Bombus, the Bumblebee

Contributed by Carol Wasserloos, Master Gardener

Ask any dedicated gardener, and she’ll tell you—“I have an abundance of bees in my summer garden.” Ask her what kind, and the answer will probably be less confident—“Honey bees and bumblebees, I think.”

And that is likely correct, as far as it goes.  These are the two most visible bees of the some 4000 bee species in the United States, the vast majority of which are small solitary creatures who do their important work largely unnoticed.  The highly social honey bee (Apris mellifera) is an introduced species, brought by European migrants, and largely dependent upon human assistance for survival.  This close relationship to humans, as well as the economic importance of honey bees, has meant that the travails of the honey bee are widely reported and generally well known.  Backyard bee keepers and avid gardeners, as well as university and industry funded melittologists (scientists who study bees) have rallied to understand and combat colony collapse disorder and provide nectar sources for the embattled honey bee.

By contrast, the bumble bee stands in need of public relations assistance.  Because we don’t look closely enough at those big, fuzzy, friendly bees in our gardens, we tend to think that “one size fits all”.  We don’t realize that there are approximately 50 Bombus species native to the Americas north of Mexico.  In Massachusetts, historically we have 11 species of native bumble bees, distinguishable by their markings, the length of their tongues (and therefore their floral preferences) and their habitat needs.

Their common names give a sense of this often overlooked diversity: Bombus affinis, the rusty-patched bumblebee; B. immaculatus, the two-spotted bumblebee; B. borealis, the mostly non-black bumblebee; B. fervidus, the yellow bumblebee; B. griseocollis, the brown-belted bumblebee; B. impatiens, the common eastern bumblebee; B. pensylvanicus, the American bumblebee; B. perplexus, the confusing bumblebee; B. ternarius, the tri-colored bumblebee; B. terricola, the yellow-banded bumblebee; and B. vagans, the half-black bumblebee.

Sadly, it is likely that our current species count has dropped to 7.  All but ONE of these 7 species are in decline.  Three of the seven species are in danger of being extirpated from the state in the next decade if current declines continue. One has already been placed on the national endangered species list.

This loss is critical.  Bombus species, especially the common and apparently flourishing B. impatiens, have been studied as possible replacements for the pollination services of honey bees.  And from that study we’ve discovered that bumble bees are at least as important agricultural pollinators as honey bees.  Even more significant, at least from an ecological perspective, these 50 Bombus species are all “keystone species” within their native habitats—their elimination could lead to the collapse of all.

We are less able to explain why Bombus is in trouble than we should be, though much good scientific work is underway.  A general explanation may lie in the Bombus life cycle.  Like the honey bee a member of the Apidae family, Bombus is a social bee, living in colonies through much of their lives.  Unlike their introduced cousins, Bombus colonies are small, 50-100 individuals.  And unlike their cousins, Bombus species maintain scant food supplies to sustain themselves over the winter season.  Instead, the colony of workers and males together with the “old” queen, perish with the hard freeze, leaving only newly fertilized queens to find a solitary nest in which to survive the winter.  In the spring (April to May, depending upon species), that female must find food for herself, a suitable nesting site for a new colony, and a sufficient supply of both nectar and pollen to begin to establish a new population of workers, then males, then females through the spring, summer and early fall.  Bombus bets everything on the survival of that queen.  Climate change, habitat loss, pesticide use, other pollution, the introduction of non- native species and the loss of natives—all are likely sources of decline for the bumblebee.

More specific explanations await more careful and systematic study of the various Bombus species.  We don’t know enough about specific pollen and nectar sources for each species, though we know that they vary considerably.  We are also relatively ignorant about the effects of pesticide exposure. Effects seem uneven, but specifically what effects and how immediate they are seem to vary not only among species, but also among queens, males and worker bees.  Dr.Robert Gegear’s studies (see companion article) describe serious sub-lethal effects of pesticide exposure, with exposed bees losing their ability to find and return from their traditional food sources.  We also have much to learn about habitat needs, and which species need what to successfully overwinter and then to establish new colonies the next season.

Finally, some recent studies seem to point us toward another unhappy conclusion.  The social cousins of the Apidae family may be in conflict with one another.  Despite our belief that our work on behalf of the honey bee serves all pollinators, the evidence for this is less than encouraging.  Recent studies have suggested that the disorders common to the honey bee may spread beyond their colonies, infecting the native bumble bee.  Others have noted unfavorable competition for pollen and nectar resources. Still others have observed that cultivation of plants for honey bees may negatively select for plants that are not useful for bumblebees, especially long tongued bees looking for tubular flowers.  All of this is fodder for more work.

In the meantime, the following suggestions will benefit all pollinators, especially bumblebees:

                Avoid pesticides

                Avoid exotic (introduced, non-native) plants

                Seek out locally sourced, neonicotinoid free, native plants.

                Plant diverse floral types (compound, tubular, etc.) and diverse colors.

                Be sure to provide sources of both pollen and nectar.

                Ensure availability of pollen and nectar through-out the growing season, early spring to late fall

                Observe—which bees visit which plants?

                Create/preserve nesting and over-wintering sites

                Rethink fall and spring cleanup—leave habitat intact as much as possible.

 

 

 

SOURCES

                Xerces Society, https://zerces.org/bumblebees

                Robert J Gegear, “Amplifying the Hum of the Bumblebee”, Massachusetts Wildlife, vol 67, no 3, pp 6-17.

                Shiela Colla, Lief Richardson and Paul Williams, “Bumblebees of the Eastern United States”, USDA Forest Service and Pollinator Partnership. https://www.fs.us/wildflowers/pollinators/documents/BumbleBeeGuideEast2011.

                Rehan Lab at the University of New Hampshire, “Native Bees of New England”, www.nativebeesofnewengland.com.

                Kim Stoner, “Information on Bees and Ways to help them”, http://www.ct.gov/caes.

                Gwen Pearson, “You’re Worrying about the Wrong Bees”, Science, 04.29.15. 

                Dan Charles, NPR Weekend Edition, January 27, 2018, “Honeybees Help Farmers, But They Don’t Help the Environment”.

                Daniel Rubinoff, “Bees Gone Wild”, January 16. 2018, blogs.scientificamerican.com.

                Heather Holm,   Bees:  An Identification and Native Plant Forage Guide, Pollination Press, 2017.  See also www.pollinatorsnativeplants.com. 

                Joseph S. Wilson and Olivia Messinger Carril, The Bees in Your Backyard:  A Guide to North America’s Bees, Princeton University Press, 2016.