Osmia bicolor the snail thatching bee

Field Paintings

I began studying solitary bees a few years ago whilst working on the ecology of their nest parasites the oil beetles. The bees are a truly fascinating group of insects which have largely been neglected by naturalists due to the lack of a current identification guide. I first learnt about the amazing behaviour of the snail shell dwelling Osmia bicolor whilst reading Val Littlewoods blog page.

After seeing her illustrations of this bee in action it became a ‘must see’ insect for me. It occurs on limestone and chalk grasslands in southern England and is not common in Devon. By chance I met some people who had seen the bee in Dorset near the Cerne Abbas Giant so I visited the site to check it out. It was autumn so no chance of seeing the bee itself but I placed a few suitably sized snail shells and marked the spot.

When I returned the following spring I was delighted to find the bees had found the shells and one female was nesting in one. I watched her turning the shell to the correct position which was fabulous to watch see video here – I was now ‘hooked’. I also found the bee in the Cotswolds on grasslands near Stroud and managed to observe the bees in action and eventually managed to watch one covering its snail shell nest with a thatch of grass and dead bramble stems see video and another recent video. This behaviour is unique amongst British bees and incredible to watch this tiny bee struggling along in flight with long grass stems up to 20 centimetres long!

Their season starts early with males emerging from late February through March. They seek out the first spring flowers on the downs including dandelions and shelter in snail shells at night and during cold weather. The females emerge a few weeks later and are quickly mated by the patrolling males. The females then seek out empty snails shells in which to nest with those of the Banded Snail Cepaea and Kentish Snail Monacha favoured. Up to 4 cells are constructed in the shell, each being provisioned with pollen before the egg is laid and a partition constructed from mashed up leaves. The outside of the shell is often camouflaged? by the bee by speckling the outside with leaf mastic.

When the nesting shell is full the female fills any remaining void with soil and small stones then seals it with more leaf mastic. The shell is then turned so the entrance faces down. The bee then collects dead grass stems, bramble stems and other items such as cast cases of beech buds and covers the shell. During warm weather this only takes an hour or two to complete as the bee brings in stems of up to 30 cm long to cover the shell. These are weaved together to make a wigwam over the now concealed shell. The female probably makes several of these nests during her lifetime of a few weeks and she is usually active until June.

Listen to a Radio 4 Living World programme on this bee.

John Walters

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