Scientists in the US have found that a new generation of insect repellents show great promise, some of which remain effective for up to three times longer than DEET which has been considered the “gold standard” for the last 50 years.

The finding is the work of chemists from the University of Florida and the US Department of Agriculture (USDA) and is to be published later today in the Proceedings of the National Academy of Sciences (PNAS).

DEET (N,N-diethyl-m-toluamide) is currently the most effective protector against mosquito bites, which can infect people with diseases like malaria, West Nile Virus, yellow fever, encephalitis and dengue fever.

The researchers found seven chemicals that show promise as the next generation of mosquito repellent. They have not been safety tested yet, but in tests on human volunteers, the researchers found many of them were still repelling mosquitos 40 or 50 days later, considerably longer than DEET which only lasts 17.5 days on average. One or two of the new chemicals lasted as long as 73 days.

Kenan Professor Alan R Katritzky of the University of Florida led the researchers to review data on hundreds of chemicals that had been collected by the USDA over the last 50 years. They gave each one a rating between 1 and 5 depending on how well the data claimed they repelled insects, and then they looked at the ones that scored 5 to see what they had in common.

They found that one of the most frequently occuring chemicals was N-acylpiperidines, which occurred in 34 of the compounds, which they reduced to 10 and then 7 using criteria like effectiveness, likely toxicity and production costs. According to the New York Times, N-acylpiperidine is related to the active ingredient in pepper.

Co-author Dr Ulrich R. Bernier, a research chemist at the USDA’s Mosquito and Fly Research unit in Gainesville, Florida, told the Associated Press in a telephone interview that he was very surprised at how well the new chemicals performed, he said some of them “were just phenomenal”.

The tests did not measure typical exposure. For example, the volunteers wore thick gloves with holes in them, on which were placed pieces of muslin cloth soaked with the target chemical. The volunteers then placed their gloved hand inside a box containing mosquitoes for one minute. This was repeated every day. The first day that five mosquitoes successfully bit through the cloth was counted as the day the chemical failed as a repellent.

Bernier said they hoped later this year to start safety tests where the chemical goes directly onto the skin.

According to the US Environmental Protection Agency (EPA), DEET is the active ingredient in many insect repellent products and is used by around one third of Americans every year to protect against being bitten by insects such as mosquitoes and ticks.

Products containing DEET come in many forms, such as liquids, lotions and sprays, and even impregnated materials like wristbands and can contain anything from 4 to 100 per cent of DEET. About 140 products containing DEET are currently registered with the EPA, made by about 40 companies.

DEET was first approved for use in the US military in 1946, and was registered for use by the general public in 1957.

Sources: Associated Press, New York Times, EPA.

Written by: Catharine Paddock,