Introducing the Tree bumblebee (Bombus hypnorum)

Tree bumblebees are fascinating creatures, and to have them nesting in your property is a real treat! The colony will only be active from spring until late July, when all the bumblebees naturally die off, and the ‘new queens’ leave in search of somewhere underground to hibernate. Their nests do not cause any structural damage to their surroundings, and the best thing to do is leave them be to carry out their natural lifecycle. Read on to find out more about Tree bumblebees…

The Tree bumblebee is a recent addition to the UK’s fauna. Despite this, it will already be familiar to many householders and beekeepers since it can be the cause of phone calls along the lines of – “Help, there’s a bee-swarm in my bird box!”

B. hypnorum has a natural distribution in mainland Europe, through Asia and up to the Arctic Circle. It was first found in the UK in 2001 in Wiltshire. It must have arrived here from mainland Europe, where its natural distribution range has also expanded. In Great Britain it has spread rapidly and it can be very common in late spring to early summer. It is now present throughout most of England and much of Wales. In 2013 it reached southern Scotland and its distribution has since expanded in the Scottish Lowlands and further north. In 2017 it was found for the first time in Ireland. Much of its rapid spread is probably due to its ability to exploit nest place locations inadvertently provided by mankind, such as bird boxes, or in the loft-spaces of buildings. The Bee, Wasps and Ants Recording Society (BWARS) has been busy monitoring its spread. See the map below or click here to see the most recent version.


  • Recognition

Male B. hypnorum, showing the most frequently seen fur colouration.

The common bumblebees can be identified from the colour patterns (banding) of their fur. B. hypnorum’s is unique amongst the UK species. The thorax is tawny to reddish brown, the abdomen is black and tail is white. Queens, workers and males (drones) all have a similar colour pattern. Drones are chunky, about twice the size of a honey bee, have blunter ends to their abdomens and longer antennae. Fresh drones have a patch of yellowish facial fur, but this wears off with time. As is normal in bumblebees, queens, workers and drones can all vary significantly in body size.  In queens the body size has a range similar to that of the White-tailed bumblebee, B. lucorum.

B. hypnorum bees inside a nest showing many bees with worn thorax fur, which makes them look visually darker

When a colony is not long started, the foraging workers tend to be fairly small, but later foragers will be larger. Foragers are usually larger than the ‘house-bees’ which stay within the colony. Thorax colour is the biggest variable and there are many dark hypnorum bees, but they always have a white tail. Sometimes bees have very worn fur on the central part of their thorax. This makes it look as if they are going bald and can add to their apparent darkness. Such fur loss is quite unusual in bumblebees, so the presence of such baldness can help with identification.


  • Lifecycle

The Tree bumblebee is one of the first bumblebee species to be seen in the spring. In nature it is a ‘woodland edge’ species, but in our human-dominated ecology it is frequently associated with man-made structures. Like all bumblebees the queens do “nest searching flights”, looking for somewhere snug to set up home. With this species the flights are usually in March and April and are often along vertical surfaces – unusual amongst bumblebees. I’ve seen them search along fences, house walls at gutter level, around the eaves and at bird box entrances.

Queen B. hypnorum working gooseberry flowers – shows their typical choice to work pendant shaped flowers.

The species is most likely to be seen from March until July, but does sometimes occur later in the year. The bees are highly active, agile, rapid and effective pollinators. I’ve been recording the flower types that hypnorum visits for more than 10 years now and my list already totals well over 100 types. Look out for flowers that hang downwards, such as raspberry and comfrey.  You’re also very likely to see them working winter heathers, pussy willow, blackcurrant, gooseberry, apple, cotoneaster, chives, simple rose flowers and snowberry. They also like lime tree, fuchsia and blackberry flowers in later summer. The queens of this species can be very enterprising in where they choose to set up home. Colonies are usually located well above ground level. Bird-boxes, containing old bird nests are commonly used. Nest-searching queens are even capable of evicting blue tits from a nest box, then re-using their nest.

Note the yellow splodges of bee-faeces and the nest material protruding at the nest-box entrance hole.

