Wildlife watch: the mason bee

A couple of years ago at Christmas my wildlife gift guide recommended buying some thermal blocks from your local builder’s merchant and drilling some holes in them to make homes for insects in your garden. I’ve made several and adorned them with roofs and cast-iron ornaments of butterflies and bees to make them look attractive. I’ve also put together a selection of other insect houses using sawn-off bamboo canes and drilled wooden blocks complete with wooden frames, all of which are fastened to the sunny side of our shed wall. In fact the shed wall has started to bow under the weight!

Anyway, did you? Did you make any insect houses? I hope so, mine helped me pass many happy hours during lockdown, watching and photographing bees and wasps. It was like having a completely independent ecosystem on the shed wall, at a convenient, watchable height. We had red mason bees in the spring then leaf-cutter bees in summer and throughout we saw a variety of parasitic and cuckoo wasps which take advantage of the abundance of food gathered by these ‘solitary’ bees.

These types of bees are referred to as solitary bees because they don’t make a communal nest or hive, each female has her own nest, or collection of them, but there could well be many different ‘solitary’ bees using the same insect house.

The red mason bees were the main stars of the show. Mason bees were named because they will make their homes in gaps between bricks in house walls. A female mason bee will collect pollen and nectar to store in her chosen hole then lay an egg with the pollen before capping the ‘cell’ with mud.

Within the cell, mason bee larvae hatch from the eggs and begin to feed on the pollen. The stay inside their protective cells and develop pupae before finally emerging as adult bees which dig their way out of their cells to take to the air.

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A pair of red mason bees mating, the female is larger than the male.

Where the holes are deep enough the mason bee will make several cells in one hole, each will be separated by a plug of mud. The female bee can choose when to lay a female egg and when to lay a male egg. To give rise to a female she fertilises the egg with sperm, if she wants male progeny she avoids fertilising the egg as she lays it. She lays the fertilised eggs deepest in the holes, this means the females, which hatch last, are deeper than the males.

Come the following spring the smaller males emerge first and can feed a little but they don’t stray far from their nest hole. Their tactic is to lie in wait for the hatching females on which they immediately pounce and try to mate. Females only need to mate once, after which they store the sperm for using as and when they choose, so the male’s work is done.

The female mason bee can lay one or two eggs each day and when one hole is full she moves onto the next. In her adult flying stage she might live for a month, if conditions allow, so that means she might give birth to 60 further bees. That’s a fair rate of productivity and is probably why so many other species of solitary and cuckoo wasps take advantage of the situation.

The ruby-tailed wasp lays its eggs in the mason bee cells.

Various parasitoid wasp species, such as the very colourful ruby-tailed wasp, come along and lay their eggs in the same holes before the mason bee has capped them. Their larvae then eat the food intended for the mason bee larvae and probably eat the mason bee larvae as well, so the mason bees don’t have it all their own way. It’s like having an Attenborough film unfolding in front of your very eyes!

Attract more wildlife into your garden with our guides to wildlife gardening, including how an organic garden works and planting to attract birds

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