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Parasitoids are a significant mortality factor in the population dynamics of many arthropods involved in key ecological processes such as herbivore-plant and predator-prey interactions. Parasitoids are therefore widely used in biocontrol programs. Global change phenomena influence these natural and anthropocentric roles of parasitoids and here we review the effects of the main drivers and their interplay. Land use intensification modifies landscape structure and elevates agroecosystem loads of fertilisers and pesticides creating risks for parasitism and loss of biocontrol services. Climate change can affect parasitoids directly, affecting physiology and survival, or indirectly via phenological and other effects (plant chemistry, herbivore-induced plant volatiles HIPVs) on their hosts, endosymbionts and plants. Biological invasions have the potential to modify native host-parasitoid systems and elevate risk of novel pest dynamics, requiring restoration of biocontrol. The interplay between these global change drivers may thus exacerbate the overall risk to parasitism in future agricultural landscapes. To make more accurate predictions, future studies could focus on the impact of interacting global change drivers on parasitoids and the biocontrol services they provide. Moreover, host and parasitoid specificity appear to be a key driver in assessing the effects of global change on parasitoids.

Landscape simplification have drastically reduced arthropods biodiversity in agricultural landscapes. Plant diversification has already been proved to be very positive for natural enemies notably through the provision of alternative food sources. At the landscape scale, natural habitats have been proved to be very positive for natural enemies and biocontrol. However, the efficacy of natural areas in enhancing biocontrol is more and more controversial. Additionally, farmers willingness to preserve or increase natural habitats in and around their fields is not systematic. Therefore, I decided during my PhD to assess the effect of increasing landscape plant diversity, by manipulating crops only in space and time, on the presence and the performance of natural enemies. In a first part, I assess the economical and social feasibility of increasing crop richness at a landscape scale. Then, in a second part, four different chapters assess the question of crop diversification in space and time on biocontrol at different scales of study. The first chapter shows a study maid at the laboratory scale, which aim is to assess the effect of crop diversification on the predator Harmonia axyridis, on its development and its predation capacity on four different types of aphids. The movement of the predator, hereafter called spillover, regarding the different food sources available is also assessed. The second chapter shows a similar study but laid at a bigger scale (greenhouse) and assesses the population dynamics, the predation capacity and the spillover of a population of Nesidiocoris tenuis regarding differential crop availability in space and time. The third chapter describes two experiments laid at the field scale which aim is to assess the effect of crop diversity, differing in space and time, on the abundance of naturally occurring pests and natural enemies. Abundances of pests and natural enemies was evaluated as a proxy of biocontrol and spillover between patches of crops was assessed in the polycultural treatment. Finally, the last chapter assessed the effect of crop diversity and crop configuration at the landscape scale on the abundance of aphids and aphidophagous predators (coccinellids, lacewings and spiders). The data were collected in the Hebei Province of China. Results at the laboratory and greenhouse scale show that individuals and populations of natural enemies are highly impacted by the functionality of the plant they are exposed to. Indeed, the natural enemies growth in polyculture was the result of the combined effect of the crops tested separately in monoculture. However, the spillover and the biocontrol were highly stimulated in polyculture, sometimes more than in monoculture. At the field scale, ladybirds spillover was higher in polyculture and populations of pests and natural enemies were strongly correlated in polyculture compare to monoculture. Finally, at the field scale, crop diversity had a positive effect on ladybirds abundance and a negative one on aphids abundance. Additionally, fields of small size increased the presence of lacewings. These results show that increasing crop diversity, and decreasing field size, might help to enhance the presence of natural enemies and increase the biocontrol through promoting the spillover of natural enemies between the different cultivated fields.; L’uniformisation du schéma agricole a eu pour conséquence une perte significative de la biodiversité des insectes utiles en