Pacia Purcell ’18

Project TitleIdentifying the potential expansion of the Walnut Twig Beetle, Pityophthorus juglandis, to Black Walnut trees, Juglans nigra
Mentor – Nathan Amador Rowley

A defining characteristic of invasive species is that they typically have significant negative impacts on the ecosystems they invade. Beetles as invasive species, can be quite harmful as they are the most abundant insects. Specifically, beetles that live in the bark of trees are a major disturbance to forests throughout the United States. Pityophthorus juglandis, the Walnut Twig Beetle, is native to Southern California, New Mexico, and Arizona and is known to feed on several different species of walnut trees. Walnut Twig Beetles feed on the phloem of walnut trees and are associated with a pathogenic fungus, Geosmithia morbida, or thousand cankers disease. Thousand cankers disease (TCD) is carried by the beetle and transported to the walnut trees through feeding and gallery formation resulting in multiple lesions in the phloem of the walnut tree. These lesions coalesce together killing off twigs and branches and eventually the entire tree. In many cases, trees showing symptoms die within 2-3 years of infection. Walnut Twig Beetles were originally found in only four counties in the southwestern US including Los Angeles County in California, Grant County in New Mexico, and Coconino and Cochise counties in Arizona, but can now be found in over 115 counties as far as Virginia and Ohio. Black Walnut trees, Juglans nigra, have the highest susceptibility to TCD and are native to the eastern half of the US. Trade and travel between the western and eastern United States has augmented the spread of the Walnut Twig Beetle. This work utilizes geographic information systems software, ArcGIS, to determine the possible further expansion of the Walnut Twig Beetle and TCD to Black Walnut trees, using data on the beetles’ current habitat range and the distribution and population densities of Black Walnut trees across the United States.