invasive plant science and...

Invasive Plant Science and Management

Ecophysiological Responses of Giant Reed(Arundo donax) to Herbivory

Georgianne W. Moore, David A. Watts, and John A. Goolsby*

Ecophysiological Responses of Giant Reed(Arundo donax) to Herbivory

Georgianne W. Moore, David A. Watts, and John A. Goolsby*

The effect of invasive species might be lessened if herbivores reduced transpiration and growth rates; however, simply

removing photosynthetic material might not ensure that the transpiration rate of active leaf tissue decreases. We

assessed whether biological control has an injurious effect on the target plant species, giant reed (Arundo donax), by

quantifying leaf photosynthetic and transpiration responses to two herbivores: an armored scale, Rhizaspidiotusdonacis, and a stem-galling wasp, Tetramesa romana. Herbivory by a sap-feeding scale and a stem-galling wasp both

separately and together, reduces the rates of leaf level physiological processes in A. donax. The effect of the wasp

increases with density and reduces photosynthesis by reducing the carboxylation rate of ribulose-1,5-bisphosphate

carboxylase oxygenase, which controls CO2 fixation in photosynthesis. The scale insect reduces photosynthesis by

decreasing the maximum rate of electron transport, which determines how much light energy can be captured in

photosynthesis. The effect of the armored scale takes approximately 5 mo after infestation, which coincides with

generation time. When both insects are present at the same time, the effect of their herbivory appears additive after

time for the scale to reproduce. We conclude that a combination of two herbivores can have a stronger physiological

effect than one type of herbivore, likely because of their different effects on leaf function.

Nomenclature: Arundo scale, Rhizaspidiotus donacis Leonardi; Arundo wasp, Tetramesa romana Walker; Giant

reed, Arundo donax L.

Key words: Biological control, leaf physiology, photosynthesis, Rhizaspidiotus donacis, Tetramesa romana,

Transpiration, Water resources effect.

Giant reed (Arundo donax L.) has been present in partsof the southwestern United States since at least the early19th century (Dudley 2000; Hoshovsky 1988). Currently,it is found throughout the southern half of the continentalUnited States, from California to Maryland, as well as onthe Hawaiian archipelago (Bell 1997). Because of its rapidgrowth rate (Dudley 2000; Perdue 1958) and ready abilityto resprout (Else 1996), A. donax is capable of formingdense, monotypic stands within a relatively short time (Bell1997; Coffman 2007; Perdue 1958; Rieger and Kreager1989), thus reducing the biodiversity of the riparian zone.

Losses in the abundance and richness of the aerialinvertebrate communities have been positively correlatedwith A. donax coverage (Herrera and Dudley 2003).Mechanisms for this reduction in the biodiversity of

communities in riparian zones that have been invaded byA. donax are not yet clearly understood. It has beensuggested that A. donax tissue is not easily digested bygeneralist herbivores (Miles et al. 1993a, 1993b; Spencer etal. 2005). As a result of the reduced biodiversity of thearthropod community in riparian areas dominated by A.donax, there is less likelihood that generalist herbivores willhave a significant effect on the growth and spread of A.donax. Mature A. donax has a mean carbon to nitrogen(C : N) ratio of 22 : 1, which is considered too high to befavorable to generalist herbivores (Spencer et al. 2007).Young shoots might be the most favorable to manyherbivores, but the rapid growth rate of this species presentsonly a small window of palatability.

One of the foremost methods for managing A. donax islikely to be biological control (Goolsby and Moran 2009;Moran and Goolsby 2009). However, Peterson et al.(2005) note that assessment of the efficacy of biologicalcontrol agents, in terms of having an injurious effect on thetarget plant species, is an important aspect in the decision-making process for managing invasive species and thatevaluation of the change in plant physiology in response toherbivory is an effective tool in determining the potentialvalue of introducing additional species to a givenecosystem. Although removal of photosynthetic material

DOI: 10.1614/IPSM-D-10-00007.1

* Assistant Professor and Graduate Student, Department of

Ecosystem Science and Management, Texas A&M University,

College Station, TX 77843-2138; and Research Scientist, U.S.

