Phytochemistry 72 (2011) 1612–1623

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Phytochemistry

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Review

Plants and insect eggs: How do they affect each other?

Monika Hilker ⇑, Torsten MeinersFreie Universität Berlin, Institute of Biology, Applied Zoology/Animal Ecology, Haderslebener Str. 9, D-12163 Berlin, Germany

a r t i c l e i n f o a b s t r a c t

Article history:Available online 23 March 2011

Keywords:Plant volatilesPlant surfacePlant boundary layerPlant defenceOviposition-induced defenceHerbivorous insectInsect ovipositionElicitorOviposition-induced transcription

0031-9422/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.phytochem.2011.02.018

Abbreviations: BA, benzoic acid; CHC, cuticular hydresponse; IG, iridoid glycoside; JA, jasmonic acid; Psalicylic acid; TPS, terpene synthase.⇑ Corresponding author. Tel.: +49 30 8385 5913; fa

E-mail address: hilker@zedat.fu-berlin.de (M. Hilk

Plant–insect interactions are not just influenced by interactions between plants and the actively feedingstages, but also by the close relationships between plants and insect eggs. Here, we review both effectsof plants on insect eggs and, vice versa, effects of eggs on plants. We consider the influence of plants onthe production of insect eggs and address the role of phytochemicals for the biosynthesis and release ofinsect sex pheromones, as well as for insect fecundity. Effects of plants on insect oviposition by contactand olfactory plant cues are summarised. In addition, we consider how the leaf boundary layer influencesboth insect egg deposition behaviour and development of the embryo inside the egg. The effects of eggs onplants involve egg-induced changes of photosynthetic activity and of the plant’s secondary metabolism.Except for gall-inducing insects, egg-induced changes of phytochemistry were so far found to be detrimen-tal to the eggs. Egg deposition can induce hypersensitive-like plant response, formation of neoplasms orproduction of ovicidal plant substances; these plant responses directly harm the eggs. In addition, eggdeposition can induce a change of the plant’s odour and leaf surface chemistry which serve indirect plantdefence with the help of antagonists of the insect eggs. These egg-induced changes lead to attraction of eggparasitoids and their arrestance on a leaf, respectively. Finally, we summarise knowledge of the elicitors ofegg-induced plant changes and address egg-induced effects on the plant’s transcriptional pattern.

� 2011 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16122. Plant effects on insect eggs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1613

2.1. Plant chemistry affects insect mating and production of eggs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16132.2. Plant chemical cues affect oviposition behaviour of herbivorous insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16142.3. Plant chemistry influences quality of insect eggs and embryo development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1615

3. Egg effects on plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1615

3.1. Insect egg deposition affects the plant’s primary metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16153.2. Insect egg deposition changes the plant’s secondary metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1617

3.2.1. Egg deposition affects plant volatile emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16173.2.2. Egg deposition affects plant surface chemistry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16183.2.3. Egg deposition affects biosynthesis of non-volatile plant internal secondary metabolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1619

4. Interacting factors of plants and insect eggs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1619

4.1. Egg-associated secretions elicit plant responses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16194.2. Phytohormones and plant defence genes respond to eggs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1620

5. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1620Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1621References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1621

ll rights reserved.

rocarbon; HR, hypersensitiveA, pyrrolizidine alkaloid; SA,

x: +49 30 8385 3897.er).

1. Introduction

Plants serve as oviposition sites for most herbivorous insectswhich deposit their eggs on almost all parts of the plants. Forexample, the sunflower stem weevil Cylindrocopturus adspersus

M. Hilker, T. Meiners / Phytochemistry 72 (2011) 1612–1623 1613

(LeConte) or the Viburnum L. leaf beetle Pyrrhalta luteola (Müller)deposit eggs on stems; nitidulid pollen beetles and many fructivo-rous insects use the reproductive parts, the flowers and fruits, asoviposition sites. The majority of herbivorous insect species includ-ing herbivorous hemipteran, lepidopteran, coleopteran and hyme-nopteran species oviposit on leaves. Only few herbivorous insectslay their eggs in the soil in the vicinity of plants, as e.g. manyOrthoptera or insects with root-feeding larvae.

Egg deposition by herbivorous insects is the start of the estab-lishment of a new herbivore generation. From the plant’s perspec-tive, strategies are beneficial which prevent herbivores from eggdeposition on the plant, which reduce the number of eggs laid orwhich impair the development of the embryo inside the eggs. Fromthe herbivore’s perspective, fitness is maximised when behaviouralresponses to plant cues lead to oviposition on plants where prog-eny will find suitable food.

In this review, we consider plant effects on egg-laying herbivo-rous insects and their eggs (Section 2). Even though herbivorousinsects are well adapted to plant cues and can efficiently exploitthem for successful reproduction, plants are no defenceless victimsof egg-laying insects. They may deter egg deposition and influencethe development of the herbivore’s embryo. We address in this re-view how plants change their primary and secondary metabolismin response to insect eggs (Section 3). The egg-induced changes ofplant secondary metabolites, especially of plant volatile terpe-noids, have been shown to serve a protective role against herbivoreattack. Furthermore, we review the knowledge of the mechanismsof plant–egg interactions (Section 4).

Fig. 1. Effects of phytochemicals on insect mating and reproduction, ovipositio

2. Plant effects on insect eggs

The various effects of plants on insect eggs are outlined in Fig. 1.Production of fertile eggs requires successful mating in non-par-thenogenously reproducing herbivorous insects. Biosynthesis andrelease of mating signals as well as production of eggs may beinfluenced or even determined by plant chemicals. Odour of plants,the plant surface and the plant’s ‘‘interior’’ (Städler, 2002) guideegg-laying herbivorous insect females to their host plants andinfluence the choice of oviposition sites. The eggs may benefit fromplant toxins sequestered by their parents from food, as the toxinsmay deter egg predators and parasitoids (reviewed by Blum andHilker, 2002). Since eggs laid on a leaf are enclosed by the leaf’sboundary layer (see Section 2.3), the concentration of leaf volatilesand atmospheric gases in this layer may specifically affect embry-onic development. Below we will address these aspects of plant ef-fects on insect eggs.

2.1. Plant chemistry affects insect mating and production of eggs

In many insect species, plant secondary metabolites provideessential direct precursors for the biosynthesis of sex pheromoneswhich mediate successful mating. One of the most famous exam-ples is the sequestration of plant pyrrolizidine alkaloids (PAs) bylarvae of arctiid moths; the plant alkaloids are maintained throughall arctiid life stages as non-toxic N-oxides. Males of several arctiidspecies can convert the plant PAs to hydroxydanaidal and release itas courtship pheromone from their coremata (Hartmann et al.,

n, egg development and protection of eggs. For details, see text Section 2.

1614 M. Hilker, T. Meiners / Phytochemistry 72 (2011) 1612–1623

2004). Besides lepidopteran species, other well known examples ofuse of plant secondary metabolites for herbivore pheromone bio-synthesis are tephritid flies (Bactrocera spp. Macquart) whichsequester methyl eugenol from their host plants. Eugenol deriva-tives are accumulated in the rectal glands of adult males (Shellyand Nishida, 2004) and serve as male sex pheromones (Hee andTan, 2004, 2006). Not only plant secondary metabolites, but alsothe fatty acid pattern of a plant can determine the blend of insectcuticular hydrocarbons (CHCs) that may serve as mate recognitioncues. For example, the CHC profile of the mustard leaf beetle, Phae-don cochleariae (F.) is significantly influenced by the host plant spe-cies the beetle is feeding upon (Geiselhardt et al., 2009). Male P.cochleariae of two laboratory lines that were fed with different hostplants (Chinese cabbage, watercress) preferred to mate ‘‘same hostplant females’’ to ‘‘different host plant females’’ and also attemptedto mate more often glass dummies treated with CHC extract of fe-males of the preferred line (Geiselhardt, Otte, Hilker, unpublisheddata). Hence, plant chemistry may influence success of mating ofherbivorous insects, and thus, production of their progeny.

Some herbivorous insects are known to adjust their productionand release of sex pheromones to the presence of plant volatileswhich indicate suitable food for the progeny (Landolt and Phillips,1997; Reddy and Guerrero, 2004). For example, volatiles from cornsilk or tomatoes, especially ethylene, 3-methyl-butan-1-ol, phenyl-acetaldehyde, trigger sex pheromone production in the abdominalglands of female Helicoverpa spp. Hardwick whose larvae feed onfruiting parts of various plants (Raina et al., 1992). The presenceof plant volatiles can not only affect the pheromone signaller, butalso the receiver. Plant volatiles may function as sexual kairo-mones (Ruther et al., 2002) and enhance the attractiveness of in-sect sex pheromones (Deng et al., 2004; Dickens, 1989; Lightet al., 1993). For example, green leaf alcohols attract swarmingcockchafer males (Melolontha spp. F.) to sites where conspecific fe-males which emerged earlier after overwintering are already feed-ing on a plant; the sex pheromones released by Melolontha F.females (1,4 benzoquinone, toluquinone) are only slightly attrac-tive or even non-alluring; however, they synergize the attractive-ness of feeding-induced green leaf alcohols (Reinecke et al.,2002; Ruther et al., 2000). Studies on the mechanism of effects ofplant volatiles on responses to insect pheromones revealed thatplant volatiles affect the firing of pheromone olfactory receptorneurons in dependence of the ratio of pheromone and plant vola-tile compound and of the temporal pattern at which pheromoneand plant volatiles are experienced by the insect (e.g. Party et al.,2009, and references therein).

Plant chemistry does not only affect mating signals and thus,reproductive success of herbivorous insects, but more directly alsothe production of eggs, i.e. oogenesis. Females need to provide thedeveloping eggs with nutritive substances to ensure successfuldevelopment of the embryo. The nutrients are provided by theplant the female is feeding upon. High plant nutritional quality isusually associated with production of a high number of eggs.Low plant nutritional quality is usually linked with less activeoogenesis. However, the eggs produced by females feeding on alow-quality plant may be larger than those from females feedingon a high-quality plant. Fox and Czesak (2000) review the modelson optimal progeny size that suggest a trade-off between progenysize and number. Larger eggs are supposed to release larger larvaewhich might have a developmental advantage on poor-qualityplants compared to smaller larvae (Ekbom and Popov, 2004).

