Reproduction of walnut twig beetle (Coleoptera: Scolytidae) on walnut and allied Juglans species

Walnut twig beetle, Pityophthorus juglandis Blackman, is a domestic invasive insect that has expanded its geographic range within the United States from the Southwest, through the West, and into isolated areas of the East.   This beetle vectors a phytopathogenic fungus, Geosmithia morbida (Kolařík et al.), found to cause cankers in walnut, butternut, and wingnut trees.  This insect-pathogen complex causes Thousand Cankers Disease (TCD), currently found in over 115 U.S. counties and northeastern Italy.  Our objective was to determine differences in reproduction between Juglans spp. that occur naturally or are cultivated in the current or potential future range of P. juglandis.  Eight Juglans and two Carya spp. were tested.  Ten trees per species were sampled in July 2014 from Texas, Missouri, Minnesota, and Indiana.  Cut branches were infested with P. juglandis adults from northern California in a randomized block design in a Biosafety Level-2 quarantine facility in St. Paul, MN.  After 12 weeks, bark was peeled from branches, and all life stages were counted.  We found a significant effect of host on mean total reproduction per established female.  Mean reproduction in Texas walnut (J. microcarpa) was significantly less than in seven other hosts.  Mean reproduction in eastern black walnut (J. nigra) and hybrid butternut (J. cinerea × [J. cinerea × J. ailantifolia]) was significantly greater than in other Juglans spp.  Reproductive differences between hosts provide more information for landscape-level questions such as the risk of natural movement pathways and potential population growth in newly established urban, peri-urban, or forested areas.