Photo Credit: Beatriz Nobua-Behrmann, UCCE Orange
Parks in Southern California are losing many of their trees to the Invasive Shot-Hole Borers (ISHB). These tiny ambrosia beetles are carrying the fungus that is causing increasingly extensive damage to urban trees, riparian natural forests, and avocado groves. Thousands of severely infested trees have been killed or removed in natural and landscaped areas.
The Invasive Shot-Hole Borers (Euwallacea spp.) are small beetles (roughly the size of a sesame seed) that belong to two closely related and physically identical species: Polyphagous Shot-Hole Borer and Kuroshio Shot-Hole Borer. Both species are believed to have been accidentally introduced into California via wood products and/or shipping material from Southeast Asia. Since ISHB was first identified in Los Angeles County in 2012, the infestation has spread to six other counties, including Orange, San Diego, Ventura, Santa Barbara, San Bernardino and Riverside. Once the beetles arrive at a new location, they steadily spread to neighboring areas, infesting more and more trees. Movement of infested firewood and green waste can spread the infestation over even longer distances.
These beetles bore into trees, creating a series of small galleries. Inside these galleries, they “farm” a fungus (Fusarium spp.) they use as their main food source. The fungus colonizes the trees’ vascular system and causes a disease called Fusarium Dieback. Infested trees have trouble transporting water and nutrients up and down their trunks, which manifests itself as branch dieback, general tree decline, and, in many cases, tree death.