Submitted to: Journal of Medical Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/7/2013
Publication Date: 6/11/2014
Citation: Carroll, J.F., Kramer, M.H., Bedoukian, R. 2014. Solvent, drying time, and the responses of lone star ticks (acari: ixodidae) to the repellents deet and picaridin. Journal of Medical Entomology. 51(3):629-637. Interpretive Summary: Tick-borne diseases pose a serious threat to humans throughout much of the habitable world. Repellents provide a critical means of personal protection against tick bite. Behavioral bioassays are an important tool in repellent discovery, development, and registration. A great variety of bioassays are used to test tick repellents. In bioassays, it is common practice to dilute a repellent in a solvent and apply the solution to filter paper to which ticks are exposed after the solution has dried. We observed that the solvent affected the effectiveness of the repellent, so we sought to define the conditions and materials associated with the conflicting results. Long drying times and use of nylon organdy instead of filter paper gave consistent results regardless of the solvent used. There is little uniformity in the bioassay methods used to evaluate tick repellents, and these results demonstrate the need for caution when comparing published tick repellent data and provide a more uniform assay. These findings are of interest to the EPA, researchers investigating tick repellents, and to manufacturers developing tick repellents.
Technical Abstract: Behavioral bioassays remain a standard tool in the discovery, development, and registration of repellents. Tick repellent bioassays tend to be rather uncomplicated, but several factors can influence their outcomes. Using lone star tick, Amblyomma americanum (L.), nymphs in climbing bioassays, we tested for the effects of substrate, solvent, and drying time on tick responses. In dose response tests, three concentrations of the widely used repellents deet and picaridin applied to Whatman No. 4 filter paper strips, were challenged by ticks at 10, 20, 30, 40, and 120 min after application of test solutions. At 10 min drying time, repellency at the intermediate concentration (500 nmol repellent/cm2 filter paper) was significantly lower for ethanol solutions of deet and picaridin (0 and 10% ticks repelled, respectively) than for solutions of deet and picaridin in acetone (96.7 and 76.7% ticks repelled, respectively). Repellency was greatest for both the acetone and ethanol solutions of deet and picaridin when 120 min elapsed between application of test solutions and start of exposure. However, the repellency of picaridin relative to deet differed at some combinations of solvent and drying time and not others. In dose response tests, ethanol and acetone solutions of deet were allowed to dry for 10 min on copier paper, Whatman No. 1 and Whatman No. 4 filter papers. At 500 nmol deet/cm2 paper, acetone solutions were significantly more repellent than ethanol solutions for all the types of paper, however, ethanol solutions of deet on Whatman No. 1 filter paper were less repellent than those on copier paper and Whatman No. 4 paper and acetone solutions of deet on Whatman No. 1 paper were less repellent than acetone solutions of deet on copier paper and Whatman No. 4 filter paper. In dose response fingertip bioassays with test solutions applied to a strip of nylon organdy doubly wrapped around an index finger, ethanol and acetone solutions of deet were similarly repellent at each concentration (drying time 10 min). Nylon organdy or another similar thin cloth may be a preferable repellent substrate to filter papers and copier paper. If the solute allows, acetone may be preferred to ethanol when paper is the substrate.