Insecticide resistance in Culex pipiens quinquefasciatus and Aedes albopictus mosquitoes from La Réunion Island

Human activities and densities shape insecticide resistance distribution and dynamics in the virus-vector Culex pipiens mosquitoes from Morocco

BACKGROUND

Mosquitoes of the Culex pipiens complex are widely distributed vectors for several arboviruses affecting humans. Consequently, their populations have long been controlled using insecticides, in response to which different resistance mechanisms have been selected. Moreover, their ecological preferences and broad adaptability allow C. pipiens mosquitoes to breed in highly polluted water bodies where they are exposed to many residuals from anthropogenic activities. It has been observed for several mosquito species that anthropization (in particular urbanization and agricultural lands) can lead to increased exposure to insecticides and thus to increased resistance. The main objective of the present study was to investigate whether and how urbanization and/or agricultural lands had a similar impact on C. pipiens resistance to insecticides in Morocco.

METHODS

Breeding sites were sampled along several transects in four regions around major Moroccan cities, following gradients of decreasing anthropization. The imprint of anthropogenic activities was evaluated around each site as the percentage of areas classified in three categories: urban, agricultural and natural. We then assessed the frequencies of four known resistance alleles in these samples and followed their dynamics in five urban breeding sites over 4 years.

RESULTS

The distribution of resistance alleles revealed a strong impact of anthropization, in both agricultural and urbanized lands, although different between resistance mutations and between Moroccan regions; we did not find any clear trend in the dynamics of these resistance alleles during the survey.

CONCLUSIONS

Our study provides further evidence for the role of anthropic activities in the selection and maintenance of mutations selected for resistance to insecticides in mosquitoes. The consequences are worrying as this could decrease vector control capacities and thus result in epizootic and epidemic outbreaks. Consequently, concerted and integrated disease control strategies must be designed that include better management regarding the consequences of our activities.

Talaromyces sp. Ethyl Acetate Crude Extract as Potential Mosquitocide to Control Culex pipiens quinquefasciatus

Vector control is considered an effective approach to controlling diseases spread by mosquito bites. Entomopathogenic fungi are widely used in agriculture to control insect pests, and fungal metabolites can potentially be developed as effective mosquitocides. In this study, a high-throughput screening method was used to search for potential mosquitocides in the Global Fungal Extract Library (GFEL). We tested the larvicidal activity of 264 fungal ethyl acetate crude extracts against Culex pipiens quinquefasciatus. Nine fungal extracts caused moderate to high mortality rates (>50%), with two fungal extracts (58A7 and 101H12) causing a 100% mortality rate. The lethal concentrations for 50% of the population (LC50) were 44.27 mg/L and 31.90 mg/L, respectively. Fraction 14 had a high mortality rate, with an LC50 value of 12.13 mg/L, and was isolated from 58A7 (Fractions 1-11) and 101H12 (Fractions 12-15). Further analyses showed that Fraction 14 was made up of vermistatin and dihydrovermistatin. In a Cx. p. quinquefasciatus larvicidal bioassay, vermistatin (LC50 = 28.13 mg/L) was more toxic than dihydrovermistatin (LC50 = 83.87 mg/L). Our findings suggested that the active fungal extract 101H12 from Talaromyces sp. and its compound vermistatin could be developed as mosquitocides.

Inference of selection pressures that drive insecticide resistance in Anopheles and Culex mosquitoes in Korea

Pyrethroids are primarily used for mosquito control in Korea. However, high frequencies of mutations conferring resistance to not only pyrethroids but also to other insecticides have been found in mosquito populations. This study aimed to examine the hypothesis that insecticides used outside of public health may play a role in selection. Briefly, the resistance mutation frequencies to three insecticide groups (pyrethroids, organophosphates, and cyclodienes) were estimated in two representative groups of mosquito species (Anopheles Hyrcanus Group and Culex pipiens complex). The relationship between these frequencies and the land-use status of the collection sites was investigated through multiple regression analysis. In the Anopheles Hyrcanus Group, the frequencies of both ace1 (organophosphate resistance) and rdl (cyclodiene resistance) mutations were positively correlated with 'proximity to golf course', possibly be due to the insecticides used for turf maintenance. They also showed positive correlations with field area and rice paddy area, respectively, suggesting the role of agricultural insecticides in the selection of these resistance traits. For the Cx. pipiens complex, the kdr (pyrethroid resistance), ace1, and rdl mutations were positively correlated with the residential area, field, and rice paddy, respectively. Therefore, pyrethroids used for public health could serve as a direct source of resistance selection pressure against kdr, whereas non-public health insecticides may pose primary selection pressure against the ace1 and rdl traits. The current findings suggest that the insecticides used in agriculture and the golf industry play a significant role in mosquito selection, despite variations in the extent of indirect selection pressure according to the mosquito groups and insecticide classes.

The Mosquito Larvicidal Activity of Lignans from Branches of Cinnamomum camphora chvar. Borneol

The chemical investigation of branches of Cinnamomum camphora chvar. Borneol guided by mosquito larvicidal activity led to the isolation of fourteen known lignans (1-14). Their structures were elucidated unambiguously based on comprehensive spectroscopic analysis and comparison with the literature data. This is the first report of these compounds being isolated from branches of Cinnamomum camphora chvar. Borneol. Compounds 3-5 and 8-14 were isolated from this plant for the first time. All compounds isolated were subjected to anti-inflammatory, mosquito larvicidal activity and cytotoxic activity evaluation. Compounds (1-14) showed significant mosquito larvicidal activity against Culex pipiens quinquefasciatus with lethal mortality in 50% (LC₅₀), with values ranging from 0.009 to 0.24 μg/mL. Among them, furofuran lignans(1-8) exhibited potent mosquito larvicidal activity against Cx. p. quinquefasciatus, with LC₅₀ values of 0.009-0.021 μg/mL. From the perspective of a structure-activity relationship, compounds with a dioxolane group showed high mosquito larvicidal activity and have potential to be developed into a mosquitocide.

Mosquito-Borne Diseases and Their Control Strategies: An Overview Focused on Green Synthesized Plant-Based Metallic Nanoparticles

Mosquitoes act as vectors of pathogens that cause most life-threatening diseases, such as malaria, Dengue, Chikungunya, Yellow fever, Zika, West Nile, Lymphatic filariasis, etc. To reduce the transmission of these mosquito-borne diseases in humans, several chemical, biological, mechanical, and pharmaceutical methods of control are used. However, these different strategies are facing important and timely challenges that include the rapid spread of highly invasive mosquitoes worldwide, the development of resistance in several mosquito species, and the recent outbreaks of novel arthropod-borne viruses (e.g., Dengue, Rift Valley fever, tick-borne encephalitis, West Nile, yellow fever, etc.). Therefore, the development of novel and effective methods of control is urgently needed to manage mosquito vectors. Adapting the principles of nanobiotechnology to mosquito vector control is one of the current approaches. As a single-step, eco-friendly, and biodegradable method that does not require the use of toxic chemicals, the green synthesis of nanoparticles using active toxic agents from plant extracts available since ancient times exhibits antagonistic responses and broad-spectrum target-specific activities against different species of vector mosquitoes. In this article, the current state of knowledge on the different mosquito control strategies in general, and on repellent and mosquitocidal plant-mediated synthesis of nanoparticles in particular, has been reviewed. By doing so, this review may open new doors for research on mosquito-borne diseases.

Evolutionary trade-offs associated with copy number variations in resistance alleles in Culex pipiens mosquitoes

Organophosphate and carbamate insecticides have largely been used worldwide to control mosquito populations. As a response, the same amino acid substitution in the ace-1 gene (G119S), conferring resistance to both insecticides, has been selected independently in many mosquito species. In Anopheles gambiae, it has recently been shown that the G119S mutation is actually part of homogeneous duplications that associate multiple resistance copies of the ace-1 gene. In this study, we showed that duplications of resistance copies of the ace-1 gene also exist in the Culex pipiens species complex. The number of copies is variable, and different numbers of copies are associated with different phenotypic trade-offs: we used a combination of bioassays and competition in population cages to show that having more resistance copies conferred higher resistance levels, but was also associated with higher selective disadvantage (or cost) in the absence of insecticide. These results further show the versatility of the genetic architecture of resistance to organophosphate and carbamate insecticides around the ace-1 locus and its role in fine-tuned adaptation to insecticide treatment variations.