2.2. Plant chemical cues affect oviposition behaviour of herbivorousinsects

Erich Städler (2002) eminently reviewed studies on the impactof plant chemical cues important for egg deposition by herbivorous

insects. He outlined the importance of both plant surface com-pounds as well as of the chemistry of the ‘‘plant interior’’ for thechoice of oviposition sites and addressed the question how herbiv-orous insects decipher the complex information on plant qualityobtained by olfactory and contact perception of both ovipositionstimulants and deterrents. Städler raised the question how anegg-laying female that does not wound the plant surface can per-ceive oviposition-stimulating plant chemicals mainly present in-side the plant cell. For example, oviposition by butterfliesspecialised on brassicaceous plants (e.g. Pieris spp. Schrank) isstimulated by secondary plant compounds typical for this taxon,the glucosinolates (Städler and Reifenrath, 2009). However, theseglucosinolates are present mainly inside the plant cell (Reifenrathet al., 2005). Prior to egg deposition, some butterflies specialised onBrassicaceae drum with their forelegs on host plant leaves. Thisdrumming behaviour may be crucial to scratch the leaf surfaceand gain access and contact to the oviposition stimulants under-neath the outermost wax layer.

In addition to plant chemicals that affect insect ovipositionupon contact, plant volatiles released from both flowers and vege-tative parts play a crucial role in guiding oviposition behaviour ofgravid insect females (e.g. Reisenman et al., 2009, 2010).

For pollination, flower volatiles attract nectar-feeding adultLepidoptera which do not only serve pollination, but also ovipositon the plant and thus threaten it by their herbivorous larval prog-eny. Hence, these plants face the dilemma of attracting pollinators(as mutualists), but avoiding larval herbivory by the same pollina-tor’s progeny (as antagonists). Some solanaceous plants addressthis problem by attracting the nocturnal hawkmoth Manduca sextaL. by flower volatiles for pollination when undamaged, but reducetheir attractiveness to adults by change of the floral odour whendamaged by hawkmoth larvae (Kessler et al., 2010). Flowering,undamaged tobacco plants attract M. sexta by benzyl acetone;when plants are damaged by moth larvae, they release less of theattractive volatile. Furthermore, feeding damaged tobacco plantscan reduce the percentage of flowers opening during night. Com-pared to undamaged plants, tobacco plants with feeding damageshow more open flowers in the morning when the hawkmothsare no longer active, but hummingbirds are attracted for pollina-tion. This shift in phenology prevents further moth oviposition(Kessler et al., 2010). Hence, the plants become pollinated at thecost of some feeding damage that can be limited by reducing therelease of flower volatiles and changing flower opening activityin response to damage.

Constitutive volatiles of the vegetative parts of a host plant mayattract gravid insect females, whereas volatiles from a feeding-damaged plant may act as repellent. Plants show a high plasticityin volatile emission, and herbivores often respond to plant volatilesdepending on the plant’s infestation status. Avoiding feeding-induced plant volatiles may be considered an adaptation of herbi-vores to prevent competition since feeding-induced volatiles sig-nalise infestation by conspecifics (De Moraes et al., 2001). Forexample, females of the elm leaf beetle Xanthogaleruca luteola Mül-ler avoid competition among their progeny by orienting to odoursfrom plants with few eggs and low feeding damage rather than toodours from highly infested plants with a high egg load and highfeeding damage (Meiners et al., 2005). In addition to induced hostplant volatiles, also constitutively released volatiles from non-hostplants can affect oviposition behaviour of both herbivorous insectsand their parasitic wasps. Such non-host plants are often used forintercropping with host plants in order to manipulate herbivorebehaviour for pest control (Cook et al., 2007).

While plant volatiles can repel or attract a gravid female from adistance and non-volatile plant cues can deter or stimulate ovipo-sition upon contact, the chemicals of the so-called leaf boundarylayer may also be important for choice of oviposition sites.

M. Hilker, T. Meiners / Phytochemistry 72 (2011) 1612–1623 1615

Humidity, concentrations of oxygen and carbon dioxide as well asof other volatiles released from the plant may be different in theboundary layer and the headspace above the plant (Schuepp,1993). The boundary layer thickness varies with the wind speedand leaf dimension and is estimated to range between 1 and5 mm. Thus, a small insect egg is usually embedded in theboundary layer (Woods, 2010).

Ovipositing insects might use gas concentrations in the bound-ary layer for their final choice of an oviposition site (Städler, 1986).For example, photosynthetically highly active plants that fix largeamounts of CO2 are expected to show a CO2 gradient between leafsurface, boundary layer and atmosphere above the boundary layer.This gradient might be used by insects to detect photosyntheticallyhighly active and thus possibly highly nutritive plants (Langanet al., 2001). Female cactus moths, Cactoblastis cactorum Berg, ac-tively probe the plant surface prior to oviposition with their CO2-sensors. This behaviour might enable the moth to detect thehealthiest, most active plant (parts) (Stange, 1997; Stange et al.,1995). The importance of CO2 concentrations around a plant hasalso been shown for M. sexta. The nectar-feeding moth prefers sur-rogate flowers that emit CO2 levels characteristic of a floweringplant (Thom et al., 2004).

2.3. Plant chemistry influences quality of insect eggs and embryodevelopment

The humidity and gas concentrations in the boundary layer of aleaf may significantly affect egg development (Woods, 2010). Highconcentrations of compounds such as volatile plant terpenes couldenter the eggs via the aeropyles in the outermost layer, the protein-aceous chorion, diffuse through the next layer, i.e. the wax layer,and thus reach the embryo behind the vitelline envelope and ser-osa. Low humidity due to dry air and closed plant stomata maylead to desiccation of the eggs. A plant may respond to insect eggsby hypersensitive response, i.e. formation of necrotic tissue at thesite of egg deposition (see also Section 3.2.2: Balbyshev andLorenzen, 1997; Little et al., 2007; Shapiro and DeVay, 1987). Atsuch a necrotic site, humidity decreases and local temperatureincreases. The metabolic rate of the embryo will increase becauseof the increase of leaf temperature, but also water loss of the eggwill increase. As a result, the egg will probably desiccate and thestressed embryo will die (Woods, 2010).

In addition to the immediate gas atmosphere provided by theleaf boundary layer enclosing an insect egg, performance of eggsmay also be affected by plant secondary compounds incorporatedinto the eggs by the females sequestering those compounds fromthe plant. The secondary plant compounds often serve as protec-tion from parasitisation and predation of eggs (Blum and Hilker,2002). For example, chrysomelid beetles of the taxon Diabroticinasequester bitter cucurbitacins from their host plants and transferthem to the eggs (Ferguson et al., 1985). Some willow leaf beetlesinclude the bitter tasting salicin from willows and poplars intotheir eggs (Pasteels et al., 1986). Numerous lepidopteran speciesare known to endow their eggs with plant toxins, e.g. aristolochicacids from Aristolochia L. plants in the eggs of the papilionid Atro-phaneura alcinous (Klug) (Nishida and Fukami, 1989), cardenolidesfrom milkweeds, Asclepias L. spp., in the eggs of the monarch but-terfly (Roeske et al., 1976), and N-oxidised alkaloids in the eggs ofseveral Lepidoptera (arctiids, nymphalids), Chrysomelidae andHemiptera that feed on plants containing PAs (Boppré, 1990;Hartmann, 1995a,b, 1999; Loaiza et al., 2007; Schaffner et al.,1994; Trigo et al., 1996). Adults of Naupactus bipes (Germar)(Curculionidae) feeding on Piper gaudichaudianum Kunth(Piperaceae) sequester chromenes from the leaves, and femalesincorporate them in the eggs (Ramos et al., 2009). Not only thefemales endow the eggs with plant toxins, but also the males can

contribute to the endowment of eggs with plant toxins; malestransfer the toxins to the eggs via mating (Eisner et al., 2002, andreferences therein).

In addition to such plant toxins, the phytohormones jasmonicacid (JA) and salicylic acid (SA), as well as a precursor of SA, benzoicacid (BA), have been detected in eggs of many insect species, oftenin much higher concentrations than in plant tissue or larval diet(Tooker and de Moraes, 2005, 2007). The function of these com-pounds in insect eggs is unknown. SA and BA have antimicrobialand antifungal activity and thus, might protect eggs from entomo-pathogens. JA, SA and BA have been detected so far only inside theeggs. However, they were not found in extracts of the exterior ofeggs (Tooker and De Moraes, 2005). Even though it is temptingto assume a role of these endogenous compounds of the eggs intriggering egg-induced plant defence, such a role is questionableat the current state of knowledge.

3. Egg effects on plants

The interaction between plants and insect eggs is not unidirec-tional. While plants can influence mating activities and ovipositionbehaviour of herbivorous insects, the eggs themselves when laidon the plant can change the plant’s primary and secondary metab-olism. So far, these egg-induced effects on plants have been shownmainly detrimental to eggs. Hence, the egg-induced effects areconsidered plant defensive responses against egg deposition byherbivorous insects. Nevertheless, insect eggs can also trigger plantresponses which are beneficial for the eggs or the hatching larvae,as is the case in some insect species inducing plant galls. Some in-sects induce the formation of a plant gall by egg deposition ratherthan by larval feeding; the gall provides shelter for the egg andnutritive tissue to the hatching larva. Oviposition-induced effectson a plant by galling insects have been reviewed by Hilker et al.(2002b) and will not be considered here.

One might expect especially short-lived, small crop plants withlow biomass to show oviposition-induced defence, as these plantsmight especially benefit from prevention or reduction of feedingdamage by larvae that will hatch from the eggs. However, defenceinduced by insect eggs is known in both trees with their huge bio-mass and long life and short-lived, small crop plants. Hence, plantswith very different life strategies evolved mechanisms to respondto herbivorous insects at such an early stage of attack, the eggdeposition. The parallels and differences of the plant–insect sys-tems studied with respect to egg-induced plant effects are summa-rised in Table 1 and will be addressed below.

3.1. Insect egg deposition affects the plant’s primary metabolism

Egg deposition by chewing and sucking herbivorous insects canlead to a reduction of the plant’s photosynthetic activity both lo-cally, i.e. directly at the leaves with eggs, and systemically, i.e. atleaves adjacent to those with eggs. A systemic reduction of thenet photosynthetic rate was found in egg-free twigs of Scots pine(Pinus sylvestris L.) adjacent to twigs laden with eggs of the herbiv-orous pine sawfly Diprion pini L. (Schroeder et al., 2005). Local pho-tosynthetic activity was measured in Brassica oleracea L. (savoycabbage) leaves laden with eggs by the herbivorous harlequinbug Murgantia histrionica (Hahn) (Velikova et al., 2010); in thisstudy, measurements of photosynthetic activity were made di-rectly at the leaf with eggs, but at parts of the leaf with access tolight and not covered by eggs. Since oviposition by M. histrionicais usually associated with feeding by the females on the leaves,the authors also measured the effect of feeding on the plant’s pho-tosynthetic activity and found that feeding reduces the plant’s pho-tosynthetic activity. Moreover, plants which experienced bothfeeding damage and egg deposition by this bug show significantly

Table 1Overview of changes of plant primary and secondary metabolism caused by insect egg deposition. For details, see text Section 3.

