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Currently, fungicides are necessary to protect potato crops against late blight, Phytophthora infestans, one of the world’s most damaging crop pathogens. The introgression of plant resistance genes from wild potato species targeted specifically to the late blight pathogen into susceptible potato varieties may alleviate the environmental impact of chemical control. Genetically modified plants are subject to an environmental risk assessment, and this includes testing for risks to the non-target arthropod community associated with the crop. The thesis begins with a review about the main plant defense responses and their role in influencing sequential interactions between herbivores and plant pathogens. The experimental chapters each focus on different aspects of the interaction between potato plants (both resistant and susceptible), the target pathogen (P. infestans) and several non-target insects. With each chapter, the scope widens: from the molecular gene expression in potato leaves in response to sequential attacks, to field scale biodiversity analyses. At the molecular level, one of the main findings was that the genomic position of the Rpi-vnt1 insertion conferring resistance to P. infestans influenced potato gene expression measured in leaves, when interacting with the non-target insect pests Myzus persicae (Green peach aphid) and Leptinotarsa decemlineata (Colorado potato beetle). Insect performance differed between the resistant GM and susceptible non-GM comparator. In the following chapter, the differences in insect performance were tested across a range of conventionally bred cultivars varying in resistance to P. infestans. Differences in M. persicae performance between several cultivars greatly outweighed the differences previously detected between the GM and non-GM comparator. These results are crucial in shaping the way risk is assessed in the context of GM crops, and these results are supported in our experiments assessing effects on biodiversity with pitfall traps in the field. The third trophic level was also addressed by comparing the performance of the parasitoid Aphidius colemani reared on GM and non-GM fed aphids, both with an without exposure to P. infestans. Differences in parasitoid performance were only found on the susceptible cultivar when inoculated with P. infestans. In the last experimental chapter the risk assessment is taken to the field comparing pitfall trap catches over two years and in two countries. Different methods for statistical analysis of biodiversity data were compared to arrive at recommendations for such analysis in the framework of environmental risk assessments. Drawing on these lessons, the discussion ends with ideas for the development of protocols for environmental risk assessments in the light of expected scientific progress in agricultural biotechnology.

Currently, fungicides are necessary to protect potato crops against late blight, Phytophthora infestans, one of the world’s most damaging crop pathogens. The introgression of plant resistance genes from wild potato species targeted specifically to the late blight pathogen into susceptible potato varieties may alleviate the environmental impact of chemical control. Genetically modified plants are subject to an environmental risk assessment, and this includes testing for risks to the non-target arthropod community associated with the crop. The thesis begins with a review about the main plant defense responses and their role in influencing sequential interactions between herbivores and plant pathogens. The experimental chapters each focus on different aspects of the interaction between potato plants (both resistant and susceptible), the target pathogen (P. infestans) and several non-target insects. With each chapter, the scope widens: from the molecular gene expression in potato leaves in response to sequential attacks, to field scale biodiversity analyses. At the molecular level, one of the main findings was that the genomic position of the Rpi-vnt1 insertion conferring resistance to P. infestans influenced potato gene expression measured in leaves, when interacting with the non-target insect pests Myzus persicae (Green peach aphid) and Leptinotarsa decemlineata (Colorado potato beetle). Insect performance differed between the resistant GM and susceptible non-GM comparator. In the following chapter, the differences in insect performance were tested across a range of conventionally bred cultivars varying in resistance to P. infestans. Differences in M. persicae performance between several cultivars greatly outweighed the differences previously detected between the GM and non-GM comparator. These results are crucial in shaping the way risk is assessed in the context of GM crops, and these results are supported in our experiments assessing effects on biodiversity with pitfall traps in the field. The third trophic level was also addressed by comparing the performance of the parasitoid Aphidius colemani reared on GM and non-GM fed aphids, both with an without exposure to P. infestans. Differences in parasitoid performance were only found on the susceptible cultivar when inoculated with P. infestans. In the last experimental chapter the risk assessment is taken to the field comparing pitfall trap catches over two years and in two countries. Different methods for statistical analysis of biodiversity data were compared to arrive at recommendations for such analysis in the framework of environmental risk assessments. Drawing on these lessons, the discussion ends with ideas for the development of protocols for environmental risk assessments in the light of expected scientific progress in agricultural biotechnology.

Het is bekend dat planten als verticale communicatiekanalen kunnen fungeren tussen onder-en bovengrondse plantetende insecten. Worteleters veroorzaken veranderingen in biomassa en de chemische samenstelling van de bovengrondse delen van een plant. Deze veranderingen kunnen de overleving, de groei en de ontwikkeling van bladeters beïnvloeden. In het promotieproject werd onderzocht of en hoe de wisselwerking tussen ruimtelijk gescheiden insecten beperkt is tot planteneters, dan wel uitgebreid kan worden naar hogere trofische niveaus zoals bijvoorbeeld parasitaire wespen, de natuurlijke vijanden van de plantenetende insecten. Tijdens de 20e Nederlandse Entomologendag (19 december 2008) is de eerste NEV dissertatieprijs uitgereikt aan dit proefschrift

The effects of a pesticide mixture (asulam, fluazinam, lambda-cyhalothrin, and metamitron) on aquatic ecosystems were investigated in 20 outdoor aquatic microcosms. Ten of the microcosms simulated mesotrophic aquatic ecosystems dominated by submerged macrophytes (Elodea). The others simulated eutrophic ecosystems with a high Lemna surface coverage (Lemna). This paper describes the fate of the chemicals as well as their effects on the growth of Myriophyllum spicatum and the periphytic algal community. In the Elodea-dominated microcosms significant increase in the biomass and alterations of species composition of the periphytic algae were observed, but no effect on M. spicatum growth could be recorded in response to the treatment. The opposite was found in the Lemna-dominated microcosms, in which decreased growth of M. spicatum was observed but no alterations could be found in the periphytic community. In the Elodea-dominated microcosms the species composition of the periphytic algae diverged from that of the control following treatment with 0.5% spray drift emission of the label-recommended rate (5% for lambda-cyhalothrin), while reduced growth of M. spicatum in the Lemna-dominated microcosms was recorded at 2% drift (20% for lambda-cyhalothrin). This study shows that the structure of the ecosystem influences the final effect of pesticide exposure.

