Methyl Salicylate Attracts Natural Enemies and Reduces Populations of Soybean Aphids (Hemiptera: Aphididae) in Soybean Agroecosystems

FIELD AND FORAGE CROPS

Methyl Salicylate Attracts Natural Enemies and Reduces Populations ofSoybean Aphids (Hemiptera: Aphididae) in Soybean Agroecosystems

RACHEL E. MALLINGER,1 DAVID B. HOGG, AND CLAUDIO GRATTON

Department of Entomology, University of WisconsinÐMadison, 1630 Linden Street, Madison, WI 53703

J. Econ. Entomol. 104(1): 115Ð124 (2011); DOI: 10.1603/EC10253

ABSTRACT Methyl salicylate, an herbivore-induced plant volatile, has been shown to attract naturalenemies and affect herbivore behavior. In this study, methyl salicylate was examined for its attrac-tiveness to natural enemies of the soybean aphid, Aphis glycinesMatsumura (Hemiptera: Aphididae),and for its direct effects on soybean aphid population growth rates. Methyl salicylate lures weredeployed in plots within organic soybean [Glycine max (L.) Merr.] Þelds. Sticky card traps adjacentto and 1.5 m from the lure measured the relative abundance of natural enemies, and soybean aphidpopulations were monitored within treated and untreated plots. In addition, exclusion cage studieswere conducted to determine methyl salicylateÕs effect on soybean aphid population growth rates inthe absence of natural enemies. SigniÞcantly greater numbers of syrphid ßies (Diptera: Syrphidae)and green lacewings (Neuroptera: Chrysopidae) were caught on traps adjacent to the methylsalicylate lure, but no differences in abundance were found at traps 1.5 m from the lure. Furthermore,abundance of soybean aphids was signiÞcantly lower in methyl salicylate-treated plots. In exclusioncage studies, soybean aphid numbers were signiÞcantly reduced on treated soybean plants when allplants were open to natural enemies. When plants were caged, however, soybean aphid numbers andpopulation growth rates did not differ between treated and untreated plants suggesting no effect ofmethyl salicylate on soybean aphid reproduction and implicating the role of natural enemies indepressing aphid populations. Although aphid populations were reduced locally around methylsalicylate lures, larger scale studies are needed to assess the technology at the whole-Þeld scale.

KEY WORDS methyl salicylate, soybean aphid, syrphid ßy, lacewing, herbivore-induced plantvolatile

Herbivore-induced plant volatiles constitute one par-ticular group of allelochemicals that are released whenherbivore elicitors induce plant defense pathways(Reymond and Farmer 1998, Turlings and Ton 2006,Bruce and Pickett 2007, Felton and Tumlinson 2008,Schmelz et al. 2009). Herbivore-induced plant vola-tiles have been shown to mediate relationships be-tween plants and insects through the attraction ofnatural enemies and the repulsion of herbivores (Turl-ings et al. 1990; Hildebrand et al. 1993; Takabayashiand Dicke 1996; Drukker et al. 2000; De Moraes et al.2001; Kessler and Baldwin 2001; Cardoza et al. 2003; deBoer and Dicke 2004a,b, 2005; Ode 2006; Turlings andTon 2006; Goggin 2007). It has long been anticipatedthat manipulation of herbivore-induced plant volatilesin synthetic or natural form can be used to concentrateand increase populations of natural enemies within anarea of interest, such as a crop Þeld, or to repel pestsfrom crop plants (Dicke et al. 1990, Turlings et al. 1990,Kessler and Baldwin 2002, Turlings and Ton 2006,Khan et al. 2008).

Methyl salicylate is a phenolic compound thatserves in plant defenses against pathogens and herbi-vores and is a common component of herbivore-in-duced plant volatile blends (Shulaev et al. 1997, Parket al. 2007, Vlot et al. 2008). Upon activation of thesalicylic acid defense pathway, liquid methyl salicylateincreases in concentration within a plant, and thevolatile form is subsequently released in high concen-trations (Kessler and Baldwin 2001, Kessler and Bald-win 2002, Park et al. 2007, Li et al. 2008, Vlot et al.2008). It is speculated to be a good volatile for use inpest management because its presence does not varyamong plant cultivars, as do other herbivore-inducedplant volatile compounds (Takabayashi et al. 1991).SpeciÞcally, feeding by the soybean aphid, Aphis gly-cines Matsumura (Hemiptera: Aphididae), has beenshown to activate the salicylic acid defense pathwaywithin soybean, Glycine max (L.) Merr., plants (Li etal. 2008), resulting in the emission of high levels ofmethyl salicylate (Zhu and Park 2005). Furthermore,methyl salicylate can itself induce the salicylic aciddefense pathway and subsequent release of methylsalicylate inhealthyplants (Shulaevet al. 1997,Farmer2001, Heidel and Baldwin 2004). The cascading release1 Corresponding author, e-mail: [email protected].

0022-0493/11/0115Ð0124$04.00/0 � 2011 Entomological Society of America

of this volatile can thereby extend the spatial range ofmethyl salicylateÕs effects in an agroecosystem. Thus,through its role in plant defensive pathways and itsrole as a volatile signal, methyl salicylate has the po-tential to inßuence herbivoreÐplant interactions.

The capacity of methyl salicylate to attract naturalenemies has been demonstrated in laboratories andagroecosystems, but the speciÞc nature of responses ishighly varied. When deployed in vineyards and hopy-ards, synthetic methyl salicylate attracted six groups ofpredatory insects and one genus of parasitoid wasp(James 2003a,b; James and Price 2004; James andGrasswitz 2005). It also has attracted predatory mitesand an egg parasitoid in laboratory experiments (deBoer and Dicke 2004a,b, 2005; Williams et al. 2008) andanthocorid predators (Hemiptera: Anthocoridae) inboth laboratories and cotton (Gossypium spp.) Þelds(Drukkeret al. 2000,Yuet al. 2008).However, only thesevenspotted lady beetle, Coccinella septempunctataL., was attracted to methyl salicylate in soybean Þelds(Zhu and Park 2005). The system-speciÞc responses ofnatural enemies to methyl salicylate suggest that fac-tors such as concentration, release rate, deploymentmethod, or deployment time during the season maystrongly inßuence the volatileÕs attractiveness to nat-ural enemies (James 2003a,b; Zhu and Park 2005; Yu etal. 2008), making it difÞcult to predict how effective itwill be in pest management across agroecosystems.

