Developing a kiln treatment schedule for sanitizing black walnut wood of the walnut twig beetle

Abstract

Geosmithia morbida is a fungus that causes numerous cankers on branches and trunks of walnut tree species (Juglans spp.), hence the common name "Thousand Cankers Disease" (TCD), which results in widespread morbidity and ultimately, tree mortality. This fungus is vectored by the walnut twig beetle (Pityophthorus juglandis) that feeds aggressively in the bark. Subsequently, cankers develop around the beetle galleries in the phloem. TCD is currently a major concern in Colorado. The beetle and fungus have been identified and confirmed in three states within the native distribution of black walnut trees; if the fungus expands beyond the quarantined counties and throughout the native range of black walnut (J. nigra), it could have devastating impacts on the nut and timber production industries. Black walnut wood products are valuable for their strength properties and rich dark color. Developing a protocol for heat-treating black walnut lumber and logs with bark intact is important so that they can be sanitized and then safely used. The purpose of this research was to determine whether the International Plant Protection Convention (IPPC) International Standards for Phytosanitary Measures (ISPM-15) standards and United States Department of Agriculture (USDA), Animal and Plant Health Inspection Service (APHIS), Plant Protection and Quarantine (PPQ) Treatment T314-a/c regulations are sufficient to kill live beetles in the bark. The thermotolerance of the walnut twig beetles was evaluated by subjecting walnut twig beetle populations at all stages (including eggs, larvae, pupae and adults) to a series of time and temperature regimens. This experiment was conducted from October 2011 to February 2012 by heating walnut twig beetle-infested black walnut wood in a laboratory oven. The heat-treatments were developed based on the ISPM-15 standards and USDA APHIS PPQ Treatment T314-a/c regulation standards centered on internal wood temperature. The treatments ranged from 42°C to 71.1°C (108°F to 160°F) and lasted between 30 and 120 minutes. The ability of adult beetles to emerge was used to evaluate if the treatment was successful. Results from the emergence trials showed that adults were able to survive up to 48°C (118°F) but no survival of any stage of beetle development was detected at 50.1°C (122°F) when wood samples were heated for 30 minutes. Results suggest that walnut twig beetle survival is variable depending on heating conditions, and an internal wood temperature of 56°C (133°F) for 30 minutes should be considered the minimum for safe treatment of walnut lumber and wood with intact bark.