Repellent effect of local indigenous knowledge-based repellent in Nakhon Si Thammarat, Thailand, against Aedes aegypti mosquito

Dengue fever has been a significant disease in Thailand for a long time, ranking it as one of the major health problems in the country. Management of the adult stage of mosquito vectors is approached by applying various synthetic chemicals such as adulticides, attractants, deterrents, and repellents. In Thailand, mosquito control and personal protection from mosquito bites are currently the most important measures for preventing and controlling mosquito-borne diseases. Although there are various control strategies for dengue disease, participation from the local community plays a vital role in the success of disease control. At present, a lot of local people have seen the value of local indigenous knowledge and used this to improve their life. The local community in the southern part of Thailand has used mosquito repellent from local knowledge for a long time. The problem regarding mosquito repellent made from local indigenous knowledge is that it has not yet been tested to determine its effectiveness. Therefore, this research aims to assess the effectiveness of mosquito repellent from local learning from Nakhon Si Thammarat provinces in Thailand. From the survey, out of 23 districts, six mosquito repellents were found in 3 communities, including Nabon, Muang, and Thasala. The repellent efficacy against the laboratory strain of Aedes aegypti by using the human-bait technique of the WHO (1996) standard method, with slight modifications. Approximately 0.1 ml of each test sample was applied evenly onto a 30 cm2 test site on one forearm of each human volunteer. Exposure experiments continued at 30 min intervals until at least two bites occurred in a 3-min period, or when a first bite was followed by a confirming bite (second bite) in the subsequent observation period. Each test was duplicated on different days for the two human volunteers. The result shows that three mosquito repellents made from local indigenous knowledge that have protection that lasts for more than 2 h are Ban Ko Sa Child Development Center's citronella spray (Nabon district, Kaew Saen subdistrict), Khun Lang's citronella spray, and Khun Lang's citronella ointment (Muang district, Pak Phun subdistrict). The result of this research was reported back to the local community to re-evaluate their self-reliance on their protection against mosquito biting.

Synergistic Repellent and Irritant Effects of a Mixture of β-Caryophyllene Oxide and Vetiver Oil against Mosquito Vectors

Repellents play a major role in reducing the risk of mosquito-borne diseases by preventing mosquito bites. The present study evaluated the mosquito-repellent activity of β-caryophyllene oxide 1% (BCO), vetiver oil 2.5% (VO), and their binary mixtures (BCO + VO (1:1), BCO + VO (2:1), BCO + VO (1:2)) against four laboratory-colonized mosquito species, Aedes aegypti (L.), Aedes albopictus (Skuse), Anopheles minimus Theobald, and Culex quinquefasciatus Say, using an excito-repellency assay system. In general, the compound mixtures produced a much stronger response in the mosquitoes than single compounds, regardless of the test conditions or species. The greatest synergetic effect was achieved with the combination of BCO + VO (1:2) in both contact and noncontact trials with An. minimus (74.07-78.18%) and Cx. quinquefasciatus (55.36-83.64%). Knockdown responses to the binary mixture of BCO + VO were observed for Ae. albopictus, An. minimus, and Cx. quinquefasciatus, in the range of 18.18-33.33%. The synergistic repellent activity of BCO and VO used in this study may support increased opportunities to develop safer alternatives to synthetic repellents for personal protection against mosquitoes.

Distribution of mosquitoes (Diptera: Culicidae) in Thailand: a dataset

Mosquitoes play a crucial role as primary vectors for various infectious diseases in Thailand. Therefore, accurate distribution information is vital for effectively combating and better controlling mosquito-borne diseases. Here, we present a curated dataset of the mosquito distribution in Thailand comprising 12,278 records of at least 117 mosquito species (Diptera: Culicidae). The main genera included in the dataset are Aedes, Anopheles, Armigeres, Culex, and Mansonia. From 2007 to 2023, data were collected through routine mosquito surveillance and research projects from 1,725 locations across 66 (out of 77) Thai provinces. The majority of the data were extracted from a Thai database of the Thailand Malaria Elimination Program. To facilitate broader access to mosquito-related data and support further exploration of the Thai mosquito fauna, the data were translated into English. Our dataset has been published in the Global Biodiversity Information Facility, making it available for researchers worldwide.

