Bee Watching – Observations of a mason bee, Osmia bicolor, concealing its nest in a snail shell

BEE WATCHING

51

Bee Watching – Observations of a mason bee, Osmia bicolor, concealing its nest in a snail shell It was the trailing length of grass stem that first caught my eye. I was eating my picnic under a tree in the King’s Forest, and a bee flew past staggering under the weight of a trailing load. It settled just a metre beyond my foot, the grass stem got caught across another, and the bee struggled to disentangle itself from its burden. Free at last, it flew off, and I thought that the show was over.

A minute later, it returned with a similar load, flying like a witch on a broomstick, the broomstick as long as a telegraph pole. It selected the same landing zone, and approached with care, offloading the stem and then fussing round a bit, before flying away again. My curiosity now aroused, I moved closer to the spot, and found a pile of similar blades of dried grass arranged like scaffold poles. Just then the bee returned with another, and I managed to photograph it as it placed the new pole horizontally, and then bumbled under the trellis into a space above what I could now see as a snail shell (Plate 7).

The bee was Osmia bicolor, a pretty black and ginger little thing, which looked like a small bumblebee, but is actually one of the mason bees. As successive poles were added to the stack, I realised most of them were of standard size, and were last year’s dried pine needles. Progressively, the construction grew from a trellis to a wigwam, and the snail shell disappeared from view. I began to wonder whether the snail shell was being used as a nest, although it was sitting upright, as if alive, in a hollow of dry moss, and I did not see the bee attempting to get inside it. The construction was still in progress, and I did not want to destroy it just to look at the snail. Just then, I noticed a similar wigwam nearby, without a bee in attendance, so I lifted the twigs off, and saw below a hollow in the moss, and an identically placed snail shell. This one I did extract, and found it lightweight, i.e. dead, although the opening was partly sealed with dried snail mucous. There was enough space for a bee to get through that veil, however. I took a photo of the pinkish orange coloured snail, which was of Cepaea nemoralis, a species rather abundant in that part of the forest. In the Breckland, C. nemoralis replaces the similar (but white-lipped) C. hortensis (Killeen, 1992). Had the snail died of natural causes, or fallen prey to the bee? Clearly, the bee could not have moved it into the hollow. As I left to resume my search for Dingy Skipper butterflies, the original bee was still making regular deliveries, having been at it for at least 20 minutes.

Adrian Knowles identified the bee for me, and told me that its behaviour was actually rather well known to hymenopterists. A Google search led to another account written by a Victorian naturalist almost exactly 117 years ago.

It is wonderful that contemporary archivists are making historic observations available to all by the World Wide Web. The precise purpose of the twig tent was not known, however. It would certainly conceal the snail shell, and would also provide shade, as presumably the inside of a snail shell in the full sun would get extremely hot.

Continuing the Dingy Skipper survey took me back to the same glade on 18 May, eleven days after the original sighting. I had explained the situation to

Trans. Suffolk Nat. Soc. 44 (2008)

52

Suffolk Natural History, Vol. 44

my companions, and we were able to find the bee’s nest wigwam. It had grown to a small pyramid, and there were no comings or goings in progress.

The construction provided camouflage, and it was not possible to see the snail shell through the arrangement of twigs. It was so well built, that it would stand up to disturbance by wind, though not to foraging by mammals. Carefully, we lifted the pyramid off the moss, and found a pine kernel balanced neatly atop the shell. The dismantled construction was placed on a tissue beside the shell (Plate 8). Expecting to find a sealed egg cell inside the shell, it was a surprise to find it part filled with loose soil, with a small entry hole (or exit?). Out of the hole popped a couple of yellow ants. Were they predators or protectors?

Loose soil below the shell suggested that there was an ant nest there now, even if it had not been present before the bee took up residence. The shell was opened, and appeared to contain just one larva at the far end of the shell. It will be interesting to discover whether it survives and emerges in due course.

A further visit on 24 May found more Osmia bicolor in the same glade, busily working on similar constructions. Elsewhere in the forest, the snails were fairly common, but the same diversity is lacking. The clearing favoured

by Dingy Skippers is particularly well sheltered, but not over shaded, by a belt of mature beech to the north and a plantation of pine to the south, on chalky soil and well provided with nectar sources, particularly bird’s-foot trefoil, which is also the larval host plant for the butterfly.

Further detail of the nesting habits may be found in Edwards (1998): “Females establish their nests in empty snail shells, including those of Helix pomatia, Cepaea nemoralis, C. hortensis and Monacha cantiana. Nests contain about four or five cells, depending on the size of shell used. Cell partitions and the closing plug consist of leaf mastic (i.e. masticated portions of green leaf). The space between the last cell partition and the closing plug is filled with a rubble containing very small snail shells and pieces of chalk, or soil. When the nest is completed the female covers the shell with a mound of dead grass stems, beech scales or leaf fragments. The reason for this behaviour is not known, but it may camouflage the nest from possible parasites and predators at a time when it may be vulnerable to such attack. Males have been found sheltering in empty snail shells during periods of inclement weather (G. R. Else, pers. obs.).”

Thanks are due to Adrian Knowles for guidance on matters hymenopterous, and to Jonathan Tyler for the photography on 18 May. References Edwards, R. ed. (1998). Provisional atlas of the aculeate Hymenoptera of Britain and Ireland. Part 2. Huntingdon: Biological Records Centre.

Killeen, I. J., (1992). The Land and Freshwater Molluscs of Suffolk. Ipswich, Suffolk Naturalists’ Society.

Rob Parker 66, Cornfield Road Bury St Edmunds IP33 3BN

Trans. Suffolk Nat. Soc. 44 (2008)

R. Parker J. Tyler

Plate 7: The mason bee Osmia bicolor at work camouflaging a snail shell Cepaea nemoralis to be used as a nest (p. 51).

Plate 8: Dismantled nest of Osmia bicolor and a shell of the snail Cepaea nemoralis (p. 52).