Thinning and urea fertilization effects on emerging grand fir (Abies grandis) foliage and growth of western spruce budworm (Choristoneura occidentalis) larvae Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1v53k0980

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  • The objective of this study was to determine how the balance of carbon to nitrogen in a grand fir ecosystem affects the chemistry of emerging grand fir foliage and the growth of western spruce budworm larvae. Forest plots in the grand fir zone of eastern Oregon were thinned, thinned and fertilized, fertilized without thinning, or left as a control to determine how increased nitrogen availability alters the efficacy of carbon-based chemical defenses at different light levels. Thinning did not significantly alter any of the foliar chemical fractions measured while fertilization increased the concentration of foliar nitrogen and free amino acids. Thinning increased tree vigor (g wood produced per m² foliage), but fertilization only increased vigor in the thinned plots. Fertilization increased the weights of western spruce budworm pupae; thinning had no effect. Male and female pupal weights correlated with foliar free amino acid concentration and the ratios of foliar free amino acids to foliar nitrogen, available carbohydrates, and lignin, but stepwise regression analysis showed that foliar free amino acid concentration alone explained most of the variation in pupal weights. The correlation of foliar free amino acid concentration with male and female pupal weights, and the lack of correlation of any indices of carbohydrate availability suggest that changes in available nitrogen rather than changes in the carbon/nitrogen balance were associated with changes in larval growth. This can be attributed to either a lack of defensive capability in the emerging foliage or a failure to measure or manipulate the variables responsible for controlling foliar defense. However, larval growth is only one aspect of plant susceptibility to insects; changing the carbon/nitrogen balance in the grand fir ecosystem may ultimately affect the susceptibility of grand fir to western spruce budworm by changing the balance between plant growth and levels of plant herbivory.
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