You can trap fleas by placing a dish of soapy water under a night light near where your pet sleeps. Fleas are attracted to warm light and will drown in the soapy water. This works for adult fleas only, but with diligence, can be effective reducing the flea population. Fleas already residing on your pet aren’t likely to leave, so you will still need to flea comb and/or bathe them in a mild shampoo (even a baby shampoo will work as fleas don’t survive well in soapy water).
Also have a look a Natural Tick and flea spray for dogs.