Fitness costs associated with a GABA receptor mutation conferring dieldrin resistance in Aedes albopictus

Understanding the dynamics of insecticide resistance genes in mosquito populations is pivotal for a sustainable use of insecticides. Dieldrin resistance in Aedes albopictus is conferred by the alanine to serine substitution (A302S or RdlR allele) in the γ-aminobutyric acid (GABA) receptor encoded by the Rdl gene. On Reunion Island, dieldrin resistance was initially reported in natural Ae. albopictus populations sampled in 2008 despite the ban of dieldrin since 1994. To monitor insecticide resistance in Ae. albopictus on the island and to identify its drivers, we measured (i) the frequency of resistance alleles in 19 distinct natural populations collected between 2016 and 2017, (ii) fitness costs associated with dieldrin resistance in laboratory-controlled experiments, and (iii) the resistance conferred by RdlR to fipronil, an insecticide widely used on the island and reported to cross-react with RdlR. The results show a persistence of RdlR in Ae. albopictus natural populations at low frequencies. Among the measured life history traits, mortality in pre-imaginal stages, adults' survival as well as the proportion of egg-laying females were significantly affected in resistant mosquitoes. Finally, bioassays revealed resistance of RdlR mosquitoes to fipronil, suggesting that the use of fipronil in natura could select for the RdlR allele. This study shows that dieldrin resistance is persistent in natural mosquito populations likely as a result of combined effects between fitness costs associated with RdlR and selection exerted by cross-reacting environmental insecticides such as fipronil.

Insecticide Resistance in Alabama Populations of the Mosquito Aedes albopictus

The insecticide sensitivity and resistance status of both adults and larvae from six Aedes albopictus samples collected in Tuskegee, Tuscaloosa, Birmingham, Dothan, Mobile, and Montgomery, Alabama, were evaluated for the levels of sensitivity and resistance to eight insecticides: β-cyfluthrin, chlorpyrifos, deltamethrin, etofenprox, fenitrothion, permethrin, resmethrin, and malathion. Adult Ae. albopictus from all locations showed similar results for the difference between the time to 100% mortality and the diagnostic time in the CDC bottle bioassay, although Ae. albopictus survive longer than the diagnostic time to permethrin, fenitrothion, and resmethrin treatments. The larval bioassay indicated that malathion was the least toxic to Ae. albopictus from all locations (LC50: ranging from 0.1 ppm to 1.2 ppm), followed by resmethrin and etofenprox (LC50: 0.05 ppm-0.4 ppm), and deltamethrin and fenitrothion (LC50: 0.01 ppm-0.06 ppm). Chlorpyrifos exhibited the highest larval toxicity (LC50: 0.003 ppm-0.05 ppm). The resistance status of Ae. albopictus from all six locations was similar to the resistance levels found in a previous survey in 2004, indicating that in Alabama the development of resistance is slow in this strain, although comparing the resistance of Ae. albopictus from Tuskegee to that of a susceptible strain showed that it is resistant to chlorpyrifos. The slopes of the dose-response curves to most of the insecticides tested for these field populations of Ae. albopictus were generally similar to or slightly higher than those measured eighteen years previously, indicating that these populations are relatively homozygous in response to all the insecticides tested.

Toxicities and Cross-Resistance of Imidacloprid, Acetamiprid, Emamectin Benzoate, Spirotetramat, and Indoxacarb in Field Populations of Culex quinquefasciatus (Diptera: Culicidae)

Culex quinquefasciatus is a major vector of several pathogens and is capable of breeding in various aquatic habitats. The extensive and injudicious use of synthetic chemicals against the mosquito species has led to the problem of insecticide resistance. To explore this resistance in detail, toxicity bioassays of imidacloprid, acetamiprid, emamectin benzoate, spirotetramat, and indoxacarb were performed on five Cx. quinquefasciatus field populations from Pakistan in addition to a laboratory susceptible strain. Compared with the susceptible strain, results for the five Cx. quinquefasciatus field populations were as follows: susceptibility to high resistance against imidacloprid (resistance ratio (RR): 0.09-11.18), susceptibility to moderate resistance against acetamiprid (RR: 0.39-8.00), susceptibility to emamectin benzoate (RR: 0.002-0.020), susceptibility to spirotetramat (RR: 0.01-0.07), and low to high resistance against indoxacarb (RR: 3.00-118.00). Correlation analyses revealed a significant positive correlation between imidacloprid, acetamiprid, and spirotetramat median lethal concentration (LC₅₀) values, indicating the possibility of cross-resistance. In contrast, there were no significant correlations between the LC₅₀ values of other tested insecticides, indicating the possible absence of cross-resistance. These results can assist public health authorities, medical entomologists, and pest managers to manage the insecticide resistance of Cx. quinquefasciatus as well as the associated pollution and human health issues.

Spatio-Temporal Dynamics of a Dieldrin Resistance Gene in Aedes albopictus and Culex quinquefasciatus Populations From Reunion Island

The control of mosquito populations using insecticides is increasingly threatened by the spread of resistance mechanisms. Dieldrin resistance, conferred by point mutations in the Rdl gene encoding the γ-aminobutyric acid receptor, has been reported at high prevalence in mosquito populations in response to selective pressures. In this study, we monitored spatio-temporal dynamics of the resistance-conferring RdlR allele in Aedes (Stegomyia) albopictus (Skuse, 1895) and Culex (Culex) quinquefasciatus (Say, 1823) populations from Reunion Island. Specimens of both mosquito species were sampled over a 12-month period in three cities and in sites located at lower (<61 m) and higher (between 503 and 564 m) altitudes. Mosquitoes were genotyped using a molecular test detecting the alanine to serine substitution (A302S) in the Rdl gene. Overall, the RdlR frequencies were higher in Cx. quinquefasciatus than Ae. albopictus. For both mosquito species, the RdlR frequencies were significantly influenced by location and altitude with higher RdlR frequencies in the most urbanized areas and at lower altitudes. This study highlights environmental factors that influence the dynamics of insecticide resistance genes, which is critical for the management of insecticide resistance and the implementation of alternative and efficient vector control strategies.

Modified Mosquito Programs’ Surveillance Needs and An Image-Based Identification Tool to Address Them

Effective mosquito surveillance and control relies on rapid and accurate identification of mosquito vectors and confounding sympatric species. As adoption of modified mosquito (MM) control techniques has increased, the value of monitoring the success of interventions has gained recognition and has pushed the field away from traditional ‘spray and pray’ approaches. Field evaluation and monitoring of MM control techniques that target specific species require massive volumes of surveillance data involving species-level identifications. However, traditional surveillance methods remain time and labor-intensive, requiring highly trained, experienced personnel. Health districts often lack the resources needed to collect essential data, and conventional entomological species identification involves a significant learning curve to produce consistent high accuracy data. These needs led us to develop MosID: a device that allows for high-accuracy mosquito species identification to enhance capability and capacity of mosquito surveillance programs. The device features high-resolution optics and enables batch image capture and species identification of mosquito specimens using computer vision. While development is ongoing, we share an update on key metrics of the MosID system. The identification algorithm, tested internally across 16 species, achieved 98.4 ± 0.6% % macro F1-score on a dataset of known species, unknown species used in training, and species reserved for testing (species, specimens respectively: 12, 1302; 12, 603; 7, 222). Preliminary user testing showed specimens were processed with MosID at a rate ranging from 181-600 specimens per hour. We also discuss other metrics within technical scope, such as mosquito sex and fluorescence detection, that may further support MM programs.

Os agrotóxicos no contexto da Saúde Única

RESUMO A industrialização da agricultura e da pecuária, além de gerar um ambiente propício à disseminação de agentes infecciosos, é responsável pelo uso generalizado de diversas substâncias tóxicas que afetam a saúde humana, animal e ambiental. O objetivo deste estudo foi promover a reflexão sobre o uso de agrotóxicos e medicamentos veterinários como elementos de debate na construção da Saúde Única. Para isso, foi realizada uma revisão exploratória literária de artigos, livros e documentos oficiais disponíveis em plataformas de banco de dados. A discussão inclui as problemáticas do uso de substâncias tóxicas em plantas e animais. Aborda, também, como os resíduos oriundos de sua utilização impactam a qualidade de alimentos, ar, solo, água com consequências à saúde humana. Embora essa discussão seja escassa na temática de Saúde Única, é fundamental que, além da participação da sociedade civil organizada, gestores públicos assegurem, por meio de políticas públicas, maior segurança e controle na utilização de substâncias tóxicas na agricultura e na pecuária.