Synergistic Behavioral Response Effect of Mixtures of Andrographis paniculata, Cananga odorata, and Vetiveria zizanioides against Aedes aegypti (Diptera: Culicidae)

Each binary mixture formulation of Vetiveria zizanioides (L.) Nash (VZ) with Andrographis paniculata (Burm.f.) Wall. ex Nees (AP) or Cananga odorata (Lam.) Hook.f. & Thomson (CO) and AP with CO at 1:1, 1:2, 1:3, and 1:4 ratios (v:v) was investigated for behavioral responses on laboratory and field strains of Aedes aegypti. Irritant and repellent activities of each formulation were compared with N,N-diethyl-3-methylbenzamide (DEET) using an excito-repellency test system. The result demonstrated that the mixture of VZ:AP in all combination ratios was the most effective in inducing an irritancy response against the laboratory strain (56.57-73.33%). The highest percentage of escaped mosquitoes exposed to the mixture at a 1:4 ratio (73.33%) was significantly different from DEET (26.67%) (p < 0.05). Against the field strain, the strongest escape response of AP:CO at a 1:1 ratio in the contact trial (70.18%) was significantly different compared with DEET (38.33%) (p < 0.05). There was a weak non-contact escape pattern in all combinations of VZ:CO against the laboratory strains (6.67-31.67%). These findings could lead to the further development of VZ and AP as active ingredients in a repellent that could advance to human use trials.

Prevalence of Plasmodium spp. in Anopheles mosquitoes in Thailand: a systematic review and meta-analysis

Background

The entomological inoculation rate (EIR) is one of the key indices used to evaluate malaria transmission and vector control interventions. One of the components of the EIR is the sporozoite rate in Anopheles vectors. A systematic review and meta-analysis was performed to identify the prevalence of Plasmodium spp. in field-collected Anopheles species across Thailand.

Methods

This systematic review was registered under the PROSPERO number CRD42021297255. Studies that focused on the identification of Plasmodium spp. in Anopheles mosquitoes were identified from the electronic databases PubMed, Web of Science, and Scopus. The quality of the identified studies was determined using the Strengthening the Reporting of Observational Studies in Epidemiology approach. The proportion of Anopheles mosquitoes collected, Anopheles vectors for Plasmodium species, and specificity of Anopheles vectors for Plasmodium species were analyzed. The pooled prevalence of Plasmodium species among the primary vectors (Anopheles dirus, Anopheles minimus, and Anopheles maculatus) was estimated using the random-effects model.

Results

Of the 1113 studies identified, 31 were included in the syntheses. Of the 100,910 Anopheles mosquitoes identified for species and sibling species, An. minimus (40.16%), An. maculatus (16.59%), and Anopheles epiroticus (9.18%) were the most prevalent Anopheles species. Of the 123,286 Anopheles mosquitoes identified, 566 (0.46%) were positive for Plasmodium species. The highest proportions of Plasmodium species were identified in Anopheles hodgkini (2/6, 33.3%), Anopheles nigerrimus (2/24, 8.33%), Anopheles balabacensis (4/84, 4.76%), An. dirus (114/4956, 2.3%), Anopheles annularis (16/852, 1.88%), Anopheles kochi (8/519, 1.54%), Anopheles vagus (3/215, 1.4%), and Anopheles baimaii (1/86, 1.16%). The pooled prevalence of Plasmodium species identified in the main Anopheles vectors was 0.4% of that of Plasmodium species identified in An. dirus was 2.1%, that of Plasmodium species identified in An. minimus was 0.4%, and that of Plasmodium species identified in An. maculatus was 0.4%.

Conclusions

We found a low prevalence of Plasmodium infection in Anopheles mosquitoes across Thailand. Therefore, the use of EIR to determine the impact of vector control intervention on malaria parasite transmission and elimination in Thailand must be undertaken with caution, as a large number of Anopheles specimens may be required.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05397-2.

Behavioral avoidance and biological safety of vetiver oil and its constituents against Aedes aegypti (L.), Aedes albopictus (Skuse) and Culex quinquefasciatus Say

Numerous plant-based repellents are widely used for personal protection against host-seeking mosquitoes. Vitiveria zizanioides (L.) Nash essential oil and its constituents have demonstrated various mosquito repellent activities. In this study, three chemical actions of vetiver oil and five constituents (terpinen-4-ol, α-terpineol, valencene, vetiverol and vetivone) were characterized against Aedes aegypti, Aedes albopictus and Culex quinquefasciatus by using the high-throughput screening assay system (HITSS). Significant contact escape responses in Ae. aegypti and Ae. albopictus to all test compounds at concentrations between 2.5 and 5% were observed. Spatial repellency responses were also observed in some tested mosquito populations depending upon concentrations. The most significant toxic response on mosquitoes was found at the highest concentration, except for vetivone which had no toxic effect on Ae. aegypti and Ae. albopictus. Results on phototoxic and genotoxic hazard revealed that vetiver oil and their constituents showed no phototoxic potential or any significant genotoxic response. In conclusion, vetiver oil and two constituents, valencene and vetiverol, are potentials as active ingredients for mosquito repellency and present no toxicity.

