Cold tolerance of walnut twig beetle (Coleoptera: Scolytidae) from northern California

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 in the East.   This beetle vectors a phytopathogenic fungus, Geosmithia morbida (Kolařík), found to cause cankers in many species of Juglans.  This insect-pathogen complex causes Thousand Cankers Disease (TCD), which is currently found in 14 states and northeastern Italy.  Our objective was to determine the cold hardiness of walnut twig beetle from California to forecast where cold will limit northerly spread of this insect.  Branch sections from a naturally infested black walnut hybrid, [Juglans hindsii X (J. nigra X J. hindsii/J. californica)], were sent monthly from August 2012 to May 2014 from Sutter Co. CA to a Biosafety Level-2 quarantine facility in St. Paul, MN.  We used contact-thermocouple thermometry to measure the supercooling point of adults and larvae and the lower lethal temperature for adults.  The lowest mean adult supercooling points in winter of 2012-2013 and 2013-2014 occurred in January (‑18.7°C ± 0.5 SEM) and February (‑18.2°C ± 0.5), respectively.  The lowest mean supercooling point measured for larvae was in December 2013 (‑17.5°C ± 0.4).  Comparisons of supercooling points and lower lethal temperatures suggest that adults are freeze-intolerant.  Phloem temperatures in eight black walnuts compared with air temperatures in winter of 2013-2014 suggest that walnut trees provide some insulation.  Walnut twig beetles in Colorado, Indiana, Ohio, and Pennsylvania are likely near the potential northern range limits for this insect.