Insecticide susceptibility status and knockdown resistance (kdr) mutation in Aedes albopictus in China

Background

Aedes (Stegomyia) albopictus (Skuse, 1894) is the main vector of dengue virus in China. The resistance to insecticides is a huge obstacle for the control of this species, and determining its resistance status and mechanisms in China is essential for the implementation of vector management strategies.

Methods

We have investigated the larval and adult resistance status of Ae. albopictus to deltamethrin in eight field populations in China. Mutations at the voltage-gated sodium channel gene, related to the knockdown resistance (kdr) effect, were detected by sequencing of PCR products. The eight field populations were examined for pyrethroid resistance using the World Health Organization standard bioassays, and the association between the mutations and phenotypic resistance was tested.

Results

The eight field populations of larvae of Ae. albopictus in China exhibited high resistance to deltamethrin; the RR₅₀ values ranged from 12 (ZJ) to 44 (GZ). Adult bioassay revealed that Ae. albopictus populations were resistant to deltamethrin (mortality rate < 90%), except ZJ population (probably resistant, mortality rate = 93.5%). Long knockdown time in the field populations was consistent with low mortality rates in adult bioassay. F1534S mutation showed increased protection against deltamethrin in all populations except BJ and SJZ populations, whereas I1532T mutation showed increased protection against deltamethrin in only BJ population.

Conclusion

There were different degrees of resistance to deltamethrin in field Ae. albopictus populations in China. The longest knockdown time and lowest mortality rate observed in Ae. albopictus population in Guangzhou indicate the severity of high resistance to deltamethrin. The patchy distribution of deltamethrin resistance and kdr mutations in Ae. albopictus mosquitoes suggests the necessity for resistance management and developing counter measures to mitigate the spread of resistance.

Graphical Abstract

Dengue fever and insecticide resistance in Aedes mosquitoes in Southeast Asia: a review

Dengue fever is the most important mosquito-borne viral disease in Southeast Asia. Insecticides remain the most effective vector control approach for Aedes mosquitoes. Four main classes of insecticides are widely used for mosquito control: organochlorines, organophosphates, pyrethroids and carbamates. Here, we review the distribution of dengue fever from 2000 to 2020 and its associated mortality in Southeast Asian countries, and we gather evidence on the trend of insecticide resistance and its distribution in these countries since 2000, summarising the mechanisms involved. The prevalence of resistance to these insecticides is increasing in Southeast Asia, and the mechanisms of resistance are reported to be associated with target site mutations, metabolic detoxification, reduced penetration of insecticides via the mosquito cuticle and behavioural changes of mosquitoes. Continuous monitoring of the status of resistance and searching for alternative control measures will be critical for minimising any unpredicted outbreaks and improving public health. This review also provides improved insights into the specific use of insecticides for effective control of mosquitoes in these dengue endemic countries.

Distribution of insecticide resistance and molecular mechanisms involved in the West Nile vector Culex pipiens in Morocco

BACKGROUND

Mosquitoes of the Culex pipiens complex are the vectors of several arboviruses and are thus subjected to insecticide control worldwide. However, overuse of insecticides selects for resistance. While assessing the resistance status of the vectors is required for effective and sustainable disease control, resistance has so far only been sparsely studied in Morocco. In this study, we establish a first countrywide assessment of the levels of resistance to various insecticides and the potential responsible mechanisms involved. Cx. pipiens larvae were collected from natural populations of five regions of Morocco, and their taxonomic status was determined (molecular forms). The level of their susceptibility to insecticides was assessed by single-diagnostic-dose bioassays. Molecular identification of known resistance alleles was investigated to determine the frequency of target-site mutations.

RESULTS

This study confirms that Moroccan populations are an interbreeding mix of pipiens and molestus forms, with large gene flow for the resistance alleles. We also found that Cx. pipiens mosquitoes are resistant to all insecticide families, all over Morocco: resistance is high for insecticides used in mosquito control, but also present for other pesticides. Resistance alleles are similarly more frequent for mosquito control insecticides. However, their distribution is heterogeneous in the five regions, with significant genetic differentiation between populations, revealing the crucial role of local insecticide treatment practices.

CONCLUSION

This study provides reference countrywide data that highlight the need for further research to refine the distribution of resistance in Morocco and to understand the role of agriculture/urban residuals in its spread. © 2020 Society of Chemical Industry.

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.

Strategic Approach, Advances, and Challenges in the Development and Application of the SIT for Area-Wide Control of Aedes albopictus Mosquitoes in Reunion Island

Simple Summary

Aedes albopictus is a well-established competent arbovirus vector in Reunion Island, a French overseas territory in the Indian Ocean, occurring in a range of natural to urban environments where it represents a major threat to public health. Following the 2006 Chikungunya outbreak and periodic occurrence of dengue epidemics, the sterile insect technique (SIT) emerged as the most environment-friendly option for integration with the current vector control strategy that relies mainly on the elimination of breeding sites and insecticide applications. This paper describes the trajectory that has been followed in assessing the feasibility of SIT against Ae. albopictus in Reunion Island, and reviews some of the main achievements since 2009. These include essential scientific information so far obtained on the biology and ecology of Ae. albopictus, and the development of the requisite technological capabilities for the production and release of sexually competitive sterile males. Furthermore, it also draws attention to the strategies established to streamline the decision-making process, including an awareness campaign to enhance public understanding, efforts to secure public acceptance and regulatory validation of SIT pilot testing for small-scale suppression of wild Ae. albopictus in selected urban sites on the island.

Abstract

The global expansion of Aedes albopictus, together with the absence of specific treatment and vaccines for most of the arboviruses it transmits, has stimulated the development of more sustainable and ecologically acceptable methods for control of disease transmission through the suppression of natural vector populations. The sterile insect technique (SIT) is rapidly evolving as an additional tool for mosquito control, offering an efficient and more environment-friendly alternative to the use of insecticides. Following the devastating chikungunya outbreak, which affected 38% of the population on Reunion Island (a French overseas territory in the southwest of the Indian Ocean), there has been strong interest and political will to develop effective alternatives to the existing vector control strategies. Over the past 10 years, the French Research and Development Institute (IRD) has established an SIT feasibility program against Ae. albopictus on Reunion Island in collaboration with national and international partners. This program aimed to determine whether the SIT based on the release of radiation-sterilized males is scientifically and technically feasible, and socially acceptable as part of a control strategy targeting the local Ae. albopictus population. This paper provides a review of a multi-year and a particularly broad scoping process of establishing the scientific and technological feasibility of the SIT against Ae. albopictus on Reunion Island. It also draws attention to some prerequisites of the decision-making process, through awareness campaigns to enhance public understanding and support, social adoption, and regulatory validation of the SIT pilot tests.

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 3^rd generation (52.7%) compared to the 6^th 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.

Malaria and Dengue Mosquito Vectors from Lao PDR Show a Lack of the rdl Mutant Allele Responsible for Cyclodiene Insecticide Resistance

The gamma-aminobutyric acid (GABA) receptor, RDL, plays important roles in neuronal signaling and is the target of highly effective insecticides. A mutation in RDL, commonly A296S, underlies resistance to several insecticides such as cyclodienes. Even though the use of cyclodienes has been banned, the occurrence of mutations substituting A296 is notably high in mosquitoes from several countries. Here, we report a survey investigating the prevalence of the Rdl mutant allele in mosquitoes from Laos, a country where mosquito-borne diseases such as malaria and dengue fever are health concerns. Anopheles and Aedes mosquitoes were collected from 12 provinces in Laos. Adult bioassays on Aedes aegypti (Linnaeus) (Diptera: Culicidae) and Aedes albopictus (Skuse) (Diptera: Culicidae) showed that all the populations tested were susceptible to dieldrin (4%) following WHO protocols. Exon 7 from a total of 791 mosquitoes was sequenced to identify the amino acid encoded for at 296 of RDL. Only one of these mosquitoes, Anopheles maculatus rampae Harbach and Somboon (Diptera: Culicidae) from Attapeu, carried the mutant allele being heterozygous for A296S. We therefore found a general lack of the Rdl mutant allele indicating that mosquitoes from Laos are not exposed to insecticides that act on the GABA receptor compared to mosquitoes in several other countries. Identifying the prevalence of the Rdl mutation may help inform the potential use of alternative insecticides that act on the GABA receptor should there be a need to replace pyrethroids in order to prevent/manage resistance.

