"We need better insect repellents to use as weapons against the spread of infectious disease," said Leslie Vosshall, head of the Laboratory of Neurogenetics and Behavior at New York's Rockefeller University.

"This finding has a direct applied potential," Vosshall added in statement. "Insects are the primary vectors for malaria, dengue fever, yellow fever, and West Nile encephalitis, and they locate human hosts largely through their exquisitely sensitive olfactory systems."

Vosshall and colleagues bred fruit flies that lack a gene known as Or83b. Writing in the journal Neuron, they said the tiny flies had no sense of smell.

The Or83b gene is found in a variety of insect species, including mosquitoes, the researchers said. It may be possible to design a compound that blocks the gene.

"Most insect repellents are based on trial and error, or folk remedies," Vosshall said. "Now we have a scientific, rational basis for designing insect repellents."

All creatures that smell detect molecules from the object they are smelling. Specialized cells detect these molecules and in humans and other similar animals they are at the top of the nasal passages.

In the fruit fly these detector cells are on the antennae and the maxillary palp, an appendage near the mouth.

The cells have receptors, or molecular doorways, designed to attach to the molecules. Many are designed to detect different smells, such as bananas or sweaty feet.

But nearly all the fly smell detector cells have the Or83b receptor, the researchers found to their surprise.

The same is true of locusts, mosquitoes, moths, honeybees and Mediterranean fruit flies, Vosshall said. "In all of these insects the gene is found in nearly all the olfactory neurons. The vast majority of cells responsible for smell have it," he said.