Insecticide resistance and detoxifying enzyme activity in the principal bancroftian filariasis vector, Culex quinquefasciatus, in northeastern India

Insights into insecticide-resistance mechanisms in invasive species: Challenges and control strategies

Threatening the global community is a wide variety of potential threats, most notably invasive pest species. Invasive pest species are non-native organisms that humans have either accidentally or intentionally spread to new regions. One of the most effective and first lines of control strategies for controlling pests is the application of insecticides. These toxic chemicals are employed to get rid of pests, but they pose great risks to people, animals, and plants. Pesticides are heavily used in managing invasive pests in the current era. Due to the overuse of synthetic chemicals, numerous invasive species have already developed resistance. The resistance development is the main reason for the failure to manage the invasive species. Developing pesticide resistance management techniques necessitates a thorough understanding of the mechanisms through which insects acquire insecticide resistance. Insects use a variety of behavioral, biochemical, physiological, genetic, and metabolic methods to deal with toxic chemicals, which can lead to resistance through continuous overexpression of detoxifying enzymes. An overabundance of enzymes causes metabolic resistance, detoxifying pesticides and rendering them ineffective against pests. A key factor in the development of metabolic resistance is the amplification of certain metabolic enzymes, specifically esterases, Glutathione S-transferase, Cytochromes p450 monooxygenase, and hydrolyses. Additionally, insect guts offer unique habitats for microbial colonization, and gut bacteria may serve their hosts a variety of useful services. Most importantly, the detoxification of insecticides leads to resistance development. The complete knowledge of invasive pest species and their mechanisms of resistance development could be very helpful in coping with the challenges and effectively developing effective strategies for the control of invasive species. Integrated Pest Management is particularly effective at lowering the risk of chemical and environmental contaminants and the resulting health issues, and it may also offer the most effective ways to control insect pests.

Transcriptome analysis reveals differential effects of beta-cypermethrin and fipronil insecticides on detoxification mechanisms in Solenopsis invicta

Insecticide resistance poses many challenges in insect pest control, particularly in the control of destructive pests such as red imported fire ants (Solenopsis invicta). In recent years, beta-cypermethrin and fipronil have been extensively used to manage invasive ants, but their effects on resistance development in S. invicta are still unknown. To investigate resistance development, S. invicta was collected from populations in five different cities in Guangdong, China. The results showed 105.71- and 2.98-fold higher resistance against fipronil and beta-cypermethrin, respectively, in the Guangzhou population. The enzymatic activities of acetylcholinesterase, carboxylases, and glutathione S-transferases significantly increased with increasing beta-cypermethrin and fipronil concentrations. Transcriptomic analysis revealed 117 differentially expressed genes (DEGs) in the BC-ck vs. BC-30 treatments (39 upregulated and 78 downregulated), 109 DEGs in F-ck vs. F-30 (33 upregulated and 76 downregulated), and 499 DEGs in BC-30 vs. F-30 (312 upregulated and 187 downregulated). Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that DEGs associated with insecticide resistance were significantly enriched in metabolic pathways, the AMPK signaling pathway, the insulin signaling pathway, carbon metabolism, peroxisomes, fatty acid metabolism, drug metabolism enzymes and the metabolism of xenobiotics by cytochrome P450. Furthermore, we found that DEGs important for insecticide detoxification pathways were differentially regulated under both insecticide treatments in S. invicta. Comprehensive transcriptomic data confirmed that detoxification enzymes play a significant role in insecticide detoxification and resistance development in S. invicta in Guangdong Province. Numerous identified insecticide-related genes, GO terms, and KEGG pathways indicated the resistance of S. invicta workers to both insecticides. Importantly, this transcriptome profile variability serves as a starting point for future research on insecticide risk evaluation and the molecular mechanism of insecticide detoxification in invasive red imported fire ants.