Insecticide resistance in Culex quinquefasciatus Say, 1823 in Brazil: a review

Culex quinquefasciatus is a successful invasive species broadly distributed in subtropical regions, including Brazil. It is an extremely annoying mosquito due to its nocturnal biting behavior, in high-density populations and it is a potential bridge between sylvatic arbovirus from birds to man in urban territories. Herein, we present a review concerning the methods of chemical control employed against Cx. quinquefasciatus in Brazil since the 1950’s and insecticide resistance data registered in the literature. As there is no specific national programme for Cx. quinquefasciatus control in Brazil, the selection of insecticide resistance is likely due in part to the well-designed chemical campaigns against Aedes aegypti and the elevated employment of insecticides by households and private companies. There are very few publications about insecticide resistance in Cx. quinquefasciatus from Brazil when compared to Ae. aegypti. Nevertheless, resistance to organophosphates, carbamate, DDT, pyrethroids and biolarvicides has been registered in Cx. quinquefasciatus populations from distinct localities of the country. Concerning physiological mechanisms selected for resistance, distinct patterns of esterases, as well as mutations in the acetylcholinesterase (ace-1) and voltage-gated sodium channel (Na_V) genes, have been identified in natural populations. Given environmental changes and socioeconomical issues in the cities, in recent years we have been experiencing an increase in the number of disease cases caused by arboviruses, which may involve Cx. quinquefasciatus participation as a key vector. It is urgent to better understand the efficiency and susceptibility status to insecticides, as well as the genetic background of known resistant mechanisms already present in Cx. quinquefasciatus populations for an effective and rapid chemical control when eventually required.

RDL A301S alone does not confer high levels of resistance to cyclodiene organochlorine or phenyl pyrazole insecticides in Plutella xylostella

Mutations in the GABA-gated chloride channel are associated with resistance to cyclodiene organochlorine and phenyl pyrazole insecticides. The best characterised of these is A301S, which was initially identified in a Dieldrin resistant strain of Drosophila melanogaster. The orthologous mutation has been found in a variety of different crop pests including the diamond back moth Plutella xylostella. However, the contribution of this mutation to resistance in this species remains unclear. We have used the CRISPR/Cas9 system in order to edit Plutella xylostella PxGABARalpha1 to Serine at the 301 orthologous position (282 in PxGABARalpha1) in an insecticide sensitive strain isolated from Vero Beach (VB) USA. In this edited line, no high level of resistance is conferred to Dieldrin, Endosulfan or Fipronil, rather only a subtle shift in sensitivity which could not confer commercially important resistance. We conclude that the high level of commercial resistance to cyclodiene organochlorine and phenyl pyrazole insecticides observed in some field isolates of Plutella xylostella cannot arise from A282S in PxGABARalpha1 alone.

A marker of glutathione S-transferase-mediated resistance to insecticides is associated with higher Plasmodium infection in the African malaria vector Anopheles funestus

Metabolic resistance to insecticides is threatening malaria control in Africa. However, the extent to which it impacts malaria transmission remains unclear. Here, we investigated the association between a marker of glutathione S-transferase mediated metabolic resistance and Plasmodium infection in field population of Anopheles funestus s.s. in comparison to the A296S-RDL target site mutation. The 119F-GSTe2 resistant allele was present in southern (Obout) (56%) and central (Mibellon) (25%) regions of Cameroon whereas the 296S-RDL resistant allele was detected at 98.5% and 15% respectively. The whole mosquito Plasmodium and sporozoite infection rates were 57% and 14.8% respectively in Obout (n = 508) and 19.7% and 5% in Mibellon (n = 360). No association was found between L119F-GSTe2 genotypes and whole mosquito infection status. However, when analyzing oocyst and sporozoite infection rates separately, the resistant homozygote 119F/F genotype was significantly more associated with Plasmodium infection in Obout than both heterozygote (OR = 2.5; P = 0.012) and homozygote susceptible (L/L119) genotypes (OR = 2.10; P = 0.013). In contrast, homozygote RDL susceptible mosquitoes (A/A296) were associated more frequently with Plasmodium infection than other genotypes (OR = 4; P = 0.03). No additive interaction was found between L119F and A296S. Sequencing of the GSTe2 gene showed no association between the polymorphism of this gene and Plasmodium infection. Glutathione S-transferase metabolic resistance is potentially increasing the vectorial capacity of resistant An. funestus mosquitoes. This could result in a possible exacerbation of malaria transmission in areas of high GSTe2-based metabolic resistance to insecticides.

Field evaluation of seasonal trends in relative population sizes and dispersal pattern of Aedes albopictus males in support of the design of a sterile male release strategy

Background

To develop an efficient sterile insect technique (SIT) programme, the number of sterile males to release, along with the spatial and temporal pattern of their release, has to be determined. Such parameters could be estimated from a reliable estimation of the wild population density (and its temporal variation) in the area to treat. Here, a series of mark-release-recapture experiments using laboratory-reared and field-derived Aedes albopictus males were carried out in Duparc, a selected pilot site for the future application of SIT in the north of La Reunion Island.

Methods

The dispersal, longevity of marked males and seasonal fluctuations in the population size of native mosquitoes were determined from the ratio of marked to unmarked males caught in mice-baited BG-Sentinel traps. The study was conducted during periods of declining population abundance (April), lowest abundance (September) and highest abundance (December).

Results

According to data collected in the first 4 days post-release, the Lincoln index estimated population size as quite variable, ranging from 5817 in April, to 639 in September and 5915 in December. Calculations of daily survival probability to 4 days after release for field and laboratory males were 0.91 and 0.98 in April, respectively, and 0.88 and 0.84 in September, respectively. The mean distance travelled (MDT) of released field males were 46 m, 67 m and 37 m for December, April and September experiments, respectively. For released laboratory males, the MDT was 65 m and 42 m in April and September, respectively.

Conclusions

Theoretically, the most efficient release programme should be started in July/August when the mosquito population size is the lowest (c.600 wild males/ha relative to 5000 wild males estimated for December and April), with a weekly release of 6000 males/ha. The limited dispersal of Ae. albopictus males highlights the nessecity for the widespread release of sterile males over multiple sites and in a field setting to avoid topographical barriers and anthropogenic features that may block the migration of the released sterile male mosquitoes.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3329-7) contains supplementary material, which is available to authorized users.

Construction of a genetic sexing strain for Aedes albopictus: a promising tool for the development of sterilizing insect control strategies targeting the tiger mosquito

Background

Aedes albopictus is an invasive mosquito species of global medical concern as its distribution has recently expanded to Africa, the Americas and Europe. In the absence of prophylaxis protecting human populations from emerging arboviruses transmitted by this mosquito species, the most straightforward control measures rely on the suppression or manipulation of vector natural populations. A number of environmental-friendly methods using mass releases of sterilizing males are currently under development. However, these strategies are still lacking an efficient sexing method required for production of males at an industrial scale.

Results

We present the first Genetic Sexing Strain (GSS) in Ae. albopictus, hereafter referred as Tikok, obtained by sex linkage of the rdl gene conferring dieldrin resistance. Hatch rate, larval survival and sex ratio were followed during twelve generations. The use of dieldrin at the third larval stage allowed selecting 98 % of males on average.

Conclusion

A good production rate of Tikok males makes this GSS suitable for any control method based on mass production of Ae. albopictus males. Despite limitations resulting from reduced egg hatch as well as the nature of the used insecticide, the construction of this GSS paves the way for industrial sex separation of Ae. albopictus.

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.

