Bumble Bees in Montana

Bumble bees are important native pollinators in wildlands and agricultural systems. Creating habitat to support bumble bees in yards and gardens can be easy and is a great way to get involved in native bee conservation.

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BUMBLE BEES ARE IMPORTANT NATIVE

pollinators in wildlands and agricultural systems. They are easily recognized by their large size and colorful, hairy bodies. Queens are active in the spring and workers can be seen throughout the summer into early fall. Creating habitat to support bumble bees in yards and gardens can be easy and is a great way to get involved in native bee conservation.

Bumble bees are in the genus Bombus and the family Apidae (includes honey bees, carpenter bees, cuckoo bees, sunflower bees, and digger bees).

There are approximately 250 bumble bee species worldwide and over 45 in North America north of Mexico. To date, 28 species have been documented in Montana. For more information about these 28 species, or to obtain a key for identifying bumble bees to species, please visit the Bumble Bees of Montana section of the Montana Entomology Collection website (mtent.org) and click on the “Bumble Bees of Montana” link.

Bombus huntii

Casey M. Delphia, Montana State University

Bumble Bees as Pollinators

While crop pollination services are normally attributed to honey bees, most people do not realize there are thousands of wild, native bees, including bumble bees, which play an important role in pollinating both wild and cultivated plants. In fact, native bees are often more efficient than honey bees at pollinating many plants.

Bumble bees are one group of bees that are able to “buzz pollinate,” which is important for certain types of plants such as blueberries and tomatoes. Within the flowers of these types of plants, pollen is held in small tube-like anthers (i.e. poricidal anthers) and is not released unless the anthers are vibrated. Bumble bees buzz pollinate by landing on the flower, grabbing the anthers with their jaws (i.e. mandibles), and then quickly vibrating their flight muscles. The vibration effect is similar to an electric toothbrush and the pollen is released. In many plants, pollination occurs as the pollen falls from the anthers to the stigma, thus fertilizing the flower. Pollination can also occur when pollen grains fall onto the bumble bee’s body hairs and are transfered to other flowers as the bee moves from one plant to the next.

Life Cycle and Social Structure

Similar to honey bees, bumble bees live in colonies with overlapping generations, social castes, and division of labor (i.e. they are a eusocial species). In a bumble bee nest, there will be only one queen, an individual that is often significantly larger than the other bees in the colony. The queen is solely responsible for laying eggs for colony growth. Throughout the summer, several generations of female workers are produced. In late summer the queen produces large females destined to be next year's queens and males whose sole purpose is reproduction.

Bombus occidentalis

RKD Peterson, Montana State University

Bombus bifarius

Casey M. Delphia, Montana State University

Bombus rufocinctus

Christopher R. Brown, Montana State University

Individual bumble bee queens spend the winter hibernating in compost piles, leaf litter, or other protected areas. In spring, queens emerge and begin foraging for sugary nectar that provides important carbohydrates needed for energy and protein-rich pollen. They also start searching for nesting sites. Nests can be found above ground in abandoned bird houses, grass tufts, and underneath wood, brush, or rock piles. But, they are most commonly found underground in abandoned rodent nests or other types of hollow cavities.

Once the queen has located a suitable nesting site, she begins constructing the nest by creating a small “honey pot” made out of wax secreted from her abdomen and filling the honey pot with nectar. Next to the honey pot, she constructs a second wax cell, or “brood clump,” fills it with a pollen/nectar mixture, and lays the first eggs of the colony into this cell. She will incubate these eggs while drinking nectar from the honey pot. The eggs will hatch in about five days and the larvae will feed together on this pollen mass, passing through four larval instars (i.e. feeding stages). In some species, the queen will feed the newly emerged larvae individually by regurgitating a pollen/nectar mixture.

The first generation of workers emerges as adults after four to five weeks. These often smaller-than-normal workers will take over the jobs of foraging for nectar and pollen, building and maintaining the nest, caring for the eggs and larvae, and defending the nest. Once there are enough workers to sustain the colony, the queen’s only job is laying more eggs. By the end of the summer there can be anywhere from 50–500 workers, depending on the species. Near the end of summer, new queens and males emerge from the colony. The new queens will feed on nectar, mate and then find a place to hibernate for the winter. All the remaining members of the colony, including the original queen (i.e. foundress), die at the end of the summer (unlike honey bees whose colonies are perennial and overwinter and survive for many years).

