Plant-mediated multitrophic interactions between aboveground and belowground insects

Transcription

1 202 entomologische erichten NEV Disserttieprijs 2008 Plnt-medited multitrophic interctions etween oveground nd elowground insects KEY WORDS Brssic nigr, Brssiccee, Deli rdicum, Dipter, Anthomyiide, Cotesi glomert, Brconide, ssocitionl resistnce, indirect defence, Pieris rssice, Pieride, Lepidopter Entomologische Berichten 69 (6): Roxin Soler Gmoren It is well documented tht plnts cn ct s verticl communiction chnnels etween elow- nd oveground plnt-feeding insects: rootfeeders induce chnges in iomss nd chemistry of the plnt s shoot, nd these chnges cn influence survivl, growth nd development of lef-feeders. The min im of my project ws to explore whether nd how this interction etween sptilly seprted insects is restricted to plnt feeders (root- nd lef-feeding insects), or cn e extended to higher trophic levels, such s insect prsitoids. My model system consisted of the wild cruciferous plnt Brssic nigr (first trophic level), elowground the specilist root herivore Deli rdicum (second level) nd its prsitoid Tryliogrph rpe (third level), nd oveground the specilist leffeeder Pieris rssice (second level) nd its prsitoid Cotesi glomert (third level) plus the hyperprsitoid Lysii nn (fourth level). The first experiments demonstrted tht root- nd lef-feeders negtively ffect ech other s performnces y incresing the levels of toxins in the host plnt, nd these negtive effects were not restricted to the herivore level, ut trnsmitted to the prsitoid nd even the hyperprsitoid. It ws remrkle tht the prsitoid ws the most ffected trophic level. I found out tht the prsitoid femle voids to prsitise hosts on root-infested plnts when hosts feeding on root-uninfested plnts were redily ville, proly sed on chnges in the voltile lend of the plnt, triggered y the root-feeders. It even ppered tht root-feeders cn influence the forging efficiency of prsitoids, even when the lef-feeders (i.e., the prsitoid s host) nd the root-feeders re not feeding on the sme plnt, ut on conspecific plnts within the sme hitt. Finlly, in field experiment the two most undnt lef-feeding insects preferred to feed nd oviposit on root-uninfested plnts. Hence, ehviourl decisions in nturlly occurring popultions of lef-feeding insects cn e influenced y root-feeders under field conditions, where multitude of interctions tke plce simultneously. Interction effects etween ove- nd elowground orgnisms clerly re not limited to insects directly ssocited with the plnt, ut such interctions operte in complex we of communiction mong vrious trophic levels. Introduction In terrestril ecosystems, indirect interctions occur when third species modifies the interction etween two species (Wooton 1994). Such indirect interctions cn e extremely importnt in strengthening or wekening the direct interctions within food wes (Schmitz et l. 2000). Indirect interctions cn lso connect species tht do not directly interct for exmple, plnts in the first trophic level nd crnivores in the third trophic level vi folir herivores in the second trophic level. In terrestril food wes, the study of indirect interctions hs trditionlly focused on orgnisms tht shre common domin: either elow or ove the soil surfce. Considering tht plnts develop oth underground nd oveground, they potentilly medite indirect interctions etween orgnisms tht elong to these sptilly seprted domins. In the lst two decdes it hs indeed een shown tht suterrnen orgnisms tht re intimtely ssocited with the roots of terrestril plnts cn influence the iomss nd levels of primry nd secondry compounds of oveground plnt prts (Bezemer & Vn Dm 2005). This in turn cn strongly influence the growth, development nd survivl of insects feeding on the shoot of the plnt (Gnge & Brown 1989,

2 entomologische erichten 203 NEV Disserttieprijs 2008 Tijdens de 20e Nederlndse Entomologendg (Ede, 19 decemer 2008) is voor de llereerste keer de NEV Disserttieprijs uitgereikt. Deze prijs estt uit een geldedrg plus een oorkonde en wordt toegekend voor het este proefschrift op het geied vn de entomologie, verdedigd n een Nederlndse universiteit in het voorgnde jr (1 septemer 31 ugustus). De NEV Disserttieprijs 2008 is uitgereikt n Dr. Roxin Soler Gmoren, voor hr proefschift Plnt-medited multitrophic interctions etween oveground nd elowground insects, op 30 oktoer 2007 verdedigd n de Wgeningen Universiteit. Het werk is grotendeels uitgevoerd n het Nederlnds Instituut voor Oecologisch Onderzoek, te Heteren. De jury ws vn oordeel dt dit proefschrift innovtief onderzoek vn een hoog niveu lt zien. Het werk is sterk entomologisch getint, mr heeft een reder ecologisch elng door de inedding in de community ecology theorie. De onderzoeksmethoden lopen uiteen vn minutieuze chemische nlyses tot n gedrgsoservties n een fors ntl soorten, in de context vn kleinschlige lortoriumexperimenten, mr ook ruimer opgezette experimentele veldproeven. De hoofdstukken vn het proefschrift vormen een duidelijk smenhngend geheel. During the 20th Annul Dutch Entomologists Meeting in Ede, Decemer 19, 2008, the very first Netherlnds Entomologicl Society (NEV) Disserttion Awrd