Effects of phytosanitation treatment on walnut twig beetle (Pityophthorus juglandis) colonization of black walnut logs

The walnut twig beetle (WTB) (Pityophthorus juglandis) is a recently introduced invader of Eastern deciduous forests and vectors the fungal pathogen Geosmithia morbida. Together, these organisms comprise the insect-pathogen complex, thousand cankers disease (TCD). TCD is a progressive and often fatal disease affecting North American Juglans species. Black walnut (J. nigra) is particularly susceptible. Movement of infested wood is the likely means of WTB introductions and long-distance dispersal, including the recent introduction of WTB into the black walnut native range. Movement of walnut has been restricted, however, the WTB range continues to expand, further threatening this valuable North American hardwood species.

This study sought to assess the efficacy of phytosanitation methods in preventing WTB colonization post-treatment. Round logs (bolts) of un-infested black walnut were cut and treated by steam heat, fumigation, kiln-dried lumber with bark on, kiln-dried lumber with bark removed, or no treatment. Bolts were baited with WTB lures and hung for 30 days in infested walnuts in Knoxville, TN. Colonization was assessed by recording WTB emergence (#WTB/100cm² bark) from each sample over 5 months. All treatments significantly reduced mean WTB density compared to the control, however, WTB successfully colonized the bark of steam heated and fumigated bolts with mean emergence densities of 26.26/100cm² and 15.56/100cm² respectively.  Sixty-three beetles were also recovered from 14 kiln-dried, bark on lumber samples. These results suggest further protection following phytosanitation treatment may be required in order to prevent further spread of the walnut twig beetle and associated pathogen.