Department of Agriculture, Agricultural Research Service, Weslaco,

TX 78596. Current address of second author: Graduate Student,

Intercollege Graduate Degree in Ecology, Pennsylvania State

University, University Park, PA 16802. Corresponding author’s E-

mail: [email protected]

Invasive Plant Science and Management 2010 3:521–529

Moore et al.: Arundo response to herbivory N 521

does not necessarily equate to reductions in photosyntheticrate (A) of active leaf tissue, several studies have shown thatis does. It is also important that such research considerplant compensation through increased A or transpiration(El) (Aldea et al. 2005; Karban and Myers 1989; Petersonet al. 2005; Spence et al. 2007).

Previous studies have shown that (1) stem-boringarthropods reduce A in Linaria dalmatica (L.) Mill.(Peterson et al. 2005) by apparently eliciting a responsethat mimics water stress (Saner and Mullerscharer 1994);(2) leaf gall mites reduce A in cherry (Prunus serotina Ehrh.)and sumac (Rhus glabra L.); (3) moth caterpillar feeding onan aster, Parthenium hysterophorus L., reduce A, El, stomatalconductance (gs), and xylem water potential (Y) (Floren-tine et al. 2005); (4) fungal infection and wasp galls on asuite of understory hardwood species reduce the efficiencyof photosystem II (WPSII), a fundamental componentprocess of photosynthesis, by over 25% (Aldea et al. 2006);and (5) a leaf-mining fly, Hydrellia pakistanae Deonier.,reduced A in Hydrilla verticillata (L. f.) Royle by up to60% with heavy (70 to 90%) damage (Doyle et al. 2002).In contrast, herbivory by the moth Hyalophora cecropia L.had no effect on A and El in both apple [Pyrus malus (L.)Mill.] and crabapple [Pyrus coronaria (L.) Mill.] (Petersonet al. 1996), and wasp galls actually increased leaf A, gs, andPsi; in drought-stressed plants (Fay et al. 1993). Herbivorycan stimulate production of axillary meristems and increaseleaf area in galled rosinweed (Silphium integrifoliumMichx.) (Fay et al. 1996) and willow (Salix eriocarpa Fr.& Sav.) (Nakamura et al. 2003).

Transpiration has also been shown to remain constant oreven increase, on a unit leaf area basis, in plants subjectedto herbivory. Damage to major veins by leaf perforationsignificantly increased gs and El in birch (Betula pendulaRoth) and alder (Alnus spp.) (Oleksyn et al. 1998).

The objective of this study was to quantify the effect ofan armored scale, Rhizaspidiotus donacis Leonardi, and astem-galling wasp, Tetramesa romana Walker, currentlybeing considered as potential biological control agents on Aand El of A. donax in the United States. These insects arecommonly found on A. donax in the native range,predominately in the Mediterranean region (Goolsby andMoran 2009; Goolsby et al. 2009a). Rhizaspidiotus donacisfeeds on phloem and congregates at the base of leaf sheathsand at nodes along the stem, whereas T. romana forms agall within the shoot meristem, generally near the apex,that is likely to acts as a sink for photosynthates. The effectof the armored scale was investigated at two different timesafter infestation and population build up, whereas theeffect of the wasps was examined at two different densities.A final experiment examined the effect of these herbivoresused in combination on A and El.

Our secondary objective was to understand the mechanismsbehind any affects on A. donax (Ni et al. 2008; Schroder et al.2005) by measuring gs and intercellular CO2 concentration(Ci) and through the examination of photosynthesis CO2 (A/Ci) response curves. The shapes of these curves reveal potentialdecreases in carboxylation capacity or electron transport rate(Lambers et al. 1998; Larcher 2003), basic component(s) ofthe photosynthetic machinery in this C3 plant.

