Native hole-nesting bees, just like any creature, have their own set of diseases and pests that, when left unchecked, can harm or kill them.
Nesting holes should be opened once a year to remove diseases like chalkbrood (a deadly fungal infection) and pests like pollen mites (they eat the pollen loaf before the larvae can). At the very least, fresh nesting holes should be provided every year.
Backyard bee houses or bee hotels have become so popular that large garden distributors have started selling quickly made nesting habitats. When these products are made from drilled blocks of wood or bamboo tubes, they actually do more harm than good for local hole-nesting bees. These companies intentions are in the right place but they lack the knowledge of the pests and diseases that can harm bees.
We hear a common argument or question about raising hole-nesting bees: In nature, these bees nest in holes in wood, why don’t I just leave them alone?
The nesting holes we are providing for bees in our bee hotels and bee houses are really different than the nesting holes found in nature. We can't build a completely natural situation for our hole-nesting bees, who are wild creatures after all, so we need to learn to maintain our man-made houses for managed wild bees.