Baseline Insecticide Susceptibility Screening Against Six Active Ingredients for Culex and Aedes (Diptera: Culicidae) Mosquitoes in the United States

Evaluation of the In2care Mosquito Station against Culex quinquefasciatus mosquitoes (Diptera: Culicidae) under semifield conditions

Culex quinquefasciatus is an important mosquito vector responsible for the transmission of filarial worms, arthropod-borne viruses like Oropouche, St. Louis encephalitis, and West Nile and protozoans that cause avian malaria. Due to insecticide resistance documented in Cx. quinquefasciatus populations worldwide, integrated vector management programs can benefit from new strategies to control this species. The In2Care Mosquito Station (In2Care station), a commercially available dissemination station containing pyriproxyfen (PPF) and Beauveria bassiana spores, has been shown to be effective against skip-ovipositing Aedes aegypti and Aedes albopictus in previously conducted semifield and field trials. To determine the potential of Cx. quinquefasciatus adult females to autodisseminate PPF and if the In2Care station could be used for Cx. quinquefasciatus control, we assessed its efficacy in a semifield setting against wild Cx. quinquefasciatus. We found that the In2Care station was attractive to gravid Cx. quinquefasciatus females, with a significantly higher percentage of egg rafts laid in the In2Care station compared to alternative ovipots. Adult females successfully autodisseminated PPF from the In2Care station to surrounding ovipots, leading to a significant increase in mosquito emergence inhibition. Additionally, adult Cx. quinquefasciatus exposure to B. bassiana spores significantly reduced mosquito survivorship. These results suggest that the In2Care station may be effective against Cx. quinquefasciatus in addition to Ae. aegypti and Ae. albopictus. Additional field evaluations are needed to assess impacts at the population level.

EFFECT OF SEX AND AGE ON SURVIVAL OF ADULT CULEX TARSALIS FROM A SUSCEPTIBLE LABORATORY STRAIN EXPOSED TO PERMETHRIN IN THE CDC BOTTLE BIOASSAY

This study investigates the effect of mosquito sex and age on the survival and resistance determination of adult Culex tarsalis exposed to permethrin, a pyrethroid commonly used for mosquito control, using the Centers for Disease Control and Prevention (CDC) bottle bioassay method. A permethrin-susceptible strain of Cx. tarsalis (Bakersfield strain) was used in this study. Survival was compared for young adult females (2-5 days old) relative to older adult females (7-10 days old) and separately for female and male mosquitoes of the same age (2-5 days old). Mortality was slightly higher for males than females during the first observation period (0-5 min) following permethrin exposure, and higher for older females relative to younger females from 5 to 10 min following permethrin exposure, with no differences in mortality by either sex or age for observation periods during the remainder of the diagnostic period. When evaluated over the full diagnostic period, survival varied with mosquito age but not sex. However, all mosquitoes, regardless of sex or age, died within the 30-min diagnostic period for this species, confirming their permethrin susceptibility per the CDC bottle bioassay. This research contributes valuable insight into the potential impact of sex and age on mosquito susceptibility to insecticides in the context of insecticide resistance determination.

Development of novel compact wind tunnel for testing efficacy of insecticide formulated products in mosquitoes

BACKGROUND

Ultra-low volume (ULV) space sprays aerosolize insecticide formulated products (FP) to contact flying mosquitoes, while barrier sprays expose mosquitoes to FP residue on vegetation and other surfaces. Centers for Disease Control and Prevention bottle bioassays used to assess insecticide resistance are based on residual active ingredient (AI) exposure and do not directly relate to FP efficacy. The current pilot study developed a novel compact wind tunnel for mosquito exposure to FP. Caged Aedes albopictus and Culex pipiens/quinquefasciatus were exposed to undiluted Biomist®3 + 15 FP (permethrin AI) or air (control) within the wind tunnel, transferred to new cages, and held in a 28 °C incubator. Separate mosquitoes were exposed to residual permethrin AI (8 μg mL-1) in bottle bioassays. Mortality was monitored 15, 30, 60, and 120 min post-exposure.

RESULTS

Chi-square tests (P < 0.05) showed significantly higher mortality in Aedes compared to Culex populations for most time points in both bioassay and wind tunnel exposure groups. As expected, mosquitoes exposed to Biomist®3 + 15 showed higher mortality rates than bottle bioassay exposure to permethrin. Two Culex colonies resistant to permethrin in bottle bioassays were susceptible to Biomist®3 + 15 in the wind tunnel.

