“Bottom-up” factors strongly influenced the spatial and temporal pattern of survival of Asphondylia atriplicis Townsend (Diptera: Cecidomyiidae) on Atriplex canescens (Pursh) Nutall (Chenopodiaceae) at three locations in Phoenix, AZ. In contrast, “top-down” effects of natural enemies did not influence the pattern of A. atriplicis mortality. A. atriplicis induces a fleshy, multilocular, rounded stem-gall near the apical meristems of A. canescens. A. atriplicis survival increased as gall size increased, and as the depth of larva in the gall increased. Larval mortality from unknown factors on A. atriplicis decreased with gall size, but the overall interval parasitism rate did not change significantly with gall size. The interval parasitism rate for the eurytomid parasitoid group with the shortest ovipositor was negatively correlated with gall size, but interval parasitism by all other parasitoids was not influenced by gall size. Gall size was strongly influenced by the bottom-up forces of environmental and plant heterogeneity. Gall size varied among seasons, sites, and plants. Larval survival and gall-size covaried in each season and site and among plants.