Friday, October 22, 2021

Bumble Bees' Year Comes to a Close

 At this time of year, bumble bees are well advised to get their affairs in order, for only the young queens who have mated will winter over and see a new spring.  The rest, including each nest’s founding queen, will slow and eventually die, repeating a cycle that has persisted for eons.

When paleontologist Manuel Dehon and his colleagues recently described a Miocene compression fossil found in lake deposits in La Cerdanya, Spain, they made a welcome addition to the meager fossil record of the bumble bee.   The fossil dated from the Miocene’s Tortonian age, making it some 12 to 7 million years old.

(Wing Shape of Four New Bee Fossils (Hymenoptera:  Anthophila) Provides Insights to Bee Evolution, PLOS ONE, Volume 9, Issue 10, October 2014.  The image shown here is from the article and can be found at Wikimedia Commons.  It is reproduced under a Creative Commons Attribution-Share Alike 4.0 International license.)

The Paleobiology Database lists only 17 collections with fossils of purported bumble bee species, all dating to the Miocene epoch (approximately 23 to 5 million years ago).  In fact, the world may have borne witness to the activities of bumble bees for a bit longer than the fossil record would have it.  Evolutionary biologist Heather Hines concludes that the bumble bee genus originated probably some 25 to 40 million years ago (at some point from the late Eocene to the middle of the Oligocene).  The climate at the time was growing colder which favored this animal which had attributes that served it well in temperate regions.  Bumble bees now (and presumably then) have the capacity to thermoregulate.  That is, by shivering with their thoracic muscles, they generate internal heat, sufficiently greater than the ambient temperature to enable them to fly.  (Historical Biogeography, Divergence Times, and Diversification Patterns of Bumble Bees (Hymenoptera:  Apidae:  Bombus), Systematic Biology, Vol. 57, No. 1, 2008.)

To my untrained eye, the fossil Dehon et al. described is quite clearly a bumble bee.  They named this new species Bombus cerdanyensis.  Its species name recognizes where the fossil was found.  Appropriately they placed this species in the longstanding bumble bee genus which bears a name that . . . well, a name I consider one of the most wonderful genus names ever.  Savor the word . . . Bombus . . . a delight to say and to hear.  So beautifully onomatopoetic, coming originally from the Greek bombus meaning “a buzzing” (Donald J. Borror, Dictionary of Word Roots and Combining Forms, 1988, p. 18).

With the wedding of their sound to their colorful fuzziness and their bouncing travel among flowers, bumble bees are irresistible.  Walt Whitman certainly found them so.  In Specimen Days, in a passage written one May, he extolled the bumble bee: 

Nature marches in procession, in sections, like the corps of an army.  All have done much for me, and still do.  But for the last two days it has been the great wild bee, the humble-bee, or ‘bumble,' as the children call him. . . .

As I write, I am seated under a big wild-cherry tree - the warm day temper'd by partial clouds and fresh breeze, neither too heavy nor light - and here I sit long and long, envelop'd in the deep musical drone of these bees, flitting, balancing, darting to and fro about me by hundreds - big fellows with light yellow jackets, great glistening swelling bodies, stumpy heads and gauzy wings - humming their perpetual rich mellow boom  (Is there not a hint in it for a musical composition, of which it should be the back-ground?  some bumble-bee symphony?)"  (Dover edition, published in 1995 as a republication of the original edition issued in 1883, p. 85.)

Bumble bees have been a constant for me this summer.  Earlier this year, some Agastache (Blue Fortune) was planted on a slope in my backyard.  Commonly known as giant hyssop, this member of the mint family features tall, lavender plumes of flowers that drew myriad pollinators, foremost among them, Bombus impatiens, the common eastern bumble bee.

It’s this species that sparked my interest in the bumble bee.  As I weed among the Agastache plants, the bees pay me little heed, going about their business of drinking nectar and gathering pollen.  These docile bees are here in startlingly large numbers, so much so that, even when the wind dies down, the hyssop flowers are in constant motion, dancing as the bees land, explore blossoms, and lift off.

In the Guide to the Bumble Bees of the Eastern United States (USDA Forest Service, FS-972, March 2011), biologist Sheila R. Colla and her colleagues have produced one of those rare treats that the web offers up:  an authoritative, beautifully assembled and edited, free resource.  It’s a highly useful introduction to the 21 Bombus species found in the eastern United States, which are just a small portion of the grand total of 250 species worldwide.  In addition to this publication, I have consulted An Identification Guide:  Bumble Bees of North America by Paul H. Williams et al., 2014.

