Insecticide resistance spectra and resistance mechanisms in populations of Japanese encephalitis vector mosquitoes, Culex tritaeniorhynchus and Cx. gelidus, in Sri Lanka

Eco-friendly biosynthesis of TiO2 nanoparticles using Desmostachya bipinnata extract: Larvicidal and pupicidal potential against Aedes aegypti and Spodoptera litura and acute toxicity in non-target organisms

The resistance to insecticides among insects, including mosquitoes and agricultural pests, and the impact of these compounds' environmental risks and health issues have motivated the proposition of eco-friendly alternatives. Thus, we aimed to explore the potential use of Desmostachya bipinnata for the biosynthesis of TiO₂NPs and evaluate their larvicidal and pupicidal activity of target (Aedes aegypti and Spodoptera litura) and acute toxicity in non-target organisms (Toxorhynchites splendens and Eisenia fetida), at distinct concentrations, after 24 h of exposure. The characterization of the biosynthesized TiO₂NPs was carried out by FT-IR, XRD, SEM, and EDX analysis. Under the UV-vis spectrum analysis, a sharp peak was recorded at 200 to 800 nm, which indicated the production of TiO₂NPs by the plant extract. The SEM analysis revealed that the synthesized TiO₂NPs were spherical with a diameter of 36.4 nm and were detected in the XRD spectrum analysis related to the TiO₂NPs. The highest percentage of mortality recorded at 900 μg/mL was 96 % and 94 % in the 2nd instar of A. aegypti and S. litura larvae, respectively, and exhibited the LC₅₀ and LC₉₀ values 5 of 458.79 and 531.01 μg/mL, respectively. The biosynthesized TiO₂NPs showed concentration-dependent increased pupal lethality for both A. aegypti and S. litura. We also observed increased detoxification enzyme activity (α esterase, β esterase, and glutathione-S-transferase) of A. aegypti and S. litura exposed to different concentrations of biosynthesized TiO₂NPs as histopathological changes in the midgut region of these animals. On the other hand, the mortality rate of non-target organisms (T. splendens and E. fetida) was lower when exposed to TiO₂NPs, compared to the high lethality induced by synthetic pesticides (cypermethrin and monocrotophos for E. fetida; and cypermethrin and temphos for T. splendens). Thus, our study provides pioneering evidence on the potential use of D. bipinnata-mediated TiO₂NPs for controlling mosquito vectors and agricultural pest management.

Detection of L1014F knockdown resistance mutation in Culex tritaeniorhynchus populations

Culex tritaeniorhynchus is a major Japanese encephalitis virus vector distributed in Southeast Asia and surrounding countries. The aim of the present study is to investigate insecticide resistance status among 10 Cx. tritaeniorhynchus populations of the Mediterranean region of Turkey. Bioassay results indicated that all of the populations were resistant or at least possibly resistant to 1,1'-(2,2,2-Trichloroethane-1,1-diyl) bis (4-chlorobenzene) (DDT) (4%), [(dimethoxyphosphorothioyl) sulfanyl] butanedioate, Diethyl (malathion) (5%), and 2-[(Propan-2-yl) oxy] phenyl methylcarbamate (propoxur) (0,1%). Whereas, some of the populations were still susceptible to 3-Phenoxybenzyl (1RS)-cis, trans-3-(2,2-dichlorovinyl)-2,2-dimethyl cyclopropane carboxylate (permethrin) (0,75%) and (S)-Cyano (3-phenoxy phenyl) methyl (1R,3R)-3-(2,2-dibromoethen-1-yl)-2,2-dimethylcyclopropane-1-carboxylate (deltamethrin) (0,05%). Biochemical analysis results showed altered alpha esterase, beta esterase, para-nitrophenyl acetate (PNPA), and glutathione-s-transferase (GST) levels in some populations while all of the populations had increased oxidase levels except for the Yumurtalık population. Additionally, all of the populations had sensitive acetylcholinesterase (AChE) levels similar to the control group except for the Erzin population. Correlation analysis showed a significant correlation between mortality rates for deltamethrin and alpha esterase, beta esterase, PNPA, and GST levels while mortality rates for permethrin were significantly correlated with GST levels. An allele-specific polymerase chain reaction (AS-PCR) detected high L1014F allele frequency in the populations. Overall results indicate the urgent need for monitoring and mapping of insecticide resistance in Cx. tritaeniorhynchus populations of the study area for effective vector control management.

Molecular Characterization of Culturable Aerobic Bacteria in the Midgut of Field-Caught Culex tritaeniorhynchus, Culex gelidus, and Mansonia annulifera Mosquitoes in the Gampaha District of Sri Lanka

Background

Larval and adult mosquito stages harbor different extracellular microbes exhibiting various functions in their digestive tract including host-parasite interactions. Midgut symbiotic bacteria can be genetically exploited to express molecules within the vectors, altering vector competency and potential for disease transmission. Therefore, identification of mosquito gut inhabiting microbiota is of ample importance before developing novel vector control strategies that involve modification of vectors.

Method

Adult mosquitoes of Culex tritaeniorhynchus, Culex gelidus, and Mansonia annulifera were collected from selected Medical Officer of Health (MOH) areas in the Gampaha district of Sri Lanka. Midgut lysates of the field-caught non-blood-fed female mosquitoes were cultured in Plate Count Agar medium, and Prokaryotic 16S ribosomal RNA partial genes of the isolated bacteria colonies were amplified followed by DNA sequencing. Diversity indices were used to assess the diversity and richness of the bacterial isolates in three mosquito species. The distribution pattern of bacterial isolates between different mosquito species was assessed by Distance-Based Redundancy Analysis (dbRDA).

