Osmia bicolor the snail thatching bee 2015

Field Paintings

I was keen to spend some more time observing this fascinating bee this year. In March I visited Cerne Abbas and put out some suitably sized banded snail shells where I had seen the bees previously. This worked again as a female was regularly bringing in loads of yellow pollen to one of these on 14th April. These bees have the pollen collecting hairs on the underside of the body so are able to collect a large amount on a single foraging trip. The bee retuned at ten minute intervals during the afternoon, bringing in 15 loads of pollen in about 3 hours. She then settled in for the night at it cooled down about 6.30pm.

I had also found another site for the bee along the undercliffs on the Devon side of Lyme Regis. Here Osmia bicolor nested alongside its close relative Osmia aurulenta and I was keen to see if they competed for suitable shells to nest in. I put out a variety of banded and the larger garden snail shells at each site. Osmia bicolor almost always went for the medium sized banded snail shells and built a single or sometimes two nest cells in each. Osmia aurulenta only used the larger garden snail shells and is known to build four or five nest cells in each shell.

Returning to Cerne Abbas on 16th April I noticed a snail shell which seemed to be ‘alive’ in the grass. It was being turned over and over by a female Osmia, she dug lumps of soil from beneath the shell and seemed to be bedding it in to the ground. After completing the nest the bee fills the rest of the shell with rubble then seals it with leaf mastic. I then watched the whole process of her thatching the shell which took 2 hours and 25 minutes to cover using 95 pieces of grass. See video here.

On 21st April there were 2 bees thatching nests about 10 metres apart. I was able to watch these bees collecting the grass for the first time. They fly to an area, often the edge of a patch or patch of bare ground and grasp pieces of dried grass with their jaws. Then they fly up and if the grass is still attached to the roots they let it go. If it is loose they quickly fly back to the shell carrying it. They are great to watch often carrying long pieces of grass in a low flight back. Once a good spot is found the bee returns repeatedly until the supply of loose stems is used up. On this occasion one of the bees found the thatch being put on the other bees shell. The carefully selected stems were ideal for it until the owner turned up and chased the thief away!

In windy conditions the grass stem acts as a sail and the bee starts flying towards its nest but gets carried away over the hillside and eventually drops the grass. It always then returns to the shell, presumably to get its bearings before going off to search again.

On 23rd April a film crew from the BBC joined me on a lovely sunny afternoon and we were able to make a short film for the One Show on the bee. Still available on the iplayer. This was broadcast in early June as the Osmia bicolor season was coming to an end. Each bee probably makes about 25 nests in her short lifetime so by the middle of June they die off leaving the developing larvae in the snail shells. The adult bees then emerge in the autumn but will wait inside the shells until the first warm days of spring before emerging.

View timelapse of shell thatching here.

John Walters

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