A biodiversity-friendly rotational grazing system enhancing flower-visiting insect assemblages while maintaining animal and grassland productivity

Grazing management is an important tool to preserve insect biodiversity. Although literature has discussed the importance of grazing pressure adjustment to support grassland insect communities for the ecosystem services they provide, little has been published on the economic sustainability of such management adjustments to date. This study compared continuous grazing (CG) to an innovative rotational grazing system (the biodiversity-friendly rotation – BR), where a subplot was excluded from grazing for two months during the main flowering period. The effects of grazing two different species (cattle and sheep) within both systems were also evaluated. The aims were to assess the effects on butterfly, bumblebee, and ground beetle assemblages, along with the impact on herbage mass and animal performance. The BR enhanced both the abundance and species richness of flower-visiting insect assemblages and it was observed that cattle provided better results than sheep grazing. A multivariate redundancy analysis highlighted that most of the flower-visiting species (including almost all the endangered and locally rare species) were favoured by BR-cattle treatment, mainly due to the high percentage of flower cover and sward heterogeneity involved in this treatment. However, grazing system and grazer species did not affect ground beetle species richness or abundance. Moreover, herbage mass and animal performance (live weight and body condition score) were comparable between CG and BR throughout the grazing season. The BR could be a useful management system to enhance grassland flower-visiting insect assemblages whilst meeting farm production objectives, especially in protected environments where insect conservation is a major target

Les abeilles dans les prairies d'exploitations d'élevage aux environnements agricoles contrastés

National audienceIn areas of less intensive agriculture, permanent grasslands can act as favourable habitat and generate consistent food resources for bees. We studied bee populations over 2 years on 4 farms. Sampling took place during 2 periods each year. There were 3 study regions: Cantal, Corrèze, and Vienne. Two farms were located in the Cantal at high elevations, the third was found in the Corrèze, and the fourth was situated in the Vienne in the flatlands. The first three were grazing-based systems exploiting long-term grasslands; the latter was a mixed crop-livestock system. Among the 489 insects captured, we observed 75 species of bees from 14 genera. Most (80%) were solitary bees; the specific percentage varied by site. Only the honey bee was observed on all 4 farms. Although the Cantal farms had fewer bees total, they did have greater numbers of rare species and species from more diverse families and genera. These 2 farms utilised permanent grasslands and had maintained numerous natural landscape features.Les abeilles sont des pollinisateurs indispensables mais leur déclin est préoccupant. Les prairies permanentes en zone peu intensifiée devraient offrir aux insectes une ressource alimentaire régulière et un environnement favorable. Une étude a été conduite dans 4 exploitations aux situations contrastées. Les populations d’abeilles ont été estimées 2 années de suite, à 2 périodes printanières et dans 4 exploitations : 2 en altitude dans le Cantal, une en Corrèze (élevages herbagers basés sur des prairies de longue durée) et une en plaine (polyculture - élevage) dans la Vienne. Au total, 75 espèces d’abeilles appartenant à 14 genres différents ont été recensées parmi les 489 insectes capturés. La majorité (4/5) des abeilles capturées sont des abeilles solitaires dont les proportions varient selon les sites ; seule l’abeille mellifère est commune aux 4 sites. Contrairement au nombre d’abeilles capturées, le nombre d’espèces rares, de familles et de genres représentés est plus élevé dans les exploitations du Cantal, basées sur les prairies permanentes qui ont conservé de nombreux éléments paysagers

Investigating a flower-insect forager network in a mountain grassland community using pollen DNA barcoding

International audienceFaced with the decline of pollinators, it is relevant to strengthen our understanding of the whole plant-pollinator web in semi-natural grasslands that serve as refuges for pollinator populations. The aim of this study was to explore the diversity of flower-foraging insects involved in pollen transfer in mountain semi-natural grasslands. Insects actively collecting pollen and/or nectar were caught in spring in six mountain semi-natural grasslands displaying a floristic richness gradient. Individual determinations of insects were made at the finest possible taxonomic scale and pollen loads were removed from the insect body. Using next-generation DNA sequencing, pollens were identified through the ribosomal DNA cistron using the ITS2 database and the ITS plant rDNA cistron sequences from Genbank. A total of 236 flower-foraging insects were collected. Diptera represented 82% of the total catches distantly followed by Hymenoptera (15%) and Apoidea (bees) (11%). Visual observations revealed that Diptera foraged on 16 of the 21 flower species visited by insects. DNA metabarcoding showed that 82% (191) of all of the collected insects were carrying pollen and 44% (104) were carrying two genera of plants or more. Our results demonstrate that Diptera are potential key-pollinators in mountain semi-natural grasslands that cannot be overlooked by the scientific community. However difficulties of taxonomic determination due to severe shortage of experts for Diptera have to be urgently overcome. Further studies on the link between pollen transfer and actual pollination in a global change context are also required. Moreover, our results support the idea that DNA metabarcoding provides accurate information about the plants-insects networks but it also pointed out sensitive issues, especially the necessity to build reliable national barcode databases

Insectes butineurs et transport de pollen : 1er maillon de l'étude du service de pollinisation rendu par l'écosystème prairial

The pollination service provided by the grasslands is very often highlighted, but it is poorly clarified and poorly quantified. The objective of this study was to quantify plant-insect interactions at the origin of this service in grasslands belonging to three contrasted agrosystems. To do this, we captured insects foraging flowers over 3 periods on transects in 18 grasslands within a gradient of floristic diversity and owned on three experimental farms of Inra located at Marcenat (Cantal), Mirecourt (Vosges) and Lusignan (Vienna). We tested in parallel an analysis method based on barcoding, allowing the simultaneous determination of the insect and pollen it carries. We analyzed 979 flowers-foraging insect couples. The results hightliht the important role of Diptera in the temperate grassland multi-species as foragers and potential pollinators, particularly at the befinning of the growing season. It also shows that interactions between insect families and plant families observed in Marcenat were more numerous and distributed more equitably than in Mirecourt and Lusignan. Moreover, analysis of pollen using DNA barcodes pointed out that a large proportion of the captured insects (82 %) carried pollen and that 44 % carried two to six different botanical genera. These first results allow identifying some actors of the pollination service in grasslands.Le service de pollinisation rendu par l'écosystème prairial est très souvent mis en avant mais il est peu explicité et peu quantifié. L'objectif de cette étude est de quantifier les interactions plante-insecte à l'origine de ce service dans des prairies appartenant à 3 agrosystèmes contrastés. Pour cela, nous avons capturé les insectes butinant les fleurs sur des transects, à 3 périodes de l'année, dans 18 prairies présentant un gradient de richesse floristique et appartenant à 3 fermes expérimentales INRA située à Marcenat (Cantal), Mirecourt (Vosges) et Lusignan (Vienne). Nous avons testé en parallèle une méthode d'analyse basée sur le méta-barcoding, permettant la détermination simultanée de l'insecte et des pollens qu'il transporte. Nous avons analysé 979 couples fleur-insecte butineur. Les résultats mettent en évidence le rôle important des Diptères dans les prairies permanentes tempérées multi-espèces en tant que butineurs et pollinisateurs potentiels, notamment en début de saison de végétation. Elle montre également que les interactions entre familles d'insectes et familles de plantes observées à Marcenat sont plus nombreuses et réparties plus équitablement qu'à Mirecourt et Lusignan. Une forte proportion des insectes capturés (82 %) transportait du pollen et l'analyse des barcodes ADN révèle que 44 % transportaient en même temps de 2 à 6 genres botaniques différents. Ces premiers résultats permettent d'identifier les acteurs potentiels du service de pollinisation dans les prairies