Results

A total of 20 bacterial species (Staphylococcus pasteuri, Bacillus megaterium, Staphylococcus cohnii, Pantoea dispersa, Staphylococcus chromogenes, Bacillus aquimaris, Staphylococcus arlettae, Staphylococcus sciuri, Staphylococcus warneri, Moraxella osloensis, Enterobacter sp., Klebsiella michiganensis, Staphylococcus hominis, Staphylococcus saprophyticus, Streptomyces sp., Bacillus niacin, Cedecea neteri, Micrococcus luteus, Lysinibacillus sphaericus, and Bacillus licheniformis) were identified. All of these species belonged to three phyla, Proteobacteria, Firmicutes, and Actinobacteria, out of which phylum Firmicutes (71.1%) was the most prominent. The least number of species was recorded from Actinobacteria. The relative distribution of midgut microbes in different mosquito species differed significantly among mosquito species (Chi-square, χ ² = 486.091; df = 36; P ≤ 0.001). Midgut microbiota of Cx. tritaeniorhynchus and Cx. gelidus indicated a similarity of 21.51%, while Ma. annulifera shared a similarity of 6.92% with the cluster of above two species. The gut microbiota of Cx. tritaeniorhynchus was also significantly more diverse and more evenly distributed compared to Ma. annulifera. Simpson's diversity, Margalef's diversity, and Menhinick's diversity indices were higher in Cx. gelidus. Of the recorded species, P. dispersa and strains of nonpathogenic species in Bacillaceae family (B. megaterium, B. niacini, B. licheniformis, and L. sphaericus) can be recommended as potential candidates for paratransgenesis.

Conclusion

The relative distribution of midgut microbes in different mosquito species differed significantly among the three studied adult mosquito species. The present data strongly encourage further investigations to explore the potential usage of these microbes through paratransgenic approach for novel eco-friendly vector control strategies.

Perspectives Regarding the Risk of Introduction of the Japanese Encephalitis Virus (JEV) in the United States

Japanese encephalitis (JE) is a zoonotic, emerging disease transmitted by mosquito vectors infected with the Japanese encephalitis virus (JEV). Its potential for emergence into susceptible regions is high, including in the United States (US), and is a reason of economic concern among the agricultural community, and to public health due to high morbidity and mortality rates in humans. While exploring the complexities of interactions involved with viral transmission, we proposed a new outlook on the role of vectors, hosts and the environment under changing conditions. For instance, the role of feral pigs may have been underappreciated in our previous work, given research keeps pointing to the importance of susceptible populations of wild swine in naïve regions as key elements for the introduction of emergent vector-borne diseases. High risk of JEV introduction has been associated with the transportation of infected mosquitoes via aircraft. Nonetheless, no JEV outbreaks have been reported in the US to date and results from a qualitative risk assessment considered the risk of establishment to be negligible under the current conditions (environmental, vector, pathogen, and host). In this work, we discuss virus-vector-host interactions and ecological factors important for virus transmission and spread, review research on the risk of JEV introduction to the US considering the implications of risk dismissal as it relates to past experiences with similar arboviruses, and reflect on future directions, challenges, and implications of a JEV incursion.

Insecticide resistance mechanisms with novel 'kdr' type gene mutations in the tropical bed bug Cimex hemipterus

BACKGROUND

The tropical bed bug, Cimex hemipterus, is a serious indoor public health pest in tropical regions causing intense physical discomfort and mental distress to humans. At present, the application of insecticides is the major control strategy. The present study was designed to evaluate the development of resistance and resistance mechanisms in Cimex hemipterus from Kandy district, Sri Lanka.

METHODS

The resistance status of the collected bed bugs was determined against the discriminative dosages of DDT, malathion, propoxur, deltamethrin and permethrin by conducting bioassays according to World Health Organization guidelines. Activities of insecticide metabolizing enzymes, i.e. esterases, glutathione S-transferases (GST) and monooxygenases, and the insensitivity of organophosphate/carbamate target site acetylcholinesterase (AChE), were evaluated by biochemical assays. Regions of the gene of the pyrethroid/DDT target site, the voltage-gated sodium channel regulatory protein (VGSC), were sequenced for possible kdr mutations.

RESULTS

Survival percentages of bed bug population were 71, 68 and 51% for DDT, malathion and propoxur respectively. KT50 and KT90 values, calculated using log-probit mortality curves for deltamethrin were 62.55 and 123.96 h, respectively. These values were much higher for permethrin where KT50 was 201.10 h and the KT90 was beyond the detectable range. Results were compared with previous values reported for the same population in 2002. Resistance to propoxur has increased significantly from 11 to 51% with about a 20-fold increase in the number of individuals with elevated esterase mechanism. No significant change has occurred in malathion and DDT resistance, in GST and monooxygenase activities, and in AChE sensitivity for the past 14 years. Six kdr associated mutations (Y/L995H, V1010L, I1011F, L1014F, V1016E, L1017F/S) and a non-kdr associated mutation (A1007S mutation) were found from the α-region of the VGSC gene. Out of the kdr type mutations, only L1014F has been reported previously form C. hemipterus while the others have been reported from other insects.

CONCLUSIONS

The bed bug population has developed high resistance to propoxur with increased esterase activities. KT50 for deltamethrin and permethrin has increased 125- and 20-fold, respectively, over the period 2002 to 2016. To the authors' knowledge, this is the first time that the possible involvement of a kdr type mutation in developing pyrethroid resistance in C. hemipterus has been shown in Sri Lanka.

Insecticide resistance status of three vectors of Japanese encephalitis in east central India

Japanese encephalitis (JE) has been reported in different districts of Odisha state (east central India) since 1992. During 2016, a major outbreak of JE and acute encephalitis syndrome (AES) occurred in the Malkangiri district of Odisha, causing 103 deaths in children, of which 37 were caused by JE and 66 by AES. Information on insecticide resistance in JE vectors is important for the selection of appropriate insecticides for use in vector control. The present study was designed to determine the resistance status of three important vectors of JE, Culex vishnui, Culex tritaeniorhynchus and Culex bitaeniorhynchus (Diptera: Culicidae), against dichlorodiphenyltrichloroethane (DDT), malathion and deltamethrin in three districts of Odisha state affected by JE. Female adult mosquitoes were collected using mouth aspirators both indoors and outdoors in JE-affected villages and used in susceptibility bioassays following World Health Organization guidelines. Knock-downs were recorded every 10 min up to 1 h and mortality rates were recorded at 24 h post-exposure. Culex vishnui and Cx. tritaeniorhynchus showed resistance to DDT, malathion and deltamethrin, whereas Cx. bitaeniorhynchus was susceptible in all study districts. The information generated by this study will be highly useful in the planning and implementing of appropriate vector control operations for the prevention and control of JE in east central India.

