The effect of temperature treatments on the survival of Pityophthorus juglandis (Coleoptera: Scolytidae)

The walnut twig beetle (Pityophthorus juglandis; WTB) vectors Geosmithia morbida, the causal agent of thousand cankers disease in black walnut (Juglans nigra).  Arizona walnut (J. major), the apparent native host of the WTB, is distributed in the southwestern USA where winter temperatures are higher than those found in the native range of black walnut.  To better understand temperature effects on survival of the WTB, we initiated studies to determine: (1) seasonal variations in cold tolerance, as measured by the supercooling point (SCP), and (2) upper and lower lethal temperatures (LT₉₉).  WTBs were exposed to decreasingly cooler temperatures and SCPs were measured, or specimens were removed at various subzero temperatures and checked for survival.  In the later study, Xyleborinus saxeseni was used for comparison.  Insects were also exposed to increasingly higher temperatures, removed at various temperatures, and checked for survival after 24 hours.  Upper and lower lethal temperatures were determined using a logistical model.  Mean SCPs for adults and larvae WTBs were between -17°C and -17.5°C.  Upper limit LT₉₉ for adults and larvae were 48.96°C and 47.44°C respectively and lower limit LT₉₉ for were -18.1 and -18.7 respectively.  X. saxeseni lower limit LT₉₉ was -19.3.  These studies, as well as beetle survival in infested Colorado trees where temperatures reached -29°C in January 2011, suggest WTB could survive the winter in much of the native range of black walnut.  This research also suggests phloem temperatures of heat-treated logs must reach at least 49°C to provide an effective control of WTB.