Host selection by the walnut twig beetle, Pityophthorus juglandis, in California
Walnut (Juglans) and wingnut (Pterocarya) trees have been declining in response to Thousand Cankers Disease (TCD). The TCD pathogen, Geosmithia morbida, is vectored by a phloeophagous bark beetle, the walnut twig beetle (Pityophthorus juglandis Blackman). To understand P. juglandis host selection, the following were investigated at two orchards in Northern California: 1) The susceptibility of two walnut species (Juglans californica and J . major) native to the western USA by comparing P. juglandis flight and landing responses to unbaited branch sections and 2) The relationship between English walnut (Juglans regia) tree health, represented by several levels of plant water stress, and P. juglandis colonization success. P. juglandis preferred to land on J. californica over J. major. With J. regia, there was no significant difference in P. juglandis activity between branches from trees in high water stress and low water stress groups (both baited with P. juglandis aggregation pheromone). No P. juglandis colonization activity was observed in unbaited branches from the negative control group (unbaited). Activity by P. juglandis was found primarily on 4 cm diameter branches. Other invasive bark and ambrosia beetles such as Hypothenemus eruditus and Xyleborinus saxeseni; and an invasive longhorned beetle, Nathrius brevipennis preferred to land on J. major over J. californica. Collections of H. eruditus and an invasive ambrosia beetle, Xyleborus affinis, in Butte County reflect the northernmost records of these species in California. No significant relationships were found in J. regia between water stress and activity by N. brevipennis and a parasitoid Gildoria sp; however, activity by N. brevipennis and Gildoria sp. was found primarily on secondary 1 cm diameter branches (twigs). In conjunction with this study, significant flight responses of N. brevipennis and X. saxeseni to ethanol were recorded in a J. regia orchard. Overall, this study demonstrated that volatile cues from Juglans may influence host selection by P. juglandis and associated insects.