Host colonization behavior and aggregation pheromones in the walnut twig beetle: Echoes of the Ph.D. research of David L. Wood

The walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Scolytidae), has emerged recently as a pest of black walnut, Juglans spp., in the western United States. As a vector of the fungal pathogen for thousand cankers disease it has been implicated in a growing number of cases of crown dieback and tree mortality of walnut trees on city streets, along rural highways, and in native forest stands. Bioassays of the flight response of P. juglandis to freshly cut branches of northern California black walnut, Juglans hindsii, infested artificially with males and females revealed that the beetle is active in northern California (Yolo County) from mid-March until early November with significant bouts of flight in April/May and August/September. At these times of year we have observed that the flight is crepuscular. The flight responses of males and females to uninfested branches or female-infested branches were not significantly different, and did not differ from the flight responses to an unbaited trap. However, males and females responded significantly to male-infested branches. When males and females were combined in either of two methods on freshly cut branches, the flight responses of males and females to the trap bait were significantly greater than the responses to male-infested branches alone. This synergistic effect indicates that there are both male- and female-contributed components to the aggregation pheromone of P. juglandis. There does not appear to be a host-based pheromone co-attractant or kairomone involved in the flight response of P. juglandis.