Importance as Pollinators

"Bumble bees aren't picky about temperature, sun, and rain like honey bees. They're hard workers - out [on crop flowers] even when it's raining and cloudy."
- Anonymous WI Fruit Grower

Bees generally are the most important pollinating animals in the world.  Their diversity, abundance, and wide geographic range allow them to be regular visitors to crop flowers across all continents, excluding Antarctica.  Bumble bees are no exception and are excellent pollinators of a variety of crops in the Northern hemisphere.  Here in Wisconsin, bumble bees visit apple, cranberry, raspberry, squashes, tomatoes, alfalfa, and of course, wildflowers.

Queen common eastern bumble bee (Bombus impatiens) visiting apple blossoms in Wisconsin.

Why are bumble bees such great pollinators?

Bumble bees have a variety of adaptations that make them great pollinators for many plant species.

  1. Buzz pollination: The charismatic buzzing sound you hear when bumble bees are on flowers gives this feature its name.  When on a flower, bumble bees have the capacity to vibrate (by shivering their flight muscles) at particular frequencies to help dislodge pollen from the anthers of flowers.  This behavior is essential for pollinating fruits like tomatoes, where the pollen is held tightly on the anthers.  Check out this great video from Smithsonian that highlights buzz-pollination.
  2. Hairy bodies: The hairs that cover bumble bees don’t only make them look cute, they’re entirely functional, as well!  Individual hairs (called “setae”) themselves are branched, increasing their surface area and making them “stickier” to pollen grains.  Additionally, the hairs help to insulate bumble bees from the cold, allowing them to retain heat and forage when conditions aren’t ideal (e.g., wet, cold, windy days).  Indeed, being so hairy is one adaptation that allows them to function in extreme conditions, like in the Himalaya!
  3. Shivering: Bumble bees, like all insects, are cold-blooded.  This means that they are incapable of regulating their internal body temperatures.  Unlike other bees, however, bumble bees have evolved a mechanism to avoid the caveats of only using external conditions to regulate their body temperature: they shiver!  Our shiver response is involuntary – we do not control it.  Bumble bees, however, choose when to shiver and, in doing so, raise their body temperature up to the threshold it must be at for flight – about 30C (86F).  Along with their insulating hairs, this “facultative endothermy” (i.e., ability to selectively regulate their body temperature) allows them to forage in cold conditions when other bees cannot.  As you might imagine, this makes bumble bees great pollinators for spring/early summer crops like apple and cranberry.
  4. Flowers are required: Pollen and nectar represent the only nutrition for bumble bees.  This makes them obligate flower visitors, meaning they have to visit flowers to successfully provision and raise their offspring.
  5. Generalist Diet: Most bumble bees are considered “generalists” with respect to their diet.  This means that they can visit a wide range of flower species for pollen and nectar, including many common crops flowers.
  6. Floral Constancy: Bumble bees are quite economical organisms.  Because it takes time to learn how to “handle” (remove pollen and nectar) from flowers, it behooves the bumble bee to visit similar flowers when possible.  This makes directed pollen transfer from like species much more likely.  Additionally, bumble bees form search images for currently rewarding flowers, and tend to keep visiting that species until the nectar it offers is depleted.