In addition to indirectly impacting herbivore pop-ulations through natural enemy attraction, methyl sa-licylate also can directly impact the behavior of her-bivores. Methyl salicylate has been shown to functionas a natural cue for aphids to alternate hosts in thespring, repelling them from their overwintering hostand into their summer host (Pettersson et al. 1994,Glinwood and Petterson 2000). When deployed inbarley (HordeumvulgareL.), a summer host, syntheticmethyl salicylate repelled aphid pests, delaying springcolonization and resulting in signiÞcantly reducedaphid abundance (Ninkovic et al. 2003). Other studieshave shown aphids to be attracted to low concentra-tions of methyl salicylate comparable with the con-stitutive levels emitted by their plant hosts (Han et al.2005, Webster et al. 2008).

Herbivore-induced plant volatiles also have the po-tential to affect herbivores through increasing plantantibiosis. The presence of synthetic volatiles as wellas the natural release of volatiles reduced the numberof eggs laid by moths on their host plants (Kessler andBaldwin 2001). Similarly, the artiÞcial induction ofsalicylic acid defenses has been shown to reduce aphidpopulation growth on tomato plants, Solanum lycop-ersicum L. (Cooper et al. 2004). Thus, herbivore-in-duced plant volatiles such as methyl salicylate may becapable of affecting changes within the host plant,including the induction of plant defenses that in turnreduce herbivore fecundity. The capacity to regulateherbivore reproduction together with the ability toattract natural enemies and repel herbivores suggestsa signiÞcant role for plant volatiles such as methylsalicylate in the management of pest populations.

The objective of this study was to examine whethermethyl salicylate formulations tested under Þeld con-ditions have the potential to reduce soybean aphidpopulations indirectly through attracting natural en-emies or directly through effects on the aphids them-selves. We hypothesized that in small Þeld plotscontaining a synthetic methyl salicylate emitter (here-after lure), there would be an overall greater abun-dance of natural enemies such as lady beetles (Co-leoptera: Coccinellidae), lacewings (Neuroptera),Orius spp. (Hemiptera: Anthocoridae), and braconidwasps (Hymenoptera: Braconidae) compared withuntreated plots. We also predicted that natural enemyabundance would be greater nearest the methyl sa-licylate lure and would decline with distance from thelure. Finally, we hypothesized that the presence ofhigh concentrations of synthetic methyl salicylatewould result in reduced soybean aphid fecundity andpopulation growth rates. The total effects of methylsalicylate would result in lower soybean aphid abun-dance within plots containing methyl salicylate lurescompared with plots without lures.

Materials and Methods

Deployment ofMethyl Salicylate.Methyl salicylateemitters were 5 g, 90 d, slow-release lures (Predalure,AgBio, Westminster, CO). In 2008, these lures weredeployed in two organically managed soybean Þelds insouth central Wisconsin in which no insect pest man-agement treatments occurred. Field 1, located nearRandolph, WI (43� 38� 08.6� N, 88� 58� 27.7� W), con-tained 45 acres of soybean (ÔBlue River 16A7�) thatwas planted on 24 May 2008. Field 2, located nearColumbus, WI (43� 18� 47.7� N, 89� 04� 28.1� W), was28 acres of soybean (ÔViking 0.1832�) planted on 3 June2008. Both of these Þelds were surrounded on threesides by other Þeld crops (corn, Zea mays L; or soy-bean) and on one side by either a road or a backyard.The experimental design was a randomized completeblock with 16 pairs of plots (2 by 2 m) dispersedthroughout the two soybean Þelds. Each plot within apair was randomly assigned as treated (receiving amethyl salicylate lure) or untreated (no lure). Treatedand untreated plots within each pair were 150 m apart,and pairs were a minimum of 50 m from any Þeldedge and 150 m from other pairs of plots. In eachtreated plot, one methyl salicylate lure was hung froma plastic stake at the center of the plot at canopyheight. Lures were put in the Þeld on 1 July 2008, �3wk after the emergence of soybean, and replaced onceat the end of July.Sampling for Natural Enemies and Aphids. One

yellow sticky card trap (Pherocon AM, Trece Inc.,Adair, OK) was attached to a plastic stake at canopyheight, adjusted throughout the growing season, andplaced in the center of each plot just below the methylsalicylate lure (hereafter adjacent to the lure) or alonein untreated plots. In addition, four yellow sticky cardtraps were hung at the corners of all treated plots(hereafter 1.5 m from the lure) and untreated plots.Traps were placed in the Þeld seven times throughout

116 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 104, no. 1

the growing season from 3 July 2008 when plants wereat theV3stage(three setsofunfolded trifoliate leaves)through 26 August 2008, plants at R5 stage (seed is 3mm long in the pod at one of the four uppermost nodeson the main stem), and on each occasion were left inthe Þeld for 4 d before collection. Once collected,traps were frozen and stored until analysis, at whichpoint all natural enemies on traps were identiÞed tofamily or species level. For corner traps, insect abun-dance is expressed as averages of the four traps.

In addition, whole plant sampling was conductedwithin all plots to determine abundance of naturalenemy immatures, natural enemy eggs and soybeanaphids. On 12 dates throughout the 2008 growing sea-son, �10Ð15 randomly selected plants in treated anduntreated plots were examined (nondestructively)from top to bottom for natural enemies and soybeanaphids. Natural enemy eggs and immatures werecounted on a per plant basis and recorded to familylevel, and the total number of soybean aphids per plantalso was recorded.Methyl Salicylate Effect on Aphid PopulationGrowth: Exclusion Cage Experiment. To assess thepotential direct effects of methyl salicylate on aphidpopulation growth, an exclusion cage experiment wasconducted. Soybean (ÔBSR 101�) was planted in 15-cm-diameter pots in which multiple strands of nylonstring, each �33 cm long, were taped to the inside toact as watering wicks. All pots contained two soybeanplants and were grown in the greenhouse. When theplants reached the V2ÐV3 stage (two to three fullyexpanded sets of trifoliate leaves), two pots wereplaced in small plastic containers for deployment inthe Þeld. Each container had two 13-cm-diameterholes cut in the lid to accommodate the two pots ofsoybean plants that sat �15 cm from the bottom of thecontainer. Water was placed in the bottom of eachcontainer, and the nylon strings extending from thepots served as watering wicks. For both experimentalpots in a container, 50 fourth-instar aphids, reared inthe laboratory on soybean plants (ÔBSR 101�) under aphotoperiod of 16:8 (L:D) h, were distributed amongthe two soybean plants per experimental pot. One potper container was then randomly assigned to be cagedto exclude natural enemies. Nylon mesh bags (200-�mmesh) were placed over the plants and closed withrubber bands around the top of the pot to prohibitmovement of insects off of and onto the soybeanplants.

