Wheat stem sawfly (Hymenoptera: Cephidae) tissue injury within wheat stems was characterized and the resulting impact on wheat primary physiology and yield under well watered and water stressed conditions was evaluated. Cross sections of injured and non-injured stems were performed on both internodal and node regions. Damage to the vascular tissue was then assessed by high definition microscopy. Physiological parameters, such as photosynthesis, stomatal conductance, intercellular carbon dioxide, transpiration, and carbon dioxide response curves were measured in the wheat flag-leaves at the flowering and grain filling stages. Concomitant chlorophyll a fluorescence measurements were taken from flag-leaves and head glumes using both dark- and light-adapted tests. Additionally, pigment compositions and quantification of total chlorophyll, chlorophyll a, chlorophyll b, chlorophyll a/b ratios, and carotenoids were performed on flag-leaves and head glumes using ethanolic extractions from a subset of plants. From the same subset of plants the impact of sawfly injury on the non-structural carbohydrate sucrose and on the reducing sugars glucose and fructose was determined. Yield parameters, such as head weight, seed weight, and the number of seeds was measured at the end of plant maturation.