Susceptibility of Aedes aegypti and Aedes albopictus (Diptera: Culicidae) to Temephos in Thailand and Surrounding Countries

Larvicidal activity of Bacillus thuringiensis strains against Aedes aegypti and Culex quinquefasciatus mosquitoes

Organophosphates, carbamates and synthetic pyrethroids are commonly used in Thailand to control mosquito vectors; however, long-term use of insecticides in vector control has led to the rapid development of insecticide resistance. In this study, we screened Bacillus thuringiensis strains as biological control agents for potential toxins against mosquito larvae as an additional control tool. Preliminary bioassays conducted on 434 strains demonstrated that 41 strains (11.64%) and 14 strains (3.97%) achieved 100% mortality against Ae. aegypti and Cx. quinquefasciatus larvae, respectively. Three strains (JC690, JC691, and JC699) showed toxicity to both mosquito species, compared with the reference Bti strain. Strain JC691 demonstrated the highest efficacy against Ae. aegypti and Cx. quinquefasciatus, with an LC50 value of 6.96 × 104 CFU/ml and 1.16 × 103 CFU/ml, respectively. A comparison of LC50 values revealed that JC691 exhibited higher efficacy against Cx. quinquefasciatus than that by Bti (Bti LC50: 8.89 × 10⁴ CFU/ml) but lower efficacy against Ae. aegypti (Bti LC50: 1.99 × 10³ CFU/ml). Scanning electron microscopy revealed that JC690, JC691, and JC699 are rod-shaped, have oval spores, and produce bi-pyramidal crystal proteins. Protein profile analysis using SDS-PAGE demonstrated distinct differences between these Thailand strains (JC690, JC691, and JC699) and the reference Bti strain. All three Thailand strains contained cry1I and cry2A genes, and only JC691 harbored the cry32 gene. Bayesian inference and maximum likelihood phylogenetic analyses of cry32 indicated that the partial sequences of cry32 in JC691 from Thailand were distinct from those of other B. thuringiensis strains from different countries. This study demonstrates the potential of JC690, JC691, and JC699 as biocontrol agents for Ae. aegypti and Cx. quinquefasciatus.

Review of dengue vectors in Cambodia: distribution, bionomics, vector competence, control and insecticide resistance

BACKGROUND

Dengue fever is one of the most prevalent mosquito-borne diseases in Cambodia. Until now, no specific vaccine nor antiviral treatment exists the virus causing Dengue fever. Consequently, its prevention relies only on vector control strategies. However, efficient vector control in turn relies on a good knowledge of the biology of the vector species. Therefore, this study aims to provide the first review of the distribution, ecology, meteorological impacts, trophic behavior, vector competence, vector control and insecticide resistance of dengue vector species in Cambodia.

METHODS

A systematic search of the Google Scholar and PubMed databases was conducted for relevant published articles. Of the 610 published articles originally identified, 70 articles were ultimately selected for inclusion in this review. We also included new data from unpublished research conducted in Cambodia between 2017 and 2023 related to dengue vector bionomics.

RESULTS

Eleven Aedes (Stegomyia) mosquito species have been recorded in Cambodia, including a new species described in 2024. Four species are associated with dengue virus transmission, among which Aedes aegypti and Ae. albopictus are the main vectors and Ae. malayensis and Ae. scutellaris are considered to be potential vectors. Aedes aegypti and Ae. albopictus are present in all provinces of Cambodia. Aedes albopictus shows a preference for forest, rural and suburban areas, while Ae. aegypti is mostly found in urban and suburban areas. The distribution of these two species is also influenced by meteorological factors, seasonality and the availability of breeding habitats and blood meals. Both species are predominant during the rainy season, and their respective density is impacted by precipitation and temperature. Aedes aegypti is characterized as anthropophilic, while Ae. albopictus exhibits zooanthropophilic behavior, and both species have been observed to be predominantly diurnal. In addition, they were found to be highly resistant to the insecticides used in Cambodia for their control, such as temephos for larvae and deltamethrin and permethrin for adult mosquitoes.

CONCLUSIONS

This review provides extensive and important knowledge on dengue vectors in Cambodia. This knowledge is derived not only from published research articles but also from many recent studies in Cambodia on the bionomics of dengue vector species. The review provides valuable information for use by public health authorities on dengue virus transmission and to develop better vector control strategies in the country.

Assessing the bioefficacy of a commercial temephos formulation (Temebate®) for controlling Aedes albopictus larvae in different land use localities in Malaysia

Temephos is the World Health Organization (WHO) recommended larvicide and is still being utilized worldwide to control larvae of dengue vectors; Aedes aegypti and Aedes albopictus. The efficacy of a commercial temephos product; Temebate® to exterminate the local populations of Ae. albopictus larvae originated from different land use particularly dengue-risk and dengue-free housing localities as well as agrarian localities including oil palm plantations, rubber estates and paddy fields was assessed to verify its bioefficacy in these localities. Field populations of Ae. albopictus larvae were attained via a larval survey at each study locality. Each Ae. albopictus larval population was subjected to a 24-h larval bioassay using Temebate® at operational dosage of 1 mg/L. Almost all Ae. albopictus larval populations demonstrated mortalities between 7.00% and 100.00% by the end of the first 4 h of Temebate® exposure with the resistance ratios between 0.94 and 8.33. After 24 h of Temebate® exposure, all sixteen Ae. albopictus larval populations exhibited increased mortalities with ten of them showing 100% mortalities. These results confirmed the relevance of Temebate® to be continuously used by the residents of these localities as their control efforts against dengue vectors. Nevertheless, Temebate® application by consumers in dengue-risk localities need to be carefully monitored to prevent further development of temephos resistance among Ae. albopictus populations and substantiated with other vector control approaches.

