The influence of species identity and herbivore feeding mode on top-down and bottom-up effects in a salt marsh system

In this study we investigated the potential importance of species identity and herbivore feeding mode in determining the strengths of top-down and bottom-up effects on phytophagous insect densities. In 1998, we conducted two factorial field experiments in which we manipulated host plant quality and intensity of parasitoid attack on three salt marsh herbivores, the planthoppers Prokelisia marginata and Pissonotus quadripustulatus (Homoptera: Delphacidae), which feed only on Spartina alterniflora and Borrichia frutescens, respectively, and the gall fly Asphondylia borrichiae (Diptera: Cecidomyiidae), which feeds only on B. frutescens. We increased plant quality through addition of nitrogen fertilizer, and decreased parasitism by trapping hymenopteran parasitoids continuously throughout the study. Herbivore densities were censused biweekly. Increasing plant quality through fertilization increased the density of all three herbivores within 2 weeks of treatment application, and higher densities were maintained for the duration of the study. Reduction of top-down pressure had no effect on either planthopper species, possibly because of compensatory mortality affecting the two species. In contrast, reduction of parasitism significantly increased the density of A. borrichiae galls, perhaps because development within gall tissue reduces the sources of compensatory mortality affecting this species. The results of this study show that the bottom-up effects of plant quality were strong and consistent for all three species, but the strength of top-down effects differed between the two feeding guilds. Thus, even for herbivores feeding on the same host plant, conclusions drawn regarding the relative importance of top-down and bottom-up effects may vary depending upon the feeding mode of the herbivore.     

Trophic cascades initiated by fungal plant endosymbionts impair reproductive performance of parasitoids in the second generation

Variation in plant quality can transmit up the food chain and may affect herbivores and their antagonists in the same direction. Fungal endosymbionts of grasses change the resource quality by producing toxins. We used an aphid-parasitoid model system to explore how endophyte effects cascade up the food chain and influence individual parasitoid performance. We show that the presence of an endophyte in the grass Lolium perenne has a much stronger negative impact on the performance of the parasitoid Aphidius ervi than on its aphid host Metopolophium festucae. Although the presence of endophytes did not influence the parasitism rate of endophyte-naïve parasitoids or their offspring’s survival to adulthood, most parasitoids developing within aphids from endophyte-infected plants did not reproduce at all. This indicates a delayed but very strong effect of endophytes on parasitoid performance, which should ultimately affect plant performance negatively by releasing endophyte-tolerant herbivores from top-down limitations.     

Root and shoot jasmonic acid induced plants differently affect the performance of Bemisia tabaci and its parasitoid Encarsia formosa

Plants employ both direct and indirect defenses to protect themselves from attacks by herbivores and pathogens. To date most aboveground and belowground interaction studies have focused on interactions between plants and leaf-chewing herbivores, while the plant defence on the performance of phloem-feeding insects, induced by above- and belowground interaction, has been less explored. Here, jasmonic acid (JA) was used to mimic herbivore-induced responses in Chinese broccoli (Brassica oleracea var. alboglabra) roots (RJA) and shoots (SJA). The effects of JA-induced plant defenses on the performance of the phloem-feeding whitefly, Bemisia tabaci, and its aphelinid parasitoid Encarsia formosa were investigated. The results indicated that SJA induction has a much larger negative effect on B. tabaci than RJA: nymphs develop slower and have a lower survivorship. Also, females live shorter and have a lower fecundity on SJA plants compared to those on RJA and untreated control (CON) plants. The intrinsic rate of increase (r_m) of B. tabaci on SJA plants was 0.089, which was significantly lower than those on CON and RJA plants (0.115 and 0.104, respectively). The parasitoid E. formosa, on the other hand, shows a significantly faster development when parasitizing whitefly hosts feeding on SJA plants, whereas parasitism rate, longevity and fecundity were similar to those on RJA and CON plants. The current study reveals that plants induced with exogenous JA vary in both their resistance to whitefly and suitability for parasitoids, depending on the organ to which the JA was applied. Root and shoot JA applications also have contrasting effects on the phloem-feeding insect B. tabaci and its parasitoid, that is, SJA induction leads to more negative effects on whitefly performance than RJA induction, but its parasitoid performs better on hosts reared on SJA plants. These results show that top-down and bottom-up processes governing herbivore populations on Chinese broccoli are working in concert to increase plant resistance when plants are induced by SJA application.     

