Hylastes nigrinus (Coleoptera: Scolytidae), Pissodes fasciatus, and Steremnius carinatus (Coleoptera: Curculionidae) as Vectors of Black-stain Root Disease of Douglas-fir

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Abstract
This study demonstrates that Hylastes nigrinus (Mannerheim), Pissodes fasciatus LeConte, and Steremnius carinatus (Boheman) are vectors of Verticicladiella wageneri Kendrick, the causal agent of black-stain root disease of Douglas-fir, Pseudotsuga menziesii (Mirbel) Franco. These insects, known associates of diseased hosts, wound and create suitable infection courts in susceptible hosts, carry inoculum in the field, and transmit the pathogen to hosts under laboratory conditions. Root systems of 12-year-old Douglas-fir, cut during precommercial thinning, were infested by these insects and were susceptible to V. wageneri infection for at least 7 months, confirming that V. wageneri may be introduced to thinned stands via these hosts. Male and female H. nigrinus created wounds suitable as infection courts on roots and root collars of crop trees for 1–2 years after precommercial thinning and may, therefore, introduce V. wageneri to thinned stands via these hosts. Insect-mediated transmission of V. wageneri to Douglas-fir by H. nigrinus in the field is documented.