galls you may find on your oak tree in an upcoming newsletter!
As always, if you see anything unusual on any tree in your yard, do not hesitate to contact our office and set up an appointment with one of our arborists to determine what it is.