Molecular interaction of the antiviral compound CW‑33 and its analogues with the NS2B‑NS3 protease of the Japanese encephalitis virus

In a previous study from our group, a novel compound, namely CW‑33 (ethyl 2‑(3',5'‑dimethylanilino)‑​4‑oxo‑4,5‑dihydrofuran‑3‑carboxylate) was identified that exhibited antiviral activity for Japanese encephalitis virus (JEV). The viral NS2B‑NS3 serine protease serves an important role in cytoplasmic cleavage events that occur during viral polyprotein maturation. The inhibition of viral RNA and protein syntheses was responsible for the antiviral activities of the novel furanonaphthoquinone derivatives that were discovered for the prevention of JEV infection. Consequently, the present study examined the molecular docking simulation of JEV protease with compound CW‑33 and its analogues, and developed quantitative structure‑activity relationship (QSAR) models to assess the potential antiviral activities of these compounds with regard to JEV. Molecular docking simulation indicated the potential ligand‑protein interactions associated with the antiviral activities of these compounds. According to the results of the QSAR models, the secondary amine group was an important moiety required for compound bioactivity, which enabled the formation of hydrogen bonding with the residue Glu155. Furthermore, the aromatic ring mapping of the phenyl moiety of each compound was predicted to form a π‑cation interaction with residue Arg76, whereas the hydrophobic feature represented by the ethyl moiety exhibited hydrophobic contacts with residue Glu74. Finally, the hydrophobic substituents in the meta‑position of the phenyl ring further contributed to the efficacy of the antiviral activity. These results unravel the structural characteristics that are required for binding of CW‑33 to the JEV protease and can be used for potential therapeutic and drug development purposes for JEV.

Japanese encephalitis in Malaysia: An overview and timeline

Japanese encephalitis (JE) is a vector-borne zoonotic disease caused by the Japanese encephalitis virus (JEV). It causes encephalitis in human and horses, and may lead to reproductive failure in sows. The first human encephalitis case in Malaya (now Malaysia) was reported during World War II in a British prison in 1942. Later, encephalitis was observed among race horses in Singapore. In 1951, the first JEV was isolated from the brain of an encephalitis patient. The true storyline of JE exposure among humans and animals has not been documented in Malaysia. In some places such as Sarawak, JEV has been isolated from mosquitoes before an outbreak in 1992. JE is an epidemic in Malaysia except Sarawak. There are four major outbreaks reported in Pulau Langkawi (1974), Penang (1988), Perak and Negeri Sembilan (1998-1999), and Sarawak (1992). JE is considered endemic only in Sarawak. Initially, both adults and children were victims of JE in Malaysia, however, according to the current reports; JE infection is only lethal to children in Malaysia. This paper describes a timeline of JE cases (background of each case) from first detection to current status, vaccination programs against JE, diagnostic methods used in hospitals and factors which may contribute to the transmission of JE among humans and animals in Malaysia.

DNA barcoding of five Japanese encephalitis mosquito vectors (Culex fuscocephala, Culex gelidus, Culex tritaeniorhynchus, Culex pseudovishnui and Culex vishnui)

Culex mosquitoes can act as vectors of several important diseases, including Japanese encephalitis, West Nile virus, St. Louis encephalitis and equine encephalitis. Besides the neurological sequelae caused in humans, Japanese encephalitis can lead to abortion in sows and encephalitis in horses. Effective vector control and early diagnosis, along with continuous serosurveillance in animals, are crucial to fight this arboviral disease. However, the success of vector control operations is linked with the fast and reliable identification of targeted species, and knowledge about their biology and ecology. Since the DNA barcoding of Culex vectors of Japanese encephalitis is scarcely explored, here we evaluated the efficacy of this tool to identify and analyze the variations among five overlooked Culex vectors of Japanese encephalitis, Culex fuscocephala, Culex gelidus, Culex tritaeniorhynchus, Culex pseudovishnui and Culex vishnui, relying to the analysis of mitochondrial CO1 gene. Variations in their base pair range were elucidated by the entropy H_x plot. The differences among individual conspecifics and on base pair range across the same were studied. The C (501-750 bp) region showed a moderate variation among all the selected species. C. tritaeniorhynchus exhibited the highest variation in all the ranges. The observed genetic divergence was partially non-discriminatory. i.e., the overall intra- and inter nucleotide divergence was 0.0920 (0.92%) and 0.125 (1.25%), respectively. However, 10X rule fits accurately intraspecies divergence <3% for the five selected Culex species. The analysis of individual scatter plots showed threshold values (10X) of 0.008 (0.08%), 0.005 (0.05%), 0.123 (1.23%), 0.033 (0.33%) and 0.019 (0.19%) for C. fuscocephala, C. gelidus, C. tritaeniorhynchus, C. pseudovishnui and C. vishnui, respectively. The C. tritaeniorhynchus haplotypes KU497604, KU497603, AB690847 and AB690854 exhibited the highest divergence range, i.e., from 0.465 -0.546. Comparatively, the intra-divergence among the other haplotypes of C. tritaeniorhynchus ranged from 0-0.056. The maximum parsimony tree was formed by distinctive conspecific clusters with appreciable branch values illustrating their close congruence and extensive genetic deviations. Overall, this study adds valuable knowledge to the molecular biology and systematics of five overlooked mosquito species acting as major vectors of Japanese encephalitis in Asian countries.