Effect of insect egg deposition Plant Herbivorous Insect References

On plant primary metabolismReduction of photosynthetic activity Scots pine, Pinus sylvestris Pine sawfly, Diprion pini Schroeder et al. (2005)Reduction of photosynthetic activity Savoy cabbage, Brassica

oleraceaHarlequin bug, Murgantiahistrionica

Velikova et al. (2010)

Reduction of transcription ofphotosynthesis-relevant genes

Thale cress, Arabidopsisthaliana

Cabbage butterfly, Pieris brassicae Little et al. (2007)

No change of net photosynthetic rate Field elm, Ulmus minor Elm leaf beetle, Xanthogalerucaluteola

Meiners (unpublished data)

On plant secondary metabolism(a) Induction of plant volatiles: attractive to egg parasitoids

Terpenoids Field elm, Ulmus minor Elm leaf beetle, Xanthogalerucaluteola

Meiners and Hilker (2000), Wegener et al. (2001)and Büchel et al. (unpublished data)

(E)-b-Farnesene Scots pine, Pinus sylvestris Pine sawfly, Diprion pini Mumm et al. (2003)(E)-b-Caryophyllene French bean, Phaseolus

vulgarisSouthern green stink bug, Nezaraviridula

Colazza et al. (2004a,b)

(E)-b-Caryophyllene Broad bean, Vicia faba Southern green stink bug, Nezaraviridula

Colazza et al. (2004a,b)

(E)-b-Caryophyllene Savoy cabbage, Brassicaoleracea var. sabauda

Harlequin bug, Murgantiahistrionica

Conti et al. (2008)

(b) Change of plant odour: attractive to larval parasitoidsReduced emission of (Z)-3-hexenylacetate

Palisade signal grass,Brachiaria brizantha

Spotted stemborer moth, Chilopartellus

Bruce et al. (2010)

(c) Induction of change of plant surface chemistryChanges of the quantitativecomposition of leaf surfacecompounds

Brussels sprouts, Brassicaoleracea var. gemmiferaThale cress, Arabidopsisthaliana

Cabbage butterfly, Pieris brassicaeCabbage butterfly, Pieris brassicae

Fatouros et al. (2005)Hilker, Blenn, Geiselhardt, Fatouros, unpublisheddata

Necrotic zones Hybrid of Solanum spp. Colorado potato leaf beetle,Leptinotarsa decemlineata

Balbyshev and Lorenzen (1997)

Necrotic zones Mustard, Brassica nigra Cabbage butterflies, Pierisbrassicae, P. rapae

Shapiro and DeVay (1987)

Accumulation of callose; productionof H2O2

Thale cress, Arabidopsisthaliana

Cabbage butterfly, Pieris brassicae Little et al. (2007)

Neoplasma formation Pea, Pisum sativum Bruchid beetles, Bruchus pisorum;Callosobruchus maculatus

Doss et al. (2000)

(d) Induction of non-volatile plant secondary metabolitesProduction of benzyl benzoate(ovicidal activity)

Rice, Oryza sativa White-backed planthopper,Sogatella furcifera

Seino et al., 1996; Suzuki et al., 1996

1616 M. Hilker, T. Meiners / Phytochemistry 72 (2011) 1612–1623

lower photosynthetic activity than plants which are egg-free, butsuffered feeding damage. These findings demonstrate that eggdeposition per se may significantly affect the plant’s primarymetabolism and reduce photosynthetic activity both in a tree spe-cies (pine) and a crop plant (savoy cabbage).

Whether transcription of genes involved in photosynthesis is af-fected locally at the leaf with eggs but next to the oviposition site,or systemically at egg-free leaves adjacent to the egg-laden oneshas not been studied so far. In Arabidopsis thaliana (L.), transcrip-tion of genes involved in photosynthesis was measured directlyat the site where Pieris brassicae (L.) had laid an egg mass; theauthors removed the egg mass 1, 2 or 3 days after egg depositionand found that transcript levels of photosynthesis-relevant genesin the tissue where the eggs have been laid are reduced at eachtime interval (Little et al., 2007). Since leaf parts covered with eggshave no or reduced access to light, the transcriptional reductionfound in this study of A. thaliana with butterfly eggs may be dueto this shadowing effect rather than to specifically egg-induced ef-fects. Thus, while transcription of photosynthesis-related genes intissue right below the eggs is changed perhaps due to the shadow-ing effect of eggs, we do not know yet how eggs affect transcriptionin tissue not shadowed by the eggs, but adjacent to them (comparelocal and systemic effects described above). Hence, the mecha-nisms of egg-induced effects in tissue adjacent to the ovipositionsite are unknown so far.

While only few studies are available that studied the effect ofinsect egg deposition on photosynthetic activity, numerous stud-ies addressed the impact of insect feeding damage on photosyn-thesis (e.g. Nabity et al., 2009; Welter, 1989; Zangerl et al.,

2002). The effect of feeding damage on photosynthetic activitydepends on a wide range of factors such as the plant and herbi-vore species, the type of feeding damage (sucking, mining, orchewing herbivores), or the site of measurement (damagedleaves or undamaged leaves of a damaged plant, or the canopy)(Delaney, 2008, and references therein). Partial plant damagewas found to cause an increase, no change, or decrease of thenet photosynthesis locally at the site of damage and systemicallyat adjacent undamaged leaves. Interestingly, systemically re-duced photosynthesis as it was found after egg deposition onpine (Schroeder et al., 2005), is very rare after feeding damage(Delaney, 2008).

Does the plant benefit from reduction of photosynthetic activityin egg-laden leaves and adjacent ones? High photosynthetic activ-ity rather than reduced one might be expected in egg-laden leavesto cover current costs of plant defence against eggs and to compen-sate in time for upcoming larval feeding damage (see Section 3.2).Compensation of plant damage by enhanced photosynthesis isconsidered a trait of plant tolerance towards herbivory (Hermsand Mattson, 1992; Karban and Baldwin, 1997). Hence, the reduc-tion of photosynthesis induced by insect egg deposition on pineand savoy cabbage indicates that defence against eggs is costlyfor these plant species and might be paid by reduced tolerance toupcoming herbivory (Schroeder et al., 2005). However, the ex-penses of egg-induced plant defence and the possibly negative ef-fects of egg-induced reduction of photosynthesis might be limitedby the fact that the time between egg deposition and larval hatch-ing is usually only a few days (except for eggs overwintering on aplant).

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3.2. Insect egg deposition changes the plant’s secondary metabolism

3.2.1. Egg deposition affects plant volatile emissionEgg deposition by herbivorous insects can induce a change of

the plant’s volatile blend. The oviposition-induced plant volatileswere shown to attract egg parasitoids killing the eggs of the herbi-vores. Mainly oviposition-induced emission of terpenoids wasfound to play a role in parasitoid attraction.

The first study which demonstrated that insect egg depositioninduces a change of plant volatile emission locally at the site ofegg deposition and systemically was a study of elm (Ulmus minorMill.) laden with eggs of the elm leaf beetle X. luteola (Meinersand Hilker, 2000). Odour from elm leaves laden with eggs wasshown to attract a parasitoid species specialised on elm leaf beetleeggs. The parasitoid responds specifically only to volatiles emittedafter elm leaf beetle egg deposition, whereas odour of leaves witheggs laid by other herbivorous insects does not attract this specia-lised parasitoid Oomyzus galerucivorus (Hedqvits), a eulophid wasp(Meiners et al., 2000). Hence, the attractiveness of elm leaves witheggs towards this specialised egg parasitoid is specific for the her-bivore species. Odour of elm leaves becomes attractive to egg par-asitoids a few hours after elm leaf egg deposition and keepsattractiveness for about 5 days, i.e. shortly before larvae hatch(Meiners, unpublished data).

These results on the attractiveness of odour from elm leaves la-den with elm leaf beetle eggs triggered several questions, i.e.

� Do the eggs indeed induce the attractive plant volatiles or doesthe ovipositional wounding associated with egg depositioninduce the attractants? The elm leaf beetle female is removingthe leaf epidermis with her mouthparts prior to egg deposition.� Do egg-free leaves that experienced regular feeding activity also

emit volatiles that attract the egg parasitoid? Regular feedingactivity is visible by holes chewed into a leaf and is usuallyaccompanied by beetle egg deposition.� Does a combination of odour released from eggs per se and from

feeding-damaged sites cause attraction of the parasitoid ratherthan an induction of plant volatiles by egg deposition?

These questions may be considered a general guide which en-ables to distinguish between egg-induced effects and wounding-or feeding-induced ones, when studying plant responses to eggdeposition by insects which feed prior to egg laying.

The studies of these questions in the tritrophic system consist-ing of elm, elm leaf beetle and egg parasitoid showed that oviposi-tional leaf damage (epidermal removal) does not induce thevolatiles attractive to the parasitoid of leaf beetle eggs. Neitherdo volatiles from egg-free leaves that have been damaged by feed-ing activity of elm leaf beetles attract the parasitic wasp, whereasleaves with eggs and feeding damage are attractive to the parasit-oid. In order to prove that attraction of the egg parasitoid is indeeddue to induction of plant volatiles by oviposition rather than to acombination of odour released from feeding-damaged leaves andfrom egg odour, the parasitoid’s behavioural response was testedto a mixture of odour from feeding-damaged leaves (without eggs)and from eggs that were removed from a leaf. The odour mixturedoes not attract the parasitoid; this study demonstrated that theattraction of the egg parasitoid is indeed due to an inductive effectof elm leaf beetle egg deposition on the plant’s volatile pattern(Meiners and Hilker, 2000).

The volatile blend of locally and systemically oviposition-induced elm leaves with feeding damage contains mainly greenleaf volatiles and terpenoids (Wegener et al., 2001). Inhibition ofterpenoid biosynthesis in elm leaves with eggs and feeding dam-age by treatment of elm with terpenoid biosynthesis inhibitors(fosmidomycin and cerivastatin) leads to a loss of the plant’s

attractiveness to the parasitoid (Büchel, Malskies, Mayer, Fenning,Gershenzon, Hilker, Meiners, unpublished data). These findingsshow the importance of induction of terpenoids necessary forattraction of the parasitoid of elm leaf beetle eggs.