CONCLUSION

The novel compact wind tunnel developed here may be an alternative to field trials for testing FP efficacy, avoiding factors such as weather, logistical planning, and extended personnel hours. The wind tunnel could allow programs to conveniently test efficacy of multiple FP. Comparisons of different insecticide exposure methods provide practical information to inform operational decisions. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Insecticide resistance of Miami-Dade Culex quinquefasciatus populations and initial field efficacy of a new resistance-breaking adulticide formulation

Sporadic outbreaks of human cases of West Nile virus (WNV), primarily vectored by Culex quinquefasciatus Say in suburban and urban areas, have been reported since introduction of the virus into Florida in 2001. Miami-Dade County, Florida is part of one of the largest metropolitan areas in the United States, supports Cx. quinquefasciatus year-round, and recently experienced over 60 human cases of WNV during one outbreak. To facilitate more effective integrated vector management and public health protection, we used the Centers for Disease Control and Prevention (CDC) bottle bioassay method to evaluate the susceptibility of adult Cx. quinquefasciatus collected from 29 locations throughout Miami-Dade County to pyrethroid and organophosphate adulticide active ingredients (AIs) used by Miami-Dade County Mosquito Control. We also determined the frequency of the 1014 knockdown resistance (kdr) mutation for Cx. quinquefasciatus from a subset of 17 locations. We detected resistance to two pyrethroid AIs in all tested locations (permethrin: 27 locations, deltamethrin: 28 locations). The 1014F allele was widely distributed throughout all 17 locations sampled; however, 29.4% of these locations lacked 1014F homozygotes even though phenotypic pyrethroid resistance was present. Organophosphate resistance was more variable; 20.7% of the locations tested were susceptible to malathion, and 33.3% of the populations were susceptible to naled. We subsequently conducted a field trial of ReMoa Tri, a recently approved multiple AI adulticide formulation labelled for resistant mosquitoes, against a mixed location field population of Miami-Dade Cx. quinquefasciatus. Average 24-hr mortality was 65.1 ± 7.2% and 48-hr mortality increased to 85.3 ± 9.1%, indicating good control of these resistant Cx. quinquefasciatus. This current study shows that insecticide resistance is common in local Cx. quinquefasciatus but effective options are available to maintain control during active disease transmission in Miami-Dade County.

Prevalence of Permethrin Resistance in Culex Tarsalis Populations in Southern California

In the western United States, Culex tarsalis is the most important vector of West Nile virus. Insecticides containing permethrin or other pyrethroid compounds are commonly used to control these mosquitoes. Because of the range of environments where Cx. tarsalis are found, this species is under insecticide pressure from both vector control and agricultural spraying. Mosquito populations may evolve resistance through mechanisms such as target site insensitivity, including the frequently identified knockdown resistance (kdr) mutations. Prevalence of permethrin resistance was determined for Cx. tarsalis from 5 southern California field sites representing 2 distinct valley regions (Coachella Valley and Inland Valley), which are geographically separated by the north-south-running Peninsular Mountain Ranges. These two valley regions are >100 km apart and vary considerably in their environmental and habitat characteristics. Permethrin resistance in mosquito populations was determined by the Centers for Disease Control and Prevention (CDC) bottle bioassay, using glass bottles coated with permethrin at 0.19 μg/cm2 of internal surface. Permethrin resistance was evident in Cx. tarsalis populations from the Coachella Valley field sites with all sites showing similar mortality in the bottle bioassay, while Cx. tarsalis from the Inland Valley field sites were largely susceptible to permethrin, with mortality rates that were similar to a susceptible lab strain of Cx. tarsalis.

Persistent variation in insecticide resistance intensity in container breeding Aedes (Diptera: Culicidae) co-collected in Houston, TX

As observed in many locations worldwide, resistance to pyrethroids is common in Aedes aegypti (L.) in the southern United States and northern Mexico. Strong resistance in Aedes albopictus (Skuse) is less common and is not as well characterized. These 2 species have been undergoing range expansion and are sympatric in many locations including Houston, Texas. They are often collected from the same locations and lay eggs in the same larval habitats. In this study, we colonized both Ae. aegypti and Ae. albopictus from 4 locations in Houston and characterized insecticide resistance using permethrin as a model pyrethroid. We found differences in resistance intensity between the species at all 4 sites. Within the Ae. aegypti, resistance ratios ranged from 3.5- to 30.0-fold when compared to the ORL1952 laboratory susceptible strain. Expression of several P450s was higher than in the ORL1952 strain, but the pattern was similar between the field strains of Ae. aegypti. Higher resistance ratios did correlate with increasing percentages of the dilocus knockdown resistance (kdr) genotype. In contrast, Ae. albopictus from the 4 locations all had very low resistance ratios (<4-fold) when compared to the same laboratory susceptible strain. Five years later, we performed additional collections and characterization from the most resistant location to assess the temporal persistence of this difference in resistance between the species. The same pattern of high resistance in Ae. aegypti and low resistance in sympatric Ae. albopictus remained 5 yr later and this may have implications for operational efficacy.

Survey of United States Mosquito Control Programs Reveals Opportunities to Improve the Operational Value of Centers for Disease Control and Prevention Bottle Bioassays

Mosquito control programs have increasingly used Centers for Disease Control and Prevention bottle bioassays (BB) to evaluate insecticide resistance (IR). The reported utility, benefits, and limitations of BB and other methods were assessed via electronic survey of U.S. mosquito control professionals to identify potential areas for improvement, future study, and professional training. Opportunities were identified to improve BB operational value and IR detection.

