At rest a bumblebee's body temperature will fall to that of its surroundings. To raise the temperature of the flight muscles high enough to enable flight the bumblebee shivers, rather the same a we do when we are cold.
This can easily be seen in a grounded bee as her abdomen will pump to ventilate the flight muscles. The rate of pumping can give an indication of the temperature of the bee. Ranging from around 1 pump per second when she is at 10oC, to 6 pumps per second when she reaches 35oC. The time taken to raise the thorax temperature has been studied and is laid out in the table below.
Bee/air temp. oC
Time taken to reach 30oC
a few seconds
Above Bombus pratorum worker warming up on a patch of bare soil.
When food is plentiful and outside temperatures fall below 10oC bumblebees generally stay inside the nest and live off their stores. At times when food is scarce or stores are low they will forage when the outside temperature is as low as 6oC, and queens will forage at even lower temperatures. In severe conditions they have even been known to vary their flying height to and from the nest to take advantage of any temperature differences.
Above left is a photograph of a Bombus pratorum worker resting on bare ground. It was a cloudy/sunny day with a cold wind. She had been foraging on nearby flowers. The ground she is resting on faced south and was warmed up by the sun when it shone, so her flattened posture would have brought her into contact with this warm soil, and enabled her to use less of her nectar store for her return flight home.
On the left are Bombus hortorum workers inside a courgette flower. Bumblebees love courgette flowers and stay in them a long time, especially on cold days as the temperature inside the flower is much higher than that of the outside. Males will even spend the night inside them.
Keeping the nest at the right temperature.
Keeping the nest at the right temperature is a very important job. Larvae and eggs must be kept between 30 - 32 oC. To do this the queen and workers "brood" the young. If the nest gets too hot the workers will stand at the entrance and fan out the hot air and in cooler air with their wings.
Generally you will find bumblebees on flowers,
in the nest, and travelling between the two. However in spring you will find
bumblebees in odd places. These are the queens and they are searching for a
suitable place to nest, or just somewhere to have a rest or spend the night.
She will investigate dark corners, mouse holes, garages and sheds. There have
even been cases where a queen has gone into a pocket. She will hover and fly
low over the ground oblivious to everything, and is quite easy to follow at
queens appear to maintain dominance purely by aggressive behaviour, though it is believed that a dominant queen secretes a pheromone that suppresses the glands in workers that would otherwise lead to their ovaries developing.
In many species the queen is bigger than the workers, she uses her size to dominate workers by opening her mandibles and
head-butting the most dominant worker from time to time. This is usually sufficient until unfertilised eggs are laid, or a
worker's ovaries develop.
Although bumblebees produce honey, the quantity
produced is not enough to make it worth while domesticating them as has been
done with honey bees. Even in the largest nests they is usually just a few teaspoons of honey.
So how do you know if you are
upsetting a bumblebee? Well it's quite simple really. If the bee is on a flower
or other surface and is feeling threatened it will raise one of its middle
legs. This is a sign that you are too close and should back off a bit.
TheBombus impatiens in the photograph on the right is showing this defensive behaviour. B. impatiens is native to the eastern U. S., but is sold commercially in the western states as a pollinator for glasshouses etc. A visitor to bumblebee.org kindly allowed me to use this image.
In cold weather a bumblebee feeling threatened may fall to the ground to avoid you, as it hasn't built up
enough heat to fly off. It is said that bumblebees don't like human breath, so
if you want to observe one closely then don't breathe on it.
While marking and measuring bumblebees I noticed that one would
sometimes give out a high pitched buzz similar to that when they sonicate to
dislodge pollen from tomatoes and other similarly constructed flowers. The bee
was trapped at this time with no chance of flying away, so I have always
believed that it was angry, perhaps I was holding it down too firmly, or
perhaps it was just fed up being handled.
Bumblebee feeding habits
Bumblebees are vegetarian at all stages of their life. The workers gather pollen and nectar for food. Pollen is 16-30% protein, 1-10% fat, 1-7% carbohydrate, and it also contains many vitamins. Nectar is mainly sugar and water with an average sugar content of 40%, but this can vary widely in a flower at different times of day and temperature. Even greater variation is found between different species of flower, e. g. the Crown imperial (Fritillaria imperialis) has extremely watery nectar with a sugar content around 8%, whilst Marjoram (Origanum vulgare) can have a sugar content as high as 76%. Nectar also contains tiny amounts of amino acids, proteins, organic acids, phosphates, vitamins and enzymes. Most of the pollen is fed to the larva. The workers and males eat very little - they live on nectar that has been turned to honey. The queen eats pollen to give her protein for egg formation, and larvae that will be raised as queens and new queens eat the rest.
A bumblebee worker is capable of carrying half her body weight in nectar carried back to the nest in her honeystomach or crop, and if she is not gathering pollen as well this load can be increased to as much as 90% of her body weight. This takes up most of her abdominal space and is why some bumblebees are predated on for this sweet load. Pollen is carried back to the nest in pollen baskets on the 2 hind legs. When both baskets are full they can weight about 50% of the worker's body weight.
So bumblebees get all their food from flowers. Most species of bumblebee show a preference for violet or blue flowers, but will forage from other colours once they have learned how to.
Metabolic rate. It is often thought that humming birds have the highest metabolic rate of all animals, however the metabolic rate of bumblebees is 75% higher than a humming bird's!
Crops pollinated by bumblebees (some of the crops are pollinated for seed).