Assessing pyrethroid resistance status in the Culex pipiens complex (Diptera: Culicidae) from the northwest suburbs of Chicago, Illinois using Cox regression of bottle bioassays and other detection tools

Culex pipiens complex is an important vector of epizootic and zoonotic pathogens, including West Nile virus. Chicago, Illinois and its suburbs have suffered high incidence of human West Nile virus infections in the past. This makes abatement programs in and around the Chicago area an essential service. The control of Cx. pipiens is often complicated by rapidly evolving resistance to pyrethroids, which are the most widely used chemical class in US mosquito abatement programs. The present study assessed Sumithrin® resistance in Cx. pipiens collected from five locations around Cook County, Illinois, neighboring the city limits of Chicago. According to CDC guidelines, samples from all five locations demonstrated some resistance to Sumithrin®. When assessed with Anvil®, a formulated product made of Sumithrin® synergized with piperonyl butoxide, susceptibility was rescued in mosquitoes from three out of the five locations, suggesting involvement of mixed-function oxidases and/or carboxylesterases in Sumithrin® resistance at these locations. Not all locations had susceptibility rescued by Anvil®, but these locations had relatively low knockdown resistance allele frequencies, suggesting that mechanisms other than knockdown resistance may be involved. Enzyme activities did not reveal any marked trends that could be related back to mortality in the bottle bioassays, which highlights the need for multiple types of assays to infer enzymatic involvement in resistance. Future directions in pyrethroid resistance management in Chicago area Cx. pipiens are discussed.

Susceptibility to Pyrethroids and the First Report of L1014F kdr Mutation in Culex quinquefasciatus (Diptera: Culicidae) in Colombia

The use of insecticides for the control of Aedes aegypti (L.) (Diptera: Culicidae) in Colombia has indirectly influenced the susceptibility status of Culex quinquefasciatus Say populations. We evaluated pyrethroid susceptibility in two populations of Cx. quinquefasciatus in the Atlantico Department of Colombia and its possible resistance mechanism (kdr mutation). Bottle bioassays were performed for permethrin, deltamethrin, and λ-cyhalothrin in female mosquitoes of Cx. quinquefasciatus. The resistance ratios (RRs) for KC50 and LC50 for each insecticide in the field populations examined were determined, using the Cartagena strain as the susceptible control. The L1014F kdr mutation was identified in the para gene of the voltage-gated sodium channel (vgsc), along with its allelic and genotypic frequency. Low knockdown resistance (RRKC50) to deltamethrin was found in Puerto Colombia and Soledad populations as well as low resistance to λ-cyalothrin in this latter population. Moderate knockdown resistance to permethrin was found in both populations. At 24 h post-exposure on the other hand, there was low resistance (RRLC50) to permethrin in Puerto Colombia and moderate resistance in Soledad. Moderate resistance to deltamethrin was found in Puerto Colombia and low resistance in Soledad. Low resistance to λ-cyhalothrin was seen in Puerto Colombia and moderate resistance in Soledad. Variability was found in the susceptibility to the pyrethroids in the populations of Cx. quinquefasciatus evaluated, and the L1014F kdr mutation is reported for the first time as a possible pyrethroid resistance mechanism in this species in Colombia.

Assessment the Changing Trend of Susceptibility to Two Insecticides among Field-Population Culex quinquefasciatus Compared with the Same Population Undergoing to Multiple Colonization

BACKGROUND

During the past decade, rapid development of insecticide resistance have been reported among many species of mosquito vectors against four main categories of insecticides worldwide. The aim of the research was to assess the variation trend of susceptibility levels of Culex quinquefasciatus to two insecticides separately for the field population compared with subsequent generations of the same sample after multiple colonization.

METHODS

Larvae and pupae of Cx. quinquefasciatus were collected from house sewages and reared to adult which blood-fed on roosters. Ten percent sucrose fed female mosquitoes aged 2-3 days were used for susceptibility tests with DDT and deltamethrin. Susceptibility levels was assessed in the adult stage of field stran Cx. quinquefasciatus against DDT 4.0% and deltamethrin 0.05% and continued up to next six generations undergoing multiple rearing at insectary condition.

RESULTS

The susceptibility levels to DDT 4.0% did not change compared to the field with the lab population to six generations. Regarding deltamethrin 0.05%, no significant difference was shown between field strain (58.3%) and 3rd generation (52.7%) compared to the 6th one (33.8%).

