The once-vibrant Sycamore Glen tree population is gradually transforming into brown leaves and falling branches due to an epidemic of beetle-inoculated fungus that has decimated California tree populations in the last few years, according to Biology Professor Gretchen North.
The Polyphagous Shot Hole Borer infestation has only recently been noticed, as it drastically affected a range of trees across campus this year, including sycamores, oaks and redbuds. The trees were already stressed from drought and were therefore more susceptible to infection, North said.
Occidental is just one of many sites along the beetles’ quickly moving path. According to the Division of Agriculture and Natural Resources (DANR) at University of California, Riverside, the beetle likely originated in Southeast Asia as a new species related to Vietnam’s Tea Shot Hole Borer. It was first found in Los Angeles County in 2003 and has since attacked over 200 species of trees in the LA area, working its way through Orange, Riverside, Santa Cruz and San Diego counties.
According to North, the epidemic hit especially hard at college campuses, horticultural orchards, botanical gardens and arboretums. University of California, Irvine had to remove 1,000 trees from campus in spring 2015.
“I don’t think you could say that any non-conifer tree in the Southwest is safe,” North said.
Given the pattern of its establishment, Biology Professor Elizabeth Braker and North speculate that the infestation could have spread from suppliers of local tree nurseries. Braker said that the nurseries’ stocks could have housed a small population of the beetles that dispersed rapidly.
“It’s one of the issues that globally we have to face,” Braker said. “Globalization and shipping plants and soil around the world is an example of unintended consequences.”
The DANR describes the Shot Hole Borer as a tiny dark brown or black beetle that drills into trees with sharp jaws and carries spores of a pathogenic fungus (Fusarium euwallacea) on its head. Pregnant females bore expansive tunnels under the bark, simultaneously inoculating the fungus and laying their eggs. The fungus spreads throughout the tree, providing food for the larvae after the eggs hatch.
“She’s a good mama; she’s making a place to live, and she’s giving them food,” Braker said about the female beetle.
When the larvae tunnel their way out of the tree, the tiny holes in the trunk create the appearance of a smattering of shotgun holes, Braker said. The fungus creates a wet dark stain surrounding the holes and discolors wood and leaves. Eventually, entire branches or even trunks can die.
“Polyphagous” means the beetle is able to feed on various kinds of food — in this case, many different tree species. North and Braker’s Organisms on Earth classes and a professional arborist hired by the college have surveyed the trees as a step toward creating a tree management plan. This fall, both parties confirmed the devastating capability of the beetle on campus.
“We found an awful lot of trees with signs of borer damage,” North said.
According to North, sycamore trees in the glen are suffering most. Two trees have already died and were recently cut down when their dropping limbs posed a risk to passersby.
“The sycamores seem to be one of the beetle’s favorite foods,” North said. “They seem to prefer deciduous trees with sort of soft wood.”
North is especially worried about the iconic oaks on the Academic Quad. The oaks show some evidence of beetle attack, North said, but currently are not suffering significantly.
Campus departments, such as Facilities and biology, are teaming up to quell the spread of the plight. But the beetle and the fungus are vigorous opponents — pesticides and fungicides are futile in combating the beetle and fungus since both reside inside the tree and chemicals only affect the surface, North said.
“At this point it doesn’t really help to take the tree out because the beetle is already here; it’s tiny, it flies, and all you need is one or two to spread the fungus,” North said.
In the prolonged battle to save Occidental’s trees, Assistant Director of Maintenance Ruben Campos has been trained in managing the infestations, and North and Braker have been tracking the infestations in a Google Maps database in their biology labs.
According to North, biocontrol agents are promising tools that Occidental may be able to use in the future. Researchers at Huntington Botanical Gardens are looking into various resistance mechanisms in some trees that can increase production of a sticky sap that deters beetle boring, as well as researching a naturally occurring microbe that competes with Fusarium euwallacea. They are not yet ready to publish the results of these possible biocontrols.
In the meantime, Braker said that continually planting a diversity of young trees will reinvigorate the immediate tree community and increase its chance of survival. Biology major Skye Harnsberger (junior) led the effort to plant species capable of strengthening the campus tree population.
“We had just a devil of a time coming up with trees that aren’t known hosts for the borer, don’t require a lot of water and that are native,” North said.
With a list of four promising tree species, Harnsberger and the biology department will initiate planting saplings this week in partnership with City Plants, a program by the city of Los Angeles aimed to expand and maintain LA’s green canopy.
Even if the young trees are successful, North and Braker said that there is no predictable time frame in which the campus could be declared a survivor of the beetle epidemic.
“I think it’s our new normal,” Braker said.