Vector Biology and Integrated Management of Malaria Vectors in China.

Malaria is an infectious disease caused by Plasmodium parasites, transmitted by Anopheles sinensis, Anopheles lesteri, Anopheles minimus, and Anopheles dirus in China. In 2021, the disease was eliminated in China after more than 70 years of efforts implementing an integrated mosquito management strategy. This strategy comprised indoor residual spray, insecticide-treated bed nets, irrigation management, and rice-fish coculture based on an understanding of taxonomic status and ecological behaviors of vector species, in conjunction with mass drug administration and promotion of public education. However, China still faces postelimination challenges, including the importation of approximately 2,000-4,000 cases of malaria into the country each year, as well as widespread resistance to pyrethroid insecticides in An. sinensis; these challenges require long-term vector surveillance to understand the distribution, population density, and development of resistance in vector mosquitoes to prevent local epidemics caused by imported malaria cases.

A semi-field evaluation in Thailand of the use of human landing catches (HLC) versus human-baited double net trap (HDN) for assessing the impact of a volatile pyrethroid spatial repellent and pyrethroid-treated clothing on Anopheles minimus landing.

BACKGROUND: The mosquito landing rate measured by human landing catches (HLC) is the conventional endpoint used to evaluate the impact of vector control interventions on human-vector exposure. Non-exposure based alternatives to the HLC are desirable to minimize the risk of accidental mosquito bites. One such alternative is the human-baited double net trap (HDN), but the estimated personal protection of interventions using the HDN has not been compared to the efficacy estimated using HLC. This semi-field study in Sai Yok District, Kanchanaburi Province, Thailand, evaluates the performance of the HLC and the HDN for estimating the effect on Anopheles minimus landing rates of two intervention types characterized by contrasting modes of action, a volatile pyrethroid spatial repellent (VSPR) and insecticide-treated clothing (ITC). METHODS: Two experiments to evaluate the protective efficacy of (1) a VPSR and (2) ITC, were performed. A block randomized cross-over design over 32 nights was carried out with both the HLC or HDN. Eight replicates per combination of collection method and intervention or control arm were conducted. For each replicate, 100 An. minimus were released and were collected for 6 h. The odds ratio (OR) of the released An. minimus mosquitoes landing in the intervention compared to the control arm was estimated using logistic regression, including collection method, treatment, and experimental day as fixed effects. RESULTS: For the VPSR, the protective efficacy was similar for the two methods: 99.3%, 95% CI (99.5-99.0) when measured by HLC, and 100% (100, Inf) when measured by HDN where no mosquitoes were caught (interaction test p = 0.99). For the ITC, the protective efficacy was 70% (60-77%) measured by HLC but there was no evidence of protection when measured by HDN [4% increase (15-27%)] (interaction test p < 0.001). CONCLUSIONS: Interactions between mosquitoes, bite prevention tools and the sampling method may impact the estimated intervention protective efficacy. Consequently, the sampling method must be considered when evaluating these interventions. The HDN is a valid alternative trapping method (relative to the HLC) for evaluating the impact of bite prevention methods that affect mosquito behaviour at a distance (e.g. VPSR), but not for interventions that operate through tarsal contact (e.g., ITC).

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.

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.

Population genetic structure of the malaria vector Anopheles minimus in Thailand based on mitochondrial DNA markers.

BACKGROUND: The malaria vector Anopheles minimus has been influenced by external stresses affecting the survival rate and vectorial capacity of the population. Since An. minimus habitats have continuously undergone ecological changes, this study aimed to determine the population genetic structure and the potential gene flow among the An. minimus populations in Thailand. METHODS: Anopheles minimus was collected from five malaria transmission areas in Thailand using Centers for Disease Control and Prevention (CDC) light traps. Seventy-nine females from those populations were used as representative samples. The partial mitochondrial cytochrome c oxidase subunit I (COI), cytochrome c oxidase subunit II (COII) and cytochrome b (Cytb) gene sequences were amplified and analyzed to identify species and determine the current population genetic structure. For the past population, we determined the population genetic structure from the 60 deposited COII sequences in GenBank of An. minimus collected from Thailand 20 years ago. RESULTS: The current populations of An. minimus were genetically divided into two lineages, A and B. Lineage A has high haplotype diversity under gene flow similar to the population in the past. Neutrality tests suggested population expansion of An. minimus, with the detection of abundant rare mutations in all populations, which tend to arise from negative selection. CONCLUSIONS: This study revealed that the population genetic structure of An. minimus lineage A was similar between the past and present populations, indicating high adaptability of the species. There was substantial gene flow between the eastern and western An. minimus populations without detection of significant gene flow barriers.