Once the colony has grown strong, the front of the nest box can sometimes become coated with yellow splodges of bee faeces. This yellow marking can be a visual indicator of the box’s use by B. hypnorum. Another indicator is if the entrance has bird-nest material bulging out the hole. Other locations they choose to nest are holes in trees and places high up in buildings, such as soffit boxes, under roof tiles and at house eaves. In such places the bees will use an existing hole to gain access, then walk inside the roof to get to their nest. Roof-space colonies will create their nest in or on the rockwool type loft insulation, using it to keep the nest snug and warm.  Sometimes a nest can be close to the back face of a ceiling and then, when the house is quiet, it is sometimes possible to hear buzzing sounds (bee-chatter) in the room below. (Further information below). Quite a range of other places are also used for nest location, such as in compost heaps and old mouse nests inside sheds – but nests at higher level are more common. A few queens each year even set up home in fluff that has accumulated in tumble drier vent pipes. Colonies based in straw accumulations in rabbit hutches, or horse stables also seem to occur fairly frequently.

Once a queen has established her nest, it will be perhaps six weeks before the workers take over the foraging. The smaller workers stay at home and become ‘House Bees’, while the larger ones forage for the colony. It can be four to five months for the colony to go full-cycle and die out. A really strong colony can build up to 300 – 400 bees, maybe more, but most colonies are likely to be much smaller. Colonies often die out early, due to attack by caterpillars of Aphomia sociella – the bumblebee wax moth. Strong colonies will rear ‘reproductives’ which are virgin queens and/or drones. Drones leave the colony and never return, living a self-sufficient life for many weeks while foraging for themselves and looking for opportunities to mate. Virgin queens will mate (which occurs away from the nest), build up in-body food reserves, then find somewhere to hibernate until the following year. A few queens start second cycle colonies which continue into the autumn, but not much is yet known about this.

  • Flight activity and behaviour causing concern

In general the public in the UK associate all matters to do with bees as things that Beekeepers know about, so their first port of call for anything to do with bees is their local Beekeepers Association.  So some behaviour traits of B. hypnorum can cause worried calls to Beekeeping Association helplines :-

  1. Nests are frequently established in bird boxes, or in parts of buildings.
  2. Apparent high level of nest flight activity due to “nest surveillance” by drones – see below.
  3. Rapid reaction and defensive behaviour when a nest suffers vibration.
  4. The sound of bee activity heard through a ceiling – which can sometimes cause concern. ……………………………………………………………………………………………..

For colonies in nest boxes, or roof-spaces the above traits can become “more edgy” because the bee activities are more obvious to their “human landlords”. However, by the time a colony has become obvious (due its general activity level, or nest surveillance, its activity will often be about to decline naturally. Spring formed colonies usually decline naturally by late July – perhaps sooner.

A “bee-sound/noise issue” can occasionally occur if a colony is in loft space above a room. The “bee-chatter” can sometimes be heard into “human-anti-social hours” and so cause annoyance. This is an area where we are still learning about such colonies, and if/how to deal with such issues.

The rate of incoming calls for advice or help to Beekeeper Associations can become overwhelming.  My BKA Swarm-line has had 40 calls a day at busy times! Some BKAs consequently try to avoid the issue by declaring that they cannot help with bumblebee issues. I view such a stance as short-sighted. Associations need to use their websites to give advice, so they can concentrate on the very small number of people who need active help. Interestingly, in mainland Europe and Asia, where B. hypnorum is an ordinary bee-fauna member, the human population appears not to have so many problems with the species.  However, I believe the provision of bird boxes is significantly less in these countries than in the UK.

Bumblebee nest flight activity is very different to the near constant flight and entrance activity that honey bees do in daytime.  With bumblebee colonies they often fly earlier and later than honey bees and in the early stages of the colony you get solely one bee, the founding queen, going off, or returning from foraging perhaps once per hour or so.  Once workers are active, flights by individual foragers every few minutes is still not very noticeable unless you happen to be looking in the right place at the right time.  So, with bumblebees it might well be two months before their flights become noticed.  With B. hypnorum though, eventually drones in the local population start doing “Nest Surveillance flights” which greatly increase the apparent activity at a colony.  However, my observations of colonies in early summer of 2018 have made clear that there does seem to be a period where you can see drones foraging in a locality, but nest surveillance activity has not yet begun.