Parasitoid genus‐specific manipulation of orb‐web host spiders (Araneae,Araneidae)

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A tritrophic approach to the preference–performance hypothesis involving an exotic and a native plant

Exotic plants often generate physical and chemical changes in native plant communities where they become established. A major challenge is to understand how novel plants may affect trophic interactions in their new habitats, and how native herbivores and their natural enemies might respond to them. We compared the oviposition preference and offspring performance of the crucifer specialist, Pieris brassicae, on an exotic plant, Bunias orientalis, and on a related native plant, Sinapis arvensis. Additionally, we studied the response of the parasitoid, Cotesia glomerata to herbivore-induced plant volatiles (HIPV) and determined the volatile blend composition to elucidate which compound(s) might be involved in parasitoid attraction. On both host plants we also compared the parasitism rate of P. brassicae by C. glomerata. Female butterflies preferred to oviposit on the native plant and their offspring survival and performance was higher on the native plant compared to the exotic. Although, headspace analysis revealed qualitative and quantitative differences in the volatile blends of both plant species, C. glomerata did not discriminate between the HIPV blends in flight-tent bioassays. Nevertheless, parasitism rate of P. brassicae larvae was higher on the native plant under semi-field conditions. Overall, P. brassicae oviposition preference may be more influenced by bottom-up effects of the host plant on larval performance than by top-down pressure exerted by its parasitoid. The potential for dietary breadth expansion of P. brassicae to include the exotic B. orientalis and the role of top-down processes played by parasitoids in shaping herbivore host shifts are further discussed.     

Differential induction of plant chemical defenses by parasitized and unparasitized herbivores: consequences for reciprocal,multitrophic interactions

Insect parasitoids can play ecologically important roles in virtually all terrestrial plant–insect herbivore interactions, yet whether parasitoids alter the defensive traits that underlie interactions between plants and their herbivores remains a largely unexplored question. Here, we examined the reciprocal trophic interactions among populations of the wild cabbage Brassica oleracea that vary greatly in their production of defensive secondary compounds – glucosinolates (GSs), a generalist herbivore, Trichoplusia ni, and its polyembryonic parasitoid Copidosoma floridanum. In a greenhouse environment, plants were exposed to either healthy (unparasitized), parasitized, or no herbivores. Feeding damage by herbivores induced higher levels of the indole GSs, glucobrassicin and neoglucobrassicin, but not any of the other measured GSs. Herbivores parasitized by C. floridanum induced cabbage plants to produce 1.5 times more indole GSs than levels induced by healthy T. ni and five times more than uninduced plants. As a gregarious endoparasitoid, C. floridanum causes its host T. ni to feed more than unparasitized herbivores resulting in increased induction of indole GSs. In turn, herbivore fitness parameters (including differential effects on male and female contributions to lifetime fecundity in the herbivore) were negatively correlated with the aliphatic GSs, sinigrin and gluconapin, whereas parasitoid fitness parameters were negatively correlated with the indole GSs, glucobrassicin and neoglucobrassicin. That herbivores and their parasitoids appear to be affected by different sets of GSs was unexpected given the intimate developmental associations between host and parasitoid. This study is the first to demonstrate that parasitoids, through increasing feeding by their herbivorous hosts, can induce higher levels of non‐volatile plant chemical defenses. While parasitoids are widely recognized to be ubiquitous in most terrestrial insect herbivore communities, their role in influencing plant–insect herbivore relationships is still vastly underappreciated.     

