BEE-ON-BEE POLLEN ROBBERY

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The cultivation of honey bee hives can have a profoundly beneficial effect on wild bee habitats. When people launch beehives, they become attuned to the needs of pollinator habitats; they cultivate pollinator gardens; and they limit insecticide use. As an immediate and long-term consequence, wild bee populations near honey bee hives often increase and diversify significantly, without regard for specific interactions between honey bees and wild bees.

 

Stone Barns Center for Food and Agriculture, located in Westchester County, New York, is one example. A large bee garden, which harbors many native pollinator plants, surrounds a honey bee apiary at Stone Barns.  Nearby lie other gardens containing a variety of native and non-native flowers. These gardens supply more than ample pollen and nectar to meet the immediate needs of the apiary's honey bees. Thus, the gardens have enough left over to attract and sustain a diverse native bee population as well. Without these gardens, wild bees' pollen sources in the area would be significantly diminished.

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The maintenance of large and varied pollinator habitats near beehives seems the key to guaranteeing such beneficial effects for wild bees.  Without plentiful pollen sources, however, our native wild bees might well lose in a competition for survival against honey bees.

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Why?  Honey bees are tough creatures that have many advantages over our wild bees.   The structuring of honey bee society into organized colonies, for example, gives honey bees an advantage over solitary wild bees, whose survival depends on their self-sufficiency.  Honey bees also tend to forage over a much wider flying range than most wild bees:  a single honey bee can fly several miles from its hive, while many wild bees never roam more than a few hundred yards from their nests.  In addition, honey bees have a significantly longer flying season than many native bees – in our area, honey bees are the first to appear at the end of winter and among the very last bee species to disappear in early November.

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Honey bees are also generalists that seem willing to gather pollen from nearly everything that flowers (a fact explored in this guide’s article on skunk cabbage and honey bees). Many native bees, by contrast, live during short periods that coincide with the bloom cycles of specific plants on which the bees specialize.  Wild bees thus can be more susceptible to frosts, droughts, heavy rain and sudden weather reversals.

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In circumstances where pollen is scarce, we have witnessed social behavior by honey bees that suggests that they might outcompete local wild bee populations.  An example can be found in honey bee behavior in our area during the fall, when pollen sources significantly diminish.  In 2016, as cold autumn weather settled in, the honey bees in Stone Barns Dooryard Garden became more aggressive:  we photographed honey bees stealing pollen directly from the pollen baskets of other honey bees as they foraged on a type of artichoke thistle known as cardoon, which blooms in the garden in late October.  This “mugging” behavior also occurs in the wild between honey bees and native bees. 

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The authors have documented honey bees pilfering honey directly from wild bees during autumn.  Two examples of honey bee “muggings” of wild bees are shown at right.  These occurred in 2016 in a southern New York nature preserve comprised of a series of small interconnected fields covered principally with goldenrod.  A limited number of late-blooming thistles flowering along field edges attracted common eastern bumblebees (Bombus impatiens); brown-belted bumblebees (Bombus griseocollis); and thistle long-horned bees (Melissodes desponsa).   

 

In repeated instances, we observed honey bees landing on the thistles in order to harvest pollen directly from the pollen baskets on the bumble bees' hind legs and from the scopal hairs located on the back legs of the long-horned bees.  The wild bees largely ignored the honey bees; rather than fight off their aggressors, the wild bees simply continued to load up with pollen, in a Sisyphean exercise, as the honey bees depleted the same pollen from the wild bees’ pollen baskets and scopal hairs. 

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Such activity raises important questions – could the wild bees die of exhaustion from such extended vain efforts at pollen gathering?  Can honey bee aggression leave adequate pollen resources beyond the reach of existing wild bee populations?  Does such aggressive honey-bee-on-wild-bee activity occur principally during periods when pollinator resources are scarce -- for example in late summer and fall, and during years such as 2016, when summer in New York experienced unusually hot and dry periods for our area?  Are native bees that specialize on a particular kind of plant -- such as thistle long-horned bees -- more endangered by honey bee aggression than generalists like the common eastern bumble bee, which can forage on a vast variety of flowers?

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Such questions can be answered only through a controlled study.  Nevertheless, a practical response to these questions might be to supplement and diversify pollinator resources in areas near honey  bee hives, with particular attention to fall-flowering plants.

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COMPOSITE FLOWERS AND HONEY BEE MUGGINGS

 

Notably, nearly all of the honey bee muggings we have observed have occurred on thistles (bull thistles and artichoke thistles).  Studies to date documenting honey bee pollen-robberies directed at wild bees suggest that this kind of activity may be restricted to certain kinds of flowers.

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A 2005 study by Robert P.  Jean characterizes honey-bee-on-wild-bee assaults as "rare" (see reference below).  Jean observed honey bees during August and September in Indiana robbing pollen directly from the pollen baskets of common eastern bumble bees.  As in our case, those bees were feeding on thistles, and the bumblebees remained passive while their pollen stores were pilfered by the honey bees.  Jean also observed a honey bee's attempted mugging of a thistle long-horned bee (Melissodes desponsa), which responded by flying away; and of a leafcutter bee (Megachile montivaga).  In both cases, the bees were on thistles. 

 

Jean noted studies in the 1970's and 1980's remarking on similar behavior directed at American bumble bees (Bombus pennsylvanicus) feeding on thistles; and on ligated sweat bees (Halictus ligatus) and sunflower chimney bees (Diadasia enavata) feeding on sunflowers.  Like thistles, sunflowers are members of the Asteraceae family and are composite blossoms (that is, blossoms composed of many closely-assembled florets).  Sunflowers share an additional characteristic with thistles:  both flowers have broad landing pads that can accommodate several bees at once.

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Composite flowers with broad landing pads offer arenas for bee interaction.  For example, in Stone Barns' sunflower fields, it is not uncommon to find several species of bees foraging together on different parts of a large sunflower.  On a single blossom on one summer day, we recorded the following species foraging harmoniously alongside one another:  a common eastern bumble bee Bombus impatiens); a Melissodes trinodis long-horned bee; a Melissodes agilis long-horned bee; a metallic green sweat bee (Agapostemon virescens); a ligated sweat bee (Halictus ligatus) and a pair of honey bees. 

 

We have witnessed inter-species aggression in such cases rarely.  Sometimes, however, a kind of king-of-the-hill behavior does crop up among bumble bees  -- we have seen bumble bees pull one another from flowers, and in one instance, we photographed a common eastern bumble bee shoving a long-horned bee off of a sunflower (as shown above right).  It may be that the landing pads of large composite flowers, in enabling several bees to forage simultaneously beside one another, are more likely forums for honey-bee muggings of other bees.  (Nonetheless, we've never witnessed pollen stealing among wild bee species in our area.)

 

Videos have appeared on YouTube showing honey bees attacking native bees such as leafcutters and sweat bees,  on both large composites and smaller and simpler flowers.   (see reference below).

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We also have recorded honey-bee muggings on dahlias at Stone Barns in very late fall.  Like sunflowers and thistles, dahlias belong to the family Asteraceae and are composite flowers.  Dahlias nonetheless offer a relatively small landing surface that is sometimes barely big enough to accommodate more than one bee.

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FURTHER INFORMATION

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We would be interested in hearing from anyone who has documented or witnessed similar honey bee interactions with wild bees.

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A wonderful video taken by Thomas Berger of a resin bee stealing pollen from the scopal hairs of a leafcutter bee can be found here:  "A Hard Day in the Life of a Leafcutter Bee".

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