Impacts of differential mosquito control treatment regimens on insecticide susceptibility status of Aedes aegypti (Diptera: Culicidae)

Abstract

Aedes aegypti (Linnaeus) is an invasive mosquito species and notable vector of several pathogens in the USA. Their cryptic and anthropophilic nature puts this species in close association with humans, where they can also be a nuisance. Mosquito control programs are the front line of defense for protecting the community from nuisance-biting and disease. However, the occurrence and prevalence of insecticide resistance in mosquitoes is a well-documented phenomenon that directly impacts the efficacy of insecticide applications. In Florida specifically, widespread resistance in Ae. aegypti has created a need for operational strategies that combat and, ideally, reverse resistance. Laboratory studies and the association between fitness costs and insecticide resistance indicate that this reversion is possible under the right conditions. For a 2.5-year period, the impact of varying operational treatment regimens on insecticide resistance in Ae. aegypti is evaluated using kdr genotyping and the CDC bottle bioassay. In an organophosphate treatment area, a decrease in frequency of a double homozygous resistant genotype was observed. CDC bottle bioassays did not reveal any clear trends in the data to indicate a reversion to insecticide susceptibility. However, the changes in genotype could indicate the first step back to insecticide susceptibility. This study provides preliminary data that has implications for resistance management in mosquito control operations.

Article Highlights

A Field Efficacy Evaluation of In2Care Mosquito Traps in Comparison with Routine Integrated Vector Management at Reducing Aedes aegypti

Aedes aegypti is the predominant vector of dengue, chikungunya, and Zika viruses. This mosquito is difficult to control with conventional methods due to its container-inhabiting behavior and resistance to insecticides. Autodissemination of pyriproxyfen (PPF), a potent larvicide, has shown promise as an additional tool to control Aedes species in small-scale field trials. However, few large-scale field evaluations have been conducted. We undertook a 6-month-long large-scale field study to compare the effectiveness and operational feasibility of using In2Care Mosquito Traps (In2Care Traps, commercially available Aedes traps with PPF and Beauveria bassiana) compared to an integrated vector management (IVM) strategy consisting of source reduction, larviciding, and adulticiding for controlling Ae. aegypti eggs, larvae, and adults. We found that while the difference between treatments was only statistically significant for eggs and larvae (P < 0.05 for eggs and larvae and P > 0.05 for adults), the use of In2Care Traps alone resulted in 60%, 57%, and 57% fewer eggs, larvae, and adults, respectively, collected from that site compared to the IVM site. However, In2Care Trap deployment and maintenance were more time consuming and labor intensive than the IVM strategy. Thus, using In2Care Traps alone as a control method for large areas (e.g., >20 ha) may be less practical for control programs with the capacity to conduct ground and aerial larviciding and adulticiding. Based on our study results, we conclude that In2Care Traps are effective at suppressing Ae. aegypti and have the most potential for use in areas without sophisticated control programs and within IVM programs to target hotspots with high population levels and/or risk of Aedes-borne pathogen transmission.

Synthesis, antimosquito activities, photodegradation, and toxic assessment of novel pyrethroids containing 2-chlorobiphenyl and 2-chlorophenylpyridine

BACKGROUND

Aedes albopictus is a mosquito species and a vector of dengue virus and malaria parasites that represents a significant threat to global public health. Although mosquito populations have been effectively controlled through the use of synthetic insecticides, the emergence of widespread insecticide resistance in wild mosquito populations is a strong motivation to explore new insecticidal chemistries.

RESULTS

In this study, A. albopictus was treated with a series of novel pyrethroids containing 2-chlorobiphenyl and 2-chlorophenylpyridine via topical application. The relative antimosquito activity of each novel compound was determined, as measured by the LC₅₀ , and compared with the synthetic pyrethroid bifenthrin. The most antimosquito activity compound (SZ-B-11) was 4.69 times more active than bifenthrin. The novel compounds were also sensitive to wild A. albopictus. In addition, in silico toxicity assessment of aquatic organisms showed that the acute toxicity and chronic toxicity of SZ-B-11 were 31.96 times and 934.40 times lower than those of bifenthrin, respectively. Cytotoxicity assessment demonstrated that all tested compounds were nontoxic against SH-SY5Y cell lines. Furthermore, photolytic results suggested that SZ-B-11 would be photodegraded more easily than bifenthrin and would reduce secondary pollution.

CONCLUSION

Novel pyrethroids containing 2-chlorobiphenyl and 2-chlorophenylpyridine, through simple synthesis steps, have better antimosquito activity, low resistance, less ecotoxicity, readily degradable characteristics, and may reduce secondary pollution. They are promising insecticides with potential application prospects in agricultural production and environmental protection. © 2021 Society of Chemical Industry.

