Wild bumblebee populations are under increasing threat from both a reduction in natural floral resources, as a direct result of anthropogenic disturbance, and an increase in disease prevalence, including emergent infectious diseases. This is of global concern as bumblebees are not only valued for their economic importance as pollinators but also culturally, as a charismatic component of the natural world. This thesis explores the impacts of phytochemicals found in the pollen and nectar of plants included in Agri-environment scheme (AES) planting strategies on bumblebee health, with a focus on their interactions with prevalent parasites. The five research chapters investigate the impact of a range of phytochemicals on two key bumblebee parasites, using a range of approaches including in vitro cellular growth experiments, in vivo experiments in individual bumblebees, both as larvae and adults, and finally epidemiological experiments on whole bumblebee colonies. In chapter one I review the pertinent literature relating to bumblebee declines and the impact of phytochemicals on pollinator health. In chapter two, larval inoculation with Crithidia bombi (Trypanasomatidae) resulted in no infected larvae seven days following inoculation. This result was critical for the design of subsequent chapters that tested the impact of phytochemicals on larval stages. In addition larvae were identified as disease transmission hubs (chapter published as Folly et al. 2017 Journal of Invertebr Pathol). Chapter three describes the identification of 62 unique phytochemicals from the pollen and nectar of AES plants. In addition, chapter three investigated the impact of four of these AES phytochemicals with known biological activity on C. bombi in vitro. Here caffeine had a significant positive effect on the growth of C. bombi at its ecologically relevant concentration before significantly reducing C. bombi growth at higher concentrations. In chapter four I designed a proof of principle investigation to ascertain if phytochemicals could impact Nosema bombi (Microsporidia) infection in B. terrestris using the isoflavone biochanin A. Biochanin A had a significant prophylactic effect in developing larvae and a significant therapeutic effect in infected adult workers. In chapter five, the phytochemicals caffeine and tricoumaroyl spermidine, which were found in nectar and pollen from