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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.

Indoor residual spray with deltamethrin remains the most common tool for reducing malaria transmission in Thailand. Deltamethrin is commonly used to spray the entire inner surfaces of the walls to prevent mosquitoes from resting. This study compared the mosquito landing responses on humans inside three experimental huts treated with deltamethrin at three different extents of wall coverage (25%, 50%, and full coverage), with one clean/untreated hut serving as a control. There were no significant differences between the numbers of Anopheles mosquitoes landing in the 50% and full coverage huts, whereas, in comparison to both of these, there was a significantly greater number landing in the 25% coverage hut. This study demonstrates that varying the percent coverage of indoor surfaces with deltamethrin-treated netting influences the blood-feeding success of wild Anopheles, and our findings suggest that it may be possible to reduce the extent of insecticide surface treatment while maintaining equivalent mosquito avoidance action to that seen in fully treated structures.

Essential oils extracted from 4 different plant species--citronella (Cymbopogon nardus), hairy basil (Ocimum americanum), sweet basil (Ocimum basilicum), and vetiver (Vetiveria zizanioides)-were investigated for their irritant and repellent activities against Anopheles minimus, using an excito-repellency test system. Pure essential oils were used in absolute ethanol at the concentrations of 0.5%, 1%, 2.5%, and 5% (v/v) compared with deet. At the lowest concentration of 0.5%, hairy basil displayed the best irritant and repellent effects against An. minimus. Citronella and vetiver at 1-5% showed strong irritant effects with80% escape, while repellent effects of both oils were observed at 1% and 2.5% citronella (73-89% escape) and at 5% vetiver (83.9% escape). Sweet basil had only moderate irritant action at 5% concentration (69.6% escape) and slightly repellent on test mosquitoes (50% escape). The results found that hairy basil, citronella, and vetiver are promising potential mosquito repellent products for protection against An. minimus.

The High Throughput Screening System (HITSS) has been applied in insecticide behavioral response studies with various mosquito species. In general, chemical or natural compounds can produce a range of insect responses: contact irritancy, spatial repellency, knock-down, and toxicity. This study characterized these actions in essential oils derived from citronella, hairy basil, catnip, and vetiver in comparison to DEET and picaridin against Aedes aegypti and Anopheles minimus mosquito populations. Results indicated the two mosquito species exhibited significantly different (P0.05) contact irritant escape responses between treatment and control for all tested compound concentrations, except with the minimum dose of picaridin (P0.05) against Ae. aegypti. Spatial repellency responses were elicited in both mosquito species when exposed to all compounds, but the strength of the repellent response was dependent on compound and concentration. Data show that higher test concentrations had greatest toxic effects on both mosquito populations, but vetiver had no toxic effect on Ae. aegypti and picaridin did not elicit toxicity in either Ae. aegypti or An. minimus at any test concentration. Ultimately, this study demonstrates the ability of the HITSS assay to guide selection of effective plant essential oils for repelling, irritating, and killing mosquitoes.

Malaria is a serious mosquito-borne disease transmitted to humans by Anopheles mosquitoes. Seven species of Anopheles are important malaria vectors in Thailand and all remain susceptible to pyrethroid insecticides, despite decades of use in public health mosquito control programs. However, for a more refined and accurate approach to temporal monitoring of susceptibility patterns to commonly used pyrethroids to control adult Anopheles mosquitoes, it is preferred to have specific concentrations for more discriminating testing. A laboratory strain of Anopheles minimus Theobald was used to establish baseline concentrations for susceptibility to six different synthetic pyrethroids-deltamethrin, permethrin, bifenthrin, cypermethrin, α-cypermethrin, and λ-cyhalothrin using the World Health Organization (WHO) adult bioassay system. Final discriminating concentrations of each active ingredient were determined based on doubling LC99 for each chemical to arrive at the following percent concentrations: deltamethrin (0.009%), permethrin (0.551%), bifenthrin (0.12%), cypermethrin (0.104%), α-cypermethrin (0.034%), and λ-cyhalothrin (0.019%). For permethrin, deltamethrin, α-cypermethrin, and λ-cyhalothrin, all revised percentages were lower than currently recommended discriminating concentrations by WHO. Using the newly derived concentrations, insecticide susceptibility tests were followed in a field population of An. minimus s.l. from Kanchanaburi Province. Deltamethrin, bifenthrin, and λ-cyhalothrin produced 100% kill, whereas permethrin, cypermethrin, and α-cypermethrin gave between 92.9 and 97.7% mortality, indicating lower sensitivity (tolerance). Routine monitoring of insecticide susceptibility in An. minimus wild populations throughout Thailand should continue to ensure that insecticides in use are effective to control this vector species.

