I spent a lot of time researching this topic and had some first hand experiences which contrasted with my fellow travelers. I thought I'd pass along my thoughts in terms of the dreaded sand flies.
Every local will have some wives tale about this or that tonic that prevents the bites and makes them better. This is much like how Avon made a killing by selling skin so soft as a fly repellent when later in blind tests it was proven to do nothing. There are a few things that actually work and obviously are sold as such.
Light long pants at night are a great way to protect your legs. In terms of repellent, Deet works the best and lasts the longest. Picaridin is also proven to work, doesn't smell as bad as deet, is less toxic though it doesn't last as long. It is my preference on island. It can't be found there that I could see though. Don't waste your money on basically anything else.
Dealing with the bites
Bites itch. They cause your body to create histamines and thus itch. The systemic way to treat these is actually allergy pills. Benadryl is great but makes you damn tired and isn't for people with high blood pressure. I took allegra. My bites didn't itch very much and didn't swell. Everyone else was itching like crazy. When I stopped taking allegra, my bites itched a ton. It's an OTC allergy medication. There are a lot of similar drugs out there like Zytec, etc that have basically no side effects. You should speak with a doctor or pharmacist about them before taking them.
In terms of the individual bites, Hydrocortisone cream is what the medical community uses to treat itch. It's proven to work. Using any local remedies is just snake oil.
Spray up, wear pants if you can, avoid being on the beach at dawn/dusk, take an allergy pill and hydrocortisone cream and bring repellent with you. Best way to foil those little creatures.