Another tree, i.e. Scots pine (P. sylvestris) was also shown torespond specifically to egg deposition by herbivorous insects.When pine sawflies like D. pini and Neodiprion sertifer (Geoffroy)slit pine needles longitudinally to insert a row of eggs into theneedle, the pine twig laden with eggs starts to change its odourlocally and systemically 3 days after oviposition. The odour re-leased after egg deposition attracts egg parasitoids specialisedon eggs of these diprionids. In contrast, odour of egg-free pineneedles which were slit by a needle to mimic the ovipositionalwounding is not attractive to the egg parasitoid (Hilker et al.,2002a). However, pine foliage releases attractive odour whensecretion associated with the eggs was applied into an artificiallyslit pine needle (Hilker et al., 2005). Thus, wounding per se doesnot induce the change of pine odour, but egg deposition or eggsecretion applied to the ovipositional wound does. Adult sawfliesdo not feed upon pine needle tissue. Odour of pine which suf-fered larval feeding damage is not attractive to the egg parasitoid(Hilker, unpublished data).

A series of bioassays showed that only pine odour released3 days after sawfly egg deposition is attractive, whereas odour re-leased earlier or later than 3 days has no behavioural effect on theparasitoid (Hilker et al., 2002a; Koepke et al., 2010; Schroederet al., 2008). In 4-day-old eggs, the diprionid embryo might havedeveloped so far that the parasitoid larva inside the host egg isprobably not able to develop successfully next to a host embryoof this age; thus, pine with 4-day-old diprionid eggs will probablynot benefit from attraction of egg parasitoids by oviposition-in-duced pine volatiles. Pine twigs with diprionid eggs might ‘‘afford’’the time-limited release of oviposition-induced plant volatilessince egg parasitoids are attracted by the sex pheromones of dipri-onid females (Hilker et al., 2000); these pheromones (acetate andpropionate of (2S,3R,7R)-3,7-dimethyl-2-tridecanol) will attractthe parasitoid to freshly laid eggs whereas the oviposition-inducedplant odour probably becomes important when no or too low con-centrations of sex pheromones are around 3 days later. Hence, re-lease of oviposition-induced plant volatiles during only a narrowtime window (i.e. 3 days after egg deposition) may be considereda very economic and efficient way of the plant to call parasitoidsfor help (Dicke and Baldwin, 2010).

The attractive oviposition-induced pine odour differs fromunattractive odour released from egg-free, slit pine needles by itsquantitative composition; no qualitative differences are detectable(Mumm et al., 2003). The attractive odour contains significantlygreater amounts of (E)-b-farnesene. This sesquiterpene is notattractive per se to the parasitoid at neither concentration tested(Mumm and Hilker, 2005). However, when (E)-b-farnesene is com-bined with the habitat (background) odour of non-induced pine,the egg parasitoid responds positively and is attracted. These find-ings show that the oviposition-induced key compound needs to beperceived in a natural odorous context to elicit a behavioural re-sponse in the parasitoid (Beyaert et al., 2010).

In addition to these tree species, changes of plant volatiles afterinsect egg deposition have been shown in several crop plants, i.e.

– in French bean Phaseolus vulgaris L. and broad bean Vicia faba L.after egg deposition by the herbivorous pentatomid bug Nezaraviridula L. (Colazza et al., 2004a,b),

– in savoy cabbage B. oleracea L. var. sabauda after egg depositionby the pentatomid M. histrionica (Conti et al., 2008), and

– in palisade signal grass Brachiaria brizantha Stapf after eggdeposition by the stemborer moth Chilo partellus (Swinhoe)(Bruce et al., 2010).

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The oviposition-induced effects on bean and cabbage showmany parallels to those described above for trees; thus, both plantswith a relatively short lifetime and those which grow for decadesand have plenty of time to regrow after feeding damage respondto the very early phase of insect attack, the egg deposition, in asimilar way. Like in elm and pine, bean plants with eggs of N. vir-idula emit the odour attractive to egg parasitoids only during a spe-cific time window after egg deposition. Bean plants with about 4–5-day-old eggs do no longer emit odour attractive to parasitoids(Colazza et al., 2004a). Like in the elm leaf beetle, egg depositionby N. viridula is associated with feeding activity of the adults (here:piercing-sucking activity). In contrast to egg deposition by the elmleaf beetle and the dipriond sawflies mentioned above, no oviposi-tional wounding of the leaf occurs when N. viridula lays eggs; theeggs are just glued on the plant cuticle. Like in the tree studies out-lined above, the volatiles released from bean plants after feedingdamage by bugs do not attract egg parasitoids. Again, the oviposi-tion-induced odour of bean plants is characterised by terpenoids,i.e. here by increased amounts of (E)-b-caryophyllene when com-pared to non-attractive plants. Also the cabbage plants with feed-ing damage and egg deposition by M. histrionica release increasedamounts of (E)-b-caryophyllene when compared to non-attractive,undamaged plants (Conti et al., 2008). Hence, oviposition-inducedplant terpenoids play a crucial role in both trees and the dicotylen-ous crops studied so far for indirect plant defence against eggs.

In contrast, the change of plant odour after egg deposition bythe stemborer C. partellus on palisade signal grass is not character-ised by increased release of terpenoid volatiles, but a significantdecrease of the emission of (Z)-3-hexenyl acetate. Female C. partel-lus moths preferred plants without eggs to plants with eggs. Odourreleased from egg-laden palisade signal grass B. brizantha causedincreased attraction of the larval parasitoid Cotesia sesamiae (Cam-eron) when compared to odour of egg-free, undamaged plants(Bruce et al., 2010). Hence, changes of plant odour after insectegg deposition are not only exploited by egg parasitoids, but canalso alert larval parasitoids and provide information on upcomingavailability of host larvae.

Like many other lepidopteran species, females of the moth C.partellus do not damage the plant when laying eggs. Nevertheless,some lepidopteran species move their ovipositor back and forth onthe leaf surface or drum with their legs on the substrate prior tooviposition (Städler, 2002). Many lepidopteran species leave hairsand scales at the site of egg deposition which may have kairomonalfunction for egg parasitoids (Fatouros et al., 2008a,b).

Hence, in order to distinguish egg-induced changes of plantmetabolism from effects caused by drumming and scratchingbehaviour of the female or by the release of scales and hairs, oneshould address the following questions when studying plant re-sponses to egg deposition by insects which do not feed upon theplant prior to egg laying, as is the case for lepidopteran species:

Do just eggs or egg-associated secretions (see Section 4.1) reallyinduce plant responses or

� Does scratching the leaf surface or drumming induce plantchanges?� Do scales and hairs left by egg-laying females on leaves induce

plant changes, or do they effectively absorb plant volatiles byenlarging the absorptive surface?� Are there combinatory effects of eggs and female scales and

hairs, respectively?

3.2.2. Egg deposition affects plant surface chemistrySpecific plant traits like a thick waxy plant cuticle may impair

release of oviposition-induced plant volatiles when obvious dam-age of tissue is lacking. Furthermore, change of plant odour in such

a way that it becomes attractive to egg parasitoids might be impos-sible in leaves that hardly produce terpenoids which seem to beimportant for attraction of egg parasitoids. Egg deposition by P.brassicae on the waxy leaves of Brussels sprouts (B. oleracea L.var. gemmifera) does not induce volatiles that attract the egg para-sitoid Trichogramma brassicae (Fatouros et al., 2005). Nor does eggdeposition by P. brassicae on leaves of A. thaliana lead to emissionof odour that attracts the egg parasitoid T. brassicae (Hilker, Blenn,Geiselhardt, Fatouros, unpublished data). Leaves of A. thaliana re-lease only low amounts of terpenoids, whereas the majority ofthe terpenoids of this plant species is emitted from the flowers(Chen et al., 2003).

However, egg deposition by P. brassicae on leaves of Brusselssprouts and A. thaliana induces changes of the leaf surface that ar-rest a host searching egg parasitoid at this leaf, and thus, intensifieshost search in the vicinity of eggs (Fatouros et al., 2005; Hilker,Blenn, Geiselhardt, Fatouros, unpublished data). In Brusselssprouts, oviposition-induced leaf surface changes are detectableonly 3 days after egg deposition. Leaves with freshly laid eggs(1–2 days old) also elicit a positive response in the egg parasitoidupon contact; however, this positive response is not due to ovipo-sition-induced leaf changes, but to kairomonal effects of residuesleft by the butterfly during egg deposition (e.g. scales). Again, ashas been discussed for the oviposition-induced release of leaf vol-atiles, also oviposition-induced leaf surface changes seem to belimited to a certain, ecologically relevant time window; this strat-egy might limit the costs for the oviposition-induced, indirect de-fence. The leaf surface changes in Brussels sprouts are restrictedto the leaf carrying an egg mass of P. brassicae; no systemic effectsare detectable (Fatouros et al., 2005). Chemical analyses of the ovi-position-induced leaf surface changes in A. thaliana revealed thatinduced leaves differ from egg-free controls by the quantitativecomposition of leaf surface compounds; no qualitative differenceshave been found so far (Hilker, Blenn, Geiselhardt, Fatouros,unpublished data).

In addition to the above-mentioned effects of egg deposition onthe plant’s surface that serve indirect plant defence involving eggparasitoids, eggs can also induce changes of plant tissue (includingthe surface) serving direct defence against the eggs or the hatchinglarvae. Egg deposition by several insects has been shown to induceformation of necrotic tissue directly at the site where eggs are at-tached to the leaf. This necrosis is associated with dehydration anddesiccation of the surface. Eggs do not stick well on a dry leaf sur-face and easily fall off the plant. Eggs that drop down suffer muchhigher risks of mortality than eggs on the plant. In humid soil, therisk of infection by pathogens may be much higher. Furthermore,when herbivorous larvae hatch in the soil instead of starting theirlife on a leaf they hardly find their way back to the host plant (Bal-byshev and Lorenzen, 1997). Formation of necrotic leaf tissue in re-sponse to eggs has been shown in several plant–herbivoreinteractions. Eggs of the butterflies P. brassicae and Pieris rapae L.can induce necrotic zones on leaves of mustard plants, Brassica ni-gra L. (Shapiro and DeVay, 1987). Leaves of A. thaliana show no vis-ible formation of necrotic tissue at the sites of egg deposition by P.brassicae, but they accumulate callose and produce H2O2 at thesesites (Little et al., 2007). Such hypersensitive response (HR) orHR-like response to eggs was also found in a clone of a hybrid ofSolanum L. spp.; plants of this clone form necrotic tissue aroundegg masses of the Colorado potato leaf beetle, Leptinotarsa decem-lineata Say (Balbyshev and Lorenzen, 1997).