The One Health Concept: 10 Years Old and a Long Road Ahead

Over the past decade, a significant increase in the circulation of infectious agents was observed. With the spread and emergence of epizootics, zoonoses, and epidemics, the risks of pandemics became more and more critical. Human and animal health has also been threatened by antimicrobial resistance, environmental pollution, and the development of multifactorial and chronic diseases. This highlighted the increasing globalization of health risks and the importance of the human-animal-ecosystem interface in the evolution and emergence of pathogens. A better knowledge of causes and consequences of certain human activities, lifestyles, and behaviors in ecosystems is crucial for a rigorous interpretation of disease dynamics and to drive public policies. As a global good, health security must be understood on a global scale and from a global and crosscutting perspective, integrating human health, animal health, plant health, ecosystems health, and biodiversity. In this study, we discuss how crucial it is to consider ecological, evolutionary, and environmental sciences in understanding the emergence and re-emergence of infectious diseases and in facing the challenges of antimicrobial resistance. We also discuss the application of the "One Health" concept to non-communicable chronic diseases linked to exposure to multiple stresses, including toxic stress, and new lifestyles. Finally, we draw up a list of barriers that need removing and the ambitions that we must nurture for the effective application of the "One Health" concept. We conclude that the success of this One Health concept now requires breaking down the interdisciplinary barriers that still separate human and veterinary medicine from ecological, evolutionary, and environmental sciences. The development of integrative approaches should be promoted by linking the study of factors underlying stress responses to their consequences on ecosystem functioning and evolution. This knowledge is required for the development of novel control strategies inspired by environmental mechanisms leading to desired equilibrium and dynamics in healthy ecosystems and must provide in the near future a framework for more integrated operational initiatives.

Human Interventions: Driving Forces of Mosquito Evolution

One of the most common strategies for controlling mosquito-borne diseases relies on the use of chemical pesticides to repel or kill the mosquito vector. Pesticide exposure interferes with several key biological functions in the mosquito and triggers a variety of adaptive responses whose underlying mechanisms are only partially elucidated. The availability of reference genome sequences opens up the possibility of tracking signatures of evolutionary changes, including the most recent, across the genomes of many vector species. In this review, we highlight the recent genomic changes, which contribute to the fascinating adaptation of malaria vectors of the sub-Saharan African region to intensive insecticide-based interventions. We emphasize the operational significance of detailed genomic knowledge for current monitoring and decision-making.

RDL mutations predict multiple insecticide resistance in Anopheles sinensis in Guangxi, China

Background

Anopheles sinensis is a major vector of malaria in China. The gamma-aminobutyric acid (GABA)-gated chloride channel, encoded by the RDL (Resistant to dieldrin) gene, is the important target for insecticides of widely varied structures. The use of various insecticides in agriculture and vector control has inevitably led to the development of insecticide resistance, which may reduce the control effectiveness. Therefore, it is important to investigate the presence and distribution frequency of the resistance related mutation(s) in An. sinensis RDL to predict resistance to both the withdrawn cyclodienes (e.g. dieldrin) and currently used insecticides, such as fipronil.

Methods

Two hundred and forty adults of An. sinensis collected from nine locations across Guangxi Zhuang Autonomous Region were used. Two fragments of An. sinensis RDL (AsRDL) gene, covering the putative insecticide resistance related sites, were sequenced respectively. The haplotypes of each individual were reconstructed by the PHASE2.1 software, and confirmed by clone sequencing. The phylogenetic tree was built using maximum-likelihood and Bayesian inference methods. Genealogical relations among different haplotypes were also analysed using Network 5.0.

Results

The coding region of AsRDL gene was 1674 bp long, encoding a protein of 557 amino acids. AsRDL had 98.0% amino acid identity to that from Anopheles funestus, and shared common structural features of Cys-loop ligand-gated ion channels. Three resistance-related amino acid substitutions (A296S, V327I and T345S) were detected in all the nine populations of An. sinensis in Guangxi, with the 296S mutation being the most abundant (77–100%), followed by 345S (22–47%) and 327I (8–60%). 38 AsRDL haplotypes were identified from 240 individuals at frequencies ranging from 0.2 to 34.8%. Genealogical analysis suggested multiple origins of the 345S mutation in AsRDL.

Conclusions

The near fixation of the 296S mutation and the occurrence of the 327I and 345S mutations in addition to 296S, in all the nine tested An. sinensis populations in Guangxi, strongly indicate a risk of multiple insecticide resistance. The haplotype diversity plus genetic heterogeneities in the geographical distribution, and multiple origins of AsRDL alleles call for a location-customized strategy for monitoring and management of insecticide resistance.

Contemporary status of insecticide resistance in the major Aedes vectors of arboviruses infecting humans

Both Aedes aegytpi and Ae. albopictus are major vectors of 5 important arboviruses (namely chikungunya virus, dengue virus, Rift Valley fever virus, yellow fever virus, and Zika virus), making these mosquitoes an important factor in the worldwide burden of infectious disease. Vector control using insecticides coupled with larval source reduction is critical to control the transmission of these viruses to humans but is threatened by the emergence of insecticide resistance. Here, we review the available evidence for the geographical distribution of insecticide resistance in these 2 major vectors worldwide and map the data collated for the 4 main classes of neurotoxic insecticide (carbamates, organochlorines, organophosphates, and pyrethroids). Emerging resistance to all 4 of these insecticide classes has been detected in the Americas, Africa, and Asia. Target-site mutations and increased insecticide detoxification have both been linked to resistance in Ae. aegypti and Ae. albopictus but more work is required to further elucidate metabolic mechanisms and develop robust diagnostic assays. Geographical distributions are provided for the mechanisms that have been shown to be important to date. Estimating insecticide resistance in unsampled locations is hampered by a lack of standardisation in the diagnostic tools used and by a lack of data in a number of regions for both resistance phenotypes and genotypes. The need for increased sampling using standard methods is critical to tackle the issue of emerging insecticide resistance threatening human health. Specifically, diagnostic doses and well-characterised susceptible strains are needed for the full range of insecticides used to control Ae. aegypti and Ae. albopictus to standardise measurement of the resistant phenotype, and calibrated diagnostic assays are needed for the major mechanisms of resistance.

Review of West Nile virus circulation and outbreak risk in Madagascar: Entomological and ornithological perspectives

West Nile fever (WNF) is a zoonotic disease, occurring nearly globally. In Madagascar, West Nile virus (WNV) was first detected in 1978 from wild birds and the virus is currently distributed across the island, but no epidemic or epizootic period has been recorded. One fatal human case of WNV infection was reported in 2011, suggesting a "tip of the iceberg" phenomenon of a possible WNF epidemic/epizootic on the island. The main objective of this literature-based survey is to review patterns of WNV circulation in Madagascar from the entomological and ornithological points of view. Among the 235 mosquito species described from Madagascar, 29 species are widely associated with WNV infection; 16 of them are found naturally infected with WNV on the island and categorized into major, candidate, and potential vectors of WNV according to their vector capacity. This study upholds the hypothesis that WNV enzooticity is independent of annual movements of migratory birds passing through Madagascar. Moreover, the lack of regular migratory bird flux between Africa and Madagascar would reduce the probability of transmission and the subsequent reintroduction of the virus into locally occurring mosquito species. Given that Palearctic migratory birds are strongly implicated in the transmission of WNV, we highlight notable differences in the movements and species diversity of these birds in Madagascar as compared to eastern and northern Africa. Risk factors from this two-pronged approach are presented for the emergence of WNF outbreak.

First identification of kdr allele F1534S in VGSC gene and its association with resistance to pyrethroid insecticides in Aedes albopictus populations from Haikou City, Hainan Island, China

Background

Aedes albopictus is distributed widely in China, as a primary vector of Dengue fever and Chikungunya fever in south of China. Chemical insecticide control is one of the integrated programmes to prevent mosquito-borne diseases. Long-term applications of pyrethroids have resulted in the development of resistance in Ae. albopictus populations in China. However, the susceptibility of Ae. albopictus to pyrethroids in Hainan Island was unclear. Knockdown resistance (kdr), caused by point mutations in the VGSC gene, is one of the mechanisms that confer resistance to DDT and pyrethroids. This study was to investigate the resistance level of Ae. albopictus populations in Haikou City to three pyrethroid insecticides, and elucidate the relationship between the resistant phenotype and kdr mutations.