Evaluation of Mosquito Attractant Candidates Using a High-Throughput Screening System for Aedes aegypti (L.), Culex quinquefasciatus Say. and Anopheles minimus Theobald (Diptera: Culicidae).

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.

Effect of Different Wall Surface Coverage With Deltamethrin-Treated Netting on the Reduction of Indoor-Biting Anopheles Mosquitoes (Diptera: Culicidae).

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.

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.

New insights into malaria vector bionomics in Lao PDR: a nationwide entomology survey.

BACKGROUND: In Laos, the malaria burden remains high despite a significant reduction of cases during the last decade. In the context of the disease elimination by 2030, a nationwide entomological survey was conducted to better understand the distribution, abundance and behaviour of major malaria vectors (Anopheles spp.) in the country. METHODS: Mosquito collections were implemented in ten villages from ten provinces during the rainy and dry seasons of 2014 and 2015 by using human landing catch (HLC) and cow bait collection (CBC) methods. After morphological identification in the field, molecular identification of the sibling species of Anopheles mosquitoes from the Funestus, Leucosphyrus, and Maculatus groups were determined using PCR specific alleles. A screening of Plasmodium falciparum and Plasmodium vivax infections in the vectors was carried out by quantitative PCR assays. RESULTS: A total of 14,146 adult mosquitoes representing 25 different Anopheles species were collected and morphologically identified. Molecular identification revealed the presence of 12 sibling species within the main primary vector groups, including Anopheles maculatus, Anopheles rampae, Anopheles sawadwongporni, Anopheles pseudowillmori, Anopheles dravidicus, Anopheles minimus, Anopheles aconitus, Anopheles pampanai, Anopheles harrisoni, Anopheles dirus, Anopheles baimaii, Anopheles nemophilous. Anopheles maculatus and An. minimus were predominant during both the dry and rainy seasons, but showed highly zoophilic preferences (Zoophilic index of 98% and 95%, respectively). Overall, 22% of the total malaria vectors were collected between 10:00 PM and 5:00 AM indoors when people are sleeping. Twenty-seven percent of primary and secondary vectors were collected outdoors before 10:00 PM or after 5:00 AM, times when people are usually awake and outdoors. Only two specimens were positive for P. falciparum, one An. aconitus from Phongsaly and one An. minimus from Vientiane Province CONCLUSIONS: The results indicate that people living in rural areas in Laos are constantly exposed to malaria vectors throughout the year and specifically outdoors. The use of LLINs/IRS remains important but innovative tools and new strategies are needed to address locally, the early and outdoor malaria transmission. Lack of expertise in general entomological methods may further exacerbate the situation.

The impact of transfluthrin on the spatial repellency of the primary malaria mosquito vectors in Vietnam: Anopheles dirus and Anopheles minimus.

