Constant itching and chewing -irritation caused by unknown source

Question
I'm so excited that your expertise is Dermatology!  We just moved into an apartment complex and our 4yr old dachshund has started to constantly scratch and chew at her stomach and hind quarters.  We think that whatever pesticide they spray on the grass is the problem but are unsure.  We took her to the vet and they gave her a steroid shot and put her on medication.  After that ran out we started medicating ourselves with Benedryl, not wanting to constantly have her on strong meds.  However, she's still having the same problem and we don't really want to keep giving her Benedryl (which is also starting to loose its effectivness).  She's getting these little blistery looking things on her stomach and insides of her thighs which get infected since she's constantly trying to lick them.  The itching is driving her nuts and the constant scratching is driving us nuts - PLEASE do you have any adivce?!  We are desperate!  

Answer
If the dog it itchy and the prednisone helped, then it is probably an allergy.  Low dose steroids during the summer allergy season do not have permanent long lasting effects on the dog.  She may drink and eat a little more, but that stops when the prednisone is stopped.  Benadryl is ineffective for dog allergies. It makes them sleepy so they don't scratch as much.  If there are infected areas on her belly, that needs to be treated with  antibiotics and baths with medicated shampoo.