Introducing the Tree bumblebee

In 2003, although I’d kept honey bees for 30 years, I knew little about bumblebees. Then, my Beekeepers Association hosted a wonderful talk about bumblebees. The speaker explained how similar, but also how staggeringly different, bumblebees are to honey bees! They grabbed my attention and afterwards I dug out my copy of ‘Bumblebees’ by Prys-Jones & Corbet to re-read. The book recommended that the best way to learn more about bumblebees was to rescue a nest, one causing trouble to a householder, and bring them home to watch and learn from. A few months later my Beekeepers Association was told of a bumblebee colony that needed to be relocated, and I was drafted in to help. I managed to find and remove the nest and brought them home. It was fascinating to watch and learn and over 20 years on I am still interested in all things bumblebee!

When are Tree bumblebees active?

Nests are usually active from spring through to June or July when the new queens leave to mate, feed and then hibernate, and all the other bumblebees, the workers and the original queen, associated with the nest naturally die off.

Bumblebees are generally quite happy to live alongside their human neighbours, as long as they aren’t disturbed, and their nests don’t cause any structural damage to their surroundings. The best thing you can do is leave them to complete their natural lifecycle.

Where are Tree bumblebees found?

Tree bumblebees are found in mainland Europe, through Asia and up to the Arctic Circle. Although this species is a relatively recent addition to the UK, having been first recorded in Wiltshire in 2001, they are now present throughout England, Wales and much of Scotland. They have become one of our ‘Big 8’ most common bumblebee species. In 2017, they were found for the first time in Ireland. Have a look at the UK distribution map.

Tree bumblebees are very common in urban settings, partly due to their ability to exploit nest locations inadvertently provided by people, such as bird boxes or roof spaces. Look out for them on a variety of flowers, such as willow catkins, raspberry, comfrey, apple blossom, open roses and many more, from early spring through to the summer months.

What is the Tree bumblebee lifecycle?

The Tree bumblebee is one of the first bumblebee species to be seen in spring. In wild places, it is a ‘woodland edge’ species but in urban settings it is frequently associated with man-made structures. After emerging from hibernation, the newly emerged queens do “nest searching flights”, looking for somewhere snug and dark to set up home. Bumblebee queens usually do these nest-searching zig-zag flights over the ground, but with Tree bumblebees they also search along vertical surfaces. I’ve seen them search along fences, house walls at gutter level, around eaves and at bird box entrances.

Queens can be very resourceful and most nests are usually located well above ground level. Bird-boxes, containing old bird nests are commonly used, as well as roof spaces, especially those with insulation material which keeps the nest snug and warm. A range of other places can be used too, such as compost heaps and old mouse nests inside sheds. Each year, a few queens even set up home in fluff that has accumulated in tumble drier vent pipes. Colonies in animal bedding, such as straw, for example in rabbit hutches and horse stables, also seem to occur fairly frequently.

Once a queen has established her nest, it could be four to five weeks before her first daughters (workers) take over the foraging. The smaller workers stay at home and become ‘House Bees’, which help to look after the nest, while the larger ones forage for pollen and nectar. It can take four to five months for the nest to go full-cycle and die out, but on average, nests last around three months. A strong nest can produce 300 – 400 bumblebee workers over its lifetime (not all at once) but most are likely to be much smaller.

Successful colonies will rear ‘reproductives’ which are new queens and males. Once they have emerged from the nest, new queens will mate, build up in-body food reserves, then find somewhere to hibernate until the following year. A few queens – instead of hibernating – start their own new nest which could continue into the autumn. Males leave the nest and never return, living a self-sufficient life for a few weeks, while foraging for themselves and looking for opportunities to mate.

Do Tree bumblebees cause any problems? Not really!

The presence of bumblebees, especially if the nest is in a bird box or roof-space, can sometimes be worrying for their “human neighbours”. However, by the time a nest has become obvious it will often be about to decline naturally.

They don’t cause any damage. Some people get worried that the bumblebees will cause damage, but they do not build any permanent structures or cause any structural damage to buildings. Their nests are small and will naturally fall apart over time. If desired, nests can be removed when the colony has finished nesting, and died out.

