Apicide, Tempo Dust, Dusts in general.

Apicide and other insecticide dusts are generally used for treating wall voids and other areas where there is a low or no flow of air that could disturb the dust. Insecticide dusts are also very effective for treating bees, wasps, and hornets in wall voids. The entrance where the bees are entering, whether is be a hornets nest entrance or the entrance to a wall, can be dusted.
The bees track this dust deep inside the nest killing the bees as they brush up against the "infested" bee or as the bees groom each other, etc. These dusts are sold in "do it yourself" stores or in "do it yourself" web stores, also any hardware store may stock a brand of dust for this purpose.
Sidenote- Treating for bees is very VERY dangerous and most times are left to the professionals. Also using dusts can be risky in general so before using them or any chemical be sure to read the label!