Differential Physiological Responses of Dalmatian Toadflax, Linaria dalmatica L. Miller, to Injury from Two Insect Biological Control Agents: Implications for Decision-Making in Biological Control

Successful biological control of invasive weeds with specialist herbivorous insects is predicated on the assumption that the injury stresses the weeds sufÞciently to cause reductions in individual Þtness. Because plant gas exchange directly impacts growth and Þtness, characterizing how injury affects these primary processes may provide a key indicator of physiological impairmentÑ which then may lead to reductions in Þtness. The objective of this study was to use physiological methods to evaluate how the invasive weed, Linaria dalmatica L. Miller (Dalmatian toadßax), 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 Þeld conditions at two sites in Montana in 2003 and 2004. ForC. lunula evaluations, a total of Þve greenhouse studies in 2003 and 2004were used.OneÞeld 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 Þeld sites revealed that the primary physiology of Dalmatian toadßax was deleteriously affected by M. janthinus larval injury. There were no signiÞcant 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 toadßax physiology. Based on the primary physiological parameters evaluated in this study,M. janthinus hadmore impact onDalmatian toadßax than C. lunula. With such information, improved risk-beneÞt decisions can be made about whether to release exotic biological control agents.