Excito-repellency of Myristica fragrans Houtt. and Curcuma longa L. extracts from Southern Thailand against Aedes aegypti (L.)

The development of local plant extracts as a mosquito repellent is environmentally safe, inexpensive, and easily accessible for residents. In this study, three extracts from two local plants, Myristica fragrans Houtt. (flesh and mace) and Curcuma longa L. (rhizome) from southern Thailand, were investigated for their inherent repellent activity using the excito-repellency (ER) assay system against insectary-colonized Aedes aegypti (L.) (Diptera: Culicidae). The escape responses of mosquitoes exposed to concentrations of 0.5% to 5.0% (w/v) were measured to determine the contact irritant and non-contact repellent properties of each extract. Both the flesh and mace extracts of M. fragrans had relatively limited contact irritants (28.1% and 34.6% escape) and non-contact repellent (16.7% and 18.3% escape) activities against Ae. aegypti, respectively. The C. longa rhizome extract produced higher escape responses in the non-contact (42.6% escape) and contact (41.4% escape) trials at concentrations of 5.0% and 1.0%, respectively. GC-MS analysis found diethyl malate (56.5%) and elemicin (11.7%) to be the main components of the flesh and mace extracts, respectively, while ar-turmerone (24.6%), β-turmerone (15.2%), α-turmerone (10.5%) were the primary constituents of the rhizome extract. Overall, our results indicate that both M. fragrans extracts primarily caused Ae. aegypti escape through contact irritation. For C. longa, lower concentrations (0.5% and 1.0%) exhibited contact irritancy, but higher concentrations (2.5% and 5.0%) exhibited non-contact repellency against Ae. aegypti. Although they had limited efficacy, further experiments (e.g., mixing with other plant-based compounds) could enhance the ER of both local plant extracts. Additional evaluation of these extracts against other mosquito species and the ER of their chemical components, either alone or in combination, would also be beneficial for the development of green repellents. Our findings emphasize the possibility of utilizing plant-based mosquito repellent as an alternative personal protection method for future mosquito control programs.

Scientific achievements and reflections after 20 years of vector biology and control research at the Pu Teuy mosquito field research station, Thailand

Additional vector control tools are needed to supplement current strategies to achieve malaria elimination and control of Aedes-borne diseases in many settings in Thailand and the Greater Mekong Sub-region. Within the next decade, the vector control community, Kasetsart University (KU), and the Ministry of Higher Education, Science, Research and Innovation must take full advantage of these tools that combine different active ingredients with different modes of action. Pu Teuy Mosquito Field Research Station (MFRS), Department of Entomology, Faculty of Agriculture, Kasetsart University (KU), Thailand was established in 2001 and has grown into a leading facility for performing high-quality vector biology and control studies and evaluation of public health insecticides that are operationally relevant. Several onsite mosquito research platforms have been established including experimental huts, a 40-m long semi-field screening enclosure, mosquito insectary, field-laboratory, and living quarters for students and researchers. Field research and assessments ranged from 'basic' investigations on mosquito biology, taxonomy and genetics to more 'applied' studies on responses of mosquitoes to insecticides including repellency, behavioural avoidance and toxicity. In the course of two decades, 51 peer-reviewed articles have been published, and 7 masters and 16 doctoral degrees in Entomology have been awarded to national and international students. Continued support of key national stakeholders will sustain MFRS as a Greater Mekong Subregion centre of excellence and a resource for both insecticide trials and entomological research.

