Varying impacts with elevation from a parasitoid guild of a montane moth, the sagebrush defoliator (Aroga websteri)

Responses of insect species to changing environmental conditions along elevational gradients are diverse and are complicated by biotic interactions. Previous studies indicate levels of parasitism and predation of herbivorous insects tend to decline with increasing altitude, yet 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. We found parasitoid species to differ strongly and predictably in host attack along elevation clines, with greatest overall parasitism occurring where parasitoid species richness was greatest. In contrast to other case studies, overall parasitism increased with increasing altitude. 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. While the presence of all parasitoids together led to greatest parasitism, conflict and competition between heterospecifics may be reduced substantially by differential use of resources along the elevation gradient.