What mosquito repellents work best? INSIDE EDITION went down to the USDA Mosquito and Fly Research Unit in Gainesville, Florida to find out which repellents work best. Three volunteers exposed their arms to hundreds of hungry Zika-free mosquitos. The first product to be tested was the chemical DEET. It's the active ingredient in many spray repellants like OFF, Cutter and Repel. They all worked fighting off the mosquitos.