Status and impact of walnut twig beetle in urban forest, orchard, and native forest ecosystems
The walnut twig beetle, a native phloem-boring bark beetle originating on Arizona walnut, has invaded urban, orchard, and native forest habitats throughout the USA as well as in Italy. Although the beetle has been associated with dead and dying walnut trees indigenous to riparian forests of the Southwest, the primary impact appears to have been on the health of landscape black walnut trees in urban and peri-urban sites in western US states, and in Pennsylvania, Tennessee, and Virginia. This has been reflected in numbers of trees removed and tree removal costs. In addition, trees have been killed in the primary US Juglans germplasm repository in northern California, and low, but measureable, tree mortality has occurred in some English walnut orchards in California’s Central Valley. As assessed under multiple circumstances, tree decline and mortality appear to develop gradually in response to infestations by this beetle.