Plastic containers holding one caged and one openpot of soybean plants were deployed on 25 June 2009in organically managed soybean Þelds at the Univer-sity of Wisconsin MadisonÕs Agricultural Research Sta-tion in Arlington, WI. Containers were placed in theÞeld in pairs, with members of the pair separated by�50 m. One container of each pair was randomlyassigned to be exposed to methyl salicylate: both theopen and caged pots received one 5-g methyl salicy-late lure (Predalure, AgBio) hung from a plastic stakeinserted into the pots. The stake and lure were placedinside nylon mesh bags on caged pots. All untreatedcontainers had identical stakes with no lures. The

experimental design included a total of 15 pairs ofcontainers (i.e., 15 treated and 15 untreated). Theseplants were left in the Þeld for 2 wk, after which periodthe mesh bags were removed from caged pots. Aphids,as well as any natural enemies, on caged and open potswere counted and expressed as aphids per pot. Thelow initial densities of aphids and the short time frameof the experiment were designed to reduce cage im-pacts on the natural emigration of winged aphids,which was found to be minimal when Þnal aphiddensities were �4,000 aphids per plant (Donaldson etal. 2007).Methyl Salicylate Effect on Aphid Life Table Pa-rameters: Clip Cage Experiment. In addition to theabove-described exclusion cage study, a clip cage ex-periment was conducted to determine the effects ofmethyl salicylate on soybean aphid population growthparameters. Soybean plants at the R1 stage (plantshaving at least one ßower on any node) in organicallymanaged soybean plots at the Agricultural ResearchStation in Arlington were used for this experiment. On23 July 2009, 28 plants were randomly selected withinthese plots, with all plants at least 50 m away from anyother selected plant. One fourth-instar aphid, rearedin the laboratory on BSR 101 soybean plants under aphotoperiod of 16:8 (L:D) h, was placed on the young-est, fully developed leaf of each selected plant andenclosed within a 5-cm-diameter clip cage as de-scribed in Myers and Gratton (2006). Half of theplants were randomly selected to be treated, and 5 gmethyl salicylate lures (Predalures, AgBio) were hungfrom plastic stakes as close to each clip cage as pos-sible. Untreated plants had stakes without a methylsalicylate lure placed close to the clip cage. A same-agecohort was achieved when all aphids in clip cages hadproduced at least two neonates on any 1 d (withprevious offspring removed daily). The experimentstarted 24 h later when the initial parent and one of theneonates were removed to leave one 2-d-old aphid ineach clip cage. All cages were subsequently checkedevery 24 h for 15 d, and after the aphids began repro-ducing (around the seventh day), the number of off-spring per aphid was recorded daily. Offspring wereleft in the cages so as to avoid disturbance and theexperiment was terminated before offspring reachedmaturity and had produced offspring themselves.Statistical Analyses. Natural enemy and aphid

counts collected through sticky traps and whole plantsamples were analyzed with repeated measures mixedmodel analyses of variance. Fixed effects includedtreatment, date, and the interaction between thesetwo variables whereas random effects included plotnested within block, and date was treated as the re-peated variable (nlme package, version 3.1-96, R De-velopment Core Team 2009). All data were log orsquare root transformed where appropriate to meetthe assumptions of normality and homogeneity ofvariance.

Differences between Þnal aphid abundance ontreated and untreated plants in the exclusion cagestudy were tested with mixed model analyses of vari-ance. Analyses were done separately for aphids on

February 2011 MALLINGER ET AL.: METHYL SALICYLATE AND SOYBEAN APHID 117

caged plants (treated versus untreated) and openplants (treated versus untreated), and each analysiswas conducted with a Þxed effect of treatment and arandom effect of individual pot nested within block(nlme package, version 3.1-96, R Development CoreTeam 2009).

In addition, a “biocontrol service index” was calcu-lated to measure the functional activity of naturalenemies on both the treated and untreated pots ofplants (Gardiner et al. 2009). The biocontrol serviceindex was determined by the relative changes in aphidabundance on caged and open pots of plants using theformula.

BSI �

�n� 1

15�Ac,n � Ao,n�

Ac,n

n

where Ac,n is the Þnal number of aphids on cagedplants for a given replicate, Ao,n is the Þnal number ofaphids on open plants for a given replicate, and n is thenumber of replicates, or pairs of caged and openplants. The biocontrol service index gives a value vary-ing from 0 to 1, with the values increasing as the levelof aphid predation increases. The biocontrol serviceindices of treated and untreated pots of plants werecompared using mixed model analyses of variancewith treatment as the Þxed effect and the container ofpots nested within block as the random effect (nlmepackage version 3.1-96, R Development Core Team2009). Data were transformed where appropriate tomeet assumptions of normality and homogeneity ofvariance.

Life table parameters including Þnite rate of in-crease, intrinsic rate of increase, net reproductive rate,and mean generation time were estimated for theaphid populations in clip cages by using PopTools 2.5(Hood 2003) with standard errors of the estimatescalculated by jackkniÞng (Myers and Gratton 2006).Statistical differences in life table parameters betweenaphids in treated versus untreated clip cages weredetermined using randomization tests (Myers andGratton 2006). The null hypothesis in such tests wasthat there was no difference in any of the life tableparameters for aphids in treated and untreated clipcages. In the randomization tests, the individual rep-licates from treated and untreated clip cages wererandomly shufßed to comprise two new groups con-taining aphids from both treatments. Life table pa-rameterswerecalculated foreachof the tworandomlyconstructed groups, and this was repeated for 1,000randomization iterations. A test statistic was com-puted as the average difference between life tableparameters in the two randomly constructed groupsover 1,000 iterations and this test statistic was thencompared with the difference in life table parameterscalculated fromtheoriginaldata.AnoverallPvalue forthe statistical difference between treated and un-treated clip cages was determined by the number oftimes that the difference between life table parame-ters in the two randomly constructed groups for each

randomization iteration exceeded the difference be-tween the life table parameters calculated from theoriginal data.