ws presented. This price comprises n mount of money nd certificte of pprecition, nd is wrded for the est doctorl thesis in the field of entomology, defended t Dutch university in the preceding yer (1 Septemer 31 August). The NEV Disserttion Awrd 2008 is presented to Dr. Roxin Soler Gmoren, for her thesis Plnt-medited multitrophic interctions etween oveground nd elowground insects, defended t Wgeningen University, on Octoer 30, The work ws lrgely done t the Netherlnds Institute of Ecology, in Heteren. The jury decided tht this thesis demonstrtes innovtive reserch of high level. The work is clerly rooted in entomology, ut it hs much wider ecologicl interest ecuse of its emedding in current community ecology theory. Reserch methods stretch from minute chemicl nlyses to ehviourl oservtions of more thn few insect species, in the context of smll scle lortory experiments, ut lso of lrger scle experimentl field trils. The vrious chpters of the thesis contriute to cler nd coherent picture. Msters et l. 1993, 2001, Msters 1995, Wrdle 2002, Vn der Putten et l. 2001, Wurst & Jones 2003, Bezemer & Vn Dm 2005). Vice vers, oveground herivory hs een shown to ffect the development of elowground insect herivores, when the shred host plnt is previously exposed to insect ttck in the shoot (reviewed in Blossey & Hunt-Joshi 2003), therey creting plnt-sed functionl links etween soil-dwelling orgnisms nd oveground lef-feeding insects. The discovery tht soil orgnisms, whose presence psses hitully unnoticed, cn ffect the functioning of plntherivore communities oveground, more recently rised the question whether lso higher trophic levels such s crnivores could e influenced. Bsed on this chllenging ide, t the end of 2003 I strted my PhD project to study the possile consequences of the interctions etween root- nd lef-feeding insects on the performnce nd ehviour of higher trophic levels, nd the mechnisms tht could medite the interctions etween such distnt orgnisms. Figure 1 presents the model system used to ddress these questions.

3 204 entomologische erichten Lysii nn (Hymenopter: Ichneumonide) Tryliogrph rpe (Hymenopter: Figitide) Cotesi glomert (Hymenopter: Brconide) Deli rdicum (Dipter: Anthomyiide) Pieris rssice (Lepidopter: Pieride) Brssic nigr (Brssiccee) 1. Model system. The system consist of: the wild cruciferous plnt species Brssic nigr (Brssiccee) (first trophic level), the specilist root herivore species Deli rdicum (Dipter: Anthomyiide) (second trophic level, elowground), its nturl enemy, the prsitoid Tryliogrph rpe (Hymenopter: Figitide) (third trophic level, elowground), the specilist folir-feeding herivore species Pieris rssice (Lepidopter: Pieride) (second trophic level, oveground), its nturl enemy the prsitoid Cotesi glomert (Hymenopter: Brconide) (third trophic level, oveground), nd the secondry prsitoid (hyperprsitoid) Lysii nn (Hymenopter: Ichneumonide) (fourth trophic level, oveground). Photos: R. Soler, expect T. rpe: photogrpher unknown, P. rssice: Sonj V. Schper 1. Modelsysteem, estnde uit: de wilde kruisloemige zwrte mosterd, Brssic nigr (1e trofische niveu), de gespeciliseerde worteletende koolvlieg, Deli rdicum (2e trofische niveu), zijn ondergrondse ntuurlijke vijnd de prsitire wesp Tryliogrph rpe (3e trofische niveu), de gespeciliseerde ldeter Pieris rssice, het grote koolwitje (2e trofische niveu, ovengronds), zijn (ovengrondse) ntuurlijke vijnd de prsitire wesp Cotesi glomert (3e trofische niveu, ovengronds), en de hyper-prsitire wesp Lysii nn (4e trofische niveu, ovengronds). Herivores in one domin ffect the performnce of herivores nd crnivores in the opposite domin The first chpters of my thesis demonstrte tht insects in the soil cn strongly interct with herivores nd crnivores oveground, nd the other wy round: folir insects oveground cn influence the development of herivores nd crnivores in the soil. This discovery emphsises the importnce of considering not only orgnisms directly ssocited to the plnt roots nd shoot, ut lso higher trophic levels when studying interctions etween sptilly seprted orgnisms. We found tht root-feeding insects negtively ffected the performnce of the lef-feeding insect (figure 2), its prsitoid (figure 2,c) nd the hyperprsitoid (figure 2c). Shoot iomss, s well s the levels of primry plnt compounds of the shoot of root-infested nd root-uninfested plnts, did not differ. However, the levels of sinigrin, the min secondry plnt compound in the shoot, were more thn folded in root-infested plnts compred with the concentrtion recorded on root-uninfested plnts, suggesting tht the negtive impct of root-herivores on the performnce of the oveground insects is (t lest prtilly) medited y root-induced chnges in the levels of secondry plnt compounds of the shoot of the plnt. The density of root herivores influenced the responses on the oveground insects (figure 2). Overll, the effects of root herivory on the development of the oveground insects were stronger t medium root herivore density (5 Deli lrve/ plnt) thn t high density (20 lrve/plnt). Plnts exposed to 20 lrve proly experienced severe stress nd were therefore unle to mintin n optiml defence system ginst herivory.