cum laude graduation

cum laude graduation

Plants have evolved intriguing defences against insect herbivores. Compared to constitutive Plants have evolved intriguing defences against insect herbivores. Compared to constitutive defences that are always present, plants can respond with inducible defences when they are attacked. Insect herbivores can induce phenotypic changes in plants and consequently these changes may differentially affect subsequent attackers and their associated insect communities. Many studies consider herbivore-feeding damage as the first interaction between plants and insects. The originality of this study was to start with the first phase of herbivore attack, egg deposition, to understand the consequences of plant responses to eggs on subsequently feeding caterpillars and their natural enemies. The main plant species used for most of the experiments was Brassica nigra (black mustard), which occurs naturally in The Netherlands. The main herbivore used was the lepidopteran Pieris brassicae, which lays eggs in clusters and feeds on plants belonging to the Brassicaceae family. This study investigated plant-mediated responses to oviposition and their effects on different developmental stages of the herbivore, such as larvae and pupae. Furthermore, the effects of oviposition were extended to four more plant species of the same family, and to higher trophic levels including parasitoids and hyperparasitoids. The experiments were conducted under laboratory, semi-field and field conditions. This study shows that B. nigra plants recognize the eggs of P. brassicae and initiate resistance against subsequent developmental stages of the herbivore. Interestingly, plant responses to oviposition were found to be species specific. Plants did not respond to egg deposition by another herbivore species, the generalist moth Mamestra brassicae. Moreover, most of the Brassicaceae species tested were found to respond to P. brassicae eggs, which indicates that plant responses against oviposition are more common among the family of Brassicaceae. To assess effects on other members of the food chain, the effects of oviposition on plant volatile emission and the attraction of parasitic wasps, such as the larval parasitoid Cotesia glomerata, were tested. It was shown that the wasps were able to use the blend of plant volatiles, altered by their hosts’ oviposition, to locate young caterpillars just after hatching from eggs. The observed behaviour of the wasps was associated with higher parasitism success and higher fitness in young hosts. Similar results were obtained in a field experiment, where plants infested with eggs and caterpillars attracted more larval parasitoids and hyperparasitoids and eventually produced more seeds compared to plants infested with caterpillars only. This study shows that an annual weed like B. nigra uses egg deposition as reliable information for upcoming herbivory and responds accordingly with induced defences. Egg deposition could influence plant-associated community members at different levels in the food chain and benefit seed production. As the importance of oviposition on plant-herbivore interactions is only recently discovered, more research is needed to elucidate the mechanisms that underlie such plant responses and how these interactions affect the structure of insect communities in nature.

Plants have evolved intriguing defences against insect herbivores. Compared to constitutive Plants have evolved intriguing defences against insect herbivores. Compared to constitutive defences that are always present, plants can respond with inducible defences when they are attacked. Insect herbivores can induce phenotypic changes in plants and consequently these changes may differentially affect subsequent attackers and their associated insect communities. Many studies consider herbivore-feeding damage as the first interaction between plants and insects. The originality of this study was to start with the first phase of herbivore attack, egg deposition, to understand the consequences of plant responses to eggs on subsequently feeding caterpillars and their natural enemies. The main plant species used for most of the experiments was Brassica nigra (black mustard), which occurs naturally in The Netherlands. The main herbivore used was the lepidopteran Pieris brassicae, which lays eggs in clusters and feeds on plants belonging to the Brassicaceae family. This study investigated plant-mediated responses to oviposition and their effects on different developmental stages of the herbivore, such as larvae and pupae. Furthermore, the effects of oviposition were extended to four more plant species of the same family, and to higher trophic levels including parasitoids and hyperparasitoids. The experiments were conducted under laboratory, semi-field and field conditions. This study shows that B. nigra plants recognize the eggs of P. brassicae and initiate resistance against subsequent developmental stages of the herbivore. Interestingly, plant responses to oviposition were found to be species specific. Plants did not respond to egg deposition by another herbivore species, the generalist moth Mamestra brassicae. Moreover, most of the Brassicaceae species tested were found to respond to P. brassicae eggs, which indicates that plant responses against oviposition are more common among the family of Brassicaceae. To assess effects on other members of the food chain, the effects of oviposition on plant volatile emission and the attraction of parasitic wasps, such as the larval parasitoid Cotesia glomerata, were tested. It was shown that the wasps were able to use the blend of plant volatiles, altered by their hosts’ oviposition, to locate young caterpillars just after hatching from eggs. The observed behaviour of the wasps was associated with higher parasitism success and higher fitness in young hosts. Similar results were obtained in a field experiment, where plants infested with eggs and caterpillars attracted more larval parasitoids and hyperparasitoids and eventually produced more seeds compared to plants infested with caterpillars only. This study shows that an annual weed like B. nigra uses egg deposition as reliable information for upcoming herbivory and responds accordingly with induced defences. Egg deposition could influence plant-associated community members at different levels in the food chain and benefit seed production. As the importance of oviposition on plant-herbivore interactions is only recently discovered, more research is needed to elucidate the mechanisms that underlie such plant responses and how these interactions affect the structure of insect communities in nature.

De resultaten van dit proefschrift laten zien dat het voor de bestrijding van bovengrondse plaaginsecten van groot belang is te weten welke ondergrondse insecten aanwezig zijn en wat er rond de wortel van de plant gebeurt, en vice versa. Zulke bovengrondse-ondergrondse interacties tussen insecten via veranderingen in de in de waardplant kunnen namelijk de efficiëntie van bestrijdingsprogramma's significant beïnvloeden

The transport and cycling of nutrients through the various pools in water, soil and sediment is controlling the long term and short term productivity of water bodies. An understanding of the size of these pools and the fluxes between them is essential for the assessment of the usefulness of management measures resulting in reduced external input and the anticipated resilience of the system towards changes in trophic character. Large pools, such as phosphorus in surficial sediments and nitrate in groundwater have a potential for prolonged stimulation of productivity. Diffuse sources, fluxes towards sinks, competition between biota and adsorbents for sparse nutrients, feedback mechanisms, non-linearities and shifts among prevailing processes are discussed.