Materials and Methods

This study was conducted as a series of greenhouseexperiments set up in an arthropod biological controlcontainment facility located within Moore Air Base,operated by USDA-APHIS (Animal and Plant HealthInspection Service) in Hidalgo County, TX (26u239390N,98u20970W), where the wasp, T. romana, and the scale, R.donacis were confined to separate greenhouses. Within eachof these two greenhouses, pots containing shoots of A. donaxwere allocated to separate cages for this experiment. All potswere filled with a fertilized, uniform potting soil1 and werewatered twice daily. Furthermore, both greenhouses weresupplemented with high-pressure sodium lighting2 toprovide 12 hr d21 of light when day length was short.Because of the mobility of T. romana, two identical cages,one each for the control and herbivory treatments, wereestablished and lined with mesh3 that prevented individualwasps from either entering or leaving the cages. By contrast,mobility was not a concern for R. donacis, so rhizomes foreach of the treatments were established in large pots thatwere not segregated by treatment within the greenhouse.With climate control, the greenhouse with T. romana waskept at temperatures between 22 and 33 C, and the R.donacis greenhouse between 18 and 35 C.

Herbivory Trials. There were two trials for R. donacis: onelong-term (24 wk) and one short-term (14 wk). The long-

Interpretive SummaryThe results of our greenhouse study clearly demonstrate that the

stem-galling wasp T. romana is effective at negatively affecting leafscale physiological processes of A. donax when present in highdensities. However, low densities of wasps had no effect on leafphysiology. This study also shows that the armored scale R. donacisis effective at reducing the rates of Al and El given enough time.The time necessary to see a physiological effect by the scale moreor less matched the time necessary for females to reproduce,suggesting a connection between that stage of life history and peakdemand for resources. Moreover, these two insect herbivores seemto affect A. donax leaf physiology differentially because T. romanaappears to reduce the carboxylation capacity of Rubisco, whereasR. donacis measurably lowers the rate of electron transport.Incorporating the time necessary for the effect of the scale to beseen, then, the combination of two herbivores can have a strongerphysiological effect than one type of herbivore, likely because oftheir different effects on leaf function.

522 N Invasive Plant Science and Management 3, October–December 2010

term trial held seven control shoots and seven treatmentshoots with an exposure level of approximately 500 to 700first-instar crawlers released onto each shoot, and the trialwas conducted from February 26 to August 16, 2007. Theshort-term trial consisted of three control shoots and threetreatment shoots, with an exposure level of at least 500first-instar crawlers released onto each shoot, and the trialwas run from November 24, 2007, to March 3, 2008(Goolsby et al. 2009b). Rhizaspidiotus donacis females donot produce offspring until at least 120 d when they areremoved from the plant artificially, and the next generationis not found on the plant until over 160 d, when femalesare allowed to remain on the shoots (Moran and Goolsby2010). Before the second trial was completed, one of thecontrol shoots was broken during handling and wastherefore not used. Leaf gas exchange survey data andCO2 response curves were collected, and the parameters ofinterest were primarily Al, El, gs, and Ci. Further details onsampling gas exchange methodology and instrumentationare provided below.

Two experiments were conducted using T. romana: aninitial low-herbivore density experiment followed by ahigh-herbivore density experiment. Within the greenhousecontaining T. romana, during the low-density treatmentexperiment, two identical cages, one per treatment, housedsix potted plants each (1 shoot plant21). The control andtreatment cages experienced similar environmental condi-tions. Three 6-wk low-density trials were run from May toJune, June to August, and October to November 2007,consisting of 12 shoots of A. donax per trial, each at least1 m tall, that were grown from rhizomes of a standardizedweight in a uniform mixture of potting soil. The shootswere paired by size, each pair being randomly dividedbetween the two treatments, using height as a proxy forbiomass (Spencer et al. 2006).

Approximately 20 to 25 adult female T. romana werereleased into the treatment cage at the start of each low-density trial. This level is considered well below theminimum release rate that is the goal of the biologicalcontrol program. Because the generation time of T. romanais approximately 4 wk (,25 d as larvae, 7 d as pupae)(Moran and Goolsby 2009), leaf gas exchange survey data,as well as CO2 response curves were collected as near to4 wk from the onset of each trial as possible, using themethodology and instrumentation described below. Dur-ing the final low-density trial, two shoots from the controlgroup were broken at the main stem before gas exchangemeasurements could be conducted and were thereforeexcluded from the study.

