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Davis, California

Tuesday, May 28, 2024

Parasitic wasps depend on campus oak trees

Consider yourself lucky that you aren’t an oak tree.

If you were, you would likely become a living host for a parasitic wasp known as Andricus quercuscalifornicus. This wasp would lay its eggs on your branches, and then induce a cancer-like growth, called a gall, around the eggs that will shelter, feed and birth the next generation of wasp.

“There are over 10,000 trees on [the UC Davis] campus,” said Melanie Gentles, the UC Davis campus arborist. “Three to four hundred of those trees have galls, and I’ve looked at every one of them.”

These wasps are from the family Cynipadae, which means gall-making wasps. The wasps do not bite and pose no danger to humans. They are extraordinarily small, only slightly larger than a grain of rice, and are golden brown in color.

The galls on the oak trees are natural woody tissue produced by the tree. It has been hypothesized that the gall is created by the tree in order to separate itself from a potential infection.

“The wasp inserts its eggs into the branch, which causes this reaction by the tree, basically a swelling of woody tissue,” Gentles said.

Gentles co-authored a study on these oak galls along with entomology graduate student Ian Pearse. According to Pearse, the galls do not noticeably affect the health of the tree.

“The galls are not an economic problem to the trees,” Pearse said. “[However], oak trees that have a whole lot of galls tend to produce fewer acorns.”

Both Pearse and Gentles have noticed that some oak trees are host to many hundreds of galls, while similar trees in close proximity have no galls at all.

“It’s a bit of a mystery to science,” Pearse said.

Pearse believes that the difference is in subtle genetic variation that makes some oaks immune to the gall-producing capabilities of the wasps. It is an evolutionary arms race between the oaks’ resistance and the wasps’ ability to induce a gall.

The galls grow in a wide range of sizes, and can be as small as the tip of your thumb or as large as a melon. Each gall can provide shelter for anywhere between zero to 40 developing wasp larvae.

Wasps are not the only organisms to benefit from a hard wooden shelter. Once the wasps vacate the gall, it can become home to various species of moth, become food for beetles that feed on the gall tissue and be a source of food for birds who feed on the wasps and beetles. The galls are also a target for other species of parasitic wasp that lay their eggs inside of the developing gall wasp larvae.

Many students have noticed the large tumorous growth on the elm tree outside of Olson Hall on campus. It is important to note that this is not a gall. This growth is known as a burl.

“Burls can be caused by stress or various organisms such as bacteria, insects and nematodes [worms], and they are not all the same,” said Valerie Williamson, a professor of nematology at UC Davis, and recently named fellow of the American Association for the Advancement of Science. “They are not usually directly harmful, but the associated organisms can cause disease.”

“Lots of those [burls] can be caused by agrobacteria, bacteria that infect plants,” Pearse said. “They can be, but are often not insect related.”

Despite the grotesque visual appearance of galls, the growths provide a tiny ecosystem for many organisms. There can be around 20 different species inhabiting the gall, feeding on the gall or living off the gall’s residents.

HUDSON LOFCHIE can be reached at science@theaggie.org.


  1. Thanks for the detailed explanation. I have seen and known of galls for years but did not really understand how they came to be.


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