The influence of time of precommercial thinning on the colonization of Douglas-fir by three species of root-colonizing insects

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Abstract
In plantations of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) in western Oregon, precommercial thinning resulted in significantly increased abundances of insect vectors of black-stain root disease. Hylastesnigrinus (Mann.), Pissodesfasciatus LeC, and Steremniuscarinatus (Boh.) were monitored for 2 years (1983 and 1984) in unthinned plots and in plots thinned in September 1982, January 1983, or May 1983. Abundances of these beetle species were significantly higher in thinned plots, relative to unthinned plots. However, the numbers of H. nigrinus and P. fasciatus caught by unbaited traps in plots thinned in May was reduced relative to plots thinned in September or January. These results suggest that time of thinning can be manipulated to reduce the activity of these vectors in plantations that are mechanically thinned.