Walnut Twig Beetle, Thousand Cankers Disease
The walnut twig beetle (Pityophthorus juglandis) is a bark-boring insect native to the western United States. Known to infest multiple species of walnut, the Arizona walnut is thought to be the original host of the walnut twig beetle. Recent research has linked the spread of Thousand Cankers, a disease affecting walnut trees, with the activity of the walnut twig beetle and a newly discovered fungus, Geosmithia morbida. Thousand Cankers Disease was first identified in Colorado and its presence was then confirmed in other western states, including Utah, New Mexico, Arizona, California, Nevada, Oregon, Washington, and Idaho. Since 2010, the walnut twig beetle and Thousand Cankers Disease have spread to Tennessee, Virginia, Pennsylvania and North Carolina through the movement of infected wood and wood packing material.
The walnut twig beetle belongs to the Scolytidae family and is one of the few beetles in the Pityophthorus genus that feeds on hardwood tree species. The adult beetles typically tunnel into branches larger than two centimeters in diameter and overwinter in bark cavities that they’ve excavated. The larvae feed underneath the bark of the walnut tree and then emerge as adults in the early summer. The walnut twig beetle is yellowish-brown with a rounded head. The beetle is smaller than a grain of rice, no longer than two millimeters.
The tiny beetle serves as a vector for the fungus by carrying the fungal spores on its wing covers. The fungus then spreads into the wood of the walnut and eventually starves the tree of nutrients as the fungus kills its nutrient-transporting tissues (phloem). Thousand Cankers Disease and the walnut twig beetle threaten the black walnut (Juglans nigra) in the eastern United States, the tree’s native range. Symptoms of Thousand Cankers Disease include crown dieback, yellowing of the upper leaves, the wilting of leaves and dark amber stains along branches and stems.
DPI is currently surveying and trapping for the walnut twig beetle in areas where black walnuts are present. Please do not tamper with the black funnel traps you may find hanging near walnuts. If you think you have found a walnut tree with Thousand Cankers Disease or a walnut twig beetle, send pictures to firstname.lastname@example.org or call DPI at 864.646.2140.