Instead of formation of necrotic tissue and HR-like responses,some lines of pea (Pisum sativum L.) form neoplasms on the surfaceof their pods in response to egg deposition by the pea weevil, Bru-chus pisorum (L.), and the cowpea weevil, Callosobruchus maculatus(F.) (Doss et al., 2000). These bruchid beetles lay eggs on the sur-face of a pea pod, the hatching larvae chew holes into the pod

M. Hilker, T. Meiners / Phytochemistry 72 (2011) 1612–1623 1619

and develop inside the pod. The neoplasm that is formed where anegg is laid elevates the egg from the pod surface. The exposed eggdoes not stick so tightly anymore to the surface and easily dropsdown. Again, like described above for eggs detached from necroticleaf tissue, eggs that drop down from a pea pod suffer a high risk ofmortality. If an egg does not fall off a pea pod with a neoplasm, thehatching larva needs to penetrate the tissue protuberance or needsto circumvent it to get inside of the pea pod. These challengesincrease the risk of larval mortality (Doss et al., 2000).

3.2.3. Egg deposition affects biosynthesis of non-volatile plant internalsecondary metabolites

While effects of eggs on the plant’s volatile emission and on thechanges of plant tissue detectable at the surface have been ad-dressed in several studies, little knowledge is available on possibleeffects of eggs on secondary plant metabolites biosynthesized andkept inside a cell. How could a plant benefit when responding toeggs by a change of its internal secondary metabolites, while theegg is touching just the surface? If the egg would induce a sustain-able change of the plant’s secondary metabolism in such a way thatthe hatching larvae would face metabolites that impair larvaldevelopment, the egg deposition would prepare direct plant de-fence against larval feeding.

In order to investigate whether egg-induced defences influenceplant defence against feeding larvae, we compared performance oftwo types of pine sawfly larvae (D. pini): (1) larvae that startedtheir life on the pine twig (P. sylvestris) where they hatched fromeggs and (2) larvae that started larval development on an egg-freepine. Two weeks after larval hatching, both types of larvae weretransferred to egg-free pine twigs since in nature, hatching larvaealso move to egg-free pine twigs after having fed up the needlesof the twig where they hatched from eggs. The results showed thatyoung larvae starting to feed on a previously egg-laden twig gainsignificantly less weight than larvae starting their life on egg-freetissue. These negative effects on neonate larvae that fed on needleswith prior eggs carry over to the entire juvenile development(about 4 weeks long): mortality rates of these larvae are higherand their duration of development is significantly longer than inlarvae that initially fed on egg-free pine. The egg-induced effectson the performance of the sawflies are even detectable in the nextgeneration. Females that developed from larvae starting their lifeon previously egg-laden pine show reduced fecundity comparedto females that spend their entire juvenile development on egg-free pine. These performance studies clearly show that egg deposi-tion can ‘‘warn’’ the plant of future feeding damage (Beyaert, I.,Koepke, D., Stiller, J., Hammerbacher, A., Schmidt, A., Gershenzon,J., Hilker, M., unpublished data).

Future studies need to elucidate the phytochemical differencesbetween pine foliage induced by both sawfly egg deposition andfeeding larvae on the one hand and egg-free pine foliage inducedby larval feeding only on the other hand. Such studies will alsoshow whether insect egg deposition can prime a plant’s defensivephytochemial response to feeding larvae. Priming of anti-herbivoredefence in an undamaged plant is well known to be triggered byvolatiles released from herbivore-infested plants that are in thevicinity of the yet undamaged plant. The undamaged plant may as-sess the risk of herbivore damage by ‘‘listening’’ to the damage ofthe infested plant (Choh and Takabayashi, 2006; Engelberthet al., 2004; Frost et al., 2008; Heil and Kost, 2006; Heil and Ton,2008; Paré et al., 2005). The study of pine and pine sawflies de-scribed above suggests that a plant ‘‘noticing’’ and responding toinsect eggs can get ‘‘ready for battle’’ against feeding herbivores(Conrath et al., 2006; Frost et al., 2008).

In contrast, a study by Bruessow et al. (2010) found that weightof 8-day-old P. brassicae larvae feeding on A. thaliana leaves treatedwith extract of crushed eggs did not differ from those feeding on

untreated leaves, whereas weight of 8-day-old larvae of the gener-alist moth Spodoptera littoralis Boiduval was even higher on ex-tract-treated leaves compared to untreated controls. Futurestudies need to elucidate whether egg-mediated plant effects onherbivore performance vary with the plant–insect system studiedand whether treatment of leaves with extracts of crushed eggshas the same effect on larval performance as egg deposition.

Egg-induced changes of the non-volatile secondary plantmetabolites may also have directly detrimental effects on the eggsif the harming metabolites exude and reach the eggs. This is thecase in rice plants which produce benzyl benzoate in response toegg deposition by the planthopper Sogatella furcifera (Horvath).When the plant tissue responds to egg deposition by formationof a watery lesion, benzyl benzoate comes in contact with the eggsand has ovicidal activity. More than 80% of the eggs die within twodays when the plant forms a watery lesion, whereas more than 80%survive and show regular embryo development when the plantdoes not respond by a watery lesions to the egg deposition. Benzylbenzoate was detected in the extract of watery oviposition lesions(Seino et al., 1996; Suzuki et al., 1996).

Egg-induced changes of iridoid glycosides (IG) of Plantagolanceolata L. have been discussed by Reudler Talsma et al. (2008).Leaves with eggs of the butterfly Melitaea cinxia L. were found tocontain higher IG concentrations than egg-free leaves. However,the results of this study suggest that the butterfly prefers leaveswith high IG concentration for oviposition rather than an inductiveeffect of eggs on IG biosynthesis.

4. Interacting factors of plants and insect eggs

4.1. Egg-associated secretions elicit plant responses

A recent review summarised and discussed current knowledgeon elicitors of plant direct and indirect defence against insect eggs(Hilker and Meiners, 2010). In short, these elicitors are either re-leased by the egg-laying female or have been detected in secretionsassociated with the eggs and adhering them to plant. Direct plantdefence against eggs by formation of neoplasms is elicited bymono- and bis-(3-hydroxypropanoate)esters of monounsaturatedC22-diols or mono- and diunsaturated C24-diols. These lipid-de-rived elicitors are called bruchins since they were isolated frombruchid beetles inducing formation of neoplasms on pea pods(see Section 3.2.2) (Doss et al., 2000). An elicitor of indirect plantdefence against eggs by change of surface chemistry arrestingegg parasitoids is benzyl cyanide, a compound present in the stickysecretion associated with eggs of P. brassicae. The P. brassicae fe-males receive these compounds from males during mating; themales transfer the compound as anti-aphrodisiac to ensure theirpaternity (Andersson et al., 2000; Fatouros et al., 2008a,b). In con-trast to these low molecular elicitors, the elicitor of indirect de-fence against eggs in the diprionid sawfly D. pini and the elm leafbeetle X. luteola is a proteinaceous compound released with ovi-duct secretion enclosing the eggs. This secretion looses elicitoractivity when treated with a proteinase (Hilker et al., 2005; Mein-ers, unpublished data). Thus, even though only few elicitors ofplant defence against eggs have been studied so far, their chemis-try varies broadly.

The plant tissue directly exposed to the elicitors of plant de-fence against eggs ranges from the plant cuticle over mesophyllcells to parenchymatic leaf tissue. The elicitor benzyl cyanide asso-ciated with egg secretion of the butterfly P. brassicae needs to pen-etrate the plant cuticle since eggs are laid on the leaf surfacewithout any visible leaf wounding; the proteinaceous elicitorassociated with eggs of the elm leaf beetle comes into contact withthe mesophyll cells of the leaves since the egg-laying female

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removes the epidermis; the proteinaceous elicitor present in thesecretion enclosing sawfly eggs is deeply inserted into a pine nee-dle and faces parenchymatic tissue in the ovipositional woundingof the needle. So far, the mode of action of the elicitors of oviposi-tion-induced plant defence is unknown, and future studies need toaddress the question whether they e.g. bind to a receptor or areforming channels in cell membranes (compare Alborn et al.,1997; Engelberth et al., 2000, for elicitors of plant defence againstfeeding insects).

Ovipositional wounding per se (without eggs or egg secretion)has so far not been shown as elicitor of induced plant defenceagainst eggs. However, even very tiny wounds like the rupture offoliar glandular trichomes in tomato plants (Solanum tuberosumL.) by an insect touching the leaf have been found to induce de-fence genes (e.g. proteinase inhibitor 2, or PIN2, a gene encodinga JA-regulated proteinase inhibitor) for several hours (Peifferet al., 2009). An insect laying eggs may also destroy these glandulartrichomes. It is yet unknown whether such effects induced by tri-chome rupture during oviposition may influence plant defensiveresponses against eggs.

4.2. Phytohormones and plant defence genes respond to eggs

Phytohormones are involved in conveying the informationabout egg deposition within the plant from the site of egg deposi-tion to other parts. Treatment of elm and pine with JA leads to re-lease of volatiles that are attractive to egg parasitoids (Hilker et al.,2002a; Meiners and Hilker, 2000). In pine, both egg depositionand treatment with JA induce an increase of emission of (E)-b-farnesene, a key component for attraction of egg parasitoidsattacking pine sawfly eggs (Mumm et al., 2003). However, whileegg deposition by a sawfly on pine leads to a reduction of ethyleneemission compared to egg-free controls, treatment of pine with JAresults in an increase of ethylene emission (Schroeder et al., 2007).These findings indicate that other phytohormones than JA are addi-tionally involved in activation of pine defence against sawfly eggs.Bruessow et al. (2010) showed that salicylic acid accumulates atthe site where eggs of P. brassicae are laying on a leaf of A. thaliana;they suggest that a cross-talk between jasmonic acid and salicylicacid mediates plant responses to insect eggs. Thus, phytohormonesacting in concert with jasmonic acid are probably important forsignalling the presence of eggs to a plant.

Transcriptional changes in response to insect egg depositionhave been shown in several plant–egg interactions:

P. sativum – bruchid eggs. The change of plant surface tissue andformation of neoplasms on pea pods after egg deposition by bru-chid beetles is associated with the up-regulation of several de-fence-relevant genes encoding enzymes involved in isoflavonephytoalexin biosynthesis, in the octadecanoid pathway and noduleformation (Cooper et al., 2005; Doss et al., 2000; Doss, 2005).

