I can be entirely wrong, but here goes, I think the oil trays is to get the SHB's, their larva is harder to kill I think. I have seen the larva crawl around in a pan of water, didn't bother them at all. You need to keep SHB's down and not see larva. As for the soap and water, I think I have heard soapy solutions can kill bees. Good luck and may you get a better answer.