In general, bumble bees are social animals spending their lives inside and out of a nest composed of a queen, female workers, and, later in the summer, males and potential queens.  (Of note, there is a subgenus of these insects which is a parasite.)  In the spring, the fertilized queen wakes from a dormant state and emerges from the hidey-hole in which she spent the winter.  She feeds and searches for a place to begin creating a small nest.  Some species nest above ground in tall grasses, others (B. impatiens among them) nest in such locations as abandoned rodent burrows, crevices amid rocks, and openings in manmade structures.  With a nest chosen, the queen lays some eggs.  When the larvae hatch, they are fed on a mixture of pollen and nectar; the larvae then pupate and subsequently emerge as adults.  The whole process from egg to adult may take five weeks.  The first adults to emerge are female workers, some of whom forage for food while others tend to in-nest duties that include attending to larvae, pupae, or honey pots.

Later in the summer, males and possible queens begin to emerge from a colony’s broods.  As the year moves into the fall, the denizens of the nest begin to die off, until only young queens who have mated remain.  These find a hiding place (in rotting logs, mulch, etc.) and go dormant until the following spring when they wake and continue the process once again.  This annual cycle has gone on some 25 million times or more.

Colla’s Guide provides phenology charts for each species of eastern bumble bee, delineating the timing of the appearance and subsequent death of each category of bee:  queens, workers (females), and males.  This is the one for B. impatiens:

I do quibble with Colla and her colleagues over one passage because they seem to rob the Bombus of a behavior that is intriguing and remarkable.  They write that bumble bee colonies are “comprised of several different ‘castes’ who divide the reproductive, foraging, defense, and other tasks necessary to their survival.”  (p. 8)  It’s that word “caste” that bothers me because it suggests a rigid or hierarchical division of labor which, it turns out, does not describe how tasks are divided among B. impatiens in-nest workers.  The reality of how chores are undertaken in at least this species is quite different.

In several studies of the interaction among the in-nest B. impatiens workers (that is, those not primarily engaged in foraging outside of the nest), zoologist Jennifer Jandt and her colleagues have shown quite clearly that responsibilities are not fixed for in-nest workers, unlike some other social insects where age or body size dictates duties.  Instead, although workers tend to repeat the same activities regarding care of larvae, of pupae, or of honey pots from one day to the next, in the long run, they change their activities to ones unrelated to what they were previously doing.  “That is, they switch tasks randomly.  We suggest that if bumble been workers do exhibit specialization within in-nest tasks, it is weak specialization.”  (Weak Specialization of Workers Inside a Bumble Bee (Bombus impatiens) Nest, Behavioral Ecology and Sociobiology, Volume 63, Number 12, October 2009, p. 1833.)

How is that accomplished?  It turns out that the in-nest workers are likely to live their entire lives within specific regions of the nest, regardless of their size or age.  Within the small nest, larvae, pupae, and honey pots – the foci of in-nest activities – are "disorganized," meaning that they are scattered and, as a result, are all in close proximity to any individual worker regardless of where she lives.  This supports workers easily shifting among different tasks.  Quite intriguing.  Are these bumble bees switching tasks instinctively in response to some stimulus in their environment or, perhaps, are they “choosing” what to do next?  Jandt’s use of the adverb “randomly” is suggestive.

Finally, one must acknowledge how critical bumble bees are to many ecosystems, doing what they were doing in the apple tree beneath which Whitman relaxed – pollinating.  They are among the most prolific and important pollinators of wild flowers and crops.  Colla captured this latter aspect of their activity in a wonderfully long, though incomplete, list of the crops bumble bees can pollinate:

tomatoes, peppers, raspberries, blueberries, chives, cucumbers, apples, strawberries, alfalfa, blackberries, soybeans, sunflowers, beans, cherries, apricots, plums, almonds, nectarines, peaches, rosehips, eggplants, and cranberries (p. 9).  

Their success as pollinators may be due to at least a couple of attributes.  They are generalists, not specializing in certain plants, but seeking out nectar and gathering pollen from a broad array of flora taxa.  Further, they are dedicated and persistent foragers.  This I discovered when I turned on my sprinklers to water the Agastache on which they were feeding.  The falling water did nothing to dissuade them from stubbornly flying from flower to flower.  Colla notes that, unlike other pollinators, bumble bees not only ignore rain, but persist in their appointed rounds despite clouds or cold.

So, it is with regret that, as the year moves into deep fall, I say goodbye to these busy, furry, summer companions.

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