Brassica L. and Arabidopsis Heynh. – Pieris Schrank eggs. Eggdeposition by P. brassicae on leaves of Brussels sprouts inducestranscriptional changes of genes that may lead to the surfacechanges arresting egg parasitoids. For example, a-expansin EXPA15and xyloglucan endotransglucosylase/hydrolase XTH6, genes in-volved in cell wall metabolism, are induced by egg deposition. Inaddition, transcription of a wide range of other genes involved inseveral metabolic processes was found to change in response toegg deposition. Since the oviposition-induced changes of leaf sur-face chemistry in Brussels sprouts were detected in the vicinityof P. brassicae egg masses, transcript levels of leaf tissue next tothe eggs were determined (Fatouros et al., 2008a,b). Little et al.(2007) took the leaf tissue directly underneath the eggs to deter-mine the transcriptional response of leaves of A. thaliana to 3-day-old P. brassicae eggs. They showed significant transcriptionalchanges of hundreds of genes involved in a wide range of meta-

bolic processes (e.g. photosynthesis, proteolysis, biosynthesis path-ways of several plant secondary metabolites, hormonebiosynthesis, fatty acid metabolism, wax biosynthesis). For threegenes encoding pathogenesis related protein PR1, trypsin inhibitorand chitinase, expression was also measured in the vicinity of eggsand in egg-free distal leaves of an egg-laden plant. Only PR1 showsa high expression level in the vicinity of the eggs similar to the oneat the site of eggs, whereas the two other genes show high expres-sion levels only directly at the site where the eggs are laid, butmuch lower levels in the vicinity of the site of egg deposition. Fu-ture studies need to address the question whether the transcrip-tional changes of tissue directly underneath the eggs is due toegg-specific factors or to the fact that tissue underneath the eggshas no access to light and has glued stomata due to the stickyegg secretion.

P. sylvestris–D. pini eggs. Scots pine (P. sylvestris) responds toeggs laid by the pine sawfly D. pini on its needles by enhancingthe transcription of two sesquiterpene synthase genes, an (E)-b-caryophyllene/a-humulene synthase (PsTPS1) and a 1(10),5-germacradiene-4-ol synthase (PsTPS2) (Koepke et al., 2008). BothPsTPS1 and PsTPS2 transcripts are significantly enhanced only inP. sylvestris, but not in Pinus nigra Arnold laden with eggs of D. pini.Interestingly, P. sylvestris laden with sawfly eggs releases volatilesattractive to egg parasitoids, whereas P. nigra laden with these eggsdoes not. Since enhanced transcription of these genes was alsofound only at those time periods when odour was attractive to par-asitoids, i.e. 3 days after oviposition, PsTPS1 and PsTPS2 are goodmarkers for the attractiveness of P. sylvestris to egg parasitoids(Koepke et al., 2010).

5. Conclusions

The close relationship between plants and insect eggs is notunidirectional limited to the well known effects of plants on insectegg deposition and eggs. Evidence is growing that egg depositionby many herbivorous insects induces changes of plant chemistrywhich serve defence of the plant against insect attack at this earlystage prior to larval feeding damage (Hilker and Meiners, 2006).

Future studies on the following key questions will broaden anddeepen our knowledge on interactions between plants and insecteggs:

1. How species specific are effects and mechanisms of interactionsbetween plants and eggs? Which common traits are detectable?So far, the effects of eggs on a plant were found to be very spe-cific to the interacting species (e.g. Koepke et al., 2010; Meinerset al., 2000; Mumm et al., 2005). Induction of sesquiterpenesplays a role in several plant–egg interactions where plants aredicotyledonous, whereas eggs laid on a monocotyledonousgrass species did not affect the terpenoid emission, but reducedemission of a green leaf volatile compound (see Section 3.2.1).However, further systems need to be studied when trying todetect common patterns. The high species specificity found sofar requires specificity of the elicitor (the eggs with their secre-tions touching the plant tissue) and of the responding mecha-nisms of the plant.

2. What happens between detection of eggs by a plant and changeof transcription of plant defence genes? We know in someplant–insect egg systems the elicitor of egg-induced defenceand the genes affected with respect to transcription, yet we can-not answer this question. Neither on the cellular nor on a sys-temic level we know how the signal ‘‘an egg has been laid’’ isconveyed inside the plant. The role of phytohormones in plantresponses to insect eggs is puzzling. A few studies indicate thatJA is involved (Hilker et al., 2002a; Meiners and Hilker, 2000),

M. Hilker, T. Meiners / Phytochemistry 72 (2011) 1612–1623 1621

others suggest a significant role of SA (Bruessow et al., 2010). Across interaction between JA- and SA-mediated processes elic-ited by oviposition may generate a specific internal plant signal.The interplay of the hormones might depend on whether eggdeposition is associated with plant damage or not. Future stud-ies need to further address the plant signalling cascadesinduced by eggs.

3. What are the costs and benefits of a plant responding to insecteggs? Plants can get rid of insect eggs by detaching them (byhypersensitive response, formation of neoplasms), by poisoningthem (by an egg-induced plant ovicidal compound) or by killingthem with the help of attracted parasitoids. They may gain ben-efit from these defensive responses to eggs, as they reduce thenumber of larvae that would damage the plant if they wouldhatch from the eggs (see Section 3.2). However, knowledge onthe costs of these egg-induced plant defences is lacking. Sofar, only very few studies investigated egg-induced effects onphotosynthetic activity which is considered one currencyamong several others to measure costs of plant defence (Cipol-lini et al., 2003). Hence, we do not know yet whether plantsresponding to insect eggs ‘‘pay’’ later on by reduced growth,reproduction or defence against other organisms.

4. Do egg-induced plant responses affect later defence against lar-vae that hatched from surviving eggs? A plant might take eggson its leaves as warning of future larval herbivory and improveits defence against larvae (Section 3.2.3). In turn, feeding byadults prior to oviposition might alert the plant to future eggdeposition, and the plant may enhance defences against insecteggs.

Intensifying our research on these questions will further eluci-date the mechanisms and ecological effects of egg-induced plantresponses.

Acknowledgements

We thank Urte Kohlhoff, Freie Universität Berlin, for correctingour typing errors and reference list. We are grateful for the supportby the German Research Foundation (Deutsche Forschungsgeme-inschaft, DFG ME 1810/4-2, DFG Hi 416/17-1,2 and 416/22).

References

Alborn, H.T., Turlings, T.C.J., Jones, T.H., Stenhagen, G., Loughrin, J.H., Tumlinson, J.H.,1997. An elicitor of plant volatiles from beet armyworm oral secretion. Science276, 945–949.

Andersson, J., Borg-Karlson, A.-K., Wiklund, C., 2000. Sexual cooperation and conflictin butterflies: a male-transferred anti-aphrodisiac reduces harassment ofrecently mated females. Proc. R. Soc. Lond. B Biol. Sci. 267, 1271–1275.

Balbyshev, N.F., Lorenzen, J.H., 1997. Hypersensitivity and egg drop, a novelmechanism of host–plant resistance to Colorado potato beetle (Coleoptera:Chrysomelidae). J. Econ. Entomol. 90, 652–657.

Beyaert, I., Wäschke, N., Scholz, A., Varama, M., Reinecke, A., Hilker, M., 2010.Relevance of resource-indicating key volatiles and habitat odour for insectorientation. Anim. Behav. 79, 1077–1086.

Blum, M.S., Hilker, M., 2002. Chemical protection of insect eggs. In: Hilker, M.,Meiners, T. (Eds.), Chemoecology of Insect Eggs and Egg Deposition. BlackwellPublishing Ltd., Berlin, Germany, pp. 61–90.

Boppré, M., 1990. Lepidoptera and pyrrolizidine alkaloids. Exemplification ofcomplexity in chemical ecology. J. Chem. Ecol. 16, 165–185.

Bruce, T.J.A., Midega, C.A.O., Birkett, M.A., Pickett, J.A., Khan, Z.R., 2010. Is qualitymore important than quantity? Insect behavioural responses to changes in avolatile blend after stemborer oviposition on an African grass. Biol. Lett. 6, 314–317.

Bruessow, F., Gouhier-Darimont, C., Buchala, A., Metraux, J.-P., Reymond, P., 2010.Insect eggs suppress plant defence against chewing herbivores. Plant J. 62, 876–885.

Chen, F., Tholl, D., D’Auria, J., Farooq, A., Pichersky, E., Gershenzon, J., 2003.Biosynthesis and emission of terpenoid volatiles from Arabidopsis flowers. PlantCell 15, 481–494.

Choh, Y., Takabayashi, J., 2006. Herbivore-induced extrafloral nectar production inlima bean plants enhanced by previous exposure to volatiles from infestedconspecifics. J. Chem. Ecol. 32, 2073–2077.

Cipollini, D., Purrington, C.B., Bergelson, J., 2003. Costs of induced responses inplants. Basic Appl. Ecol. 4, 79–85.

Colazza, S., Fucarino, A., Peri, E., Salerno, G., Conti, E., Bin, F., 2004a. Insectoviposition induces volatile emission in herbaceous plants that attracts eggparasitoids. J. Exp. Biol. 207, 47–53.

Colazza, S., McElfresh, J.S., Millar, J.G., 2004b. Identification of volatile synomones,induced by Nezara viridula feeding and oviposition on bean spp., that attract theegg parasitoid Trissolcus basalis. J. Chem. Ecol. 30, 945–964.

Conti, E., Zadra, C., Salerno, G., Leombruni, B., Volpe, D., Frati, F., Marucchini, C., Bin,F., 2008. Changes in the volatile profile of Brassica oleracea due to feeding andoviposition by Murgantia histrionica (Heteroptera: Pentatomidae). Eur. J.Entomol. 105, 839–847.

Conrath, U., Beckers, G.J.M., Flors, V., Garcia-Agustin, P., Jakab, G., Mauch, F.,Newman, M.A., Pieterse, C.M.J., Poinssot, B., Pozo, M.J., Pugin, A., Schaffrath, U.,Ton, J., Wendehenne, D., Zimmerli, L., Mauch-Mani, B., Prime, A.P.G., 2006.Priming: getting ready for battle. Mol. Plant Microbe Interact. 19, 1062–1071.

Cook, M.S., Khan, Z.R., Pickett, J.A., 2007. The use of push–pull strategies inintegrated pest management. Annu. Rev. Entomol. 52, 375–400.

Cooper, L.D., Doss, R.P., Price, R., Peterson, K., Oliver, J.E., 2005. Application ofbruchin B to pea pods results in the up-regulation of CYP93C18, a putativeisoflavone synthase gene, and an increase in the level of pisatin, an isoflavonephytoalexin. J Exp Bot 56, 1229–1237.

Delaney, K.J., 2008. Injured and uninjured leaf photosynthetic responses aftermechanical injury on Nerium oleander leaves, and Danaus plexippus herbivory onAsclepias curassavica leaves. Plant Ecol. 199, 187–200.