Results

Abundance of Natural Enemy Adults. Sticky cardtraps placed at the center of treated plots, adjacent tothe methyl salicylate lure, had signiÞcantly more nat-ural enemy adults compared with traps placed at thecenter of untreated plots (F� 39.0; df � 1, 7;P� 0.001;Fig. 1A). There was also a signiÞcant time treatmentinteraction (F � 3.0; df � 6, 84; P � 0.01), with dif-ferences in natural enemy abundance being most pro-nounced during the month of July (Fig. 1A). Datafrom both soybean farms were analyzed together asthere was no signiÞcant interaction between treat-ment and farm (F � 1.1; df � 1, 6; P � 0.34). Amongindividual natural enemy taxa, Syrphidae (Diptera),syrphid ßies, and Chrysopidae (Neuroptera), greenlacewings, were both signiÞcantly more abundant onsticky card traps placed in the center of treated plotscompared with untreated plots with no signiÞcanttime treatment interactions for either taxon (Table1). There was no signiÞcant difference in abundanceof other natural enemy taxa such as the lady beetlesCoccinella septempunctata L. and Harmonia axyridis(Pallas); the insidious ßower bug, Orius insidious(Say); braconid wasps (Hymenoptera: Braconidae);and brown lacewings (Neuroptera: Hemerobiidae),with no signiÞcant time treatment interactions forany taxon (Table 1).

Therewasno signiÞcantdifference in the totalnum-ber of natural enemy adults caught in sticky card trapsplaced at the corners of treated plots, 1.5 m from themethyl salicylate lure, compared with untreated plots(F� 0.2; df � 1, 7;P� 0.67; Fig. 1B), with no signiÞcanttime treatment interaction (F� 0.4; df � 6, 84; P�0.88). Again, data from both farms were pooled be-cause there was no signiÞcant interaction betweenfarm and treatment (F � 0.4; df � 1, 6; P � 0.55).Furthermore, the abundance of speciÞc natural en-emy taxa found in these corner traps did not differbetween treated and untreated plots, including C.septumpunctata, H. axyridis, O. insidiosus, Braconidae,Syrphidae, Chrysopidae, and Hemerobiidae (Table2). There was a signiÞcant time treatment interac-tion for H. axyridis but not for the other aforemen-tioned taxa (Table 2).Abundance ofNatural Enemy Immatures andEggs.

Abundance of natural enemy immatures, includinglarvae of Syrphidae, Coccinellidae, Chrysopidae, andHemerobiidae, as well as nymphs ofO. insidiosus,wasnot signiÞcantly different in treated versus untreatedplots (F� 0.1; df � 1, 7; P� 0.72; Fig. 2A), and therewas no signiÞcant time treatment interaction (F �0.4; df � 11, 154; P � 0.95). In addition, there was nosigniÞcant difference in the abundance of natural en-emy eggs, including those of Syrphidae, Chrysopidae,Hemerobiidae, and Coccinellidae, in treated versusuntreated plots (F� 0.04; df � 1, 7; P� 0.85; Fig. 2B),with no signiÞcant time treatment interaction (F�


1.1; df � 11, 154; P � 0.40). Immature and egg abun-dances were pooled across both farms because therewere no signiÞcant interactions between farm andtreatment for immatures (F� 0.6; df � 1, 6; P� 0.47)or eggs (F � 0.3; df � 1, 6; P � 0.59).

Aphid Abundance. Aphids were signiÞcantly moreabundant in untreated plots compared with treatedplots (F � 8.0; df � 1, 7; P � 0.03; Fig. 3), with nosigniÞcant time treatment interaction (F� 1.0; df �11, 154; P � 0.31). Data of aphid abundance were

Fig. 1. Average abundance (per 4-d period), means SEM, of natural enemies captured on sticky card traps during the2008 growing season in soybean Þelds in southern Wisconsin. (A) Sticky card traps placed in the center of untreated andtreated plots (adjacent to the methyl salicylate lure). (B). Sticky card traps placed at the corners of untreated and treatedplots (1.5 m from the methyl salicylate lure).

Table 1. Average abundance of natural enemy taxa captured on sticky card traps placed in the center of untreated and treated plots(adjacent to the methyl salicylate lure) over seven 4-d periods during the 2008 growing season

Taxa TreatmentAvg per-trapabundance

(mean SEM)

ANOVA results

Treatment Time Time treatment

C. septempunctata Treated 0.07 0.04 F � 0.04, P � 0.85 F � 2.8, P � 0.02* F � 0.3, P � 0.93Untreated 0.08 0.04

H. axyridis Treated 0.31 0.10 F � 1.2, P � 0.31 F � 0.8, P � 0.61 F � 1.8, P � 0.12Untreated 0.38 0.08

O. insidiosus Treated 0.91 0.25 F � 0.00003, P � 1.00 F � 27.0, P � 0.001*** F � 0.4, P � 0.86Untreated 1.00 0.28

Braconidae Treated 0.39 0.10 F � 7.0, P � 0.49 F � 1.0, P � 0.45 F � 1.1, P � 0.36Untreated 0.26 0.07

Syrphidae Treated 12.09 1.58 F � 45.0, P � 0.001*** F � 14.0, P � 0.001*** F � 1.0, P � 0.26Untreated 4.46 0.64

Chrysopidae Treated 0.63 0.13 F � 5.3, P � 0.05* F � 2.0, P � 0.08 F � 1.5, P � 0.20Untreated 0.16 0.06

Hemerobiidae Treated 0.04 0.03 F � 0.8, P � 0.39 F � 2.3, P � 0.04* F � 1.1, P � 0.40Untreated 0.07 0.04

Treatment df � 1, 7; time df � 6, 84; and time treatment df � 6, 84.


pooled for both farms as there was no farm by treat-ment interaction (F � 1.9; df � 1, 6; P � 0.22).Exclusion Cage Study: Aphid Population Growth.

Aphid abundance at the end of the 2-wk exclusioncage study was not signiÞcantly different between the

treated and untreated caged plants (F� 0.9; df � 1, 14;P � 0.36; Fig. 4) but was signiÞcantly different be-tween the treated and untreated open plants (F� 6.1;df � 1, 14; P � 0.03; Fig. 4). The biocontrol serviceindex, a measure of the functional activity level of

Table 2. Average abundance of natural enemy taxa captured on sticky card traps placed in the corners of untreated and treated plots(1.5 from the methyl salicylate lure) over seven 4-d periods during the 2008 growing season

Taxa TreatmentAvg per-trapabundance

(mean SEM)

ANOVA results

Treatment Time Time treatment

C. septempunctata Treated 0.09 0.02 F � 0.1, P � 0.77 F � 5.2, P � 0.001*** F � 0.3, P � 0.94Untreated 0.08 0.02

H. axyridis Treated 0.41 0.08 F � 0.3, P � 0.60 F � 1.9, P � 0.09 F � 2.4, P � 0.04*Untreated 0.33 0.05

O. insidiosus Treated 1.05 0.24 F � 0.2, P � 0.66 F � 40.0, P � 0.001*** F � 0.2, P � 0.97Untreated 0.95 0.23

Braconidae Treated 0.37 0.06 F � 0.3, P � 0.6 F � 1.0, P � 0.27 F � 0.3, P � 0.93Untreated 0.41 0.06

Syrphidae Treated 4.91 0.46 F � 0.1, P � 0.73 F � 20.0, P � 0.001*** F � 0.8, P � 0.61Untreated 5.32 0.53

Chrysopidae Treated 0.40 0.07 F � 0.5, P � 0.51 F � 6.6, P � 0.001*** F � 0.9, P � 0.53Untreated 0.32 0.06

Hemerobiidae Treated 0.04 0.02 F � 0.003, P � 0.96 F � 2.7, P � 0.02* F � 1.6, P � 0.16Untreated 0.03 0.01

Treatment df � 1, 7; time df � 6, 84; and time treatment df � 6, 84.

