Budworm densities natural

Our objective Is to examine some aspects of current plant herbivore theory using Douglas-flr (Pseudotsuga menzlesll) and western spruce budworm (Chorlstoneura occldentalls). Both plant and herbivore are widespread In western North America. Natural hosts of the budworm Include Douglas-flr, species of Abies, and, on occasion, other conifers (9). Variation In budworm density occurs on both a geographic and local scale. We have frequently observed differential defoliation In trees having overlapping crowns at sites In Montana, Idaho, and New Mexico. 

Studies to determine the level of resistance or susceptibility due to foliage quality and tree physical parameters of Douglas-flr to the budworm were conducted at the Boulder, Montana and Barley Canyon, New Mexico sites. The study at the Montana site Involved trees exposed to moderate budworm densities, while the study at Barley Canyon was done using budworm that were placed on trees that had very low natural densities of budworm. 

Natural budworm densities were determined by sampling 6 sprays, each 40 cm long, In the same quarter of the tree used to collect tissue for chemical analysis and to collect defoliation data. Densities were expressed as the average number of budworm larvae per 100 buds per tree. A visual estimate of the amount of defoliation eilso was made In the same area of the crown where the densities and needle tissue were collected. Since budworm may disperse from heavily defoliated trees, (Greenback, 1963) budworm densities from each tree were weighted by the level of defoliation that each tree sustained. This resulted In an Infestation Intensity measurement (dependent variable) which was subjected to multiple stepwise correlation analysis using various foliage quality and physical tree parameters as the Independent variables. Thirty-one parameters were used as Independent variables In this analysis. 

Resistance - Susceptibility Studies Using Natural Budworm Densities and Defoliation... 

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