Attraction of walnut twig beetle adults and larvae to the fungus Geosmithia morbida

Geosmithia morbida causes thousand cankers disease of black walnut (Juglans nigra) and it is vectored by the walnut twig beetle (Pityophthorus juglandis). The fungus often is observed coating pupae in galleries, but the relationship between the fungus and beetle is unclear. We initiated studies to determine whether adults and larvae are attracted to G. morbida and other bark fungi when given a choice. For adult studies, a box containing an infested log was constructed with collection tubes on opposite sides that were filled with either potato dextroxe agar (PDA) or PDA colonized by G. morbida. No differences (P=0.16) in adult numbers were detected between the two tubes. For larval studies, plugs of PDA or PDA colonized by G. morbida or other fungi were placed on opposite sides in a Petri dish containing water agar. A larva was placed in the center of the dish and movement was monitored. In these tests, larvae preferentially (P=0.00002) migrated towards agar plugs colonized by G. morbida. Similarly, larvae also migrated toward plugs colonized by Fusarium spp. (P=.000063). However, when given a choice between G. morbida and Penicillium or Fusarium, no preference in larval movement was observed (P=.371 and .067 respectively). These results suggest that larvae are attracted to bark fungi in general, but not specifically to G. morbida.