NEVBD Pesticide Resistance Monitoring Network: Establishing a Centralized Network to Increase Regional Capacity for Pesticide Resistance Detection and Monitoring

Pesticide resistance in arthropod vectors of disease agents is a growing issue globally. Despite the importance of resistance monitoring to inform mosquito control programs, no regional monitoring programs exist in the United States. The Northeastern Regional Center for Excellence in Vector-Borne Diseases (NEVBD) is a consortium of researchers and public health practitioners with a primary goal of supporting regional vector control activities. NEVBD initiated a pesticide resistance monitoring program to detect resistant mosquito populations throughout the northeastern United States. A regionwide survey was distributed to vector control agencies to determine needs and refine program development and in response, a specimen submission system was established, allowing agencies to submit Culex pipiens (L.) (Diptera:Culicidae) and Aedes albopictus (Skuse) (Diptera: Culicidae) for pesticide resistance testing. NEVBD also established larvicide resistance diagnostics for Bacillus thuringiensis israelensis (Bti) and methoprene. Additional diagnostics were developed for Cx. pipiens resistance to Lysinibacillus sphaericus. We received 58 survey responses, representing at least one agency from each of the 13 northeastern U.S. states. Results indicated that larvicides were deployed more frequently than adulticides, but rarely paired with resistance monitoring. Over 18,000 mosquitoes were tested from six states. Widespread low-level (1 × LC-99) methoprene resistance was detected in Cx. pipiens, but not in Ae. albopictus. No resistance to Bti or L. sphaericus was detected. Resistance to pyrethroids was detected in many locations for both species. Our results highlight the need for increased pesticide resistance testing in the United States and we provide guidance for building a centralized pesticide resistance testing program.

A Novel Allele Specific Polymerase Chain Reaction (AS-PCR) Assay to Detect the V1016G Knockdown Resistance Mutation Confirms Its Widespread Presence in Aedes albopictus Populations from Italy

Polymerase chain reaction (PCR)-based genotyping of mutations in the voltage-sensitive sodium channel (vssc) associated with resistance to pyrethroid insecticides is widely used and represents a potential early warning and monitoring system for insecticide resistance arising in mosquito populations, which are vectors of different human pathogens. In the secondary vector Aedes albopictus-an Asian species that has invaded and colonized the whole world, including temperate regions-sequencing of domain II of the vssc gene is still needed to detect the V1016G mutation associated with pyrethroid resistance. In this study we developed and tested a novel allele-specific PCR (AS-PCR) assay to genotype the V1016G mutation in this species and applied it to the analysis of wild populations from Italy. The results confirm the high accuracy of the novel AS-PCR and highlight frequencies of the V1016G allele as >5% in most sampling sites, with peaks of 20-45% in coastal touristic sites where pyrethroid treatments are extensively implemented, mostly for mosquito nuisance reduction. The high frequency of this mutation observed in Italian Ae. albopictus populations should serve as a warning bell, advocating for increased monitoring and management of a phenomenon which risks neutralizing the only weapon today available to counteract (risks of) arbovirus outbreaks.

Collection and Rearing of Container Mosquitoes and a 24-h Addition to the CDC Bottle Bioassay

Container mosquitoes (Diptera: Culicidae) oviposit their eggs in both natural and artificial containers. Many container mosquito species also serve as important vectors of disease-causing pathogens including Aedes aegypti, Ae. albopictus, and Ae. triseriatus. Control of these species can be done through the use of adulticide sprays. The efficacy of these treatments is highly dependent on the insecticide susceptibility status of the local mosquito populations. This paper provides protocols on collecting and rearing container mosquitoes for use in the Centers for Disease Control and Prevention (CDC) bottle bioassay. A brief description of the CDC bottle bioassay is provided as well as a standardized protocol for the incorporation of a 24-h mortality to the CDC bottle bioassay. Results from this 24-h holding addition to the CDC bottle bioassay reveal that some forms of resistance may be missed without the incorporation of the additional mortality reading. These protocols provide a foundation for new laboratories to establish rearing protocols and begin conducting resistance monitoring.

Baseline Susceptibility Status of Florida Populations of Aedes aegypti (Diptera: Culicidae) and Aedes albopictus

Resistance to insecticides used to control mosquito vectors threatens the ability of mosquito-control organizations to protect public health. Aedes aegypti (Linnaeus) and Aedes albopictus (Skuse) are invasive species widely distributed throughout Florida and have been implicated in recent epidemics of Zika, dengue, and chikungunya viruses. Knowledge of the susceptibility status of these mosquito species to pyrethroid and organophosphate active ingredients (AIs) is needed to inform product selection and treatment decisions. The susceptibility of 37 Ae. aegypti and 42 Ae. albopictus populations from Florida was assessed in response to six pyrethroid and three organophosphate AIs using the CDC bottle bioassay method. Of all bioassays completed with a pyrethroid AI, 95% for Ae. aegypti and 30% for Ae. albopictus resulted in a resistant outcome. For organophosphate AIs, ~31% of assays conducted for both species were classified as resistant. The highest frequency of susceptibility for both species was observed in response to the organophosphate AI, naled. Lambda-cyhalothrin was the only pyrethroid to result in a susceptible status for Ae. aegypti and also had the highest frequency of susceptibility for Ae. albopictus. Resistance was detected to every AI tested for both Ae. aegypti and Ae. albopictus, but there was a pronounced trend of pyrethroid resistance in Florida populations of Ae. aegypti. The results of this work provide evidence for the need to decrease reliance on pyrethroids and to implement different methods of control of Ae. aegypti in Florida.

