BY ANNE RAVER
Two closely related Asian beetles are boring their way through Southern California’s trees.
FROM THE MARCH 2018 ISSUE OF LANDSCAPE ARCHITECTURE MAGAZINE.
Smaller than sesame seeds, two beetle species are spreading through Southern California, killing hundreds of thousands of trees and infecting many thousands more with a pathogenic fungus.
At first, scientists thought the pests were the same species because they look exactly alike, but they carry different pathogenic fungi, and DNA analysis revealed genetic differences. But their damage to trees is so spectacularly similar that the two beetles?the polyphagous shot hole borer and the Kuroshio shot hole borer?are now referred to collectively as the invasive shot hole borer (ISHB). A 2017 U.S. Forest Service survey estimated that 23 million trees are vulnerable to the ISHB that is working its way through Los Angeles, Orange, Riverside, and San Bernardino Counties?or 33 percent of Southern California?s urban canopy. It?s impossible to know how many trees will die, but the projected losses are catastrophic.
The polyphagous shot hole borer (PSHB) was first caught in a California Department of Food and Agriculture trap in Whittier Narrows in Los Angeles in 2003. Identified as a type of ambrosia beetle, of which there are 3,500 species?most of which partner with fungi to decompose dead trees?the insect didn?t appear to be doing any damage, particularly to economic crops, so it raised no alarms.
But by 2010, hund...