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A system of indirect control of the Douglas fir beetle, Dendroctonus pseudotsugae Hopk Walters, John


The paper describes a Douglas-fir classification for the interior of British Columbia which was developed for the purpose of identifying trees susceptible to the attacks of the Douglas-fir beetle, Dendroctonus pseudotsugae Hopk. The significance of the depredations of the beetle to the objectives of sustained-yield forestry is discussed and the need for control and continuous protection stressed. A method of direct control of bark beetles is exemplified and receives criticism for its laborious-ness, high cost, and temporary protection. It is noted that foresters and entomologists are increasingly cognizant that forest management should strive for insect control through the development and maintenance of forest conditions unfavourable to insects. These conditions become manifest in vigorous forests which possess an inherent resistance to insect attack. By harvesting on a selective and critical basis the forest may be made relatively resistant and insects can be controlled by indirect means. Detailed reference is made to a system of indirect control developed for ponderosa pine forests against the attacks of the western pine beetle, Dendroctonus brevicomis Lec. The theory, development, and application of the system is considered and later referred to in the light of the results of the present study. The possibility of developing similar systems for other insects and hosts is demonstrated from the literature. Similarly, the ability of other workers, to classify Douglas fir into vigour and age groups is shown. In view of the apparent feasibility of judging the susceptibility of classes of Douglas fir to attack by beetles an attempt was made to develop a classification which might have value in this regard. A classification was developed and tabulated from data collected at Westwold, B. C. in the interior Dry Belt at an elevation of 3,000 feet. The method is described in detail and the limits of the four vigour classes and of the four age groups are statistically justified. The classification was tested for its accuracy in judging actual and relative age and vigour at Westwold and also at Lumby in the interior Wet Belt. The results of the tests are tabulated and the reason for the high degree of accuracy in judging actual vigour in terms of diametral growth attributed to the measurements of phloem streaks. Infested trees were objectively classified on an area of eight square miles at Westwold to determine which classes of vigour and age were susceptible to attack. The results reveal that the older, slower growing trees are most susceptible. Specifically, trees of over 150 years of age with a ten-year diametral growth of less than 11 millimetres are most likely to be infested. The type of host selection made by the Douglas-fir beetle is compared to a combined thinning from below and a selection cutting of the older age classes. It is suggested that, in forests subjected to endemic populations of beetles, trees of classes 2D, 3C, 3D, 4C, and 4D be harvested to increase the vigour and resistance of the residual stand and to implement indirect control.

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