Assessing the repellent efficacy of essential oils
Environmental management and protection from insect bites remain some of the main strategies shared with communities to prevent mosquito-borne diseases such as dengue and break transmission cycles. The search for effective novel tools for the protection from mosquitoes is ongoing, with researchers increasingly exploring alternatives to existing insecticides and repellents.
Applying insect repellent remains one of the most favoured strategies chosen by individuals to protect themselves against mosquito bites. The U.S. Environmental Protection Agency (EPA) strictly regulates repellents as pesticides, to mitigate any toxicity. In 1996, the EPA compiled a list of 44 active ingredients and 200 inert ingredients considered to have minimum risk for human health - the ingredients on this list, also known as EPA 25 (B), can be used as pesticides or repellents without formal assessment through the EPA registration process. However, essential plant oils have been known to present adverse effects and toxicity while DEET, a synthetic chemical, has generally been considered safe and highly effective at repelling Aedes mosquitoes throughout the scientific literature.
In a recent study, researchers from the Department of Biology at the New Mexico State University tested 21 of those common active ingredients as well as 5 commercially available products containing exclusively ingredients from this list in order to assess their efficacy in repelling female Aedes aegypti (Table 1 and Table 2). The active ingredients were tested after initial application and then at 30 min intervals until no repellency was observed, and this was compared to a positive control of 100% DEET. Unlike many previous studies, often providing contradictory conclusions, this study used similar assays, active ingredients from a single source, and a unique species of mosquitoes.
Table 1: List of the active ingredients from the 25(b) list used in this experiment. Shown are the Sigma Aldrich order numbers and the Chemical Abstracts Service (CAS) numbers of the individual ingredients.
Table 2: Composition of repellent sprays
Repellency was tested using a Y-tube olfactometer (fig. 1); this setup and the fact that essential oils can be irritant or exhibit toxicity with direct skin application, however, did not permit for contact testing:
Of the essential oils, five had an impact in terms of the mosquitoes' attraction to humans, but with variable length of efficacy: peppermint, spearmint, garlic oil and lemongrass oil were effective for 30 minutes, with spearmint and garlic oil presenting a strong initial effect. Cinnamon oil was effective in significantly reducing mosquito attraction for considerably longer at 90 minutes. Based on this research in this set-up, the remaining 16 active ingredients did not demonstrate any effect. The commercial products all demonstrated some weak repellent efficacy for 30 minutes, with Honest Bug Spray reducing attraction for 60 minutes.
Further testing and work is needed to determine the conditions under which both strength and length of repellent efficacy can be harnessed in these active ingredients. In addition, some of the commercial products composed of a number of essential oils showed efficacy even though those essential oils did not show efficacy when used separately - further testing is needed to better understand synergistic effects between essential oils and how combinations can provide a higher repellent efficiency.
For further details:
Efficacy of Active Ingredients From the EPA 25(B) List in Reducing Attraction of Aedes aegypti (Diptera: Culicidae) to Humans Soumi Mitra, Stacy D Rodriguez, Julia Vulcan, Joel Cordova, Hae-Na Chung, Emily Moore, Yashoda Kandel, Immo A Hansen Journal of Medical Entomology, tjz178