Methods

The Aedes albopictus samples were collected in Xinbu Island (XI), Longtang Town (LT), Shishan Town (ST), Baishamen Park (BP), and Flower Market (FM) from Haikou City, Hainan Island, China. The larval susceptibility to deltamethrin, permethrin and beta-cypermethrin was tested by larval bioassays, and adult susceptibility to deltamethrin and DDT was determined by adult bioassays. The degree of resistance was determined by resistance ratio value (RR₅₀ > 3) for larvae and by mortality for adult. The kdr alleles at codon 1534 of the VGSC gene were genotyped. The relationship between kdr genotypes and resistant phenotypes was analyzed by Chi-square test.

Results

Out of five populations, assessed by larval bioassays, XI was susceptible to deltamethrin and permethrin; LT was susceptible to permethrin and beta-cypermethrin; and ST was susceptible to permithrin. FM and BP both were resistant to all of the three pyrethroids, and FM showed the highest degree of resistance, with RR₅₀ values from 65.17 to 436.36. A total of 493 individuals from the larval bioassays were genotyped for kdr alleles. Five alleles were detected, including two wildtype alleles, TTC(F) (67.04 %) and TTT(F) (0.41 %), and three mutant alleles, TGC(C) (0.30 %), TCC(S) (31.54 %) and TTG(L) (0.71 %). There was a clear correlation between mutant alleles (or F1534S) and resistant phenotypes (P < 0.01).

Conclusion

Two novel kdr mutant alleles F1534S and F1534L were detected in the pyrethroid resistant populations of Ae. albopictus in Haikou Hainan, China. For the first time, the mutant F1534S was associated with pyrethroid resistance in Ae. albopictus.

Electronic supplementary material

The online version of this article (doi:10.1186/s40249-016-0125-x) contains supplementary material, which is available to authorized users.

Seasonal dynamics of insecticide resistance, multiple resistance, and morphometric variation in field populations of Culex pipiens

Resistance to insecticides that impairs nervous transmission has been widely investigated in mosquito populations as insecticides are crucial to effective insect control. The development of insecticide resistance is also of special interest to evolutionary biologists since it represents the opportunity to observe the genetic consequences of a well-characterized alteration in the environment. Although the frequencies of resistance alleles in Culex pipiens populations against different groups of insecticides have been reported, no detailed information is available on the relative change in these allele frequencies over time. In this study, we collected mosquitoes of the Cx. pipiens complex from six locations in three seasons in the Aegean region of Turkey and examined the i) seasonal variations in resistance to four different chemical classes of insecticides, ii) seasonal fluctuations in frequencies of resistance-associated target-site mutations of the three genes (ace-1, kdr, and Rdl), and iii) potential seasonal variations in wing morphometric characters that may be modified in resistant mosquitoes. Our bioassay results indicated the presence of different levels of resistance to all tested insecticides for all three seasons in all locations. The results of the PCR-based molecular analysis revealed low frequencies of mutations in ace-1 and Rdl that are associated with resistance to malathion, bendiocarb, and dieldrin and no obvious seasonal changes. In contrast, we detected high frequencies and striking seasonal changes for two kdr mutations associated with resistance to DDT and pyrethroids. In addition, the evaluation of the field populations from all seasons in terms of the combinations of polymorphisms at four resistance-associated mutations did not reveal the presence of insects that are resistant to all pesticides. Results from the morphological analysis displayed a similar pattern for both wings and did not show a clear separation among the samples from the three different seasons. The results of this study have advanced our knowledge of the potential dynamics of insecticide resistance among populations of the Cx. pipiens complex. The implications of these results to the understanding of the evolution of insecticide resistance and the management of resistance in mosquitoes are discussed.

An updated checklist of mosquito species (Diptera: Culicidae) from Madagascar

An updated checklist of 235 mosquito species from Madagascar is presented. The number of species has increased considerably compared to previous checklists, particularly the last published in 2003 (178 species). This annotated checklist provides concise information on endemism, taxonomic position, developmental stages, larval habitats, distribution, behavior, and vector-borne diseases potentially transmitted. The 235 species belong to 14 genera: Aedeomyia (3 species), Aedes (35 species), Anopheles (26 species), Coquillettidia (3 species), Culex (at least 50 species), Eretmapodites (4 species), Ficalbia (2 species), Hodgesia (at least one species), Lutzia (one species), Mansonia (2 species), Mimomyia (22 species), Orthopodomyia (8 species), Toxorhynchites (6 species), and Uranotaenia (73 species). Due to non-deciphered species complexes, several species remain undescribed. The main remarkable characteristic of Malagasy mosquito fauna is the high biodiversity with 138 endemic species (59%). Presence and abundance of species, and their association, in a given location could be a bio-indicator of environmental particularities such as urban, rural, forested, deforested, and mountainous habitats. Finally, taking into account that Malagasy culicidian fauna includes 64 species (27%) with a known medical or veterinary interest in the world, knowledge of their biology and host preference summarized in this paper improves understanding of their involvement in pathogen transmission in Madagascar.

Transstadial Effects of Bti on Traits of Aedes aegypti and Infection with Dengue Virus

Most mosquito control efforts are primarily focused on reducing the adult population size mediated by reductions in the larval population, which should lower risk of disease transmission. Although the aim of larviciding is to reduce larval abundance and thus recruitment of adults, nonlethal effects on adults are possible, including transstadial effects on phenotypes of adults such as survival and pathogen infection and transmission. In addition, the mortality induced by control efforts may act in conjunction with other sources of mosquito mortality in nature. The consequences of these effects and interactions may alter the potential of the population to transmit pathogens. We tested experimentally the combined effects of a larvicide (Bacillus thuringiensis ssp. israelensis, Bti) and competition during the larval stages on subsequent Aedes aegypti (Linnaeus) traits, population performance, and susceptibility to dengue-1 virus infection. Ae. aegypti that survived exposure to Bti experienced accelerated development, were larger, and produced more eggs with increasing amounts of Bti, consistent with competitive release among surviving mosquitoes. Changing larval density had no significant interactive effect with Bti treatment on development and growth to adulthood. Larval density, but not Bti or treatment interaction, had a strong effect on survival of adult Ae. aegypti females. There were sharper declines in cumulative daily survival of adults from crowded than uncrowded larval conditions, suggesting that high competition conditions of larvae may be an impediment to transmission of dengue viruses. Rates of infection and dengue-1 virus disseminated infections were found to be 87±13% and 88±12%, respectively. There were no significant treatment effects on infection measurements. Our findings suggest that larvicide campaigns using Bti may reduce the number of emerged adults, but survivors will have a fitness advantage (growth, development, enhanced production of eggs) relative to conspecifics that are not under larvicide pressure. However, under most circumstances, these transstadial effects are unlikely to outweigh reductions in the adult population by Bti and altered risk of disease transmission.

Effective control of mosquito-borne diseases like dengue fever has historically been achieved by controlling vector populations. The use of larvicides that kill the larval stages of the yellow fever mosquito is a critical component of effective control. However, larvicides may act together with other components of the environment like larval crowding and alter the biology of adult mosquitoes. The authors found that mosquitoes that survived exposure to the larvicide Bacillus thuringiensis spp. israelensis (Bti) experienced faster development, were bigger, and produced more offspring. Larval crowding, but not larvicide Bti, had a strong effect on survival of adult Aedes aegypti (Linnaeus) females. Adult mosquitoes from crowded larval conditions had reduced survival compared to individuals from uncrowded larval conditions. Exposure to the larvicide Bti or larval crowding did not alter the ability of the surviving adult mosquitoes to become infected with dengue virus. Our findings suggest that larvicide campaigns may reduce the number of emerged adults, but survivors will have a fitness advantage (growth, development, enhanced production of offspring) relative to mosquitoes not exposed to the larvicide. However, under most circumstances, these effects are unlikely to outweigh reductions in the adult population by larvicide Bti and altered risk of disease transmission.