Fig. 2. Average abundance, means SEM, of natural enemies sampled through whole plant sampling in methylsalicylate-treated and untreated plots over 12 sampling dates during the 2008 growing season. (A) Natural enemy immatures,including larvae of Coccinellidae, Chrysopidae, Hemerobiidae, and Syrphidae as well as nymphs of insidious ßower bug. (B)Natural enemy eggs, including those of Syrphidae, Chrysopidae, Hemerobiidae, and Coccinellidae.


natural enemies (Gardiner et al. 2009), calculated forplants paired with methyl salicylate lures (0.941 0.003), was marginally signiÞcantly greater than thatcalculated for untreated plants (0.869 0.008) (F �4.4; df � 1, 14;P� 0.06). Although few natural enemieswere found on the open plants when aphids werecounted, whole plant samples of natural enemiestaken during the 2-wk experiment revealed the pres-ence of two lady beetle species, Hippodamia conver-gens Guerin-Meneville, and Hippodamia variegata(Goeze), O. insidiosus, eggs of Syrphidae and Chry-sopidae, harvestmen (Arachnida: Opiliones), and spi-ders in the study Þeld.

Clip Cage Study: Aphid Life Table Parameters.Aphids reared in clip cages on treated and untreatedplants showed no signiÞcant differences in any lifetable parameters including aphid Þnite rate of popu-lation increase (�), intrinsic rate of population in-crease (r), net reproductive rate (Ro), and mean gen-eration time (T) (Table 3).

Discussion

Herbivore-induced plant volatiles have the poten-tial to inßuence crop pests directly through decreasedfeeding performance and changes in host plant pref-erence, or indirectly by affecting the third trophiclevel. Traps placed adjacent to methyl salicylate luresin treated soybean plots caught signiÞcantly morenatural enemies compared with traps in untreatedplots where no lures were deployed. Moreover, am-bient aphid populations were lower in methyl salicy-late treated plots and predation intensity, as measuredwith predator-excluders, was higher on methyl salic-ylate treated plants, suggesting that methyl salicylateinßuences pest abundance through effects on naturalenemies. However, this effect may be highly localizedbecause at a distance of 1.5 m from the lure, there wasno difference in natural enemy abundance betweentraps in treated and untreated plots. Although theresults illustrate the efÞcacy of methyl salicylate at asmall spatial scale, they also raise questions on scalingthis technology to the whole Þeld.Effect ofMethyl Salicylate on Natural Enemies andAphids.Syrphid ßies (Syrphidae) and green lacewings(Chrysopidae) were the only natural enemy taxa thatwere more abundant on traps adjacent to methyl sa-licylate lures compared with traps in untreated plots.This was consistent with previous studies that dem-onstrated the attraction of both of these groups tomethyl salicylate in grapes and hops (James 2003a,b;James and Price 2004). In contrast, we found no at-traction to methyl salicylate by the sevenspotted ladybeetle, a predator of the soybean aphid (Rutledge etal. 2004), found previously to be attracted to methylsalicylate in soybean (Zhu and Park 2005). However,populations of all lady beetles were unusually low inour treated and untreated plots during the 2008 grow-ing season (R.E.M., unpublished data), probably as aresult of the late establishment of soybean aphids(Cullen 2008). Sticky card traps adjacent to methylsalicylate lures showed greater natural enemy abun-dance most noticeably in the Þrst half of the growingseason, which suggests that methyl salicylate can at-tract natural enemies before herbivore outbreaks.Early attraction of natural enemies into an agroeco-system is believed to be an important attribute con-tributing to effective biological control of herbivores(Khan et al. 2008).

Increased abundance of natural enemies on traps inmethyl salicylate-treated plots was associated withlower soybean aphid abundance in treated plots. Therelative impact of natural enemies on aphid abun-dance, as measured by an exclusion cage study, sug-gests that one mechanism through which methyl sa-

Fig. 4. Final average aphid abundance, means SEM, oncaged and uncaged pots of soybean plants exposed to methylsalicylate or untreated. All pots began with 50 aphids andwere left in the Þeld for 14 d after which time aphid abun-dances were recorded. SigniÞcant differences are deÞned atthe P � 0.05 level.

Fig. 3. Average abundance, means SEM, of soybeanaphids per soybean plant sampled by whole-plant samplingin methyl salicylate-treated and untreated plots during the2008 growing season.


licylate affects aphid populations is via the attractionof natural enemies. In our study, the natural enemygroup displaying the strongest attraction to methylsalicylate was syrphid ßies, which as adults do not feedon aphids but rather on pollen and nectar (Sommaggio1999). Green lacewings also were attracted to methylsalicylate, but they have not been implicated as havingstrong effects on soybean aphid populations (Fox et al.2004, Rutledge et al. 2004, Costamagna and Landis2007, Costamagna et al. 2007) and also are predaceousprimarily in the larval stage (Principi and Canard 1984,Costamagna and Landis 2007). Moreover, we did notÞnd greater numbers of green lacewing or syrphidlarvae and eggs in treated plots. Thus, we remainuncertain as to which natural enemies are actuallyaffecting aphid populations. However, our samplingmethods may not have been adequate for capturingnatural enemy immatures and eggs, because they arerarely found on sticky traps and can be difÞcult toidentify or locate in whole-plant sampling. Other taxathat may have been present in our system but were notadequately captured using our collection technique(sticky cards), such as spiders or ground beetles(Carabidae), also may have signiÞcantly contributedto the predation of soybean aphids (Fox et al. 2005,Hajek et al. 2007, Birkhofer et al. 2008, Hannam et al.2008, Schmidt et al. 2008, Oelbermann and Scheu2009).

