Bumblebee behaviour 2

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 shivering 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
24 a few seconds
13 5 minutes
6 15 minutes

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.

Keeping warm

Below 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.

Bomus pratorum bumblebee worker warming up

Below 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.

Bombus hortorum inside a courgette flower to stay warm

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.

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.

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.

Bombus impatiens in defensive posture

The Bombus impatiens in the photograph above 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.

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 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).

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

More bumblebee behaviour,

Distances flown
Foraging preferences
Male behaviour
Nectar robbing
Profit and loss
Scent marking visited flowers
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