Selection, Realized Heritability, and Fitness Cost Associated With Dimethoate Resistance in a Field Population of Culex quinquefasciatus (Diptera: Culicidae)

Mosquitoes are known to be vectors of numerous diseases leading to human morbidity and mortality at large scale in the world. Insecticide resistance has become a serious concern in controlling the insect vectors of public health importance. Dimethoate is an organophosphate insecticide used to control different insect pests including mosquitoes. Biological parameters of susceptible, unselected, and dimethoate-selected strains of Culex quinquefasciatus Say were studied in the laboratory to recognize resistance development potential and associated fitness cost. The dimethoate-selected strain showed 66.48-fold resistance to dimethoate compared with the susceptible strain after three continuous selections of generations. Realized heritability estimates of dimethoate resistance in Cx. quinquefasciatus yielded a value of 0.19. In dimethoate-selected strain, the biological traits including larval weight, survival from first instar to pupae, fecundity, number of next-generation larvae, relative fitness, net reproductive rate, intrinsic rate of natural increase, and biotic potential were significantly reduced as compared with the unselected strain. However, adult longevity, mean relative growth rate, weight of egg raft, female ratio, pupal duration, and emergence rate of the dimethoate-selected strain did not differ significantly compared with that of the unselected strain. This study provides useful information to devise retrospective management strategy for dimethoate resistance in Cx. quinquefasciatus.

Mechanisms of acaricide resistance in the cattle tick Rhipicephalus (Boophilus) microplus in Sri Lanka

High tolerance of ticks to acaricides is increasingly becoming a problem to cattle farmers. Resistance status of Rhipicephalus (Boophilus) microplus (Acari: Ixodidae)from two cattle farms of Sri Lanka were determined against different concentrations of pyrethroid permethrin, organophosphate malathion, organochlorine DDT and carbamate propoxur using Larval Packet Test (LPT) as recommended by Food and Agriculture Organization (FAO). Mechanisms of acaricide resistance were studied by conducting biochemical and PCR assays. Tick larvae were tested for the activity levels of acaricide metabolizing enzymes i.e.esterases, glutathione S-transferases (GSTs) and monooxygenases, and for altered target sites i.e.acetylcholinesterase (target site of organophosphates and carbamates) and sodium channel regulatory proteins (target site of pyrethroids and DDT). According to discriminating dosages specified by FAO for ticks both populations were 24-56% resistant to DDT. LC values showed that the both populations were susceptible to permethrin and resistant to malathion. Moderate insensitivity of AChEs and knock-down resistance (kdr) mutations were found as resistance mechanisms. GSTs and monooxygenases were not elevated. The kdr type mutation G72V (G215T in the gene) found in the sodium channel regulatory protein of R. (B.) microplus samples may be responsible for DDT resistance. Systematic and sophisticated insecticide resistance monitoring programmes and a better understanding on the mechanisms which govern resistance development are vital for future tick control programmes.

Functional and immunohistochemical characterization of CCEae3a, a carboxylesterase associated with temephos resistance in the major arbovirus vectors Aedes aegypti and Ae. albopictus

Temephos is a major organophosphate (OP) larvicide that has been used extensively for the control of Aedes albopictus and Aedes aegypti, the major vectors for viral diseases, such as dengue fever, zika and chikungunya. Resistance to temephos has been recently detected and associated with the upregulation of carboxylesterases (CCEs) through gene amplification, in both species. Here, we expressed the CCEae3a genes which showed the most striking up-regulation in resistant Aedes strains, using the baculovirus system. All CCEae3a variants encoded functional enzymes, with high activity and preference for p-nitrophenyl butyrate, a substrate that was shown capable to differentiate temephos resistant from susceptible Aedes larvae. Enzyme kinetic studies showed that CCEae3as from both Ae. aegypti and Ae. albopictus (CCEae3a_aeg and CCEae3a_alb, respectively) strongly interact with temephos oxon and slowly released the OP molecule, indicating a sequestration resistance mechanism. No difference was detected between resistant and susceptible CCEae3a_aeg variants (CCEae3a_aegR and CCEae3a_aegS, respectively), indicating that previously reported polymorphism is unlikely to play a role in temephos resistance. HPLC/MS showed that CCEae3as were able to metabolize temephos oxon to the temephos monoester [(4-hydroxyphenyl) sulfanyl] phenyl O,O-dimethylphosphorothioate. Western blot and immunolocalization studies, based on a specific antibody raised against the CCEae3a_alb showed that the enzyme is expressed at higher levels in resistant insects, primarily in malpighian tubules (MT) and nerve tissues.

The Environmentally Benign form of Pesticide in Hydrodispersive Nanometric form with Improved Efficacy Against Adult Mosquitoes at Low Exposure Concentrations

Permethrin, a poorly water-soluble synthetic pesticide belonging to the pyrethroid family, was formulated into water-dispersive nanometric form by rapid evaporation of pesticide loaded oil-in-water microemulsion. The mean hydrodynamic diameter of Nanopermethrin was found to be 199.01 ± 1.4 nm. The efficacy of the Nanopermethrin was comparatively investigated with its bulk form against 2-3 days old adult mosquitoes by WHO cone bioassay for 60 min. The median knockdown concentration of Culex tritaeniorhynchus, Culex quinquefasciatus and Aedes albopictus were found to be 7.20 × 10(4), 7.53 × 10(4), 0.42 × 10(3) mg/L for Bulk permethrin, and 0.98 × 10(4), 1.17 × 10(4), 0.05 × 10(3) mg/L for Nanopermethrin, respectively. The obtained results extrapolate the improved efficacy of Nanopermethrin even at low-level concentrations. Hence, the formulated Nanopermethrin will serve as an effective alternative pesticide in controlling the mosquito population with reduced environmental toxicity.

