Diversification spatiale des agroécosystèmes pour la lutte biologique contre les insectes ravageurs: Intérêts des associations de culture et des bandes fleuries

Facing the limits of input-intensive agriculture, agroecology aims at thinking ways to design a sustainable agriculture that is economically viable and socially relevant. It notably invites to mobilize ecological processes within agroecosystems in order to enhance the delivery of ecosystem services towards reducing the use of external inputs – among others insecticides. For enhancing biological control of insect pests, a strategy is to spatially diversify agroecosystems at the field scale. Whereas increasing plant diversity could directly negatively affect pest development on the one hand (i.e. bottom-up effect), providing flowering features could allow the enhancement of natural enemies and their direct effect on pest populations on the other hand (i.e. top-down effect). The present thesis focused on intercropping (i.e. the cultivation of at least two crop species simultaneously in a same field) as a way to diversify crop habitat, and the sowing of wildflower strips as a non-crop feature. A systematic analysis of the literature revealed that, in most of studies, wheat (Triticum aestivum L.)-based intercropping allows a reduction of insect pests on crops, without necessarily favouring their natural enemies, compared to pure-stands. Besides, the provision of flowering resources, by for instance sowing wildflower strips, can attract and support flower visiting predators and parasitoids. Hence, in a first set of field experiments, combining the two tactics of increasing crop diversity and providing flowering resources was tested. First in China, wheat crop was associated with oilseed rape (Brassica napus L.), but it neither allowed reducing aphid (Hemiptera: Aphididae) abundance nor enhancing their natural enemies compared to pure stands. Instead, aphid density – independently from the treatments – affected natural enemy abundance, and interspecific relations between aphids and their natural enemies were observed. Second in Belgium, wildflower strips were sown within a wheat field, which led to a reduction of aphid density in wheat plots in between flowering features and an increase of aphidophagous hoverflies (Diptera: Syrphidae) compared to pure-stand wheat. Nevertheless, the presence of flowering strips did not affect the other natural enemies, i.e. lacewings (Neuroptera: Chrysopidae), ladybeetles (Coleoptera: Coccinellidae) and parasitoid wasps (Hymenoptera: Braconidae). Therefore, a second set of field experiments focussed on ways to compose mixtures of wild flowers attractive to a diversity of natural enemies. Flower functional traits were considered due to their effect on insect behaviour. First, the hypothesis that mixtures with high functional diversity attract and support a high abundance and diversity of aphid flower visiting predators was tested. This hypothesis was not verified. Instead, the high density in the plots of some flower species (especially the Asteraceae Leucanthemum vulgare Lam.) known to be attractive to flower visitors was supposed to have overwhelmed the effect of functional diversity. Second, a methodology was developed to identify which flower traits significantly affect natural enemy abundance – in this experiment parasitoids of oilseed rape beetle pests (i.e. Meligethes spp. [Coleoptera: Nitidulidae] and Ceutorhynchus spp. [Coleoptera: Curculionidae]) – in flower mixtures. Among seven traits, visual traits (i.e. colour, ultra-violet reflectance) and the one related to food availability (i.e. corolla morphology) were found to significantly affect parasitoid abundance. These results highlight that (i) increasing plant diversity at the field scale can – but not systematically – favour a reduction of insect pests, (ii) including flowering features can enhance some – but not all – of their natural enemies, and (iii) in order to compose flower mixes attractive to natural enemies, specific flower traits – rather than functional diversity at the mixture level – can be considered. These results are discussed in a broader perspective. Indeed, strategies to spatially diversify crop and non-crop habitats in agroecosystems are various, as well as the ways to compose, manage and design such habitats. Also, processes at larger scales than the field may be determinant. Moreover, insects are not the only pests, and pests are not the only biotic or abiotic elements that need to be regulated in agroecosystems. Controlling multiple pests simultaneously but also enhancing the provision of multiple regulating services represent challenges for future research in agriculture.Face aux limites de l’agriculture intensive en intrants, l’agroécologie propose un cadre pour penser la conception d’une agriculture écologiquement durable, mais aussi économiquement viable et socialement responsable. Notamment, elle invite à mobiliser les processus écologiques au sein des agroécosystèmes dans le but d’accroitre la fourniture de services écosystémiques, pour