Unsuccessful development of young mason bees
This nesting tray contains 5 nesting tunnels. The upper nesting tunnel contains cocoons with a mud partition between each cocoon. The mud plug at the left hand side of the nesting tray indicates the entrance /exit. The larger or female cocoons are at the back of the tunnel (RHS). Note in the next two nesting tunnels, there are quite a number of compartments, not with a cocoon, but with a pollen lump.
The presence of a pollen lump means that the bee larvae died and did not continue its development into an adult bee. This can be caused by disease, but can also be caused by cool weather. Young bees need warmth to feed. A two week spell of cold weather usually means the demise of these bees. Unfortunately in this case, the pollen lumps were at the far end of the tunnel and these are the female bees.
This year, many people were not able to produce many mason bee cocoons. I am sure the weather played a big part in this story.