“Our Mosquito Project Takes Flight,” reads a baby-blue billboard off US-1 in the Florida Keys, alongside an image of an insect tracing a path in the shape of a heart. Sponsored by the local mosquito control board and U.K.-based biotech firm Oxitec, the ad promotes a contentious plan to release millions of genetically modified Aedes aegypti mosquitoes here to test a new method of bioengineered pest control.