Mosquito repellence induced by tarsal contact with hydrophobic liquids

Mosquito legs have a unique highly water-repellent surface structure. While being beneficial to mosquitoes, the water-repellence of the tarsi enhances the wettability of hydrophobic substances such as oils. This high wettability induces strong attraction forces on a mosquito’s legs (up to 87% of the mosquito’s weight) towards the oil. We studied the landing behaviour of mosquitoes on oil-coated surfaces and observed that the mosquito contact time was reduced compared to that on hydrophilic-liquid-coated surfaces, suggesting that the oil coating induces an escape response. The observed escape behaviour occurred consistently with several hydrophobic liquids, including silicone oil, which is used globally in personal care products. As the repellent effect is similar to multiple hydrophobic substances, it is likely to be mechanically stimulated owing to the physical properties of the hydrophobic liquids and not due to chemical interactions. On human skin, the contact time was sufficiently short to prevent mosquitoes from starting to blood-feed. The secretion of Hippopotamus amphibius, which has physical properties similar to those of low-viscosity silicone oil, also triggered an escape response, suggesting that it acts as a natural mosquito repellent. Our results are beneficial to develop new, safe, and effective mosquito-repellent technologies.

. For DEET, the angle showed an equilibrium state, whereas the PDMS continuously spread on the surface after 1 s. The contact angle of DEET averaged 65.7° ( = 36.1 mN/m), suggesting relatively low wettability. We anticipated that large application ratios would be required to shorten the mosquito contact-time via wetting-based repellence, implying that receptor-based mechanism likely contributes to contact-based repellence of DEET [S1]. The horizontal line represents the mean ± s.e.m. *Contact angle varied significantly between liquids (Student's ttest, P = 10 −15 ). b. Determination of the critical surface tension (c) of mosquito tarsi. Contact angles, except for L-PDMS, were obtained with droplet deposition of each liquid. For the measurement with L-PDMS, the wetting did not reach equilibrium within the measurement time; thus, its contact angle was calculated using the attractive force measured with a force tensiometer (Fig. 1e). Fitting line (solid black line) was obtained with the least-square method using four plots, except for L-PDMS, because the PDMS spreads completely owing to its low surface tension ( = 0°). Critical surface tension c was 20.9 mN/m; when   c  liquids spread completely on mosquito tarsi (total wetting:  = 0°). Supplementary Fig. S3 Frame-by-frame photos of mosquito on substrate coated with squalane oil. Application ratio: 0.25 mg/cm 2 . Contact time was defined as the elapsed time between the recorded time when at least one tarsi or proboscis contacted the substrate and the time when all mosquito tarsi were removed.

Supplementary Fig. S5 Ground-glass substrates used in mosquito contact-time test. a., b.
Image of surface of uncoated glass substrate and same substrate after application of liquids specified in Fig. 2c and 3a. Application rate: 0.25 mg/cm 2 . Scale bars: 100 m. c. Mean height of uncoated substrate and liquid-coated substrates (n = 3). The letters (A, B, or C) indicate the significant variance among the different liquids (one-way ANOVA with the Tukey post hoc test, P = 10 −13 ).

Supplementary Fig. S6 Flow curve of shear-thinning fluids.
Flow curve (viscosity as a function of shear rate) of hippopotamus secretion and fumed silica suspension in L-PDMS exhibited shear-thinning behaviour. These two curves were comparable. L-PDMS is a Newtonian fluid in which the viscosity did not vary by shear rate.

Supplementary Table S2
Contact angle of droplets on the scale carpet. The values were measured 1 s after liquid deposition.

Supplementary Table S3
Contact-time of mosquitoes on ground-glass substrates coated with the liquids addressed in this study. The measurement results of contact-time of each trial are displayed. The ratio (C/C ratio) of mosquitoes that made ceasing wing motions after making contact with the substrate was calculated.

Supplementary Table S4 Preparation conditions and film thickness of spin-coated
PDMSs on a silicon-wafer used in AFM measurement.