Liquid sprays of preventive insecticides can be applied to wood surfaces that are attracting bees. They usually last only one to two weeks, so the treatment may need to be repeated. Treat existing tunnels by puffing an insecticidal dust into the nest opening. Aerosol sprays labeled for wasp or bee control are also effective. Leave the hole open for a few days after treatment to allow the bees to contact and distribute the insecticide throughout the nest galleries. Then plug the entrance hole with a piece of wooden dowel coated with carpenter's glue or wood putty. This will prevent future use of the old nesting tunnels and reduce the chances of wood decay. Contact your local Extension office for insecticide recommendations.