Differential effects of flower feeding in an insect host–parasitoid system

In many insect host–parasitoid systems, both the host and its parasitoids forage on shared floral resources. As a result of insect behaviour, morphology and physiology, flower species may act selectively at different levels of such systems, e.g., between the trophic levels of hosts and parasitoids, between species within a guild, between sexes or individuals within a species or between life history traits within an individual. We asked if effects of selectivity are consistent across levels in the horse chestnut leafminer, Cameraria ohridella, and its parasitoid complex. Insects were exposed singly in no-choice feeding trials to twelve common flower species and their survival and reproduction were recorded. Only one of twelve flower species (Ranunculus acris) tended to selectively favour the longevity of leafminers, but not of parasitoids. No flower species were found to favour parasitoids only. Both trophic levels profited from feeding on Anthriscus sylvestris, however, parasitoids benefited up to eight times more than their hosts. No differences were found among the species of the parasitoid guild, but females lived significantly longer than males, and single individuals within species were able to exploit generally unfavourable flower species. Out of the seven flower species that increased the longevity of leafminer females, only Chaerophyllum hirsutum significantly enhanced the number of eggs laid. Fecundity was generally positively correlated with longevity of leafminer females, but two flower species (C. hirsutum, Taraxacum officinale) had an additional positive effect on fecundity. In conclusion, we demonstrated that flowers act differently on life history traits in a host–parasitoid system at a multitude of biological levels and that these effects are not always consistent across levels. Selective plant-derived resources can therefore modify herbivore–natural enemy interactions in ways that are more complex than currently appreciated.     

Complementary effects of resident natural enemies on the suppression of the introduced moth Epiphyas postvittana

Generalist predators may disrupt or complement biological control by parasitoids. Past studies have examined how predators and parasitoids interact to affect aphid suppression, but more information is needed from other host taxa. Here, we explore the interactive effects of a spider (Cheiracanthium mildei) and a generalist parasitoid (Meteorus ictericus) on the light brown apple moth (Epiphyas postvittana), a recent introduction to North America. The spider negatively affected adult parasitoids in a field experiment, and reduced numbers of parasitized larvae in the laboratory. Nonetheless, the combined effects of parasitoids and spiders on larval mortality of the moth were additive. Percent parasitism was not affected by the presence of the spider in field or laboratory experiments, and results were similar when single or multiple larval instars of the moth were included. The spider’s lack of prey preference for unparasitized or parasitized larvae likely precluded any disruptive effects on parasitism. Results suggest that resident generalist parasitoids and predators can work in conjunction to hinder the invasion success of a novel herbivore prey species.     

Plant‐mediated indirect effects and the persistence of parasitoid–herbivore communities

We have examined the effects of herbivore diversity on parasitoid community persistence and stability, mediated by nonspecific information from herbivore‐infested plants. First, we investigated host location and patch time allocation in the parasitoid Cotesia glomerata in environments where host and/or nonhost herbivores were present on Brassica oleracea leaves. Parasitoids were attracted by infochemicals from leaves containing nonhost herbivores. They spent considerable amounts of time on such leaves. Thus, when information from the plant is indistinct, herbivore diversity is likely to weaken interaction strengths between parasitoids and hosts. In four B. oleracea fields, all plants contained herbivores, often two or more species. We modelled parasitoid–herbivore communities increasing in complexity, based on our experiments and field data. Increasing herbivore diversity promoted the persistence of parasitoid communities. However, at a higher threshold of herbivore diversity, parasitoids became extinct due to insufficient parasitism rates. Thus, diversity can potentially drive both persistence and extinctions.     

Can hyperparasitoids cause large‐scale outbreaks of insect herbivores?

