UC Davis

Huanglongbing (HLB) is an invasive disease of citrus trees associated with the bacterium “Candidatus Liberibacter asiaticus” and transmitted by the Asian citrus psyllid (ACP), Diaphorina citri, that is threatening citrus production in California and other citrus-producing areas of the world. Current strategies to prevent citrus trees from being infected with HLB are based on the application of coordinated insecticide treatments for the insect vector, detection and removal of HLB-positive trees and use of certified plant material. These measures are most effective if applied on an area-wide scale by all citrus growers in a region, yet little is known about the California citrus growers’ willingness to coordinate measures across property boundaries. When individuals need to make contributions to achieve a collective effort but may benefit from the efforts of others without bearing the costs, they may be tempted to free-ride on others’ efforts, giving rise to a collective action problem. This type of problem has been extensively studied by the social-ecological systems literature, but it has rarely been addressed in the context of plant diseases. In this dissertation, a social-ecological systems perspective was used to integrate the social and ecological dimensions of HLB management in order to explore what strategies may be more effective to achieve collective action for this disease. The first chapter introduces the idea of plant health provision as a public good collective action problem. Ostrom’s design principles for long-enduring common-pool resource institutions are used as a reference to compare the institutional approaches that have been developed to achieve collective action for HLB in California and other citrus-producing areas, illustrating how these principles could be applied to other plant diseases that are threatening food security, and suggesting a link between institutional approaches that follow the design principles and successful collective outcomes. The second chapter explores the California citrus industry’s propensity to adopt voluntary measures to manage HLB. A multivariate ordinal logistic regression model is used to analyze a survey distributed to 300 participants, showing that the propensity to adopt management measures may depend on the citrus stakeholder’s perceived vulnerability to HLB, as well as their intention to stay informed and communicate with the regional coordinators of the HLB control program and their neighbors. In addition, the analysis sheds light into what combinations of management measures may be adopted together as an integrated pest management approach to HLB. The third chapter focuses on the area-wide management (AWM) program for ACP in Southern California, examining the individual perceptions and group-level determinants of collective action for AWM. It shows that citrus stakeholders are aware of the benefits of coordinating insecticide treatments for ACP, but they identify the lack of participation as the main obstacle for collective action, and some do not believe that their neighbors will coordinate. To face this collective action problem, two distinct institutional approaches have been developed to coordinate insecticide treatments for ACP, one in which treatments are voluntary, and one in which they are mandatory. An analysis of participation in AWM in Southern California over nine seasons shows that these two institutional approaches have followed a different trajectory over time. In addition, group-level variables from collective action theory, such as the size of the group or the heterogeneity in grove size, have had a negative impact on participation and may be relevant for the design of future AWM programs. This dissertation contributes to answering the question of what institutional approaches and strategies might be more effective to deal with the spatial and temporal dynamics of plant diseases while staying aligned with the preferences, values and needs of the societies affected, setting the basis for further interdisciplinary research that will likely benefit the management of HLB and other plant diseases that give rise to collective action problems.