When we lived in the south, we used Frontline. It was recommended by the vet that was used at the time by SENC rescue. It turns out he had done the clinical trials on the product. He also told me that it would keep the critters at bay for about 3 months, so there was little need to apply it sonner than that. It worked great when I was using it.
Fortunately here in Maine we have little need for the product. My guys are inside dogs, and there seem to be few fleas here. I do, however, need to be on tick alert fairly vigilently. When the ticks are found, it's out with the tick tweezers, alcohol, and lavender essential oil for the doctoring.
Finally winter has arrived here in Maine...
Hannah