I found that diatomaceous earth and tea tree oil were not effective for a flea infestation. Luckily, I tried lemon juice spray and was amazed by how effective it was.
I took a few small whole lemons, skin, seeds and all, and blended them with some water (not too much) and strained the mixture. I spray some on my hands and rub it into my kitty from head to tail. I also spray it outside and indoors. Smells great. She licks it off and is perfectly healthy and much happier now that the fleas are gone.
Plz read my post under tea tree oil for more info.
I've had incredible good luck ridding my cats of fleas with lemon juice. You can dunk a flea comb into a small dish of juice until the tines are submerged and comb daily until no more fleas are seen. Or you can just pour it on your pet and massage down to the skin 'tho when I tried this on one of my cats, I did dilute the lemon juice by 1/4 with water.
Before spraying on furniture and carpets, check a small area to make sure it won't stain. I can't tell you how many thousands of dollars I've spent on topical treatments, pills, shampoos, sprays etc. and nothing worked. Then a friend suggested lemon juice, which did make sense, since it's a key ingredient in citronella candles, horse fly wipes etc. to keep bugs away.