  • Nest surveillance flights

A small B. hypnorum nest surveillance ‘drone cloud’ at a bird-box entrance.

This looks like a cloud of bees doing an ‘aerial dance’ close to the nest’s flight point, as shown in the photo. Such activity catches the eye, draws attention to the colony and can cause public concern. To an untutored eye it looks like honey bee colony flight: but honey bees wouldn’t choose a bird-box (it’s too small) and the bees look too big and are  the wrong shape.  This behaviour, known technically as ‘Nest Surveillance‘ or ‘Lekking’ is a mating-preparation characteristic of B. hypnorum.  The ‘cloud’ bees are drones (males), about twice the size of honey bees, noticeably furry and they have white tails – take a photo and check for these details.  This activity happens mainly in May / June / July.  It can occur over most daylight hours and may last several weeks.  There might be one bee doing it, or 20+. Warm temperatures and sunshine increase the number of bees, cool damp weather, or rain, reduces numbers.  It stops at dusk and starts a bit after dawn.  Bees often join / leave the cloud as they move from nest to nest: they are probably following a ‘patrol route’.  If you look at slow motion film of the activity, the drones are facing towards the nest.  When such ‘dancing’ is going on, a few bees fly directly to/from the colony straight through the cloud – these are workers and are usually smaller than the drones.  Occasionally drones dart towards each other and fall out of the air with an audible bang – this is erroneous mating activity. Finally, if the colony has produced virgin queens, when these fly, drones attempt to mate. It looks like fighting. Paired bees fall to the ground, where they can remain coupled a considerable time. Virgin queens can even fly carrying the drone to somewhere more ‘private’ !

  • Defensive behaviour due to nest vibration

This occurs sometimes with colonies located in bird boxes on, or in sheds.  It does not seem to occur with colonies in house loft-spaces, or elsewhere in buildings, very likely since the building structure will absorb vibration, not transmit it.  Some householders can get ‘jumpy’ at the very idea of a bumblebee colony living so close-by, but most people quickly get used to the bees and such situations almost always work out to be a non-problem in real-life.  Most people grow to enjoy watching their bee colony’s antics and feel sad when it dies out naturally, a few weeks after it was first spotted.

With Tree bumblebee colonies in bird boxes fixed on light-weight sheds, or based somewhere inside a shed, or fixed to an insecure fence, sometimes these colonies may react quickly if subject to vibration.  The triggers for such defensiveness will be when someone opens the shed door, or moves about in the shed, so causing the whole shed to shake a bit.  I’ve also seen it with a colony located inside a shed, in place which was a little insecure.  In such cases the bees are capable of reacting defensively because of the vibration.  They can rush to the entrance and may fly at, or sting people nearby whenever the vibration occurs.  In bad cases the bees can ‘boil out of the nest‘ which is highly intimidating – especially if you hadn’t realised the colony was there!

It might be that for defensiveness to occur, the colony has to exceed some colony-number threshold – say 30 or more bees – because I’ve not seen any tendency to defensiveness with early stage colonies, with just a few workers.


B. hypnorum colony in an old mouse-nest in a clutter-filled box from a largely abandoned shed. That colony was reacting strongly to vibration caused by movement within the shed. Wearing beekeeping gear and working in the dark by red-light illumination, it was moved into a ‘bumblebee nest box’ and brought home. Its new location was also in a shed, but I placed the nest box on top of a slab of flexible furniture foam, which isolated the box from vibration. The nest box was provided with an entrance tube leading through the shed wall. In that new vibration-free location the temperament of the bees was never ‘defensive’ and it was a pleasure to watch them.

  • Beekeeper help

Discussing and solving issues due to B. hypnorum can be an excellent source of ‘brownie points’ and grateful financial donations for a Beekeeper’s Association.  If your association is a Registered Charity, try to get donations Gift Aided.  It may be necessary to move the nest (for example if someone has been stung).  See the advice on nest moving below. If the person is just concerned because of the drone clouds, I aim to educate the caller to enjoy the spectacle and feel honoured to be a “Bee Landlord”. Unless they fiddle with the nest, they should be perfectly safe.

