With increasing pressures on sagebrush steppe ecosystems, the sagebrush defoliating Aroga moth, Aroga websteri Clarke (Lepidoptera: Gelechiidae) has become a critical organism of concern. Despite the cyclic nature of A. websteri outbreaks throughout the Great Basin, there is limited information on the moth’s population dynamics. The goal of this dissertation was to develop effective means of assessing and describing population trends of the Aroga moth across space and time, and potentially promoting biological control of the moth to prevent unnaturally large, prolonged and destructive outbreaks. Field studies were conducted to: 1) monitor and quantify activity of the Aroga moth and its damage to sagebrush across a montane landscape, 2) assess the effects of parasitoid and floral diversity on parasitism of the moth, and 3) develop a degree-day (D) model to describe the phenology of the insect, as well as field populations studied previously. North-facing stands of sagebrush, characterized by low values of solar radiation, appear to be especially suitable local habitats for the Aroga moth. High habitat suitability may result from favorable microclimate, both in its direct effects on the Aroga moth and in indirect effects tied to sagebrush plant community productivity and performance. Parasitoid and floral diversity differed strongly and predictably across space and time, with greatest overall parasitism occurring when three major parasitoid species were present. Field experiments revealed individual species of parasitoids differed substantially and complemented one another in their patterns of attack among local populations of the Aroga moth across the montane landscape. Differing responses to provision of floral resources and methyl salicylate (an herbivore- induced plant volatile) support the general hypothesis that over large scales of space and time, species diversity of natural enemies promotes suppression of insect herbivores. Lastly, degree-day models were developed and least variation among years in (D) phenology resulted with the single-sine method with base temperature of 5C. Years of historical Aroga moth outbreaks had characteristic seasonal patterns of D accumulation that were intermediate and characterized by high precipitation in June and July during late stage larval development. Thus, it appears that periodic outbreaks of the defoliator are due to favorable weather conditions.