The high-density experiment was conducted as part of asingle long-term trial that maintained a T. romana–onlytreatment as described by Goolsby and Moran (2009).Roughly one-third of the T. romana greenhouse wasblocked off with the same fine black mesh used in cage

construction, and this space was further divided to separatetreatment and control sections. Seven potted plants of A.donax were added to each section. Tetramesa romana werereleased in high density into the treatment section, with aninitial minimum of 60 wasps. Seven wasps per shoot perweek were added, including additional wasps for shootsthat appeared in the course of the trial. This treatment wasconsidered high density because it approximates the levelof the intended future mass releases of T. romana atestablished sites of A. donax.

In addition to the high-density T. romana–only trial, asimultaneous trial added another treatment—combinedherbivory by R. donacis in addition to the high densities ofT. romana. This trial consisted of seven additional pots,each starting out with a single shoot, and was also locatedwithin the same T. romana treatment section of thegreenhouse. These pots were not separated from the wasp-only high-density treatment because of the negligiblemobility of R. donacis, even in the first-instar crawlingstage. At least 500 first-instar R. donacis crawlers werereleased on the internodes of each of these shoots at theonset of the trial. Leaf gas exchange survey data and CO2

response curves were recorded after 10 and 26 wk ofexposure to herbivory on all 21 plants, enough time for atleast two generations of T. romana and one generation of R.donacis, respectively. Details about how plants respondedto herbivory (e.g., changes in stem or leaf length, numbersof galls, etc.) over the course of this and the otherexperiments are reported in Goolsby et al. (2009b).

Leaf Gas Exchange Methodology. All leaf gas exchangedata were collected using a LI-6400 open-pathway systemwith a 6400-02B LED light source and 6400-01 CO2

injector.4 Using the automated cooling system, leaf andblock temperatures were maintained near ambient. For eachshoot, only the second fully expanded leaf was used formeasurements to control for the effect of leaf age (Hikosaka2005). Sampled shoots within treatment cages werepreferentially selected above galls and, when present, clustersof R. donacis, but sampled leaves did not have insects onthem or any localized herbivory damage from either T.romana or R. donacis. All measurements were conductedbetween 10:30 A.M. and 2:30 P.M. central standard timewith a photosynthetic photon flux density (PPFD) fixed at1,500 mmol m22 s21 and a CO2 concentration at370 mmol mol21. Leaves were given a minimum of 5 minto adjust to the conditions in the cuvette. Leaf gas exchangemeasurements were repeated on at least two, and usuallythree, consecutive days to obtain an average value per shoot.Calculations of the physiological parameters Al, El, gs, and Ci

were as described by vonCaemmerer and Farquhar (1981)and adjusted for sampled leaf area.

To understand through what mechanism the twoherbivores, separately and in combination, affected leaf-


level physiology, we examined the shapes of A/Ci responsecurves for any changes in the ribulose-1,5-bisphosphatecarboxylase oxygenase (Rubisco)–limited carboxylationcapacity or electron transport rate (Lambers et al. 1998).These measurements were taken from one leaf for eachplant in each treatment. A/Ci response curves weregenerated at intervals of 50 mmol mol21 from 50 to400 mmol mol21 and intervals of 100 mmol mol21 from400 to 800 mmol mol21. Parameter values to model themaximum carboxylation rate of Rubisco (Vcmax) and themaximum rate of electron transport (Jmax) were taken fromSharkey et al. (2007). The activation energy (Ea) of A.donax was assumed to be 60 kJ mol21, a value expected fora C3 plant from a warm environment (Sage 2002).