réduire l’usage des intrants externes – parmi d’autres les insecticides. Une stratégie pour favoriser la lutte biologique contre les insectes ravageurs est de diversifier spatialement les agroécosystèmes à l’échelle parcellaire. D’un côté, accroitre la diversité végétale peut directement pénaliser le développement des ravageurs (effet « bottom-up »), d’un autre côté fournir une resource florale peut favoriser le développement d’ennemis naturels et potentiellement leur action de prédation et de parasitisme sur les ravageurs (effet « top-down »). Dans ce but, la présente thèse s’intéresse aux associations de cultures (cultiver au moins deux cultures simultanément dans la même parcelle) comme un outil de diversification des habitats cultivés, et au semis de bandes de fleurs sauvages comme habitat semi-naturel. Une analyse systématique de la littérature montre que, dans la plupart des études, les associations de cultures intégrant le blé (Triticum aestivum L.) permettent une réduction de l’abondance en insectes ravageurs, sans pour autant favoriser leurs ennemis naturels, en comparaison à des cultures pures. Par ailleurs, la fourniture de ressource florale, en semant par exemple des bandes de fleurs sauvages, peut attirer et maintenir des populations de prédateurs et de parasitoïdes. D’où, un premier ensemble d’expériences consista à évaluer en champ l’effet combiné de l’accroissement de la diversité végétale et de la fourniture de ressources florales sur les populations de ravageurs et d’ennemis naturels. Dans un premier temps en Chine, du blé fut associé à du colza (Brassica napus L.), mais cela ne permit ni de réduire les populations de pucerons (Hemiptera : Aphididae), ni d’accroitre celles de leurs ennemis naturels, en comparaison à des parcelles en culture pure. Les ennemis naturels furent plutôt affectés par l’abondance en pucerons – indépendamment des traitements – et des relations interspécifiques entre pucerons et ennemis naturels furent observées. Dans un second temps en Belgique, des bandes de fleurs sauvages furent semées au sein d’une parcelle de blé. Une moindre abondance en pucerons et un accroissement du nombre de syrphes aphidiphages (Diptera : Syrphidae) fut observé dans la culture entre les bandes fleuries en comparaison à des parcelles en culture pure. Néanmoins, la présence de fleurs sauvages n’affecta pas les autres ennemis naturels, précisément les chrysopes (Neuroptera : Chrysopidae), les coccinelles (Coleoptera : Coccinellidae) et les parasitoïdes (Hymenoptera : Braconidae). C’est pourquoi, un second ensemble d’expériences en champ se pencha sur la composition des mélanges fleuris et leur attractivité pour une diversité d’ennemis naturels. Les traits fonctionnels des fleurs furent considérés du fait de leur effet sur le comportement des insectes. Dans un premier temps, l’hypothèse que des mélanges fleuris ayant une diversité fonctionnelle élevée attirent et maintiennent une grande diversité et abondance de prédateurs de pucerons fut testée. Cette hypothèse ne fut pas vérifiée. Il fut supposé que la densité élevée, dans quelques unités expérimentales, de certaines espèces de fleurs (en particulier l’Asteraceae Leucanthemum vulgare Lam.) connues pour être particulièrement attractives, a pu prendre le dessus sur l’effet de la diversité fonctionnelle. Dans un second temps, une méthodologie fut développée dans le but d’identifier les traits fonctionnels qui effectivement affectent l’abondance des ennemis naturels – dans cette expérience les parasitoïdes des ravageurs coléoptères du colza (Meligethes spp. [Coleoptera : Nitidulidae] et Ceutorhynchus spp. [Coleoptera : Curculionidae]) – dans les mélanges fleuris. Parmi sept traits, ceux relatifs à la vision (couleur, réflectance ultra-violette) et à la disponibilité en ressource alimentaire (morphologie de la corolle) eurent un effet significatif sur l’abondance en parasitoïdes. Ces résultats soulignent que (i) accroitre la diversité végétale à l’échelle parcellaire peut – mais non systématiquement – favoriser une diminution de l’abondance en insectes ravageurs, (ii) inclure des espèces florales peut accroitre certains – mais pas tous les – ennemis naturels, et (iii) pour composer des mélanges fleuris attractifs pour les ennemis naturels, certains traits floraux particuliers – plutôt que la diversité fonctionnelle à l’échelle du mélange – pourraient être considérés. Ces résultats sont discutés dans une optique plus large. En effet, les stratégies de diversification des habitats cultivés et non-cultivés au sein des agroécosystèmes sont diverses, tout autant que les moyens de composer, gérer et agencer ces habitats. De plus, des processus à des échelles supérieures que celle du champ cultivé peuvent être déterminants. Enfin, les insectes ne sont pas les seuls ravageurs, et les ravageurs les seuls éléments biotiques ou abiotiques qui doivent être régulés dans les agroécosystèmes. Réguler les populations de ravageurs et offrir une diversité de services écosystémiques de manière conjointe représente un défi scientifique pour les recherches futures

