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

Abstract

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.

Chemicals mediate the interaction between plant‐associated fungi and insects

“…This cross-kingdom communication facilitates fungal dispersal. Similar observations have been reported for bluestaining fungi, Fusarium solani, Raffaelea lauricola, Puccinia punctiformis, and Geosmithia morbida (Kluth et al, 2002;Luna et al, 2014;Zhao et al, 2015;Blood et al, 2018;Guillen Sanchez et al, 2021). In agriculture and forestry, communication between plant-associated fungi and insects is predominantly mediated by chemical signals, which contribute to plant diseases and pest outbreaks.…”

Thousand Cankers Disease in Walnut Trees in Europe: Current Status and Management

“…Based on knowledge from other insect–fungus interactions, various hypotheses have been made: the fungus might be used as a nutritional source by the insect, or it might give other advantages, increasing the success of beetle attacks [ 32 ]. In any case, WTB adults exploit not only aggregation pheromones and walnut volatiles to find suitable hosts but also GM volatiles [ 33 ], showing how close the relationship between the two organisms is.…”

Why do entomologists and plant pathologists approach trophic relationships so differently? Identifying biological distinctions to foster synthesis

“…In other cases, abilities to detect invasive pathogens may be improved by exploiting herbivore behavior, such as using host volatiles and pheromones to identify vector presence and hence probable new infection sites. For example, pheromone-based trapping of beetle vectors of the thousand canker disease fungus (Geosmithia morbida) of walnut has facilitated rapid detection and sanitation of this otherwise cryptic pathogen (Blood et al, 2018).…”