Het is bekend dat planten als verticale communicatiekanalen kunnen fungeren tussen onder-en bovengrondse plantetende insecten. Worteleters veroorzaken veranderingen in biomassa en de chemische samenstelling van de bovengrondse delen van een plant. Deze veranderingen kunnen de overleving, de groei en de ontwikkeling van bladeters beïnvloeden. In het promotieproject werd onderzocht of en hoe de wisselwerking tussen ruimtelijk gescheiden insecten beperkt is tot planteneters, dan wel uitgebreid kan worden naar hogere trofische niveaus zoals bijvoorbeeld parasitaire wespen, de natuurlijke vijanden van de plantenetende insecten. Tijdens de 20e Nederlandse Entomologendag (19 december 2008) is de eerste NEV dissertatieprijs uitgereikt aan dit proefschrift

cum laude graduation (with distinction)

De resultaten van dit proefschrift laten zien dat het voor de bestrijding van bovengrondse plaaginsecten van groot belang is te weten welke ondergrondse insecten aanwezig zijn en wat er rond de wortel van de plant gebeurt, en vice versa. Zulke bovengrondse-ondergrondse interacties tussen insecten via veranderingen in de in de waardplant kunnen namelijk de efficiëntie van bestrijdingsprogramma's significant beïnvloeden

cum laude graduation (with distinction)

The Marine Trophic Index (MTI) is proposed by the CBD as an indicator for sustainable fisheries and ecosystem integrity. This report explores the usefulness of the Marine Trophic Index (MTI) as an indicator for sustainable fisheries and ecosystem integrity for the Dutch situation

Herbivory can induce changes in plant growth, physiology, morphology, and phenology. Such herbivore-induced changes by insects are commonly observed in many plant species. Changes in the host plant can alter interaction linkages between plants, herbivores and higher trophic levels such as carnivores of the herbivores. These interaction linkages shape community organization and biodiversity in plant-based terrestrial communities. Considering that plants have both aboveground and belowground parts, systemic herbivore-induced changes within the plant could potentially link spatially separated herbivores associated with plants in the soil and in the aboveground (sub) system. Relatively recently, an increasing number of studies have shown that root herbivores, among other soil dwelling organisms, can indeed interact with foliar herbivores. Root herbivores change the quality and biomass of aboveground plant parts, which in turn influences the growth, development and survival of foliar herbivores. Similarly, foliar-feeding insects have been shown to influence root-feeding insects. This thesis explores whether, and mechanistically how, these feed-backs between root and foliar herbivores are transmitted through the soil and the aboveground food chains influencing the performance and behaviour of higher trophic levels. In Chapter 1, I introduce plant-insect interactions in a multitrophic context, briefly describing how the plant mediates these intricate interactions, and I describe the general aim and the outline of the thesis. In Chapter 2 I study the effects of root herbivores, at different densities, on the biomass and quality of the plant shoot, and on the performance of a foliar herbivore, its primary parasitoid and its secondary parasitoid (hyperparasitoid) sharing the host plant. In Chapter 3 I study the reverse effect; the effects of foliar herbivores on a root herbivore and its parasitoid mediated by the host plant. The results of these Chapters show that the spatially separated herbivores negatively affect each other's performances by increasing the levels of secondary plant compounds within the host plant. These negative effects are not restricted to the herbivore level, but are transmitted to the parasitoids and the hyperparasitoids, reducing in particular the growth and size of the off-spring of the aboveground parasitoid. Based on the results of Chapter 2, where I observed a marked suboptimal performance of the off-spring of the aboveground parasitoid when sharing the host plant with root herbivores, I investigate in Chapter 4 whether female parasitoids of foliar herbivores can exploit root herbivore-related cues to select the most suitable hosts for their offspring. In this Chapter I show that female parasitoids avoid to parasitise hosts feeding on plants shared with root herbivores, when hosts feeding on root-undamaged plants are available. The mechanism points to changes in the volatile blend of the plant triggered by the root-feeding insects. After observing this strong interaction between root herbivores and aboveground parasitoids mediated by the host-infested plant, in Chapter 5, I investigate whether root herbivores influence the behaviour of female parasitoids also indirectly via changes in the surrounding habitat. I provide evidence that root-feeding insects can influence the behaviour and foraging efficiency of parasitoids of foliar herbivores, even when the foliar (parasitoid-host) and the root herbivores do not feed on the same plant, but on neighbouring conspecific plants within the same habitat. The above described results are based on experiments carried out under controlled conditions in the laboratory or semi-field experiments in large, enclosed cages. To test some of the lab-based results under more natural conditions, I conducted a field experiment in which several cohorts of mustard plants with and without root herbivores were placed in the field and were monitored aboveground for the presence of foliar herbivores over the course of a summer season. To investigate whether the spatial distribution of root-damaged plants influences the plant preference of the foliar-feeders, I arranged the plants in the field in plots either in clusters of root-undamaged and root-damaged plants, or root-undamaged and root-damaged plants homogeneously mixed. Specialised foliar herbivores were observed feeding and ovipositing preferentially on root- ____________________________________________________________ Summary uninfested plants over plants shared with root herbivores. Interestingly, one of the species only discriminates against root-damaged plants when the plants were grouped in clusters. This thesis shows that the strong feedbacks between root and shoot associated organisms can significantly influence the performance and behaviour of organisms of higher trophic levels. Therefore, interactive effects of aboveground and belowground organisms cannot be limited to organisms directly associated with the shared host plant because they occur throughout a complex multitrophic chain of organisms. Furthermore, this thesis underlines the importance of considering not only the mere presence or absence of the soil dwelling organisms, but show that the density and distribution of root herbivores can determine the outcome of the interactions with their aboveground counterparts. These interactions are mediated by the shared host plant, via induced changes in the secondary plant compounds within the plant and in the plant volatile blend. Consumers in one compartment can change the quality of the plant in the opposite compartment, as well as induce changes in the emission of the plant volatiles used by carnivores that forage for hosts. Moreover, I provide evidence that spatially separated insects can interact not only via changes in the shared host plant, but also more indirectly via changes in the quality of the surrounding habitat. This suggest that aboveground-belowground multitrophic interactions can occur and interact in much more complex ways than what has been reported so far. In the last experimental Chapter I provide evidence that behavioural decisions in naturally occurring populations of foliar-feeding insects can also be influenced by root-feeding insects, under field conditions where several complex interactions take place simultaneously. In the final Chapter, I summarise the results presented in this thesis and provide a general conclusion of the work. I identify a number of important areas and topics for future research to further gain insight into aboveground-belowground multitrophic interactions.