We can reject in part the hypothesis that methylsalicylate-induced plant responses affect aphid popu-lations directly. In the two studies where aphids werecaged on plants in proximity of methyl salicylate (ex-clusion cage study and clip cage experiment), therewas no difference in the Þnal number of aphids or inaphid life table parameters on treated versus un-treated plants, offering no support for methyl salicy-lateÕs role in reducing aphid fecundity. However, nei-ther of these studies could determine whether methylsalicylate reduces soybean aphid abundance by repel-ling colonizing aphids or by increasing emigration ofwinged aphids via changes in host plant preference.The lower numbers of aphids on open, treated plantsin the exclusion cage study as well as on plants withintreated plots could be the result of multiple mecha-nisms such as methyl salicylate increasing the emigra-tion of aphids and attracting natural enemies.Methyl Salicylate in Aphid Pest Management. The

efÞcacy of methyl salicylate in pest management isdependent on its ability to consistently and predict-ably attract natural enemies that can suppress pests atÞeld scales. The heterogeneous response of naturalenemies to herbivore-induced plant volatiles found

across multiple studies suggests that methyl salicy-lateÕs attractiveness could be modulated by environ-mental factors as well as by the release rate and de-ploymentmodeof thevolatile.Forexample,predatorymites were attracted to intermediate doses of methylsalicylate but showed no response to low doses andwere repelled by high doses (de Boer and Dicke2004b). In addition, the ability of an herbivore-in-duced plant volatile to attract a particular naturalenemy taxon can depend on the natural enemyÕs pre-vious exposure to volatiles (de Boer and Dicke 2004a).Volatile blend, in turn, depends not only on the spe-cies of plant but also the plant cultivar and plant age(Takabayashi and Dicke 1996) as well as factors suchas light intensity, temperature, soil moisture, and si-multaneous attack by other pests or pathogens (Car-doza et al. 2002, Gouinguene and Turlings 2002).Therefore, natural enemies of aphids may display dif-ferential attraction to synthetic plant volatiles basedon the species, cultivar, and plant age of their preyÕshost and on environmental factors affecting plant vol-atile blend. Results of in-Þeld studies are thus highlyspeciÞc to the agroecosystem and may not translate toother crop Þelds.

In addition, the range at which methyl salicylateattracts natural enemies will inßuence its role in pestmanagement and the ability to scale this managementapproach to grower Þelds. In this study, we wereunable to determine the spatial extent of methyl sa-licylateÕs attractiveness. Natural enemies found ingreater abundance on traps adjacent to the methylsalicylate lures may have originated from areas ofsoybean bordering the experimental plots or may havebeen attracted at longer distances from the off-crophabitat. Although we did not Þnd greater abundanceof natural enemies at traps 1.5 m from the methylsalicylate lure, this result does not conclusively ad-dress the issue of range. Natural enemies may still havebeen attracted at a long distance, orienting directly tothe methyl salicylate lures and resulting in no in-creased abundance at traps 1.5 m from the lure. Al-ternatively, our results could indicate that methyl sa-licylate has only a short-range attractiveness andoperates to redistribute natural enemies within thecrop Þeld, resulting in no overall increase in naturalenemy abundance or reduction in pest numbers. Formethyl salicylate to function at grower-relevant scalesby increasing natural enemy abundance throughoutthe agroecosystem, natural enemies must respond tothe volatile from off-crop habitats. The landscapecontext within which methyl salicylate is deployedcould then affect the volatileÕs role in pest manage-

Table 3. Life table parameters of soybean aphids raised in clip cages on methyl salicylate treated and untreated soybean plants

Treatment n � (95% CI) r (95% CI) Ro (95% CI) T (95% CI)

Untreated 14 1.293a (1.289, 1.297) 0.257a (0.254, 0.260) 12.929a (12.713, 13.262) 9.217a (9.111, 9.341)Treated 12 1.285a (1.282, 1.288) 0.251a (0.249, 0.253) 13.833a (13.567, 14.1) 10.003a (9.976, 10.033)

n, number of female soybean aphids in the treatment group; �, Þnite rate of population increase; r, intrinsic rate of population increase; Ro,

net reproductive rate; and T, mean generation time. The 95% CI were constructed using jackknife estimates of the parameters. Means withina column followed by the same letter are not signiÞcantly different (P � 0.05) as determined using Monte Carlo randomization tests.


ment by supporting variable numbers of natural en-emies. In this study, the organic Þelds used were bor-dered by impervious surface (roads) and other Þeldcrops, with virtually no Þeld margins containing non-crop vegetation. It is possible that methyl salicylatewould have had larger effects in an agroecosystemsituated in a more complex landscape, bordered bywoodlots and perennial vegetation that are known tosupport a higher abundance of natural enemies (Bi-anchi et al. 2006).

In summary, the Þndings of this study illustratemethyl salicylateÕs efÞcacy in attracting natural ene-mies to a lure source and show its potential in reducingherbivore populations at small spatial scales. Researchaddressing the most appropriate method and timing ofdeployment, as well as how surrounding landscapeand farm management decisions interact with methylsalicylate, will advance the potential for using thisvolatile lure in pest management. Ultimately, experi-ments performed at larger spatial scales, comparingtreated and untreated crop Þelds, are needed to testthe applicability of this technology for pest manage-ment.

Acknowledgments

We thank Katlyn Arnett, John Carlee, Dan Endow, DaleJacques, Emily Houtler, Pat Moriarty, Merritt Singleton,Sarika Sharma, Matt Stangl, Brent Wittig, and Flora Zeng forassistance in the Þeld and laboratory. We also thank CamilaBotero, Ting-Li Lin, and Seung Cheon Hong for help withstatistics.Methyl salicylate luresweredonatedbyAgBio.Thismanuscript was improved by comments from two anony-mous reviewers. This work was funded through Hatch grantWIS04956 (awarded to D.B.H. and C.G.).

References Cited

Birkhofer, K., E. Gavish-Regev, K. Endlweber, Y. D. Lubin,K. von Berg, D. H. Wise, and S. Scheu. 2008. Cursorialspiders retard initial aphid population growth at low den-sities in winter wheat. Bull. Entomol. Res. 98: 249Ð255.

Bianchi, F.J.J.A., C.J.H. Booij, and T. Tscharntke. 2006. Sus-tainable pest regulation in agricultural landscapes: a re-view on landscape composition, biodiversity and naturalpest control. Proc. R. Soc. B 273: 1715Ð1727.

