Bumblebee behaviour 2

Bomus pratorum bumblebee worker warming up

Warming up the flight muscles

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 10^oC, to 6 pumps per second when she reaches 35^oC. 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 30^oC
24	a few seconds
13	5 minutes
6	15 minutes

A Bombus pratorum worker warming up on a patch of bare soil.

When food is plentiful and outside temperatures fall below 10^oC 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 6^oC, 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.

Bombus hortorum inside a courgette flower to stay warm

Keeping warm

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.

Nest searching by queens

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 this time.

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.

Maintaining dominance

Bumblebee 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.

Bumblebee honey

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.

Feeling threatened

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.

The Bombus 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.

Above is a Bombus pratorum male who spent the night hanging on to a spike of flowers. Look at his hind leg, the tibia is convex as opposed to the concave tibia that forms the female pollen baskets.

Crops pollinated by bumblebees (some of the crops are pollinated for seed).

Kiwi fruit	Oil seed rape	Runner beans
Cranberry	Turnip rape	Lima beans
Blueberry	Sunflowers	Broad beans
Gooseberry	Cotton	Soya bean
Currants	Vetches	Aubergine
Cherry	Lucerne (alfalfa)	Tomato
Pear	Clovers	Peppers
Plum	Lupin	Cucumber
Apple	Buckwheat	Squash
Blackberry		Pumpkin
Orange		Gourds
Lemon		Celery
Melon		Coriander
Watermelon		Fennel
Raspberry		Gherkin
Peach		Marrow
Strawberry		Mustard

Male bumblebee spending the night under a teasel flower head

Angry buzz

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 a good source of protein. Most of the pollen is fed to the larva, and 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.

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!

Male behaviour

Above you can see a photograph of a Bombus hortorum male deep inside a flower. The photograph was taken in the morning before the sun had warmed up the garden and the bee could barely move.

Once males leave the nest they do not go back, so they have to find somewhere to spend the night. Hanging underneath the heads of flowers or even getting right into them is what they normally do. Below is another male hanging underneath the head of a teasel.

Their temperatures will drop and by morning they will have used up their stores of energy, so until they warm up by either drinking nectar or sitting in the sun or both, they will appear listless and sick.

I get a huge number of emails from people asking me why their bees are sick, when in fact they are just males who have spent the day chasing queens and drinking nectar and then stayed out all night. Sometimes it rains and they get soaking wet, but they will recover once they drink or get warmed up by the sun.

Sleeping inside a disk or bowl shaped flower is a good strategy for these bumblebees and other insects as research has shown that the temperature at the base of the bowl, near the source of nectar, can be as much as 10 °C higher than the surrounding air temperature (see the bumblebees inside the courgette flower at the top of the page).

Males patrol circuits

All bumblebee males patrol mating circuits laying down a pheromone to attract new queens. The pheromone is used to scent-mark prominent objects (tree trunks, rocks, posts, etc) on the circuit.

The circuit is marked in the morning, and after rain. The scent of some species can be detected by some humans. Usually they patrol at species specific heights. Bombus lapidarius, terrestris and lucorum males patrol at tree-top height. Bombus sylvestris and hortorum within 1 m of the ground. However this depends on the habitat.