  • Moving colonies in a next box

We do not recommend moving nests unless absolutely necessary. Wait for any flying bees to return home – late dusk, they fly noticeably later than honey bees. Work in the dark wearing bee-gear. Beware – you might get stung, but bumblebee stings are un-barbed, so you only get a small dose of venom. Use red light from a cycle rear-light, or even better a head torch – so you can see what you are doing, but the bees (who don’t see red) can’t see what’s going on. Quickly stop up the nest-box entrance. (I use a roll of Scotchbrite kitchen scouring pad for this, because it is very air-porous and when rolled-up it can expand nicely to fill the entrance hole; but the flexible foam used in furniture cushions, or bathroom sponges also works OK.)  Lift the box from it’s hook. Check for, and quickly tape over any gaps bees could get through.  Keep the box upright.  Now there are two options:

Re-locating close-by. Re-locate the nest box onto a stable surface close to it’s original location (say 1 – 3 metres) but in a position less awkward for human passers-by. The following day, quickly remove the bung to release the bees and retire to a safe distance. When they come out of the box the bees will quickly ‘re-orientate’ – and shutting them in for a few hours will help them realise their location is a bit different.

Re-locating further away. Keep the box upright and somewhere cool and dark overnight, while they are shut-in.  The following day, fix the box to a firm surface not liable to vibration and ideally a mile or more from the original location.  Remove the bung and release the bees – they will re-orientate.

The reason for the mile spatial separation is to minimise the risk of bees returning to the original nest location to which they used to come home. In either case at the original location, a few bees might return. These will be ones who camped-out overnight then came back to find ‘home’ gone, but these will soon diminish. (Only bumblebees camp-out, not honey bees.) Lost bees will either die of quite quickly, or perhaps go and join another local colony of their species.

At it’s new location the hypnorum colony can be a fascinating learning opportunity.  Drones doing nest surveillance in the locality are quite likely to find the colony at it’s new location within about 20 minutes!

Tumble drier colonies

There seem to be several cases of this each year in the UK.  You see bee-traffic entering the vent pipe grill.  The nest will probably be in a vent-pipe side arm, that had previously filled with fluff. It is possible to move such colonies, but this needs beekeeping skills and is extremely time-consuming, so a more practical approach is to leave the colony alone, to reach full cycle and die out naturally.


  • More FAQs

How long do TBB colonies last?

Somewhere around three to four months.  TBBs are known as ‘a short-cycle species’ amongst bumblebee scientists.  Queens are likely to emerge from hibernation in say late February, or March.  A few days later they will start to nest-search and establish colonies.  These reach the end of their life-cycle by around late July, or so.  In years when we have a ‘late spring’, as has occurred in 2018, the spring-founded colonies may last into August.

Will they come back again next year? 

I’ve had several previous reports, of TBBs nesting in a roof that has been used previously, but as far as I know they do not contruct a nest in exactly the same place in the loft-space.  Certainly, I’m not aware of them re-using a bird box without it being cleaned out beforehand.

Are TBBs causing problems for our other bumblebee species? 

To my knowledge they are using new ecological niches for their nest locations in things like bird boxes and house roofs, in sheds etc, that are hardly used by other bumblebees.  We get a small number of reports most years of colonies using Tumble Dryer fluff as their nest location – so the queens are remarkably free-thinking in where they choose to set up home.  And the Early BB, which is their main potential competitor seems still to be doing well here despite TBBs now being here.

Is there a major risk of being stung by TBBs?

In general bumblebees are remarkably docile. TBB colonies can sometimes behave defensively if you get really close to them and they sense you as a potential aggressor.  Human breath can cause them to react.  Defensive behaviour is most likely to occur in response to vibrations reaching the nest structure – as described above.  Most of the flight activity at a colony once Nest Surveillance has become established will be by male bees doing ‘nest surveillance’ and hoping to get an opportunity to mate with a new virgin queen.  Because they are male, they have no sting, so are totally unable to sting.  You have to be a female, or worker bee to have a sting. These will only be flying directly in / out of the nest location, knowing very closely where they are going to.  It is the male bees that just float about, although if you look closely, they are often following each other.  Bees out foraging should only sting if they are picked up and handled roughly – but male foragers cannot sting.