4 entomologische erichten 205 lrvl development (dys) 19 lrvl development (dys) 19.5 pupl weight (mg) 3.7 pupl weight (mg) (control) medium high 0 (control) medium high 0 (control) medium high root herivore density c d 0 (control) medium high 2. Fitness correltes of the folir herivore (), its prsitoid ( nd c) nd the hyperprsitoid (d) on root-uninfested plnts (green rs), nd on plnts previously exposed for 13 dys to medium (violet r) nd high (red r) root herivore densities. 2. Fitnessmten (lrvle ontwikkelingsduur en popgewicht) vn de ldeter (), zijn prsitoïd ( en c) en de hyper-prsitire wesp (d) op plnten wrvn de wortels niet zijn ngevreten (control), dn wel op plnten wrvn de wortels gedurende twee weken zijn ngevreten door wortelkngers nwezig in een mtige (medium) of een hoge (high) dichtheid. survivl (%) root herivore prsitoid 3. Fitness correltes of the root herivore () nd its prsitoid () on shoot-uninfested plnts (violet rs) nd on plnts previously exposed to folir herivory y three first instr lrve during 3 dys (green rs). 3. Fitnessmt (overleving) vn de worteleter () en zijn prsitoïd () op plnten wrvn de spruit onngetst is (prs), dn wel gedurende drie dgen is ngevreten door drie jonge lrven (groen). More ttention should e pid to the effects of density of root herivores or the severity of dmge, when linking interctions etween sptilly seprted insects, since this my profoundly impct the outcome of such studies. In nturlly occurring popultions of Brssic nigr in The Netherlnds, s well s in Brssic olerce (Brussels sprouts) crop fields, out 5-9 Deli rdicum (Linneus) (Dipter: Anthomyiide) lrve re often found in infested plnts (R. Soler, pers. os., summer 2006 nd 2007; K. Winkler, pers. os.). Selecting nturlly occurring system, comined with field-knowledge of the dynmics of this system, is n essentil prerequisite for the design of controlled experiments, if we im to otin relistic nswers to questions on how ove- nd elowground orgnisms interct vi shred host plnts. Remrkly, in my studies the oveground prsitoid ws the trophic level most ffected y the presence of root herivores on the host plnt, even though the folir herivores were feeding directly on the ffected plnts. This shows tht chnges in host plnt qulity induced y root herivores ecome not only mnifest one step up in the food chin, ut cn even e mgnified t higher trophic levels. Bsed on these results lso considering tht femle prsitoids hve evolved complex mechnisms to find their hosts nd usully prefer to ttck those tht mximise the performnce of their off-spring (Godfry 1994) we then investigted whether such negtive effects of root herivores on the performnce of prsitoid progeny might hve consequences for host preference nd oviposition ehviour of forging prsitoids. The reverse effect lso tkes plce (figure 3). Lef-feeding insects cn negtively ffect the performnce of root-feeding insect nd its prsitoid. In this cse lso the interctions were medited vi induced chnges in the levels of secondry plnt compounds in the roots of the plnt. Previous studies hve shown tht severe folir dmge y insects cn strongly reduce the iomss of the plnt roots, negtively influencing the growth nd development of the elowground insects feeding on the limited root system (Msters et l. 1993, Tindll & Stout 2001, Blossey & Hunt-Joshi 2003). However, plnts my respond in different wys when suffering from removl of tissue y insect herivores, nd the effects of herivory on plnts cn exceed the simple loss of tissues. Dmge from folir herivores, for exmple, cn led to systemic increse in the expression of secondry plnt compounds in the roots (Collntes et l. 1999), ut most studies report no chnges in root secondry chemistry following oveground dmge (Bezemer & Vn Dm 2005). In our experiments, we exposed the plnt shoot to only three young cterpillrs for short period of time to ssure tht the physicl mount of folir tissue removed ws negligile, efore growing the soil food-chin elowground. Therefore, we cn conclude tht minor dmge cused y folir-feeding insects cn e sufficient to impct the performnce of soil insects, y inducing defence response in the plnt resulting in incresed levels of secondry plnt compounds in the roots. This suggests tht, even when the mount of dmge in the plnt shoot is negligile, plnt-medited effects of oveground herivory cnnot e ignored in soil ecology. In nture, virtully ll plnts experience t lest some level of folir dmge. This cn led to ltered performnce of soil dwelling orgnisms over severl trophic levels, which cn ultimtely result in chnges in the composition nd functioning of soil communities. Recently, soil ecologists hve ecome incresingly wre of this, ut the studies hve focused predominntly on differences in nutritionl qulity etween plnt species (Brdgett & Shine 1999, Wrdle 2002, De Deyn et l. 2004, Bezemer et l. 2006), nd not on the effects of chnges within single plnt species. There is n urgent need, therefore, for more studies tht investigte how chnges in root nutritionl qulity, induced y oveground herivory, ffect elowground multitrophic interctions.

