Local officials in Florida have approved the release of 750 million mosquitoes that have been genetically modified to reduce local populations. The aim is to reduce the number of mosquitoes that carry diseases like dengue or the Zika virus.

The green-lighting of a pilot project after years of debate drew a swift outcry from environmental groups, who warned of unintended consequences. But the company involved says there will be no adverse risk to humans or the environment, and points to a slate of government-backed studies.

The plan to release the mosquitoes in 2021 in the Florida Keys, a string of islands, comes months after the modified mosquitoes were approved by federal regulators.

In May, the US Environmental Agency granted permission to the British-based, US-operated company Oxitec to produce the genetically engineered, male Aedes aegypti mosquitoes, which are known as OX5034. Aedes aegypti mosquitoes are known to spread deadly diseases to humans such dengue, Zika, chikungunya and yellow fever.

Only female mosquitoes bite humans because they need blood to produce eggs. So the plan is to release the male, modified mosquitoes who will then hopefully breed with wild female mosquitoes.

However the males carry a protein that will kill off any female offspring before they reach mature biting age. Males, which only feed on nectar, will survive and pass on the genes.

Over time, the aim is to reduce the population of Aedes aegypti mosquitoes in the area and thereby reduce the spread of disease to humans.