Behavioral responses of Pityophthorus juglandis to volatiles of walnut and Geosmithia morbida, the causal agent of thousand cankers disease
Thousand cankers disease (TCD) has caused widespread death of black walnut (Juglans nigra) throughout the West and was recently detected in the midwestern and eastern United States, the native range of black walnut. This disease complex is caused by a fungal pathogen, Geosmithia morbida Kolařík, Freeland, Utley and Tisserat, vectored by the walnut twig beetle (WTB), Pityophthorus juglandis (Coleoptera: Curculionidae: Scolytinae). Male WTB locate suitable black walnuts by orienting to host odors and, once on an appropriate tree, 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, and plant- and fungal-derived volatile organic compounds (VOCs) that may mediate host location remain poorly understood. The long-term goal of this research is to develop a detection tool for WTB to enhance surveys in both the native range of black walnut and the non-native range in the West. The rationale for this study is that the identification of VOCs used by WTB to locate suitable hosts will lead to the development of an enhanced tool to more effectively detect, delimit and monitor WTB.^ Behavioral bioassays were performed to explore the chemically-mediated host location behavior of WTB. I performed olfactometry bioassays to determine the extent to which genotype influences the attraction of adult WTB and determined WTB were attracted to all genotypes. The beetles also exhibited varying levels of attraction to genotypes. By mechanically injuring the most attractive genotype with girdling, the degree to which damage may enhance this attraction was established. Adult WTB responded more readily to volatile extracts from girdled branches than intact branches. Laboratory bioassays were also performed to test the hypothesis that adult WTB are attracted to volatiles of the pathogenic fungus G. morbida and other commonly associated fungi and determined WTB are attracted to crude extracts from the fungi and individual synthetic fungal VOCs.^ Field experiments were designed to test the extent to which volatiles of black walnut and G. morbida increased the attraction of adult WTB to traps baited with a commercially-available aggregation pheromone lure. No synthetic VOCs tested increased trap capture of WTB; in fact, some compounds even repelled WTB from traps baited with the pheromone lure. Racemic limonene decreased trap capture of WTB by three times in the field and could be utilized as a component in a push-pull tactic. A repellent (“push”) stimulus such as limonene is located in a commodity planting and acts to repel pest species toward an attractive stimulus (“pull”) from where the pest is later removed. Furthering this research holds promise for the creation of an attractive lure to enhance the utility of the pheromone lure. ^
Matthew D. Ginzel, Purdue University.