Officially endangered: the rusty-patched bumble bee

It’s official: the US Fish and Wildlife Service has listed the rusty-patched bumble bee, Bombus affinis, as an endangered species. This bee, in the family Apidae, is the first insect from the continental United States to make the list. Last year, seven species of Hylaeus were the first bees to make the list, but they occur only in Hawaii.

Like many things that were once abundant such as bison and passenger pigeons, it was easy to take the rusty-patched bumble bee for granted. It was once a common sight in eastern North America, occurring in 28 states and 2 provinces. But populations began to crash in the mid-1990s. The most recent data, collected in the early 2000s, show small populations remaining in only 13 states and Ontario, a decrease in range of about 87%.

Small populations lose genetic variability

Since no thorough counts have occurred since the early 2000s, populations are likely even lower. In fact, very few colonies have been spotted in recent years. When colonies are few and far between, outcrossing between non-related colonies is less likely to occur and inbreeding increases. Loss of vigor, often called inbreeding depression, makes the colonies more prone to disease, stress, and poor nutrition among other things.

Since accurate data on insect populations and distribution was not a priority in the past, little data is available to show trends over time. Many entomologists suspect that the number of rusty-patched bumble bee colonies was declining long before it came to anyone’s attention.

A large bee with a short tongue

The rusty-patched bumble bee is one of the large bumble bees. According to Bumble Bees of North America (2014) it is a short-tongued species that prefers to live in prairies, woodland, marshes, parks, urban gardens, and agricultural areas. They prefer to nest in abandoned burrows that once belonged to chipmunks or rabbits.

The rusty-patched bumble is polylectic, meaning it forages on a wide range of flowering plants. Depending on the local climate, individuals may be seen from April through October, but it is most abundant in July and August.

Habitat loss is crucial

According to a report by the Natural Resources Defense Council, the primary reason for the population decrease is habitat loss. But other factors such as the disease Nosema bombi, which has spread from commercial bumble bee stocks, may have had a large impact.

Other detrimental conditions include heavy use of pesticides, habitat degradation, loss of forage, and climate change. Climate change causes mismatched timing between flowering plants and the bees that depend on them. It may also cause increased flooding that destroys underground burrows, and prolonged drought that can interfere with the food supply.

Ultimately, as colonies become more scattered, the dynamics of small populations—like inbreeding depression and large numbers of sterile males—causes what is known as an extinction vortex. An extinction vortex is easy to visualize: just imagine a population of any animal swirling around a drain until it finally disappears. As the number of individuals gets lower and lower, the remainder disappear faster and faster.

How can you help the rusty-patched bumble bee?

The US Fish and Wildlife Service lists three things that anyone can do to aid the rusty-patched bumble bee. But the most important thing to remember is that these three items can help any population of pollinator, no matter where you live.

According to Bumble Bees of North America, the rusty-patched bumble bee prefers flowers in the following families: Aesculus, Agastache, Dalea, Eupatorium, Helianthus, Impatiens, Lonicera, Monarda, Prunus, Solidago, and Vaccinium. Do your part for the remaining pollinators and plant a wide diversity of flowers.

Rusty
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