The endangered designation is made by the U.S. Fish and Wildlife Service under the Endangered Species Act for species that are in danger of becoming extinct throughout all or a portion of their range. Once common and abundant across 28 states from Connecticut to South Dakota, the District of Columbia and two Canadian provinces, the rusty patched bumble bee has experienced a swift and dramatic decline since the late 1990s. Abundance of the rusty patched bumble bee has plummeted by 87 percent, leaving small, scattered populations in 13 states and one province.
Causes of the decline in rusty patched bumble bee populations are believed to be loss of habitat; disease and parasites; use of pesticides that directly or indirectly kill the bees; climate change, which can affect the availability of the flowers they depend on; and extremely small population size. Most likely, a combination of these factors has caused the decline in rusty patched bumble bees.
— source fws.gov