The Potential Use of Wolbachia-Based Mosquito Biocontrol Strategies for Japanese Encephalitis

Japanese encephalitis virus (JEV) is a zoonotic pathogen transmitted by the infectious bite of Culex mosquitoes. The virus causes the development of the disease Japanese encephalitis (JE) in a small proportion of those infected, predominantly affecting children in eastern and southern Asia. Annual JE incidence estimates range from 50,000–175,000, with 25%–30% of cases resulting in mortality. It is estimated that 3 billion people live in countries in which JEV is endemic. The virus exists in an enzootic transmission cycle, with mosquitoes transmitting JEV between birds as reservoir hosts and pigs as amplifying hosts. Zoonotic infection occurs as a result of spillover events from the main transmission cycle. The reservoir avian hosts include cattle egrets, pond herons, and other species of water birds belonging to the family Ardeidae. Irrigated rice fields provide an ideal breeding ground for mosquitoes and attract migratory birds, maintaining the transmission of JEV. Although multiple vaccines have been developed for JEV, they are expensive and require multiple doses to maintain efficacy and immunity. As humans are a “dead-end” host for the virus, vaccination of the human population is unlikely to result in eradication. Therefore, vector control of the principal mosquito vector, Culex tritaeniorhynchus, represents a more promising strategy for reducing transmission. Current vector control strategies include intermittent irrigation of rice fields and space spraying of insecticides during outbreaks. However, Cx. Tritaeniorhynchus is subject to heavy exposure to pesticides in rice fields, and as a result, insecticide resistance has developed. In recent years, significant advancements have been made in the potential use of the bacterial endosymbiont Wolbachia for mosquito biocontrol. The successful transinfection of Wolbachia strains from Drosophila flies to Aedes (Stegomyia) mosquitoes has resulted in the generation of “dengue-refractory” mosquito lines. The successful establishment of Wolbachia in wild Aedes aegypti populations has recently been demonstrated, and open releases in dengue-endemic countries are ongoing. This review outlines the current control methods for JEV in addition to highlighting the potential use of Wolbachia-based biocontrol strategies to impact transmission. JEV and dengue virus are both members of the Flavivirus genus, and the successful establishment of Drosophila Wolbachia strains in Cx. Tritaeniorhynchus, as the principal vector of JEV, is predicted to significantly impact JEV transmission.

Entomopathogenic marine actinomycetes as potential and low-cost biocontrol agents against bloodsucking arthropods

A novel approach to control strategies for integrated blood-feeding parasite management is in high demand, including the use of biological control agents. The present study aims to determine the efficacy of optimized crude extract of actinomycetes strain LK1 as biological control agent against the fourth-instar larvae of Anopheles stephensi and Culex tritaeniorhynchus (Diptera: Culicidae) and adults of Haemaphysalis bispinosa, Rhipicephalus (Boophilus) microplus (Acari: Ixodidae), and Hippobosca maculata (Diptera: Hippoboscidae). Antiparasitic activity was optimized using the Plackett-Burman method, and the design was developed using the software Design-Expert version 8.0.7.1. The production of the optimized crude actinomycetes LK1 strain extract was performed using response surface methodology to optimize the process parameters of protease inhibitor activity of marine actinobacteria for the independent variables like pH, temperature, glucose, casein, and NaCl at two levels (-1 and +1). The potential actinomycetes strain was identified as Saccharomonas spp., and the metamodeling surface simulation procedure was followed. It was studied using a computer-generated experimental design, automatic control of simulation experiments, and sequential optimization of the metamodels fitted to a simulation response surface function. The central composite design (CCD) used for the analysis of treatment showed that a second-order polynomial regression model was in good agreement with the experimental results at R (2) = 0.9829 (p < 0.05). The optimized values of the variables for antioxidant production were pH 6.00, glucose 1.3%, casein 0.09%, temperature 31.23 °C, and NaCl 0.10%. The LK1 strain-optimized crude extract was purified using reversed-phase high-pressure liquid chromatography, and the isolated protease inhibitor showed antiparasitic activity. The antiparasitic activity of optimized crude extract of LK1 was tested against larvae of A. stephensi (LC₅₀ = 31.82 ppm; r(2) = 0.818) and C. tritaeniorhynchus (LC₅₀ = 26.62 ppm; r(2) = 0.790) and adults of H. bispinosa (LC₅₀ = 106.58 ppm; r(2) = 0.871), R. (B.) microplus (LC₅₀ = 92.96 ppm; r(2) = 0.913), and H. maculata (LC₅₀ = 84.90 ppm; r(2) = 0.857).

Insecticide resistance and, efficacy of space spraying and larviciding in the control of dengue vectors Aedes aegypti and Aedes albopictus in Sri Lanka

Unprecedented incidence of dengue has been recorded in Sri Lanka in recent times. Source reduction and use of insecticides in space spraying/fogging and larviciding, are the primary means of controlling the vector mosquitoes Aedes aegypti and Ae. albopictus in the island nation. A study was carried out to understand insecticide cross-resistance spectra and mechanisms of insecticide resistance of both these vectors from six administrative districts, i.e. Kandy, Kurunegala, Puttalam, Gampaha, Ratnapura and Jaffna, of Sri Lanka. Efficacy of the recommended dosages of frequently used insecticides in space spraying and larviciding in dengue vector control programmes was also tested. Insecticide bioassay results revealed that, in general, both mosquito species were highly resistant to DDT but susceptible to propoxur and malathion except Jaffna Ae. aegypti population. Moderate resistance to malathion shown by Jaffna Ae. aegypti population correlated with esterase and malathion carboxylesterase activities of the population. High levels of acetylcholinesterase (AChE) insensitivity in the absence of malathion and propoxur resistance may be due to non-synaptic forms of AChE proteins. Moderate pyrethroid resistance in the absence of high monooxygenase levels indicated the possible involvement of 'kdr' type resistance mechanism in Sri Lankan dengue vectors. Results of the space spraying experiments revealed that 100% mortality at a 10 m distance and >50% mortality at a 50 m distance can be achieved with malathion, pesguard and deltacide even in a ground with dense vegetation. Pesguard and deltacide spraying gave 100% mortality up to 50 m distance in open area and areas with little vegetation. Both species gave >50% mortalities for deltacide at a distance of 75 m in a dense vegetation area. Larval bioassays conducted in the laboratory showed that a 1 ppm temephos solution can maintain a larval mortality rate of 100% for ten months, and the mortality rate declined to 0% in the eleventh month. In the field, where 1 ppm concentration is gradually decreased with water usage, 100% mortality was observed only for the first four months, <50% mortality for the next two months, and 0% mortality was observed eight months after the application of temephos. Deltacide can be effectively used for space spraying programmes in Sri Lanka. Larval control can be successfully achieved through temephos with public participation.

