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