Analysis. SPSS 14.05 was used to analyze the data in whichexperimental units, individual potted plants, were assignedto cages by initial plant height. Data were screened foroutliers and checked for normality, then one-tailedStudent’s t tests were used at a significance level of a 50.05 to compare Al, El, gs, and Ci between treatments.When the Levene’s test for equality of variances failed (P ,0.05), data were compared assuming unequal variances.One-tailed tests were deemed appropriate on the basis ofthe hypothesis that, in all cases, physiological processes willbe reduced because of the method of herbivory by each ofthe two insects (Larson 1998; Meyer and Whitlow 1992;Peterson 2001; Peterson et al. 2005; Raghu and Dhileepan2005; Raghu et al. 2006), and doing so enabled us tominimize Type II errors. Separate Student’s t tests wereconducted for the R. donacis experiment (pooled for bothtrials because of low sample size), the low- and high-densityT. romana trials, and the combined R. donacis/T. romanatrials. Additionally, component slopes of the A/Ci responsecurves corresponding to Rubisco-limited carboxylationcapacity or electron transport rate were compared toelucidate the mechanistic differences between the waydifferent insects elicit responses in A. donax (Macedo et al.2005; Peterson et al. 2005).

Results and Discussion

Rhizaspidiotus donacis Trials. Rhizaspidiotus donaciseffectively reduced leaf gas exchange rates in A. donaxdespite only minor effects on growth at the whole-plantlevel (Goolsby et al. 2009b). Mean El was 40% lower inleaves of plants with R. donacis than in control plants afterseveral months of exposure to the herbivore (Table 1).Furthermore, mean gs was 46% lower in plants subject toherbivory. These strong reductions in water use wereexpected because of the nature of the method of feeding byR. donacis. As plants lose sap to scale herbivory, leaves aremore likely to wilt or distort (Gullan and Cranston 1994).Because scale insects feed directly on nutrients in fluids T

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from the phloem, they can act as a sink for photosynthatesthat would otherwise be directed toward plant growth,maintenance, or storage (Risebrow and Dixon 1987).

We found only weak evidence of a reduction in Al

(Table 1). Mean Al in leaves of plants with R. donacis was32% lower than mean Al in control leaves (Table 1);however, this reduction in treated plants was not significant(t 5 1.719, df 5 17, P 5 0.052). This is likely a result ofthe large variance between leaves in the same treatment,given the low sample size of seven per treatment. Thestandard error was 15.5 and 16.2% of the mean for thecontrol and scale treatments, respectively. Meyer andWhitlow (1992) also reported no effect of a phloem-feeding insect on rates of Al in goldenrod, Solidago altissimaL., normalized by leaf weight. The aphid used in Meyerand Whitlow’s study actually had no measurable effect onany investigated parameter, including El, gs, and Ci,although it should be noted the duration for which theirplants were exposed to herbivory was limited to 12 d.

Because R. donacis had not yet reproduced at the time ofthe measurement in the short-term trial (Moran andGoolsby 2009), we investigated whether this problemaffected our results. When those short-term data wereexcluded from the analysis, Al differed between treatments(t 5 2.279 , df 5 12, P , 0.025). Leaf responses toherbivory by R. donacis were apparently sensitive to time ofexposure, which is consistent with other studies (Schafferand Mason 1990). Rhizaspidiotus donacis might not becapable of slowing physiological processes until after theyreproduce, which occurred between the 14- and 24-wktrials (Moran and Goolsby 2010). It has already beenshown elsewhere that, as scale insects reach sexual maturityor some other significant stage in their life history, theirdemand for resources increases (Boggs 1992).

The Al /Ci response curve yields further evidence for thenegative effects of 24 wk of herbivory by R. donacis on leafscale physiological processes. The maximum rate ofelectron transport, Jmax, is drastically reduced by R. donacis(Table 2; Figure 1), so the presence of this herbivoreclearly reduced the ability to capture energy for photosyn-thesis when CO2 availability is unlimited. The carboxyl-ation rate, Vcmax, was also lower in the attacked plants, butthe effect was smaller than for Jmax. Rhizaspidiotus donacismay act as a sink for photosynthates that prevents A. donax

from growing new roots that could increase nutrient uptake(Li et al. 2006). Because Jmax is controlled by the supply ofnitrogen-rich Rubisco enzyme (vonCaemmerer and Farqu-har 1981), a reduction in Jmax could indicate that lessnitrogen is being allocated to Rubisco (Hikosaka 2004;Hikosaka and Terashima 1996).