CONCLUSION

This finding may reflect the role of the kdr gene in resistance to organochlorine which has cross-resistance with pyrethroid insecticides. The results of this study clearly showed the irreversible trend of pyrethroid resistance among colonized mosquitoes. This is the first study of the resistance status of Cx. quinquefasciatus in Iran.

Pyrethroids Use: Threats on Metabolic-Mediated Resistance Mechanisms in the Primary Dengue Vector Aedes aegypti (Diptera: Culicidae)

The emergence of pyrethroid resistance in Aedes aegypti (L.) has limited the success of vector control. Early detection of resistance could assist authorities in deciding well-suited control strategies to minimize operational failures of Ae. aegypti control. Herein, biochemical analysis was performed to investigate the mechanisms involved in pyrethroid resistance in nine populations of Indonesian Ae. aegypti. Enzymes of adult Ae. aegypti such as esterases (ESTs), glutathione-S-transferases (GSTs), and mixed-function oxidases (MFOs) were characterized. Elevated MFO activity was correlated with resistance phenotype, indicating the role of this enzyme in contributing to pyrethroid resistance. No significant correlations were shown between pyrethroid resistance phenotype and α-ESTs, suggesting that marginally exceeded enzyme levels relative to the reference strain in some pyrethroid-susceptible populations were causative factor for insecticide resistance in other groups of insecticides. However, significant correlation was demonstrated between β-ESTs and pyrethroid resistance phenotype. The lowest enzyme levels in GSTs indicated that this enzyme was not predominant in causing pyrethroid resistance, despite the presence of significant correlations. Because metabolic detoxification fails to comprehensively explain the pyrethroid resistance in some Indonesian Ae. aegypti, additional mechanisms such as altered target sites in voltage-gated sodium channel may also contribute to the high pyrethroid resistance in Ae. aegypti.

Insecticide susceptibility status of invasive Aedes albopictus across dengue endemic districts of Odisha, India

BACKGROUND

Aedes albopictus is currently the most invasive mosquito species in the world. Keeping in view the wide emergence of insecticide resistance, it is imperative to focus on the current susceptibility status for various insecticides in Ae. albopictus. This study focused on understanding the insecticide resistance mechanism of Ae. albopictus collected from dengue-endemic districts of Odisha.

RESULTS

Insecticide resistance was evaluated by using standardized bioassay kits (WHO) and biochemical analysis. Larval bioassays revealed the highest level of resistance from Jaipur (JP) population with a RR₅₀ of 15.3 and LC₅₀ of 1.177 ppm compared with an LC₅₀ of 0.077 for the susceptible strain LabS. Results indicated the presence of dichlorodiphenyltrichloroethane resistance in the majority of adult populations. Elevated activity of nonspecific esterases and cytochrome P450s MFO indicated probable resistance to organophosphates and pyrethroids. Molecular screening for common insecticide target-site mutations confirmed the absence of the 'knockdown resistance' response for pyrethroid insecticide in Ae. albopictus population, suggesting its continual effectiveness as the major insecticide of significant importance in future vector-control programmes.

CONCLUSION

This is the first report of a kdr mutation in Ae. albopictus in India and highlights the need for intensive research on other unexplored target-site mutations that might also contribute to pyrethroid resistance. Effective management and sustainable use of insecticides can be implemented by understanding resistance mechanisms and development of appropriate diagnostic tools. © 2017 Society of Chemical Industry.