BACKGROUND: The complexity of mosquito-borne diseases poses a major challenge to global health efforts to mitigate their impact on people residing in sub-tropical and tropical regions, to travellers and deployed military personnel. To supplement drug- and vaccine-based disease control programmes, other strategies are urgently needed, including the direct control of disease vectors. Modern vector control research generally focuses on identifying novel active ingredients and/or innovative methods to reduce human-mosquito interactions. These efforts include the evaluation of spatial repellents, which are compounds capable of altering mosquito feeding behaviour without direct contact with the chemical source. METHODS: This project examined the impact of airborne transfluthrin from impregnated textile materials on two important malaria vectors, Anopheles dirus and Anopheles minimus. Repellency was measured by movement within taxis cages within a semi-field environment at the National Institute of Hygiene and Epidemiology in Hanoi, Vietnam. Knockdown and mortality were measured in adult mosquito bioassay cages. Metered-volume air samples were collected at a sub-set of points in the mosquito exposure trial. RESULTS: Significant differences in knockdown/mortality were observed along a gradient from the exposure source with higher rates of knockdown/mortality at 2 m and 4 m when compared with the furthest distance (16 m). Knockdown/mortality was also greater at floor level and 1.5 m when compared to 3 m above the floor. Repellency was not significantly different except when comparing 2 m and 16 m taxis cages. Importantly, the two species reacted differently to transfluthrin, with An. minimus being more susceptible to knockdown and mortality. The measured concentrations of airborne transfluthrin ranged from below the limit of detection to 1.32 ng/L, however there were a limited number of evaluable samples complicating interpretation of these results. CONCLUSIONS: This study, measuring repellency, knockdown and mortality in two malaria vectors in Vietnam demonstrates that both species are sensitive to airborne transfluthrin. The differences in magnitude of response between the two species requires further study before use in large-scale vector control programmes to delineate how spatial repellency would impact the development of insecticide resistance and the disruption of biting behaviour.

Excito-repellent activity of ß-caryophyllene oxide against Aedes aegypti and Anopheles minimus.

Contact irritant and non-contact repellent activities of ß-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. ß-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 ß-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 ß-caryophyllene oxide. The analysis of escape responses showed significant differences between mosquito species at all concentrations (P < 0.05) except for 0.1%. For both species, there were significant differences in irritant and repellent responses between ß-caryophyllene oxide and DEET at higher concentrations (0.5 and 1.0%).

Diversity and biting patterns of Anopheles species in a malaria endemic area, Umphang Valley, Tak Province, western Thailand.

Malaria is highly endemic in Umphang Valley, a district in the western edge of Tak Province, along the boundary with Kayin State of Myanmar. Although there are high indigenous malaria cases in this area every year, nothing about malaria vectors and their transmission role have been investigated before this study. The objective of this work is to characterize the Anopheles species diversity and trophic behavior of malaria vectors in the transmission area of Umphang Valley. Females of Anopheles mosquitoes were collected every two months during a two-year period. Mosquito collections were using standard collection technique, indoor and outdoor human landing collections and outdoor cattle bait collection. Anopheles mosquitoes were identified using morphological characters and multiplex AS-PCR assay for the identification of sibling species within groups and complexes present. From a total of 16,468 Anopheles females, 2723 specimens (16.54%) were collected from humans and 13,745 specimens (83.46%) were captured from cattle. From human landing collections, 2447 specimens (89.86%) of Anopheles minimus were obtained, followed by 119 Anopheles peditaeniatus (4.37%), 62 Anopheles maculatus (2.28%), 17 Anopheles dirus (0.6%), 15 Anopheles aconitus (0.5%) and 6 Anopheles sawadwongporni (0.2%) respectively. Seven putative malaria vectors, including An. minimus, An. dirus, An. baimaii, An. sawadwongporni, An. maculatus, An. pseudowillmori and An. aconitus were documented from this study and trophic behavior of each respective species were observed. Such information is definitely crucial for defining the vector capacity of each single species and for designing appropriate vector prevention and control strategies against target vector species.

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.

Toxicity of several botanical essential oils and their combinations against females of Aedes albopictus (Skuse) and Anopheles minimus (Theobald): Oviposition deterrent, ovicidal and adulticidal efficacies