Aromatic plants in East-Asia to enhance natural enemies towards biological control of insect pests. A review

Introducing flowering plants in fields may attract and benefit predators and parasitoids of insect pests and potentially enhance biological control when associated with crops. Through a vote counting analysis, the present review evaluates whether aromatic plants of East Asia could be used in this purpose. Through a systematic search of the scientific literature, we identified 64 papers published worldwide considering 32 aromatic plant species. We found a significant number of studies reporting that Apiaceae aromatic plants (Anethumgraveolens L., Coriandrumsativum L., Foeniculumvulgare L.) attract and benefit (in terms of longevity and/or fecundity) insect predators. Contrasting results were found for parasitoids, as well as with plant species from other families (mostly Asteraceae and Lamiaceae). Functional traits of plants (corolla shape, colour, volatiles) and of insects (mouthparts mainly) were considered to discuss these results. Additionally, we have not found a significant number of studies reporting an enhancement of biological control in crop or fruit trees cultivated adjacent to aromatic plants. However, the number of studies was limited, suggesting a need for further field-based research. Still, the retrieved studies highlight a recent interest for sowing aromatic plants in fields and orchards towards biological control of insect pests in East Asia and especially in China

Do wildflower strips favor insect pest populations at field margins ?

Reducing pesticide use is one the major issues of today’s agriculture. Among other possibilities, attracting and conserving pest natural enemies in agricultural landscapes by providing them habitats is promising. Wildflower strips (WFS) sown at field margins are one of these potential habitats. They are known to attract and conserve a large diversity of insects, as they provide them food resources such as pollen and nectar, as well as shelter and overwintering sites. However, the risk of attracting insect pests at field margins may represent an obstacle to their adoption by farmers. Conversely, it would be interesting if such WFS could play the role of pest trap crops. In an experimental field sown with WFS intercropped with oilseed rape (OSR) (Brassica napus L.), its coleopteran pests were trapped in both WFS and OSR using yellow pan traps between April and June 2014. More than 130 000 Meligethes spp., Ceutorhynchus spp. and Psylliodes chrysocephalla (L.) adults were trapped. Meligethes spp., Ceutorhynchus spp. were significantly more abundant in the OSR compared with WFS when adults emerged and populations reached their abundance peak. Before and between these periods, the few adults trapped were significantly more abundant in the WFS compared with the OSR. Concerning P. chrysocephala, too few individuals were caught for analysis. Results showed that OSR was more attractive than WFS when coleopteran pests were abundant. In this study, WFS sown for insect conservation may neither favour insect pest conservation at field margin, nor be considered as trap crops

Biodiversity and ecosystem services: think functional!

