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Bark beetles are casual agents of tree mortality, often affecting landscapes with diverse topography. The Douglas-fir beetle (Dendroctonus pseudotsugae Hopkins) (DFB) uses Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco), as its primary host. Populations often increase following disturbances such as defoliation, fires, or blowdowns and then transition into live healthy trees. Mitigating tree mortality where high mortality levels are incompatible with land manager objectives is of relevance. In this study, we compared mortality levels in eighteen thinned and twelve unthinned pure Douglas-fir stands in the Helena National Forest in Montana. Managed stands were thinned during the summers of 2014 and 2015 and tree mortality evaluated in 2017 and 2018. Douglas-fir beetle populations were at endemic levels in the study area, as represented by low observed tree mortality. Even under endemic populations, 3.6% of Douglas-firs > 12 in diameter at breast height (dbh) were killed in unthinned stands compared to 1.6% in thinned stands. Basal area of trees killed in unthinned stands was 5.4 (± 2.4) ft2/ac compared with 0.5 (± 0.3) ft2/ac in thinned stands. Although insect pressure was low, this study indicates that vegetation management could be a tool to mitigate DFB-caused tree mortality. Additional studies conducted under higher insect populations are needed. Study Implications: Douglas-fir beetle (DFB) can cause extensive mortality of Douglas-fir, particularly in dense stands of large-diameter trees with a high percentage of hosts. Vegetation management through silvicultural thinnings has been proposed as a long-term strategy to mitigate tree mortality from bark beetles. In this study, we compared tree mortality caused by DFB in thinned and unthinned Douglas-fir stands. Douglas-fir beetle populations were at low levels post-thinning and mortality levels were limited. Regardless, we observed increased mortality in denser, unthinned stands compared with thinned stands with lower tree density, suggesting that stand density reductions may mitigate DFB-caused tree mortality.

The distribution range of the western pine beetle Dendroctonus brevicomis LeConte (Coleoptera: Curculionidae) is supported only by scattered records in the northern parts of Mexico, suggesting that its populations may be marginal and rare in this region. In this study, we review the geographical distribution of D. brevicomis in northern Mexico and perform a geometric morphometric analysis of seminal rod shape to evaluate its reliability for identifying this species with respect to other members of the Dendroctonus frontalis Zimmermann (Coleoptera: Curculionidae) complex. Our results provide 30 new records, with 26 distributed in the Sierra Madre Occidental and 4 in the Sierra Madre Oriental. These records extend the known distribution range of D. brevicomis to Durango and Tamaulipas states in northern Mexico. Furthermore, we find high geographic variation in size and shape of the seminal rod, with conspicous differences among individuals from different geographical regions, namely west and east of the Great Basin and between mountain systems in Mexico. Key words: seminal rod shape, geometric morphometry, Dendroctonus frontalis complex

Data collected from Douglas-fir (Pseudotsuga menziesii) stands infested by the Douglas-fir beetle (Dendroctonus pseudotsugae) in Wyoming, Montana, Idaho, and Utah, were used to develop models to estimate amount of mortality in terms of basal area killed. Models were built using stepwise linear regression and regression tree approaches. Linear regression models using initial Douglas-fir basal area were built for all study sites but produce low precision estimates. Regression tree models using initial Douglas-fir basal area or stand density index or both were also built for all sites. Regression tree models provide a more realistic approach to estimate potential mortality by creating more homogenous mortality classes with reduced variance. The models developed provide land managers with a basis for determining the potential mortality should a Douglas-fir beetle outbreak develop. West. J. Appl. For. 14(3):121-127.