Synchronous population fluctuations occur in many species and have large economic impacts, but remain poorly understood. Dispersal, climate and natural enemies have been hypothesized to cause synchronous population fluctuations across large areas. For example, insect herbivores cause extensive forest defoliation and have many natural enemies, such as parasitoids, that may cause landscape‐scale changes in density. Between outbreaks, parasitoid‐caused mortality of hosts/herbivores is high, but it drops substantially during outbreak episodes. Because of their essential role in regulating herbivore populations, we need to include parasitoids in spatial modelling approaches to more effectively manage insect defoliation. However, classic host‐parasitoid population models predict parasitoid density, and parasitoid density is difficult to relate to host‐level observations of parasitoid‐caused mortality. We constructed a novel model to study how parasitoids affect insect outbreaks at the landscape scale. The model represents metacommunity dynamics, in which herbivore regulation, colonisation and extinction are driven by interactions with the forest, primary parasitoids and hyperparasitoids. The model suggests that parasitoid spatial dynamics can produce landscape‐scale outbreaks. Our results propose the testable prediction that hyperparasitoid prevalence should increase just before the onset of an outbreak because of hyperparasitoid overexploitation. If verified empirically, hyperparasitoid distribution could provide a biotic indicator that an outbreak will occur.     

Bottom-up and top-down herbivore regulation mediated by glucosinolates in Brassica oleracea var. acephala

Quantitative differences in plant defence metabolites, such as glucosinolates, may directly affect herbivore preference and performance, and indirectly affect natural enemy pressure. By assessing insect abundance and leaf damage rate, we studied the responses of insect herbivores to six genotypes of Brassica oleracea var. acephala, selected from the same cultivar for having high or low foliar content of sinigrin, glucoiberin and glucobrassicin. We also investigated whether the natural parasitism rate was affected by glucosinolates. Finally, we assessed the relative importance of plant chemistry (bottom-up control) and natural enemy performance (top-down control) in shaping insect abundance, the ratio of generalist/specialist herbivores and levels of leaf damage. We found that high sinigrin content decreased the abundance of the generalist Mamestra brassicae (Lepidoptera, Noctuidae) and the specialist Plutella xylostella (Lepidoptera, Yponomeutidae), but increased the load of the specialist Eurydema ornatum (Hemiptera, Pentatomidae). Plants with high sinigrin content suffered less leaf injury. The specialist Brevicoryne brassicae (Hemiptera, Aphididae) increased in plants with low glucobrassicin content, whereas the specialists Pieris rapae (Lepidoptera, Pieridae), Aleyrodes brassicae (Hemiptera, Aleyrodidae) and Phyllotreta cruciferae (Coleoptera, Chrysomelidae) were not affected by the plant genotype. Parasitism rates of M. brassicae larvae and E. ornatum eggs were affected by plant genotype. The ratio of generalist/specialist herbivores was positively correlated with parasitism rate. Although both top-down and bottom-up forces were seen to be contributing, the key factor in shaping both herbivore performance and parasitism rate was the glucosinolate concentration, which highlights the impact of bottom-up forces on the trophic cascades in crop habitats.     

Partitioning of herbivore hosts across time and food plants promotes diversification in the Megastigmus dorsalis oak gall parasitoid complex

Communities of insect herbivores and their natural enemies are rich and ecologically crucial components of terrestrial biodiversity. Understanding the processes that promote their origin and maintenance is thus of considerable interest. One major proposed mechanism is ecological speciation through host‐associated differentiation (HAD), the divergence of a polyphagous species first into ecological host races and eventually into more specialized daughter species. The rich chalcid parasitoid communities attacking cynipid oak gall wasp hosts are structured by multiple host traits, including food plant taxon, host gall phenology, and gall structure. Here, we ask whether the same traits structure genetic diversity within supposedly generalist parasitoid morphospecies. We use mitochondrial DNA sequences and microsatellite genotypes to quantify HAD for Megastigmus (Bootanomyia) dorsalis, a complex of two apparently generalist cryptic parasitoid species attacking oak galls. Ancient Balkan refugial populations showed phenological separation between the cryptic species, one primarily attacking spring galls, and the other mainly attacking autumn galls. The spring species also contained host races specializing on galls developing on different host‐plant lineages (sections Cerris vs. Quercus) within the oak genus Quercus. These results indicate more significant host‐associated structuring within oak gall parasitoid communities than previously thought and support ecological theory predicting the evolution of specialist lineages within generalist parasitoids. In contrast, UK populations of the autumn cryptic species associated with both native and recently invading oak gall wasps showed no evidence of population differentiation, implying rapid recruitment of native parasitoid populations onto invading hosts, and hence potential for natural biological control. This is of significance given recent rapid range expansion of the economically damaging chestnut gall wasp, Dryocosmus kuriphilus, in Europe.     

