Several types of olfactometers have been used to evaluate mosquito responses to agents that mimic natural volatiles that repel or attract. The Y-tube olfactometer has been widely used to study repellents and attractants, while the high-throughput screening system assay has only been used to study repellents. Whether the high-throughput screening system assay is suitable for evaluating attractants is unknown. We evaluated the responses to four lactic-acid-based mixtures and two non-lactic-acid-based chemical lure candidates using the high-throughput screening system (HITSS) for three mosquito species (laboratory strains and field populations of both Aedes aegypti (L.) and Culex quinquefasciatus Say.; laboratory strain of Anopheles minimus Theobald) under laboratory-controlled conditions. HITSS assay results showed that KU-lure #1 elicited the greatest percent attraction for pyrethroid-resistant and -susceptible Ae. aegypti. KU-lure #6 elicited the strongest attractive response for pyrethroid-susceptible and -resistant Cx. quinquefasciatus and pyrethroid-susceptible An. minimus. The response to the lures from each species was independent of the pyrethroid susceptibility status (Ae. aegypti, p = 0.825; Cx. quinquefasciatus, p = 0.056). However, a significant difference in attraction to KU-lure #6 was observed between diurnal and nocturnal mosquitoes (Cx. quinquefasciatus vs. Ae. aegypti, p = 0.014; An. minimus vs. Ae. aegypti, p = 0.001). The laboratory-level HITSS assay effectively selects potential lure candidates. Because the host-seeking behavior differs between mosquito species, further studies are needed to develop species-specific attractants. Additional studies in semi-field screen houses using commercial traps are necessary to evaluate the accuracy of these laboratory assay results.

Excito-repellency activity of plant extracts have been increasingly studied as mosquito repellents. In this study, the crude extract of Andrographis paniculata was evaluated for its noncontact repellency, contact excitation (irritancy + repellency), and knockdown/toxicity response against five colonized mosquitoes; Aedes aegypti (L.), Aedes albopictus (Skuse), Anopheles dirus Peyton & Harrison, Anopheles epiroticus Linton & Harbach, and Culex quinquefasciatus Say (Diptera: Culicidae) using an excito-repellency assay system under laboratory-controlled conditions. The escape responses were observed at four different concentrations (0.5–5.0% w/v) with A. paniculata showing strong spatial repellency against Ae. albopictus (96.7% escape) and Ae. aegypti (71.7% escape) at the 2.5% and 0.5% concentrations, respectively. At 0.5% and 5.0% concentrations, the greatest repellency was seen for An. dirus (48.2% escape) and Cx. quinquefasciatus (59.7% escape), respectively. Comparatively, low repellency action was observed against An. epiroticus (1.6–15.0% escape). Escape in contact assays (before adjustment) was generally less pronounced compared to noncontact spatial repellency, with Ae. albopictus showing highest percent escape (71.4% escape) in the contact assay at 1.0% concentration. After adjusting for spatial repellency, escape due to contact irritancy alone was either not present or an insignificant contribution to the overall avoidance response for all species. No knockdown or mortality at 24-h postexposure was observed in any trials. These findings indicate that the A. paniculata crude extract is more active against day-biting mosquitoes; however, this may be a reflection of the time of testing. This study demonstrates compelling evidence that A. paniculata extract performs primarily as a spatial repellent. Further investigations exploring the use A. paniculata as a potential active ingredient in repellent products are needed.

Natural volatile host cues play a critical role for mosquito orientation and locating a blood source for egg production. Similar olfactory activation responses have allowed the use and development of artificial chemical attractants to lure mosquitoes to trapping devices. Using a pre-formulated commercial product mixture of different attractant chemicals, a high-throughput screening system (HITSS) is used to screen varying doses of chemical required to activate behavioral responses. Two strains of Aedes aegypti (L.): permethrin-susceptible (USDA) and -resistant (Pu Teuy) phenotypes and one Culex quinquefasciatus Say. (NIH) laboratory strain were tested. Overall, mosquitoes showed repellency between 1.0 g and to 10.0 g dose of each compound. However, by progressively reducing the dose, Cx. quinquefasciatus showed a greater positive percent attraction (88.9%) at 0.025 g, whereas the USDA and Pu Teuy Ae. aegypti produced optimum attractant activation at 0.005 g (72.6% and 58.9%, respectively) without significant difference within species (p > 0.05). In parallel control assays, Cx. quinquefasciatus was significantly attracted to 1 g of dry ice (carbon dioxide) (76%) more than Ae. aegypti (USDA) (12.2%). The HITSS was originally designed to measure three chemical actions to sublethal concentrations of chemicals by mosquitoes: toxicity and the two primary behavior avoidance responses (contact excitation and spatial repellency). These findings demonstrate that the HITSS assay, with only minor modifications, allows comparison screening of candidate compounds as potential attractants for anemotactic responses under laboratory-controlled conditions. Further investigations will be required to equate measurements obtained from controlled laboratory assays to more varied field conditions for attracting natural mosquito populations.

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Background Entomological surveillance is an important means of assessing the efficacy of insect vector management programs and estimating disease transmission thresholds. Among baited traps, Biogents’ BG-Sentinel (BGS) trap baited with BG-Lure is considered to have the most similar outcome to, and be a possible replacement for, human-landing catches for the epidemiologically relevant monitoring of adult Aedes aegypti and Culex quinquefasciatus. In contrast to the BGS trap, the Black Hole ultraviolet (UV) light trap, which is widely used to catch nocturnal flying insects, is not baited with synthetic human odor-mimicking lures. Methods We evaluated the l-lactic acid-based Kasetsart University (KU)-lures nos. 1–6 as novel candidate chemical lures for the diurnal species Ae. aegypti and the nocturnal species Cx. quinquefasciatus using two commercial traps (the BGS trap and the Black Hole UV light trap) in a semi-field screen (SFS) house. Firstly, we optimized the dose of each KU-lure in an SFS house (140 m3). Secondly, six different candidate KU-lures were screened by comparing their percent attraction using a single discriminating dose (0.5 g). Finally, we evaluated the synergism of the KU-lures selected in this way with commercially available traps. Results BGS traps baited with KU-lure no. 1 exhibited the greatest percent attraction for Ae. aegypti (29.5% ± 14.3%), whereas those baited with KU-lure no. 6 most strongly attracted Cx. quinquefasciatus (33.3% ± 10.7%). Interestingly, BGS traps treated with 10 g BG-Lure did not significantly attract more Ae. aegypti or Cx. quinquefasciatus than the untreated BGS traps. CO2 at a flow rate of 250 ml/min most strongly attracted both Ae. aegypti and Cx. quinquefasciatus (42.2% ± 14.2% and 75.1% ± 16.9%, respectively). BGS and Black Hole UV light traps with KU-lure no. 6 exhibited a stronger attraction for Cx. quinquefasciatus than untreated traps, and the percent attraction did not differ between the treated traps. Conclusions Synergistic effects of KU-lures nos. 1 and 6 with the mosquito traps were demonstrated for both the diurnal and nocturnal species in the SFS house assays. However, further studies are urgently needed for the development of species-specific lures to increase trap efficacy in the field for local vector mosquitoes in Thailand. Graphical Abstract