Successful biological control of invasive weeds with specialist herbivorous insects is predicated on the assumption that the injury stresses the weeds sufficiently to cause reductions in individual fitness. Because plant gas exchange directly impacts growth and fitness, characterizing how injury affects these primary processes may provide a key indicator of physiological impairment—which then may lead to reductions in fitness. The objective of this study was to use physiological methods to evaluate how the invasive weed, Linaria dalmatica L. Miller (Dalmatian toadflax), is affected by two introduced biological control agents within different injury guilds: the stem-boring weevil, Mecinus janthinus Germar, and the defoliating moth, Calophasia lunula Hufnagel. All studies with M. janthinus were conducted under field conditions at two sites in Montana in 2003 and 2004. For C. lunula evaluations, a total of five greenhouse studies in 2003 and 2004 were used. One field study in 2003 and two studies in 2004 also were conducted. Variables measured included net CO₂ exchange rate, stomatal conductance, and transpiration rate. Results from both field sites revealed that the primary physiology of Dalmatian toadflax was deleteriously affected by M. janthinus larval injury. There were no significant differences among treatments for any of the gas exchange variables measured in all eight experiments with C. lunula. Our results indicate that insect herbivores in two distinct injury guilds differentially affect Dalmatian toadflax physiology. Based on the primary physiological parameters evaluated in this study, M. janthinus had more impact on Dalmatian toadflax than C. lunula. With such information, improved risk-benefit decisions can be made about whether to release exotic biological control agents.