Date of Award:

5-1-1977

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Department name when degree awarded

Life Sciences:Biology

Committee Chair(s)

Ting H. Hsiao

Committee

Ting H. Hsiao

Committee

Wilford J. Hanson

Committee

Keith L. Dixon

Abstract

The demography of the sagebrush defoliator, Aroga websteri Clarke, and its impact on the productivity of Artemisia tridentata were studied during 1974 and 1975. Four objectives were pursued: 1) to continue demographic studies begun in 1971 to determine causes for population fluctuations, 2) to correlate temperature and precipitation records from the study site with changes in the defoliator populations and with sagebrush quality, 3) to assess the impact of defoliator infestation on the subsequent productivity of Artemisia tridentata, and 4) to assess the long-term impact of severe defoliation on the survival and regrowth of Artemisia tridentata. The defoliator population was found to have both density-dependent and density-independent controls. Parasites, particularly the Hymenopterans, Apanteles cacocciae, Temelucha sp., and Copidosoma bakeri, killed higher percentages of defoliator larvae when the host density was high. Parasitism, however, was vii very low during 1974 (6.8 percent parasitized) and 1975 (4.0 percent parasitized). Climatic conditions were found to influence larval progression, adult emergence and longevity, larval hatch, and "crashes" in the population density. Higher temperatures accelerated all of these events. In 1974 and 1975, population "crashes" were noted following two-week periods of high temperatures. Precipitation also determined the production of green tissue in sagebrush. Sagebrush productivity was negatively correlated with insect infestation. Both the foliage percentage of the plant and the number of flower stalks produced declined at high defoliator density. Severe defoliation caused mortality in 32.5 percent of plants defoliated and initially examined in 1973, with a higher percentage of mortality in smaller than larger plants. Medium- sized plants had the highest survival rate. Chances of long-term survival and regrowth were very good for those plants that had survived one year following the defoliation.

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