5 206 entomologische erichten plnts with prsitized hosts (%) % prsitoids 4. Percentge of plnt-host complexes (plnts with 10 lrve of the prsitoid host), 2 hours fter relesing nïve femle prsitoids in lrge tent contining 13 root-uninfested (violet r) nd 13 root-infested (green r) plnts (). Percentge of choices of the prsitoid, in twochoice experiment etween host-infested nd host-uninfested plnts (ove) nd etween root-infested nd root-uninfested plnts (elow) (). 4. () Prsiteringssucces (% plnten met geprsiteerde gstheren), 2 uur n loslting vn nïeve vrouwelijke prsitire wespen in een tent met 13 plnten wrvn de wortels onngetst zijn (prs) en 13 plnten met ngevreten wortels (groen). () Verdeling vn prsitire wespen over plnten met en zonder gstheren nwezig (ovenste) en over plnten met wel of niet ngetste wortels (onderste). Consequences for forging prsitoids Optiml forging theory predicts tht crnivores prefer to ttck hosts nd prey species tht re most rewrding for them in terms of their fitness, mesured s numer nd qulity of their offspring (Stephens & Kres 1986). Similrly, within host species, prsitoid femles re expected to select the most profitle individuls (Godfry 1994, Brodeur et l. 1998). The results of the study summrized in figure 4 indicte tht, overll, the prsitoid exhiited cler preference for () prsitizing nd () serching for hosts on plnts tht hd not een ttcked y lrve of the root herivore. In the erlier study summrized in figure 2 we report tht the performnce, i.e. the development time nd pupl weight of the prsitoids, is negtively ffected y folir increses in phytotoxins induced y the root herivore shring the host plnt. In this study we find tht the prsitoid is le to recognize plnts sed on the presence or sence of root herivores, nd prefers to serch nd ly their offspring on hosts feeding on root-undmged plnts. Hence, we find cler preference-performnce pttern for the prsitoid. Theoreticl models predict tht oviposition decisions y prsitoid femles led to the selection of the most profitle host for their offspring (Vn Alphen & Visser 1990, Godfry 1994). These models hve een exclusively sed on oveground model systems, wheres our results suggest tht Cotesi glomert (Linneus) (Hymenopter: Brconide) femles re lso le to exploit rootinduced signls to evlute nd select the most suitle host for their off-spring. The underlying mechnism mediting the interctions etween the root herivores nd the oveground prsitoid points to chnges in the voltile lend of the plnt (figure 5). Overll, plnts exposed to lef herivory were chrcterised y high mount of et-frnesene nd dimethylnontriene, nd plnts exposed to root herivory were chrcterised y high mounts of dimethyl disulfide nd dimethyl trisulfide. Betfrnesene nd dimethylnontriene re voltile compounds reported to ct s ttrctnts for herivorous nd crnivorous insects (Fukushim et l. 2002, Anseo et l. 2005). By contrst, dimethyl di- nd trisulfide re reported to exert insecticidl neurotoxicity through mitochondril dysfunction (Dugrvot et l. 2003). Plnts exposed to oth root nd lef herivory hd voltile lends with higher levels of sulfides nd lower levels of ttrctnts compred with plnts exposed to only lef herivory y the host. Consequently, the voidnce y the prsitoid of the plnts exposed to root herivory my e prtly ttriuted to the higher mount of toxic voltiles comined with the lower production of ttrctnts present in the lend of the rootdmged host-infested plnts. Contrry to this result, Poved et l. (2005), using relted plnt species (wild mustrd, Sinpis rvensis) ut with different ove- nd elowground insects, showed tht phid prsitoids were ttrcted to plnts with phids tht were lso exposed to root-feeding insects over conspecific root-undmged plnts. The uthors did not investigte the effects of the soil dwelling orgnisms on the performnce of the prsitoid offspring or the possile mechnisms mediting these interctions, thus mking it difficult to compre their study with the results of my thesis. There re still reltively few studies including crnivores when exmining interctions etween sptilly seprted orgnisms (e.g., Gnge et l. 2001, Rsmnn & Turlings 2007), nd thus generl conclusions or generlistions cnnot e drwn. Assemling ll these summrized results suggests the possiility of ssocitionl resistnce etween the sptilly seprted herivores, where Pieris rssice Linneus (Lepidopter: Pieride) gins protection from its prsitoid C. glomert y ssocition with its (elowground) competitor D. rdicum. This phenomenon my hve importnt consequences for the popultion dynmics mong hosts nd their specilised nturl enemies. By protecting proportion of the host popultion from prsitism, the non-host competitor provides the host with refuge, vi the presence of enemy-free spce. For intimte hostprsitoid interctions, sptil refuges cn e stilising y preventing over-exploittion of the host popultion tht would otherwise led to locl extinction of the host nd ultimtely the prsitoid (Begon et l. 1996). Further studies re needed to find out whether P. rssice femles prefer to oviposit on rootdmged plnts, nd whether ssocitionl resistnce indeed tkes plce. Hitt-medited ove-elowground interctions Thus fr, interctions etween sptilly seprted orgnisms hve een reported in situtions where the root nd the lef orgnisms shre the sme plnt (e.g., Bezemer et l. 2003, Guerrieri et l. 2004). We investigted whether the interction etween the root-feeding insect nd the oveground prsitoid could still tke plce if the root-feeding insect nd the

