What are oak galls?
WHAT’S GROWING ON
Plenty of animals build their homes in oak trees. But some teeny, tricky insects make the tree do all the work. “What nerve!” you might say.
What ... gall! And you’d be right. Oak galls are caused by a group of small insects known as gall makers. Contrary to public perception, they don’t endanger an oak’s health. Oaks aren’t the only host plants for galls. You can find them on many other plants and trees such as willows, alders, manzanitas or pines.
Who causes galls? Lots of creatures; midges, mites, aphids, flies, even bacteria and viruses. But the undisputed champs are a big family of little wasps called Cynipids — rarely exceeding the size of a mosquito. In California and the other western states, there are more than 200 gall inducing arthropods and there are more than 90 cynipid wasps known to cause galls in oaks in California. There have been more than 700 species of gall wasps documented in North America.
Out of the thousands of species of insects and other organisms that cause galls, each one creates its own distinctive type of gall. This allows biologists called “cecidologists” to identify and study these galls with a high degree of specificity.
Adult cynipids usually are purple or black, small, stout insects that have clear wings with few veins. The life cycle of gall wasps is impressive and well timed. Since adult gall wasps live only about a week and don’t eat during that time, their most important task is to find the proper host organ (a bud, leaf or new stem) and lay eggs in it. This means the adults’ emergence must coincide with the proper stage in the development of the host tree species. The tree’s development varies with the weather, so the wasps’ must as well. When the time is right, a female deposits an egg and injects a chemical into the tree that causes it to form a nutritive shell around each egg. Nothing else happens until the egg hatches within that nutritive shell, on which the larva begins to feed. In the process, it releases a compound that induces the host to produce a larval nursery of specific size, shape and color for each species of wasp.
The galls serve as an ideal environment for wasp larvae, whether it’s a single offspring, or dozens. Much research has been done to determine the exact nature and causes of speciesspecific gall formation. Scientists suspect these tiny wasps are able to manipulate genes in specific plant tissues, inducing the plant to create galls characteristic of their species.
Some galls are fuzzy, some are round or lumpy, some spiny, others flattened and dish-shaped. Sizes can range from a fraction of an inch to several inches in diameter. Often, the larger galls will grow together on stems forming large masses that weigh the branch down. They come in a wide range of colors from red and pink to yellow and green.
One of the most common in our region is the California gall wasp (Andricus querc us calif or nic us ). These wasps cause galls that start out green on the oak branch then turn reddish, brown or sometimes black as they increase in size to as large as 4 inches in diameter. Your oak tree may have individual galls or dense clusters. These are the biggest insect galls found in the western United States. Only females are known for this species, and they reproduce parthenogenetically, laying eggs without mating.
This time of year, the ground beneath many Valley oaks is covered with pinhead-sized yellow or brown seed-like objects, most of which are hopping around due to the activity of the larva inside. Presumably this movement helps situate the larvae into the proper area for pupation. The tiny things are called “jumping oak galls” and are formed by a tiny, dark wasp.
As with other gall wasps, no control measure is necessary or recommended. The galls don’t harm the tree or weaken it any way. Gall wasps and the gall itself are important hosts for other predators including parasitic wasps, birds, spiders and lizards. Therefore, chemical applications would do more harm than good by disrupting this complex food chain.
Next time you’re around a California oak, take a close look at the leaves and branches. You might just discover an entire microcosm growing right before your eyes.
Our Master Gardeners are available to answer your gardening questions at (209) 953-6112 or visit our website: https:// ucanr.edu/sjmg.