Differential Physiological Responses of Dalmatian Toadflax, Linaria dalmatica L. Miller, to Injury from Two Insect Biological Control Agents: ...
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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
ROBERT K.D. PETERSON,1, 2 SHARLENE E. SING,3 AND DAVID K. WEAVER1
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 Zwölfer 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@
montana.edu).
believe that the characterization of plant physiological
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 䉷 2005 Entomological Society of America900 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 ⫽ 1798 m) and Melstone (eleva- injury treatment because of a lack of sufÞcient plants.
tion ⫽ 939 m), MT, in rangeland that had been ex- We also conducted three Þeld experiments: one in
tensively and intensively burned by wildÞre in 2000. 2003 (Bozeman, MT) and two in 2004 (Missoula, MT).
The Boulder site has a lower average temperature and The experimental design for all studies was completely
receives slightly more precipitation per year than the random (Table 1). Treatments for two experiments
Melstone site. Dalmatian toadßax was the dominant consisted of C. lunula larval injury, artiÞcial defolia-
plant species at both sites. Observers with pre- and tion injury, and uninjured control. In a third experi-
postÞre knowledge of the vegetation community at ment, only two treatments, C. lunula injury and con-
these sites suggest that toadßax had increased signif- trol, were used (Table 1). Treatments were replicated
icantly in both density and biomass postburn, sup- six times in 2003. In 2004, treatments were replicated
porting the inference that Dalmatian toadßax is a Þre- six times in one experiment and eight times in the
enhanced invasive weed (Jacobs and Sheley 2003aÐ c, other. Individual plants served as experimental units.
Phillips and Crisp 2000). Insects were released at each For seven of the eight experiments, a leaf at the bottom
site in 2002, and follow-up observations made later in one-third of each stem also was measured for physi-
the same year conÞrmed that the insects had fed and ological parameters to determine if there were inter-August 2005 PETERSON ET AL.: PHYSIOLOGICAL RESPONSES TO BIOCONTROL AGENTS 901
Table 1. Experimental details and statistical summaries for each C. lunula herbivory experiment
Treatment Treatment ⫻ Mean percentage
Treatment Treatment
Year Type Treatments Experimental design ⫻ leaf leaf position defoliation by C.
Fd P⬎F
position F P⬎F lunula ⫾ SEM
2003 Field 3a CRD, factorial, repeated 2.23 0.13 0.32 0.73 38 ⫾ 6.16
measures
2004 Field 3a CRD, factorial 1.97 0.15 0.66 0.52 45 ⫾ 5.98
2004 Field 2b CRD, factorial 1.94 0.19 0.001 0.96 75 ⫾ 15.44
2003 Greenhouse 3a RCBD, factorial, repeated 0.24 0.79 3.31 0.07 52 ⫾ 6.37
measures
2003 Greenhouse 2c RCBD, factorial, repeated 0.57 0.47 1.91 0.2 NA
measures
2003 Greenhouse 2c RCBD, factorial, repeated 0.47 0.51 0.91 0.37 NA
measures
2003 Greenhouse 3a RCBD, factorial, repeated 2.73 0.11 0.25 0.78 43 ⫾ 7.4
measures
2004 Greenhouse 2b RCBD 1.67 0.25 NA NA 13 ⫾ 2.8
a
Treatments ⫽ C. lunula larval injury, artiÞcial defoliation injury, and control.
b
Treatments ⫽ C. lunula larval injury and control.
c
Treatments ⫽ artiÞcial defoliation injury and control.
d
Treatment effect.
CRD, completely random design; RCBD, randomized complete block design; NA, not applicable.
actions between defoliated (or undefoliated control) tance rate, and transpiration rate. The leaves were
upper leaves and undefoliated lower leaves. illuminated with a light intensity of 1400 mol pho-
For most greenhouse and Þeld experiments, pre- tons/m2/s from a light source inside the 2-cm2 leaf
starved, C. lunula fourth instars were securely con- chamber. The leaf chamber reference CO2 concen-
tained within leaf cages made from stiff, Þne-mesh tration was 400 mol CO2/mol, generated from a 12-g
nylon netting (tuile) and allowed to feed for at least CO2 cylinder connected to the LI-6400.
12 h. The leaf cages were ⬇11 by 11 cm and covered Statistical Analysis. Because gas exchange variables
the top portion of the stem, usually enclosing from usually were observed on the same leaf over time,
four to six large leaves. The open end of each cage was most analyses were conducted using a repeated mea-
closed tightly around the stem with a drawstring. Leaf sures multivariate analysis of variance (MANOVA;
cages could not enclose single leaves because the ␣ ⫽ 0.05; SAS 9.0, Cary, NC).
Dalmatian toadßax leaves are small and lack a petiole.
