The walnut twig beetle, Pityophthorus juglandis (Coleoptera: Scolytidae): distribution, genetic diversity, and impact on native walnuts of the southwestern U.S

The walnut twig beetle, Pityophthorus juglandis Blackman (WTB), is native to Arizona (AZ), California (CA), Mexico, and New Mexico (NM), where its original hosts were indigenous western black walnut trees (e.g., Juglans californica, J. major, and J. microcarpa) (Seybold et al., 2010). WTB is associated with a newly described fungus, Geosmithia morbida (Kolarík et al., 2011), which colonizes and kills the phloem around WTB feeding and reproductive galleries in walnut branches and stems (Graves et al., 2009, 2010; Utley et al., 2009). When populations of WTB are high, enough galleries are colonized by the fungus that the resulting cankers coalesce and girdle branches and stems. This “thousand cankers disease” was first observed in 2001 in the Española Valley (NM) when walnut trees declined and died in association with the feeding of the WTB. We have collected over 50 populations of WTB in the western U.S. and in Knoxville, Tennessee for a comparative analysis of the mitochondrial COI gene sequence. The analysis revealed that populations in AZ, NM, and Colorado were more diverse than populations from Tennessee and the Pacific Coast states. The exact origin of the Tennessee population has yet to be determined. Survey plots were established throughout the ranges of J. californica, J. major, and J. microcarpa (native hosts of WTB in CA, AZ, NM, OK, and TX) to determine the presence and incidence of attack and subsequent mortality attributed to this complex.