Effect of freezing temperatures on survival of the walnut twig beetle (Pityophthorus juglandis)

The walnut twig beetle (Pityophthorus juglandis) is the vector of Geosmithia morbida, the cause of thousand cankers disease of black walnut (Juglans nigra). The beetle is native to Arizona walnut (J. major), a tree species that is widely distributed in the southwestern United States and Mexico, and where winter temperatures are higher than those found in much of the native range of black walnut. No information is available regarding the cold tolerance of the walnut twig beetle, but we hypothesized that low winter temperatures could influence its population dynamics. The objective of this study was to assess the survival rate of walnut twig beetle adults and larvae when exposed to subzero temperatures and establish a median lethal temperature (LT₅₀). As a comparison, adult ambrosia beetles (Xyleborinus saxeseni) were also tested. Extracted or emerged walnut twig and ambrosia beetles were placed individually into 1.5 ml tubes and exposed to subzero temperatures in an environmental test chamber. Beetles were removed after exposure to increasingly lower temperatures, transferred to 4°C for 30 minutes, left at ambient temperature for 30 minutes, and then checked for survival. Based on liner regression models, the LT₅₀ was -16°C for both adults (R²=0.73) and larvae (R²=0.80) of the walnut twig beetle and -24°C for adult ambrosia beetles (R²=0.84). These studies, as well as adult and larval survival in infested trees in Colorado, where temperatures reached -29°C in January 2011, suggest the walnut twig beetle could survive the winter in much of the native range of black walnut.