I once had a terrible flea problem in a carpeted area of my basement. Nothing worked - sprays, foggers, etc. Then I worked diatomacious earth into the carpet and the fleas disappeared and never came back. (1) It is effective, (2) it is a mechanical method of pest control, and (3) I fail to understand how ground up seashells cold possible be carcinogenic. Now about using it in the hives, that's another question. I tried it around the legs of the hive stand for ants, but it didn't work as well as I had hoped. I have no need for it in the tray under the screened bottom because the only reason I can think of to have a tray at all is varroa monitoring, and the sticky board keeps them from moving.