Comparative Bio-Efficacy and Synergism of New Generation Polyfluorobenzyl and Conventional Pyrethroids Against Culex quinquefasciatus (Diptera: Culicidae)

Intensive exposure to insecticides has resulted in the evolution of insecticide resistance in the mosquitoes. We tested the bio-efficacy of two Culex quinquefasciatus Say (Diptera: Culicidae) laboratory strains differentially bio-responsive to pyrethroids to understand the comparative efficacy of different polyfluorobenzyle and conventional pyrethroid molecules and the role of piperonyl butoxide (PBO) in synergizing these molecules in increased tolerance of mosquitoes to these molecules. We have taken deltamethrin (α-cyano pyrethroid with phenoxybenzyl moiety); permethrin (phenoxybenzyl pyrethroid without an α-cyano group); transfluthrin, dimefluthrin, metofluthrin, and meperfluthrin (polyfluorinated benzyl compounds); and prallethrin (modified cyclopentadienone compound) for this study. We found higher bio-efficacy in dimefluthrin, metofluthrin, and meperfluthrin compared with transfluthrin against tested mosquito strains. We found that transfluthrin exhibited synergism with PBO, which supports the hypothesis that P450 enzymes could play a role in the detoxification process of transfluthrin, which was earlier not believed. However, other polyfluorobenzyl pyrethroids with a 4-(methoxymethyl) phenyl capping in the tetrafluorobenzyl ring (dimefluthrin, metofluthrin, and meperfluthrin) exhibit greater synergism with PBO compared with transfluthrin. Further study is required to understand the mechanism for higher synergistic ratios in polyfluorobenzyl pyrethroids with 4-(methoxymethyl) phenyl moiety and ascertain the possible involvement of novel mechanisms that may involve in developing resistance. This is the first report of comparative bio-efficacy of multiple polyfluorobenzyl pyrethroids and PBO synergism against mosquitoes.

Larval Age and Nutrition Affect the Susceptibility of Culex quinquefasciatus (Diptera: Culicidae) to Temephos

Larval age and nutrition significantly affected the insect's physiology. These influences are important when rearing a population of vectors that is used to monitor the resistance level, in which standardized conditions are crucial for a more harmonized result. Little information has been reported on the effects of larval age and nutrition on the susceptibility of insects to insecticides, and therefore, we studied the effects on the susceptibility of Culex quinquefasciatus Say's (Diptera: Culicidae) larvae to temephos by comparing the median lethal concentration (LC50) after 24 hr between the second and fourth instar larvae and between the larvae that fed on protein-based and carbohydrate-based larval diets. The susceptibility of the larvae was significantly affected by the larval diets, as the larvae that fed on protein-based beef food and milk food demonstrated significantly higher LC50 value compared with the larvae that fed on carbohydrate-based food: lab food and yeast food. The larval diet interacted significantly with the larval age: while the second instar larvae were susceptible to temephos when supplied with carbohydrate-based food, the second and fourth instar larvae had no significant effect when supplied with protein-based diets, implying that a protein-rich environment may cause the mosquito to be less susceptible to temephos. This study suggested the importance of standardizing nutrition when rearing a vector population in order to obtain more harmonized dosage-response results in an insecticide resistance monitoring program. Future research could focus on the biochemical mechanism between the nutrition and the enzymatic activities of the vector.

Pharmacological potential of Bidens pilosa L. and determination of bioactive compounds using UHPLC-QqQLIT-MS/MS and GC/MS

Background

Research of natural products from traditionally used medicinal plants to fight against the human ailments is fetching attention of researchers worldwide. Bidens pilosa Linn. var. Radiata (Asteraceae) is well known for its folkloric medicinal use against various diseases from many decades. Mizoram, North East India, has high plant diversity and the use of this plant as herbal medicine is deep rooted in the local tribes. The present study was executed to understand the pharmacological potential of B. pilosa leaves extract.

Methods

The antimicrobial potential was determined using agar well diffusion and broth microdilution method against bacterial and yeast pathogens. Cytotoxicity was evaluated using MTT and apoptotic DNA fragmentation assays. Further, the antioxidant ability of the extract was analysed using DPPH and ABTS free radical scavenging assay. Mosquitocidal activity was evaluated against third in-star larvae of C. quinquefasciatus using dose response and time response larvicidal bioassay. Additionally, the major phenolic and volatile compounds were determined using UHPLC-QqQ_LIT-MS/MS and GC/MS respectively.

Results

We found that the extract showed highest antimicrobial activity against E. coli (MIC 80 μg/mL and IC₅₀ 110.04 μg/mL) and showed significant cytotoxicity against human epidermoid carcinoma (KB-3-1) cells with IC₅₀ values of 99.56 μg/mL among the tested cancer cell lines.