Plant Quantity Affects Development and Survival of a Gregarious Insect Herbivore and Its Endoparasitoid Wasp

Virtually all studies of plant-herbivore-natural enemy interactions focus on plant quality as the major constraint on development and survival. However, for many gregarious feeding insect herbivores that feed on small or ephemeral plants, the quantity of resources is much more limiting, yet this area has received virtually no attention. Here, in both lab and semi-field experiments using tents containing variably sized clusters of food plants, we studied the effects of periodic food deprivation in a tri-trophic system where quantitative constraints are profoundly important on insect performance. The large cabbage white Pieris brassicae, is a specialist herbivore of relatively small wild brassicaceous plants that grow in variable densities, with black mustard (Brassica nigra) being one of the most important. Larvae of P. brassicae are in turn attacked by a specialist endoparasitoid wasp, Cotesia glomerata. Increasing the length of food deprivation of newly molted final instar caterpillars significantly decreased herbivore and parasitoid survival and biomass, but shortened their development time. Moreover, the ability of caterpillars to recover when provided with food again was correlated with the length of the food deprivation period. In outdoor tents with natural vegetation, we created conditions similar to those faced by P. brassicae in nature by manipulating plant density. Low densities of B. nigra lead to potential starvation of P. brassicae broods and their parasitoids, replicating nutritional conditions of the lab experiments. The ability of both unparasitized and parasitized caterpillars to find corner plants was similar but decreased with central plant density. Survival of both the herbivore and parasitoid increased with plant density and was higher for unparasitized than for parasitized caterpillars. Our results, in comparison with previous studies, reveal that quantitative constraints are far more important that qualitative constraints on the performance of gregarious insect herbivores and their gregarious parasitoids in nature.     

Potential for climate effects on the size-structure of host–parasitoid indirect interaction networks

Communities of insect herbivores are thought to be structured mainly by indirect processes mediated by shared natural enemies, such as apparent competition. In host–parasitoid interaction networks, overlap in natural enemy communities between any pair of host species depends on the realized niches of parasitoids, which ultimately depend on the foraging decisions of individuals. Optimal foraging theory predicts that egg-limited parasitoid females should reject small hosts in favour of future opportunities to oviposit in larger hosts, while time-limited parasitoids are expected to optimize oviposition rate regardless of host size. The degree to which parasitoids are time- or egg-limited depends in part on weather conditions, as this determines the proportion of an individual''s lifespan that is available to foraging. Using a 10-year time series of monthly quantitative host–parasitoid webs, we present evidence for host-size-based electivity and sex allocation in the common secondary parasitoid Asaphes vulgaris. We argue that this electivity leads to body-size-dependent asymmetry in apparent competition among hosts and we discuss how changing weather patterns, as a result of climate change, may impact foraging behaviour and thereby the size-structure and dynamics of host–parasitoid indirect interaction networks.     

Influence of fungal isolates infecting tall fescue on multitrophic interactions

The epichloae are ascomycetous fungi in the genera Epichloë and Neotyphodium that live within grasses. Some of these fungi produce alkaloids that can help protect the host from herbivores. The alkaloids may also travel up the food web and affect members of the third trophic level. In this way they can produce trophic cascades which are rippling effects when a trophic level impacts those above or below it. We briefly summarize the general patterns of multitrophic effects of endophytes and highlight the most recent studies on this topic. Further, we report on our study in which we tested if different fungal strains in tall fescue (cultivar Jesup) affect multitrophic interactions involving aphids and their parasitoid natural enemies. Using both the common strain of N. coenophialum and a novel isolate (AR577), we allowed potted plants to be colonized by aphids and parasitoids in a semi-natural setting. We found that endophyte infection of tall fescue resulted in greater vegetative growth of the plant. We also found that N. coenophialum modified bottom-up cascades by depressing both aphid and parasitoid densities. Finally, we found that multitrophic effects were modified by fungal isolate: the common strain had stronger negative impacts on aphid and parasitoid densities than did the novel isolate.     

