A PRECIPITOUS DECLINE IN RANGE AND ABUNDANCE OF MULTIPLE SPECIES OF NATIVE BUMBLE BEES IS OF GRAVE CONCERN TO BIOLOGISTS AND AGRICULTURALISTS. THE VITAL ROLE PLAYED BY THESE POLLINATORS IN THE REPRODUCTIVE SUCCESS OF NATIVE AND CULTIVATED FOOD PLANTS UNDERSCORES THE URGENT NEED TO UNDERSTAND THE CAUSES OF DECLINE. CORRELATIONAL DATA SUGGEST THE PATHOGEN NOSEMA BOMBI COULD BE A FACTOR IN THE U.S. DECLINES, BUT NO EMPIRICAL DATA YET SHOW THAT IT HAS PLAYED A CAUSAL ROLE. NEONICOTINOID INSECTICIDES, SUCH AS IMIDACLOPRID USED WIDELY IN CONTEMPORARY AGROECOSYSTEMS, ARE HYPOTHESIZED TO EXACERBATE PATHOGEN EFFECTS ON POLLINATOR HEALTH. THIS PROPOSAL ADDRESSES HOW BUMBLE BEE SPECIES VARY IN SUSCEPTIBILITY TO PATHOGENS AND PESTICIDES, INVESTIGATING HOW THESE STRESSES CAN INTERACT TO THE DETRIMENT OF BEE VIABILITY. AN INTEGRATION OF COLONY-LEVEL, WHOLE-ORGANISM, CELLULAR AND TRANSCRIPTOMIC APPROACHES WILL ADDRESS HOW SUSCEPTIBILITY IS LINKED TO DECLINE STATUS. THESE INDIVIDUAL AND SYNERGISTIC EFFECTS HAVE NOT BEEN EXPLORED IN RELATION TO U.S. BUMBLE BEE DECLINES. BY TESTING HYPOTHESES THAT (1) DECLINING BEE SPECIES ARE MORE SUSCEPTIBLE TO NOSEMA INFECTION THAN SPECIES WITH STABLE POPULATIONS, AND (2) THAT IMIDACLOPRID ENHANCES NEGATIVE PATHOGEN EFFECTS, ESPECIALLY IN DECLINING SPECIES, OUR OBJECTIVES ADDRESS PRIORITIES OF AFRI'S PESTS AND BENEFICIAL SPECIES IN AGRICULTURAL PRODUCTION SYSTEMS PROGRAM AND THE 2015 FEDERAL POLLINATOR RESEARCH ACTIONPLAN: TO UNDERSTAND FACTORS ASSOCIATED WITH DECLINE OF INSECT POLLINATORS. UNDERSTANDING DISEASE DYNAMICS IN MULTIPLE SPECIES HAS IMPLICATIONS FOR APPROPRIATE SPECIES SELECTION IN COMMERCIAL REARING, FACILITATING DEVELOPMENT OF ANTI-MICROSPORIDIAL MEDICATIONS FOR THE TREATMENT OF DISEASE IN REARING FACILITIES, AND APPROPRIATE AGROCHEMICAL USE.