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We evaluated the behavioral responses of Aedes albopictus and Anopheles minimus to 3 isovaleric acid and lactic acid-based chemical lure blends and 2 individual alcohols, using Spatial Repellency Assay in a high-throughput screening system (HITSS). Five doses of 0.0002, 0.001, 0.0025, 0.005, and 0.01 g were tested per lure. A BG-lure was used as a reference standard. After 10-min exposure, the number of mosquitoes moving toward or away from the treated chamber was calculated. The results showed that all lures were repellent against Ae. albopictus except for Lure-4 (4% w/v isovaleric acid + 2% w/v lactic acid + 0.0025% w/v myristic acid + 2.5% w/v ammonium hydroxide) which showed a nonsignificant attractancy at the lowest dose. Significantly high spatial repellency was observed at the highest dose of all the tested lures including BG-lure. Lure-2 (isoamyl alcohol) was significantly repellent at all the tested doses. Against An. minimus, Lure-5 (0.02% w/v isovaleric acid + 2% w/v lactic acid) showed significant spatial repellency while Lure-4 was significantly attractant, at all the tested doses. All lures, except Lure-4, showed strong spatial repellency at high doses and attractancy or weak spatial repellency at the lowest dose of 0.0002 g. In summary, our study demonstrated that spatial repellency and attractancy of the tested lures were influenced by both the dose tested and the mosquito species. Lure-2 and Lure-4 are potential spatial repellents and attractants, respectively, for malaria and dengue vectors. However, further studies are necessary to confirm these results at a semifield and open field level.

Contact irritant (locomotor excitation) and noncontact spatial repellent avoidance behavior to deltamethrin and cypermethrin at dosages 0.025, 0.05, and 0.1 g/m2 impregnated on papers were evaluated in the laboratory against deltamethrin- and cypermethrin-resistant field populations of female Aedes aegypti (L.) from Rayong and Chanthaburi Provinces, Thailand. Pyrethroid-resistant populations were compared with a susceptible laboratory strain (NIH-Thai) using an ‘excito-repellency’ (ER) test system. Both NIH-Thai and field mosquitoes had stronger contact irritancy responses compared to the relatively weak noncontact repellency effects. Contact assays with deltamethrin and cypermethrin at 0.1 g/m2 showed high escape rates for Rayong (80.1 and 83.4%, respectively) and Chanthaburi (84.6 and 73.1%, respectively) mosquitoes. Cypermethrin produced significantly different (P < 0.05) percent escape responses in contact tests between NIH-Thai and field mosquitoes. Only deltamethrin contact at 0.05 g/m2 produced a significant escape response (P < 0.001) between NIH-Thai and Rayong mosquitoes. These results suggest that there may not be an overall significant effect of background pyrethroid resistance on escape response, and the differences by comparisons may reflect inherent individual variation when using the ER bioassay system. The results show that pyrethroid resistance in Ae. aegypti does not appear to influence or reduce contact avoidance responses with the compounds tested. In particular, deltamethrin at 0.1 g/m2 was an effective contact irritant and toxic compound against pyrethroid-resistant populations of Ae. aegypti. Therefore, 0.1 g/m2 deltamethrin could be considered for residual applications of either fixed surfaces or materials (e.g., curtains) as a supplemental control measure against adult dengue vectors.

Chemicals can protect humans from the bites of hemophagous arthropods through three different primary actions: irritancy (excitation), repellency, or toxicity, actions that can be evaluated using a laboratory-based assay system. In this study, the deterrent and toxic actions of three synthetic pyrethroids and DDT were characterized on six field strains of Aedes aegypti from Thailand under laboratory-controlled conditions using the high throughput screening system. All six strains showed significant contact irritant responses to the three synthetic pyrethroids, but significantly weaker irritant responses to DDT. Marked repellency responses were seen in all six Ae. aegypti test strains exposed to DDT, whereas the synthetic pyrethroids resulted in greater toxicity than DDT under similar test conditions. Although significantly different in actions, irritancy and repellency may reflect and be influenced by the background insecticide susceptibility status of a particular mosquito population. Results from this study can be used to guide decision making regarding more effective Ae. aegypti adult control in Thailand.