Behavioral Responses of Pityophthorus juglandis (Coleoptera: Curculionidae: Scolytinae) to Volatiles of Black Walnut and Geosmithia morbida (Ascomycota: Hypocreales: Bionectriaceae), the Causal Agent of Thousand Cankers Disease

Thousand cankers disease (TCD) is a pest complex formed by the association of the walnut twig beetle (WTB), Pityophthorus juglandis Blackman (Coleoptera: Curculionidae: Scolytinae), with the fungal pathogen Geosmithia morbida Kolařík, Freeland, Utley and Tisserat (Ascomycota: Hypocreales: Bionectriaceae). Current monitoring and detection efforts for WTB rely on a pheromone lure that is effective over a limited distance while plant- and fungal-derived volatiles that may facilitate host location remain poorly understood. In this study, we test the hypothesis that adult beetles are attracted to volatiles of black walnut, Juglans nigra L. (Juglandaceae), and the pathogen, G. morbida. We measured the response of beetles to head-space volatiles collected from leaves and stems of 12 genotypes of black walnut and found genotypic variation in the attractiveness of host trees to adult WTB. Volatile profiles of the most attractive genotypes contained more α-pinene and β-pinene, and adult beetles were attracted to both of these compounds in olfactometer bioassays. In bioassays, we also demonstrated that adult WTB are attracted to volatiles of G. morbida. These findings suggest that, in addition to the aggregation pheromone, dispersing WTB potentially use host plant and fungal volatiles to locate suitable larval hosts. Finally, we conducted a field experiment to determine the extent to which ethanol, a common attractant for bark beetles, and limonene, a known bark beetle repellent, influence the behavior of adult WTB to pheromone-baited traps. Although ethanol did not increase trap capture, WTB were repelled by limonene, suggesting that this compound could be used to manipulate and manage WTB populations.