6 entomologische erichten host infested plnts time (minutes) st 2nd 3rd 4th 5st -3 0% of plnts without root herivores in the environment 100% of plnts without root herivores in the environment Mhlonois distnces Two-dimensionl cnonicl discriminte nlysis (CDA) plot of the voltile lends of plnts exposed to root herivory (red circles) or lef herivory (green circles), no herivory (light green circles, control) or oth types of herivory (violet circles). Ech point represents the voltile lend of smpled plnt. Below, Mhlonois distnces etween the voltile lends re presented to quntify the reltive (dis)similrities etween the voltile lends of the four plnt tretments, which mesures the distnce etween tretment-cncroids in multivrite spce. 5. Digrm wrin de gelijkenis stt weergegeven tussen de mengsels vn vluchtige stoffen fkomstig vn plnten met ngevreten wortels (rode cirkels) of spruit (groen), onngetste plnten (lichtgroen; controle), of met ngetst wortels en spruit (prs). Elke cirkel geeft het mengsel weer vn een enkele plnt. De getllen in de tel geven een ndere mt weer voor de gelijkenis: hoe groter het getl, des te groter het verschil tussen twee vergeleken mengsels. 6. Time spent y the femle prsitoids to find five host-plnt complexes in lrge tent, within surrounding environment consisting on 25 root-infested plnts (violet rs, right scheme elow), or within surrounding environment consisting on 25 root-uninfested plnts (green rs, left scheme elow). 6. Invloed vn de omgeving: tijd (in minuten) die vrouwelijke prsitire wespen nodig heen om vijf plnten met gstheren te ezoeken in een grote tent, wnneer de plnten omgeven zijn door 25 plnten wrvn de wortels zijn ngevreten (prs; ruimtelijke situtie zols rechtsonder), dn wel door 25 plnten met onngetste wortels (groen; linksonder). leffeeding insect (i.e., the prsitoid host) were feeding on neighouring conspecific plnts. In n experiment conducted in lrge tent, the proportion of neighouring plnts tht were root-uninfested nd root-infested ws mnipulted, nd the time tht prsitoids spent to locte five dditionl hostinfested plnts within the vrious surrounding environments ws recorded. It ppers tht root herivores cn indeed influence oveground host-prsitoid interctions vi chnges in the ttrctiveness of surrounding conspecific plnts (figure 6). Complexes of herivore species on single plnt cn diminish the serching efficiency of forging prsitoids, either y mking the voltile lend of host-infested plnts less relile for serching prsitoids or y cusing prsitoids to wste time serching on non-host lrve (McCnn et l. 1998, Vos et l. 2001). We found tht the presence of non-host (root) herivores on surrounding plnts enhnced the serching efficiency of the prsitoid, strengthening the host-prsitoid interction. Becuse B. nigr plnts exposed to root herivory emit voltile lend chrcterised y high levels of specific sulphur voltile compounds, nd ecuse these voltile cues seem to e used y the prsitoid femles to identify nd void plnts with root herivory (figures 4 nd 5), we propose tht the chrcteristic voltile lend emitted y plnts exposed to root herivory tht surrounded the host-infested plnts my hve provided more contrsting ckground thn the lend emitted y nery plnts without root herivory, thus enefiting prsitoids tht re serching for hosts. In hitt with plnts of the sme species, the differences in voltile lends etween plnts with nd without hosts my not e clerly detectle t lrger distnces for serching prsitoids. Serching femle prsitoids my therefore wste time on or ner plnts without hosts. In our study system plnts without hosts, ut exposed to root herivory, repel serching prsitoids, nd we suggest tht in our experiment this hs improved their ility to locte host plnts y wsting less time on hitt plnts without hosts. Here we provide evidence tht femle prsitoids cn exploit qulittive chrcteristics of the surrounding environment, triggered y elowground insects, to mximise their serching efficiency. These results suggest tht sptilly seprted orgnisms cn interct in much more complex nd indirect wys thn hs een proposed so fr.

7 208 entomologische erichten Aoveground-elowground root nd folir insect interctions in the field Severl studies hve shown tht ove- nd elowground insects cn interct y influencing ech others growth, development nd survivl when they re forced to feed on the sme host-plnt. In nturl systems, however, insects cn choose on which plnts to oviposit nd feed. We crried out field experiment to determine whether root-feeding insects cn influence feeding nd oviposition decisions of nturlly colonising leffeeding insects, plcing in the field lrge groups of root-infested nd root-uninfested B. nigr plnts, nd monitoring the nturlly colonising popultions of lef-feeding insects over the course of summer seson. We found tht two of the most undnt lef-feeding insects on B. nigr, the fle eetle Phyllotret spp. nd the phid Brevicoryne rssice (Linneus) (Hemipter: Aphidide), significntly preferred to feed nd oviposit on the shoots of root-uninfested plnts rther thn on the shoots of root-infested plnts. Bering in mind tht the levels of folir phytotoxins (glucosinoltes) of root-infested B. nigr plnts y D. rdicum re more thn twice s high s compred with root-uninfested plnts, nd tht the specilised folir herivore P. rssice develop suoptimlly on these plnts (figure 2), root-infested B. nigr plnts my represent suoptiml nutritionl source for other species of folir herivores s well. Considering tht Deli spp. commonly ttck rssicceous plnts, folir herivores of these plnts my e under strong selection