Responses of insect species to changing environmental conditions along elevational gradients are diverse and are complicated by biotic interactions. Previous studies indicate that levels of parasitism and predation of herbivorous insects tend to decline with increasing altitude, but the processes and adaptations shaping this general pattern are unclear. We examined patterns of abundance along an altitudinal gradient for the sagebrush defoliator (Aroga websteri; Lepidoptera), parasitoid species in the guild attacking the moth, and flowering understory plants throughout a 14,000 acre wildlife management area in a sagebrush-steppe ecosystem dominated by the shrub Artemesia tridentata, big sagebrush.
Results/Conclusions
Over the course of three years (2008-2010), we found three major parasitoid species to differ strongly and predictably in attacking the moth along elevation clines, with greatest overall parasitism occurring when all three species were present. In contrast to other case studies, overall parasitism increased with increasing altitude. The associated floral richness also increased strongly with elevation and was correlated with rates of parasitism by two major parasitoid species. Field experiments indicate that species of pupal parasitoids may differ in their responses to cues associated with their insect host and with floral nectaries along the altitudinal gradient. It appears that in this system, the highest rates of parasitism occur at or near the defoliator’s altitudinal limit. Further, for the two species of parasitoids that respond to floral resources, highest floral species richness occurs at the most interior and pristine sites at the highest elevations within the study area. Finally, while the presence of all major parasitoids together led to greatest parasitism, conflict and competition between heterospecifics may be reduced substantially by their differential use of resources along the elevation gradient.