Efficacy and Spatial Extent of Yard-Scale Control of Aedes (Stegomyia) albopictus (Diptera: Culicidae) Using Barrier Sprays and Larval Habitat Management

The Asian tiger mosquito, Aedes (Stegomyia) albopictus (Skuse), is a peridomestic, container-ovipositing mosquito commonly found throughout the southeastern United States. In the United States, Ae. albopictus is typically considered a nuisance pest; however, it is capable of transmitting multiple pathogens. Ae. albopictus is an important pest species and the target of numerous mosquito control efforts in the United States. Here, we evaluate the effectiveness and spatial extent of Ae. albopictus population reduction using a bifenthrin (AI Bifen IT, 7.9%) barrier spray and larval habitat management (LHM) in a temperate, suburban setting. Sixteen pairs of adjoining neighbors were randomly assigned to treatment groups with one neighbor receiving a treatment and the other monitored for evidence of a spillover effect of the treatments. Ae. albopictus populations in both yards were monitored for 33 d, with treatments occurring on the eighth day. Barrier sprays, both alone and combined with LHM, resulted in a significant reduction in Ae. albopictus abundance posttreatment. While LHM alone did not result in a significant reduction over the entire posttreatment period, Ae. albopictus populations were observed to be in decline during this period. No treatments were observed to have any reduction in efficacy 25 d posttreatment, with treatments involving LHM having a significantly increased efficacy. Yards neighboring treated yards were also observed to have reduced population sizes posttreatment, but these differences were rarely significant. These results provide insights into the population dynamics of Ae. albopictus following two common treatments and will be useful for integrated pest management plans.

Reducing West Nile Virus Risk Through Vector Management

Over 50,000 human West Nile virus (WNV) (Flaviviridae: Flavivirus) clinical disease cases have been reported to the CDC during the 20 yr that the virus has been present in the United States. Despite the establishment and expansion of WNV-focused mosquito surveillance and control efforts and a renewed emphasis on applying integrated pest management (IPM) principles to WNV control, periodic local and regional WNV epidemics with case reports exceeding 2,000 cases per year have occurred during 13 of those 20 yr in the United States. In this article, we examine the scientific literature for evidence that mosquito control activities directed at either preventing WNV outbreaks or stopping those outbreaks once in progress reduce WNV human disease or have a measurable impact on entomological indicators of human WNV risk. We found that, despite a proliferation of research investigating larval and adult mosquito control effectiveness, few of these studies actually measure epidemiological outcomes or the entomological surrogates of WNV risk. Although many IPM principles (e.g., control decisions based on surveillance, use of multiple control methodologies appropriate for the ecosystem) have been implemented effectively, the use of action thresholds or meaningful public health outcome assessments have not been used routinely. Establishing thresholds for entomological indicators of human risk analogous to the economic injury level and economic thresholds utilized in crop IPM programs may result in more effective WNV prevention.

Phenotypic and genotypic pyrethroid resistance of Aedes albopictus, with focus on the 2017 chikungunya outbreak in Italy

BACKGROUND

The highly invasive mosquito species Aedes albopictus has become a major health concern in temperate areas due to its role as vector of exotic arboviruses. Pyrethroid insecticides represent the main tools for limiting the circulation of such mosquito-borne viruses. The present work aim to extend previous reports on phenotypic pyrethroid-resistance in European Ae. albopictus, to identify its genetic basis and to monitor the geographical distribution of resistant genotypes, with a particular focus on sites experiencing the 2017 chikungunya outbreak in Italy.

RESULTS

Bioassays, performed according to World Health Organization protocols, showed full susceptibility to deltamethrin (concentration = 0.05%) and varying levels of resistance to permethrin (0.75%) and/or α-cypermethrin (0.05%) across Italy, with highest levels in the core of the 2017 chikungunya outbreak. Partial genotyping of the VSSC gene revealed widespread distribution of V1016G mutation and confirmed its association with pyrethroid resistance.

CONCLUSION

The results obtained show that the condition for the spread of pyrethroid resistance in Ae. albopictus in Europe exists under strong selective pressure due to intensive insecticide spraying to control exotic arbovirus outbreak or high levels of nuisance. The results draw attention to the need for an evidence-based implementation of mosquito nuisance control, taking insecticide resistance management into consideration. © 2019 Society of Chemical Industry.

