The Red Mason bee is well over in the garden. They have been replaced by their smaller relatives, Osmia leaiana. They have started nesting in the bee hotel and there are several females stocking their nests with nectar and pollen and sealing their nests with chewed up leaves. I removed the side panel of the nest box temporarily to see the progress of the nests. Below is a female with a clump of chewed leaves starting another cell. The yellow lumps are the food supply necessary to nurture one of her larvae to develop into an adult bee. The bee needs many pollen collecting trips to complete a cell, and then many other trips to collect the leaves to build the cell that will seal each of her nests compartments.
Well, that's the mason bee's plan. However, there are other bees and wasps with their own intentions regarding these laboriously collected clumps of pollen and nectar: they are cleptoparasites. They will take advantage of a female bee outing in search of flowers, enter the nest and lay an egg next to the bees egg. The cleptoparasite egg either hatches earlier, or the parasite larva kill the bee larva to have the food to itself. One of these cleptoparasites is the wasp Sapyga quinquepunctata (above), a black, thin, long wasp with iridescent blue/purple wings and white marks on the abdomen. A female was around the bee hotel today. She inspected each hole, checking it there were bees in them and if they had cells in construction. One of the times, as she left the hole, a mason bee appeared in its entrance. The wasp is careful not to enter a nest containing a bee, presumably because the bees are aggressive to intruders (and have their stings to defend themselves).
This bee hotel with observation panels was designed by George Pilkington, check his website, where he has many informative videos and posts on solitary bees and bumblebees.