Do mason bees have any predators?

Photo by gbohne
Yes, there are several other species that can pose threats to the survival of the mason bee. This is why we bring our bees into a temperature controlled room during the summer and a walk-in refrigerator during the winter so that we can control pest population and ensure high survival rates of our bees.
The monodontomerus wasp, or mono wasp, is a parasitic wasp that can be extremely detrimental to mason bee populations. The mono wasp will insert its ovipositor into the mason bee nests and lay its eggs on the mason bee larvae. Once the young wasps hatch, they will eat the developing mason bees inside of their cocoons. 
Pollen mites also pose threats to developing bees. When blocks are not cleaned, adult mason bees will carry pollen mites from their surroundings into their nesting cavities. Normally adult bees will knock off most of the mites but there is a section on their thorax behind their head that they cannot reach. Those mites will fall off onto the developing mason bees and consume their food source. If the mason bee egg has no food to consume, it will not survive. To ensure these predators are not spread into local bee populations, we sanitize all of our nesting blocks and clean all of the cocoons. We also try to provide education to the community about why it is important to maintain clean responsible practices when it comes to raising bees.