During the last years, several studies and reviews have considered the relation between biodiversity and ecosystem functioning or the provision of ecosystem services. Many studies found that plant functional traits and plant functional diversity (FD) are key drivers in this relation in terrestrial ecosystems. Researchers used different methods to obtain a gradient in plant FD to examine the effect on ecosystem services, going from observational studies of natural communities to synthetic assemblages. Furthermore, different methods exist to quantify plant FD going from simple functional trait richness to indices, distance-based frameworks and the division into FD components. In the AgricultureIsLife project, we set up a field experiment aiming to examine the biodiversity – ecosystem service relation in agricultural context. The experiment consists of perennial wildflower strips with different plant functional diversities in an arable field with conventional crop production. The wildflower strips were sown as synthetic assemblages but are subject to natural succession during the following years. We monitor the evolution of FD from the sowing to the establishment of a typical wildflower strip using Rhao’s quadratic entropy index to quantify FD. In addition, the flower strips will be monitored for four ecosystem services they are expected to provide: pollination, pest control, biodiversity support and provision of valuable compounds.AgricultureIsLif

Checklist des Aphidiinae (Hymenoptera: Braconidae) et Aphelinus (Hymenoptera: Aphelinidae) de Belgique avec respectivement quatre et trois nouveaux enregistrements d'espèces

Aphid parasitoids have good potential for crop protection. However, they have been poorly studied in Belgium, especially in terms of species diversity. Therefore, the aim of this work was to establish the first checklist for the country. To complete the list, aphid parasitoids were sampled in wheat and pea fields near Gembloux (Belgium), in 2013 and 2014. Among the identified species, Aphelinus asychis Walker, Aphelinus daucicola Kurdjumov, Aphelinus fusciscapus (Förster), Aphidius asteris Haliday, Aphidius eadyi Starý, Gonzalez & Hall, Praon barbatum Mackauer, and Trioxys auctus (Haliday) were recorded for the first time in Belgium. Thirty-two Aphidiinae and seven Aphelinus species were included in the checklist. It is hoped this study will stimulate further research, as species diversity is still low compared with neighbouring countries

Checklist des Aphidiinae (Hymenoptera: Braconidae) et Aphelinus (Hymenoptera: Aphelinidae) de Belgique avec respectivement quatre et trois nouveaux enregistrements d'espèces

peer reviewedAphid parasitoids have good potential for crop protection. However, they have been poorly studied in Belgium, especially in terms of species diversity. Therefore, the aim of this work was to establish the first checklist for the country. To complete the list, aphid parasitoids were sampled in wheat and pea fields near Gembloux (Belgium), in 2013 and 2014. Among the identified species, Aphelinus asychis Walker, Aphelinus daucicola Kurdjumov, Aphelinus fusciscapus (Förster), Aphidius asteris Haliday, Aphidius eadyi Starý, Gonzalez & Hall, Praon barbatum Mackauer, and Trioxys auctus (Haliday) were recorded for the first time in Belgium. Thirty-two Aphidiinae and seven Aphelinus species were included in the checklist. It is hoped this study will stimulate further research, as species diversity is still low compared with neighbouring countries

Des mélanges fleuries présantant une diversité fonctionnelle élevée favorisent-ils les prédateurs des pucerons dans les bandes fleuries?

peer reviewedAmong semi-natural elements in agricultural landscapes, wildflower strips sown at field margins or within fields represent potential habitats for the natural enemies of insect pests. As insects are sensitive to a variety of flower traits, we hypothesised that mixtures with high functional diversity attract and support a higher abundance and species richness of aphid flower visiting predators compared to mixtures with low functional diversity. During a field experiment, repeated over two years (2014 and 2015) in Gembloux (Belgium), aphid predators (i.e., lacewings, ladybeetles and hoverflies) were pan-trapped in five sown flower mixtures (including a control mixture, with three replicates of each mixture) of low to high functional diversity based on seven traits (i.e., flower colour, ultra-violet reflectance and pattern, blooming start and duration, height and flower class, primarily based on corolla morphology). In both years, flower species in the sown mixtures (i.e., sown and spontaneous flowers) were listed, and the realised functional diversity of each plot was calculated. Over the two years, an increase in functional diversity did not result in an increase in the abundance and richness of aphid predators. Moreover, ladybeetles, representing the majority of trapped predators, were more abundant in mixtures with very low or intermediary functional diversity at sowing, especially in 2014. We hypothesise that certain flower species, which were abundant in certain mixtures (and not in those exhibiting the highest functional diversity), attracted predators and were sufficiently represented to support them. Our results present novel information that could be used to the development of flower mixtures that provide effective ecosystem services, such as pest control