Hello and welcome to Just Answer. While itching can be a sign of a flea allergy it can also be a sign of seasonal allergies, or red mange.

I would take a flea comb and go over the rump a few times, then take the hair out of the come and and put on a white piece of paper. If you see black specs it is flea dirt and is a sign of fleas. http://www.petplace.com/dogs/flea-allergy-dermatitis-in-dogs/page1.aspx

Red mange is prone to some breeds of dog and is not contagious but will need to be treated by your vet. http://www.petplace.com/dogs/demodicosis-red-mange-in-dogs/page1.aspx

Allergies are very common in the spring and can give dogs a moth eaten appearance , they can scratch through the season or sometimes it lasts longer depending on what the dog is allergic to. http://www.petplace.com/dogs/seasonal-allergies-mean-big-headaches-for-dogs-too/page1.aspx

At this time frontline and an oatmeal bath may not help but it won't hurt if your dog has used Frontline before. I would bring the dog in for a skin scraping to be sure what you are dealing with. If it is fleas the frontline will take care of it, but if it is a flea allergy where the dog is allergic to the flea saliva it can get worse, Benadryl allergy will help if it is an allergy and if mange is involved you'll need specific shampoo from your vet.