Osmia aurulenta - the leaf mastic bee

Field Paintings

__p. During the spring and early summer I made several trips to a lovely spot on the undercliffs near Lyme Regis to observe another snail shell nesting bee Osmia aurulenta also known as the Golden-haired Mason Bee. It is about the same size as Osmia bicolor but from what little I knew had slightly different behaviour.

I put out a selection of banded and garden snail shells on a steep south-facing bank where the bees had been seen last year. Osmia bicolor also nests here and the first shell used was by one of these bees. Osmia aurulenta flies slightly later with the first males and a few females flying on 20th April but none visited any shells that day.

The males are slightly smaller than the females with a snug furry band of white hairs around their neck and thorax. One seemed to have a territory on this bank and repeatedly returned to a bare patch of earth to sun himself. From here he would fly out to intercept passing females.

The females began investigating the snail shells in early May and preferred the larger garden snail shells. I had seen Osmia bicolor sometimes coat the outside of her shell nest with a few dots of green leaf mastic. Osmia aurulenta are much keener on doing this and as I was to find spend hours collecting leaf mastic whilst nesting. See video here. They coat the outside of the shell with a mass of green speckles and I was able to watch them collect these. They fly a short distance and settle on a favourite plant – usually a strawberry leaf and chew sections of the leaf which are held in the jaws and carried back to the shell. Here the bee busily applies the fresh green mastic to the surface of the shell.

I knew that this bee didn’t cover up her shell with thatch but what I didn’t realise was that on completion of the nest the bee collects a mass of leaf mastic and makes a green bung to seal up the completed shell nest. I found three shells sealed up when I visited in early June and was then really keen to see the bee doing this.

On 18th June I just missed it as I arrived in the early afternoon to find a female putting the finishing touches to a bung Then again on 25th June another was just finishing off her nest but luckily another was busy taking leaf mastic into an open shell so I suspected she was just starting the seal. A little later she began to add mastic to the rim of this large garden snail shell but it clouded over so activity was slow.

It was cloudy the next day so I returned mid-morning on 27th June to see if I could see her complete the job. Again there were sunny spells so the activity was a bit stop start as she flew to a nearby patch of strawberry plants to collect the mastic. I had to leave at 3.30pm so hoped she would finish by then. She worked away at the bung from the inside of the shell until there was just a bee-sized hole left then she quickly filled this hole about 3pm and put the finishing touches to the seal about half an hour later – perfect timing! I sketched her at work and also set up my camera to record a timelapse of this behaviour.

Why she goes to all this effort is still a bit of a mystery. Obviously it protects the nest from intruding insect predators but it may also be some form of insulation to keep the nest cool in the summer and prevent freezing over the winter months. The adult bees will emerge in the autumn and like its close relative will wait for a warm spring day before emerging.

John Walters

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