Variation of supercooling points in a northern California population of walnut twig beetle (Pityophthorus juglandis Blackman)
Cold temperatures can limit range expansions of invasive insects. The walnut twig beetle (Pityophthorus juglandis Blackman) and its associated fungus, Geosmithia morbida, appear to be native to the southwestern United States (U.S.) but have spread to new regions of the U.S. These agents act together in a condition called thousand cankers disease, which has killed thousands of trees within the genus Juglans. The beetle has been detected in nine states in the western U.S. and four states in the eastern U.S. Expansion of the range of the beetle and pathogen throughout eastern North America, where the commercially-valuable black walnut (J. nigra) and endangered butternut (J. cinerea) are native, is a concern to nut growers, timber producers, and land managers. We are investigating the cold hardiness of walnut twig beetle to determine if cold will preclude its spread into the northern limits of the geographic ranges of the eastern hosts. Branch sections from naturally infested walnut (J. hindsii x nigra) were sent monthly from August 2012 to March 2013 from Sutter Co. California to a Biosafety Level-2 quarantine facility (MAES/MDA) in St. Paul, MN. Adults and larvae were dissected from the host material and held at room temperature for 48 hr to allow for gut evacuation. We used contact-thermocouple thermometry to measure the supercooling point, defined as the lowest temperature observed before the latent release of heat.
Supercooling points for both life stages were approximately –16°C for all months except January. We observed a temporal shift in the cold tolerance of adult walnut twig beetle. The adult mean supercooling point in January 2013 (n=19, –18.7°C ± 0.5 SEM) was significantly different than mean supercooling points observed in August (n=16, –15.8°C ± 0.3) and December (n=18, –16.1°C ± 0.3) of 2012 and February (n=22, –16.5°C ± 0.3) and March (n=16, –17.3°C ± 0.5) of 2013. Larval mean supercooling points observed in November 2012 through February 2013 were not significantly different. These data, when integrated with other measures of cold tolerance, provide insight about where the insect might overwinter and how it might do so.