Just a note to be -very- careful with essential oils...Fourth: Herbal shampoos/essential oils. Warning: do not use essential oils on cats! Cedar, citronella, neem, eucalyptus, orange, rosemary, and several other oils/herbals are great against fleas. I'm fond of Ark Natural's "Neem Protect" spray and shampoo, Touch of Mink's "3 in 1 Pet Shampoo", and Snook's (www.snookdog.com) "Herbal Shampoo" and "Flea Tea Dip". On dogs, you can even get away with applying the essential oils on the back of the shoulder blades, but do research on any necessary dilutions.
I used to use pennyroyal on my dog, very very diluted to keep fleas off, you can even grow it in your yard to keep fleas away. Works and is a member of the mint family so doesn't smell bad.. Many dog shampoos use it. Used it for years in shampoos and even made my own rinse for her. She lived happy and healthy past 16 years.
But straight pennyroyal oil or some other straight oil extracts of plants can be -fatal- very quickly if enough is applied to a dog's skin or ingested, and even moreso for cats, and it doesn't take much sometimes just a few drops. For that matter they can be fatal to you too.
http://csuvets.colostate.edu/pain/Articlespdf/Natural Approaches for Flea Control.pdf
As for Diatomaceous earth, I have never tried it on anything but Fire ants, if that works you might want to look into the pool store variety, many many pools use DE in the filter system and every pool store out there sells it by the box.