Check prices on Diatomacious earth on the net or at a swimming pool supply store.  I found places on the net where it is half the price of the products sold as organic pest killer, and it is all the same thing.

Diatomacious earth is used in lots of food products, ice cream is the first thing that comes to mind.  It helps to stabilize a food product by giviing it structure, and is harmless to humans.  It kills insects by scratching their exoskeleton as they crawl over it, according to what I have read and heard.

It is used in high-end swimming pool filters which is why I mention pricing the product in swimming pool supply stores.