Jo F. writes:
I have a garden in Neighborhood Nine that has dahlias. I have often noticed bumblebees “sleeping” in the petals. What stood out to me was that they’d often be quite close together and quite sluggish, which didn’t strike me as typical behavior. I wonder if there’s some chemical specific to dahlia pollen that makes bees sluggish? Thanks for the great column!
Jo also included a photo of the sluggish bees in October.
To answer your question, I contacted Dr. Nick Dorian, formerly of the Tufts Pollinator Initiative and currently an assistant professor at the City College of New York (CCNY).
Dorian says that the bees in your photo are common eastern bumble bees (Bombus impatiens), a queen bee on the left and a male on the right. Bumble bees are endoheterotherms. That’s a long word with a simple meaning: at rest they rely on the air to warm their bodies, but they can also produce heat. When it is cool, bees become sluggish, so Dorian thinks you probably took the photo early in the morning or late in the day, when the bees are resting on the flowers.
It’s interesting that you photographed the queen, which is larger, together with a male. It’s perhaps an indication that they have mated. It is not uncommon to see several male bees sleep together on flowers at night, but during the day they are very competitive and chase each way from their patrol area.
October is the time of year that bumble bees prepare for winter. The female worker bees are dying off, and newly hatched males and virgin queens become responsible for perpetuating the lineage. The males feed on flower nectar and do not return to the nest, even at night. Males of many bumble bee species deposit scents (pheromones) on trees, rocks, fences, and other prominent locations hoping to attract a queen. Then they patrol the area, waiting for a new queen to appear.
As the weather cools, bumble bees warm themselves by shivering their flight muscles. To fly, bumble bee flight muscles must reach 86° F. A bumble bee can warm up its mid-section, or thorax, where the flight muscles are, from 55° F to the 86° needed for flight in just five or six minutes. Circulating blood (hemolymph) carries warmth throughout the body. Bumble bee hair (called setae) insulates them from heat loss. Unsurprisingly, bumble bees from colder climates have longer hairs.
Warming up takes a lot of energy, so if bees do not need to fly, they rest. The new queens do need to forage for both nectar (sugar) and pollen (protein) to increase the fat stores in their body. The more mass a queen has, the better her chances of surviving the winter, so she may collect pollen in 50° F temperatures, or even lower. (Honeybees do not fly unless it is at least 55° F.)
When she is ready to mate, she may follow a male’s pheromone trail to the lucky male. The female then emits her own pheromones to encourage mating. Despite all these pheromones, many male bumble bees never find a mate.
Only the newly mated queens survive winter. They do so by digging a small chamber in loose soil and entering diapause (a kind of hibernation). During this phase, a queen’s metabolism is only five percent of her summer resting state, though she still loses body mass because she no longer eats. When the temperature gets cold, queens produce an antifreeze-like substance in their hemolymph (insect blood) to protect against tissue damage. It’s a little difficult to image queen bees in the frozen ground a few inches beneath our feet in January and February, but they are there, waiting patiently for spring.
In about April in Massachusetts, the common eastern bumble bee queen emerges from her underground chamber. She finds a suitable site for a nest and deposits eggs in it. One bee cannot heat up the entire nest, so she wraps her abdomen around the eggs to warm them up, much like birds incubate their eggs to keep them warm. Of course, the queen bee also has to forage for nectar and pollen, so the eggs cool off whenever she leaves. But when she returns, the eggs return to at least 86°F. As worker bees hatch and develop, they help incubate the eggs. In some species, such as brown-belted bumble bees, even the males help incubate the eggs.
So, to answer your question, there is not a chemical specific to dahlia pollen that makes bees sluggish. When the temperature cools down, bees slow down, too. Late-blooming dahlias provide shelter for bumble bees to spend the night, and they provide food as the day warms up. Therefore, if you are a bumble bee, dahlias make a wonderful place to reside.
A stronger
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