There are many, many sources that will tell you DE works. I haven't had any success with it, so I thought what Zaz wrote made sense about the jar and the fleas.
We once moved into a house that had previously been rented to a family with a lot of dogs and apparently no flea control program. The carpets were steam cleaned before we moved in. The fleas almost ate us alive. I tried everything... LOTS of DE, more steam cleaning... It was AWFUL. I would walk through the house in pantyhose, and the fleas were so thick that it looked like I sprinkled pepper on my legs. It turns out the steam cleaning was causing more flea eggs to hatch
Who knew? The DE did nothing. I think we finally ended up going with a chemical assault and more steam cleaning to get rid of residues, eventually got the house flea free. Meanwhile, the dog we had then developed a flea allergy of life threatening proportions, and eventually developed additional health problems as a result of the allergies and meds.
I don't find that DE works.