PLANTÐINSECT INTERACTIONS 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 1, 2 3 1 ROBERT K.D. PETERSON, SHARLENE E. SING, AND DAVID K. WEAVER Environ. Entomol. 34(4): 899Ð905 (2005) ABSTRACT 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. For C. lunula evaluations, a total of Þve greenhouse studies in 2003 and 2004 were used. One Þeld 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 phys- iological parameters evaluated in this study, M. janthinus had more impact on Dalmatian toadßax than C. lunula. With such information, improved risk-beneÞt decisions can be made about whether to release exotic biological control agents. KEY WORDS Calophasia lunula, Mecinus janthinus, herbivory, photosynthesis, plant gas exchange BIOLOGICAL CONTROL OF WEEDS through the intentional An improved set of measurable indicators of bio- introduction of nonindigenous herbivorous insects logical control impact on weed densities would aid has reached a crossroads, both in terms of research and decision-makers in objective evaluation of tangible application. The research community has long ac- beneÞts versus potential risks when deciding whether knowledged the potential for classical biological con- to release nonindigenous organisms. Therefore, a trol of weeds to result in emerging or increased en- quantitative evaluation of beneÞts should be an im- vironmental risks (Harris and Zwo¨lfer 1968, Wapshere portant part of the overall risk assessment for agents. 1974, Howarth 1991, Louda et al. 2003, Sheppard et al. The lack of demonstrable beneÞts from the release of 2003). However, current regulatory attitudes and Þs- biological control agents can have substantial conse- cal shortfalls (Briese 2004) are reßected in the narrow quences (Thomas and Willis 1998). SpeciÞcally, if a focus of agent prerelease evaluations on host speci- nonindigenous organism does not deleteriously affect Þcity, at the expense of a more holistic screening and the targeted weed population, the economic costs or assessment process (but see Louda 1998). The lack of environmental risks associated with its release and evaluation of agent efÞcacy, as well as potential eco- establishment may always be greater than its beneÞts. logical risks, emphasize the need for formal, well- Therefore, it is crucial that agents approved for release quantiÞed risk-beneÞt evaluations of insect agents in- will actually reduce target weed populations and not troduced to manage invasive weeds. simply proliferate on them. Although several strategies exist for determining 1 Department of Entomology, Montana State University, Bozeman, the potential efÞcacy of weed biological control, using MT 59717Ð3020. methods that characterize changes in weed growth 2 Corresponding author: Department of Entomology, 333 Leon and Þtness can be very costly and time-consuming. We Johnson Hall, Bozeman, MT 59717Ð3020 (e-mail: bpeterson@ believe that the characterization of plant physiological montana.edu). 3 U.S. Forest Service, Rocky Mountain Research Station, Bozeman, response to herbivory provides a tenable alternative MT 59717. approach as a valuable indicator of the ability of bi- 0046-225X/05/0899Ð0905$04.00/0 ᭧ 2005 Entomological Society of America 900 ENVIRONMENTAL ENTOMOLOGY Vol. 34, no. 4 ological control agents to reduce target weed popu- oviposited in the target weed. Our physiological stud- lations. The delineation of physiological mechanisms ies were conducted 3Ð19 July 2003 and 7Ð14 July 2004. underlying plant responses to insect injury has been Two study groups were evaluated: injured and un- crucial to the explanation of yield loss and to the injured plants. In 2003, there were 18 group replicates development of general models of insect-induced at the Melstone site and 24 replicates at the Boulder plant stress response in crop plants (Boote 1981; Peter- site. In 2004, there were 18 replicates per group at each son 2001; Peterson and Higley 2001). Plant gas ex- site. Individual plants served as sample units at all sites. change processes such as photosynthesis, water vapor Injured plants were chosen based on the presence of transfer, and respiration represent a subset of a plantÕs ovipositional scars and swelling of stems, which indi- primary physiological processes. Understanding how cated the presence of larvae in the stems. Injured insect injury inßuences these parameters is important plants that were chosen had similar amounts of injury because these are the primary processes determining within each location, but plants at the Boulder site plant growth, development, and, ultimately, Þtness were not as visually injured as those at the Melstone (Peterson and Higley 1993). Although individual leaf site. Uninjured plants, those absent of oviposition scars photosynthetic rates typically are not accurate pre- and stem swelling, were chosen near infested plants. dictors of plant yield and Þtness (e.g., Irvine 1975, All plants at both sites were at approximately the same Elmore 1980, Baker and Ort 1992, Higley 1992), they developmental stage: early to mid ßower. can be used to objectively quantify physiological im- The primary limitation in our Þeld research with M. pairmentÑwhich may lead to reductions in Þtness. janthinus was that it did not involve experimental Boote (1981), Pedigo et al. (1986), and Higley et al. manipulation to create treatments. We chose injured (1993) emphasized the use of categorizing plant biotic plants based on the presence of oviposition scars and stressors, such as weed biological control agents, based stem swelling, rather than caging a group of plants and on injury type and physiological response, rather than assigning treatments for two main reasons: (1) Dal- on the taxonomic classiÞcation of the stressors or phys- matian toadßax plants are deleteriously affected (al- ical appearance of injury (as conventionally had been terations in stem and leaf thickness and whole plant done). Furthermore, Peterson and Higley (2001) ar- architecture) by caging whole plants (N. J. Irish, per- gued that similarities of plant response to speciÞc sonal communication), and (2) the Þeld sites were injury types, also known as injury guilds, are effective inaccessible when the adults Þrst emerged and fe- foci for addressing many basic and applied research males began ovipositing. questions. Calophasia lunula. We conducted a total of Þve The objective of this study therefore was to use greenhouse experiments in 2003 and 2004 (Table 1). physiological methods to evaluate how the invasive The experimental design for all greenhouse experi- weed, Dalmatian toadßax, Linaria dalmatica L. Miller, ments was a randomized complete block, with the area is affected by two introduced biological control agents under individual metal halide lamps serving as the in different injury guilds: the stem-boring weevil, blocking factor. Four of the experiments incorporated Mecinus janthinus Germar, and the defoliating moth, repeated measures into the experimental design. Calophasia lunula Hufnagel. Characterizing Dalma- Treatments consisted of C. lunula larval injury, artiÞ- tian toadßax primary metabolic impairment to her- cial defoliation injury, and uninjured control. We in- bivory would provide an initial step toward determin- corporated an artiÞcial defoliation treatment to de- ing the impact of biological control agents on the termine if manual defoliation could be used as a weedÕs Þtness and population dynamics. surrogate for C. lunula feeding. Treatments were rep- licated Þve or more times, and an individual pot con- taining one plant grown from seed served as the ex- Materials and Methods perimental unit. In two greenhouse experiments, we Mecinus janthinus. All studies with M. janthinus were not able to include the actual insect injury treat- were conducted under Þeld conditions at two sites in ment because of a lack of larvae. In one greenhouse both 2003 and 2004. The two sites were located near study, we did not include the artiÞcial defoliation Boulder (elevation ϭ 1798 m) and Melstone (eleva- injury treatment because of a