Insecticide susceptibility of field-collected populations of Culex tritaeniorhynchus in the Republic of Korea

The toxicities of 10 insecticides were examined against late third instar Culex tritaeniorhynchus Giles (Diptera: Culicidae) using the direct-contact mortality bioassay. Six geospatially-distant field mosquitoes were collected from Chuncheon-si, Hwaseong, Seosan. Jeonju, Daegu, and Busan in the Republic of Korea. Marked regional variations of insecticide susceptibility were observed. Field populations of Seosan, Jeonju, and Daegu from agricultural areas showed higher to extremely higher insecticide susceptibility to pyrethroids than those of Chuncheon-si, Hwaseong, and Busan strains from non-agricultural areas. Extremely high to low levels of susceptibility were measured: bifenthrin, susceptible ratio (SR) = 2.7-896.3; β-cyfluthrin, SR = 1.8-633.3; α-cypermethrin, SR = 1.2-1,051.9; deltamethrin, SR = 1.3-711.1; permethrin, SR = 1.5-1,053.4; etofenprox, SR = 2.2-29.3; chlorfenapyr, SR = 5.1-103.6; chlorpyrifos, SR = 2.3- 337.0; fenitrothion, SR = 2.0-142.3; and fenthion, SR = 1.4-186.2. Cx. tritaeniorhynchus populations from rice paddies had been under heavy selection pressure due to the agricultural insecticides, and that's why the mosquito species demonstrated high resistance to pyrethroids, which were used for a long time to control agricultural pests in the localities. These results indicate that careful selection and rotational use of these insecticides may result in continued satisfactory control against field populations of Japanese encephalitis vector mosquitoes.

Mechanisms of organophosphate resistance in a field population of oriental migratory locust, Locusta migratoria manilensis (Meyen)

The susceptibilities to three organophosphate (OP) insecticides (malathion, chlorpyrifos, and phoxim), responses to three metabolic synergists [triphenyl phosphate (TPP), piperonyl butoxide (PBO), and diethyl maleate (DEM)], activities of major detoxification enzymes [general esterases (ESTs), glutathione S-transferases (GSTs), and cytochrome P450 monooxygenases (P450s)], and sensitivity of the target enzyme acetylcholinesterase (AChE) were compared between a laboratory-susceptible strain (LS) and a field-resistant population (FR) of the oriental migratory locust, Locusta migratoria manilensis (Meyen). The FR was significantly resistant to malathion (57.5-fold), but marginally resistant to chlorpyrifos (5.4) and phoxim (2.9). The malathion resistance of the FR was significantly diminished by TPP (synergism ratio: 16.2) and DEM (3.3), but was unchanged by PBO. In contrast, none of these synergists significantly affected the toxicity of malathion in the LS. Biochemical studies indicated that EST and GST activities in the FR were 2.1- to 3.2-fold and 1.2- to 2.0-fold, respectively, higher than those in the LS, but there was no significant difference in P450 activity between the LS and FR. Furthermore, AChE from the FR showed 4.0-fold higher activity but was 3.2-, 2.2-, and 1.1-fold less sensitive to inhibition by malaoxon, chlorpyrifos-oxon, and phoxim, respectively, than that from the LS. All these results clearly indicated that the observed malathion resistance in the FR was conferred by multiple mechanisms, including increased detoxification by ESTs and GSTs, and increased activity and reduced sensitivity of AChE to OP inhibition.

Synergist efficacy of piperonyl butoxide with deltamethrin as pyrethroid insecticide on Culex tritaeniorhynchus (Diptera: Culicidae) and other mosquitoe species

Continuous and indiscriminate use of pesticides, especially in tropical countries for public health or agriculture purpose, has led many vector populations to become resistant to organochlorides, organophosphates, and even to carbamates and pyrethroids. Development of resistance by a vector population has been one of the reasons for the failure of the control measures in many countries. This investigation demonstrates the efficacy of piperonyl-butoxide (PBO) with deltamethrin, as pyrethroid insecticide, against the field-collected mosquitoe larvae of five species, Aedes aegypti, Anopheles culicifacies, An. stephensi, An. vagus, and Culex quinqufasciatus, and two morphological variants of Cx. tritaeniorhynchus (type A from grand pools of Mysore city and type B from rice fields of Mandya district). For testing the synergistic effect of PBO, stock solutions of deltamethrin and PBO were mixed in 1:6 ratio. The synergistic ratio and the percent suppression in deltamethrin tolerance were calculated by using LC(50) values. From the results, it is clear that, PBO is an effective synergist with deltamethrin against all of species undertaken in this investigation. So, it is suggested that PBO is a good synergist in this area for decreasing the use of pesticides in environment in vector control.

Past, present, and future of Japanese encephalitis

Japanese encephalitis (JE), a vector-borne viral disease, is endemic to large parts of Asia and the Pacific. An estimated 3 billion people are at risk, and JE has recently spread to new territories. Vaccination programs, increased living standards, and mechanization of agriculture are key factors in the decline in the incidence of this disease in Japan and South Korea. However, transmission of JE is likely to increase in Bangladesh, Cambodia, Indonesia, Laos, Myanmar, North Korea, and Pakistan because of population growth, intensified rice farming, pig rearing, and the lack of vaccination programs and surveillance. On a global scale, however, the incidence of JE may decline as a result of large-scale vaccination programs implemented in China and India.