Bruce, T.J.A., and J. A. Pickett. 2007. Plant defense signalinginduced by biotic attacks. Curr. Opin. Plant Biol. 10:387Ð392.

Cardoza, Y.,H.T.Alborn, and J.H. 2002. Tumlinson. In vivovolatile emissions from peanut plants induced by simul-taneous fungal infection and insect damage. J. Chem.Ecol. 28: 161Ð174.

Cardoza, Y., P.E.A. Teal, and J. H. Tumlinson. 2003. Effectof peanut plant fungal infection on oviposition prefer-ence by Spodoptera exigua and on host-searching behav-ior by Cotesia marginiventris. Environ. Entomol. 32: 970Ð976.

Cooper,W. C., L. Jia, and F. L. Goggin. 2004. Acquired andR-gene mediated resistance against the potato aphid intomato. J. Chem. Ecol. 30: 2527Ð2542.

Costamagna, A. C., andD.A. Landis. 2007. Quantifying pre-dation on soybean aphid through direct Þeld observa-tions. Biol. Control 42: 16Ð24.

Costamagna, A. C., D. A. Landis, and C. D. Difonzo. 2007.Suppression of soybean aphid by generalist predatorsresults in a trophic cascade in soybeans. Ecol. Appl. 17:441Ð451.

Cullen, E. 2008. Soybean aphid update for late August2008. Wisconsin Crop Manager. (http://ipcm.wisc.edu/WCMNews/tabid/53/EntryId/624/Soybean-Aphid-Update-for-Late-August-2008.aspx).

deBoer, J. G., andM.Dicke. 2004a. Experience with methylsalicylate affects behavioral responses of a predatory miteto blends of herbivore-induced plant volatiles. Entomol.Exp. Appl. 110: 181Ð189.

de Boer, J. G., and M. Dicke. 2004b. The role of methylsalicylate in prey searching behavior of the predatorymite Phytoseiulus persimilis. J. Chem. Ecol. 30: 255Ð271.

de Boer, J. G., and M. Dicke. 2005. Information use by thepredatory mite Phytoseiulus persimilis (Acari: Phytosei-idae), a specialized natural enemy of herbivorous spidermites. Appl. Entomol. Zool. 40: 1Ð12.

De Moraes, C. M., M. Mescher, and J. H. Tumlinson. 2001.Caterpillar-induced nocturnal plant volatiles repel con-speciÞc females. Nature 410: 577Ð580.

Dicke, M., M. W. Sabelis, J. Takabayashi, J. Bruin, and M. A.Posthumus. 1990. Plant strategies of manipulating pred-ator-prey interactions through allelochemicals Ð pros-pects for application in pest control. J. Chem. Ecol. 16:3091Ð3118.

Donaldson, J. R., S. W. Myers, and C. Gratton. 2007. Density-dependent responsesof soybeanaphid(AphidglycinesMat-sumura) populations to generalist predators in mid to lateseason soybean Þelds. Biol. Control 43: 111Ð118.

Drukker, B., J. Briun, and M. Sabelis. 2000. Anthocoridpredators learn to associate herbivore-induced plantvolatiles with presence or absence of prey. Physiol. En-tomol. 25: 260Ð265.

Farmer, E. E. 2001. Surface to air signals. Nature 411: 854Ð856.

Felton, G. W., and J. H. Tumlinson. 2008. Plant-insect dia-logs: complex interactions at the plant-insect interface.Curr. Opin. Plant Biol. 11: 457Ð463.

Fox,T.B.,D.A.Landis,F.F.Cardoso, andC.D.Difonzo. 2004.Predators suppress Aphis glycines Matsumura populationgrowth in soybean. Environ. Entomol. 33: 608Ð618.

Fox, T. B., D. A. Landis, F. F. Cardoso, and C. D. Difonzo.2005. Impact of predation on establishment of the soy-bean aphid, Aphis glycines, in soybean, Glycine max. Bio-control 50: 545Ð563.

Gardiner, M. M., D. A. Landis, C. Gratton, C. D. DiFonzo,M.O’Neal, J.M.Chacon,M. T.Wayo, N. P. Schmidt, E. E.Mueller, and G. E. Heimpel. 2009. Landscape diversityenhances biological control of an introduced crop pest inthe north-central USA. Ecol. Appl. 19: 143Ð154.

Glinwood, R. T., and J. Petterson. 2000. Change in responseof R. padi spring migrants to the repellent winter hostcomponent methyl salicylate. Entomol. Exp. Appl. 94:325Ð330.

Goggin, F. 2007. Plant-aphid interactions: molecular and eco-logical perspectives. Curr. Opin. Plant Biol. 10: 399Ð408.

Gouinguene, S. P., and T.C.J. Turlings. 2002. The effects ofabiotic factors on induced volatile emissions in cornplants. Plant Physiol. 129: 1296Ð1307.

Han, B. Y., C. S. Zhou, and L. Cui. 2005. Attraction ofwinged tea aphids to tea plant volatiles measured byolfactometry and electro-antennography. Trop. Agric. 82:138Ð142.

Hannam, J. J., J. K. Liebherr, and A. E. Hajek. 2008. Climb-ing behavior and aphid predation by Agonum muelleri(Coleoptera: Carabidae). Can. Entomol. 140: 203Ð207.


Hajek, A. E., J. J. Hannam, C. Neilsen, A. J. Bell, and J. K.Liebherr. 2007. Distribution and abundance of Carabi-dae (Coleoptera) associated with soybean aphid(Hemiptera: Aphididae) populations in central NewYork. Ann. Entomol. Soc. Am. 100: 876Ð886.

Heidel, A. J., and I. T. Baldwin. 2004. Microarray analysis ofsalicylic acid and jasmonic acid signaling in responses ofNicotiana attenuata to attack by insects from multiplefeeding guilds. Plant Cell Environ. 27: 1362Ð1373.

Hildebrand, D. F., G. C. Brown, D. M. Jackson, and T. R.Hamiltonkemp. 1993. Effects of some leaf-emitted vol-atile compounds on aphid population increase. J. Chem.Ecol. 19: 1875Ð1887.

Hood,G.M. 2003. PopTools version 2.6.4. (http://www.cse.csiro.au/poptools).

James, D. G. 2003a. Field evaluation of herbivore-inducedplant volatiles as attractants for beneÞcial insects; methylsalicylate and the green lacewing, Chrysopa nigricornis.J. Chem. Ecol. 29: 1601Ð1609.

James, D. G. 2003b. Synthetic herbivore-induced plantvolatiles as Þeld attractants for beneÞcial insects. Environ.Entomol. 32: 977Ð982.

