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I actually am shocked at myself for not mentioning DE, it's one of my all-time favorite remedies. Thank you for the reminder @jamesc, I'll add it to the post

The hardware stores in the US sell (or sold, I'm not sure) a version of DE that was treated with heat and turned into more of a sharp glass. It probably works better for bug control but it is more dangerous to breath and you don't want to consume it! So I make sure people know to get food grade for large treatments especially indoors. DE really works only in dry areas too..