agriculture. La diversification des plantes a de nombreux avantages agronomiques, notamment celui d’augmenter la présence d’ennemis naturels au champ en leur fournissant des sources diversifiées de nourriture. A l’échelle du paysage, la préservation des zones naturelles est un levier très efficace dans le maintien des ennemis naturels et le biocontrôle. Cette efficacité étant parfois contestée et difficilement gérable par les agriculteurs, lors de cette thèse j’ai étudié les effets de la diversification des cultures, dans l’espace et dans le temps, sur l’abondance des ennemis naturels, leur mouvement, et le service de biocontrôle. Dans une première partie, je pose la question de la faisabilité économique et sociale de systèmes agricoles basés sur une diversité culturale accrue. Il s’ensuit une seconde partie constituée de 4 chapitres mettant en application la question à 4 échelles de complexité croissante. Le premier chapitre pose la question de l’effet de la diversification des cultures, dans l’espace et le temps, à l’échelle de l’individu (Harmonia axyridis) sur le biocontrôle de plusieurs espèces pucerons, le spill-over et l’abondance du prédateur. Le chapitre 2 étudie l’effet de la diversification des cultures dans l’espace et le temps, à l’échelle d’une population du prédateur omnivore, Nesidiocoris tenuis, sur sa dynamique de population, son spill-over, et sur son service de biocontrôle d’œufs de lépidoptères (en condition contrôlée sous serre). Le chapitre 3 décrit des expérimentations mises en place à l’échelle du champ permettant de comprendre les effets de la diversité spatiale et temporelle des plantes cultivées sur des populations et les mouvements d’ennemis naturels et ravageurs spontanément présents en environnement agricole. Enfin, le chapitre 4 s’intéresse aux effets de la diversité des plantes cultivées à l’échelle d’une région, en Chine, sur l’abondance d’ennemis naturels et de pucerons en champs de coton et de blé. Les résultats à l’échelle du laboratoire et de la serre montrent que les individus et les populations d’ennemis naturels sont fortement impactés par la nature de la plante à laquelle ils sont exposés. En effet, la croissance des populations résultait de l’effet combiné des plantes testées en monoculture. Cependant le spill-over et le biocontrôle ont été plus fortement stimulés en polyculture. En champ, le spill-over des coccinelles et des chrysopes était plus important en polyculture. De surcroit, les populations de ravageurs et d’ennemis naturels étaient plus fortement corrélées en polyculture qu’en monoculture ; dans certains cas le biocontrôle s’est avéré plus important en polyculture. Enfin, à l’échelle du paysage, la diversité des cultures avait un effet positif sur les coccinelles et négatif pour les pucerons. De plus, les champs cultivés de petite taille étaient positifs pour le maintien des chrysopes dans les champs cultivés. L’ensemble de ces résultats montrent donc que l’augmentation de la diversité des plantes cultivées, ainsi que la réduction de la taille des champs cultivés, peuvent permettre une augmentation des mouvements des ennemis naturels entre les champs, ainsi qu’un accroissement des services de biocontrôle.

Landscape simplification have drastically reduced arthropods biodiversity in agricultural landscapes. Plant diversification has already been proved to be very positive for natural enemies notably through the provision of alternative food sources. At the landscape scale, natural habitats have been proved to be very positive for natural enemies and biocontrol. However, the efficacy of natural areas in enhancing biocontrol is more and more controversial. Additionally, farmers willingness to preserve or increase natural habitats in and around their fields is not systematic. Therefore, I decided during my PhD to assess the effect of increasing landscape plant diversity, by manipulating crops only in space and time, on the presence and the performance of natural enemies. In a first part, I assess the economical and social feasibility of increasing crop richness at a landscape scale. Then, in a second part, four different chapters assess the question of crop diversification in space and time on biocontrol at different scales of study. The first chapter shows a study maid at the laboratory scale, which aim is to assess the effect of crop diversification on the predator Harmonia axyridis, on its development and its predation capacity on four different types of aphids. The movement of the predator, hereafter called spillover, regarding the different food sources available is also assessed. The second chapter shows a similar study but laid at a bigger scale (greenhouse) and assesses the population dynamics, the predation capacity and the spillover of a population of Nesidiocoris tenuis regarding differential crop availability in space and time. The third chapter describes two experiments laid at the field scale which aim is to