Tetramesa romana Trials. Herbivory by low densities ofthe galling wasp T. romana did not reduce the rates ofeither of the basic physiological processes of Al or El inleaves of A. donax (Table 1). Parameters Ci and gs werereduced by 9 and 26%, respectively (Table 1). Thedecrease in Ci without a significant reduction of Al couldbe due to some compensation on the part of A. donax ifleaves are able to more effectively utilize the available CO2

in the intercellular airspaces. This situation would onlyplausibly arise if stomata are closed more in leaves of plantswith galls, which the reduction in gs suggests. It wasunexpected, though, that such a strong reduction in gs

would not lead to a similar reduction in El. It might be thecase that changes occurred below the detectable limit,especially because gs is largely derived from El. Leaftemperature and atmospheric pressure also factor into the

Table 2. The efficacy of two insect herbivores, separately and in combination, in reducing the carboxylation rate of ribulose-1,5-bisphosphate carboxylase oxygenase (Vcmax) and the rate of electron transport (Jmax). Mean and percent difference (% Diff.) shown.

Agent

Vcmax Jmax

Control Treatment % Diff. Control Treatment % Diff.

Rhizaspidiotus donacis 59.9 49.1 218.0 78.7 30.9 260.7Tetramesa romana 63.3 67.7 7.0 90.7 97.0 6.9T. romana/R. donacis 38.7 12.6 267.5 48.5 24.5 249.4

Figure 1. CO2 response of leaves of Arundo donax to 24 wk ofexposure to herbivory from the armored scale Rhizaspidiotusdonacis. Mean values 6 standard error are given. Photosyntheticphoton flux density was 1,500 mmol m22 s21.


calculation of gs, so these variables were also comparedbetween treatments, but no differences were found (datanot shown). Although this is likely an indication that the El

data have large enough variances to preclude statisticaldifferentiation between treatments, it is possibly anindication that leaves of galled shoots are susceptible toincreased water loss through parts of the leaf other than thestomata. Nonetheless, the ability of A. donax to balance thefundamental physiological processes of Al and El does notseem to be perturbed by herbivory by T. romana at lowdensity. Additionally, the A/Ci response curves for the low-density treatment of T. romana demonstrate that there islittle effect on A. donax physiology. Neither Vcmax nor Jmax

were much different in the plants experiencing herbivory(Table 2; Figure 2).

By contrast, when T. romana was present in highdensity, the response of A. donax to herbivory was quitedistinct. Parameters Al, El, and gs were all lower in shootsthat experienced herbivory, whereas Ci was not differentbetween the two treatments (Table 1). In the high-densitytreatment, Al, El, and gs were 19, 27, and 32% lower,respectively, compared with the same parameters in controlplants. The observation that both Al and El are markedlylower in galled shoots in the high-density treatment, incontrast with the low-density experiment, clearly indicatesthat the effect of herbivory on A. donax by T. romana isdensity dependent. This phenomenon has been document-ed for galls by cynipid wasps on leaves (Bagatto et al. 1996;Dorchin et al. 2006), but to our knowledge, this is the firststudy to have documented this type of relationship forstem-galling wasps. Moreover, high densities of T. romanasuppress leaf and stem lengths and stimulate side shootproduction (Goolsby et al. 2009b).

Combination Trials. After 10 wk of exposure to both R.donacis and high-density T. romana, Al, El, Ci, and gs werenot significantly different between the two treatments(Table 1). These results are in contrast with the experimentof herbivory by T. romana alone at high density. In thiscase, clear differences in these physiological parameterswere evident within 10 wk (Table 1). Because these trialsoccurred simultaneously and within the same enclosurespace, both the T. romana high-density trials (with andwithout R. donacis) experienced similar levels of herbivory,having about the same number of galls per plant (Goolsbyet al. 2009b). This suggests that leaves of A. donax exhibit acompensatory response when the shoot experiencesmultiple forms of herbivory. Another plausible explanationis that adult T. romana avoided shoots with R. donacispresent to reduce interspecific competition and that, astime passed, there was a cumulative effect on leafphysiology that led to the difference between thesetreatments in leaf physiology after 10 wk.

