I mentioned that I had bought some cardboard tubes (http://www.ebay.com/itm/Mason-Bee-Nest- ... 0921219110
). I will definitely never use those again, for the following reasons:
1. They are very difficult to open.
2. 99% of the bees had been killed by parasitic wasps (monodontomerus). The wasps had no trouble at all piercing the cardboard.
I don't think I have lost any bees to mono wasps when using teasel stems.
I guess the mono wasps become active around June 1, and you are supposed to protect your tubes from them by putting them in a fine mesh cloth bag or something. I have a few of the cardboard tubes left, I'll definitely do that this year.