Immo Hansen (right), associate professor in New Mexico State University’s Department of Biology, and graduate student Soumi Mitra.

By Andrew Porterfield

Andrew Porterfield

Humans have probably sought ways to avoid mosquito bites throughout their existence. And for good reason: the insects are the world’s most dangerous animals, feeding on blood and in doing so spreading deadly pathogens like malaria, yellow fever, and others.

While chemical repellents containing DEET (N,N-diethyl-m-toluamide) are widely used, known to be effective, and result in few health problems, more people are looking at essential plant oils as a more “natural” and safe way to repel mosquitoes. However, many of these oils have well-documented toxicity, and each oil’s capacity as a repellent is unknown.

Meanwhile, the U.S. Environmental Protection Agency (EPA) regulates all insect repellents as pesticides under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). The agency has listed 44 active ingredients (and 200 inert ingredients) as exempt from FIFRA, on the “EPA 25(B) list.” Many of these exempted chemicals include essential plant oils. The exemption list was made to “reduce the cost and regulatory burdens on businesses and the public for pesticides posing little or no risk,” according to EPA’s website.

To investigate the ability of essential oils to repel mosquitoes, Immo Hansen, associate professor of biology at New Mexico State University, and his colleagues conducted tests on 21 oil active ingredients and five commercial repellents from the EPA 25(B) list. They found that five oils reduced mosquitoes’ attraction to human odor, and just one commercial repellent reduced attraction to human odor. Their findings were published in the Journal of Medical Entomology.

Hansen and his team used a Y-tube olfactometer bioassay, in which the sample substance was placed in one tube, and a human volunteer was placed so that her hand’s odor was detected along with the oil/sample. All 21 active ingredients were tested this way at 30-minute intervals until they observed no repellency. The commercial products were applied directly to the volunteer’s hand and tested with the Y-tube device. Female Aedes aegypti mosquitoes were evaluated in the study.

The team found that on average, mosquito attraction to human odor in the presence of oil repellents ranged from 74 to 86 percent, while DEET-based repellents ranged from 30 to 34 percent attraction. Of the oils tested, only five of them—peppermint, lemongrass, spearmint, garlic, and cinnamon—showed any impact on mosquito attraction. Cinnamon oil could reduce mosquito attraction for 90 minutes, peppermint and lemongrass oil worked for 60 minutes, and spearmint and garlic oil lost their efficacy after the 30-minute mark. For commercial sprays only the OLE (Cutter Lemon Eucalyptus Insect Repellent) reduced attraction to 22 to 31 percent, though all five showed some amount of reduction of attraction.

The team was surprised to find that citronella, derived from tropical grasses and long used to repel insects, was not effective against mosquitoes. “We were surprised by the complete failure of citronella,” said Hansen. “The second surprise was that when we tested commercial products that contained only essential oils that tested negative in our initial test, we found that they repelled mosquitoes at the initial time point. One explanation is that it was the mixture of different essential oils that did the job, another is that the inactive ingredients in the sprays covered up the human smell of the treated hand.”

Hansen has compared DEET-containing substances and oils in other experiments, but he emphasized that direct comparisons with DEET may not be a fair evaluation. “We tested the pure essential oils which are very volatile. It would not be fair to compare the protection time with DEET because much of the oils probably evaporated quickly. The trick with repellents using this sort of essential oils is to mix them with inactive ingredients to achieve slow release,” he said.

Oils are also a very complex mixture of a variety of chemicals. Dozens, maybe even hundreds of compounds may comprise an essential oil mix. “In many cases, we don’t know which of the compounds found in essential oils are the active ingredients that repel mosquitoes,” Hansen said.

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.