Cananga odorata (Magnoliales: Annonaceae) Essential Oil Produces Significant Avoidance Behavior in Mosquitoes

Essential oil of Cananga odorata Hook. F. & Tomson is a source of insect repellent, but contact irritancy and noncontact repellency actions that stimulate insect's avoidance behavior (escape away from chemical source after direct physical contact or without making physical contact, respectively) have not been investigated. Therefore, an excito-repellency test chamber was used for measuring avoidance behavior of four insectary-reared mosquito species (Diptera: Culicidae) that escape from esposure to four concentrations (0.5, 1.0, 2.5, and 5.0% v/v) of C. odorata oil. The oil strongly repelled both Culex quinquefasciatus Say (85-97% escape) and Anopheles minimus Theobald (97-99%) at high concentrations (2.5-5.0%). For Anopheles dirus Peyton & Harrison and Aedes aegypti (L.), highest repellency (64 and 39% escape, respectively) was demonstrated at 2.5% concentration. For contact irritancy, the oil produced relatively high percent escape found in Cx. quinquefasciatus (90-100% escape) and An. minimus (83-100%). Whereas moderate contact irritancy was observed against An. dirus (40-50% escape) and Ae. aegypti (51-59%). The percent escape was then adjusted with repellency to estimate the effect of contact irritancy alone. We found that highest contact irritancy was presented at 0.5% concentration against An. minimus (67% escape). Knockdown and toxic actions were only found in Anopheles mosquitoes at 5.0% concentration. The results revealed that An. minimus and Cx. quinquefasciatus were more prone to be repelled by C. odorata oil. Detailed analysis of oil identified primary compounds as methyl benzoate (14.6%), α-gurjunene (12.8%), p-methyl-anisole (11.3%), and benzyl acetate (9.9%). Further investigations are needed to assess excito-repellency actions of these compounds alone or in combination.

Avoidance Behavior to Guava Leaf Volatile Oil by Three Medically Important Mosquito Vectors

Volatile organic compounds from various plants have received popular interest as one of the vector control tools due to their eco-friendliness and insect-repellent activities. In this study, an excito-repellency assay system was used to examine the noncontact repellency, contact excitation, and knockdown (KD) effects of guava leaf (Psidium guajava L.) oil against Anopheles minimus (Theobald), Anopheles epiroticus (Linton & Harbach), and Culex quinquefasciatus (Say). The organic components of guava oil were identified by gas chromatography-mass spectrometry analysis with dl-limonene (17.4%), cymene (5.49%), and α-terpinene (5.20%) as the major constituents. At concentrations of 2.5 and 5.0%, 100% escape of An. minimus was recorded in the contact assay and 96-98% escape in the noncontact assay. Guava oil stimulated potent irritant (92% escape) and repellent (61-86% escape) effects against Cx. quinquefasciatus. A lower repellency action was observed against An. epiroticus (17-20% escape). No KD effect was observed for guava oil against An. minimus and Cx. quinquefasciatus at any concentration. However, An. epiroticus was more prone to KD effects, with the highest percentage KD (44% in nonescape group) observed with 5.0% guava oil in the noncontact assay. Mortalities of 35% and 11% were observed for An. epiroticus in the nonescape groups in the contact and noncontact assays, respectively. Concentrations of 1.0% and 2.5% guava oil led to <2% mortality in An. minimus. Our findings highlight guava oil as a promising plant-based mosquito repellent that can be included in insecticide formulations for future mosquito control programs.

Forced Egg Laying Method to Establish F1 Progeny from Field Populations and Laboratory Strains of Anopheles Mosquitoes (Diptera: Culicidae) in Thailand

Successful monitoring of physiological resistance of malaria vectors requires about 150 female mosquitoes for a single set of tests. In some situations, the sampling effort is insufficient due to the low number of field-caught mosquitoes. To address this challenge, we demonstrate the feasibility of using the forced oviposition method for producing F1 from field-caught Anopheles mosquitoes. A total of 430 and 598 gravid Anopheles females from four laboratory strains and five field populations, respectively, were tested. After blood feeding, gravid mosquitoes were individually introduced into transparent plastic vials, containing moistened cotton balls topped with a 4 cm2 piece of filter paper. The number of eggs, hatching larvae, pupation, and adult emergence were recorded daily. The mean number of eggs per female mosquito ranged from 39.3 for Anopheles cracens to 93.6 for Anopheles dirus in the laboratory strains, and from 36.3 for Anopheles harrisoni to 147.6 for Anopheles barbirostris s.l. in the field populations. A relatively high egg hatching rate was found in An. dirus (95.85%), Anopheles minimus (78.22%), and An. cracens (75.59%). Similarly, a relatively high pupation rate was found for almost all test species ranging from 66% for An. minimus to 98.7% for Anopheles maculatus, and lowest for An. harrisoni (43.9%). Highly successful adult emergence rate was observed among 85-100% of pupae that emerged in all tested mosquito populations. The in-tube forced oviposition method is a promising method for the production of sufficient F1 progeny for molecular identification, vector competence, insecticide resistance, and bioassay studies.

