Scientists Develop Successful Lure to Attrack Beetle That Causes Thousand Cankers Disease in Walnut Trees
Walnut trees are cultivated throughout the western hemisphere and Eurasia. The primary orchard species grown for its edible nuts is Persian, or English, walnut. Various black walnuts are grown in forest plantations or harvested wild for fine wood products. Several black walnut species are also used as hybrids or pure strains to provide grafting rootstock for other varieties. The emergence of the walnut twig beetle, which spreads thousand cankers disease, threatens worldwide cultivation of walnut. In the U.S. alone, the beetle's distribution expanded from four counties in 1960 to 100 counties by Sept. 2013. A key to this pest's expanding range is its association with the pathogen, Geosmithia morbida, which the walnut twig beetle introduces into a host tree when it bores into the phloem of its branches and main stem. The resulting disease, thousand cankers disease, has caused widespread mortality among native and adventive walnuts throughout the U.S. At this time there is no control method for the disease, other than cutting and destroying the infected trees. Scientists at the Forest Service's Pacific Southwest Research Station developed the male-produced aggregation pheromone of walnut twig beetle as a detection tool for incipient populations of the beetle. They developed guidelines for the use of the pheromone and identified the movement of barked raw walnut wood as the potential primary source of new foundation populations of walnut twig beetle around the world. The trapping guidelines suggest trapping in residential areas, especially older neighborhoods; near lumber and veneer mills; green waste facilities; firewood lots; and parks, arboreta, orchards, or plantations. In wildland forest habitats, look for riparian areas where walnut trees often grow in higher densities. Trap from March to November, with a focus on April to June and late August to late October; and replace lures every 2-3 months. There is potential for the walnut twig beetle to spread thousand cankers disease further within North America and internationally via human-mediated transport of any raw walnut wood materials with bark attached, including saw and veneer logs, burls, and firewood. The beetle may also expand its range from Arizona and New Mexico by dispersing eastward. Germplasm trials in California suggest that native walnuts in South America, Europe, and Asia are likely to be colonized by walnut twig beetles and infected with thousand cankers disease.