pressure to void ovipositing or feeding on plnts with Deli spp., prticulrly if this is correlted with significnt reduction in their fitness, nd rootuninfested plnts re redily ville. The oservtion tht oveground herivores void plnts tht experience root herivory implies the interesting possiility tht the voidnce ehviour of C. glomert prsitoids to root herivore dmged plnts is not necessrily relted to fitness consequences for their offspring, ut ecuse, under nturl conditions, hosts re less likely to e found on such plnts. Generl conclusion nd future directions An incresing numer of studies pulished over the pst 15 yers hs clerly shown tht the soil system should not e seen s lck ox, ecuse the orgnisms tht live there, such s decomposers, ntgonistic root feeders, nd mutulistists, cn influence oveground plnt iomss nd qulity which, in turn, ffect the performnce of insects feeding oveground (Vn der Putten et l. 2001, Wrdle 2002, Wrdle et l. 2004, Bezemer & Vn Dm 2005, Kpln et l. 2008). The results of my thesis contriuted to show, together with other contemporry studies (e.g., Omncini et l. 2001, Guerrieri et l. 2004, Bezemer et l. 2005), tht the indirect interctions etween sptilly seprted orgnisms re not restricted to the orgnisms directly shring the host plnt. Rther, herivory cn significntly influence the performnce of higher trophic level orgnisms in the opposite comprtment. Therefore, the interctive effects etween ovend elowground orgnisms cnnot e limited to orgnisms directly ssocited to the plnt ecuse they occur throughout complex chin of multitrophic orgnisms. Ecologicl studies tht clim tht etter understnding of terrestril ecosystems requires n oveground-elowground pproch ecuse of the strong feedck etween root nd folir orgnisms directly ssocited with the plnt should emphsise the need to incorporte higher trophic levels. The results of this thesis lso show tht hidden root-feeding insects cn influence the ehviour nd serching efficiency of oveground folir herivores nd prsitoids. These results dd to the empiricl dt generted from numer of studies (Omcini 2001, Msters et l. 2001, Gnge et l. 2003, Wurst & Jones 2003, Guerrieri et l. 2004, Bezemer et l. 2005) nd provide evidence tht soil orgnisms cn e mjor component in the interply of forces tht shpe community structure. Soil orgnisms, however, re still not included in minstrem ecologicl theory. It is time now to develop theoreticl frmework tht more rodly integrtes oveground-elowground orgnism interctions into the field of ecology. This is, in my opinion, one of the most chllenging issues in this field. It remins lrgely unstudied whether innte responses of oveground insect prsitoids to ntgonistic root-feeding insects chnge over time, nd whether nd how plnt-odour lerning in prsitoids is influenced y interctions tht occur in the rhizosphere. I think tht this is novel re of study. Figure 6 summrizes tht root-feeding insects influence the forging efficiency of prsitoids of folir herivores vi the shred hitt. From fundmentl reserch perspective, these interctions nd the mechnisms tht medite them re nother exciting re for future studies. From n pplied perspective, elucidting the fctors tht influence preference nd forging ehviour of insect prsitoids is importnt for their ppliction in iologicl control progrms, since insect prsitoids cn effectively control pestpopultions in griculturl systems. So fr, reserch on refining nd improving modern Integrted Pest Mngement (IPM) progrmmes hs focused on experiments in which ove- nd elowground systems hve een studied independently. The results of this thesis cnnot e directly pplied to improve iologicl control strtegies, ut suggest tht to look into the soil might contriute to optimise io-control progrms. Wht hppen in the soil my lter the interctions etween the plnt shoot, the herivore pests nd their nturl enemies. Similrly, elucidting the fctors tht influence plnt preference y leffeeding insects is extremely importnt to improve push-pull strtegies in IPM progrmmes in cropping systems. To dte, IPM progrmmes hve een sed on reserch focused on independent ove- nd elowground systems ignoring the fct tht soil dwelling nd oveground insects my strongly interct. The reserch crried out in this thesis imed t disentngling the feed-cks tht occur etween elow- nd oveground insects over severl trophic levels. It is likely tht horizontl interctions, such s those with other species within the sme feeding guild nd/or trophic level, will lso ply significnt role in shping oveground-elowground interctions. This hs not een ddressed in this thesis. The literture is still not very undnt in this re, since most pulished studies hve focused on the effects of single species or functionl groups of soil orgnisms (ut see Wurst et l. 2004, 2006), on single species of oveground insects. Considering tht soil orgnisms re likely to interct with ech other in mny nd complex wys, nd lso considering tht it hs een recently shown tht oveground plnt-insect interctions cn drmticlly chnge when second species enters the scene (Rodriguez-Son et l. 2003, Beckers & Spoel 2006, Zheng et l. 2007), inter-species interctions per trophic level will likely ffect the outcome of the interctions etween ove- nd elowground food wes. Acknowledgements Promotors: prof. Dr. W.H. vn der Putten, prof. Dr. L.E.M. Vet; Co-promotors: Dr. J.A. Hrvey, Dr. T.M. Bezemer, ll t the Netherlnds Institute of Ecology, NIOO-KNAW, Deprtment of Multitrophic Interctions, in Heteren, The Netherlnds, where I ws sttioned during my PhD project.

