The western bumble bee (Bombus occidentalis) is named after its historic range; it was once common in 14 western US states and four Canadian provinces. Since the late 1990's this bee has undergone a dramatic decline in the western part of its range and populations are currently found only in the northern and eastern portions of its range. The rapid decline is believed to be caused by an introduced pathogen, possibly Nosema bombi, which was likely picked up by queens sent to European rearing facilities and then sent back to the United States in the early 1990's. It is hypothesized that after the return of these newly infected bees, the disease then spread to wild populations of western bumble bees, Franklin's bumble bees (B. franklini), rusty patched bumble bees (B. affinis), and yellowbanded bumble bees (B. terricola). A decline in each of these species was noticed in the late 1990's. Additional threats to bumble bees include other pests and diseases, habitat destruction, pesticides, invasive species, natural pests or predators, and climate change.