The leaf-cage fabric intercepted ⬍5% of photosyn-
Results
thetically active radiation (Peterson et al. 1998). The
artiÞcial defoliation injury treatment was imposed by Mecinus janthinus. Although establishment of M.
clipping leaf tissue with scissors from the plant in a janthinus was relatively recent at both sites, injury was
spatial and temporal pattern consistent with C. lunula more widespread at Melstone than at Boulder. Evi-
feeding; however, because of variability in C. lunula dence of M. janthinus injury to plants was compara-
feeding (Table 1), we standardized artiÞcial defolia- tively intermittent at Boulder. Results from both Þeld
tion injury at 50% of leaf area within each cage. Leaves sites over the 2-yr study period revealed that the
of both the control and artiÞcially defoliated plants primary physiology of Dalmatian toadßax was signif-
also were enclosed in leaf cages, ensuring appropri- icantly affected by M. janthinus larval injury (Table 2).
ately comparable treatments. Larvae were removed, In particular, over both years and locations, photo-
and gas exchange variables were recorded on injured synthetic rates for plants with injury from larvae were
leaves at 1 and 48 h after the termination of injury. signiÞcantly lower than rates for uninjured plants (Ta-
Plant Primary Metabolism. Repeated gas exchange ble 2; Fig. 1). At the Melstone site, the mean reduction
measurements were made from the same leaf per plant in photosynthetic rates associated with larval injury
at approximately the apical one-third of the stem. was 29.5% in 2003 and 27.3% in 2004. At the Boulder
When it was not possible to take readings from the site, the mean photosynthetic rate reduction associ-
same leaf per plant because of breakage, the nearest ated with larval injury was 38.6% in 2003 and 17% in
leaf was measured. For all C. lunula experiments, mea- 2004. There was a signiÞcant time effect between
surements were recorded from injured (partially de- measurement dates in both years at the Boulder site,
foliated) leaves. If defoliated leaves had too little leaf but only a signiÞcant time effect in 2003 at the Mel-
area remaining to measure, the nearest undefoliated stone site.
leaf was used. At the Melstone site, transpiration rates and stoma-
All gas exchange measurements were made within tal conductances were signiÞcantly lower for injured
3 h of solar noon using a portable photosynthesis plants in 2004, but not in 2003 (Table 2). These trends
system (model LI-6400; LI-COR, Lincoln, NE). Vari- were reversed at the Boulder site, where we found that
ables measured and analyzed included: net CO2 ex- transpiration rates and stomatal conductances were
change rate (photosynthetic rate), stomatal conduc- signiÞcantly lower for injured plants in 2003, but not902 ENVIRONMENTAL ENTOMOLOGY Vol. 34, no. 4
Table 2. Mean gas exchange responses ⴞ SEM of Dalmatian toadflax to M. janthinus larval injury
2003 2004
Location
treatment 3 Jul 7 Jul 7 Jul 12 Jul
Melstone Site
Stomatal conductance (mol H2O/m2/s)
Uninjured 0.51 ⫾ 0.025 0.23 ⫾ 0.014 0.19 ⫾ 0.032 0.16 ⫾ 0.017
Injured 0.48 ⫾ 0.035 0.17 ⫾ 0.031 0.12 ⫾ 0.019 0.11 ⫾ 0.014
F 1.29 6.34
P⬎F 0.27 0.017
df 1,22 1,34
Transpiration rate (mmol H2O/m2/s)
Uninjured 17.3 ⫾ 0.68 4.5 ⫾ 0.66 6.5 ⫾ 0.76 5.3 ⫾ 0.41
Injured 16.2 ⫾ 0.74 4.1 ⫾ 0.69 4.4 ⫾ 0.48 3.6 ⫾ 0.36
F 1.12 12.97
P⬎F 0.3 0.001
df 1,28 1,31
14-Jul 19-Jul 8-Jul 14-Jul
Boulder Site
Stomatal conductance (mol H2O/m2/s)
Uninjured 0.17 ⫾ 0.01 0.17 ⫾ 0.01 0.25 ⫾ 0.01 0.22 ⫾ 0.02
Injured 0.12 ⫾ 0.02 0.13 ⫾ 0.01 0.22 ⫾ 0.02 0.22 ⫾ 0.02
F 9.9 0.2
P⬎F 0.003 0.65
df 1,36
Transpiration rate (mmol H2O/m2/s)
Uninjured 3.5 ⫾ 0.26 6.4 ⫾ 0.42 4.5 ⫾ 0.18 6.5 ⫾ 0.62
Injured 3.3 ⫾ 0.52 4.8 ⫾ 0.34 3.8 ⫾ 0.23 6.6 ⫾ 0.39
F 9.28 0.41
P⬎F 0.004 0.53
df 1,37 1,34
2004. The inconsistency in these responses suggests may be affected if stem injury by M. janthinus larvae
that stem-boring injury by M. janthinus larvae may not prevents translocation of carbohydrates to the roots.
be severely impairing the ability of the leaves to up- Our results support the contention that M. janthinus
take CO2 and transpire H2O (stomatal limitations). injury affects photosynthesis of Dalmatian toadßax.
