How do the bees know to come back to the block?

The male mason bees will scent mark the outside of where they hatched from (in our case the white tube) with a brown pheromone which will let them know to come back to find the later emerging females. Once they have mated, females will look for places to lay their eggs. The 5/16” holes in the nesting blocks are the ideal size for these bees. Additionally, being gregarious bees they want to nest around each other so they will be naturally attracted to laying their eggs in the nesting blocks. Some of the females will naturally disperse, which means they will lay their eggs in your backyard (in woodpecker holes, fences, rockeries, etc.). This is another great result of hosting bees because you are also helping to boosting native populations of these bees in your neighborhood.