Place a decoy hive with a queen and a few worker bees nearby to attract bees emerging from the wall. Do not use products with longer residual effects. After 2 or 3 days, remove the trap to allow bees in the decoy hive to retrieve the honey stores in the wall. About 2 weeks later, remove the decoy hive and stop up the entance hole in the structure. The wax comb in the wall will likely be destroyed by wax moths, leaving only webbing and some fecal material.
Insecticides are the safest, quickest and most effective materials for killing bees in buildings.
Do not use fumigants or flammable compounds in structures. The pyrethroid insecticides are often used for their rapid knockdown of bees. Either dust or spray formulations can be used within a wall or other cavity, but dusts generally disperse better. Apply the insecticide at night through the entrance hole if the colony is fairly close to the entrance or if the nest can not be located. An extremely large colony may have to be treated again about 10 days later to kill emerging young bees.