The IC₅₀ values for scavenging DPPH and ABTS was 80.45 μg/mL and 171.6 μg/mL respectively. The extract also showed the high phenolics (72 μg GAE/mg extract) and flavonoids (123.3 μg Quercetin /mg extract). Lastly, five bioactive and six volatile compounds were detected using UHPLC-QqQ_LIT-MS/MS and GC-MS respectively which may be responsible for the plant’s bioactivities. An anticancerous compound, Paclitaxel was detected and quantified for the first time from B. pilosa leaves extract, which further showed the anticancerous potential of the tested extract.

Conclusion

On the basis of the present investigation, we propose that the leaf extract of B. pilosa might be a good candidate for the search of efficient environment friendly natural bioactive agent and pharmaceutically important compounds.

Insecticides resistance in the Culex quinquefasciatus populations from northern Thailand and possible resistance mechanisms

The mosquito vector Culex quinquefasciatus is known to be resistant to insecticides worldwide, including Thailand. This study was the first investigation of the insecticide resistance mechanisms, involving metabolic detoxification and target site insensitivity in C. quinquefasciatus from Thailand. Adult females reared from field-caught larvae from six provinces of northern Thailand were determined for resistant status by exposing to 0.05% deltamethrin, 0.75% permethrin and 5% malathion papers using the standard WHO susceptibility test. The overall mortality rates were 45.8%, 11.4% and 80.2%, respectively. A fragment of voltage-gated sodium channel gene was amplified and sequenced to identify the knock down resistance (kdr) mutation. The ace-1 gene mutation was determined by using PCR-RFLP. The L1014F kdr mutation was observed in all populations, but the homozygous mutant F/F1014 genotype was found only in two of the six provinces where the kdr mutation was significantly correlated with deltamethrin resistance. However, none of mosquitoes had the G119S mutation in the ace-1 gene. A laboratory deltamethrin resistant strain, Cq_CM_R, has been established showing a highly resistant level after selection for a few generations. The mutant F1014 allele frequency was significantly increased after one generation of selection. A synergist assay was performed to assess the metabolic detoxifying enzymes. Addition of bis(4-nitrophenyl)-phosphate (BNPP) and diethyl maleate (DEM), inhibitors of esterases and glutathione S-transferases (GST), respectively, into the larval bioassay of the Cq_CM strain with deltamethrin showed no significant reduction. By contrast, addition of piperonyl butoxide (PBO), an inhibitor of cytochrome P450 monooxygenases, showed a 9-fold reduction of resistance. Resistance to pyrethroids in C. quinquefasciatus is widely distributed in northern Thailand. This study reports for the first time for the detection of the L1014F kdr mutation in wild populations of C. quinquefasciatus in Thailand. At least two major mechanisms, kdr and cytochrome P450 monooxygenases, confer resistance to deltamethrin in Thai C. quinquefasciatus populations.

Susceptibility status of Culex quinquefasciatus (Diptera: Culicidae) populations to the chemical insecticide temephos in Pernambuco, Brazil

BACKGROUND

Culex quinquefasciatus is the vector of many agents of human diseases, including Wuchereria bancrofti, the parasite that causes bancroftian filariasis, an endemic disease in Pernambuco State, Brazil. Although temephos is not currently used to control C. quinquefasciatus, the species might be under a selection process from incidental exposure to this compound. This study aimed to evaluate the susceptibility status of C. quinquefasciatus to temephos, using bioassays, and to investigate its putative resistance mechanisms through biochemical assays and screening of the G119S mutation in the acetylcholinesterase gene, which is associated with organophosphate resistance, carried out by PCR and sequencing.

RESULTS

The results showed that only mosquitoes from Santa Cruz do Capibaribe (SC) had an alteration in their susceptibility status (RR = 7.2-fold), while the other populations were all susceptible to the insecticide. Biochemical assays showed increased activity for all esterases in SC, as well as evidence of acetylcholinesterase insensitivity. The G119S mutation was detected in this population with a frequency of 0.11, but it was not found in the remaining populations.