The Marine Trophic Index (MTI) is proposed by the CBD as an indicator for sustainable fisheries and ecosystem integrity. This report explores the usefulness of the Marine Trophic Index (MTI) as an indicator for sustainable fisheries and ecosystem integrity for the Dutch situation

Herbivory can induce changes in plant growth, physiology, morphology, and phenology. Such herbivore-induced changes by insects are commonly observed in many plant species. Changes in the host plant can alter interaction linkages between plants, herbivores and higher trophic levels such as carnivores of the herbivores. These interaction linkages shape community organization and biodiversity in plant-based terrestrial communities. Considering that plants have both aboveground and belowground parts, systemic herbivore-induced changes within the plant could potentially link spatially separated herbivores associated with plants in the soil and in the aboveground (sub) system. Relatively recently, an increasing number of studies have shown that root herbivores, among other soil dwelling organisms, can indeed interact with foliar herbivores. Root herbivores change the quality and biomass of aboveground plant parts, which in turn influences the growth, development and survival of foliar herbivores. Similarly, foliar-feeding insects have been shown to influence root-feeding insects. This thesis explores whether, and mechanistically how, these feed-backs between root and foliar herbivores are transmitted through the soil and the aboveground food chains influencing the performance and behaviour of higher trophic levels. In Chapter 1, I introduce plant-insect interactions in a multitrophic context, briefly describing how the plant mediates these intricate interactions, and I describe the general aim and the outline of the thesis. In Chapter 2 I study the effects of root herbivores, at different densities, on the biomass and quality of the plant shoot, and on the performance of a foliar herbivore, its primary parasitoid and its secondary parasitoid (hyperparasitoid) sharing the host plant. In Chapter 3 I study the reverse effect; the effects of foliar herbivores on a root herbivore and its parasitoid mediated by the host plant. The results of these Chapters show that the spatially separated herbivores negatively affect each other's performances by increasing the levels of secondary plant compounds within the host plant. These negative effects are not restricted to the herbivore level, but are transmitted to the parasitoids and the hyperparasitoids, reducing in particular the growth and size of the off-spring of the aboveground parasitoid. Based on the results of Chapter 2, where I observed a marked suboptimal performance of the off-spring of the aboveground parasitoid when sharing the host plant with root herbivores, I investigate in Chapter 4 whether female parasitoids of foliar herbivores can exploit root herbivore-related cues to select the most suitable hosts for their offspring. In this Chapter I show that female parasitoids avoid to parasitise hosts feeding on plants shared with root herbivores, when hosts feeding on root-undamaged plants are available. The mechanism points to changes in the volatile blend of the plant triggered by the root-feeding insects. After observing this strong interaction between root herbivores and aboveground parasitoids mediated by the host-infested plant, in Chapter 5, I investigate whether root herbivores influence the behaviour of female parasitoids also indirectly via changes in the surrounding habitat. I provide evidence that root-feeding insects can influence the behaviour and foraging efficiency of parasitoids of foliar herbivores, even when the foliar (parasitoid-host) and the root herbivores do not feed on the same plant, but on neighbouring conspecific plants within the same habitat. The above described results are based on experiments carried out under controlled conditions in the laboratory or semi-field experiments in large, enclosed cages. To test some of the lab-based results under more natural conditions, I conducted a field experiment in which several cohorts of mustard plants with and without root herbivores were placed in the field and were monitored aboveground for the presence of foliar herbivores over the course of a summer season. To investigate whether the spatial distribution of root-damaged plants influences the plant preference of the foliar-feeders, I arranged the plants in the field in plots either in clusters of root-undamaged and root-damaged plants, or root-undamaged and root-damaged plants homogeneously mixed. Specialised foliar herbivores were observed feeding and ovipositing preferentially on root- ____________________________________________________________ Summary uninfested plants over plants shared with root herbivores. Interestingly, one of the species only discriminates against root-damaged plants when the plants were grouped in clusters. This thesis shows that the strong feedbacks between root and shoot associated organisms can significantly influence the performance and behaviour of organisms of higher trophic levels. Therefore, interactive effects of aboveground and belowground organisms cannot be limited to organisms directly associated with the shared host plant because they occur throughout a complex multitrophic chain of organisms. Furthermore, this thesis underlines the importance of considering not only the mere presence or absence of the soil dwelling organisms, but show that the density and distribution of root herbivores can determine the outcome of the interactions with their aboveground counterparts. These interactions are mediated by the shared host plant, via induced changes in the secondary plant compounds within the plant and in the plant volatile blend. Consumers in one compartment can change the quality of the plant in the opposite compartment, as well as induce changes in the emission of the plant volatiles used by carnivores that forage for hosts. Moreover, I provide evidence that spatially separated insects can interact not only via changes in the shared host plant, but also more indirectly via changes in the quality of the surrounding habitat. This suggest that aboveground-belowground multitrophic interactions can occur and interact in much more complex ways than what has been reported so far. In the last experimental Chapter I provide evidence that behavioural decisions in naturally occurring populations of foliar-feeding insects can also be influenced by root-feeding insects, under field conditions where several complex interactions take place simultaneously. In the final Chapter, I summarise the results presented in this thesis and provide a general conclusion of the work. I identify a number of important areas and topics for future research to further gain insight into aboveground-belowground multitrophic interactions.