Bumblebees don’t swarm. Male Tree bumblebees exhibit a behaviour called ‘nest surveillance‘ or ‘lekking’ towards the end of the nesting period. They are hanging around, waiting for a mating opportunity with a new queen. This looks like a cloud of bumblebees hovering outside the nest entrance. Such activity catches the eye, draws attention to the nest and can cause concern. Some people might think that it looks like a honeybee “swarm” but honeybees wouldn’t choose a bird-box (it’s too small) and the bees look too big, have white tails and are the wrong shape.

This activity happens mainly in May/June/July. It can occur over most daylight hours and may last several weeks. There might be one male doing it or 20 plus. Warm temperatures and sunshine increase the number of bumblebees, whereas cool damp weather, or rain, reduces numbers. Males often join and leave the cloud as they move from nest to nest: they are probably following a ‘patrol route’. They can also be seen to drop out of the throng, to visit flowers to get some more nectar – like popping into a garage to get some more ‘fuel’.

In slow motion film of this activity, the males can be seen 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 males.  Occasionally males dart towards each other and fall out of the air with an audible bang – this is mistaken-identity mating activity. When new queens emerge, they look distinctly large. The males attempt to mate which can, at first glance, look like fighting. A mating pair might fall to the ground, where they can remain coupled for a considerable time. Queens can even fly, carrying the male to somewhere more ‘private’!

Bumblebee nests can make a little noise. A “bee-sound” can occasionally occur if a colony is in a roof space above a room. The “bee-chatter” can sometimes be heard into “human anti-social hours” and so cause annoyance. Sounds of fanning for nest ventilation (which can sound like a buzzing or a low gentle roar) are common, as well as intermittent ‘peep’ sounds. Earplugs can help reduce disturbance for the time the nest is active.

What happens when Tree bumblebees are disturbed?

Tree bumblebee nests in bird boxes fixed to sheds, or based somewhere inside a shed, or fixed to an insecure fence, may be disturbed by, and react defensively to vibration. They can rush to the entrance all at once, and may fly at, or sting people nearby whenever the vibration occurs. The triggers for such defensiveness can be when someone opens/shuts the shed door which causes the whole shed to shake a bit. They can also be disturbed by nearby lawn mowers, or by the box being knocked directly. If you can, it’s best to give them a bit of space, by minimising whatever caused the reaction, until they are finished nesting.

This behaviour 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 bumblebee neighbours and feel sad when it dies out naturally, a few weeks after it was first spotted.

Can I move a nest box of Tree bumblebees?

We do not recommend moving nests unless absolutely necessary. It is much better for the bees if they can stay where they are, and thoughtful education of the “human landlords” can change their mindset from consternation about the bees into great interest and wonder! However, in a few cases it might be necessary to relocate a colony so here’s how go about it.

Wait for any flying bees to return home – usually at very late dusk. Be aware that you might get stung, but bumblebee stings are un-barbed, so you only get a small dose of venom. Work in the dark wearing protective clothing, including washing-up type rubber gloves.  Use red light, from a cycle rear-light or a head torch, to see what you are doing.  Bees are unable to see red unless it is incredibly bright on them – so to them it will be dark. Then they can’t see what’s going on, and only crawl, not fly.

Quickly stop up the nest-box entrance using something air-porous, like a Kitchen Scourer pad, or a piece a flexible foam sponge pad. Lift the box from its hook. Check for, and put something over any gaps that bees could get through. Keep the box upright.

Now there are two options:

• Re-locating close-by (3 – 4 metres). Re-locate the nest box onto a stable surface close to its original location but in a position less awkward for human passers-by. The following day, quickly remove the sponge and any other covers to release the bees and retire to a safe distance. When they come out of the box the bees will quickly ‘re-orientate’ by making repetitive arcing flights around the nest entrance to learn their new surroundings.
• Re-locating further away. Keep the box upright and somewhere cool and dark overnight while the bumblebees 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 stand back – again the bumblebees will re-orientate. Make sure you have landowner permission to do this. 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.