Thousand Cankers Disease: Attraction of Pityophthorus juglandis to volatiles associated with black walnut and Geosmithia morbida

Thousand cankers disease (TCD) is a pest complex formed by the association between the walnut twig beetle (WTB), Pityophthorus juglandis (Coleoptera: Curculionidae: Scolytinae), and the fungal pathogen Geosmithia morbida. TCD has caused the widespread death of walnut trees throughout the West and has recently been introduced to the midwestern and eastern US, and threatens black walnut (Juglans nigra) throughout its native range. WTB locate suitable black walnut trees by orienting to host odors and girdling increases the production of these compounds. Once on an appropriate tree, males release volatile aggregation pheromones that coordinate mass attack and mating. Currently, monitoring and detection efforts for WTB rely on a pheromone lure that is effective from a very limited distance, while plant- and fungal-derived kairomones that may facilitate host location remain poorly understood. In this study, we performed both olfactometer and field experiments to test the hypothesis that adult beetles are attracted to volatile organic compounds (VOCs) of black walnut and Geosmithia morbida. Through dynamic-headspace sampling, we collected VOCs of intact and girdled walnut trees and compared their profiles by coupled gas chromatography-mass spectrometry. We then performed olfactometer bioassays to evaluate the attraction of adult WTB to authentic standard of those compounds that were unique or more highly represented in the collections from girdled walnut. We also tested the capacity of volatiles of black walnut to increase the attraction of WTB to pheromone baited traps in the field. Finally, we characterized the VOCs of G. morbidaand, through behavioral bioassays, discovered a suite of pathogen-specific compounds that are attractive to adult WTB. This work may lead to the development of a kairomone lure to enhance current detection and monitoring efforts and aid in the early detection of WTB.

Keywords: walnut twig beetle, Scolytinae, kairomone, Juglans nigra, vector