assess the effect of crop diversity, differing in space and time, on the abundance of naturally occurring pests and natural enemies. Abundances of pests and natural enemies was evaluated as a proxy of biocontrol and spillover between patches of crops was assessed in the polycultural treatment. Finally, the last chapter assessed the effect of crop diversity and crop configuration at the landscape scale on the abundance of aphids and aphidophagous predators (coccinellids, lacewings and spiders). The data were collected in the Hebei Province of China. Results at the laboratory and greenhouse scale show that individuals and populations of natural enemies are highly impacted by the functionality of the plant they are exposed to. Indeed, the natural enemies growth in polyculture was the result of the combined effect of the crops tested separately in monoculture. However, the spillover and the biocontrol were highly stimulated in polyculture, sometimes more than in monoculture. At the field scale, ladybirds spillover was higher in polyculture and populations of pests and natural enemies were strongly correlated in polyculture compare to monoculture. Finally, at the field scale, crop diversity had a positive effect on ladybirds abundance and a negative one on aphids abundance. Additionally, fields of small size increased the presence of lacewings. These results show that increasing crop diversity, and decreasing field size, might help to enhance the presence of natural enemies and increase the biocontrol through promoting the spillover of natural enemies between the different cultivated fields.; L’uniformisation du schéma agricole a eu pour conséquence une perte significative de la biodiversité des insectes utiles en agriculture. La diversification des plantes a de nombreux avantages agronomiques, notamment celui d’augmenter la présence d’ennemis naturels au champ en leur fournissant des sources diversifiées de nourriture. A l’échelle du paysage, la préservation des zones naturelles est un levier très efficace dans le maintien des ennemis naturels et le biocontrôle. Cette efficacité étant parfois contestée et difficilement gérable par les agriculteurs, lors de cette thèse j’ai étudié les effets de la diversification des cultures, dans l’espace et dans le temps, sur l’abondance des ennemis naturels, leur mouvement, et le service de biocontrôle. Dans une première partie, je pose la question de la faisabilité économique et sociale de systèmes agricoles basés sur une diversité culturale accrue. Il s’ensuit une seconde partie constituée de 4 chapitres mettant en application la question à 4 échelles de complexité croissante. Le premier chapitre pose la question de l’effet de la diversification des cultures, dans l’espace et le temps, à l’échelle de l’individu (Harmonia axyridis) sur le biocontrôle de plusieurs espèces pucerons, le spill-over et l’abondance du prédateur. Le chapitre 2 étudie l’effet de la diversification des cultures dans l’espace et le temps, à l’échelle d’une population du prédateur omnivore, Nesidiocoris tenuis, sur sa dynamique de population, son spill-over, et sur son service de biocontrôle d’œufs de lépidoptères (en condition contrôlée sous serre). Le chapitre 3 décrit des expérimentations mises en place à l’échelle du champ permettant de comprendre les effets de la diversité spatiale et temporelle des plantes cultivées sur des populations et les mouvements d’ennemis naturels et ravageurs spontanément présents en environnement agricole. Enfin, le chapitre 4 s’intéresse aux effets de la diversité des plantes cultivées à l’échelle d’une région, en Chine, sur l’abondance d’ennemis naturels et de pucerons en champs de coton et de blé. Les résultats à l’échelle du laboratoire et de la serre montrent que les individus et les populations d’ennemis naturels sont fortement impactés par la nature de la plante à laquelle ils sont exposés. En effet, la croissance des populations résultait de l’effet combiné des plantes testées en monoculture. Cependant le spill-over et le biocontrôle ont été plus fortement stimulés en polyculture. En champ, le spill-over des coccinelles et des chrysopes était plus important en polyculture. De surcroit, les populations de ravageurs et d’ennemis naturels étaient plus fortement corrélées en polyculture qu’en monoculture ; dans certains cas le biocontrôle s’est avéré plus important en polyculture. Enfin, à l’échelle du paysage, la diversité des cultures avait un effet positif sur les coccinelles et négatif pour les pucerons. De plus, les champs cultivés de petite taille étaient positifs pour le maintien des chrysopes dans les champs cultivés. L’ensemble de ces résultats montrent donc que l’augmentation de la diversité des plantes cultivées, ainsi que la réduction de la taille des champs cultivés, peuvent permettre une augmentation des mouvements des ennemis naturels entre les champs, ainsi qu’un accroissement des services de biocontrôle.

