1. Ecological research has increasingly highlighted the importance of intraspecific variation in shaping the structure and function of communities and ecosystems. Indeed, the effects of intraspecific variation can match or exceed those of interspecific variation. Previous reviews of intraspecific variation in plant traits across heterogeneous environments have focused primarily on mean phenotypic effects. We propose that a richer and fuller understanding of the ecological causes and consequences of intraspecific variation would be provided by partitioning trait variance into its subcomponents (genetic, environment, genotype by environment interaction). 2. We used a meta-analysis of 352 sets of genetic, environment, and genotype by environment (GxE) variation estimates from 72 studies of Salicaceae to compare these sources of variation across plant traits (growth, foliar nitrogen, defense compounds), insect herbivore performance metrics (e.g., survival, growth, fecundity), and environmental conditions (e.g., soil nutrients, water, defoliation). 3. Our findings revealed that variation in levels of defense compounds (both condensed tannins and salicinoids) and insect herbivore performance were primarily genetically determined, while variation in plant growth and foliar nitrogen were more environmentally determined. 4. Plasticity in plant growth, foliar nitrogen levels, and insect herbivore performance varied substantially across different sites (year x location), and nutrient, water, and carbon dioxide environments. Plasticity was lowest for chemical defense traits and all traits in contrasting ozone and defoliation environments. 5. Our quantitative review also revealed several gaps in the literature, including a need for surveying more mature plants (>2 years-old), a wider variety of insect herbivore species (e.g., leaf-modifiers, specialist insects), and underrepresented environmental treatments (e.g., competition, defoliation, disease, light, water). This work will help to assess how the patterns within this meta-analysis may or may not be confined within particular parameters (e.g., plant maturity). 6. Findings from this analysis further highlight the importance of and patterns within intraspecific variation in shaping the evolvability and plasticity of traits and in governing plant-insect interactions.