Classical biological control of weedy species involves the introduction of natural enemies from a weed’s native range to the weed’s area of introduction. For biocontrol to be successful, established agents must stress individual plants to the point where the population effects are realized. Dalmatian toadflax (Linaria dalmatica) is a widespread invasive weed across western North America. The most promising control agent on Dalmatian toadflax appears to be the stem boring weevil, Mecinus janthinus. Although M. janthinus has been credited with controlling L. dalmatica at several sites in Canada in the United States, the mechanism behind the control is not well understood. Therefore, we measured the impact of different M. janthinus infestation rates on plant growth and physiology. We measured change in stem length, biomass of plant parts, photosynthetic rate, conductance, and transpiration on plants with four levels of infestation. We observed that increasing infestation rates led to reduced shoot growth, decreased reproductive biomass, and decreased stem biomass. The results also indicated trends of decreasing photosynthetic rates with increasing injury. Adult feeding at the medium and high infestation rate caused tip kill on many plants and could have led to decreased stem growth and reproductive biomass. At high infestation rates, the stems were completely mined out, which could lead to interruption of the movement of photosynthate from the leaves through the phloem. The build up of photosynthate in the leaves may trigger a feedback loop that reduces photosynthetic rates.