De Moraes, C.M., Mescher, M.C., Tumlinson, J.H., 2001. Caterpillar induced nocturnalplant volatiles repel conspecific females. Nature 410, 577–580.

Deng, J.Y., Wei, H.Y., Huang, Y.P., Du, J.W., 2004. Enhancement of attraction to sexpheromones of Spodoptera exigua by volatile compounds produced by hostplants. J. Chem. Ecol. 30, 2037–2045.

Dicke, M., Baldwin, I.T., 2010. The evolutionary context for herbivore-induced plantvolatiles: beyond the ‘‘cry for help’’. Trends Plant Sci. 15, 167–175.

Dickens, J.C., 1989. Green leaf volatiles enhance aggregation pheromone of the bollweevil Anthonomus grandis. Entomol. Exp. Appl. 52, 191–203.

Doss, R.P., 2005. Treatment of pea pods with bruchin B results in up-regulation of agene similar to MtN19. Plant Physiol. Biochem. 43, 225–231.

Doss, R.P., Oliver, J.E., Proebsting, W.M., Potter, S.W., Kuy, S.R., Clement, S.L.,Williamson, R.T., Carney, J.R., Devilbiss, E.D., 2000. Bruchins–Insect-derivedplant regulators that stimulate neoplasm formation. Proc. Natl. Acad. Sci. USA97, 6218–6223.

Eisner, T., Rossini, C., González, A., Iyengar, V.K., Siegler, M.V.S., Smedley, S.R., 2002.Paternal investment in egg defense. In: Hilker, M., Meiners, T. (Eds.),Chemoecology of Insect Eggs and Egg Deposition. Blackwell Publishing Ltd.,Berlin, Germany, pp. 91–116.

Ekbom, B., Popov, S.Y.A., 2004. Host plant affects pollen beetle (Meligethes aeneus)egg size. Physiol. Entomol. 29, 118–122.

Engelberth, J., Alborn, H.T., Schmelz, E.A., Tumlinson, J.H., 2004. Airborne signalsprime plants against insect herbivore attack. Proc. Natl. Acad. Sci. USA 101,1781–1785.

Engelberth, J., Koch, T., Kühnemann, F., Boland, W., 2000. IonenkanalbildendePeptaibole sind hochwirksame Elicitoren des pflanzlichenSekundärstoffwechsels und der Rankenkrümmung. Angew. Chem. 112, 1928–1930.

Fatouros, N.E., Broekgaarden, C., Bukovinszkine’Kiss, G., Van Loon, J.J.A., Mumm, R.,Huigens, M.E., Dicke, M., Hilker, M., 2008a. Male-derived butterfly anti-aphrodisiac mediates induced indirect plant defense. Proc. Natl. Acad. Sci.USA 105, 10033–10038.

Fatouros, N.E., Bukovinszkine’Kiss, G., Kalkers, L.A., Soler Gamborena, R., Dicke, M.,Hilker, M., 2005. Plant synomone induced by butterfly eggs arrestsTrichogramma wasps. Entomol. Exp. Appl. 115, 207–215.

Fatouros, N.E., Dicke, M., Mumm, R., Meiners, T., Hilker, M., 2008b. Foragingbehavior of egg parasitoids exploiting chemical information. Behav. Ecol. 19,677–685.

Ferguson, J.E., Metcalf, R.L., Fischer, D.C., 1985. Disposition and fate of cucurbitacin Bin five species of Diabroticites. J. Chem. Ecol. 11, 1307–1321.

Fox, C.W., Czesak, M.E., 2000. Evolutionary ecology of progeny size in arthropods.Annu. Rev. Entomol. 45, 341–369.

Frost, C.J., Mescher, M.C., Carlson, J.E., De Moraes, C.M., 2008. Plant defense primingagainst herbivores: getting ready for a different battle. Plant Physiol. 146, 818–824.

Geiselhardt, S., Otte, T., Hilker, M., 2009. The role of cuticular hydrocarbons in malemating behavior of the mustard leaf beetle, Phaedon cochleariae (F.). J. Chem.Ecol. 35, 1162–1171.

Hartmann, T., Theuring, C., Beuerle, T., Ernst, L., Singer, M.S., Bernays, E.A., 2004.Acquired and partially de novo synthesized pyrrolizidine alkaloids in twopolyphagous arctiids and the alkaloid profiles of their larval food-plants. J.Chem. Ecol. 30, 229–254.

Hartmann, T., 1995a. Pyrrolizidine alkaloids between plants and insects: a newchapter of an old story. Chemoecology 5, 139–146.

Hartmann, T., 1995b. Alkaloids: Chemistry, biology and chemoecology of thepyrrolizidine alkaloids. In: Pelletier, S.W. (Ed.), Alkaloids: Chemical andBiological Perspectives. Elsevier, New York, pp. 155–232.

1622 M. Hilker, T. Meiners / Phytochemistry 72 (2011) 1612–1623

Hartmann, T., 1999. Chemical ecology of pyrrolizidine alkaloids. Planta 207, 483–495.

Hee, A.K.W., Tan, K.H., 2004. Male sex pheromonal components derived frommethyl eugenol in the hemolymph of the fruit fly Bactrocera papayae. J. Chem.Ecol. 30, 2127–2138.

Hee, A.K.W., Tan, K.H., 2006. Transport of methyl eugenol-derived sex pheromonalcomponents in the male fruit fly, Bactrocera dorsalis. Comp. Biochem. Physiol. C:Toxicol. Pharmacol. 143, 422–428.

Heil, M., Kost, C., 2006. Priming of indirect defences. Ecol. Lett. 9, 813–817.Heil, M., Ton, J., 2008. Long-distance signalling in plant defence. Trends Plant Sci. 13,

264–272.Herms, D.A., Mattson, W.J., 1992. The dilemma of plants: to grow or defend. Quart.

Rev. Biol. 67, 283–335.Hilker, M., Bläske, V., Kobs, C., Dippel, C., 2000. Kairomonal effects of sawfly sex

pheromones on egg parasitoids. J. Chem. Ecol. 26, 2591–2601.Hilker, M., Kobs, C., Schrank, K., Varama, M., 2002a. Insect egg deposition induces

Pinus to call for egg parasitoids. J. Exp. Biol. 205, 455–461.Hilker, M., Meiners, T., 2006. Early herbivore alert: insect eggs induce plant defense.

J. Chem. Ecol. 32, 1379–1397.Hilker, M., Meiners, T., 2010. How do plants ‘‘notice’’ attack by herbivorous

arthropods? Biol. Rev. 85, 267–280.Hilker, M., Rohfritsch, O., Meiners, T., 2002b. The plant’s response towards insect

egg deposition. In: Hilker, M., Meiners, T. (Eds.), Chemoecology of Insect Eggsand Egg Deposition. Blackwell Publishing Ltd., Berlin, Germany, pp. 205–233.

Hilker, M., Stein, C., Schroeder, R., Varama, M., Mumm, R., 2005. Insect eggdeposition induces defence responses in Pinus sylvestris: characterization of theelicitor. J. Exp. Biol. 208, 1849–1854.

Karban, R., Baldwin, I.T., 1997. Induced Responses to Herbivory. The University ofChicago Press, Chicago.

Kessler, D., Diezel, C., Baldwin, I.T., 2010. Changing pollinators as a means ofescaping herbivores. Curr. Biol. 20, 237–242.

Koepke, D., Beyaert, I., Gershenzon, J., Hilker, M., Schmidt, A., 2010. Species-specificresponses of pine sesquiterpene synthases to sawfly oviposition.Phytochemistry 71, 909–917.

Koepke, D., Schroeder, R., Fischer, H.M., Gershenzon, J., Hilker, M., Schmidt, A., 2008.Does egg deposition by herbivorous pine sawflies affect transcription ofsesquiterpene synthases in pine? Planta 228, 427–438.

Landolt, P.J., Phillips, T.W., 1997. Host plant influences on sex pheromone behaviorof phytophagous insects. Annu. Rev. Entomol. 42, 371–391.

Langan, A.M., Wheater, C.P., Dunleavy, P.L., 2001. Does the smale white butterfly(Pieris rapae L.) aggregate eggs on plants with greater gas exchange activity? J.Insect Behav. 14, 459–468.

Light, D.M., Flath, R.A., Buttery, R.G., Zalom, F.G., Rice, R.E., Dickens, J.C., Jang, E.B.,1993. Host–plant green-leaf volatiles synergize the synthetic sex pheromonesof the corn earworm and codling moth (Lepidoptera). Chemoecology 4, 145–152.

Little, D., Gouhier-Darimont, C., Bruessow, F., Reymond, P., 2007. Oviposition bypierid butterflies triggers defense responses in Arabidopsis. Plant Physiol. 143,784–800.

Loaiza, J.C.M., Cespedes, C.L., Beuerle, T., Theuring, C., Hartmann, T., 2007.Ceroplastes albolineatus, the first scale insect shown to sequester pyrrolizidinealkaloids from its host–plant Pittocaulon praecox. Chemoecology 17, 109–115.

Meiners, T., Hacker, N., Anderson, P., Hilker, M., 2005. Response of the elm leafbeetle to host plants induced by oviposition and feeding: the infestation ratematters. Entomol. Exp. Appl. 115, 171–177.

Meiners, T., Hilker, M., 2000. Induction of plant synomones by oviposition of aphytophagous insect. J. Chem. Ecol. 26, 221–232.

Meiners, T., Westerhaus, C., Hilker, M., 2000. Specificity of chemical cues used by aspecialist egg parasitoid during host location. Entomol. Exp. Appl. 95, 151–159.

Mumm, R., Hilker, M., 2005. The significance of background odour for an eggparasitoid. Chem. Senses 30, 1–7.

Mumm, R., Schrank, K., Wegener, R., Schulz, S., Hilker, M., 2003. Chemical analysis ofvolatiles emitted by Pinus sylvestris after induction by insect oviposition. J.Chem. Ecol. 29, 1235–1252.

Mumm, R., Tiemann, T., Varama, M., Hilker, M., 2005. Choosy egg parasitoids:specificity of oviposition-induced pine volatiles exploited by an egg parasitoidof pine sawfly. Entomol. Exp. Appl. 115, 217–225.

Nabity, P.D., Zavala, J.A., Delucia, E.H., 2009. Indirect suppression ofphotosynthesis on individual leaves by arthropod herbivory. Ann. Bot. 103,655–663.

Nishida, R., Fukami, H., 1989. Ecological adaptation of an Aristolochiaceae-feedingswallowtail butterfly, Atrophaneura alcinous, to aristolochic acids. J. Chem. Ecol.15, 2549–2563.

