MNDA VS DEET IN MOSQUITO REPELLENCY 285 The effectiveness of each repellent versus the vehicle control was calculated according to Abbott's formula (11): Percent effectiveness - C-T ---x 100 where C represents the number of mosquitoes biting/probing the vehicle-treated hand/ forearm and T the number of insects biting/probing the repellent-treated hand/forearm during the five-minute challenge session. Percent effectiveness was compared between MNDA and Deet at each time point using Student's paired t-test (o• = 0.05). A Shapiro-Wilks test indicated that the data, in almost all cases, were normally distrib- uted. RESULTS AND DISCUSSION Table I summarizes the results of the "forced-choice" repellency experiment. Each data point represents the mean of three separate trials involving four subjects. Almost 60% of the available A. stephensi and C. quinquefasciatus landed on or probed the subjects' skin within the five-minute challenge period. For A. aegypti, only 40% of the mosquitoes participated in the blood meal search. Although the number of mosquitoes landing or probing remained relatively constant during the first three to four hours, thereafter the numbers of mosquitoes involved decreased substantially, regardless of species. Repletion and adaptation to the stimulus are likely explanations for the diminished activity. Nevertheless, we believe that the "forced-choice" test with a fixed population of mos- quitoes, particularly during the early stages of the experiment (i.e., •4 hours) where 20 or more mosquitoes are involved, provides a reasonable test of a mosquito repellent's efficacy. The mosquito-repelling efficacy of MNDA and Deet relative to the alcohol vehicle is plotted in Figure 1 as a function of time. Both repellents provided 100% protection Table I Number of Mosquitoes Landing on or Probing Repellent/Vehicle-Treated Forearms During Five-Minute Exposure Period* Number of mosquitoes landing or probing (Repellent-treated skin/vehicle-treated skin) A. stephensi C. quinquefasciatus A. Aegypti Time (hr) MNDA Deet MNDA Deet MNDA Deet 1.0 0/29 0/31 0/28 0/29 0/19 1/23 2.0 0/24 0/29 0/27 0/26 0/18 5/22 3.0 0/26 2/26 0/27 1/25 1/17 6/21 4.0 0/23 4/27 1/23 3/23 1/18 7/16 5.0 1/20 4/24 1/21 3/20 2/15 10/14 6.0 1/19 6/23 2/16 4/19 7/15 13/13 7.0 1/20 7/21 2/13 3/15 8/13 12/14 8.0 2/17 7/19 2/11 3/13 9/12 12/13 * Results represent the means of three separate trials (four subjects per trial). Test chambers were charged with 50 mosquitoes.

286 JOURNAL OF COSMETIC SCIENCE 11o lOO 90 8o 7o 60 5o ß MNDA--•- DEET] Anopheles ', lOO 90 80 70 lOO 80 60 40 20 Culex Aedes .• .... ß .... ß 1 2 3 4 5 6 7 8 Time (Hrs) Figure 1. Efficacy of MNDA ( ) and Deer ( ..... ) as a function of time versus Anopheles stephensi, Culex quinquefasciatus, and Aedes aegypti. Each data point represents the global mean derived from three trials with four subjects per trial. Asterisk denotes statistical significance (p -- 0.05) as determined by Student's paired t-test. Percent effectiveness = (C - T)/C x 100. against A. stephensi and C. quinquefasciatus for the first two hours. However, only MNDA provided 100% protection against A. aegypti during the same period. In terms of persistency, both repellents begin to lose efficacy approximately two to four hours after