Ecosystem engineering and predation: the multi-trophic impact of two ant species

1. Ants are ubiquitous ecosystem engineers and generalist predators and are able to affect ecological communities via both pathways. They are likely to influence any other terrestrial arthropod group either directly or indirectly caused by their high abundance and territoriality. 2. We studied the impact of two ant species common in Central Europe, Myrmica rubra and Lasius niger, on an arthropod community. Colony presence and density of these two ant species were manipulated in a field experiment from the start of ant activity in spring to late summer. 3. The experiment revealed a positive influence of the presence of one ant colony on densities of decomposers, herbivores and parasitoids. However, in the case of herbivores and parasitoids, this effect was reversed in the presence of two colonies. 4. Generally, effects of the two ant species were similar with the exception of their effect on Braconidae parasitoid densities that responded positively to one colony of M. rubra but not of L. niger. 5. Spider density was not affected by ant colony manipulation, but species richness of spiders responded positively to ant presence. This effect was independent of ant colony density, but where two colonies were present, spider richness was significantly greater in plots with two M. rubra colonies than in plots with one colony of each ant species. 6. To test whether the positive ecosystem engineering effects were purely caused by modified properties of the soil, we added in an additional experiment (i) the soil from ant nests (without ants) or (ii) unmodified soil or (iii) ant nests (including ants) to experimental plots. Ant nest soil on its own did not have a significant impact on densities of decomposers, herbivores or predators, which were significantly, and positively, affected by the addition of an intact nest. 7. The results suggest an important role of both ant species in the grassland food web, strongly affecting the densities of decomposers, herbivores and higher trophic levels. We discuss how the relative impact via bottom-up and top-down effects of ants depends on nest density, with a relatively greater top-down predatory impact at higher densities.     

Manduca sexta experience high parasitoid pressures in the field but minor fitness costs of consuming plant secondary compounds

1. Plant‐herbivore coevolutionary interactions have led to a range of plant defenses that minimize insect damage and a suite of counter adaptations that allow herbivores to feed on defended plants. Consuming plant secondary compounds results in herbivore growth and developmental costs but can have beneficial effects such as deterrence or harm of parasitoid enemies. Therefore, the role of secondary compounds on herbivore fitness must be considered in the context of the abundance and level of harm from natural enemies and the costs herbivores incur feeding on plant secondary compounds.
2. In this study, I combined field measurements of Cotesia congregata wasp parasitism pressure with detailed measurements of the costs of plant secondary compounds across developmental stages in the herbivore host, Manduca sexta.
3. I show that C. congregata parasitoids exert large negative selective pressures, killing 31%–57% of M. sexta larvae in the field. Manduca sexta developed fastest during instars most at risk for parasitoid oviposition but growth was slowed by consumption of plant secondary compounds. The negative effects of consuming plant secondary compounds as larvae influenced adult size traits but there were no immune, survival, or fecundity costs.
4. These results suggest that developmental costs experienced by M. sexta herbivores consuming defensive compounds are minor in comparison to the strong negative survival pressures from abundant parasitoid enemies.