Disruption of xylem tissues because of larval feeding More detailed physiological measurements will be
within the stem apparently was insufÞcient to consis- required to determine the mechanisms underlying
tently close stomata and reduce transpiration. There- reductions in photosynthetic rates of Dalmatian toad-
fore, larval injury did not seem to physiologically ßax. Measurements of light-response curves, assimila-
mimic drought stress. Reductions in stomatal conduc- tion-intercellular CO2 curves, and chlorophyll ßuo-
tances and transpiration, when they occurred, seemed rescence can be used to determine precisely where
to follow, rather than cause, photosynthetic reduc- biochemical limitations to photosynthesis are occur-
tions. This type of primary metabolic response also ring and therefore can provide mechanistic explana-
was observed with injury from Epilachna varivestis tions of physiological impairment. Stem-boring injury
Mulsant on Phaseolus vulgaris L. and Glycine max L. is so poorly understood that, to date, the injury guild
Merrill (Peterson et al. 1998). Jeanneret and Schroe- is characterized by the physical appearance of injury
der (1991) suggested that photosynthetic mechanisms rather than on physiological effects of the injury (Wel-
Fig. 1. Photosynthetic responses of L. dalmatica to injury by M. janthinus larvae (A, Melstone, MT; B, Boulder, MT). Note
statistically signiÞcant differences in all tests between injured/uninjured plants.August 2005 PETERSON ET AL.: PHYSIOLOGICAL RESPONSES TO BIOCONTROL AGENTS 903
ter 1989; Peterson and Higley 1993). Additional stud- ation of whole plants is known to alter the pattern of
ies within this system therefore will assist in charac- normal progressive leaf senescence of some plant spe-
terizing plant physiological responses to this injury cies (Higley 1992, Peterson et al. 1992). Consequently,
guild. Dalmatian toadßax may respond differently to defo-
Because we did not experimentally impose the liation injury at different levels of biological organi-
treatments, we cannot discount the possibility that the zation, as has been observed for other plant-insect
plants containing M. janthinus larvae were previously systems (Peterson and Higley 1993, Peterson and
stressed. Adult females may have selected speciÞc Higley 2001).
plants for oviposition because those plants were al- Because there were no statistically signiÞcant phys-
ready stressed. Therefore, the gas exchange responses iological differences between leaves from C. lunula
observed in this study may not have been caused by injured and artiÞcial defoliation injured plants (Table
larval injury, but by other factors. However, that ex- 1), it is possible to simulate C. lunula defoliation. This
planation is unlikely for the following reasons: (1) the is important because simulating insect defoliation of-
uninjured and injured plants were often ⬍20 cm apart, ten allows for better experimental control and a more
indicating that abiotic or other biotic stress factors rapid quantiÞcation of injury than using actual insects
likely were not different between treatments, and (2) (Pedigo et al. 1986, Peterson et al. 2004). However, if
the injured plants were chosen based only on the the objective of a study is to characterize whole plant
presence of ovipositional scars and stem swelling; no and Þtness responses of Dalmatian toadßax to C. lunula
other visual differences between plants were evident. injury, the artiÞcial defoliation techniques must ade-
Future experimental research will need to determine quately reproduce the spatial and temporal pattern of
conclusively if the responses we observed are solely injury on the plant (Pedigo et al. 1986).
the result of larval injury.
Calophasia lunula. Defoliation by C. lunula larvae
Discussion
varied considerably among plants within and among
experiments (Table 1). There were signiÞcant time Injury Guilds. The injury guild concept has evolved
effects in all Þve of the experiments in which repeated from grouping herbivores by taxonomic category to
measures statistical analysis was used. In all eight ex- grouping them by categories of plant physiological
periments, there were no signiÞcant differences impact (Peterson 2001). Our results indicate that in-
among treatments for any of the gas exchange vari- sect herbivores in two distinct injury guilds differen-
ables measured (Table 1). Therefore, our results sug- tially impact Dalmatian toadßax physiology. Based on
gest that defoliation injury did not affect Dalmatian the primary physiological parameters evaluated in this
toadßax primary metabolism (i.e., photosynthesis, sto- study, the stem borer, M. janthinus, seemed to affect
matal conductance, transpiration) over the observa- Dalmatian toadßax more than the defoliator, C. lunula.