Evaluation of Insecticide Resistance in Aedes albopictus (Diptera: Culicidae) in North Carolina, 2017

Mosquitoes may develop resistance to insecticide active ingredients (AIs) found in formulated products (FPs) due to environmental exposure from insecticides in mosquito control and/or unrelated to mosquito control, e.g., agricultural, household pest control. Mosquito control programs should implement resistance management strategies by assessing resistance in targeted populations, rotating different classes of insecticides based on resistance testing, and/or increasing insecticide concentration (i.e., saturation, using maximum labeled rate) to overcome emerging resistance. Resistance testing is often done solely on AIs, but should, in some cases, include both AIs and FPs at the concentrations mosquitoes may encounter in the field. The resistance/susceptibility status was determined for adulticides used in mosquito control. Centers for Disease Control and Prevention (CDC) bottle bioassays were used to assess resistance/susceptibility status for eight AIs (i.e., bifenthrin, permethrin, sumethrin/prallethrin, deltamethrin, tau-fluvalinate, chlorpyrifos, malathion, and naled) and eight FPs (TalStar, Biomist 3 + 15, Duet, Suspend Polyzone, Mavrik, MosquitoMist, Fyfanon, and Dibrom) that respectively contain the AIs. Current CDC guidelines were utilized: susceptible (97-100% mortality at diagnostic time [DT]), developing resistance (90-96% mortality at DT), or resistant (<90% mortality at DT). Significant differences were observed in mosquito susceptibility/resistance among and between AIs and FPs.

Quantification of permethrin resistance and kdr alleles in Florida strains of Aedes aegypti (L.) and Aedes albopictus (Skuse)

Recent outbreaks of locally transmitted dengue and Zika viruses in Florida have placed more emphasis on integrated vector management plans for Aedes aegypti (L.) and Aedes albopictus Skuse. Adulticiding, primarily with pyrethroids, is often employed for the immediate control of potentially arbovirus-infected mosquitoes during outbreak situations. While pyrethroid resistance is common in Ae. aegypti worldwide and testing is recommended by CDC and WHO, resistance to this class of products has not been widely examined or quantified in Florida. To address this information gap, we performed the first study to quantify both pyrethroid resistance and genetic markers of pyrethroid resistance in Ae. aegypti and Ae. albopictus strains in Florida. Using direct topical application to measure intrinsic toxicity, we examined 21 Ae. aegypti strains from 9 counties and found permethrin resistance (resistance ratio (RR) = 6-61-fold) in all strains when compared to the susceptible ORL1952 control strain. Permethrin resistance in five strains of Ae. albopictus was very low (RR<1.6) even when collected from the same containers producing resistant Ae. aegypti. Characterization of two sodium channel kdr alleles associated with pyrethroid-resistance showed widespread distribution in 62 strains of Ae. aegypti. The 1534 phenylalanine to cysteine (F1534C) single nucleotide polymorphism SNP was fixed or nearly fixed in all strains regardless of RR. We observed much more variation in the 1016 valine to isoleucine (V1016I) allele and observed that an increasing frequency of the homozygous V1016I allele correlates strongly with increased RR (Pearson corr = 0.905). In agreement with previous studies, we observed a very low frequency of three kdr genotypes, IIFF, VIFF, and IIFC. In this study, we provide a statewide examination of pyrethroid resistance, and demonstrate that permethrin resistance and the genetic markers for resistance are widely present in FL Ae. aegypti. Resistance testing should be included in an effective management program.

Aedes aegypti (Yellow-fever mosquito) and Aedes albopictus (Asian Tiger mosquito) can vector a variety of arboviruses that cause diseases and are thus a public health concern. Pyrethroid insecticide resistance is common in Ae. aegypti in many locations worldwide and can adversely affect vector control operations. However, the resistance status of these vectors in Florida is largely unreported and recent local transmission of dengue and Zika viruses has made this information critical for effective control operations. In this study, we showed that permethrin resistance and two common SNPs of the voltage gated sodium channel (V1016I and F1534C) previously associated with pyrethroid resistance were widely present in Florida Ae. aegypti strains. We also observed a strong correlation between the dilocus knock down response (kdr) genotype and resistance ratio (RR) as determined by topical application, which suggests, as have others, that kdr frequency may be a useful indicator of resistance in Aedes aegypti.

Impact of Mosquito Age and Insecticide Exposure on Susceptibility of Aedes albopictus (Diptera: Culicidae) to Infection with Zika Virus

Zika virus (ZIKV) is primarily transmitted to humans by Aedes aegypti and Ae. albopictus. Vector⁻virus interactions influencing vector competence vary and depend on biological and environmental factors. A mosquito's chronological age may impact its immune response against virus infection. Insecticides, source reduction, and/or public education are currently the best defense against mosquitoes that transmit ZIKV. This study assessed the effects of a mosquito's chronological age at time of infection on its response to ZIKV infection. We exposed young (6⁻7 d post-emergence) and old (11⁻12 d post-emergence) Ae. albopictus to a sublethal dose of bifenthrin prior to oral exposure to blood meals containing ZIKV (7-day incubation period). Old mosquitoes experienced a significantly (p < 0.01) higher rate of mortality than young mosquitoes. Significantly higher ZIKV body titers (p < 0.01) were observed in the old control group compared to the young control group. Significantly higher (p < 0.01) ZIKV dissemination rates and leg titers (p < 0.01) were observed in old bifenthrin-exposed mosquitoes compared to old control mosquitoes or young bifenthrin-exposed or control mosquitoes. Hence, bifenthrin exposure may increase the potential for virus transmission; however, the degree of these impacts varies with mosquito age. Impacts of insecticides should be considered in risk assessments of potential vector populations.

