Thinning and Nitrogen Fertilization in a Grand Fir Stand Infested with Western Spruce Budworm. Part I: Insect Response

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Thinning and nitrogen fertilization of a grand fir (Abies grandis [Dougl. ex D. Don] Lindl.) stand were tested as possible alternative techniques for managing outbreaks of the western spruce budworm (Choristoneura occidentalis Freeman). Treatment effects on an infestation of budworm and associated defoliating insects were evaluated over 5 years by a replicated split-plot experiment. Mean densities of budworm egg masses and larvae tended to be higher on fertilized plots than on control plots, but differences were not significant during the course of the experiment. Total herbivore biomass was, however, 40-60% greater and individual larvae were 32-38% heavier for 2-4 years after fertilization. Thinning alone produced a significantly greater dry mass of budworms in one year although the difference was relatively small. Fertilization also significantly increased the weight of male and female pupae by 6-23% for 4 years whereas thinning had no effect on pupal weight. Parasitization of late larvae and pupae was significantly reduced in one year by fertilization but was not affected by thinning. In general, defoliating insects seemed to benefit from the applied treatments, especially fertilization; however, overall budworm survival and prevailing trends of the outbreak were unaffected by the treatments and seemed to be determined mostly by other environmental factors, probably natural enemies. The ultimate value of thinning and fertilizing infested forests as a method of pest management must be viewed in the larger context of the total effect on tree and stand productivity. For. Sci. 38(2):235-251.

Keywords: Insect defoliators; outbreaks; silvicultural control

Document Type: Journal Article

Affiliations: Forestry Technician, Forestry and Range Sciences Laboratory, 1401 Gekeler Lane, La Grande, OR 97850

Publication date: April 1, 1992

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