Objective: To investigate the efficacies of 12 essential oil (EO) formulations from three Zingiberaceae plants (Alpinia galanga, Curcuma zedoaria, and Zingiber cassumunar) individually and in combination with an augmenting Eucalyptus globulus (E. globulus) EO against females of Aedes albopictus (Ae. albopictus) and Anopheles minimus (An. minimus). Methods: These formulations were evaluated for their ovicidal, oviposition deterrent and adulticidal activities against Ae. albopictus and An. minimus by a topical method, a double-choice method and a WHO susceptibility test, respectively. Results: It was found that all formulations of Zingiberaceae plants EOs augmented with E. globulus EO were more effective in oviposition deterrent, ovicidal, and adulticidal activities against the two mosquito species than all of the formulations used without E. globulus EO. Their oviposition deterrent, ovicidal and adulticidal activities were equivalent to those of 10% w/v cypermethrin. In contrast, 70% v/v ethyl alcohol as a control alone was not effective at all. The highest synergistic effect in effective repellency against Ae. albopictus was achieved by 5% Alpinia galanga EO + 5% E. globulus EO and against An. minimus was 5% Zingiber cassumunar EO + 5% E. globulus EO. Moreover, the highest synergistic effects in ovicidal activities against Ae. albopictus and An. minimus were achieved by 10% Zingiber cassumunar EO + 10% E. globulus EO and 5% Curcuma zedoaria EO + 5% E. globulus EO, respectively. For the adulticidal activities, the highest synergistic effect against two mosquitoes was achieved by 5% Curcuma zedoaria EO + 5% E. globulus EO. Conclusions: These results suggest that Zingiberaceae plant EOs augmented with E. globulus EO have a high potential to be developed into oviposition deterrent, ovicidal, and adulticidal agents for controlling populations of Ae. albopictus and An. minimus.

Toxicity of several botanical essential oils and their combinations against females of Aedes albopictus (Skuse) and Anopheles minimus (Theobald): Oviposition deterrent, ovicidal and adulticidal efficacies

Objective:To investigate the efficacies of 12 essential oil (EO) formulations from three Zingiberaceae plants (Alpinia galanga,Curcuma zedoaria,and Zingiber cas.sumunar) individually and in combination with an augmenting Eucalyptus globulus (E.globulus) EO against females of Aedes albopictus (Ae.adbopictus) and Anopheles minimus (An.minimus).Methods:These formulations were evaluated for their ovicidal,oviposition deterrent and adulticidal activities against Ae.albopictus and An.minimus by a topical method,a double-choice method and a WHO susceptibility test,respectively.Results:It was found that all formulations of Zingiberaceae plants EOs augmented with E.globulus EO were more effective in oviposition deterrent,ovicidal,and adulticidal activities against the two mosquito species than all of the formulations used without E.globulus EO.Their oviposition deterrent,ovicidal and adulticidal activities were equivalent to those of 10％ w/v cypermethrin.In contrast,70％ v/v ethyl alcohol as a control alone was not effective at all.The highest synergistic effect in effective repellency against Ae.albopictus was achieved by 5％ Alpinia galanga EO + 5％ E.globulus EO and against An.minimus was 5％ Zingiber cassumunar EO + 5％ E.globulus EO.Moreover,the highest synergistic effects in ovicidal activities against Ae.albopictus and An.minimus were achieved by 10％ Zingiber cassumunar EO + 10％ E.globulus EO and 5％ Curcuma zedoaria EO + 5％ E.globulus EO,respectively.For the adulticidal activities,the highest synergistic effect against two mosquitoes was achieved by 5％ Gurcuma zedoaria EO + 5％ E.globulus EO.Conclusions:These results suggest that Zingiberaceae plant EOs augmented with E.globulus EO have a high potential to be developed into oviposition deterrent,ovicidal,and adulticidal agents for controlling populations of Ae.albopictus and An.minimus.

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.

