This was my third year of releasing Osmia lignaria and Osmia californica bees and it was to be the first year to set-up a sustaining lineage by not purchasing natal tubes from a vendor. I was satisfied with the results of the past two years. The bees seemed to like the location where I had put the tubes and had in prior years successfully repopulated clean tubes in that spot. This year I had 33 natal tubes set from 2011. I decided to let them stay outdoors in the habitat in which the females had populated them and not bring them in to hibernate in the refrigerator. The sad story is that all the bees succumbed to something. I did see one male in the spring but no other activity. I just finished investigating the cocoons and the majority seemed to have desiccated. When I open the cocoons the bodies fall out in pieces. It appears that a few did get out of their cocoons but could not make their way out of the tube and therefore died in the tube. In some cases it's clear that large pollen plugs (unsuccessful larvae? and they were still moist) were in the way. Most died in their cocoons. The soil around here has a lot of clay though my garden soil is quite loamy from composting and general vegetable gardening. What happened?

My thinking is that the winter was too warm and the bees simply dried out. Is that what others may be experiencing? Is there a species that should be able to overwinter successfully outdoors in the Los Angeles area?

Another thought is that the clay in the soil caused the plugs and partitions in the natal tube to be too hard to break through. It's a thought anyway.

Thanks so much for feedback. I'll be purchasing natal tubes again this year to start over.