PhD Defense: Safiatou Barro

Title: Weed Management and Herbicide Resistance: An Economic Analysis Abstract: Agriculture has undergone a major transformation driven by the massive use of pesticides. This intensification has made it possible to increase yields and stabilize production, thereby addressing global food security concerns. However, the dependence on pesticides has generated worrying environmental and health consequences. Beyond these effects, the very effectiveness of pesticides is now being challenged by the growing emergence of resistant pest populations, which increase control costs and threaten the economic viability of many farms. In light of this situation, a more rational use of pesticides has become necessary. This dissertation, organized into three chapters, seeks to shed light on this issue by focusing on herbicides. Chapter 1 analyzes the factors influencing the willingness of farmers engaged in weed control to adopt new sustainable agricultural practices, particularly collective approaches such as collective agri-environmental schemes. We show that farmers who already use certain weed control methods—such as mechanical treatment, combined strategies (mechanical and chemical), and crop rotation—are more inclined to participate in these new collective frameworks. In Chapter 2, we first examine the potential of Integrated Pest Management (IPM) to mitigate the development of resistance in the presence of intertemporal production externalities, showing that IPM contributes to reducing the overall level of resistance. Second, we analyze two types of policy instruments designed to internalize the negative intertemporal production externality: one input-based (taxing each unit of herbicide use) and one output-based (taxing the overall level of resistance). We derive the conditions under which both policy approaches lead to socially optimal strategies. Chapter 3 evaluates the effect of lacking information on the level of weed development, more specifically the interaction between susceptible and resistant weeds, on farmers’ strategies when they adopt a mixed approach (combined herbicide and mechanical control). We show that the absence of information leads farmers to rely more heavily on both herbicide and mechanical treatments than necessary. We then assess a taxation mechanism based on the total stock of susceptible and resistant weeds. We show that such a tax encourages more rational use of herbicide and mechanical treatments. Keywords: Herbicide Resistance, Collective Agri-Environmental Measures, Integrated Pest Management (IPM), Information, Public Policies.