[Thesis defence] 5/12/2025 - Estelle BRIDOUX: "Effects of planting flowering cover crops on bees' exposure to pesticides and the pollination of apple trees". (UR INRAE ABEILLES ET ENVIRONNEMENT)

Ms Estelle BRIDOUX will publicly defend her thesis entitled "The effects of planting flowering cover on bees' exposure to pesticides and the pollination of apple trees", supervised by Mr Axel DECOURTYE and Mr Fabrice REQUIER, on Friday 5 December 2025.

Laboratory

Bees and the Environment Research Unit

Composition of the jury

Mr Axel DECOURTYE	ITSAP bee institute	Thesis co-director
Ms Emmanuelle PORCHER	MNHN	Rapporteur
Mrs Annie OUIN	UMR Dynafor	Rapporteur
Ms Marie-Pierre CHAUZAT	ANSES	Examiner
Mr Jean-Pierre SARTHOU	UMR 1248 INRAE/Toulouse INP-ENSAT AGIR	Examiner
Mr Fabrice REQUIER	UMR EGCE	Thesis co-director
Mr James DESAEGHER	INRAE-UR0406	Thesis co-supervisor

Summary

Pollinating insects are essential to the functioning of ecosystems and agriculture. However, their decline has been documented. In agricultural environments, they can face a number of threats, including habitat loss, a reduction in the quantity and diversity of floral resources, and exposure to pesticides. Agro-ecological developments can help to maintain them. In France, for example, there are plans to encourage the planting of flowering cover crops to mitigate the unintended effects of pesticides. We conducted a field study over three consecutive years in the Provence-Alpes-Côte-d'Azur region to explore the extent to which flowering canopies influence (1) entomophilic pollination of apple trees and (2) pesticide exposure of bees in orchards. Seven flowering canopies ranging in size from 700 to 4270m² and composed of mustard (Sinapis alba), phacelia (Phacelia tanacetifolia) and faba bean (Vicia faba) were sown in orchards. Apple tree pollination and the abundance and diversity of flower visitors were assessed as a function of distance from the flower canopy. We observed a greater diversity of flower visitors in flowered canopies than in apple trees at a distance. In addition, we observed a greater abundance of flower visitors in nearby apple trees, in particular a greater abundance of wild bees. Our results therefore suggest that the wild bees were attracted by the flower cover and that they overflowed into the nearby apple trees. We observed a non-linear effect of distance from the flower canopy on the initial fruiting rate, which was improved for apple trees located less than 33 m from the flower canopy. Pesticide exposure was examined for three species of managed bees Apis mellifera, Bombus terrestris and Osmia cornuta by analysing pesticide residues in samples of flowers, pollen, nectar and individuals. We observed that flowering canopies were less contaminated than crops, and that the proportion of flowering canopies in the bees' foraging area reduced the contamination of pollen collected by the bees. A. mellifera and O. cornuta. These results suggest that flowering canopies can reduce the exposure of pollinating insects to pesticides. Finally, we used structural equations to explore the potential direct and indirect effects of landscape composition (flowering cover, crops), pollen supply (quantity and diversity) and pesticide risk on the reproduction of O. cornuta in orchards. Although we did not observe any significant effect of flower cover on the reproduction of O. cornutaA positive direct marginal effect was observed. Overall, the results of this thesis suggest that flowering canopies can be effective in reducing pesticide exposure and could improve bee reproduction while improving pollination of nearby crops.

Keywords Mitigation measures, ecotoxicology, biodiversity, pollinating insects, agricultural production