The first open field tests of genetically engineered mosquitoes, carried out in the Cayman Islands, show that the insects can successfully compete with their wild counterparts for mates. The mosquitoes are designed to help combat dengue fever, a potentially fatal infection that afflicts 50 to 100 million people per year, mostly in tropical countries. No drugs or vaccines exist to treat or prevent the disease, so prevention efforts are focused on eradicating the carriers, a single species of mosquito.

The mosquitoes, created by a U.K. based start-up called Oxitec, are engineered to carry a mutation that makes them die if not fed a specific chemical. Larval insects can survive in the lab with special food; but once the males are released in the wild and mate, subsequent generations die, reducing the population as a whole.

According to unpublished research from Oxitec, releasing 3.3 million engineered mosquitoes in Grand Cayman Island reduced population by 80 percent. The findings follow a smaller study carried out in 2009, whose results were published Sunday in the journal Nature Biotechnology.

Previous insect control efforts have included sterilizing male animals and releasing them to the wild, where they mate but produce no offspring. Sterile male Mediterranean fruit flies, for example, are used world­wide to protect fruit and vegetable crops. But this does not work for mosquitoes, because the technique used to sterilize the insects also weakens them, meaning they cannot compete for mates.