Larvicidal efficacy of crude and fractionated extracts of Dracaena loureiri Gagnep against Aedes aegypti, Aedes albopictus, Culex quinquefasciatus, and Anopheles minimus mosquito vectors

Objective: To evaluate the larvicidal efficacy of crude and fractionated extracts of Dracaena loureiri endocarp against Aedes aegypti, Aedes albopictus, Culex quinquefasciatus, and Anopheles minimus mosquitos. Methods: Larvicidal activity was tested according to World Health Organization standard protocol. The third-stage larvae of each mosquito species were exposed to various concentrations of Dracaena loureiri crude extract and six groups of Dracaena loureiri fractionated extracts (RC-DT 009-014). Larval mortality rates were observed after 24 h and 48 h of exposure. Then, a computerized probit analysis of the mortality data was performed to determine lethal concentration 50 (LC

Larvicidal efficacy of crude and fractionated extracts of Dracaena loureiri Gagnep against Aedes aegypti, Aedes albopictus, Culex quinquefasciatus, and Anopheles minimus mosquito vectors

Objective: To evaluate the larvicidal efficacy of crude and fractionated extracts of Dracaena loureiri endocarp against Aedes aegypti, Aedes albopictus, Culex quinquefasciatus, and Anopheles minimus mosquitos. Methods: Larvicidal activity was tested according to World Health Organization standard protocol.The third-stage larvae of each mosquito species were exposed to various concentrations of Dracaena loureiri crude extract and six groups of Dracaena loureiri fractionated extracts (RC-DT 009–014). Larval mortality rates were observed after 24 h and 48 h of exposure.Then, a computerized probit analysis of the mortality data was performed to determine lethal concentration 50 (LC50) and lethal concentration 90 values. Results: Anopheles minimus larvae (24-h LC5077.88 mg/L) had the highest susceptibility to crude extract, whereas others (Aedes aegypti, 24-h LC50224.73 mg/L; Aedes albopictus, 24-h LC50261.75 mg/L; and Culex quinquefasciatus, 24-h LC50282.86 mg/L) were significantly less susceptible. The most effective groups of fractionated extracts were RC-DT 012 and RC-DT 013. The mosquito species most susceptible to fractionated extracts was Culex quinquefasciatus, with 24-h LC50 values of 0.66 and 0.94 mg/L for RC-DT 012 and RC-DT 013, respectively. Conclusions: The larvicidal activity of fractionated extracts is more effective than that of crude extract against all tested mosquito species. For the most effective alternative larvicide, purification and a phytochemical constituent analysis must be performed.

An effective method for the identification and separation of Anopheles minimus, the primary malaria vector in Thailand, and its sister species Anopheles harrisoni, with a comparison of their mating behaviors.

BACKGROUND: Species of the Anopheles minimus complex are considered to be the primary vectors of malaria in South and Southeast Asia. Two species of the complex, Anopheles minimus and Anopheles harrisoni, occur in Thailand. They are sympatric and difficult to accurately distinguish based on morphological characters. The aim of this study was to investigate the potential of antennal sensory organs to distinguish these two species. Additionally, we investigated their ability to mate in cages of different sizes, as well as the possible mechanism(s) that evokes stenogamous behavior. METHODS: Large sensilla coeloconica present on the antennae of females of An. minimus and An. harrisoni were counted under a conventional light microscope and various types of antennal sensilla were examined under a scanning electron microscope (SEM). Determinations of mating ability were carried out in 20 and 30 cm3 cages with a density resting surface (DRS) of 7.2. The insemination rate, frequency of clasper (gonocoxopodite) movement of the male genitalia during induced copulation and duration of mating of the two species were compared. RESULTS: The mean numbers of large sensilla coeloconica on antennal flagellomeres 1-8 and the mean number of large sensilla coeloconica on each flagellum in An. minimus (26.25) and An. harrisoni (31.98) were significantly different. Females of both species bear five types of antennal sensilla: chaetica, trichodea, basiconica, coeloconica and ampullacea. Marked differences in the structure of the large sensilla coeloconica were observed between the two species. Furthermore, only An. minimus could copulate naturally in the small cages. The frequency of clasper movement in the stenogamous An. minimus was significantly higher than in An. harrisoni, but there was no difference in the duration of mating. CONCLUSIONS: To our knowledge, this study is the first to examine and discover the usefulness of large sensilla coeloconica on the antennae of females and the frequency of clasper movement in males for distinguishing the sibling species An. minimus and An. harrisoni. The discovery provides an effective and relatively inexpensive method for their identification. Additionally, the greater frequency of clasper movement of An. minimus might influence its ability to mate in small spaces.