Permethrin Susceptibility for the Vector Culex tarsalis and a Nuisance Mosquito Aedes vexans in an Area Endemic for West Nile Virus

In 2016, we compared susceptibility to the insecticide, permethrin, between the West Nile virus vector, Culex tarsalis Coquillett, and a major nuisance mosquito, Aedes vexans (Meigen), using baseline diagnostic dose and time values determined using the CDC bottle bioassay protocol. Mosquitoes were collected in the wild in Brookings County, South Dakota, situated in the Northern Great Plains of the USA. The determined diagnostic dose and time were then used in 2017 to validate these measurements for the same 2 mosquito species, collected at a second location within Brookings County. The diagnostic dose was determined for multiple time periods and ranged from 27.0 µg/ml at 60 min to 38.4 µg/ml at 30 min. There was no significant difference detected in mortality rates between Cx. tarsalis and Ae. vexans for any diagnostic time and dose. For practical purposes, mosquitoes in 2017 were tested at 38 µg/ml for 30 min; expected mortality rates were 93.38% for Cx. tarsalis and 94.93% for Ae. vexans. Actual 2017 mortality rates were 92.68% for Cx. tarsalis and 96.12% for Ae. vexans, validating the usefulness of this baseline at an additional location and year.

Toxicity of different fatty acids and methyl esters on Culex quinquefasciatus larvae

The Culex quinquefasciatus mosquito is a vector of several diseases, and its control has been performed with synthetic insecticides, which may have human and environmental side effects. Thus, the use of new and safe molecules are important, and this study evaluated the toxicity of active substances against this mosquito. The oleic, linoleic, linolenic, palmitic and stearic acids and their respective methyl esters were tested against fourth instar C. quinquefasciatus larvae. Oleic, linoleic and linolenic acids had LC50 values of 8.58, 10.04 and 19.78 mg/L, respectively. Histological analysis showed that these three compounds caused cell instability with an increase in the number of vesicles in the fat body and in the midgut cells. Based on these results, glucose, triglyceride, and protein levels were evaluated after 1 h of acid exposure. These compounds decreased in insects treated with linoleic acid. Linolenic acid also caused a significant increase in acetylcholinesterase activity. These results show that oleic, linoleic, and linoleic acids have a lower LC50 for C. quinquefasciatus, affecting its metabolism and the morphology of midgut and fat body.

First evidence of resistance to pyrethroid insecticides in Italian Aedes albopictus populations 26 years after invasion

BACKGROUND

Aedes albopictus has spread during the last few decades all over the world. This has increased significantly the risk of exotic arbovirus transmission (e.g. chikungunya, dengue, and Zika) also in temperate areas, as demonstrated by the Chikungunya 2007 and 2017 outbreaks in northeastern and central Italy. Insecticides are an important tool for limiting the circulation of these mosquito-borne viruses. The aim of the present study was to address the gap in current knowledge of pyrethroid insecticide resistance of European Ae. albopictus populations, focusing on populations from Italy, Albania and Greece.

RESULTS

Bioassays for resistance to permethrin (0.75%), α-cypermethrin (0.05%) or deltamethrin (0.05%) were performed according to World Health Organization (WHO) protocols and showed reduced susceptibility (<90% mortality) of some Italian populations to permethrin and α-cypermethrin, but not to deltamethrin.

CONCLUSION

This study reports the first evidence of resistance to pyrethroids in adult Italian Ae. albopictus populations. Results refer to the season preceding the Chikungunya 2017 outbreak in central Italy and highlight the need to increase efforts to monitor the spread of insecticide resistance and the need to develop strategies to limit the spread of insecticide resistance, particularly in areas where extensive treatments have been carried out to contain disease outbreaks. © 2017 Society of Chemical Industry.

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.

Insecticide Susceptibility Screening Against Culex and Aedes (Diptera: Culicidae) Mosquitoes From the United States

Mosquitoes exposed to sublethal doses of insecticides may be selected for resistance to insecticide active ingredients (AIs). Mosquitoes are exposed to AIs through agricultural, public/private mosquito control programs, homeowners, and other sources. Hence, mosquito control programs should routinely measure the resistance/susceptibility status of mosquito populations of public health concern. The objectives here were to determine resistance status for six AIs used in adult mosquito control in the United States to assess how resistance/susceptibility differs between AI, mosquito species (states where > 1 species collected), and between years (some populations sampled for 2 yr). Field-collected eggs from 21 mosquito populations of six different species or hybrid species (Aedes albopictus Skuse [Diptera: Culicidae], Aedes aegypti L. [Diptera: Culicidae], Culex nigripalpus Theobald, Culex pipiens L. [Diptera: Culicidae], Culex quinquefasciatus Say [Diptera: Culicidae], Cx. pipiens/quinquefasciatus) were obtained. Centers for Disease Control and Prevention bottle bioassays were used to assess the resistance/susceptibility status for six AIs (bifenthrin, deltamethrin, etofenprox, malathion, permethrin, and phenothrin). World Health Organization guidelines were used to classify mosquitoes as susceptible (98-100% mortality at diagnostic time [DT]), possibly resistant (80-97% mortality at DT), or resistant (<80% mortality at DT). Significant differences were observed in mosquito susceptibility/resistance between species and AIs. In states where both Aedes and Culex were collected, the odds of exhibiting resistance in Culex were 68-69 times higher than Aedes (Texas odds ratio: 69.30; 95% confidence interval: 5.86, 819.44; P = 0.001; North Carolina odds ratio: 67.99; 95% confidence interval: 15.21, 303.94; P < 0.0001). Some level of resistance was detected against all tested AIs in several mosquito populations and some varied between 2015 and 2016.

