Washington, Aug 17 - After more than 50 years of research, scientists have finally discovered a number of new mosquito repellants that keep the dangerous pests away much longer than before.

The new repellants beat DEET, the current gold standard for warding off mosquitoes. DEET is N,N-Diethyl-meta-toluamide, a slightly yellow oil. It is the most common active ingredient in insect repellants.

The new repellants make mosquitoes stay away three times longer than DEET, and without the latter's odour. Nor does it cause DEET's sticky-skin sensation.

But there's a fly in the ointment: The odds may be stacked against any of the new repellants finding a place on store shelves this year or next -- or ever.

Ulrich Bernier from the Centre for Medical, Agricultural, and Veterinary Entomology (CMAVE), who led a new study on the subject, said the costly, time-consuming pre-market testing and approval process is a hurdle that will delay availability of the repellants, which were developed last year.