Comparison of Irradiation and Wolbachia Based Approaches for Sterile-Male Strategies Targeting Aedes albopictus

The global expansion of Aedes albopictus together with the absence of vaccines for most of the arboviruses transmitted by this mosquito has stimulated the development of sterile-male strategies aiming at controlling disease transmission through the suppression of natural vector populations. In this context, two environmentally friendly control strategies, namely the Sterile Insect Technique (SIT) and the Wolbachia-based Incompatible Insect Technique (IIT) are currently being developed in several laboratories worldwide. So far however, there is a lack of comparative assessment of these strategies under the same controlled conditions. Here, we compared the mating capacities, i.e. insemination capacity, sterilization capacity and mating competitiveness of irradiated (35 Gy) and incompatible Ae. albopictus males at different ages and ratios under laboratory controlled conditions. Our data show that there was no significant difference in insemination capacity of irradiated and incompatible males, both male types showing lower capacities than untreated males at 1 day but recovering full capacity within 5 days following emergence. Regarding mating competitiveness trials, a global observed trend is that incompatible males tend to induce a lower hatching rate than irradiated males in cage controlled confrontations. More specifically, incompatible males were found more competitive than irradiated males in 5:1 ratio regardless of age, while irradiated males were only found more competitive than incompatible males in the 1:1 ratio at 10 days old. Overall, under the tested conditions, IIT seemed to be slightly more effective than SIT. However, considering that a single strategy will likely not be adapted to all environments, our data stimulates the need for comparative assessments of distinct strategies in up-scaled conditions in order to identify the most suitable and safe sterilizing technology to be implemented in a specific environmental setting and to identify the parameters requiring fine tuning in order to reach optimal release conditions.

Genotype to phenotype, the molecular and physiological dimensions of resistance in arthropods

The recent accumulation of molecular studies on mutations in insects, ticks and mites conferring resistance to insecticides, acaricides and biopesticides is reviewed. Resistance is traditionally classified by physiological and biochemical criteria, such as target-site insensitivity and metabolic resistance. However, mutations are discrete molecular changes that differ in their intrinsic frequency, effects on gene dosage and fitness consequences. These attributes in turn impact the population genetics of resistance and resistance management strategies, thus calling for a molecular genetic classification. Mutations in structural genes remain the most abundantly described, mostly in genes coding for target proteins. These provide the most compelling examples of parallel mutations in response to selection. Mutations causing upregulation and downregulation of genes, both in cis (in the gene itself) and in trans (in regulatory processes) remain difficult to characterize precisely. Gene duplications and gene disruption are increasingly reported. Gene disruption appears prevalent in the case of multiple, hetero-oligomeric or redundant targets.

Pyrethroid resistance in Culex pipiens mosquitoes

Mosquitoes within the Culex pipiens complex are widely distributed and important in the transmission of many human diseases. Insecticides, pyrethroids in particular, remain a mainstay for control of these important vectors. In this paper we review what is known about the levels, mechanisms and fitness costs of pyrethroid resistance in Cx. pipiens. Pyrethroid resistance in Cx. pipiens is a global problem, and resistance ratios of up to 7000-fold have been found in larvae of field collected mosquitoes. However, there is considerable variation between populations, indicating significant geographic heterogeneity of the resistance. The two major mechanisms of resistance to pyrethroids in Culex are mutations in Vssc (target site insensitivity) and overexpression of cytochrome P450(s) (increased detoxification). The most frequently reported Vssc mutation is L1014F (i.e. kdr), which has been found throughout the world. The L1014S mutation has been found in Cx. p. pallens from Japan and China, and in Cx. p. pipiens from China. The L1014C mutation has only been reported for Cx. p. pipens molestus from China and the V1016G mutation has only been reported from Saudi Arabia. Studies on the P450s of Cx. pipiens have identified several that are overexpressed (measured as transcript levels) in pyrethroid resistant strains. CYP9M10 is consistently overexpressed in pyrethroid resistant Cx. pipiens from at least seven countries, suggesting this P450 might be of global importance in resistance. Both CYP9M10-mediated pyrethroid resistance and kdr have fitness costs in the absence of insecticides under certain environmental conditions. Research needs and future directions are discussed.

Target-site resistance mutations (kdr and RDL), but not metabolic resistance, negatively impact male mating competiveness in the malaria vector Anopheles gambiae

The implementation of successful insecticide resistance management strategies for malaria control is currently hampered by poor understanding of the fitness cost of resistance on mosquito populations, including their mating competiveness. To fill this knowledge gap, coupled and uncoupled Anopheles gambiae s.l. males (all M form (Anopheles coluzzii)) were collected from mating swarms in Burkina Faso. This multiple insecticide resistant population exhibited high 1014F kdr^R allele frequencies (>60%) and RDL^R (>80%) in contrast to the Ace-1^R allele (<6%). Kdr heterozygote males were more likely to mate than homozygote resistant (OR=2.36; P<0.001), suggesting a negative impact of kdr on An. coluzzii mating ability. Interestingly, heterozygote males were also more competitive than homozygote susceptible (OR=3.26; P=0.006), suggesting a heterozygote advantage effect. Similarly, heterozygote RDL^R/RDL^S were also more likely to mate than homozygote-resistant males (OR=2.58; P=0.007). Furthermore, an additive mating disadvantage was detected in male homozygotes for both kdr/RDL-resistant alleles. In contrast, no fitness difference was observed for the Ace-1 mutation. Comparative microarray-based genome-wide transcription analysis revealed that metabolic resistance did not significantly alter the mating competitiveness of male An. coluzzii mosquitoes. Indeed, no significant difference of expression levels was observed for the main metabolic resistance genes, suggesting that metabolic resistance has a limited impact on male mating competiveness. In addition, specific gene classes/GO terms associated with mating process were detected including sensory perception and peroxidase activity. The detrimental impact of insecticide resistance on mating competiveness observed here suggests that resistance management strategies such as insecticide rotation could help reverse the resistance, if implemented early.

Molecular markers for analyses of intraspecific genetic diversity in the Asian Tiger mosquito, Aedes albopictus

Background

The dramatic worldwide expansion of Aedes albopictus (the Asian tiger mosquito) and its vector competence for numerous arboviruses represent a growing threat to public health security. Molecular markers are crucially needed for tracking the rapid spread of this mosquito and to obtain a deeper knowledge of population structure. This is a fundamental requirement for the development of strict monitoring protocols and for the improvement of sustainable control measures.

Methods

Wild population samples from putative source areas and from newly colonised regions were analysed for variability at the ribosomal DNA internal transcribed spacer 2 (ITS2). Moreover, a new set of 23 microsatellite markers (SSR) was developed. Sixteen of these SSRs were tested in an ancestral (Thailand) and two adventive Italian populations.

Results

Seventy-six ITS2 sequences representing 52 unique haplotypes were identified, and AMOVA indicated that most of their variation occurred within individuals (74.36%), while only about 8% was detected among populations. Spatial analyses of molecular variance revealed that haplotype genetic similarity was not related to the geographic proximity of populations and the haplotype phylogeny clearly indicated that highly related sequences were distributed across populations from different geographical regions. The SSR markers displayed a high level of polymorphism both in the ancestral and in adventive populations, and F_ST estimates suggested the absence of great differentiation. The ancestral nature of the Thai population was corroborated by its higher level of variability.

Conclusions

The two types of genetic markers here implemented revealed the distribution of genetic diversity within and between populations and provide clues on the dispersion dynamics of this species. It appears that the diffusion of this mosquito does not conform to a progressive expansion from the native Asian source area, but to a relatively recent and chaotic propagule distribution mediated by human activities. Under this scenario, multiple introductions and admixture events probably play an important role in maintaining the genetic diversity and in avoiding bottleneck effects. The polymorphic SSR markers here implemented will provide an important tool for reconstructing the routes of invasion followed by this mosquito.

Contrasting patterns of insecticide resistance and knockdown resistance (kdr) in the dengue vectors Aedes aegypti and Aedes albopictus from Malaysia

Background

Knowledge on the extent, distribution and mechanisms of insecticide resistance is essential for successful insecticide-based dengue control interventions. Here, we report an extensive resistance profiling of the dengue vectors Aedes aegypti and Aedes albopictus across Malaysia and establish the contribution of knockdown resistance mechanism revealing significant contrast between both species.

