King’s College London – improving nesting sites for bees

Urban Bees was asked by King’s College London to come up with recommendations for improving biodiversity across its campuses. After visiting each campus, we suggested places where bee hotels and bee observation boxes could be installed for cavity-nesting wild solitary bees.

In early May 2024, we installed some 20 bee hotels across campus gardens, the grounds of residential apartments and at the back of a sports ground. And six bee observation boxes for education purposes.

What is a bee hotel?

It’s a wooden nesting box for cavity-nesting solitary bees designed to keep the rain out, which is packed with about 20 x 15cm long bamboo tubes 6mm – 8mm diameter wide. It is positioned at least a metre off the ground in a warm spot.

What is bee observation box?

It is a larger wooden nesting box designed to enable you to see the nesting habits of some wild, solitary bees through a Perspex cover. Its removable ‘window’ panels allows you to observe the action without moving the inner two-sided, slide-out wood cartridge which has channels to create ‘tunnels’.

Which bees could I see?

The most likely inhabitants are Red Mason Bee, (Osmia bicornis) which build their nests from late April to the end of June. Later in summer, Willughby’s Leafcutter Bee, (Megachile willughbiella), or the Patchwork Leafcutter Bee, (Megachile centuncularis) may nest. These bees are excellent pollinators and docile. They don’t sting. They nest alone, but often next door to each other, so one bee may check in to three tubes and lay 10 eggs in each, and other few bee do the same.

What might I observe?

Mason bees build mud walls along their particular ‘tunnel’, creating cells in which larvae develop.

Mated females will load up the hairs on their abdomen with as much pollen as they can – often seeming bright yellow in the process. They crawl to the end of their chosen tunnel and create a neat ball of pollen, on which they will lay their egg (see photo above). Then they will fly backwards and forwards holding mud balls and ‘brick up’ that cell.

They will continue this exhausting work, filling each cell and building a mud wall between each until the tunnel is full of perhaps nine or ten cells (see photo above) The eggs containing males are laid right at the front of the tunnel and females at the back.

Then they will plug the front of the tunnel with mud, so you can see that it is being used.

Life cycle Over the next few months the larvae will hatch and slowly eat the pollen: by September each will form a brown cocoon. This is how they will overwinter.

In March or April, depending on temperatures, males will hatch first. It’s amazing to watch the bees bite their way out through the hard mud walls, the outermost bees first (see photo above).

The males go to collect nectar from nearby flowers to build up their energy for mating and hang around the box waiting for the females to emerge.

Leafcutter bees have a similar approach, but use pieces of leaf instead of mud.  Females will find tough leaves – roses are favourites – and bite away a neat circle of the leaf.

They then fly backwards and forwards carrying the leaf discs in their front legs (see photo above) and stuff each one – with some effort – down the tunnel. Using their saliva, they overlap and stick the discs together, making a cylinder about the size and shape of a cigarette butt.

Into this they create a pollen ball, lay an egg on it, then seal the ‘butt’ with another few layers of leaf discs. This process repeats until the tunnel is full of egg-filled cylinders, then the front is plugged with layers of chewed leaves.

The following year, new bees will chew their way out of their containers and the cycle will start again.

We have designed our own Urban Bees flat-pack Bee Hotels.

Our bee observation boxes are supplied by Bee Equipment. More information here.

Mining bees

Of the 250 wild solitary bee species in the UK, many nest in tunnels in the ground and are mining bees. While we were doing the install at Great Dover Street Apartments we noticed a small hole in the soil below our feet and a bee flying low around it. On closer inspection we discovered that it was a Grey-patched mining bee (Andrena nitida) making her nest in the lose, bare soil. It reminded us that it’s important to have bare patches of soil where mining bees can burrow into the ground and lay their eggs.

We didn’t get any good photos, but here is one of the Grey-patched mining bee by Penny Metal.

Feeding the bees

Wild bees need nesting sites and nectar and pollen-rich flowers.

Now we are going to be working with the gardeners across all the campuses and residential settings to greatly improve the forage for solitary bees throughout the year. Without the nectar and pollen-rich flowers in early spring to late summer, the solitary bees won’t forage and they won’t use the nesting sites.

We are also embarking on a series of educational event to introduce students and staff to the bees and how to help them. The first one is a walkabout on Guy’s campus on 28 May. Details are here

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