The Brazilian city of Piracicaba has a potent new weapon in the ongoing fight against Dengue Fever, which infects more than a million people annually: genetically modified mosquito lotharios Created by Oxitec of Abingdon, UK and bred locally within Brazil, these GM mosquitoes (all of which are male) are designed to crash the local population before they can spread the tropical disease. More than six million have been released throughout Piracicaba since April. When a GM male mates with a wild female, his sapper genetics cause the resulting larvae to die before they can reach adulthood. What's more, the larvae also carry a genetic mutation that causes them to glow red under UV light, giving researchers an easy way to identify them on sight. "It gives an instant readout of how successfully you're driving down the native population," Hadyn Parry, chief executive of Oxitec, told New Scientist.