International audience; Contact irritant and non-contact repellent activities of β-caryophyllene oxide were evaluated against laboratory strains of female Aedes aegypti (USDA strain), a major arbovirus vector and Anopheles minimus (KU strain), a major malaria parasite vector, compared with the synthetic repellent DEET, using an excito-repellency test system. β-caryophyllene oxide and DEET were tested at concentrations of 0.1, 0.25, 0.5 and 1.0% (v/v). Anopheles minimus was found to be more sensitive to β-caryophyllene oxide than that of Ae. aegypti and exhibited high avoidance response rates (86–96% escape) at 0.5% and 1.0% concentrations in contact and non-contact trials compared with Ae. aegypti (22–59% escape). However, at the same concentrations, DEET displayed lower irritancy and repellency capacities against these two mosquito species (range 0–54% escape) compared to β-caryophyllene oxide. The analysis of escape responses showed significant differences between mosquito species at all concentrations (P

Escape responses, knockdown (KD), and toxicity of laboratory strains of Anopheles minimus Theobald and Culex quinquefasciatus Say to three synthetic mosquito repellents, DEET (N, N-diethyl-3-methylbenzamide), IR3535, or picaridin, at 5% v/v concentrations, were evaluated using repellent-treated papers in standard WHO tube assays and an excito-repellency (ER) test chamber system. The tube assays recorded knockdown effects of each repellent immediately after 30-min exposure and the final morality following a 24-h holding period. DEET showed 100% KD at 30 min and complete toxicity at 24 h against both species. Both actions were either minimal or absent for IR3535 and picaridin, respectively. Culex quinquefasciatus showed significantly greater escape with DEET compared with the other compounds in both contact irritancy (excitation) and noncontact spatial repellency trials. Anopheles minimus showed much more pronounced irritancy and repellency flight escape to IR3535 than picaridin. DEET was the most active irritant and repellent compound against Cx. quinquefasciatus. When adjusting contact test responses based on paired noncontact repellency assays, DEET and IR3535 showed much stronger spatial repellent properties than irritancy with An. minimus. Picaridin performed poorly as an irritant or repellent against both species. We conclude that DEET, followed by IR3535, act as strong spatial repellents at 5% concentration. DEET also performs as a strong toxicant. Our findings show that different mosquitoes can respond contrastingly to repellents, thus the importance to test a wider range of species and populations to assess the full range of chemical action.

The essential oils of kaffir lime (Citrus hystrix DC.) at four different concentrations (0.5, 1.0, 2.5, and 5.0% v/v) were studied for their repellency, excitation, and knockdown properties against laboratory strains of Aedes aegypti (L.) and Anopheles minimus Theobald using an excito-repellency test system. Both contact and noncontact escape responses to leaf- and peel-derived kaffir lime oils were observed. Comparing unadjusted escape responses for An. minimus, leaf oil had strong combined irritant and repellent activity responses at 1-5% concentrations (90.0-96.4% escape) and the strongest spatial repellent activity at 1% and 2% (85.9% and 87.2% escape, respectively). The peel oil exhibited good excitation with repellency at concentrations of 2.5% (89.8% escape) and 5% (96.28% escape), while concentrations 1-5% showed more moderate repellent activity against An. minimus. For Ae. aegypti, 2.5% leaf oil produced the greatest response for both contact (56.1% escape) and noncontact (63.3% escape) trials, while 2.5% produced the strongest response among all concentrations of peel oil, with 46.5% escape. However, after adjusting the contact trial escape (a measure of combined excitation and repellency), the estimated escape due to contact alone was a much weaker response than spatial repellency for both species. Knockdown responses above 50% were only observed in Ae. aegypti exposed to 5% leaf oil. Kaffir lime oils were more active against An. minimus than Ae. aegypti mosquitoes. There were statistically significant differences between leaf (more active) and peel oils at each concentration against An. minimus in contact and noncontact trials, except at the highest (5%) concentration.