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.

Expansive and Diverse Phenotypic Landscape of Field Aedes aegypti (Diptera: Culicidae) Larvae with Differential Susceptibility to Temephos: Beyond Metabolic Detoxification

Arboviruses including dengue, Zika, and chikungunya are amongst the most significant public health concerns worldwide. Arbovirus control relies on the use of insecticides to control the vector mosquito Aedes aegypti (Linnaeus), the success of which is threatened by widespread insecticide resistance. The work presented here profiled the gene expression of Ae. aegypti larvae from field populations of Ae. aegypti with differential susceptibility to temephos originating from two Colombian urban locations, Bello and Cúcuta, previously reported to have distinctive disease incidence, socioeconomics, and climate. We demonstrated that an exclusive field-to-lab (Ae. aegypti strain New Orleans) comparison generates an over estimation of differential gene expression (DGE) and that the inclusion of a geographically relevant field control yields a more discrete, and likely, more specific set of genes. The composition of the obtained DGE profiles is varied, with commonly reported resistance associated genes including detoxifying enzymes having only a small representation. We identify cuticle biosynthesis, ion exchange homeostasis, an extensive number of long noncoding RNAs, and chromatin modelling among the differentially expressed genes in field resistant Ae. aegypti larvae. It was also shown that temephos resistant larvae undertake further gene expression responses when temporarily exposed to temephos. The results from the sampling triangulation approach here contribute a discrete DGE profiling with reduced noise that permitted the observation of a greater gene diversity, increasing the number of potential targets for the control of insecticide resistant mosquitoes and widening our knowledge base on the complex phenotypic network of the Ae. aegypti response to insecticides.

Entomological Risk Assessment for Dengue Virus Transmission during 2016-2020 in Kamphaeng Phet, Thailand

Individual houses with high risks of dengue virus (DENV) transmission might be a source of virus transmission within the neighborhood. We conducted an entomological risk assessment for DENV transmission at the household level, comprising family cohort members residing in the same location, to assess the risk for dengue virus transmitted by mosquito vectors. The studies were conducted in Kamphaeng Phet Province, Thailand, during 2016-2020. Entomological investigations were performed in 35 cohort families on day 1 and day 14 after receiving dengue case reports. DENV was found in 22 Aedes samples (4.9%) out of 451 tested samples. A significantly higher DENV infection rate was detected in vectors collected on day 1 (6.64%) compared to those collected on day 14 (1.82%). Annual vector surveillance was carried out in 732 houses, with 1002 traps catching 3653 Aedes females. The majority of the 13,228 water containers examined were made from plastic and clay, with used tires serving as a primary container, with 59.55% larval abundance. Larval indices, as indicators of dengue epidemics and to evaluate disease and vector control approaches, were calculated. As a result, high values of larval indices indicated the considerably high risk of dengue transmission in these communities.

Widespread multiple insecticide resistance in the major dengue vector Aedes albopictus in Hainan Province, China

BACKGROUND

Aedes albopictus is a highly invasive mosquito and has become a potential vector of dengue, chikungunya and Zika viruses. Insecticide-based mosquito interventions are the main tools for vector-borne disease control. However, mosquito resistance to insecticides is a major threat to effective prevention and control. Five Ae. albopictus populations across Hainan Province, China were investigated for susceptibility to multiple insecticide and resistance mechanisms.

RESULTS

Larval bioassays indicated that resistance to pyrethroids was common in all larval populations. Adult bioassays revealed all populations were either resistant or highly resistant to at least four of the six synthetic insecticides (deltamethrin, permethrin, cyfluthrin, propoxur, malathion, and DDT) tested. Pre-exposure of mosquitoes to the synergistic agent piperonyl butoxide (PBO) increased mosquito mortality by 2.4-43.3% in bioassays to DDT, malathion, and permethrin and rendered mosquito sensitive to deltamethrin, cyfluthrin, and propoxur. The frequency of knockdown resistance (kdr) mutations (F1534S and F1534C) ranged from 69.8% to 89.3% and from 38.1% to 87.0% in field-resistant and sensitive populations, respectively. F1534S mutation was significantly associated with pyrethroid resistance. No mutation was detected in the acetylcholinesterase (ace-1) gene in the two examined populations.

CONCLUSION

This study provides evidence of widespread resistance to multiple insecticides in Ae. albopictus in Hainan Province, China. Both kdr mutations and metabolic detoxification were potential causes of insecticide resistance for Ae. albopictus. Our findings highlight the need for insecticide resistance management and mosquito control measures that do not entirely depend on synthetic insecticides. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.