Risk of invasion by walnut twig beetle throughout eastern North America

Abstract

The walnut twig beetle (Pityophthorus juglandis Blackman) is a domestic alien invasive bark beetle in the United States of America (USA) that vectors a phytopathogenic fungus. Together, the beetle and fungus cause thousand cankers disease in species of Juglans and Pterocarya. Geographic range expansion by P. juglandis from its native range in the southwestern USA throughout the western United States and isolated areas of the eastern United States provides evidence for human-mediated movement. The disease is now present in more than 120 counties on naïve native and cultivated hosts in the eastern and western USA and in northern Italy. This research describes the cold mortality and host suitability of P. juglandis. I measured the cold tolerance of P. juglandis adults and larvae from a northern California population monthly from January 2013 to May 2014. I found significant seasonal changes in adult supercooling points in fall, winter, and spring. I observed a shift in cold-tolerance strategy in P. juglandis adults from freeze-intolerance (December 2013 and January 2014) to partial freeze-tolerance (February 2014). Adults appear to be more cold-hardy than larvae. Predicted winter survivorship in the southeastern USA is higher than in the northeastern USA. I conducted field and laboratory trials to determine if reproduction by P. juglandis varies between two black walnut (Juglans nigra L.) parent trees and black walnut and butternut (Juglans cinerea L.). Fewer adult offspring developed in branch sections of the black walnut maternal ‘Sparrow’ parent than the paternal ‘Schessler’ parent over three summer months and one winter month in the lab. In the field, P. juglandis reproduction in black walnut and butternut did not differ. In an expanded laboratory study of host suitability, I screened 11 Juglans spp., one Pterocarya sp., and two Carya spp. over two years. Eleven Juglans and one Pterocarya species supported complete brood development. Julgans nigra, J. californica, and J. hindsii had the greatest levels of reproduction. Less suitable hosts include native southwestern United States hosts (J. major and J. microcarpa), Eurasian species (J. regia), Asian butternuts (J. ailantifolia, J. mandshurica, and J. cathayensis), and native eastern United States butternut (J. cinerea) and Japanese walnut-butternut hybrid (J. ailantifolia × cinerea). The two Carya species were not hosts. Finally, I present a framework that provides strategies for accessing stakeholder knowledge of unspecified pathways that may move forest insect pests. Using social science, stakeholder analysis, and design principles, the framework provides risk managers a tool to consult stakeholders for pathway information. The result is a list of pathways that can be validated independently. My research provides biological information of the potentially limiting factors of the spread and establishment of P. juglandis. Although the impacts of thousand cankers disease appear variable, the probability of exposure of walnut to P. juglandis appears to be limited by cold temperatures and host species. The overall risk of P. juglandis to the eastern United States is not determined by this body of work. The national perception of risk or concern over P. juglandis to walnut has decreased since I began this dissertation. Despite the shift in national perception, however, the completion of this work provides state and federal regulators information for improved decision-making regarding trapping and monitoring, quarantines, and how to research unspecified pathways of movement.