Multiway analysis of covariance revealed ecological determinants of gall morphology operating hierarchically, with plant effects primary, insect effects secondary, and habitat effects tertiary. Plant effects included Tetradymia DC. (Asteraceae) species identity, timing and duration of growth pulses, and differential vegetation quality among height striata. Insect effects included a numerical response in gall density and a functional response in individual feeding duration and intensity. Alterations in Gnorimoschema tetradymiella Busck 1903 (Lepidoptera: Gelechiidae) feeding caused by parasitization created parasitoid-specific gall sizes. Habitat effects indexed by topographic and climatic features operated most strongly in relatively southern latitudes.