Paré, P.W., Farag, M.A., Krishnamachari, V., Zhang, H.M., Ryu, C.M., Kloepper, J.W.,2005. Elicitors and priming agents initiate plant defense responses. Photosyn.Res. 85, 149–159.

Party, V., Hanot, C., Said, I., Rochat, D., Renou, M., 2009. Plant terpenes affectintensity and temporal parameters of pheromone detection in a moth. Chem.Senses 34, 763–774.

Pasteels, J.M., Daloze, D., Rowell-Rahier, M., 1986. Chemical defence in chrysomelideggs and neonate larvae. Physiol. Entomol. 11, 29–37.

Peiffer, M., Tooker, J.F., Luthe, D.S., Felton, G.W., 2009. Plants on early alert:glandular trichomes as sensors for insect herbivores. New Phytol. 184, 644–656.

Raina, A.K., Kingan, T.G., Mattoo, A.K., 1992. Chemical signals from host plant andsexual behavior in a moth. Science 255, 592–594.

Ramos, C., Vanin, S., Kato, M., 2009. Sequestration of prenylated benzoic acid andchromenes by Naupactus bipes (Coleoptera: Curculionidae) feeding on Pipergaudichaudianum (Piperaceae). Chemoecology 19, 73–80.

Reddy, G.V.P., Guerrero, A., 2004. Interactions of insect pheromones and plantsemiochemicals. Trends Plant Sci. 9, 253–261.

Reifenrath, K., Riederer, M., Müller, C., 2005. Leaf surface wax layers of Brassicaceaelack feeding stimulants for Phaedon cochleariae. Entomol. Exp. Appl. 115,41–50.

Reinecke, A., Ruther, J., Hilker, M., 2002. The scent of food and defense: green leafvolatiles and toluquinone as sex attractant mediate mate finding in theEuropean cockchafer Melolontha melolontha. Ecol. Lett. 5, 257–263.

Reisenman, C.E., Riffell, J.A., Bernays, E., Hildebrand, J.G., 2010. Antagonistic effectsof floral scent in an insect–plant interaction. Proc. R. Soc. Lond. B Biol. Sci. 277,2371–2379.

Reisenman, C.E., Riffell, J.A., Hildebrand, J.G., 2009. Neuroethology of ovipositionbehavior in the moth Manduca sexta. Ann. N. Y. Acad. Sci. 1170, 462–467.

Reudler Talsma, J.H., Biere, A., Harvey, J.A., van Nouhuys, S., 2008. Oviposition cuesfor a specialist butterfly: plant chemistry and size. J. Chem. Ecol. 34, 1202–1212.

Roeske, C.N., Seiber, J.N., Brower, L.P., Moffit, C.M., 1976. Milkweed cardenolides andtheir comparative processing by monarch butterflies (Danaus plexippus L.). Rec.Adv. Phytochem. 10, 93–167.

Ruther, J., Reinecke, A., Thiemann, K., Tolasch, T., Francke, W., Hilker, M., 2000. Matefinding in the forest cockchafer, Melolontha hippocastani, mediated by volatilesfrom plants and females. Physiol. Entomol. 25, 172–179.

Ruther, J., Meiners, T., Steidle, J.L.M., 2002. Rich in phenomena lacking in terms – aclassification of kairomones. Chemoecology 12, 161–167.

Schaffner, U., Boevé, J.L., Gfeller, H., Schlunegger, U.P., 1994. Sequestration ofVeratrum alkaloids by specialist Rhadinoceraea nodicornis Konow(Hymenoptera, Tenthredinidae) and its ecoethological implications. J. Chem.Ecol. 20, 3233–3250.

Schuepp, P.H., 1993. Leaf boundary layers. New Phytol. 125, 477–507.Schroeder, R., Forstreuter, M., Hilker, M., 2005. A plant notices insect egg deposition

and changes its rate of photosynthesis. Plant Physiol. 138, 470–477.Schroeder, R., Cristescu, S.M., Harren, F.J.M., Hilker, M., 2007. Reduction of ethylene

emission from Scots pine elicited by insect egg secretion. J Exp Bot 58, 1835–1842.

Schroeder, R., Wurm, L., Varama, M., Meiners, T., Hilker, M., 2008. Unusualmechanisms involved in learning of oviposition-induced host plant odours inan egg parasitoid? Anim. Behav. 75, 1423–1430.

Seino, Y., Suzuki, Y., Sogawa, K., 1996. An ovicidal substance produced by rice plantsin response to oviposition by the whitebacked planthopper, Sogatella furcifera(Horvath) (Homoptera: Delphacidae). Appl. Entomol. Zool. 31, 467–473.

Shapiro, A.M., DeVay, J.E., 1987. Hypersensitivity reaction of Brassica nigra L.(Cruciferae) kills eggs of Pieris butterflies (Lepidoptera: Pieridae). Oecologia 71,631–632.

Shelly, T.E., Nishida, R., 2004. Larval and adult feeding on methyl eugenol and themating success of male oriental fruit flies, Bactrocera dorsalis. Entomol. Exp.Appl. 112, 155–158.

Stange, G., 1997. Effects of changes in atmospheric carbon dioxide on the location ofhosts by the moth, Cactoblastis cactorum. Oecologia 110, 539–545.

Stange, G., Monro, J., Stowe, S., Osmond, C.B., 1995. The CO2 sense of the mothCactoblastis cactorum and its probable role in the biological control of the CAMplant Opuntia stricta. Oecologia 102, 341–352.

Städler, E., 1986. Oviposition and feeding stimuli in leaf surface waxes. In: Juniper,B.E., Southwoud, T.R.E. (Eds.), Insects and the Plant Surface. Edward Arnold,London, pp. 105–121.

Städler, E., 2002. Plant chemicals important for egg deposition in herbivorousinsects. In: Hilker, M., Meiners, T. (Eds.), Chemoecology of Insect Eggs and EggDeposition. Blackwell Publishing Ltd., Berlin, pp. 171–197.

Städler, E., Reifenrath, K., 2009. Glucosinolates on the leaf surface perceived byinsect herbivores: review of ambiguous results and new investigations.Phytochem. Rev. 8, 207–225.

Suzuki, Y., Sogawa, K., Seino, Y., 1996. Ovicidal reaction of rice plants against thewhitebacked planthopper, Sogatella furcifera Horvath (Homoptera:Delphacidae). Appl. Entomol. Zool. 31, 111–118.

Thom, C., Guerenstein, P.G., Mechaber, W.L., Hildebrand, J.G., 2004. Floral CO2

reveals flower profitability to moths. J. Chem. Ecol. 30, 1285–1288.Trigo, J.R., Brown, K.S., Witte, L., Hartmann, T., Ernst, L., Barata, L.E.S., 1996.

Pyrrolizidine alkaloids: different acquisition and use patterns in Apocynaceaeand Solanaceae feeding ithomiine butterflies (Lepidoptera: Nymphalidae). Biol.J. Linn. Soc. 58, 99–123.

Tooker, J.F., De Moraes, C.M., 2005. Jasmonate in lepidopteran eggs and neonates. J.Chem. Ecol. 31, 2753–2759.

Tooker, J.F., De Moraes, C.M., 2007. Jasmonate, salicylate, and benzoate in insecteggs. J. Chem. Ecol. 33, 331–343.

Velikova, V., Salerno, V., Frati, F., Peri, E., Conti, E., Colazza, S., Loreto, F., 2010.Influence of feeding and oviposition by phytophagous pentatomids onphotosynthesis of herbaceous plants. J. Chem. Ecol. 36, 629–641.

Wegener, R., Schulz, S., Meiners, T., Hadwich, K., Hilker, M., 2001. Analysis ofvolatiles induced by oviposition of elm leaf beetle Xanthogaleruca luteola onUlmus minor. J. Chem. Ecol. 27, 499–515.

Welter, S.C., 1989. Arthropod impact on plant gas exchange. In: Bernays, E.A. (Ed.),Insect–Plant Interactions. CRC Press, Boca Raton, FL, pp. 135–151.

Woods, H.A., 2010. Water loss and gas exchange by eggs of Manduca sexta: tradingoff costs and benefits. J. Insect Physiol. 56, 480–487.

M. Hilker, T. Meiners / Phytochemistry 72 (2011) 1612–1623 1623

Zangerl, A.R., Hamilton, J.G., Miller, T.J., Crofts, A.R., Oxborough, K., Berenbaum, M.R.,de Lucia, E.H., 2002. Impact of folivory on photosynthesis is greater than thesum of its holes. Proc. Natl. Acad. Sci. USA 99, 1088–1091.

Monika Hilker studied biology and chemistry at theUniversity of Goettingen, Germany. After her diplomathesis on bark beetle chemical ecology, she started herPh.D. studies on infochemicals influencing insect eggdeposition. She finished her Ph.D. studies in 1986 (Dr.rer. nat.) and took a position as Assistant Professor atthe University of Bayreuth in Bavaria; here she focusedon chemical ecology of the juvenile stages of leaf beetles(Chrysomelidae). In 1993, she earned her habilitation inZoology (Dr. habil.). Since 1994, she is Full Professor andhead of the Department of Applied Zoology/AnimalEcology at the Freie Universitaet Berlin. Her major

interest is in the field of chemoecology of plant–insect interactions, and she putsspecial emphasis on eggs of herbivorous insects and factors affecting insect eggdeposition. Currently, she is President of the International Society of Chemical

Ecology and serves as Associate Editor of the Journal of Chemical Ecology.

Torsten Meiners studied biology at the University ofWuerzburg, Germany, and finished his diploma thesison malachiid beetle behavioural ecology in 1994. Hestarted his Ph.D. studies on infochemicals in plant–herbivore–parasitoid interactions at the Freie Universi-taet Berlin in 1995 and finished them with a Dr. rer. nat.in the beginning of 1999. From 1999 to end of 2000, heworked first as post-doc on the ecology of ticks at theHumboldt Universitaet Berlin. Later, he went for a post-doc to the USA where he studied odour learningcapacities of parasitoids at the University of Georgia.Furthermore, he stayed for six months at the Chemical

Ecology lab of the Swedish University of Agricultural Sciences in Alnarp where heworked on insect electrophysiology and behaviour. In the end of 2000, he returnedto Berlin and took a position as Assistant Professor at the Freie Universitaet Berlin;

here he works at the Department of Applied Zoology/Animal Ecology on theChemical Ecology of multitrophic interactions. His special research interests are theroles of chemical and structural diversity of the vegetation and of induced plantdefences on host and host plant location strategies of herbivores and their para-sitoids. He is member of the International Society of Chemical Ecology and Asso-ciate Editor of BioControl.