[dkmn]

Having a bee house (for mason bees) has worked very well for us. Harvesting the cocoons in fall helps prevent concentration and spread of parasites, so I find the split-and-drilled layered wooden blocks work very well and pay for themselves over time.

I've heard some same the solid drilled wood, though cost-effective, can make the problem worse (they call them "bee tombs" though I suspect it's a bit hyperbolic).

Agree with stinos about coarse bird mesh and using non-treated wood, but it hasn't been a problem for us.