Understanding how human activities affect species’ nutritional ecology is central to predicting responses to environmental change and conserving vulnerable species. Developing this understanding in bees is important because many bee species are experiencing widespread declines that may be driven, in part, by poor nutrition. This proposal aims to improve our understanding of nutritional ecology in the bumble bee Bombus terrestris and how it is affected by land-use and disease – a growing threat as commercial apiculture promotes pathogen spread. This overall aim will be achieved by addressing three Objectives. Objective 1: tests how 20 amino acids (AAs) interact with one another to affect B. terrestris’ fitness and nutrient regulation. This Objective applies a technique at the forefront of nutritional ecology called exome matching, where sequence data are used to predict individual AA requirements. This Objective will strengthen the foundation of bee nutrition research while optimizing exome matching methods for bees. Building on Objective 1, Objective 2 first tests how AAs interact with the two other major components of bee diets (carbohydrates and lipids) to affect key B. terrestris fitness traits, including immunity. Next, Objective 2 tests how control and immune challenged bumble bees regulate their intake of AAs, carbohydrates and lipids when given a choice of diets. This will reveal how these nutrients affect overall bumble bee performance and if bee dietary requirements may be altered by pathogen exposure, as well as if bumble bees self-select an optimal nutrient blend when they are healthy or immune challenged. This will be achieved using a powerful dietary mapping approach - the Geometric Framework of Nutrition. Finally, Objective 3 determines how pollen AA and lipid composition affects realized foraging decisions in diverse agricultural landscapes. This will illustrate how land-use affects bee nutrition. By uniting field and lab data, this Objective also tests if exome matching and the Geometric Framework of Nutrition predict foraging behavior in nature. Collectively, this work advances our understanding of bumble bee nutrition. More generally, this proposal will illustrate how environmental change can shift a pollinator’s nutritional ecology, while generating data that may help prevent pollinator loss.