Biocontrol of larval mosquitoes by Acilius sulcatus (Coleoptera: Dytiscidae)

Background

Problems associated with resistant mosquitoes and the effects on non-target species by chemicals, evoke a reason to find alternative methods to control mosquitoes, like the use of natural predators. In this regard, aquatic coleopterans have been explored less compared to other insect predators. In the present study, an evaluation of the role of the larvae of Acilius sulcatus Linnaeus 1758 (Coleoptera: Dytiscidae) as predator of mosquito immatures was made in the laboratory. Its efficacy under field condition was also determined to emphasize its potential as bio-control agent of mosquitoes.

Methods

In the laboratory, the predation potential of the larvae of A. sulcatus was assessed using the larvae of Culex quinquefasciatus Say 1823 (Diptera: Culicidae) as prey at varying predator and prey densities and available space. Under field conditions, the effectiveness of the larvae of A. sulcatus was evaluated through augmentative release in ten cemented tanks hosting immatures of different mosquito species at varying density. The dip density changes in the mosquito immatures were used as indicator for the effectiveness of A. sulcatus larvae.

Results

A single larva of A. sulcatus consumed on an average 34 IV instar larvae of Cx. quinquefasciatus in a 24 h period. It was observed that feeding rate of A. sulcatus did not differ between the light-on (6 a.m. – 6 p.m.), and dark (6 p.m. – 6 a.m.) phases, but decreased with the volume of water i.e., space availability. The prey consumption of the larvae of A. sulcatus differed significantly (P < 0.05) with different prey, predator and volume combinations, revealed through univariate ANOVA. The field study revealed a significant decrease (p < 0.05) in larval density of different species of mosquitoes after 30 days from the introduction of A. sulcatus larvae, while with the withdrawal, a significant increase (p < 0.05) in larval density was noted indicating the efficacy of A. sulcatus in regulating mosquito immatures. In the control tanks, mean larval density did not differ (p > 0.05) throughout the study period.

Conclusion

the larvae of the dytiscid beetle A. sulcatus proved to be an efficient predator of mosquito immatures and may be useful in biocontrol of medically important mosquitoes.

Biocontrol efficiency of odonate nymphs against larvae of the mosquito, Culex quinquefasciatus Say, 1823

An estimation of the predatory efficiency of the nymphs of five coexisting odonate species Aeshna flavifrons, Coenagrion kashmirum, Ischnura forcipata, Rhinocypha ignipennis and Sympetrum durum using the fourth-instar larvae of Culex quinquefasciatus as prey, was made under laboratory and semi-field conditions. The daily feeding rate varied among the odonate species, at laboratory conditions. The mean number of IV instars Cx. quinquefasciatus larvae killed per day, ranged between 14 and 64 (64 mosquito larvae for I. forcipata, 57 for A. flavifrons, 45 for R. ignipennis, 25 for S. durum and 14 for C. kashmirum). The prey consumption was linearly related to the number of predators and prey available but inversely related with space. It was also noted that the feeding rates varied significantly between dark and light conditions, in all the odonate species. The presence of nymphs in semi-field conditions significantly lowered the mosquito larval density in dipper samples after 15 days from the introduction, followed by a significant increase of larval mosquito density after 15 days from the withdrawal of the nymphs. The results of the present observations are suggestive of the use of odonate nymphs in temporary pools or larger habitats where they can be a potential biological resource in regulating the larval population of the vector and pest mosquitoes.

Effects of lambda-cyhalothrin on mosquito larvae and predatory aquatic insects

Agricultural insecticides can affect mosquito production in rice fields by controlling mosquitoes, disrupting biological control or contributing to selection of insecticide resistance. The duration of insecticidal activity of the pyrethroid lambda-cyhalothrin was quantified on predatory insects in rice fields and on three kinds of mosquito larva: a pyrethroid-susceptible strain of Culex tarsalis Coquillet, a pyrethroid-resistant strain of Cx pipiens L. (sensu lato) and non-resistant Cx pipiens s.l. Lambda-cyhalothrin killed most caged, susceptible mosquitoes for up to 21 days. It killed fewer resistant Cx pipiens s.l., but suppressed their survival for over a week. Lambda-cyhalothrin suppressed field populations of predatory insects through day 29. Agricultural use of lambda-cyhalothrin can provide incidental mosquito control. However, the pyrethroid persisted in sediment and gradually decreased in activity, which could contribute to selection of pyrethroid-resistant mosquitoes. Because caged mosquitoes showed good survival before predators recovered, disruption of biological control is possible. It is therefore advisable for growers and mosquito control agencies to communicate about pesticide use.

Spatiotemporal distribution of insecticide resistance in Anopheles culicifacies and Anopheles subpictus in Sri Lanka

The malaria situation in Sri Lanka worsened during the 1990s with the emergence and spread of resistance to the drugs and insecticides used for control. Chloroquine resistance has increased rapidly over this period, but adverse changes in malaria transmission are more closely associated with insecticide use rather than drug resistance. Insecticide susceptibility tests were routinely carried out in key anopheline vectors across the country for more than a decade. These sentinel data were combined with data collected by other research programmes and used to map the spatial and temporal trends of insecticide resistance in the main vectors, Anopheles culicifacies and A. subpictus, and to examine the relationship between insecticide resistance, changes in national spraying regimens and malaria prevalence. Both species had widespread resistance to malathion, the insecticide of choice in the early 1990s. Both species were initially susceptible to the organophosphate and pyrethroid insecticides used operationally from 1993, but some resistance has now been selected. The levels of malathion and fenitrothion resistance in A. subpictus were higher in some ecological regions than others, which may be related to the distribution of sibling species, agricultural pesticide exposure and/or environmental factors. The study highlights that the emergence and spread of insecticide resistance is a constant threat and that active surveillance systems are vital in identifying key vectors and evidence of resistance.