The fate and effects of the pyrethroid insecticide lambda-cyhalothrin were compared in mesotrophic (macrophyte-dominated) and eutrophic (phytoplankton-dominated) ditch microcosms (approximately 0.5 m3). Lambda-cyhalothrin was applied three times at one-week intervals at concentrations of 10, 25, 50, 100, and 250 ng/L. The rate of dissipation of lambda-cyhalothrin in the water column of the two types of test systems was similar. After 1 d, only 30% of the amount applied remained in the water phase. Initial, direct effects were observed primarily on arthropod taxa. The most sensitive species was the phantom midge (Chaoborus obscuripes). Threshold levels for slight and transient direct toxic effects were similar (10 ng/L) between types of test systems. At treatment levels of 25 ng/L and higher, apparent population and community responses occurred. At treatments of 100 and 250 ng/L, the rate of recovery of the macroinvertebrate community was lower in the macrophyte-dominated systems, primarily because of a prolonged decline of the amphipod Gammarus pulex. This species occurred at high densities only in the macrophyte-dominated enclosures. Indirect effects (e.g., increase of rotifers and microcrustaceans) were more pronounced in the plankton-dominated test systems, particularly at treatment levels of 25 ng/L and higher.

Keywords: vegetational diversity, intercropping, plant quality, herbivores, D. semiclausum, foraging behaviour, time-allocationReduction of herbivore populations is more often observed in vegetationally diverse than in simple ecosystems. Consequently, plant-species mixtures (i.e. intercropping) may be used to manage pest problems in agro-ecosystems, thereby contributing to the development of ecologically sustainable crop production. Compared with monotypic stands, increasing vegetation diversity may suppress herbivore populations through the reduction in the apparency and quality of crop plants and by the increased success of natural enemies. Unfortunately, the highly variable responses of both herbivores and natural enemies to intercropping provide insufficient information to make generalisations for the selection of pest-suppressive plant-mixtures. An explanation for this lack of information is that predictions of the hypotheses explaining the pest-suppression are relevant at the population level, while the underlying mechanisms at the behavioural (i.e. functional) level remain largely untested. In this thesis I attempted to further our understanding of how vegetation diversity leads to pest suppression in agro-ecosystems. The aims were (1) to understand the cause(s) of lower herbivore numbers in vegetationally more diverse cropping systems compared to monocultures and (2) to study the behavioural bases of the responses of natural enemies to vegetation diversity. The studied system includedBrusselssprout ( Brassica oleracea gemmifera), its herbivores, and Diadegma semiclausum , a parasitoid of the diamondback moth. Vegetational diversity was characterised by mixingBrusselssprout with either barley ( Hordeum vulgare ) or mustard ( Sinapisalba ). Numbers of several herbivore species were reduced when Brussels sprout was mixed with barley. A study showed that the plant competition in the species mixture influenced herbivore responses by altering plant quality compared with the monocrop. Field and simulation studies showed that responses of herbivores in diversified habitats were influenced by species-related differences in foraging behaviour. Behavioural and analytical studies showed the importance of inter- and intraspecific variation in volatiles of both damaged and undamaged plants in the attraction of the parasitoid D. semiclausum . Plant mixtures interacted with the searching behaviour and time-allocation of wasps. Compared with pureBrusselssprout patches, mustard attracted and retained individuals longer, whereas barley reduced their tendency to enter the plant patch. Although both mustard and barley reduced the tendency of wasps to locate hosts onBrusselssprout, parasitoids improved their foraging efficiency through oviposition experiences and became equally efficient in finding further hosts. In conclusion, this thesis demonstrates the importance of foraging behaviour in explaining variable responses of herbivores and parasitoids to plant-species mixtures.

Keywords: vegetational diversity, intercropping, plant quality, herbivores, D. semiclausum, foraging behaviour, time-allocationReduction of herbivore populations is more often observed in vegetationally diverse than in simple ecosystems. Consequently, plant-species mixtures (i.e. intercropping) may be used to manage pest problems in agro-ecosystems, thereby contributing to the development of ecologically sustainable crop production. Compared with monotypic stands, increasing vegetation diversity may suppress herbivore populations through the reduction in the apparency and quality of crop plants and by the increased success of natural enemies. Unfortunately, the highly variable responses of both herbivores and natural enemies to intercropping provide insufficient information to make generalisations for the selection of pest-suppressive plant-mixtures. An explanation for this lack of information is that predictions of the hypotheses explaining the pest-suppression are relevant at the population level, while the underlying mechanisms at the behavioural (i.e. functional) level remain largely untested. In this thesis I attempted to further our understanding of how vegetation diversity leads to pest suppression in agro-ecosystems. The aims were (1) to understand the cause(s) of lower herbivore numbers in vegetationally more diverse cropping systems compared to monocultures and (2) to study the behavioural bases of the responses of natural enemies to vegetation diversity. The studied system includedBrusselssprout ( Brassica oleracea gemmifera), its herbivores, and Diadegma semiclausum , a parasitoid of the diamondback moth. Vegetational diversity was characterised by mixingBrusselssprout with either barley ( Hordeum vulgare ) or mustard ( Sinapisalba ). Numbers of several herbivore species were reduced when Brussels sprout was mixed with barley. A study showed that the plant competition in the species mixture influenced herbivore responses by altering plant quality compared with the monocrop. Field and simulation studies showed that responses of herbivores in diversified habitats were influenced by species-related differences in foraging behaviour. Behavioural and analytical studies showed the importance of inter- and intraspecific variation in volatiles of both damaged and undamaged plants in the attraction of the parasitoid D. semiclausum . Plant mixtures interacted with the searching behaviour and time-allocation of wasps. Compared with pureBrusselssprout patches, mustard attracted and retained individuals longer, whereas barley reduced their tendency to enter the plant patch. Although both mustard and barley reduced the tendency of wasps to locate hosts onBrusselssprout, parasitoids improved their foraging efficiency through oviposition experiences and became equally efficient in finding further hosts. In conclusion, this thesis demonstrates the importance of foraging behaviour in explaining variable responses of herbivores and parasitoids to plant-species mixtures.