The repellent and irritant effects of three essential oils-clove, hairy basil, and sweet basil-were compared using an excito-repellency test system against an insecticide-resistant strain of Aedes aegypti (L.) females from Pu Teuy, Kanchanaburi Province. DEET was used as the comparison standard compound. Tests were conducted under field and controlled laboratory conditions. The most marked repellent effect (spatial noncontact assay) among the three test essential oils was exhibited by sweet basil, Ocimum basilicum L. (53.8% escaped mosquitoes in 30-min exposure period) under laboratory conditions while hairy basil, Ocimum americanum L. and clove, Syzygium aromaticum (L.) Merill et. L.M. Perry from laboratory tests and sweet basil from field tests were the least effective as repellents (0-14%). In contrast, the contact assays measuring combined irritancy (excitation) and repellency effects found the best contact irritant response to hairy basil and DEET in field tests, whereas all others in laboratory and field were relatively ineffective in stimulating mosquitoes to move out the test chambers (0-5.5%). All three essential oils demonstrated significant differences in behavioral responses between field and laboratory conditions, whereas there was no significant difference in contact and noncontact assays for DEET between the two test conditions (P > 0.05).

Excito-repellency responses of Aedes aegypti (L.) exposed to deltamethrin and cypermethrin were assessed using an excito-repellency test system. Contact irritancy and non-contact repellency assays compared non-bloodfed (unfed) parous (post-gravid), nulliparous, early blood-fed, late blood-fed, sugar-fed, and unmated female mosquitoes for behavioral responses based on nutritional and physiological conditions at the time of testing. Rates of escape during contact exposure with either compound were most pronounced in parous mosquitoes, followed by unmated mosquitoes, when compared to other conditional states. Significantly higher numbers of parous females also escaped from control chambers compared to other cohorts (P

Knowledge on test conditions that may influence behavioral responses of mosquitoes is critical when excito-repellency tests are conducted. The objective of this study was to investigate the effect of test time differences on normal circadian activity and behavioral responses of field and colonized Aedes aegypti (L.) (=Stegomyia aegypti) and Culex quinquefasciatus Say to DEET, one of the most common synthetic repellent active ingredients available. Two field populations of Ae. aegypti and Cx. quinquefasciatus from Kanchanaburi and Nonthaburi provinces, respectively, and two long-standing laboratory populations, Ae. aegypti obtained from the U.S. Department of Agriculture, and Cx. quinquefasciatus from the Ministry of Public Health, Thailand, were used. Each population was exposed to DEET during two different periods of time (0900-1500 hours) and (2100-0300 hours). Both field and laboratory Cx. quinquefasciatus showed marked differences in spatial repellent escape responses between day and nighttime periods but none in direct contact tests. No significant differences between day and nighttime testing periods were observed with field or laboratory Ae. aegypti, except a higher daytime escape response from noncontact DEET treatment. This study indicates that test time may influence the behavioral avoidance responses and is a potential confounder of excito-repellency evaluations.

The objective of this study was to compare the behavioral responses (contact irritancy and noncontact spatial repellency) between susceptible and resistant populations of Aedes aegypti (L.) (=Stegomyia aegypti) to essential oils, citronella, and eucalyptus, Eucalyptus globulus, extracts, using an excito-repellency test system. N, N-diethyl-meta-toluamide (DEET) was used as the standard reference repellent. Mosquitoes included two long-standing insecticide susceptible colonies (U.S. Department of Agriculture and Bora Bora) and two pyrethroid-resistant populations recently obtained from Phetchabun and Kanchanaburi provinces in Thailand. Both DEET and citronella produced a much stronger excitation ("irritancy") and more rapid flight escape response in both pyrethroid-resistant populations compared with the laboratory populations. Noncontact repellency was also greater in the two resistant populations. Eucalyptus oil was found to be the least effective compound tested. Differences in responses between long-established pyrethroid-susceptible colonies and newly established and naturally resistant colonies were clearly demonstrated. These findings also demonstrate the need for further comparisons using natural pyrethroid-susceptible populations for elucidation of factors that might contribute to different patterns of escape behavior.