8 entomologische erichten 209 Literture Anseo L, Ignell R, Lofqvist J & Hnsson BS Responses to sex pheromone nd plnt odours y olfctory receptor neurons housed in sensill uricillic of the codling moth, Cydi pomonell (Lepidopter: Tortricide). Journl of Insect Physiology 51: Brdgett RD & Shine A Linkges etween plnt litter diversity, soil microil iomss nd ecosystem function in temperte grsslnds. Soil Biology nd Biochemistry 31: Beckers GJM & Spoel SH Fine-tuning plnt defence signlling: Slicylte versus jsmonte. Plnt Biology 8: Begon M, Hrper JL & Townsend CR (1996) Ecology: individuls, popultions nd communities. Blckwell Science, Oxford. Bezemer TM, Wgenr R, Vn Dm NM & Wäckers FL Interctions etween ove- nd elowground insect herivores s medited y the plnt defense system. Oikos 101: Bezemer TM & Vn Dm NM Linking oveground nd elowground interctions vi induced plnt defenses. Trends in Ecology nd Evolution 20: Blossey B & Hunt-Joshi TR Belowground herivory y insects: influence on plnts nd oveground herivores. Annul Review of Entomology 48: Brodeur J, Geervliet JBF & Vet LEM Effects of Pieris host species on life history prmeters in solitry specilist nd gregrious generlist prsitoid (Cotesi species). Entomologi Experimentlis et Applict 86: Collntes HG, Ginoli E & Niemeyer HM Defolition ffects chemicl defenses in ll plnt prts of rye seedlings. Journl of Chemicl Ecology 25: De Deyn GB, Rijmkers CE, Vn Ruijven J, Berendse F & Vn der Putten WH Plnt species identity nd diversity effects on different trophic levels of nemtodes in the soil food we. Oikos 106: Dugrvot E, Grolleu F, Mcherel D, Rocheting A, Hue B, Stnkiewicz M, Huignrd J & Lpied B Dimethyl disulfide exerts insecticidl neurotoxicity through mitochondril dysfunction nd ctivtion of insect K-ATP chnnels. Journl of Neurophysiology 1: Fukushim J, Kinoh Y, Hond H & Tkyshi J Lerning of herivoreinduced nd nonspecific plnt voltiles y prsitoid, Cotesi kriyi. Journl of Chemicl Ecology 28: Gnge AC & Brown VK Effects of root herivory y n insect on folir-feeding species, medited through chnges in the host plnt. Oecologi 81: Gnge AC, Brown VK & Aplin DM Multitrophic links etween rusculr mycorrhizl fungi nd insect prsitoids. Ecology Letters 6: Godfry HCJ Prsitoids. Behviorl nd Evolutionry Ecology. Princeton University Press, New Jersey. Guerrieri E, Lingu G, Digilio MC, Mss N & Bert G Do interctions etween plnt roots nd the rhizosphere ffect prsitoid ehviour? Ecologicl Entomology 29: Kpln I, Ryko H, Kessler A, Srdnelli S & Denno R Constitutive nd induced defenses to herivory in ove- nd elowground plnt tissues. Ecology 89: Msters GJ, Brown VK & Gnge AC (1993) Plnt medited interctions etween oveground nd elowground insect herivores. Oikos 66: Msters GJ The effect of herivore density on host-plnt medited interctions etween two insects. Ecologicl Reserch 10: Msters GJ, Jones TH & Rogers M Hostplnt medited effects of root herivory on insect seed predtors nd their prsitoids. Oecologi 127: McCnn K Hstings A & Huxel GR Wek trophic interctions nd the lnce of nture. Nture 395: Omcini M, Chneton EJ, Ghers CM & Muller CB Symiotic fungl endophytes control insect host-prsite interction wes. Nture 409: Poved K, Steffn-Dewenter I, Scheu S & Tschrntke T Effects of decomposers nd herivores on plnt performnce nd oveground plnt-insect interctions. Oikos 108: Rsmnn S & Turlings TCJ Simultneous feeding y oveground nd elowground herivores ttenutes plnt-medited ttrction of their respective nturl enemies. Ecology Letters 10: Rodriguez-Son C, Chlmers JA, Rj S & Thler JS Induced plnt responses to multiple dmgers: differentil effects on n herivore nd its prsitoid. Oecologi 143: Schmitz OJ, Hmck PA & Beckermn AP Trophic cscdes in terrestril systems: A review of the effects of crnivore removls on plnts. Americn Nturlist 155: Soler R, Bezemer TM, Vn der Putten WH, Vet LEM & Hrvey JA 2005 Root herivore effects on ove-ground herivore, prsitoid nd hyperprsitoid performnce vi chnges in plnt qulity. Journl of Animl Ecology 74: Soler R, Hrvey JA, Kmp AFD, Vet LEM, Vn der Putten WH, Vn Dm NM, Stuefer JF, Gols R, Hordijk CA & Bezemer TM 2007 Root herivores influence the ehviour of n oveground prsitoid through chnges in plnt-voltile signls. Oikos 116: Soler R, Hrvey JA & Bezemer TM 2007c Forging efficiency of prsitoid of lef herivore is influenced y root herivory on neighouring plnts. Functionl Ecology 21: Soler R, Hrvey JF, Bezemer TM & Stuefer JF Plnts s Green Phones. Novel insights into plnt-medited communiction etween elow- nd oveground insects. Plnt Signling & Behvior 3: Stephens DW & Kres JR 1986 Forging Theory. Princeton University Press, Princeton, New Jersey. Tindll KV & Stout MJ 2001 Plnt-medited interctions etween the rice wter weevil nd fll rmyworm in rice. Entomologi Experimentlis et Applict 101: Vos M, Berrocl SM, Krmoun F, Hemerik L & Vet LEM Plnt-medited indirect effects nd the persistence of prsitoidherivore communities. Ecology Letters 4: Vn Alphen JJM & Visser ME Superprsitism s n dptive strtegy for insect prsitoids. Annul Review of Entomology 35: Vn der Putten WH, Vet LEM, Hrvey JA.