Activity of Hylastesnigrinus, a vector of Verticicladiellawageneri, in thinned stands of Douglas-fir

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Abstract
Live adults of Hylastesnigrinus (Mann.), a root-feeding bark beetle suspected of being a vector of Verticicladiellawageneri Kendr., were trapped in infection centers of black stain root disease and allowed to feed on seedlings of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) in the greenhouse. Three of 22 beetles that were artificially contaminated with conidia of V. wageneri introduced the pathogen into seedlings. One of 47 seedlings infested with pairs of H. nigrinus that were not artificially contaminated became infected with V. wageneri. The activity of H. nigrinus in thinned plots at three sites near the North Coast of California was monitored with sticky traps. In 1981, 100 adults were trapped in plots mechanically thinned on 13 April 1981. Only two H. nigrinus were trapped in unthinned, control plots. Twenty-two adults were trapped in plots chemically thinned by the injection of cacodylic acid. In 1982, as many or more adults were trapped in plots that were thinned after the seasonal peak of H. nigrinus in 1981 as in plots that were freshly thinned. Roots of living Douglas-fir trees examined in a mechanically thinned plot had feeding wounds similar to those described as feeding wounds made by H. nigrinus. Roots of living Douglas-fir in an unthinned plot had no evidence of insect feeding. Although no black stain was found associated with feeding wounds on living trees, the previously noted association of black stain root disease in thinned stands may still be due to an increased activity of H. nigrinus.