California Gall Wasps
By Tami Reece UCCE Master Gardener
What are the brown balls that look like fruit growing on my oak trees? Mary B. Atascadero
The apple size structures on the ground under your oak trees are probably oak galls caused by small wasps. These wasps are harmless to pets and humans and there are a variety of species of wasp species that commonly infest oak trees. Most galls are initiated by insects, but can also be caused by bacteria, fungi, or mistletoe.
Different gall wasp species form different types of oak galls on different areas of the tree, during different parts of the season. One of the most common in our region is the apple gall wasp (Andricus quercuscalifornicus). 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.
The lifecycle of California gall wasps alternate between sexual and asexual reproduction. The springtime generation consists of both males and females. The generation of the summer and fall consists of females only that reproduce parthenogenetically, or without male genes. The spring generation gives rise to galls forming in March and April and newly emerged adults in May through June. The summer and fall all female generations typically emerge as adults the following spring. There are hundreds of insects that use California oaks as a food source, many of which are gall formers.
As with other gall wasps, no control measure is necessary or recommended. The galls do not harm the tree or weaken it any way. Gall wasps and the gall itself are important host 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.
On Saturday August 20, the Master Gardeners held their 10th Annual Tomato Extravaganza and Plant Sale. We would like to thank everyone who attended and announce the winners of the tomato and basil voting. The top tomato for taste was Brandywine. The second was Black Cherry and the third Yellow Pear. The top basil voted for taste was Sweet Thai with Lime Basil and Queenette Thai as a close second and third. See you next year!