& Wäckers FL Linking ove- nd elowground multitrophic interctions of plnts, herivores, pthogens, nd their ntgonists. Trends in Ecology nd Evolution 16: Wrdle DA Communities nd Ecosystems. Linking the Aoveground nd Belowground Components. Princeton University Press, New Jersey. Wrdle DA, Yetes GW, Willimson WM, Bonner KI & Brker GM Linking oveground nd elowground communities: the indirect influence of phid species identity nd diversity on three trophic level soil food we. Oikos 107: Wootton JT The nture nd consequences of indirect effects in ecologicl communities. Annul Review of Ecology nd Systemtics 25: Wurst S & Jones TH Indirect effects of erthworms (Aporrectode cliginos) on n ove-ground tritrophic interction. Pedoiologi 47: Wurst S, Dugss-Goen D, Lngel R, Bonkowski M & Scheu S Comined effects of erthworms nd vesiculrrusculr mycorrhizs on plnt nd phid performnce. New Phytologist 163: Wurst S, Lngel R, Rodger S & Scheu S Effects of elowground iot on primry nd secondry metolites in Brssic olerce. Chemoecology 16: Zheng SJ, Vn Dijk JP, Bruinsm M & Dicke M Sensitivity nd speed of induced defense of cge (Brssic olerce L.): Dynmics of BoLOX expression ptterns during insect nd pthogen ttck. Moleculr Plnt-Microe Interctions 20: Received: 21 Ferury 2009 Accepted: 16 Octoer 2009

9 210 entomologische erichten Smenvtting Plnten ls spin in een we vn intercties tussen oven- en ondergrondse insecten Het is ekend dt plnten ls vertikle communictieknelen kunnen fungeren tussen onder- en ovengrondse plntenetende insecten: worteleters veroorzken vernderingen in de iomss en de chemische smenstelling vn de ovengrondse delen vn een plnt, en deze vernderingen kunnen de overleving, de groei en de ontwikkeling vn ldeters eïnvloeden. Het elngrijkste doel vn mijn promotieproject ws te onderzoeken of en hoe de wisselwerking tussen ruimtelijk gescheiden insecten eperkt is tot plnteneters (wortel- en spruiteters), dn wel uitgereid kn worden nr hogere trofische niveus, zols ijvooreeld prsitire wespen, de ntuurlijke vijnden vn de plntenetende insecten. Mijn modelsysteem kende de volgende soorten: zwrte mosterd, Brssic nigr, een in het wild voorkomende kruisloemige (1e trofische niveu), ondergronds de gespeciliseerde worteletende koolvlieg, Deli rdicum (2e niveu) en de prsitire wesp Tryliogrph rpe (3e niveu), en ovengronds het grote koolwitje, Pieris rssice, een gespeciliseerde ldeter (2e niveu), de prsitire wesp Cotesi glomert (3e niveu) en de hyper-prsitire wesp Lysii nn (4e niveu). De eerste experimenten lieten zien dt de wortel- en spruitetende insecten elkr negtief eïnvloeden vi de toenme n fweerstoffen in de plnt. Deze effecten leven niet eperkt tot het niveu vn de plnteneters, mr werkten door op het niveu vn de prsitoiden en zelfs op dt vn de hyper-prsitoide. Het ws opmerkelijk dt de grootste effecten werden gevonden op de prsitire wespen. Om een vooreeld te geven: vrouwelijke wespen leken gstheren op de spruit vn plnten wrvn de wortels wren ngetst te vermijden, wnneer e rook gstheren eschikr wren op plnten wrvn de wortels niet wren ngevreten. Wrschijnlijk stellen door de wortelvrt geinduceerde vernderingen in het geurmengsel vn de plnt de wespen hiertoe in stt. Worteleters leken zelfs de foergeereslissingen vn wespen te kunnen eïnvloeden in het gevl de ovengrondse gstheren nwezig wren op onngetste plnten in de omgeving. Tenslotte werd in een veldexperiment gevonden dt de twee meest voorkomende ldetende insecten ij voorkeur ten vn en hun eieren legden op plnten wrvn de wortels niet wren ngevreten. Kortom: ook in ntuurlijke situties, wrin op elk moment tlloze intercties dooreenlopen, kn het gedrg vn spruiteters worden einvloed door worteleters. De wisselwerking tussen onder- en ovengrondse insecten lijft duidelijk niet eperkt tot de directe elgers vn de plnt. Roxin Soler Gmoren Netherlnds Institute of Ecology, NIOO-KNAW P.O. Box ZG Heteren Current ddress: Lortory of Entomology Wgeningen University P.O. Box EH Wgeningen roxin.soler@wur.nl