Chlorpyrifos resistance in mosquito Culex quinquefasciatus

Two mosquito strains of Culex quinquefasciatus Say, MAmCq and HAmCq, were collected from Mobile and Huntsville, AL, respectively, after the control of mosquitoes with insecticides proved difficult. A synergism study showed that resistance to chlorpyrifos in MAmCq and HAmCq was not suppressed by piperonyl butoxide (PBO) and S,S,S,-tributylphosphorotrithioate (DEF), suggesting that P450 monooxygenase- and hydrolase-mediated detoxication does not contribute to chlorpyrifos resistance in either strain. Diethyl maleate (DEM) did not cause any significant change in the level of chlorpyrifos toxicity to HAmCq. However, DEM enhanced toxicity of chlorpyrifos to MAmCq 2.5-fold, indicating that glutathione S-transferase (GST)-mediated detoxication may play a minor role in the resistance of MAmCq. An inhibition study of acetylcholinesterase (AChE) by chlorpyrifos showed that bimolecular rate constants (Ki) of chlorpyrifos for the inhibition of AChE in adults and larvae of the susceptible S-Lab strain were 2.2- and 1.9-fold higher, respectively, than in the HAmCq strain and 3.4- and 3.8-fold higher than in the MAmCq strain. The single mutation, G119S, resulting from a single nucleotide polymorphism (SNP), G to A, in ace-1 acetylcholinesterase gene was present in HAmCq and MAmCq mosquitoes. The frequency of the heterozygote for the G119S mutant allele in the HAmCq and MAmCq mosquito populations was 0.25 and 0.45, respectively, and no individuals in either of these mosquito strains were homozygous for the A allele. It thus seems likely that the presence of heterozygous individuals for the G119S allele in HAmCq and MAmCq populations may be a response to the insensitivity of AChE observed in these two mosquito strains.

Molecular and biochemical characterization of a sand fly population from Sri Lanka: evidence for insecticide resistance due to altered esterases and insensitive acetylcholinesterase

With an increasing incidence of cutaneous leishmaniasis in Sri Lanka, particularly in northern provinces, insecticide-mediated vector control is under consideration. Optimizing such a strategy requires the characterization of sand fly populations in target areas with regard to species composition and extant resistance, among other parameters. Sand flies were collected by human bait and cattle-baited net traps on Delft Island, used as an illegal transit location by many refugees returning to the north of Sri Lanka from southern India where leishmaniasis is endemic. For species identification, genomic DNA was extracted and a fragment of the ribosomal 18S gene amplified. The sequence from all flies analysed matched that of Phlebotomus argentipes Annandale & Brunetti, the primary vector in India and the most likely vector in Sri Lanka. Independent morphological analysis also identified P. argentipes. To establish the current susceptibility status of vector species, data were obtained at the biochemical level, from which potential cross-resistance to alternative insecticides can be predicted. The Delft Island collection was assayed for the activities of four enzyme systems involved in insecticide resistance (acetylcholinesterase, non-specific carboxylesterases, glutathione-S-transferases and cytochrome p450 monooxygenases), establishing baselines against which subsequent collections can be evaluated. There was preliminary evidence for elevated esterases and altered acetylcholinesterase in this population, the first report of these resistance mechanisms in sand flies to our knowledge, which probably arose from the malathion-based spraying regimes of the Anti-Malarial Campaign.

Temephos-resistant larvae of Simulium sanctipauli associated with a distinctive new chromosome inversion in untreated rivers of south-western Ghana

Larvae of the Simulium damnosum Theobald complex (Diptera: Simuliidae) were sampled in June 1996 from two sites in south-west Ghana where larviciding has not been applied: Sutri Rapids on the Tano river (05 degrees 23 minutes N 02 degrees 38 minures W) and Sekyere-Heman on the Pra river (05 degrees 11 minutes N 01 degrees 35 minutes W). All specimens were identified as Simulium sanctipauli Vajime & Dunbar sensu stricto (Diptera: Simuliidae). Bioassays with temephos (organophosphorus larvicide employed by the Onchocerciasis Programme for systematic treatment of most rivers across West Africa since the 1970s) showed about five-fold resistance in the Tano population (LC95 2.37-3.14 mg/L) and slight tolerance to temephos in the Pra population (LC95 0.67-0.76 mg/L), vs. the diagnostic concentration of 0.625 mg/L. Larval salivary polytene chromosomes of S. sanctipauli showed fixed inversions 1S-24/24, standard IIL-6 and a new inversion IL/36 polymorphism at Sutri on the Tano. These karyotype characteristics differ from those of temephos-resistant S. sanctipauli in rivers of C te d'Ivoire and other sites on the Tano in Ghana. Thus, temephos resistance in S. sanctipauli at Sutri is associated with distinct chromosomal configurations, showing that immigration was unlikely. This resistance could have been locally selected by exposure of S. sanctipauli larval populations to agrochemicals run-off from cocoa, coffee and oil plantations flanking the rivers.

Effects of organic and inorganic fertilisers on mosquito populations in rice fields of southern India

The effects of nitrogenous (inorganic) fertilisers, organic manures and blue-green algae (BGA) biofertiliser on mosquito populations (Diptera: Culicidae) were studied in rice fields of Madurai, Tamil Nadu, south India, with particular attention to Culex vishnui Theobald, Cx. pseudovishnui Colless and Cx. tritaeniorhynchus Giles, the vectors of Japanese encephalitis (JE). The application of urea, a nitrogenous fertiliser, in rice fields significantly increased the grain yield and the population densities of mosquito larvae and pupae (anophelines as well as culicines) in a dose-related manner. Fields treated with inorganic fertilisers (N, P, K) had significantly higher population densities of mosquito immatures than fields treated with organic manures (farmyard manure and green manure). Without nitrogenous fertiliser, BGA increased paddy yield without enhancing mosquito production. Therefore, the use of BGA with less nitrogenous fertiliser is recommended, which is beneficial economically and agronomically to the farming community and also significantly reduces mosquito production in rice fields. Increased use of nitrogenous fertiliser over the past two decades may have contributed to the increased severity of Japanese encephalitis epidemics, vectors of which breed in rice fields.