Ian Pearse

DAVIS — Many insects that target California's native oak trees will also feed on non-native oaks planted near them, but with one distinct difference: The insects tend to do more damage to the non-native oaks that are closely related to the natives, than they do to the distant relatives.

So says University of California, Davis researcher Ian Pearse in a recently published article and cover story in the journal Proceedings of the National Academy of Sciences (PNAS). 

"This is a confirmation of the ideas dating back to Darwin," said Pearse, a doctoral candidate in the Department of Entomology who for three summers studied insect damage to 57 species of introduced (non-native) oaks in the Peter J. Shields Oak Grove botanical garden, located along Garrod Drive in the UC Davis Arboretum. The dominant oak species is the native Valley oak, Quercus lobata Née.

"The insects were mostly small moths, fairly inconspicuous," Pearse said. "They don’t cause a lot of damage. This was more of a theoretical study, of how insects on native oaks also tend to interact with non-native oaks that are similar."

Pearse, with co-author Andrew Hipp of the Illinois-based Morton Arboretum and Field Museum of Natural History, Chicago, asked "Why do herbivores interact with some non-native plant species but not others?"

"Relatedness to the native oak is a strong predictor of herbivory," Pearse said, "as are several other plant traits such as the phenology of leaf development."

The researchers determined, through molecular methods, which oaks were closely related and which were distantly related and then assessed herbivory and leaf traits. They divided the insects into leaf chewers and leaf miners. Leaf chewers, predictably, incurred more damage. The larvae of mining insects live and feed between the epidermal layers of a leaf and thus have more space constraints, Pearse noted.

"Introduced plants tend to experience less herbivory than natives, although herbivore load vary widely," the researchers wrote in their abstract. If the non-native plant is similar, insects may recognize it as a potential host. Alternately, they also may feed on which plants "provide the best nutrition, irrespective of similarity to native species."

"Introduced oaks that are more closely related to the native oak received more chewing and mining damage than distantly related oaks," Pearse said, "and introduced oaks that had greater overall similarity in leaf traits (such as tannins, phenolics or other bitter-tasting chemicals) also received higher chewing damage but not mining damage."

"Ian's study is important for several reasons," said noted insect ecologist Rick Karban, professor of entomology at UC Davis and Pearse's major professor. "Our collective intuition about what makes some introduced plants, including crop species, more susceptible to herbivores than others is poorly developed. By using a large number of oak species planted in a common environment, and accounting for the relatedness of the species, Ian can answer that question with a great deal of elegance and power. His finding that relatedness of the various oaks to the native species explains a lot of the picture and provides considerable insight."

The journal cover features an image of the mural "Oak Family Tree," from the UC Davis Arboretum oak collection. The mural, created through the UC Davis Art/Science Fusion Program, directed by entomology professor and artist Diane Ullman and artist Donna Billick, depicts the evolutionary relationships of 29 oak species and the animals associated with each species.

"The project was a collaboration with the arboretum," Ullman said, noting that Emily Griswold, a national leader in oak conservation and the arboretum's assistant director of horticulture, "provided the leadership and knowledge base from the arboretum."

"Shields Oak Grove is the arboretum’s most scientifically significant collection," Griswold said, "and we’re thrilled to have research happening in the collection that takes advantage of its depth and diversity."

The PNAS article is drawing widespread interest from ecologists, taxonomists and oak enthusiasts. Pearse is the first person to create a phylogeny of the oaks in Shields Oak Grove. Internationally recognized oak expert John Tucker (1916-2008), former UC Davis botany professor and a former director of the arboretum, helped plant the trees nearly a half century ago. The UC Davis Plant Biology Department’s herbarium bears his name, the J. M. Tucker Herbarium.

"More than 80 kinds of oaks are found in the Peter J. Shields Oak Grove, including scientifically documented trees native to the United States, Central America, Europe and Asia," said Griswold. "The arboteum holds one of the most diverse mature oak collections in the United States and has been recognized as a collection of national significance through the North American Plant Collections Consortium."

Pearse received a fellowship from the National Science Foundation for his research. Long interested in plant biochemistry, he joined the UC Davis graduate program in entomology in 2006, after studying a year in Germany with the Max Planck Society for Chemical Ecology. He received his bachelor of science degree in plant biology from the University of Illinois, Urbana-Champaign, in 2004.