Developing larvae feed off of "bee bread" (pollen and nectar) regurgitated by the female bee. Larvae become adults by late August and September, but do not emerge until the following spring. Individual holes may not cause much damage, but cumulative effects of numbers of bees can weaken structures. Painting wood surfaces can make them less attractive to bees. Stains seem to have little effect.
Insecticides, such as Sevin, can be used to treat openings. Sprays and dusts are both effective but sprays may only last for 1 to 2 weeks and require retreatment. Dusts are most easily applied with a puffer duster. It is best to treat near sundown when the bees have returned to their tunnel. (Ward Upham)