Carl Linnaeus first described six species of the Bombus genus in 1758 and these beautiful creatures have captivated our attention ever since. Bumble bees, genus Bombus, evolved as a cold-adapted alpine Holarctic group and are essential for pollination in cooler habitats (Evans et al., 2008; Hines, 2008). In 2006, native Bombus pollinators of North America were estimated to be worth 3 billion to our annual agricultural economy alone (Losey & Vaughan, 2006). Bumble bees are generalist foragers, perform sonication or buzz pollination, and are adapted to fly in lower temperatures (Couvillon et al. 2010, Cameron et al. 2011). Bombus includes 250 species across the world with 49 species present in North America, half of these species occur in Colorado (Koch et al., 2012; Williams et al., 2014).
Photo: Bombus terarius foraging on Asteraceae
Bombus bees have four life stages, in cooler regions most Bombus species produce a single generation during the summer season (Hartfield et al. 2012; Koch et al. 2012). In stage 1, the queen emerges from winter hibernation and begins a new monogynous colony on her own. Workers (females) are reared and start to forage leading the colony to develop and grow, this concludes stage 2 (Hartfield et al. 2012). Through-out stage 3 unfertilized eggs (males) are laid and some worker larva develop into new queens. In the final stage, stage 4, developed males and new queens mate, the colony disintegrates and the entirety of the old colony expires (previous seasons queen, workers, and males)(Hartfield et al. 2012). It is believed that the delayed production of queens and males yields a higher reproductive output than would be obtained if Bombus colonies attempted continual production of offspring throughout the year (Macevicz & Oster, 1976; Oster & Wilson, 1978).
The inceptive diversification of extant Bombus lineages is estimated at around 25 to 40 Ma, near the Eocene-Oligocene boundary 34 Ma, during an interval of dramatic global cooling (Hines, 2008). Numerous intercontinental dispersal events occurred mostly in the direction of Old World to New World and North America to South America (Hines, 2008). Cooler climates have been instrumental in the evolutionary history of bumble bees. Bombus have developed thermoregulatory adaptions enabling them to thrive in alpine ecosystems located in some of the highest elevations known to contain insects (Macevicz & Oster, 1976; Oster & Wilson, 1978).
Figure: Simplified summary of historical dispersal events inferred across bumble bee lineages. Arrow thickness between continents is scaled relative to the number of lineages that dispersed in the direction indicated (Hines, 2008).
Bombus are eusocial bumble bees, they have reached high levels of diversity in the arctic, subarctic, and boreal regions of the world (Hines, 2008; Koch et al. 2012). Consequently, many alpine flora found at higher temperatures have coevolved to develop a floral syndrome suited for the anatomically long tongues and learning-based foraging behaviors of Bombus (Hines, 2008). Bombus contains specialized pollen baskets as a part of the tibia (arthropod leg) known as corbicula, this structure is utilized in harvesting pollen and enables these bees to travel greater distances while foraging (Colla et al. 2011, Koch et al. 2012). In addition to colder climates, there are species that deviate from common bumble bee habitats occurring in dry arid ecosystems as well as in warm tropical lowlands with high levels of annual precipitation (Ponchau et al., 2006; Koch et al. 2012). Some species also have considerable variation in their habitat flexibility and are capable of occupying a diverse range of ecosystems (Ponchau et al., 2006; Hines, 2008).
There is substantial evidence of North American Bombus species decline, several are facing extinction (Colla et al. 2011; Koch et al. 2012). One such example, Franklin’s bumble bee (Bombus franklini), has recently been listed as Critically Endangered by the International Union for Conservation of Nature. Species that are most vulnerable to loss are organisms with smaller climate tolerances and less of an ability to adapt to new habitats, bumble bees at the edge of their climatic niches. The most common threats to Bombus species survival is loss of habitat, habitat degradation, pesticide use, and climate change (Colla et al. 2011; Koch et al. 2012). The importance of the Bombus genus to the ecology of this earth cannot be overstated, it is incumbent on us all to shepherd the protection of these organisms as we continue to develop the natural world. For additional information regarding the ecology of bumble bees please review the following publications below.
References
Cameron, S., Lozier, J., Strange, J., Koch, J., Cordes, N., Solter, L., Griswold, T., 2011. Patterns of Widespread Decline in North American Bee. Proceedings of the National Academy of Sciences, 108 (2) 662- 667; DOI: 10.1073/pnas.1014743108.
Colla, S., Gadallah, F., Richardson, L., Wagner, D., Gall, L., 2012. Assessing Decline of North American Bumble Bee (Bombus spp.) using museum specimen. Biodiversity and Conservation. 21. 10.1007/s10531-012-0383-2.
Colla, S., Richardson, L., Williams, P., 2011. Bumble Bees of the Eastern United States. A product of the USDA Forest Service and the Pollinator Partnership with funding from the National Fish and Wildlife Foundation.
Couvillon, M. & Dornhaus, A., 2010. Small worker bumble bees (Bombus impatiens) are hardier against starvation than their larger sisters. Insectes Sociaux. 57. 193-197. 10.1007/s00040-010-0064-7.
Evans, E., Thorp, R., Jepsen, S., Black S., 2008. Status Review of Three Formerly Common Species of Bumble Bee in the Subgenus Bombus. 63 pp.
Hartfield, R., Mader, E., Black, S., Shepherd, M., 2012. Conserving Bumble Bees: Guidelines For Creating and Managing Habitat For American Declining Pollinators. The Xerces Society for Invertebrate Conservation, Portland, OR.
Hines, Heather M., 2008. Historical Biogeography, Divergence Times, and Diversification Patterns of Bumble Bees (Hymenoptera: Apidae: Bombus). Department of Entomology, University of Illinois Urbana-Champaign, Urbana, IL.
Koch J., Strange J., Williams P., 2012. Bumble Bees of the Western United States. A product of the USDA Forest Service and the Pollinator Partnership with funding from the National Fish and Wildlife Foundation.
Losey, J., & Vaughan, M., 2006. The Economic Value of Ecological Services Provided by Insects, BioScience, Volume 56, Issue 4, Pages 311–323.
Macevicz, S., & Oster, G. F., 1976. Modelling social insect populations. II. Optimal reproductive strategies in annual eusocial insect colonies. Behav. Ecol. Sociobiol. 1:265-82.
Oster, G. F., & Wilson, E. O., 1978. Caste and Ecology in the Social Insects. Princeton: Princeton University.
Ponchau, O., Iserbyt, S., Verhaeghe, J-C., Rasmont, P., 2006. Is the caste-ratio of the oligolectic bumblebee Bombus gerstaeckeri Morawitz (Hymenoptera: Apidae) biased to queens? University of Mons-Hainaut, Laboratory of Zoology, Avenue Maistriau 19, B-7000 mons, Belgium. Fr. 42:207–214.
Williams, P., Thorp, R., Richardson, L., Colla, S., 2014. Bumble Bees of North America: An Identification Guide.
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