Titre de la dissertation: « Evaluation of nutritional and agrochemical stressors on bee health ».

Promoteur de thèse: Monsieur Denis Michez

Résumé: With more than 2,000 species currently recorded in Europe, bees are a highly diversified and efficient group of pollinating insects. However, populations decline recorded worldwide is associated to a risk for ecosystems functioning and crop yields. Multiple drivers of this decline have been pointed out, such as pesticides, pathogens, poor diet or climate change, and their respective interactions. The present Ph-D focussed on two main factors of decline, nutritional stress and pesticides, and their interactions in agro-ecosystems. Bees obtain their nutrients exclusively from nectar and pollen of flowers. A reduction in floral resource abundance and diversity due to landscape simplification and habitat loss is generally observed in agro-ecosystems. At the same time, bees are exposed to various agrochemical products. While negative impact of those stressors is expected, variation among species, physiological responses and intensity of the impact are still poorly understood. To address these knowledge gaps, we developed original experiments in laboratory conditions to evaluate (i) the interspecific variations in bee nutritional requirements and in bee sensitivity to pesticides, (ii) the impact of nutrition on bumblebee’s sensitivity to agrochemicals and (iii) the impact of agrochemicals on nutritional intake. Overall, we found that nutritional optimum, as well as pesticide sensitivity are different across species. Moreover, stress interactions induced modifications of the proteome and impacted reproduction parameters in bumblebees. Bees highly exposed to pesticides were similarly affected regardless of the pollen diet, indicating that a good diet may not compensate for the impact of an agrochemical exposure. Nutritional intake was altered during chronic pesticide exposure by impairing the frequency of flower visits and reducing the number of proboscis extensions, with the strongest effects when exposed to pesticide mixture. While already of concern, these results could be exacerbated when performing more complex behaviours (e.g. flying, orientation, finding mates…) under natural conditions with exposure to additional stresses like parasites or temperature. Our study supports the need of conserving and/or restoring plant diversity into managed ecosystems to meet the nutritional optimum of bees at species and community level, along with an advanced monitoring of the use of agrochemicals. To reach these objectives, new protocols to rear and maintain wild bee species under laboratory conditions are needed.