Bumblebee behaviour 1, 2

Bombus terrestris/lucorum worker foraging Bombus terrestris/lucorum worker foraging. The tongue is entering the flower nectary.

communication, profit and loss, scent marking visited flowers, distances flown, foraging preferences - nectar robbing


A returning honeybee forager performs a figure-of-eight dance on the combs which tells her workmates the whereabouts of a good source of food. Recent work has shown that a returning bumblebee forager does perform a dance, or a series of runs over the combs, often wing fanning at the same time. This excites unoccupied workers making them leave the nest to forage on flowers with the same scent as the dancing bee.

The method of communication is a pheromone (a chemical substance secreted by one animal which influences the behaviour of other animals), however it seems only to encourage other workers to go out to forage; there does not seem to be any other information conveyed by the pheromone. The pheromone contains eucalyptol, farnesol and ocimene, and has been produced artificially. The release of this compound, for short periods at regular intervals, increased foraging traffic 3 fold. This could be used commercially in greenhouses to induce workers in nest newly introduces to the greenhouse to go out and pollinate the crop by gathering pollen. This is especially useful in crops which produce no nectar such as tomatoes.

Profit and loss

Bumblebees need energy to fly, so when they leave on a foraging trip they carry nectar in their stomach to fuel them. The amount of nectar needed is roughly about 10% by volume of the amount collected in one foraging trip.

The average mass of pollen and nectar carried by bumblebees returning to the nest is around 25% of their body weight. However some bumblebees fly back carrying as much as 75% or more of their body weight! A bumblebee making about ten average foraging trips would expect to provide the nest with about 3 ml of honey a day. However practically all the pollen collected in the pollen basket is stored for larval consumption. So to a bumblebee time is honey!

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!

Foraging preferences

Bumblebees have to learn how to get the nectar from flowers. In some flowers such as the daisy-like Compositae this is fairly easy, and involves repeated probing for small rewards while standing on the platform-like flower. Other flowers are more of a challenge, for example monkshood (Aconitum napellus) is a very complicated flower, and the bumblebee must learn just how to get inside to reach the comparatively large reward. So because the bumblebee must spend time learning how to get at the nectar in the various shapes and colours of flower available, they tend to specialize on one or two types or species of flower at a time. In fact many of them will visit only one species of flower as long as there are sufficient of them to provide enough nectar.

Bombus pascuorum pollinating antirrhinum

It is this behaviour, called "constancy" that makes the bumblebee an invaluable pollinator of crops, as pollen deposited on the stigma (female part leading to ovary) of a different species is just wasted pollen and will not fertilize the flower, so will not lead to fruit or seed production.

The two photographs on the right show Bombus pascuorum pollinating antirrhinum. The bee in the top photograph was just learning how to use the flower and took a while to get into the flower, as you can see she is holding the top open and hesitated quite a while before entering. The location of the nectary is right down at the bottom of the gullet-like flower and once inside the bee will be fully enclosed by the flower. It must be quite an experience and a great leap of faith the first time a bee lets the top petal fall down behind her. This female has very little pollen in her baskets, so she may be at the start of her foraging trip. In contrast, the photograph below shows an experienced forager. She did not hesitate, nor did she try to hold up the top petal, she just used her weight to squeeze her way down into the flower. Also she came out rear end first, whereas the inexperienced forager above turned round to come out head first.

I did my thesis on bumblebee foraging preferences, and it can be found here.

Foraging preferences of a colony can be quantified by examining the husks of pollen grains smeared on to the inside of the cocoon by the larva after it has defecated for the only time as a larva before it pupates.

Bombus pacuorum pollinating antirrhinum

Distances flown

Bumblebees are what are called "central place foragers" the central place being the nest. They fly out to the source of nectar/pollen then fly back. Bombus pratorum tends to forage fairly close to the nest, while others, Bombus terrestris, lucorum and lapidarius, will often ignore or desert apparently "good" foraging sites to travel great distances to other sites.

The distance a bumblebee is willing to travel to gather food obviously has great implications when considering sites for the planting of genetically modified plants with non-sterile pollen. Studies on distances flown by bumblebees show that the foraging range varies considerably according to species and food availability, but most workers tend to stay within 5 km of the nest. However distances as large as 20 km have been recorded.

In spring masses of bumblebee queens have been seen flying the 80 km over the Gulf of Finland between Finland and Estonia. Reasons for the exodus are not yet known, but could be due to competition for nest sites, or fleeing from one of their major predators in the area - the vole - whose population cycles build to huge numbers in some years, then crash before building again.

Scent marking visited flowers

Foraging bumblebees tend to avoid flowers recently visited by other bumblebees, though they often visit the same patch of flowers, or even the same spike, as in foxglove. This is because bumblebees can scent mark the flowers they have visited. A recent study (Goulson et al, 1998) has shown that these scent markings can also be recognized by other species of bumblebee. The scent is secreted from a gland in the bumblebee's tarsus. Scent marking reduces the time spent probing unprofitable flowers.

You can see this behaviour quite easily if you watch their visits to flowers on a spike. They generally start to forage near the bottom of the spike and work their way up, but they do not visit every flower. It is possible to give the flowers numbers, then you can see that recently visited flowers will not be revisited.

Nectar robbing

Bombus terrestris/lucorum nectar robbing

The photograph above shows a bumblebee cheating. The flower is an antirrhinum. It has a relatively large amount of nectar, but this nectar is right at the bottom of the gullet-shaped flower. The bumblebee cannot see the nectar. To get at it she must learn how to use the flower. She must sit on the bottom petal and using her weight either hold up the top petal with a leg then squeeze in (see the photograph above), or just use her body as a wedge and push her way in. Once inside the petals will come together and the bee will be wholly enclosed by the flower. Even after that the bee must extend her tongue if she's a long-tongued bee, or crawl a little further down, then extend her tongue to reach the nectar.

It takes time and energy to learn how to get nectar from complicated flowers, however the rewards are great for both flower and bee. The bee gets a lot of nectar in one visit. As it has taken so much effort to learn and as the rewards are so great a bee that has learned how to use such a flower will tend to specialize on that particular type of flower. She will visit the same species again and again. This is termed constancy. A highly constant pollinator is very valuable to flowers and to crop growers. To be fertilized the flower must have pollen of the same species. Pollen from another species is not just useless, but may also clog up the style.

Some bumblebees however, usually Bombus terrestris or lucorum, do not bother to put in the work needed to learn how to use such flowers. They rob the flower of its nectar without pollinating it in return.

Nectar robbing hole

Both B. terrestris and B. lucorum are short-tongued bumblebees. The bee will crawl on the outside of the flower close to where she thinks the nectary is located, and then with her tongue sheath and mandibles she bites and pokes a hole in the flower (see above and right). Then she inserts her tongue sheath, extends her tongue and mops and sucks up the nectar. Later other bumblebees may use the hole. And, depending on the type of flower, other insects may also use the hole. One of the most complete examples of nectar robbing I have ever seen was in the Cruickshank Botanical Gardens at Aberdeen University. There is a large strawberry tree (Arbutus unedo). This tree is actually related to the heathers and is covered in little white bell-shaped flowers. Though not deep the nectar in these flowers is just out of reach of Bombus terrestris/lucorum workers, so they rob. And after them come the honeybees, the wasps, and the hoverflies. Using a robbing hole made by another bee is called secondary robbing. Towards the end of the summer it is hard to find a single flower without a little hole.

On the right is photograph of comfrey flowers, almost every one of these had robbing holes.

nectar robbing holes made by bumblebees in comfrey flowers
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