     

Wildflower companion plants increase pest parasitation and yield in cabbage fields: Experimental demonstration and call for caution

Monocultures typical of intensive agriculture offer ideal conditions to specialized herbivores while depriving their natural enemies of habitat and nutritional resources. The resulting release of herbivores from both bottom-up and top-down control causes pest outbreaks in economically important crops. Boosting locally occurring natural enemy populations through species-specific habitat management to restore natural herbivore control has been much advocated but remains rarely tested in the field. Here, we investigated whether adding specifically selected flowering plants to monocultures increases parasitation rates of herbivores and crop yield. We performed replicated field experiments in 2 years and found that adding cornflowers (Centaurea cyanus) into cabbage (Brassica oleracea) fields significantly increased larval and egg parasitation and egg predation of the herbivore, reduced herbivory rates, and increased crop biomass in at least 1 year. These findings show that addition of a single, well-chosen flowering plant species can significantly increase natural top-down pest control in monocultures but success is variable. This is relevant on two applied levels. First, well chosen companion plants may partially substitute pesticides in agriculture if the approach is optimized, reducing negative effects such as unspecific killing of non-target organisms, residues in food, contamination of soils and water-bodies and increasing pesticide resistances. Our results suggest that, from an agro-economical point of view, egg parasitoids or predators may be the best targets for habitat management because strong natural selection acts on larval parasitoids to keep their hosts alive for their own development. Second, the addition of non-crop vegetation to monocultures benefits biodiversity conservation directly through resource diversification and indirectly through the reduction of pesticide application that increased natural control makes possible.     

Parasitism on seed predators overcomes the detrimental effects of defoliation on plant fitness in a tritrophic system

It is widely recognised that the interaction between plants and herbivores cannot be completely understood if the natural enemies of the latter are not included. Most studies looking at the effects of herbivores and their enemies on plant fitness only consider one herbivore species or guild; however, plants in nature usually face the attack of more than one herbivore guild simultaneously and these herbivores may have a non-additive effect on the attraction with bodyguards and plant fitness. In this study, we asked whether folivory affects the activity of parasitoids on seed predators and whether this effect cascades down to plant fitness. We assessed these questions in a tritrophic system: the plant Ruellia nudiflora, its pre-dispersal seed predators and the parasitoids of the latter. Plants were submitted to either 50 % artificial defoliation or no defoliation (control). The number of seeds, fruit production and parasitoid incidence was assessed periodically in both sets of plants. Parasitoids indirectly and positively affected seed number, while defoliation had a direct negative effect on the number of seeds and an indirect negative effect on parasitoid incidence. However, the combined effect of defoliation and seed predation increased the indirect positive effect of the parasitoids on seed production, which overcame the negative effects of defoliation.     

Effects of glucosinolates on a generalist and specialist leaf-chewing herbivore and an associated parasitoid

Glucosinolates (GLS) are secondary plant metabolites that as a result of tissue damage, for example due to herbivory, are hydrolysed into toxic compounds that negatively affect generalist herbivores. Specialist herbivores have evolved specific adaptations to detoxify GLS or inhibit the formation of toxic hydrolytic products. Although rarely studied, GLS and their breakdown products may also affect parasitoids. The objectives were to test the effects of GLS in a multitrophic system consisting of the generalist herbivore Spodoptera exigua, the specialist herbivore Pieris rapae, and the endoparasitoid Hyposoter ebeninus. Three ecotypes of Arabidopsis thaliana that differ in their GLS composition and concentrations and one transformed line that constitutively produces higher concentrations of aliphatic GLS were used, the latter allowing a direct assessment of the effects of aliphatic GLS on insect performance.Feeding by the generalist S. exigua and the specialist P. rapae induced both higher aliphatic and indole GLS concentrations in the A. thaliana ecotypes, although induction was stronger for indole than aliphatic GLS. For both herbivores a negative correlation between performance and aliphatic GLS concentrations was observed. This suggests that the specialist, despite containing a nitrile-specifier protein (NSP) that diverts GLS degradation from toxic isothiocyanates to less toxic nitriles, cannot completely inhibit the formation of toxic GLS hydrolytic products, or that the costs of this mechanism are higher at higher GLS concentrations. Surprisingly, performance of the parasitoid was positively correlated with higher concentrations of aliphatic GLS in the plant, possibly caused by negative effects on host immune responses. Our study indicates that GLS can not only confer resistance against herbivores directly, but also indirectly by increasing the performance of the parasitoids of these herbivores.