There is astounding variation in the abundance and diversity of insect herbivores among plant individuals within plant species in natural systems. One of the most well studied hypotheses for this pattern, the plant architecture hypothesis, suggests that insect community patterns vary with plant structural complexity and plant traits associated with structure. An important limitation to our understanding of the plant architecture hypothesis has been that most studies on the topic confound plant size and plant age. This occurs because, for most plant species, larger individuals are older individuals. This is a limitation because it prevents us from knowing whether insect community patterns are more dependent on traits associated with plant size, like resource quantity or plant apparency, or traits associated with plant age, like ontogenetic changes in phytochemistry. To separate these effects, we characterized galling insect communities on sagebrush (Artemisia tridentata)—a shrub in which age and size are not tightly correlated. We identified gall insects and recorded morphological measurements from 60 plants that varied separately in size and age. We found that plant size explained significantly more variation in insect gall abundance and species richness than did plant age. These results suggest that processes supporting the plant architecture hypothesis in this system are driven primarily by plant size and not plant age per se. Resource qualities associated with host-plant ontogeny may be less important than resource quantity in the assembly of herbivorous insect communities.