Avocado growers have faced serious threats in the past, including thrips, mites and root rot, which threatened to destroy the industry during the 1940s and 1950s, until trees based on resistant rootstocks — and a cost-effective way of propagating them rapidly — were developed. However, a new pest and disease complex is spreading across Southern California, jeopardizing the avocado industry as well as a wide range of native vegetation and popular landscape trees.
Tiny beetles known as the polyphagous shot hole borer (PSHB) and Kuroshio shot hole borer (KSHB) burrow into trees and deposits spores of a fusarium fungus. Then, they lay their eggs in the tunnel and the emerging larvae feed on the fungus. As the fungus grows it quickly clogs the tree’s vascular system, causing branches to die and eventually killing the tree.
Besides avocados, PSHB is known to attack more than 200 species of woody plants. More than 30 of these are reproductive hosts, including some of our most beloved natives: oaks, sycamores, cottonwoods, and alders. The beetles and fungus thus pose a serious threat both to the urban landscape and to natural habitat. So far, no control method for either the beetle or the fungus has been found. It is easily transported in loads of firewood and improperly handled green waste; the best way to slow its spread is to avoid moving these materials from one area to another.
The beetles originated in Southeast Asia and likely made their way to this region in freight shipments containing
wooden packing material. There have been at least two introductions, PSHB first showing up in ornamental
plantings in Los Angeles County in 2003 and KSHB appearing more recently in San Diego County, where it
infested numerous commercial avocado groves. In late 2015, PSHB was found to have infested commercial avocado
groves in the Santa Paula area.