Insect-borne diseases like dengue and malaria, including agricultural pests, could soon be controlled by genetically engineered insects, scientists say.

Recently, U.S. researchers reported initial signs of success from the first release of engineered mosquitoes into the environment that pass a lethal gene to their offspring that kills them before they mature.

This result could herald a new age in the use of genetically modified insects for insect-borne diseases, especially dengue, which has 50 to 100 million cases each year causing an estimated 25,000 deaths.

Florida Keys authorities want to conduct an open-air test on the modified mosquitoes as early as December while the Department of Agriculture is looking at using genetic engineering to help control farm pests like the Mediterranean fruit fly, medfly and bollworms.

"It's a more ecologically friendly way to control mosquitoes than spraying insecticides," said Coleen Fitzsimmons, a spokesman for the Florida Keys Mosquito Control District. In 2009, the Keys experienced the first cases of dengue in decades.

The study published looked at how successfully the lab-reared, genetically modified insects could mate. About 19,000 engineered mosquitoes were released over four weeks in 2009 in a 25-acre area on Grand Cayman island.

The study showed the genetically engineered males accounted for 16 percent of the overall male population in the test zone, and the lethal gene was found in almost 10 percent of larvae, suggesting that the genetically engineered males were about half as successful in mating as wild ones, a rate sufficient to suppress the population.

However, the research is arousing concern about possible unintended effects on public health and the environment, because once genetically modified insects are released, they cannot be recalled.

Alfred M. Handler, a geneticist at the Agriculture Department in Gainesville, Fla., said the mosquitoes can evolve resistance to the lethal gene while being bred for generations in the lab and might be released inadvertently.

In response to the concerns of the different groups, experts assembled by the World Health Organization are preparing guidelines on how field tests of genetically modified insects should be conducted.