Stemborers are major pests of maize and sorghum in tropical Africa. Various management approaches to stemborer control are being sought with emphasis on those that are environmentally sound, sustainable and economically feasible. Studies reported in this thesis were conducted to explore the potential of employing habitat management techniques to enhance the effectiveness of stemborer parasitoids. Emphasis was placed on the chemical and behavioural ecology of stemborer parasitoids in a diversified system that consisted of the stemborer host plants, maize and sorghum, stemborers and a non-host plant, molasses grass ( Melinis minutiflora ).Olfactometric studies on the stimuli involved in long-range foraging behaviour of the parasitoids Cotesia sesamiae and Dentichasmias busseolae revealed that host plant volatiles play a major role in guiding searching parasitoids to habitats that harbour their hosts. Volatiles from herbivore-injured plants were the most attractive. Sorghum was more attractive to the parasitoids than maize. Molasses grass volatiles were attractive to C. sesamiae but repellent to D. busseolae . Combining host plants and molasses grass did not have an additive effect of increasing the response of the parasitoids. Local growth conditions influenced the volatile blend produced by molasses grass grown in two different locations in Kenya, Thika and Mbita. This was manifested by their differential attractiveness to C. sesamiae . Dentichasmias busseolae did not discriminate between host species at a distance based on the volatiles released by infested host plants. Headspace analyses of volatiles emitted by uninfested and infested host plants and molasses grass grown at Thika and Mbita, revealed qualitative differences in their compositions. Infested host plants released a richer volatile blend than the uninfested host plants. Most of the compounds identified in the herbivore-injured plants were previously reported to play a role in parasitoid recruitment. Molasses grass from Thika had a number of its identified compounds that were similar to some in the blend of infested host plants. These compounds might have played a role in attracting C. sesamiae . Molasses grass from Mbita had very few compounds common to the other plants.Semi-field studies on the close-range searching behaviour of C. sesamiae showed that the parasitoid was not arrested by molasses grass although the grass had previously been demonstrated to be attractive to this parasitoid. The presence of molasses grass in a patch with host plants did not interfere with the foraging behaviour of D. busseolae . Both parasitoids recognized host plants at close range and were arrested on infested host plants where they spent most of their time engaged in host searching activities. Field studies showed that stemborer densities were lower in the intercrop than in the monocrop, while diversifying the habitat with molasses grass had no effect on stemborer parasitism. Larval and pupal parasitism was 2.1 and 11.0% in the monocrop compared to 2.0 and 9.8% in the intercrop.The main conclusion of this thesis is that intercropping maize or sorghum with molasses grass does not enhance the foraging behaviour of stemborer parasitoids. Molasses grass seems to be of more importance to the herbivore than the parasitoids. Hence, to clearly understand why stemborer densities were lower in the intercrop than the monocrop, future studies should focus on the interrelations between molasses grass, stemborers and host plants.

Stemborers are major pests of maize and sorghum in tropical Africa. Various management approaches to stemborer control are being sought with emphasis on those that are environmentally sound, sustainable and economically feasible. Studies reported in this thesis were conducted to explore the potential of employing habitat management techniques to enhance the effectiveness of stemborer parasitoids. Emphasis was placed on the chemical and behavioural ecology of stemborer parasitoids in a diversified system that consisted of the stemborer host plants, maize and sorghum, stemborers and a non-host plant, molasses grass ( Melinis minutiflora ).Olfactometric studies on the stimuli involved in long-range foraging behaviour of the parasitoids Cotesia sesamiae and Dentichasmias busseolae revealed that host plant volatiles play a major role in guiding searching parasitoids to habitats that harbour their hosts. Volatiles from herbivore-injured plants were the most attractive. Sorghum was more attractive to the parasitoids than maize. Molasses grass volatiles were attractive to C. sesamiae but repellent to D. busseolae . Combining host plants and molasses grass did not have an additive effect of increasing the response of the parasitoids. Local growth conditions influenced the volatile blend produced by molasses grass grown in two different locations in Kenya, Thika and Mbita. This was manifested by their differential attractiveness to C. sesamiae . Dentichasmias busseolae did not discriminate between host species at a distance based on the volatiles released by infested host plants. Headspace analyses of volatiles emitted by uninfested and infested host plants and molasses grass grown at Thika and Mbita, revealed qualitative differences in their compositions. Infested host plants released a richer volatile blend than the uninfested host plants. Most of the compounds identified in the herbivore-injured plants were previously reported to play a role in parasitoid recruitment. Molasses grass from Thika had a number of its identified compounds that were similar to some in the blend of infested host plants. These compounds might have played a role in attracting C. sesamiae . Molasses grass from Mbita had very few compounds common to the other plants.Semi-field studies on the close-range searching behaviour of C. sesamiae showed that the parasitoid was not arrested by molasses grass although the grass had previously been demonstrated to be attractive to this parasitoid. The presence of molasses grass in a patch with host plants did not interfere with the foraging behaviour of D. busseolae . Both parasitoids recognized host plants at close range and were arrested on infested host plants where they spent most of their time engaged in host searching activities. Field studies showed that stemborer densities were lower in the intercrop than in the monocrop, while diversifying the habitat with molasses grass had no effect on stemborer parasitism. Larval and pupal parasitism was 2.1 and 11.0% in the monocrop compared to 2.0 and 9.8% in the intercrop.The main conclusion of this thesis is that intercropping maize or sorghum with molasses grass does not enhance the foraging behaviour of stemborer parasitoids. Molasses grass seems to be of more importance to the herbivore than the parasitoids. Hence, to clearly understand why stemborer densities were lower in the intercrop than the monocrop, future studies should focus on the interrelations between molasses grass, stemborers and host plants.