Permethrin treated clothing to protect outdoor workers: evaluation of different methods for mosquito exposure against populations with differing resistance status

Minimizing arthropod exposure (e.g. mosquito and tick bites) is vital to protect health of outdoor workers. Personal protective measures can help protect against exposure. Here, the quantity of permethrin was evaluated for different fabric types after washing. Cone and petri dish exposure assays were used to investigate the knockdown/mortality of permethrin-susceptible and permethrin-resistant populations of mosquitoes. Permethrin-treated clothing was effective against the tested mosquito population that was susceptible to permethrin but not a permethrin-resistant population. Permethrin quantity was significantly highest in the 100% cotton fabric and for the 0 wash group. Permethrin quantity in fabrics decreased with washing. No significant differences (p > 0.05) were observed in knockdown/mortality rates for either exposure method. The protective effect of permethrin-treated clothing against mosquitoes is impacted by many factors, e.g. wash frequency, fabric type, and the susceptibility/resistance status of local mosquito populations.

Regional Survey of Mosquito Control Knowledge and Usage In North Carolina

Mosquitoes are a nuisance and can transmit pathogens to humans and other animals, resulting in notable morbidity and mortality. Organized mosquito control programs conduct surveillance, source reduction, treatment of mosquito oviposition sites, and adulticiding to protect public health. However, in some regions, there has been a decline in county and municipal mosquito control programs, and homeowners increasingly contract with private mosquito control companies or address mosquito-related issues themselves at the household level. The extent to which these services are being used, the potential for mosquito and/or insecticide exposure around the home, and the access to these services by individuals of different socioeconomic categories are not known. We conducted regional (western, central, eastern) assessments of public perception and knowledge of mosquito control services in a variety of communities of different socioeconomic categories (low, moderate, high) and types (urban, suburban, rural) in 3 North Carolina counties. We also assessed the respondents' basic knowledge of mosquito-borne diseases. Most respondents thought mosquito control was important to protect against both nuisance mosquitoes and mosquito-borne disease. The majority of people would pay ≤$50/year for mosquito control and most thought this should be covered by municipal/county taxes. Many respondents (31%, 95% confidence interval [CI] 26-35%) personally undertake mosquito control on their properties, but only 5.3% (95% CI 3.1-7.5%) contract with professional mosquito control services, with no significant differences between region, community type, or socioeconomic category in the proportion using private mosquito control services. While there were significant differences between factor levels (3 regions, 3 community types, 3 socioeconomic categories) in some responses, there were no significant differences observed between respondents, regardless of factor level, in the willingness to contribute funds to a community-based mosquito control program.

Potential for sublethal insecticide exposure to impact vector competence of Aedes albopictus (Diptera: Culicidae) for dengue and Zika viruses

Chikungunya, dengue, and Zika viruses (CHIKV, family Togaviridae, genus Alphavirus; DENV and ZIKV, family Flaviviridae, genus Flavivirus) are arboviruses that cause human epidemics. Due to the lack of vaccines for many mosquito-borne diseases, there is a need for mosquito control. In the US and other regions, residual barrier insecticide sprays are applied to foliage where female mosquitoes rest and/or sugar feed between blood meals. Residual sprays are an important control method for anthropogenic day-active sylvan mosquitoes such as Aedes albopictus (vector of CHIKV, DENV, and ZIKV) that are difficult to control using ultralow-volume sprays applied only at dusk or dawn when these mosquitoes are not active. In this exploratory study, we analyzed the extent to which ingestion of a sublethal dose of the active ingredient bifenthrin affected vector competence (i.e., infection, dissemination, and transmission) of Ae. albopictus for DENV and ZIKV. Two incubation periods (IPs; 7 and 14 d) were tested at 28°C for insecticide-fed and sugar-fed mosquitoes. We show that mosquitoes that were fed bifenthrin (0.128 µg/mL) mixed with sucrose solution exhibited significantly lower DENV infection rates and body titers after a 14-d IP. During the 7-d IP, one mosquito (sugar group) transmitted ZIKV. During the 14-d IP, 100% of mosquitoes showed body and leg ZIKV infections, and one mosquito (sugar+bifenthrin group) transmitted ZIKV. This is a preliminary communication, and more studies will be required to further investigate these findings. We expect the findings of this small-scale study to spur larger-scale investigations of the impacts of insecticides on mechanisms regulating vector competence, and exposure to other active ingredients, and aid in development of new insecticides.