tion period. Larval defoliation reduced leaf area, but The differences we observed in host-plant physio-
did not affect the photosynthetic apparatus of remain- logical response to the two herbivore species, repre-
ing tissue. There was no statistically signiÞcant inter- senting two different injury guilds, may help explain
action between injury of upper leaves and photosyn- their relative effectiveness in controlling the target
thesis of lower, uninjured leaves on the same plant weed. Weed ecological studies indicate that crude
(Table 1). This indicates that there was no photosyn- tissue removal or consumption, such as is produced by
thetic response by lower leaves linked to defoliation C. lunula larvae, seldom signiÞcantly affects trenchant
injury on upper leaves. weed infestations (Cousens and Mortimer 1995; My-
Many studies have reported that there are no ers and Bazely 2003); this also holds true for Dalmatian
changes in photosynthetic rate in the remaining leaf toadßax (Robocker et al. 1972; Vujnovic and Wein
tissue of injured leaves in response to insects in the 1997). Furthermore, weed species with both vegeta-
defoliator injury guild (e.g., Davidson and Milthorpe tive and seed reproduction, such as Dalmatian toad-
1966, Poston et al. 1976, Syvertsen and McCoy 1985, ßax, are particularly resilient to simple biomass reduc-
Welter 1989, 1991, Higley 1992, Peterson et al. 1992, tion (Burdon et al. 1980, Burdon and Marshall 1981,
Peterson et al. 1996, Peterson and Higley 1996, Burk- Lajeunesse et al. 1993).
ness et al. 1999, Peterson et al. 2004). The similarity in Jeanneret and Schroeder (1992) reported that lar-
response across these studies suggests that the pho- val mining in high density or outbreak populations of
tosynthetic processes of many plant species is not M. janthinus caused premature stem wilting and sup-
affected directly by defoliation. The principal effect of pression of ßower formation, and consequently, re-
this injury type seems to be linked more to the re- duction in seed production (Jeanneret and Schroeder
duction in photosynthesizing leaf area than to a re- 1991). Larval feeding also was correlated with reduced
duction or enhancement of photosynthetic capacity of stem biomass and with an increased incidence of veg-
remaining tissue of injured leaves (Peterson and etative stem mortality during prerelease tests (Saner
Higley 1996, Peterson 2001, Peterson et al. 2004). et al. 1994). According to Robocker (1970), the veg-
Measurement of the potential impact from the de- etative stems of Dalmatian toadßax contribute signif-
foliation injury in these experiments was limited to icantly to both the root carbohydrate reserves and
relatively few leaves on the distal end of one stem of vigor of each individual toadßax plant.
a plant. Therefore, the results may not translate to Conclusions. Successful biological control of inva-
whole plant performance. Indeed, substantial defoli- sive weeds with herbivorous insects is predicated on904 ENVIRONMENTAL ENTOMOLOGY Vol. 34, no. 4
the assumption that insect-induced injury will stress less costly than measurements of biomass, yield, and
the weed sufÞciently to cause reductions in individual Þtness reduction, potentially improve the process of
Þtness and, eventually, weed population density. agent selection from among numerous available can-
These reductions in Þtness can be caused through didate species.
direct and/or indirect consequences of herbivory
(Crawley 1989, Sheppard 1996). Herbivores may
cause physiological impairment to the weed host, Acknowledgments
which may lead directly to reductions in Þtness
through feeding activity. Alternatively, herbivores This study was funded, in part, by the National Fire Plan,
may impair the weed hostÕs ability to compete effec- Project 01.RMS.B.3, the USDA-USFS Rocky Mountain Re-
search Station RM-4151, Project 03-JV-11222022-249, and the
tively with other plants. Regardless of the mechanism
Montana Agricultural Experiment Station, Montana State
underlying Þtness loss, because plant gas exchange University. D.K.W. would like to acknowledge funding in
directly affects growth, development, and Þtness, we support of biological control of Dalmatian toadßax from
argue that characterizing how agent injury impacts three United States Department of the Interior coopera-
these primary physiological processes may be a key tors, speciÞcally Cooperative Agreement 98-FC-60-10740
indicator of the agentÕs potential efÞcacy. However, with the Bureau of Reclamation, Cooperative Agreement
plant physiological impacts by the agent do not nec- ESA04DD31 with the Bureau of Land Management, and
essarily predict agent efÞcacy. Indeed, the type of Memorandum of Agreement AG4C5000584 with the Bureau
injury does not solely determine yield or Þtness loss. of Indian Affairs. We thank D. FitzGerald and J. Dinkins for
technical support.
In addition to injury type (impact per unit injury), the
magnitude and duration of injury (intensity of injury)
also is an important determinant (Pedigo et al. 1986,
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Photosynthetic responses of legume species to leaf-mass Received for publication 22 December 2004; accepted 12
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