Time of Test Periods Influence the Behavioral Responses of Anopheles minimus and Anopheles dirus (Diptera: Culicidae) to DEET

Information on factors influencing the behavioral responses of mosquitoes to repellents is lacking and poorly understood, especially in the Anopheles species, night-biting mosquitoes. Our goal was to investigate the impact of different time periods on circadian activity and behavioral responses of two malaria vectors, Anopheles minimus and An. dirus, to 5% DEET using an excito-repellency test system. Each mosquito species was exposed to the repellent during the daytime (06.00-18.00) and nighttime (18.00-06.00), and time of observation was further divided into four 3-h intervals. Significant escape responses were observed between daytime and nighttime for An. minimus in both noncontact and contact tests. An. dirus showed statistical differences in contact irritancy escape response, whereas no significant difference was found in noncontact repellency tests. Both mosquito species showed more significantly higher escape responses when exposed to DEET during the afternoon and late in the night. This finding indicates that the time of testing may affect the behavioral responses of mosquitoes to repellents, especially in An. minimus and An. dirus. A better understanding of nocturnally active mosquito behavioral responses spanning from dusk to dawn would assist in optimizing product development, screening, and effective evaluation.

Field Evaluation of a Spatial Repellent Emanation Vest for Personal Protection Against Outdoor Biting Mosquitoes

Exophilic vectors are an important contributor to residual malaria transmission. Wearable spatial repellents (SR) can potentially provide personal protection in early evening hours before people retire indoors. An SR prototype for passive delivery of transfluthrin (TFT) for protecting humans against nocturnal mosquitoes in Kanchanaburi, western Thailand, is evaluated. A plastic polyethylene terephthalate (PET) sheet (676 cm2) treated with 55-mg TFT (TFT-PET), attached to the back of short-sleeve vest worn by human collector, was evaluated under semifield and outdoor conditions. Field-caught, nonblood-fed female Anopheles minimus s.l. were released in a 40 m length, semifield screened enclosure. Two collectors positioned at opposite ends conducted 12-h human-landing collections (HLC). The outdoor experiment was conducted between treatments among four collectors at four equidistant positions who performed HLC. Both trials were conducted for 30 consecutive nights. TFT-PET provided 67% greater protection (P < 0.001) for 12 h compared with unprotected control, a threefold reduction in the attack. In outdoor trials, TFT-PET provided only 16% protection against An. harrisoni Harbach & Manguin (Diptera: Culicidae) compared with unprotected collector (P = 0.0213). The TFT-PET vest reduced nonanophelines landing by 1.4-fold compared with the PET control with a 29% protective efficacy. These findings suggest that TFT-PET had diminished protective efficacy in an open field environment. Nonetheless, the concept of a wearable TFT emanatory device has the potential for protecting against outdoor biting mosquitoes. Further development of portable SR tools is required, active ingredient selection and dose optimization, and more suitable device design and materials for advancing product feasibility.

Excito-repellency and biological safety of β-caryophyllene oxide against Aedes albopictus and Anopheles dirus (Diptera: Culicidae)

The activity of β-caryophyllene oxide as either a contact or noncontact repellent was evaluated against two laboratory strains (Aedes albopictus and Anopheles dirus) using an excito-repellency test system. N, N-Diethyl-3-methylbenzamide (DEET) was used as a standard reference baseline for comparative purposes. β-Caryophyllene oxide and DEET were tested at concentrations of 0.1, 0.25, 0.5 and 1.0% (v/v). In addition, the phototoxic and genotoxic effects of β-caryophyllene oxide were investigated on Balb/c 3T3 mouse fibroblasts (3T3-L1) and Chinese hamster ovary cell line (CHO-K1). The results demonstrated that the higher concentrations of test compounds (0.5 and 1.0%) produced greater behavioral responses. Aedes albopictus was more sensitive to β-caryophyllene oxide than An. dirus. Moderate avoidance response rates (25-56% escape) of Ae. albopictus at 0.5% and 1.0% β-caryophyllene oxide were observed in contact and noncontact trials compared with low response rates from An. dirus (26-31% escape). DEET at ≤1% displayed lower irritancy and repellency (1-38%) than β-caryophyllene oxide when tested against the two mosquito species. Knockdown responses (37%) were only observed in An. dirus exposed to 1% β-caryophyllene oxide in the contact trial. β-Caryophyllene oxide did not show any phototoxic potential (PIF= 0.38) nor was there any significant genotoxic response as indicated by no increase in micro-nucleated cells with or without metabolic activation. β-Caryophyllene oxide could be considered as a safe repellent, effective against mosquitoes.

