Researcher Ian Pearse looks at an oak apple gall. His research team at UC Davis studied galls on valley oaks in Davis, Woodland and Vacaville. Kathy Keatley Garvey/Courtesy photo

Local News

Researchers learn more about oak apple galls

By March 31, 2011

Those valley oak trees in California’s Central Valley have a lot of gall.

Scientists Ian Pearse, Maxwell Joseph and Melanie Gentles of the UC Davis department of entomology knew that going into their research, but in a survey of 1,234 oak apple galls, what they found has led to a better understanding of the gall-making wasps and the organisms that prey upon them or live with them.

“Oak apple galls are themselves a complex ecosystem, with over 20 species of insects, that are in many people’s back yards,” said Pearse, who is studying for his doctoral degree in entomology with major professor Rick Karban. “The galls and their wasps are not a major problem for oaks but are themselves food for other organisms such as birds and other insects.”

The wasp, a member of the Cynipidae family, lays her eggs on the leaves or twigs of a valley oak, which then forms a gall or a structure that resembles an apple hanging from the tree.

“The gall is actually very beneficial, and necessary, for the insect,” Pearse said. In reality, the insect “ ‘coerces’ the plant to make it a great home.”

Their research, published in a recent edition of the international journal Biodiversity and Conservation, shed light on the natural history of the common oak apple gall and its parasitoid and inquilines community. They found that the composition of the insect community varies with galls of different size, phenology and location. Their collection sites included Davis (around Putah Creek), Woodland and Vacaville.

The researchers discovered that the gall maker “most often reached maturity in larger galls that developed later in the season.”

Galls, which hang on the valley oaks like apples, are especially visible this time of year.

Researcher Joseph, who worked closely with UCD professors Sharon Lawler and Rick Karban, recently received his bachelor of science degree in wildlife, fish and conservation biology. Joseph is studying for his doctorate in ecology and evolutionary biology at the University of Colorado.

Gentles, who worked with major professor (now emeritus) Les Ehler at UCD, received her master’s degree in entomology. She is the UCD campus arborist.

Kathy Keatley Garvey

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