Methods

Aedes mosquitoes were collected from four states in Malaysia in 2010 using ovitraps and tested against six major insecticides using WHO bioassays. Knockdown resistance (kdr) was investigated in both species.

Results

A moderate resistance to temephos was detected from samples collected in 2010 in Penang, Kuala Lumpur, Johor Bharu and Kota Bharu (1.5 < RR < 3.3). A widespread and multiple resistances was observed in Ae. aegypti particularly against pyrethroids, DDT and bendiocarb. Mosquitoes from Kuala Lumpur consistently had the highest resistance levels and was the only population showing a moderate resistance to malathion (91% mortality). The resistance profile of Ae. albopictus contrasted to Ae. aegypti with full susceptibility to pyrethroids except in Kuala Lumpur where moderate resistance is observed. PBO synergist assays suggest metabolic resistance mechanisms play a major role in resistance in both species. Two kdr mutations, F1534C and V1016G, were detected in Ae. aegypti across Malaysia but neither of these mutations were found in Ae. albopictus. Additionally, signatures of selection were detected on the Voltage-gated sodium channel gene in Ae. aegypti but not in Ae. albopictus. The presence of the 1534C allele was significantly associated with pyrethroid resistance and an additive effect to pyrethroid resistance was observed in individuals containing both kdr alleles.

Conclusions

Findings from this study will help to design and implement successful insecticide-based interventions against Ae. aegypti and Ae. albopictus to improve dengue control across Malaysia.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-0797-2) contains supplementary material, which is available to authorized users.

Wolbachia-based population control strategy targeting Culex quinquefasciatus mosquitoes proves efficient under semi-field conditions

In mosquitoes, the maternally inherited bacterial Wolbachia induce a form of embryonic lethality called cytoplasmic incompatibility (CI). This property can be used to reduce the density of mosquito field populations through inundative releases of incompatible males in order to sterilize females (Incompatible Insect Technique, or IIT, strategy). We have previously constructed the LR[wPip(Is)] line representing a good candidate for controlling field populations of the Culex quinquefasciatus mosquito in the islands of the south-western Indian Ocean. The main purpose of the present study was to fill the gap between laboratory experiments and field implementation, i.e. assessing mating competitiveness of these incompatible males under semi-field conditions. In a first set of experiments, we analyzed crossing relationships between LR[wPip(Is)] males and La Réunion field females collected as larvae in 19 distinct localities throughout the island. This investigation revealed total embryonic mortality, confirming the strong sterilizing capacity of LR[wPip(Is)] males. Subsequently, mating competitiveness of LR[wPip(Is)] males was assessed under semi-field conditions in the presence of field males and females from La Réunion. Confrontations were carried out in April and December using different ratios of LR[wPip(Is)] to field males. The results indicated that the LR[wPip(Is)] males successfully compete with field males in mating with field females, displaying even higher competitiveness than field males in April. Our results support the implementation of small-scale field tests in order to assess the feasibility of IIT against Cx. quinquefasciatus in the islands of southwestern Indian Ocean where this mosquito species is a proven competent vector for human pathogens.

Over-transcription of genes in a parathion-resistant strain of mosquito Culex pipiens quinquefasciatus

Insecticide resistance is an evolutionary adaptation that develops quite quickly in mosquitoes because of the high selection pressure of chemical insecticides, rapid generation time and large population size. Identification of genes associated with insecticide resistance is fundamental to understand the complex processes responsible for resistance. We compared the gene transcriptional profiles of parathion-resistant and -susceptible Culex pipiens quinquefasciatus using a combination of suppression subtractive hybridization and complementary DNA (cDNA) microarray techniques. A total of 278 colonies were selected from the resistant-susceptible mosquito subtractive library, 38 of which showed more than two fold stronger immunoblotting signals in the resistant strain than in the susceptible strain using cDNA microarray selection. The sequencing results showed that the 38 colonies can be matched to 12 genes of C. p. quinquefasciatus. Eight genes were confirmed to be overexpressed by more than two fold in the resistant strain. These genes encode chymotrypsin-1, theta glutathione S-transferase, lipase 3, larval serum protein 1 β chain, cytochrome b, mitochondrial ribosomal large subunit, 28S rRNA, and a protein with unknown function. This study serves as a preliminary attempt to identify new genes associated with organophosphate resistance in this mosquito species and provides insights into the complicated physiological phenomenon of insecticide resistance.

Effects of Croton rhamnifolioides essential oil on Aedes aegypti oviposition, larval toxicity and trypsin activity

Although numerous reports are available concerning the larvicidal potential of essential oils, very few investigations have focused on their mechanisms of action. In the present study, we have investigated the chemical composition of the leaf oil of Croton rhamnifolioides during storage and its effects on oviposition and survival of larvae of the dengue fever mosquito Aedes aegypti. In addition, we have established a possible mechanism of action for the larvicidal activity of the essential oil. GC-MS analyses revealed marked differences in the composition of oil that had been freshly isolated and that of a sample that had been stored in a sealed amber-glass vial under refrigeration for three years. However, both fresh and stored oil exhibited substantial larvicidal activities with LC50 values of 122.35 and 89.03 ppm, respectively, and oviposition deterrent effects against gravid females at concentrations of 50 and 100 µg·mL-1. These results demonstrate that the larvicidal effect of the essential oil was unchanged during three years of storage even though its chemical composition altered. Hence, the essential oil could be used in the preparation of commercial products. In addition, we observed that the trypsin-like activity of mosquito larvae was inhibited in vitro by the essential oil of C. rhamnifolioides, suggesting that the larvicidal effect may be associated with inhibition of this enzyme.

Point mutations associated with organophosphate and carbamate resistance in Chinese strains of Culex pipiens quinquefasciatus (Diptera: Culicidae)

Acetylcholinesterase resistance has been well documented in many insects, including several mosquito species. We tested the resistance of five wild, Chinese strains of the mosquito Culex pipiens quinquefasciatus to two kinds of pesticides, dichlorvos and propoxur. An acetylcholinesterase gene (ace1) was cloned and sequenced from a pooled sample of mosquitoes from these five strains and the amino acids of five positions were found to vary (V185M, G247S, A328S, A391T, and T682A). Analysis of the correlation between mutation frequencies and resistance levels (LC₅₀) suggests that two point mutations, G247S (r² = 0.732, P = 0.065) and A328S (r² = 0.891, P = 0.016), are associated with resistance to propoxur but not to dichlorvos. Although the V185M mutation was not associated with either dichlorvos or propoxur resistance, its RS genotype frequency was correlated with propoxur resistance (r² = 0.815, P = 0.036). And the HWE test showed the A328S mutation is linked with V185M, also with G247S mutation. This suggested that these three mutations may contribute synergistically to propoxur resistance. The T682A mutation was negatively correlated with propoxur (r² = 0.788, P = 0.045) resistance. Knowledge of these mutations may help design strategies for managing pesticide resistance in wild mosquito populations.

Guidelines to site selection for population surveillance and mosquito control trials: a case study from Mauritius

Many novel approaches to controlling mosquito vectors through the release of sterile and mass reared males are being developed in the face of increasing insecticide resistance and other limitations of current methods. Before full scale release programmes can be undertaken there is a need for surveillance of the target population, and investigation of parameters such as dispersal and longevity of released, as compared to wild males through mark-release-recapture (MRR) and other experiments, before small scale pilot trials can be conducted. The nature of the sites used for this field work is crucial to ensure that a trial can feasibly collect sufficient and relevant information, given the available resources and practical limitations, and having secured the correct regulatory, community and ethical approvals and support. Mauritius is considering the inclusion of the sterile insect technique (SIT), for population reduction of Aedes albopictus, as a component of the Ministry of Health and Quality of Life's 'Operational Plan for Prevention and Control of Chikungunya and Dengue'. As part of an investigation into the feasibility of integrating the SIT into the Integrated Vector Management (IVM) scheme in Mauritius a pilot trial is planned. Two potential sites have been selected for this purpose, Pointe des Lascars and Panchvati, villages in the North East of the country, and population surveillance has commenced. This case study will here be used to explore the considerations which go into determining the most appropriate sites for mosquito field research. Although each situation is unique, and an ideal site may not be available, this discussion aims to help researchers to consider and balance the important factors and select field sites that will meet their needs.

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.