ARCHIVAL: This post has been re-dated from 2015 in order to position it closer in the blog to Mason Bee Information
I find that if I remove a set of Mason Bees from the refrigerator and put them out by the colonies to emerge from their cocoons as the weather warms up, if they are not hatched within a week, and yet look like viable cocoons, you can suspect that they may have Monodontomerus wasp parasites,
I opened several cocoons today and the results are shown below. In many of the cocoons the bee was still alive, but must be weakened so that they can’t chew their way out. If I got them soon enough before they were being thoroughly eaten, often with small white eggs attached to the outside which the bee brushes off easily, they can fly away after a few minutes. In the handful of about 100 cocoons that I looked through today, less than 5 % seem to be affected like this.
This external website has a good video on how Monodontomerus wasps attack mason bees.
This external website provides a method of “candling” to check for Monodontomerus parasitic wasp larvae.
Other links on the websites to parasites