47. What are those? Oak Galls

You see these ball shaped things on Oak trees all over the San Francisco Bay Area.  They are especially noticeable in winter time when deciduous Oaks have no leaves.  I get asked all the time what these are and what to do about them.

They are called Galls, and it may be hard to believe but they are actually homes for the larvae of tiny wasps and other insects.  The most notable ones seen around Oak trees are ball shaped growths like the above pictures.  These are Oak Galls, or Oak Apples, and they serve as the homes for the development of wasps from the family Cynipidae.  These are very tiny wasps - 1 to 8 millimeters in size.  They form when an adult wasp lays eggs into the tissue of the tree triggering a swelling around the eggs.  When the larvae emerge from the eggs they further stimulate swelling and create the Gall, an almost tumor-like growth of tree tissue.  This structure protects and nourishes the wasp larvae as it grows into an adult wasp.

A Wide Variety

It is amazing how many different varieties of insects use galls as a part of reproduction, approximately 800 different types just on Oak trees in North America and over 200 wasp species that associate with California Oaks.  There are also a wide variety of shapes and locations for galls, with some interesting spiky pink ones, cup shapes, sea urchin shapes, very small ones on leaves, and many others.  Not to mention that there are also insects that invade and parasitize the galls of other insects, all part of that wonderful web of life we are a part of.

A Hazard to the Tree?

It is generally thought that galls don’t harm Oak trees unless there are other stress factors already weakening the tree.  Some birds are natural predators of these wasps and usually there is a balance of predator and prey that keeps the gall wasps from becoming overpopulated to the point of causing problems.  However there are cases of extreme infestation where treatment may be needed.  Heavily infested branches can be removed in the early spring and disposed of to prevent the wasps from developing.  There is a chemical insecticide called carbaryl that can be sprayed on trees, but like all insecticides this should be a last resort since it kills other beneficial insects and is a toxic carcinogen for humans.

A Hazard to People?

Because the wasps are so small, and they do not sting, the insects are not thought to be a hazard to people.  The galls themselves are sometimes called Oak Apples, but they are definitely not tasty.  Apparently they contain tannin compounds and have a bitter taste, but I have not read anything about them being poisonous.  It is said that they were once used for inks and dyes, and even cleaning wounds.

Beneficial Insects?

I couldn’t find much about the benefits of having these insects in your garden. In part it depends on which of the hundreds of insects that make galls you are talking about.  There are definitely wasps that prey on other insects that damage plants such as aphids and caterpillars, so it seems like a good thing to have a few gall wasps around. I can’t help but wonder about the connection between different meanings of the word “gall”.  In a way these wasps do have some gall in using the tree to create a home for their eggs!  I wonder if there is any etymological connection here, perhaps one use of the word led to the other?

References:

Oaks of California, authors Pavlik, Muick, Johnson and Popper

Sunset Western Garden Problem Solver,  by the Editors of Sunset Books

KQED Science article: What Gall!  The Crazy Cribs of Parasitic Wasps, by Craig Rosa https://ww2.kqed.org/science/2014/11/18/what-gall-the-crazy-cribs-of-parasitic-wasps/

Wikipedia article on Gall Wasps:  https://en.wikipedia.org/wiki/Gall_wasp