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Research shows bumblebees on lavender have it licked
New research by the University of Sussex shows why lavender flowers attract more bumblebees than honey bees – it’s down to the bumble bee’s longer tongue and ability to visit flowers more quickly.
The tubular flowers of lavender make nectar extraction harder for the shorter-tongued honey bee, which has to jam its head into each flower to get to the nectar, Nick Balfour and fellow researchers at the University of Sussex observed when studying bees visiting an experimental flower garden at the University.1
Bumblebees spent 1.1-1.4 seconds per lavender flower, as opposed to the slower honey bee’s 3.5 seconds per flower.
Nick Balfour, who is studying for a doctorate in the Laboratory of Apiculture and Social Insects (LASI), supported by Waitrose and the CB Dennis Research Trust, says: “Lavender is a very popular plant, but with so many visitors the amount of nectar available per lavender flower is miniscule. We estimate it would take a bee one week and 300,000 flower visits to collect just one teaspoon of lavender nectar. With such small rewards on offer the faster-handling bumblebees can do much better than honey bees on lavender."
Professor Francis Ratnieks, who is LASI Director and who supervised the research, says: “It has been known for a long time that bumblebees have longer tongues than honey bees. Our research shows that on lavender this difference is crucial.”
The length of time needed is crucial because bee foraging is about efficiency. The quicker a bee can visit a flower, the more food it can bring back for its colony. Two seconds faster might not seem much, but multiplied over thousands of flowers the benefit is significant.
The project arose from earlier research carried out by Professor Ratnieks and PhD student Mihail Garbuzov comparing the attractiveness of different garden plants to insects by observing insects on flowers in the experimental garden.
They found that lavender and borage were both very attractive to bees, but that lavender had 10 times as many bumblebees as honey bees, whereas, just a few yards away, the borage had 10 times as many honey bees as bumblebees.
Nick Balfour’s research looked at what made the lavender so much more attractive to bumblebees than to honey bees and whether this was because the bumblebees were better at foraging on lavender. To investigate, Nick determined how quickly bees could “handle” lavender flowers.
Four species of bumble bee and the honey bee were videotaped as they foraged for nectar on lavender (var. Grosso) in the experimental garden on the Sussex campus. The lavender flower is a narrow tube. To get the nectar, a bee lands on and grasps the flowering head and inserts its tongue into the flower to reach the nectaries (where the nectar is made) at the base of the flower tube. The bee then visits the next flower and so on.
By playing back the videos the researchers could determine how long each bee took to handle each flower, that is to land on or walk to a new flower (search time), to get its body into the right position on the flower (orientation time), and to remain in position to extract the nectar (extraction time).
In addition, bee tongue length was measured in the laboratory by getting a bee to drink diluted honey from a plastic tube. By gradually withdrawing the liquid down the tube, the bee extended its tongue into the tube as far as it could, allowing the maximum to be determined.
Researchers also modified lavender flowers to make the nectary more accessible (that is to shorten the tube) by making two cuts down the side of the tube. Control flowers had cuts made on the main petal in a way that did not shorten the tube.
The researchers found that:
- Bumblebees of different species did not differ significantly in the time they spent handling the lavender flowers, taking on average 1.1 to 1.4 seconds per flower versus 3.5 seconds for honey bees.
- Bumblebees were quicker than honey bees at all three stages of lavender flower handling: search time, orientation time, and extraction time.
- When on a flower, and unlike the bumblebees, the honey bees appeared to be jamming their head into the lavender flower as far as possible in order to reach for the nectar.
- Grosso lavender flowers were approximately 7.5mm deep. Honey bee tongues were 6-7mm long whereas bumblebee tongues were 7.5-9mm long.
- When the lavender flowers were shortened, this significantly reduced the extraction time of honey bees, but had no effect on the extraction time of bumblebees. This showed that the honey bees, but not the bumblebees, were being slowed down because their tongues could not easily reach the nectar.
- Because each flower contains a minute amount of nectar, the 2 seconds difference in handling time is magnified many times. On average, each lavender flower with nectar contained just 0.02 microlitres. As a honey bee can hold 50 microlitres in its honey stomach, a bee would need to visit 2,500 flowers to fill up. At 3.5 seconds per flower that would take several hours.
Professor Ratnieks says: “It is very easy to see by eye how much quicker bumblebees are than honey bees to visit lavender flowers, and how the honey bees seem to push their heads into the flower more than bumblebees. Have a look this summer.”
Notes for Editors
The study draws on data collected at LASI in the summer of 2011 by PhD student Nick Balfour, using lavender planted on the University campus. The study was conducted under the supervision of Professor Francis Ratnieks and with the assistance of PhD student Mihail Garbuzov.
‘Longer tongues and swifter handling: why more bumblebees (Bombus spp.) than honey bees (Apis mellifera) forage on lavender (Lavandula spp.)’, Balfour, N. J.; Garbuzov, M.; Ratnieks, F. L.W., in Ecological Entomology (online)
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