Excito-repellency Activity of Andrographis paniculata (Lamiales: Acanthaceae) Against Colonized Mosquitoes

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.

Discriminating Lethal Concentrations for Transfluthrin, a Volatile Pyrethroid Compound for Mosquito Control in Thailand

Pyrethroids are commonly used to control malaria and dengue vectors in Thailand. The lack of specific lethal discriminating concentrations (DCs) for specific mosquito species has possibly compromised more accurate assessments of physiological susceptibility to various chemicals over time. Previous studies have established DCs of various residual pyrethroids against specific mosquitoes in Thailand. However, DCs for transfluthrin (TFT), a highly volatile pyrethroid compound, against mosquito vectors in Thailand has been lacking. The aim of this study was to determine the DCs and susceptibility baselines of TFT against pyrethroid-susceptible laboratory strains of Aedes aegypti, Anopheles minimus, and An. dirus using the World Health Organization adult susceptibility tube method. Final DCs of TFT of each species were determined based on doubling the 99% lethal concentration at the following percentages: Ae. aegypti (0.06824%), An. minimus (0.06382%), and An. dirus (0.01508%). Subsequently, the respective TFT DCs were used to test field-collected populations of Ae. aegypti, An. harrisoni (Minimus Complex species), and An. dirus. Anopheles harrisoni and An. dirus were found completely susceptible (100% mortality), whereas Ae. aegypti from Nonthaburi Province was resistant to TFT. The suitability of the testing system and procedures is discussed. Routine assessment of insecticide susceptibility should include pyrethroids with high-vapor-pressure characteristics for informing control programs and consumers of product and chemical effectiveness.

Optimal Discriminating Concentrations of Six Synthetic Pyrethroids for Monitoring Insecticide Susceptibility in Anopheles minimus (Diptera: Culicidae), a Primary Malaria Vector in Thailand

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.

Evaluation of the Constituents of Vetiver Oil Against Anopheles minimus (Diptera: Culicidae), a Malaria Vector in Thailand

The development of resistance by mosquitoes to current synthetic compounds has resulted in reduced effectiveness of prevention and control methods worldwide. An alternative nonchemical based control tools are needed to be evaluated particularly plant-derived essential oils. Several components of vetiver oil have been documented as insect repellents. However, detailed knowledge of those components action against insect remains unknown. In this study, behavioral response of Anopheles minimus to four constituents of vetiver oil (valencene, terpinen-4-ol, isolongifolene, vetiverol) was evaluated by using the high-throughput screening assay system. Vetiverol and isolongifolene exhibited strong contact irritancy action at 1.0% (80.2% escaping) and 5.0% (81.7% escaping) concentration, respectively, while moderate action was found in both valencene and terpinen-4-ol at 5.0% (57.6% escaping). Only at 1.0% (0.7 spatial activity index [SAI]) and 5.0% (1.0 SAI) of valencene and 0.5% (0.7 SAI) of isolongifolene showed spatial repellency activity. High mortality (58.9-98.2%) was recorded in all concentration of vetiverol and isolongifolene. Meanwhile, valencene exhibited high mortality only at 5.0%, terpinen-4-ol showed very low toxic action (0-4.3%) in all concentration. These proved that valencene in vetiver oil is the promising constituent that can be developed as an alternative mosquito control mean in efforts to prevent disease transmission.

Physical influence on larvicidal and pupicidal activity of the silicone-based monomolecular film

Although silicone-based monomolecular film (MMF) has been accepted as larvicide in several countries, its mosquito control potential has never been investigated in Thailand. Laboratory assessment in this study was conducted to determine the MMF efficacy against Aedes aegypti. At the recommended dosage (1mL/m(2) of water surface), mortality of pupae (99.17±0.83%) was significantly greater than mortality of old and young larvae (73.33±9.13, 11.67±3.47%; respectively). Pupicidal activity was rapidly exhibited within hours while larvicidal activity took at least one day. Interestingly, among the survived mosquitoes after MMF exposure, larval length (3.6±0.18mm), pupation (0%) and adult emergence (0%) were significantly less than the control group. Gravid females also avoided laying eggs in MMF-treated oviposition cups. There was no influence of physical factors on MMF efficacy and no toxic effects on fish and plants. These results indicated the MMF is promising to provide not only larvicidal and pupicidal activity but also inhibition of larval development as indicated by both larval length and stage transformation.