CONCLUSION

These data show that mechanisms of temephos resistance have been selected in natural C. quinquefasciatus populations from Pernambuco, which could undermine future control actions.

Enzymatic characterization of insecticide resistance mechanisms in field populations of Malaysian Culex quinquefasciatus say (Diptera: Culicidae)

Background

There has been no comprehensive study on biochemical characterization of insecticide resistance mechanisms in field populations of Malaysian Culex quinquefasciatus. To fill this void in the literature, a nationwide investigation was performed to quantify the enzyme activities, thereby attempting to characterize the potential resistance mechanisms in Cx. quinquefasciatus in residential areas in Malaysia.

Methodology/Principal Findings

Culex quinquefasciatus from 14 residential areas across 13 states and one federal territory were subjected to esterases, mixed function oxidases, glutathione-S-transferase and insensitive acetylcholinesterase assays. Enzyme assays revealed that α-esterases and β-esterases were elevated in 13 populations and 12 populations, respectively. Nine populations demonstrated elevated levels of mixed function oxidases and glutathione-S-transferase. Acetylcholinesterase was insensitive to propoxur in all 14 populations. Activity of α-esterases associated with malathion resistance was found in the present study. In addition, an association between the activity of α-esterases and β-esterases was also demonstrated.

Conclusions/Significance

The present study has characterized the potential biochemical mechanisms in contributing towards insecticide resistance in Cx. quinquefasciatus field populations in Malaysia. Identification of mechanisms underlying the insecticide resistance will be beneficial in developing effective mosquito control programs in Malaysia.

Insecticide susceptibility status in three medically important species of mosquitoes, Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus, from Bruhat Bengaluru Mahanagara Palike, Karnataka, India

BACKGROUND

Development of resistance among mosquito vectors of diseases is of growing concern in many countries of the subtropics, including much of India. The distributions of susceptibility/resistance to insecticides are scarcely recorded and are unknown in many parts of India. This manuscript reports the susceptibility status among strains of Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus collected from different geographical areas of Bruhat Bengaluru Mahanagara Palike (BBMP), Karnataka, to six conventionally used insecticides.

RESULTS

An. stephensi, Ae. aegypti and Cx. quinquefasciatus from different geographic areas showed susceptibility towards temephos. However, susceptibility was meagre for most of the other insecticides used in the study. Strong resistance to propoxur was reported in Ullala (RR(90) = 467.1) and Bannerghatta (RR(90) = 12,265) populations of An. stephensi and Ae. aegypti respectively. RR(90) ≥ 12,830 was recorded as higher in the Yelahanka (YLK) population of Cx. quinquefasciatus to neem. Significant differences at LC(50) and LC(90) concentrations were observed against various insecticides in different mosquito populations. Resistance was observed against some of the larvicides for the diagnostic dose recommended by WHO.

CONCLUSION

The results form baseline data on the susceptibility status of three medically important mosquito vectors of diseases to commonly used insecticides in the field at present emphasise the importance of carrying out more effective and planned vector control measures.

Multiple insecticide resistance/susceptibility status of Culex quinquefasciatus, principal vector of bancroftian filariasis from filaria endemic areas of northern India

OBJECTIVE

To understand the insecticide resistance status of Culex quinquefasciatus Say (Diptera: Culicidae) (Cx. Quinquefasciatus) to deltamethrin, cyfluthrin, permethrin, lambdacyhalothrin, DDT and malathion in filarial endemic areas of Uttar Pradesh, India.

METHODS

Insecticide susceptibility assays were performed on wild-caught adult female Cx. quinquefasciatus mosquitoes to deltamethrin (0.05%), cyfluthrin (0.15%), permethrin (0.75%), lambdacyhalothrin (0.05%), malathion (5.0%) and DDT (4.0%), the discriminating doses recommended by the World Health Organisation (WHO).

