Renter: Existing cavities; Dead Wood nester. Females of this bee nest, usually gregariously, in existing burrows in decaying wood, such as dead tree trunks and fence posts, and in reeds used in thatching. Also known to nest in mud walls (Friese, 1923; Bischoff, 1927). The bee has an apparent preference for shaded situations for its nest sites (Käpylä, 1978).
The following account has been based on van Lith (1957), Käpylä (1978) and Vardy (1989). Before cell construction begins a preliminary plug of soil may be added at the back end of the burrow. The cell partitions are very thin, strongly concave in front, and are made from soil moistened with nectar and possibly saliva, with grit, small pebbles, sand grains, small snail shells and other particles embedded in the outer plug surface. Nectar is added to the mud for the construction of partitions and plug. The addition of nectar may well be responsible for the unusual and long-lasting plasticity of the partitions. The closing plug also consists of soil mixed with nectar and possibly saliva; added grit tends to be coarser than in the cell partitions. The appearance of these terminal plugs in timber nest sites is very characteristic, as they project a little from the surface of the nesting substrate and extend laterally over the margins of the nest entrance. The outer part of the plug may drop away from these nests by late summer, leaving a loamy ring around the nest entrance. Terminal closing plugs in nests located in trap-nests and in stems are usually flush with the rim of the entrance.
In common with other Megachilinae which nest in linear galleries, there is a distinct tendency for cells nearest the nest entrance to be occupied by male offspring, those further back by females. This usually results in a pronounced protandry. The prepupae spin thin, whitish, opaque cocoons which are rounded at each end. A nest of this species, a mature larva and a female sealing a nest entrance are figured by Westrich (1989). The winter is passed in either of two different immature stages. A random proportion of prepupae changes to pupae towards the end of the summer and these become very dark within only a few days. Remarkably, these black pupae (which actually are pharate adults) remain in this condition throughout the winter, the adults finally eclosing the following spring. The remaining C. florisomne prepupae, however, do not pupate until late summer in the succeeding year, and these give rise to adults the following spring. Because the distribution of these two forms is random within the linear nest this results in some adults emerging from behind prepupae, causing the death of the latter. In most other bees from temperate regions the winter is generally passed as either a prepupa or a freshly eclosed, diapausing adult. The pupal period in these species is considerably shorter than in C. florisomne, usually lasting for less than a month (Else & Edwards, 2018).