This thesis describes research on multitrophic interactions between parasitoids, herbivores and plants. These are all involved in a dynamic game of hide-and-seek. Insect herbivores may occur in different spatial distributions across plants. Some species occur in local clusters, while others have a more even distribution. Parasitoids are challenged to deal effectively with such variation in the spatial distribution of their victims. I studied how different species and strains of Cotesia parasitoids make use of chemical information and foraging experience under varying spatial distributions of hosts.The parasitoids Cotesia glomerata and Cotesia rubecula show clear interspecific variation in foraging decisions when exploiting patches with herbivores. C. glomerata tunes its foraging decisions to the current environment as it gains experience with hosts during subsequent patch visits. In contrast, foraging decisions in C. rubecula seem insensitive to acquired experience. These Cotesia species prefer different herbivore species, that occur in different spatial distributions across plants. C. glomerata prefers to attack Pieris brassicae , that occurs in rare clusters of highly variable density. The parasitoid C. rubecula specialises on Pieris rapae that mostly feeds solitarily on plants. This leads the parasitoid C. glomerata to experience a more variable environment, with a wider scope for increased efficiency through learning.Both Pieris species are present in Europe, but the clustered host is absent in North America. The parasitoid C. glomerata was introduced to North America in 1883 to control solitarily feeding Pieris rapae , and about 350 generations of C. glomerata parasitoids have now foraged there. The experimental results suggest that American C. glomerata have adapted to forage for solitary host larvae, mostly through a loss of costly traits for finding rare clustered larvae. The European parasitoids spend a lot of time on explorative flights, while their American conspecifics seem to minimise travel costs and effectively focus on the exploitation of patches with solitary hosts.Herbivores are hard to find victims for parasitoids, and parasitic wasps may use any reliable information from the environment to locate them. Plants play a key role in providing this information. Plants may emit volatile infochemicals, when they are damaged by herbivores. Such volatiles are attractive to parasitoids and may guide them to their hosts. However, this research shows that the reliability of plant infochemicals may be very low when a complex of herbivores feeds on the plant. This is important, as plants are mostly attacked by several herbivore species, both in agricultural and natural ecosystems. The presence of nonhost herbivores causes parasitoids to waste time on damaged plants without hosts. A model study shows that the interaction between this 'wasted time' effect and herbivore diversity stabilises communities of parasitoids and herbivores. However, above a certain diversity threshold parasitoids may waste too much time on nonhost herbivores. This may lead to the local extinction of several parasitoid species. Thus, diversity may promote both stability and extinctions, when plants provide unreliable information.This thesis shows that the flow of information between all these actors is crucial for the understanding of insect behaviour and community dynamics. This information flow involves plants, parasitoids and, surprisingly, both host and nonhost herbivores.

This thesis describes research on multitrophic interactions between parasitoids, herbivores and plants. These are all involved in a dynamic game of hide-and-seek. Insect herbivores may occur in different spatial distributions across plants. Some species occur in local clusters, while others have a more even distribution. Parasitoids are challenged to deal effectively with such variation in the spatial distribution of their victims. I studied how different species and strains of Cotesia parasitoids make use of chemical information and foraging experience under varying spatial distributions of hosts.The parasitoids Cotesia glomerata and Cotesia rubecula show clear interspecific variation in foraging decisions when exploiting patches with herbivores. C. glomerata tunes its foraging decisions to the current environment as it gains experience with hosts during subsequent patch visits. In contrast, foraging decisions in C. rubecula seem insensitive to acquired experience. These Cotesia species prefer different herbivore species, that occur in different spatial distributions across plants. C. glomerata prefers to attack Pieris brassicae , that occurs in rare clusters of highly variable density. The parasitoid C. rubecula specialises on Pieris rapae that mostly feeds solitarily on plants. This leads the parasitoid C. glomerata to experience a more variable environment, with a wider scope for increased efficiency through learning.Both Pieris species are present in Europe, but the clustered host is absent in North America. The parasitoid C. glomerata was introduced to North America in 1883 to control solitarily feeding Pieris rapae , and about 350 generations of C. glomerata parasitoids have now foraged there. The experimental results suggest that American C. glomerata have adapted to forage for solitary host larvae, mostly through a loss of costly traits for finding rare clustered larvae. The European parasitoids spend a lot of time on explorative flights, while their American conspecifics seem to minimise travel costs and effectively focus on the exploitation of patches with solitary hosts.Herbivores are hard to find victims for parasitoids, and parasitic wasps may use any reliable information from the environment to locate them. Plants play a key role in providing this information. Plants may emit volatile infochemicals, when they are damaged by herbivores. Such volatiles are attractive to parasitoids and may guide them to their hosts. However, this research shows that the reliability of plant infochemicals may be very low when a complex of herbivores feeds on the plant. This is important, as plants are mostly attacked by several herbivore species, both in agricultural and natural ecosystems. The presence of nonhost herbivores causes parasitoids to waste time on damaged plants without hosts. A model study shows that the interaction between this 'wasted time' effect and herbivore diversity stabilises communities of parasitoids and herbivores. However, above a certain diversity threshold parasitoids may waste too much time on nonhost herbivores. This may lead to the local extinction of several parasitoid species. Thus, diversity may promote both stability and extinctions, when plants provide unreliable information.This thesis shows that the flow of information between all these actors is crucial for the understanding of insect behaviour and community dynamics. This information flow involves plants, parasitoids and, surprisingly, both host and nonhost herbivores.