RESULTS

The data showed that Cx. quinquefasciatus is highly resistant to DDT and malathion; the mortality was 28.33% and 27.5%, respectively and incipient resistance to synthetic pyrethroids (deltamethrin, cyfluthrin, permethrin, and lambdacyhalothrin), where mortality ranged from 95.83% in permethrin to 98.33% in cyfluthrin and lambdacyhalothrin. Knockdown times (KDT(50)) in response to synthetic pyrethroids varied significantly between different insecticides (P<0.01) from 31.480 min for permethrin to 21.650 for cyfluthrin.

CONCLUSIONS

The results presents here provide the status report of the insecticide resistance/susceptibility of Cx. quinquefasciatus in major filaria endemic areas of northern India.

Field and Laboratory Evaluation of Bioefficacy of an Insect Growth Regulator (Dimilin) as a Larvicide against Mosquito and Housefly Larvae

The inhibitory function of Dimilin (Diflubenzuron), mostly a chitin synthesis regulator, on the ecdysis of mosquitoes (Anopheles gambiae s.l., Culex quinquefasciatus) and housefly was evaluated in the field and in laboratory. Three formulations of Diflubenzuron were evaluated in this study: Dimilin, Wettable powder (25%), Dimilin granules (2%), and Dimilin tablets (2%). The laboratory and field evaluation used different rates of concentrations of these formulations. Generally, at higher dosages larvae developments, eggs hatchability and pupation were impossible. The development of mosquitoes was significantly higher in control while highly depressed in different dosages of treatment in both laboratory and field experiments. In houseflies, the adult population decreased sharply after treatment of their breeding sites while pupae mortality was noticed to be high in laboratory-treated samples. Dimilin could be opted as one of the choice of the larval control chemicals to be incorporated in the integrated vector control programmes in urban and rural areas.

Evaluation of time toxicity, residual effect, and growth-inhibiting property of Carapa guianensis and Copaifera sp. in Aedes aegypti

Oils of Carapa guianensis and Copaifera spp. are well-known in the Amazonian region as natural insect repellents, and studies have reported their efficiency as larvicide against some mosquito species. However, toxicity persistence and effect on mosquito development have not yet been evaluated. Thus, the objective of this study was to evaluate the initial time of larvicidal activity, residual effect, and the effect of very low concentrations of these oils on Aedes aegypti. Different concentrations of the oils were used to evaluate the initial time of larval mortality and residual effect, as well as, the development of larvae, pupae, and adults. Results demonstrated that the lethal effect started mainly between the first 2 and 3 h of larvae exposure to oils, when using concentrations which ranged from 500 mg/L of C. guianensis and 90 mg/L of Copaifera sp. The toxic effect remained with total efficiency (100% mortality) until the sixth day for Copaifera sp. and 12th day for C. guianensis. When using sublethal dosages (ranging from 140 mg/L of C. guianensis to 26 mg/L of Copaifera sp.) mortality was observed after the larval molt. Also, imperfection of pupae and adult development and unsuccessful emergence of adults were observed. A product of botanical origin that could break the development of immature stage of mosquitoes and inhibit the emergence of adults should be essential in vector control. Thus, our results provide new information for a better understanding in using C. guianensis and Copaifera sp. oils with a potential to be used as a natural insecticide.

Evaluation of the pyrrole insecticide chlorfenapyr for the control of Culex quinquefasciatus Say

Culex quinquefasciatus Say (Diptera: Culicidae) is a widely distributed mosquito vector species in India and also in other tropical regions of the world. This species is implicated in the transmission of lymphatic filariasis in many countries. This species is reported to be widely resistant to insecticides of different classes in current use. In the present study, bio-efficacy of chlorfenapyr, an insecticide of pyrrole class with a novel mode of action was tested for the control of Cx. quinquefasciatus. Studies were performed to determine the diagnostic dosage; residual efficacy on different artificially fabricated substrates, namely wood, mud, mud+lime, cement and cement+distemper; to assess cross-resistance with different insecticides; and synergism/antagonism using piperonyl butoxide (PBO). A dosage of 5.0% chlorfenapyr was determined as diagnostic dosage with 2 h exposure and 48 h holding period for assessing the susceptibility of mosquitoes. The residual efficacy was observed up to 34 weeks on wood and mud+lime substrates while on other substrates, it was about 15 weeks at a dosage of 400mg a.i./m(2). Laboratory-reared strains of Cx. quinquefasciatus showed cross-resistance, whereas field-collected mosquitoes showed absence of cross-resistance to chlorfenapyr. Potentiation bioassays showed antagonistic effect of PBO to chlorfenapyr toxicity owing to the involvement of oxidases in the initial step of a conversion of pro-insecticide chlorfenapyr to toxic form CL 303268. The present study results have shown that chlorfenapyr can be a potential insecticide for the control of multiple insecticide resistant strains of Cx. quinquefasciatus. However, in countries where indoor residual spray (IRS) is not targeted for the control of this species, like in India, chlorfenapyr used in IRS for the control of malaria vectors in rural and peri-urban areas can additionally provide control of Cx. quinquefasciatus also.

