Responses of lone star tick (acari: ixodidae) nymphs to the repellent deet applied in acetone and ethanol solutions in vitro bioassays

Authors: Carroll, John; Kramer, Matthew

Submitted to: Journal of Entomological Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/23/2011
Publication Date: 4/1/2012
Citation: Carroll, J.F., Kramer, M.H. 2012. Responses of lone star tick (acari: ixodidae) nymphs to the repellent deet applied in acetone and ethanol solutions in vitro bioassays. Journal of Entomological Science. 47(2):193-196.

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. ARS has a memorandum of understanding with the EPA to cooperate in developing standardized methods for testing tick repellents. In bioassays, it is common practice to dilute a repellent in a solvent, such as acetone or ethanol, and apply the solution to filter paper to which ticks are exposed. We noticed that in a bioassay we often use acetone solutions of the widely used repellent deet were more repellent to lone star ticks than we had previously observed with ethanol solutions of deet. To obtain a better profile of these differences, we tested ethanol and acetone solutions of deet over a range of concentrations. When test solutions were allowed to dry for 10 min before exposing them to ticks, intermediate concentrations of deet in acetone solutions repelled considerably more lone star ticks than did ethanol solutions of deet. 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. 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. Although tick repellent bioassays tend to be rather uncomplicated, several factors can influence their outcomes. Typically repellent bioassays use a solvent, such as acetone or ethanol, to dispense desired concentrations of the active solute evenly on a substrate. Time is usually allowed for the solvent to evaporate before test organisms are exposed to the treatment. Using lone star tick, Amblyomma americanum (L.), nymphs in climbing bioassays (vertical filter paper), we tested ethanol and acetone solutions of the widely used repellent N,N-diethyl-3-methyl benzamide (deet) over a range of concentrations. The test solutions were applied to filter paper and allowed to dry 10-12 min before exposure to ticks. The dose-response relationship from the two solvents differed greatly, and was similar only for low (zero) and high concentrations of deet. Ethanol solutions of deet applied at 500 and 1000 nmol compound/cm2 filter paper were essentially not repellent to A. americanum nymphs (0 and 13.3% respectively), whereas the same concentrations of deet in acetone repelled 90 and 96.7% the nymphs tested. The differences we observed in the proportions of A. americanum repelled when deet was applied in acetone or ethanol solutions indicate that that it is inadvisable to compare repellent data obtained by different bioassay methods.