Threats to Bumble Bees

Natural Enemies. Like any other animal, there are many natural threats to bumble bee colonies, including pathogens, parasites, predators, and parasitoids. Pathogens include viruses, bacteria, and fungi. Parasites include mites and cuckoo bumble bees. Cuckoo bumble bees are social parasites, meaning a queen will “hijack” an established colony of another bumble bee species, kill the queen, and rely on the already present workers to rear her offspring. Other insects, spiders, birds, and mammals are all known predators of bumble bees. Certain fly species, wasps, and some nematodes are known parasitoids of bumble bees.

Human-caused Stressors. Populations of some bumble bee species are in decline, and in some areas, bumble bees’ native ranges are shrinking. These declines are likely due to a variety of factors. Urbanization and agricultural intensification have fragmented bumble bee habitat and caused a shortage of high quality food. Pathogens and parasites from non-native and commercial bees have been shown to spread to native bee populations. Competition from non-native bees may also increase stress on a colony. Chemicals in the environment like pesticides can have sub-lethal effects on bees, including reduced immunity, foraging capabilities, and overall health. Currently, there is a great deal of research being conducted on specific pesticides’ effects on bees. If pesticides must be used, make sure to follow all labels directions carefully and spray in the evening when bees are less active. Changes in local climate may also impact bumble bee health in the future. Especially at risk from climate change are certain species, like Bombus kirbiellus and Bombus sylvicola, known to only inhabit high-altitude or arctic environments.

Creating Habitat for Bumble Bees

Home-made bumble bee houses are typically not effective at attracting bumble bee tenants. However, there are some things homeowners, gardeners, and landscapers can do to attract and support these native pollinators. Namely, plant some of their favorite flowers. Bumble bees forage on a diverse array of plants that include flowering trees, shrubs, herbaceous perennials, and annuals (Table 1). It’s important to choose several plant species with different bloom times in order to provide a continuous supply of food resources from early spring until late fall when bees are active. Early spring and late fall floral resources are especially important because food resources can be limited at these times of year. Additionally, leaving the yard and surrounding landscape a bit “messy” can create essential nesting and overwintering habitat (e.g. compost, leaf, and brush piles). It is also important to provide access to fresh water either in a bird bath or shallow dish filled with pebbles if no other sources of water are available..

Fear of Bees?

Human’s fear of bees is often unwarranted. Despite their large size and loud buzzing, bumble bees are important pollinators and should not be feared. Only female bumble bees (queens and workers) have the ability to sting, but they are usually not aggressive and rarely sting unless threatened (e.g. getting too close to their nests). Although stings are rare, bumble bees can sting multiple times due to the smooth structure of their sting; honey bees, which have a barbed sting, can only sting once as it results in their death due to the sting becoming lodged in the recipient. Honey bees have much larger colonies and tend to be more aggressive when defending their nests and honey stores. In addition, some social wasps like yellow jackets, which are often mistaken for bees, are more aggressive because they are scavengers often found interacting more frequently with humans. However, caution should be taken near all bees in case of allergic reactions to bee stings.

Additional Resources

References

Bumble Bees of North America: An Identification Guide. 2014. Paul H. Williams, Robbin W. Thorp, Leif L. Richardson, and Sheila R. Colla.

One example of a bumble bee friendly garden

Casey M. Delphia, Montana State University

Nest of Bombus impatiens

Elaine Evans, University of Minnesota, used with persmission

FUN FACTS

Bumble bees are able to thermoregulate better than most other insects. When temperatures are too low for flight, they warm their bodies up by “shivering.” Basically, they vibrate their thoracic flight muscles until enough heat is generated to allow for flight.

Some scientists once claimed that bumble bees defy the laws of physics by being able to fly with their large bodies and tiny wings. Obviously bumble bees can fly, even though they are not the most graceful of aeronauts. The trick is in the way the wings move – it’s not an up/down motion, but more of a figure 8. The tiny changes in air pressure caused by these wing movements are what keep the bumble bees aloft.

While honey bees are the most economically important commercial pollinators, they are not native to North America. Colonists brought them from Europe in the early 1600s.

TABLE 1. Floral resources for bumble bees. The following list includes Montana native plants as well as non-native plants that grow well in our region and are not considered invasive in Montana. Some of the plants on this list are toxic to livestock, and are therefore recommended for use in the home garden or other similar landscapes.