Walmart should have a garden section that carries the Safer products. I have not seen Serenade but it should work as well. Safer is supposedly a safe organic product that can be sprayed right on the plant. Just one application usually works on the amount of pm you are seeing. Since you are growing indoors you need to get used to treating pm since it will be a recurring problem. Be sure to watch for spider mites as well when growing indoors. There is also a supposedly organic Safer product for that as well.