Essentail oils are great repellents and unlike man made repellents insects dont seem to get immune.
The big problem with synthetic pyrethroids is that many species of flies now demonstrate resistance to them. Jerry Butler, PhD, a medical and veterinary entomologist with the Entomology and Nematology Department at the University of Florida, has done extensive studies on fly repellents and insecticides. He notes, "Permethrin used to last three days. Now it’s only effective for a couple of hours. As resistance has built up, the repellent action has been lost first." So, it’s not your imagination; your fly spray probably is less effective than it used to be. But interestingly, no resistance has yet been noted for natural pyrethrin.
Natural Oils are very biodegradable and are often put with formulas to try and make the oil stay around for longer such as sun cream and different carrier oils these often actually attract flies.
New products are coming to the market like FLY-OFF inserts these are designed to continuosly release the oils slowely over several days. Other methods are using foam pads to put oils on these work like a car air freshener releasing small amounts of oil constantly.