James, D. G., and T. R. Grasswitz. 2005. Synthetic herbi-vore-induced plant volatiles increase Þeld captures ofparasitic wasps. Biocontrol 50: 871Ð880.

James, D. G., and T. S. Price. 2004. Field-testing of methylsalicylate for recruitment and retention of beneÞcial in-sects in grapes and hops. J. Chem. Ecol. 30: 1613Ð1628.

Kessler, A., and I. T. Baldwin. 2001. Function of herbivore-induced plant volatile emissions in nature. Science 291:2141Ð2144.

Kessler,A., and I.T.Baldwin. 2002. Plant responses to insectherbivory: the emerging molecular analysis. Annu. Rev.Plant Biol. 53: 299Ð328.

Khan, Z. R., D. G. James, C.A.O. Midega, and J. A. Pickett.2008. Chemical ecology and conservation biological con-trol. Biol. Control 45: 210Ð224.

Li, Y., J. Zou, M. Li, D. D. Bilgin, L. O. Vodkin, G. L.Hartman, and S. J. Clough. 2008. Soybean defense re-sponses to the soybean aphid. New Phytologist 179: 185Ð195.

Mye Myers, S. W., and C. Gratton. 2006. Inßuence of po-tassium fertility on soybean aphid, Aphis glycines Mat-sumura (Hemiptera: Aphididae), population dynamics ata Þeld and regional scale. Environ. Entomol. 35: 219Ð227.

Ninkovic, V., E. Ahmed, R. Glinwood, and J. Pettersson.2003. Effects of two types of semiochemical on popula-tion development of the bird cherry-oat aphid Rhopalo-siphum padi in a barley crop. Agric. For. Entomol. 5:27Ð33.

Oelbermann, K., and S. Scheu. 2009. Control of aphids onwheat by generalist predators: effects of predator densityand the presence of alternative prey. Entomol. Exp. Appl.132: 225Ð231.

Ode, P. J. 2006. Plant chemistry and natural enemy Þtness:effects on herbivore and natural enemy interactions.Annu. Rev. Entomol. 51: 163Ð185.

Park, S-W., E. Kaimoyo, D. Kumar, S. Mosher, and D. F.Klessig. 2007. Methyl salicylate is a critical mobile signalfor plant systemic acquired resistance. Science 318: 113Ð116.

Pettersson, J., J. A. Pickett, B. J. Pye, A. Quiroz, L. E. Smart,L. J. Wadhams, and C. M.Woodcock. 1994. Winter host

component reduces colonization by bird-cherry-oataphid, Rhopalosiphum padi (L.) (Homoptera, Aphidi-dae), and other aphids in cereal Þelds. J. Chem. Ecol. 20:2565Ð2574.

Principi, M. M., and M. Canard. 1984. Feeding habits, pp.76Ð92. In M. Canard, Y. Semeria, and T. R. New (eds.),Biology of Chrysopidae. Dr W Junk, The Hague, TheNetherlands.

Reymond, P., and E. F. Farmer. 1998. Jasmonate and salic-ylate as global signals for defense gene expression. Curr.Opin. Plant Biol. 1: 401Ð411.

Rutledge, C. E., R. J. O’Neil, T. B. Fox, and D. A. Landis.2004. Soybean aphid predators and their use in inte-grated pest management. Ann. Entomol. Soc. Am. 97:240Ð248.

Schmelz, E. A., J. E. Engleberth, H. T. Alborn, J. H. Tum-linson, III, and P.E.A. Teal. 2009. Phytohormone-basedactivity mapping of insect herbivore-produced elicitors.Proc. Natl. Acad. Sci. U.S.A. 106: 653Ð657.

Schmidt, N. P., M. E. O’Neal, and P. M. Dixon. 2008. Aphi-dophagous predators in Iowa soybean: a community com-parison across multiple years and sampling methods. Ann.Entomol. Soc. Am. 101: 341Ð350.

Shulaev, V., P. Silverman, and I. Raskin. 1997. Airbornesignaling by methyl salicylate in plant pathogen resis-tance. Nature 385: 718Ð721.

Sommaggio, D. 1999. Syrphidae: can they be used as envi-ronmental bioindicators? Agric. Ecosyst. Environ. 74:343Ð356.

Takabayashi, J., M.Dicke, andM. A. Posthumus. 1991. Vari-ation in composition of predator attracting allelochemi-cals emitted by herbivore-infested plants: relative inßu-ence of plant and herbivore. Chemoecology 2: 1Ð6.

Takabayashi, J., and M. Dicke. 1996. Plant-carnivore mutu-alism through herbivore-induced carnivore attractants.Trends Plant Sci. 1: 109Ð113.

Turlings, T.J.C., and J. Ton. 2006. Exploiting scents of dis-tress: the prospect of manipulating herbivore-inducedplant odours to enhance the control of agricultural pests.Curr. Opin. Plant Biol. 9: 421Ð427.

Turlings, T. C., J. H. Tumlinson, and W. J. Lewis. 1990.Exploitation of herbivore-induced plant odors by host-seeking parasitic wasps. Science 250: 1251Ð1253.

Vlot, A. C., D. F. Klessig, and S-W. Park. 2008. Systemicacquired resistance: the elusive signal(s). Curr. Opin.Plant Biol. 11: 436Ð442.

Webster, B., T. Bruce, S. Dufour, C. Birkemeyer, M. Birkett,J. Hardie, and J. Pickett. 2008. IdentiÞcation of volatilecompounds used in host location by the black bean aphid,Aphis fabae. J. Chem. Ecol. 34: 1153Ð1161.

Williams III, L., C.Rodriguez-Saona, S.C.Castle, and S. Zhu.2008. EAG-active herbivore-induced volatiles modifybehavioral responses and host attack by an egg parasitoid.J. Chem. Ecol. 34: 1190Ð1201.

Yu, H., Y. Zhong, K. Wu, X. W. Gao, and Y. Y. Guo. 2008.Field-testing of synthetic herbivore-induced plant vola-tiles as attractants for beneÞcial insect. Environ. Entomol.37: 1410Ð1415.

Zhu, J., and K.-C. Park. 2005. Methyl salicylate, a soybeanaphid induced plant volatile attractive to the predatorCoccinella septempunctata. J. Chem. Ecol. 31: 1733Ð1746.

Received 6 July 2010; accepted 8 September 2010.


Documents

Methyl Salicylate Attracts Natural Enemies and Reduces Populations of Soybean Aphids (Hemiptera: Aphididae) in Soybean Agroecosystems