My daughter was cleaning out the polytunnel before winter comes upon us. She uses the polytunnel for growing on seedlings for the small flower business which she runs.
Beneath a plastic sheeting on a table she found these rolls of portions of leaves.
“What can these be?” she asked me. They looked very like cases in which live caddis fly larva in streams. Obviously they were not. But then it came to me – could they be nests of a leaf cutting bee? And indeed they are.
Very occasionally I have seen the results of the activity of these bees on plants, particularly on rose leaves. The bees cut almost circular holes in leaves, perhaps over a centimetre in diametre. They carry these away and construct these quite extraordinary nests. I have never seen such nests before and naturally I searched the books for more information.
Quite often, it seems the nests are made in old tunnels in wood made by beetles. One can imagine a bee lining a pre-existing tunnel with pieces of leaf. I understand that some species make a tunnel in sand and then line that tunnel. But in this case there was only a gap between the plastic sheet and a table top beneath. There was no circular walls on which to apply the morsels of leaves. The fragments are clearly stuck together with some kind of ‘glue’ which maust be some exudation from some gland on the bee.
When some part of the leafy tube is completed the bee packs in a quantity of pollen and lays a egg. This is then sealed with a small packing of leaf fragments and then another deposit of pollen is made with an egg. This is seen in the next photograph. One would hardly think that there is enough sustenance in the pollen mass to see a larva through to maturity but it must be so!
Further it is obviously necessary that the leaf material does not rot – Water must not enter the nest. Again one wonders how there can be enough water to see the larva through its development. Can such a dry situation always be found by the parent? Or is it that there is an incorporated preservative in the construction?
As to the species – I guess that it is Megachile pilidens.