In agroecosystems, predatory ladybeetles play an important role in restraining aphid population growth and suppressing aphid populations. They can adapt to various habitats and make use of various aphid species associated with multiple host plants during their life cycle. Agricultural landscapes in China are composed of a mosaic of small fields with a diverse range of crops, and how ladybeetles make use of host plant diversity in such landscapes has rarely been documented. In this study, we examined the relationship between aphid densities and ladybeetle densities in two different settings: (i) on the majority of plant species (including crops, trees, and weeds) at a local field scale in 2013 and 2014, and (ii) in paired cotton and maize crop fields at a regional landscape scale in 2013. Overall, we found that aphid abundance determined predatory ladybeetle abundance at both the local field and landscape scales, and there was a positive correlation between aphid densities and ladybeetle densities. However, plant taxa had no significant influence on the predatory ladybeetle abundance at the local field scale. In addition, the effect of aphids on ladybeetles abundance was influenced by the crop type and growing season at the regional landscape scale. There was a significant positive correlation between aphids and ladybeetles populations on cotton only in July and August, whereas the correlation was significant for maize throughout the whole growing season. We also conducted an analysis of the stable carbon isotope ratios of the adult ladybeetles caught in cotton and maize fields (C3 and C4 crops, respectively) in a regional landscape-scale survey in 2013. The δ13Cvalue indicated that most prey aphids for ladybeetles originated from crops where aphids are abundant (cotton in June and July; both maize and cotton in August).These findings improved our understanding of the migration and dispersal of ladybeetles among different habitats and plant species and provided insight into the promotion of the regional conservation and pest control of natural enemies in northern China.

High plant biodiversity and landscape food provision stability is known to have a positive impact on biocontrol services provided by natural enemies. However, few studies have assessed how differential spatial and temporal crop richness actually impact biocontrol services, and notably how natural enemies may spill-over among crops. Within this framework, using a four-cage maze system under laboratory conditions, we evaluated the effect of a diversified crop system (four crops, namely cotton, tomato, squash and soybean) and low diversified crop systems (one single crop), on the generalist predator Harmonia axyridis (Pallas) and its predation capacity on aphids. The system varied food availability both in space, i.e. different cages, and in time, i.e. different dates of resources implementation. In general, the effects of crops on the natural enemies’ traits observed in the diversified crop system resulted from the average effects of the individual low diversified crop systems; the impact of the predator was highly dependent on the plant. Low diversified crop systems actually proved to be either very suitable for the predator’s development (tomato and squash), or not at all (soybean and cotton), but inversely suitable in reducing pest population with lower efficacy in tomato and squash and higher efficacy in soybean and cotton. The spillover of the ladybird was strongest in the squash low diversified crop system and lowest in the cotton one, other systems showing intermediate spillover values. In the diversified crop system, the ladybird presence was always closely related to plant presence, and aphid populations were maintained at a stable population increase. Still, the predator was also found in cages lacking plants, as opposed to the low diversified crop systems; this hinted at the potential of the highly diversified crop system to promote ladybird dispersal and increase foraging activity. We demonstrate that increasing crop diversity in agroecosystems may help promote biocontrol services provided by H. axyridis by promoting its spillover between crops (e.g. while the plants are senescing and/or when they are harvested).

High plant biodiversity and landscape food provision stability is known to have a positive impact on biocontrol services provided by natural enemies. However, few studies have assessed how differential spatial and temporal crop richness actually impact biocontrol services, and notably how natural enemies may spill-over among crops. Within this framework, using a four-cage maze system under laboratory conditions, we evaluated the effect of a diversified crop system (four crops, namely cotton, tomato, squash and soybean) and low diversified crop systems (one single crop), on the generalist predator Harmonia axyridis (Pallas) and its predation capacity on aphids. The system varied food availability both in space, i.e. different cages, and in time, i.e. different dates of resources implementation. In general, the effects of crops on the natural enemies’ traits observed in the diversified crop system resulted from the average effects of the individual low diversified crop systems; the impact of the predator was highly dependent on the plant. Low diversified crop systems actually proved to be either very suitable for the predator’s development (tomato and squash), or not at all (soybean and cotton), but inversely suitable in reducing pest population with lower efficacy in tomato and squash and higher efficacy in soybean and cotton. The spillover of the ladybird was strongest in the squash low diversified crop system and lowest in the cotton one, other systems showing intermediate spillover values. In the diversified crop system, the ladybird presence was always closely related to plant presence, and aphid populations were maintained at a stable population increase. Still, the predator was also found in cages lacking plants, as opposed to the low diversified crop systems; this hinted at the potential of the highly diversified crop system to promote ladybird dispersal and increase foraging activity. We demonstrate that increasing crop diversity in agroecosystems may help promote biocontrol services provided by H. axyridis by promoting its spillover between crops (e.g. while the plants are senescing and/or when they are harvested).