Midwestern bark and ambrosia beetles (Curculionidae: Scolytinae) that attack stressed black walnut and their fungal associates

Walnut twig beetles (Pityophthorus juglandis; WTB) transmit the fungal pathogen Geosmitiha morbida to walnut trees. Consequently, mass attacks by infested beetles lead to Thousand Cankers Disease (TCD) in susceptible Juglans species. TCD has been reported in three eastern states and threatens the black walnut industry. We used girdled black walnut trees to detect WTB, document other Curculionidae taxa that attack black walnut, and explored the possibility that other bark and ambrosia beetles could spread the Geosmithia pathogen. In May 2011, we girdled 104 black walnut trees at 27 locations within Indiana and Missouri. After three months, we removed stem and canopy wood pieces and collected emerging beetles. These beetles were identified to species and their fungal associates were isolated using a serial dilution plating method. Representatives of fungal types were collected and the number of colonies counted.

WTB was not among the 16, 315 bark and ambrosia beetles specimens collected.  However, our collection did contain a closely related species, Pityophthorus lautus. Most beetles collected (94%) represented 7 taxa exotic to the eastern US. We found no reports of walnut as a host for seven of the 18 bark and ambrosia beetle species collected, although most species attacked closely related tree genera. We isolated fungi from 1-10% of each of the four most abundant beetle species, Xylosandrus crassiusculus, Xylosandrus germanus, Xyleborus affinis, and Xyleborinus saxeseni, and from P. lautus and an abundant weevil species. DNA-based identification techniques are being used to identify these fungi with emphasis on Geosmithia spp. detection.