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- Naturalist's Feature -

Photos: Oak Galls on Valley Oak Trees in Spring and Fall

Oak galls, oak trees, oh my!
By: Martha Cerda, Naturalist

Nature is full of intricate and awing relationships among organisms. It takes two to form a connection and in a symbiotic relationship one or both organisms may benefit from such interaction. Sometimes only one of the organisms may benefit while the other doesn’t gain or lose anything. Other times, one inflicts harm on the other.

At the Alviso Adobe Community Park it is easy to find such relationships between organisms. One example is the connection between the oak tree and the tiny gall wasp. The gall wasp is an insect that resembles an ant, but has wings. Female gall wasps lay their eggs in leaves or branches causing a lump or swelling around the eggs. When the gall wasp larvae hatch, they excrete chemicals that trigger a response from the host plant. The small lumps then turn into galls. These abnormal plant growths vary in size, shape, and color. Gall wasps appear to have little to no negative impact on the host plant. The gall is very beneficial and necessary for the insect as its spongy mass serves as both a consistent food source and a cozy home protecting the larvae from any dangers.

If you’ve taken a stroll through the Alviso Adobe Community Park lately, you’ve probably noticed the Valley Oak trees are almost bare of leaves. The Valley Oak (Quercus lobate) and the California gall wasp (Andricus quercus californicus) have a unique relationship indeed. Hanging from the branches, you can see clusters of insect galls. The galls are woody and strike a resemblance to rotting apples, although they smell nothing like them and are tasty only to wasps. At this time of year, you can see many of them on the ground with clear tiny exit holes, a signal that the wasp has developed and taken off. Female wasps lay eggs in the stems of the oak tree in the fall. In spring, larvae hatch and begin to feed and produce a plant growth hormone that prompts the gall to form. Galls do not begin to form until spring; they are about 1-2 inches in diameter and green in color. By fall, the galls are shades of brown and fall to the ground.

This symbiotic relationship between the insect and the oak tree demonstrates how all living organisms depend on one another. Oak galls have been used throughout time to treat cuts, wounds, produce dye, and ink. Never seen an oak gall before? Join us on Hiking with a Naturalist on February 16th to learn more about relationships in nature and more! See the ads below for more programs this February including Hiking with a Naturalist, Nature Storytime, and Morning Walks and Tweets. 
Recipe for oak gall ink:
●       5 galls- 5 g. 
●       Distilled water- 65 ml.
●       Ferrous sulphate- 1 g.
●       Gum Arabic – 1 g.
●       Heating component
●       Pot

1.     Crush galls into a fine powder
2.     Place galls in container with water and heat until the water turns dark brown
3.     Strain out the oak gall powder
4.     Add the ferrous sulfate
5.     Add the gum arabic
6.     Use as an ink!

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Volunteering at Alviso Adobe Community Park
The Alviso Adobe Community Park is always looking for volunteers to assist around the park and docents to help with our special events and school programs. Look at our volunteer opportunities online at Better Impact and create your volunteer account if you want to be involved!
Follow this link to get started in applying to be a volunteer:
Alviso Adobe Community Park
3465 Old Foothill Road
Pleasanton, CA 94588

Facility Hours:

Contact our Staff:

Martha Cerda

Nick Carozza
Assistant Naturalist

Heather Smalley
Camp Coordinator

Amanda Schmidt
Volunteer Coordinator and Administrative Assistant


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City of Pleasanton · 3465 Old Foothill Road · PO Box 520 · Pleasanton, CA 94566 · USA