Great for solitary native bees .. We've used from 3/8" holes up to 1/2 (explanation in video)
We will be building small clay habitats with moss and hay/straw etc for the bumblebees.
Bumblebee House Experiment in a 6" Peanut Tin .. Thick Cardboard cut out with an entrance hole for the front.. small holes punched on the back side for air .. and a soil bedding inside. Placed on the ground in a fairly shaded spot .. it's a good idea to cover the exposed tin so as not to heat up from direct sun contact.