High-throughput approach to detection of knockdown resistance (kdr) mutation in mosquitoes, Culex quinquefasciatus, based on real-time PCR using single-labelled hybridisation probe/melting curve analysis

BACKGROUND

Knockdown resistance (kdr) mutation (L1014F) is a well-defined mechanism of resistance to pyrethroids and DDT in many insect species. Sensitive detection of the mutations associated with resistance is a prerequisite for resistance management strategies. The authors have developed a new real-time molecular diagnostic assay based on SimpleProbe(®)/melting curve analysis for large-scale kdr genotyping in the wild population of Culex quinquefasciatus Say, the principal vector of bancroftian filariasis. Melting curve analysis is based on the thermal stability difference between matched and mismatched DNA duplexes. The application of SimpleProbe(®) chemistry in insects described here is novel in entomology research.

RESULTS

The mosquitoes homozygous for knockdown-resistant and knockdown-susceptible allele showed melting peaks at 60.45 °C (±0.25) and 64.09 °C (±0.24) respectively. The heterozygous mosquitoes yielded both peaks at approximately 60.5 °C (±0.2) and 64.20 °C (±0.23). Among the 92 samples genotyped, 16 were found to be homozygous resistant, 44 homozygous susceptible and 32 heterozygous. Comparative assessments were made of all the reported methods for kdr genotyping.

CONCLUSION

The present method is cheaper, faster, more reliable and versatile than other alternatives proposed in detecting correct kdr genotypes in mosquitoes. This is the first report using a single-labelled hybridisation probe to detect point mutations in insect populations.

Molecular analysis of knock down resistance (kdr) mutation and distribution of kdr genotypes in a wild population of Culex quinquefasciatus from India

OBJECTIVE

To investigate the presence of knock down resistance (kdr) mutation, its frequency distribution in the principal vector of bancroftian filariasis, Culex quinquefasciatus from northeastern India, and to relate kdr genotypes with susceptibility and/or resistance to DDT and deltamethrin in this vectors.

METHODS

Adult female mosquitoes were collected by aspiration from human dwellings in two villages, Benganajuli and Rikamari, and two military establishments, Field Units I and II. Insecticide susceptibility tests were performed following WHO methods with 4% DDT and 0.05% deltamethrin. Molecular identification of kdr mutation and genotyping of kdr locus was performed by allele-specific PCR (AS-PCR) and direct sequencing in a subset of samples.

RESULTS

Mosquitoes were resistant to DDT and showed 11.9-41.2% mortality, whereas the knock down bioassay for deltamethrin suggests complete susceptibility to this insecticide in all study sites except Benganajuli. The result of AS-PCR confirmed the presence of three genotypes: susceptible (SS), resistant (RR) and heterozygous (SR) in the population. Genotype frequencies at kdr locus for DDT-resistant individuals conformed with the Hardy-Weinberg proportion, whereas DDT and deltamethrin susceptible individuals differed significantly (P < 0.05). The efficacy of AS-PCR in detecting the correct genotype was not encouraging.

CONCLUSIONS

This is the first report from India on kdr genotyping in C. quinquefasciatus, and it confirms the occurrence of kdr allele in this vector in northeastern India. This finding has serious implications for the filariasis control programmes in India.