The exact cause of the rapid decline of B. affinis is still not entirely understood. Past studies have suggested a series of plausible explanations, but further study is needed to identify threats, assess their severity, and to develop effective strategies to mitigate or eradicate future impacts.

Below are hypothesized factors leading to the decline of B. affinis populations in Canada and the United States.

1. Exposure to pesticides

Fig 6. Use of the insecticide imidacloprid in U.S counties in 1995 (left) and 2012 (right) Counties colored darker red have a higher incidence of imidacloprid use. Map via Xerces Society for Invertebrate Conservation.

Use of insecticides such as Imidacloprid (a pesticide designed to chemically mimic nicotine, which is toxic to insects) can have extremely harmful impacts on bee populations. While these pesticides are designed to limit pests, they also have been shown to be harmful to bees and other pollinating insects. Instead of using insecticide in your garden, manage pests by planting fragrant herbs such as lemon grass to naturally deter pests or introduce populations of native predator insects such as lady bugs!

2. Exposure to disease

Commercially reared bees such as the European Honey Bee have been found to carry diseases which are easily transmittable to native bee species. According the the Xerces Society for Invertebrate Conservation, this may be the leading cause of B. affinis decline. Bees are often bred in Europe or other foreign locations before being transported to the U.S. for commercial pollination, and these bees may act as vectors of deadly infections such as the fungal parasite Nosema bombi. 

3. Habitat loss and fragmentation

Large portions of B. affinis native range has been lost due to land development for agriculture and urbanization, and due to the economic benefits of these developments, there has been little consideration of potential impacts on native bee populations. 

Fig 7. An Estimated 87% of the historical range of Bombus affinis has been lost in recent years. Map depicting historical range (left) versus current range (right) of Bombus affinis. Map via University of Wisconsin Fruit Program.

4. Invasive species

As early as 1622, English settlers began transporting non-native bees such as the European Honey Bee across the ocean to North America. While this introduction has proven beneficial for agriculture, it can devastate already established native populations. The European Honey Bee, Apis mellifera, is a social species which can often outcompete native, solitary species for pollen resources. These bees have also been found to carry harmful disease. Rusty patched bumble bees are one of the few social species of bees native to the United States, which is why it is so important to specifically focus conservation efforts on this species.

5. Effects of climate change