What is the purpose of Nest Surveillance?

It is the way male bees look for opportunities to mate – so is part of species procreation.  In simpler terms the males  in the ‘nest surveillance cloud (sometimes called a ‘Lek’ are hoping for an opportunity to mate with a new virgin queen when she flies from, or returns to the colony.

I’ve seen what looks like bees fighting on the ground, what’s going on?

You’ve spotted a mating attempt between a drone (male) bee and a virgin queen.  When this occurs people often think the bees are fighting.  The drone seems to start by landing on the queen’s abdomen.  She is able to decide whether to knock him off, or to let him stay.  They might fall to the ground immediately, or sometimes she flies off, carrying him, to somewhere ‘more private’.  This is often into vegetation.  Eventually they link genitalia, abdomen tip to abdomen tip, he appears to lie back away from her body.  They stay paired for around an hour or more, so you might need to have a suitable ‘Birds & Bees’ explanation ready to explain to curious children who might ask leading questions !  At the end of mating, both bees fly off.  In bumblebees the drones survive and could mate again.  (This is very different to honey bees, in which the drone dies immediately on mating.)

What happens to the newly impregnated queen bee?

The queen will return to her natal colony.  There she will feed up and build up in-body food stores.  She may also go foraging on flowers for nectar and pollen.  Once  she is ready, she the leaves the colony and searches for somewhere safe to rest until the following spring.  This will most likely be in a small chamber she has dug in the ground, on a north facing slope, or in leaf-litter.  She will choose a place where she won’t be awoken by warmth from the sun.  In the following spring she will hopefully emerge to start a TBB colony of her own.  Not all queens will be successful.  A fair proportion are likely to die because of parasite infection, or perhaps be found and eaten by badgers.  Her new colony may be in the area where she originates from, but she could also fly away (disperse) to somewhere more distant.

Will my TBB colony cause damage to my building?

I’ve not heard of any species of bumblebee causing any damage to a building.  One or two people have told me they have found the remains of a nest in their loft-space and that it was a bit sticky.  Under dry conditions the nest should eventually just dry out.

Will TBBs return to my loft or bird-box next year?

TBBs seem to come back more often to house loft-spaces than to sheds. As far as I know they never re-use a bird-box unless it has been cleaned out. They don’t re-use an old TBB nest, but may go somewhere close by.  There is a chance that the same entrance point into a roof-space could be used by one of next year’s nest-searching queens.  Sometimes more than one colony can be based in the same roof-space but usually they will have separate access points.

I hope you find our new bumblebee interesting and a good source of donation income for your association. My personal experience is that re-locating colonies to my garden has been a source of much satisfaction and extension to my bee- knowledge. Other beekeepers who have moved colonies have also told me they have greatly enjoyed the experience.

Clive Hill, High Wycombe BKA.


BBCT Tree Bumblebee Overview article. Version: 2018 v2

Photo credits: All by Clive Hill.


This article is a much updated version of my earlier article “Introducing the Tree Bumblebee” published in the beekeeper’s magazine BeeCraft, in May 2013.  Two previous articles published in BBKA News issues 189 and 190 carried articles by myself about dealing with ‘Bumblebee Swarm Calls’ and about B. hypnorum.

Websites. The Bumblebee Conservation Trust website has a wealth of information about bumblebees. There is also extra bumblebee know-how in the ‘Get Involved’ section of their website. BBCT and others have put videos about the Tree bumblebee onto YouTube.

If you do a search on Google using : “Clive Hill, Bumblebee Trust, Tree Bumblebee, Bombus hypnorum” you will find some short video clips of the nest surveillance activity.

Should you want to learn more about bumblebees, BBCT’s new book “Bumblebees an Introduction”  will be a good start for complete beginners.  An easy to read and use technical book for amateur use is : Bumblebees (3rd Edition) by Prys-Jones & Corbet. ISBN 978-1-907807-06-0.

My thanks to BWARS for the map. You will find identification and mapping information on their website.

Comments are closed.

Registered Charity No. 1115634 / Scottish Charity No. SC042830

Copyright © 2020 Bumblebee Conservation Trust. All Rights Reserved.