Many insect species use an aggregation pheromone to form groups with conspecifics in certain localities of the environment. This type of behaviour has a variety of implications for ecological interactions, both directly through the effect of the pheromone on the behaviour of con- and heterospecifics, and indirectly through the consequential aggregative distributions that may affect species interactions. The evolutionary ecology of the use of aggregation pheromone has received only little attention. Yet, these pheromones may play an intricate role in food web interactions by providing an accompanying information web.The aim of this thesis is to further our understanding on the ecological and evolutionary aspects of the use of aggregation pheromone in insects. By unravelling costs and benefits that arise from the use of aggregation pheromone in our ecological model organism, Drosophila melanogaster , we strive to answer why they use an aggregation pheromone and elucidate the ecological consequences of an aggregation pheromone in a food web context.In laboratory and field studies, we identified behaviours and interactions of the fruit fly D. melanogaster that were affected by its aggregation pheromone. The pheromone affected the distribution of adults, their eggs, competitor species and parasitoids. Moreover, a number of costs and benefits to the use of aggregation pheromone were indicated. In subsequent studies, the major hypotheses on costs and benefits were examined.A major benefit of using aggregation pheromone was shown to be aggregated oviposition. Aggregated oviposition enhanced the quality of the larval resource, as indicated by a higher survival of the larvae and larger size of the emerging flies. This Allee effect was characterised by a positive effect of adult density on larval fitness components, and may have arisen from the interaction between adult flies and micro-organisms (yeasts and fungi). Fungi antagonise yeast and larval development, while adults can inoculate yeast on a substrate and temper fungal growth. Larvae also tempered fungal growth, but an increased larval density did not result in an Allee effect but in competition instead.A major cost of using aggregation pheromone arose from an increased risk of parasitism. The parasitoid Leptopilina heterotoma uses the aggregation pheromone of adult fruit flies to localise the larval hosts, and based on this information this parasitoid can differentiate quantitatively at long range between substrates that differ in profitability. After arrival on a substrate, the pheromones no longer play a role in the host searching behaviour. A behaviour-based model was developed to predict the individual risk of parasitism for hosts in differently sized host aggregations. The functional and numerical responses of the parasitoids were combined with a flexible patch leaving decision rule for the parasitoid, to assess whether aggregation could also comprise a benefit to the hosts in terms of a diluted risk ( sensu Hamilton 1971). The model prediction reads that aggregation is not beneficial in the context of the Drosophila - Leptopilina interaction, and these predictions were supported by field data.In a simple spatio-temporal simulation model, the population dynamics arising from several modes of dispersal, food competition and an Allee effect were explored. The model is a first step towards a more extensive model that incorporates the responses of insects to spatially heterogeneous resources and chemical information (e.g., aggregation pheromone).The main conclusion from this thesis is that the aggregation pheromone of D. melanogaster plays an intricate role within a foodweb context, and that a variety of costs and benefits arise from multitrophic interactions. To understand the dynamic interactions in this and many other ecological systems, it is essential to gain more insight into the effect of aggregation pheromone on the behaviour of individuals.

Many insect species use an aggregation pheromone to form groups with conspecifics in certain localities of the environment. This type of behaviour has a variety of implications for ecological interactions, both directly through the effect of the pheromone on the behaviour of con- and heterospecifics, and indirectly through the consequential aggregative distributions that may affect species interactions. The evolutionary ecology of the use of aggregation pheromone has received only little attention. Yet, these pheromones may play an intricate role in food web interactions by providing an accompanying information web.The aim of this thesis is to further our understanding on the ecological and evolutionary aspects of the use of aggregation pheromone in insects. By unravelling costs and benefits that arise from the use of aggregation pheromone in our ecological model organism, Drosophila melanogaster , we strive to answer why they use an aggregation pheromone and elucidate the ecological consequences of an aggregation pheromone in a food web context.In laboratory and field studies, we identified behaviours and interactions of the fruit fly D. melanogaster that were affected by its aggregation pheromone. The pheromone affected the distribution of adults, their eggs, competitor species and parasitoids. Moreover, a number of costs and benefits to the use of aggregation pheromone were indicated. In subsequent studies, the major hypotheses on costs and benefits were examined.A major benefit of using aggregation pheromone was shown to be aggregated oviposition. Aggregated oviposition enhanced the quality of the larval resource, as indicated by a higher survival of the larvae and larger size of the emerging flies. This Allee effect was characterised by a positive effect of adult density on larval fitness components, and may have arisen from the interaction between adult flies and micro-organisms (yeasts and fungi). Fungi antagonise yeast and larval development, while adults can inoculate yeast on a substrate and temper fungal growth. Larvae also tempered fungal growth, but an increased larval density did not result in an Allee effect but in competition instead.A major cost of using aggregation pheromone arose from an increased risk of parasitism. The parasitoid Leptopilina heterotoma uses the aggregation pheromone of adult fruit flies to localise the larval hosts, and based on this information this parasitoid can differentiate quantitatively at long range between substrates that differ in profitability. After arrival on a substrate, the pheromones no longer play a role in the host searching behaviour. A behaviour-based model was developed to predict the individual risk of parasitism for hosts in differently sized host aggregations. The functional and numerical responses of the parasitoids were combined with a flexible patch leaving decision rule for the parasitoid, to assess whether aggregation could also comprise a benefit to the hosts in terms of a diluted risk ( sensu Hamilton 1971). The model prediction reads that aggregation is not beneficial in the context of the Drosophila - Leptopilina interaction, and these predictions were supported by field data.In a simple spatio-temporal simulation model, the population dynamics arising from several modes of dispersal, food competition and an Allee effect were explored. The model is a first step towards a more extensive model that incorporates the responses of insects to spatially heterogeneous resources and chemical information (e.g., aggregation pheromone).The main conclusion from this thesis is that the aggregation pheromone of D. melanogaster plays an intricate role within a foodweb context, and that a variety of costs and benefits arise from multitrophic interactions. To understand the dynamic interactions in this and many other ecological systems, it is essential to gain more insight into the effect of aggregation pheromone on the behaviour of individuals.