Can Bumble Bees Survive Cold Temperatures?

Date:  20th April 2021


Questions:
"It's very cold, and I have found a bumble bee.  It's not moving.  Will it be survive?"
"Can bumble bees survive cold weather?"

The short answer is:
Scientists have found that buff tailed bumble bees (Bombus terrestris audax) workers could freeze at temperatures of -7.1°C, whilst queens would freeze at about −7.4 °C.   There may be some similarities with other bumble bee species.  Thus, if temperatures drop to that level, they are unlikely to survive.  Here we look at the research, and how to help bees in cool temperatures, even if it's not quite so cold.


Survival Of Winter Active Bumble Bees In Cold Temperatures

At the time of writing, the weather is typically hitting about 15°C, a temperature fine for bumble bees.  However, during the winter every year, I am asked whether bumble bees can survive in very cold conditions.  This query usually arises from people who have found an inactive queen bumble bee.  People become concerned about whether the bee will survive or will it die from exposure to the cold.  They are often wondering what they can do to help the bee.

The question of the survival of bees in cool temperatures has been the subject of published research in 2013 by Owen et al (1), a team of scientists who were primarily pondering this topic.  They wrote:
 

" ...it is crucial to determine whether key pollinator species such as bumblebees will actually benefit from climate change, or if winter activity might have a negative impact on their abundance, distribution and pollination service provision."

Owen et al Scientific Research Summary


Firstly, an explanatory note:

Scientists measuring what is called the supercooling point (SCP), which is a standard procedure to describe the cold tolerance of freeze-avoiding arthropods. 

Owen et al specifically studied Bombus terrestris audax (buff tailed bumble bee), one of the earliest species of bumble bee to emerge from winter hibernation in the UK.  Other bumble bee species may respond in a similar way to cool temperatures, but differences may also occur.

They found that:

  • Bombus terrestris audax workers could freeze at  temperatures of -7.1°C (the SPC), whilst queens would freeze at about −7.4 °C (SPC).  

  • Queens were able to tolerate cool temperatures for longer than workers, probably due to having greater fat reserves.

  • Despite this, both workers and queens were able to tolerate exposures to temperatures close to these temperatures, but only for short periods.  For example, about 80% of the samples survived after exposure to −5°C for 2 hours.  

  • When subjected to acute (15 min) sub-zero cold stress, 90% of workers were able to survive at −5.0 °C and 50% at −7.8°C, but the researchers remarked that tolerance of sub-zero conditions is actually greater than this result might suggest, for extended periods e.g. 65 mins below 0°C for a −5°C exposure. Such temperature conditions can and do occur across much of the native Northern European range of Bombus terrestris audax during winter.  The researchers comment that the ability to remain active could have a significant impact on survival  and abundance of this species.  

  • The researchers note that bumble bees spending a night away from the warmth of the colony and nest, could be at risk in cool temperatures.  Temperatures have been known to dip as low as −10°C in the UK.

  • The researchers also commented on the risk of chronic low colony temperatures. While bumble bees do possess the ability to thermoregulate their colonies, a lack of winter floral resources or an excessive thermoregulatory demand, may mean colonies are unable to consistently maintain a favourable temperature. In the case of Bombus terrestris audax which typically construct nests underground at a range of depths, winter soil temperatures (10 cm depth) in the UK have been known to consistently fall below 5°C for many weeks, and below 0°C for periods of several days, even in the warmer, southern parts of England.  

  • Low day time temperatures when bees need to forage for food, could decrease the survival chances of bees.


Do all bee species have the same temperature tolerance levels?

Actually, no! For example, solitary species Osmia cornuta and Osmia rufa, are both hardier with SCPs typically below −24°C.


Helping bees to survive cold temperatures

bumble bee queen foraging on pink daphne flowerThis stunning and exquisitely fragrant winter flowering shrub Daphne bholua 'Jacqueline Postill' is popular with bumble bee queens and honey bees in winter. I have even seen Comma butterflies foraging on during cold weather.


The most important thing you can do for the longer term, is provide winter flowering shrubs and plants for bees.  This will enable bees foraging in cool temperatures, to have ample access to food which will help to sustain them. 

There are many choices available, from low-growing, hardy heathers that are versatile in the garden or in pots, to attractive shrubs, such as Daphne.  Bulbs such as crocus are invaluable.  Read more about winter flowering shrubs for bees, and winter flower borders.

bumble bee foraging on pink camellia flowerPlant winter flowering shrubs for bees, such as camellia.


Finding an immobile bee

I receive many emails from worried members of the public about finding bumble bees that are exposed to the cold, and apparently not moving.

There could be a number of reasons for this, including disease or internal parasite, or the bee may be resting.  Queens sometimes remain stationary on a flower or on the ground whilst they rest and conserve energy.

My advice is as follows:

  • Preferably, leave the bee alone.  If you are concerned it will be stepped on or disturbed by an animal (such as a dog or cat), then by all means, you can gently scoop it on to a leaf of flower, and put it to one side, perhaps onto a flower head, then let it be.

  • If after some hours it is very, very cold or the temperature is due to drop below freezing, and the bee is still in the same place by the evening, you could scoop her into a shoe box lined with dry grass and a lid with holes, and take her into a sheltered spot, perhaps a cool spot in the house.  If the weather is not cold (e.g. warm spring or summer) please leave the bee alone.

  • The next day, once it has warmed up, set the bee free.  Put her in a sheltered spot and allow her to fly off on her own if she wishes.  She may take some time to move. 

    If you are thinking of offering sugar water, please read this page first.

  • It is possible that despite your best efforts, the bee will not move.  It may simply be resting or have a parasite.  Please leave the bee alone and allow nature to take its course.  They have survived for years without humans interfering in matters like this.  If the bee has not moved by the evening, I may place an upturned plant pot with a hole in it over the bee to provide additional shelter and a means to escape if necessary.  

  • The best thing we can all do to help bees is to start gardening with bees in mind, as stated above.  


References

(1). Owen EL, Bale JS, Hayward SAL (2013) Can winter-active bumblebees survive the cold? Assessing the cold tolerance of Bombus terrestris audax and the effects of pollen feeding. PLoS ONE 8(11): e80061


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