Apparently, compensation, if any, only lasts for a limitedtime. Twenty-six weeks after the two herbivores werereleased onto A. donax, very strong differences between thetreatments were evident. Parameters Al and El were 67 and42% lower in leaves of attacked shoots than in leaves ofshoots without herbivores, respectively (Table 1). Con-versely, Ci was greater in the herbivory treatment byapproximately 25%, suggesting reduced carbon assimila-tion; but gs was not different between treatments (Table 1).Such strong reductions in Al support earlier explanationsfrom the effect of second-generation R. donacis. Thisobservation suggests that the photosynthetic machinery inthe leaves subjected to prolonged herbivory by both insectsis less capable of utilizing available carbon (Meyer andWhitlow 1992).

The A/Ci response of A. donax after 26 wk of exposure tothe two herbivores also points to a large cumulative effectof both insects, each likely acting in a different capacity.For example, the combination of the two insectsdemonstrated a similar reduction in Jmax as R. donacisalone, suggesting that R. donacis negatively affects theprocess of electron transport (Table 2; Figure 3). Con-versely, the much greater reduction in Vcmax with theadditional presence of high densities of T. romana suggeststhat the carboxylation capacity of Rubisco was reducedlargely as a result of the presence of a large number of galls(Table 2; Figure 3). Plants are already known to reduceboth Vcmax and Jmax in the presence of herbivores (Schroderet al. 2005), so this outcome is expected. The presence oftwo herbivores in large numbers might also induce theproduction of plant secondary metabolites in A. donax.This could act as a further sink for energy gained fromphotosynthesis (Karban and Myers 1989), although thisquestion was not explicitly addressed in this work.However, this study is only the second to document that

Figure 2. CO2 response of leaves of Arundo donax to herbivoryfrom low densities of the stem-galling wasp Tetramesa romana.Mean values 6 standard error are given. Photosynthetic photonflux density was 1,500 mmol m22 s21.


two insect herbivores differentially affect the physiology oftheir host species (Peterson et al. 2005). Because neither ofthese two insects consume leaves, understanding the effectson leaf physiology is particularly valuable (Karban 1997).

Little research has been done on the combined herbivoryby two or more phytophagous insects from differentfeeding guilds. We have shown that effects are not alwaysdirectly additive (cf. Meyer and Whitlow 1992). It is wellknown that plants respond differently to these variousforms of herbivory (Gavloski and Lamb 2000; Trumble etal. 1993). Our study suggests that the stem-galling waspreduced A. donax capacity to optimize carbon gain andwater loss, but that the addition of a sap-feeding insectmight stimulate temporary compensatory responses in theplant. It appears to take a comparatively long time for sap-feeding scale insects to affect leaf physiological processes,but once realized, we found additive responses betweenthese insects from different feeding guilds on characteristicsof leaf gas exchange. Arundo donax might compensate forscale insects by using stored carbon resources to produceyoung tissue (Boose and Holt 1999; Decruyenaere andHolt 2001), the vigor of individual plants declining onlyonce these resources become exhausted. Further research isneeded to determine whether these types of damage aresufficient to control this weed in the field.

Sources of Materials1 Potting soil, Sunshine Mix #1, Sungrow Horticulture, Bellevue,

WA.2 High-pressure sodium lighting, GE Multi-Vapor 400 watt, General

Electric Co., Cleveland, OH.3 Mesh, sheer black silk, Los Dos Rios Fabric Store, McAllen, TX.

4 6400-02B LED lights, 6400-01 CO2 injector, Li-Cor Inc.,Lincoln, NE.

5 SPSS software 14.0, IBM Company, Chicago, IL.

Acknowledgments

Funding for this project was provided by the Rio GrandeBasin Initiative via the U.S. Department of Agriculture(USDA) CSREES Program, the USDA–Agricultural ResearchService via the Arundo Biological Control Program, and theU.S. Department of Homeland Security via the Science andTechnology Directorate. We are grateful to USDA-APHIS foruse of the quarantine facility and temporary lodging.Technical assistance was provided by Ann Vacek, KiraZhaurova, Sarah Snavely, Ila Billman, and Crystal Salinas.

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Received January 21, 2010, and approved June 1, 2010.


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