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.

Evolution of a Mosquito's Hatching Behavior to Match Its Human-Provided Habitat

AbstractA subspecies of the yellow fever mosquito, Aedes aegypti, has recently evolved to specialize in biting and living alongside humans. It prefers human odor over the odor of nonhuman animals and breeds in human-provided artificial containers rather than the forest tree holes of its ancestors. Here, we report one way this human specialist has adapted to the distinct ecology of human environments. While eggs of the ancestral subspecies rarely hatch in pure water, those of the derived human specialist do so readily. We trace this novel behavior to a shift in how eggs respond to dissolved oxygen, low levels of which may signal food abundance. Moreover, we show that while tree holes are consistently low in dissolved oxygen, artificial containers often have much higher levels. There is thus a concordance between the hatching behavior of each subspecies and the aquatic habitat it uses in the wild. We find this behavioral variation is heritable, with both maternal and zygotic effects. The zygotic effect depends on dissolved oxygen concentration (i.e., a genotype-environment interaction, or G×E), pointing to potential changes in oxygen-sensitive circuits. Together, our results suggest that a shift in hatching response contributed to the pernicious success of this human-specialist mosquito and illustrate how animals may rapidly adapt to human-driven changes in the environment.

Consumption of endemic arbovirus mosquito vectors by bats in the southeastern United States

Mosquitoes affect human health and well-being globally through their roles as disease-causing pathogen vectors. Utilizing genetic techniques, we conducted a large-scale dietary study of three bat species common to the southeastern U.S.A., Lasiurus seminolus (Seminole bat), Nycticeius humeralis (evening bat), and Myotis austroriparius (southeastern myotis). Through next-generation sequencing of a 180 bp portion of cytochrome oxidase subunit I (COI) of mitochondrial DNA from 180 bat guano samples, we documented consumption of 17 species of mosquitoes by bats, including six endemic arbovirus vectors. Culex quinquefasciatus, Culex coronator, Culiseta melanura, Culex salinarius, Culex erraticus, and Coquillettidia perturbans were consumed by 51.3%, 43.7%, 27.2%, 22.8%, 18.0%, and 12.7% of bats sampled, respectively. Consumption of two of these mosquito species was explained by spatial variables reflecting the prevalence of mosquito larval habitat, five were explained by bat traits (bat mass, bat species), and two were explained by these factors plus temporal variables (maximum daily temperature, time since sunset, date), making it challenging to offer specific guidance on how best to promote bats as a means of reducing arbovirus vector species. Our results show that common bat species of the southeastern U.S.A. consume endemic, but not exotic, arbovirus mosquito vectors. Future studies are needed to understand the impact of bat consumption on mosquito numbers and public health.

Intrinsic variation in the vertically transmitted core virome of the mosquito Aedes aegypti

Virome studies among metazoans have revealed the ubiquity of RNA viruses in animals, contributing to a fundamental re-thinking of the relationships between organisms and their microbiota. Mosquito viromes, often scrutinized due to their public health relevance, may also provide insight into broadly applicable concepts, such as a "core virome," a set of viruses consistently associated with a host species or population that may fundamentally impact its basic biology. A subset of mosquito-associated viruses (MAVs) could comprise such a core, and MAVs can be categorized as (i) arboviruses, which alternate between mosquito and vertebrate hosts, (ii) insect-specific viruses, which cannot replicate in vertebrate cells, and (iii) viruses with unknown specificity. MAVs have been widely characterized in the disease vector Aedes aegypti, and the occurrence of a core virome in this species has been proposed but remains unclear. Using a wild population previously surveyed for MAVs and a common laboratory strain, we investigated viromes in reproductive tissue via metagenomic RNA sequencing. Virome composition varied across samples, but four groups comprised >97% of virus sequences: a novel partiti-like virus (Partitiviridae), a toti-like virus (Totiviridae), unclassified Riboviria, and four orthomyxo-like viruses (Orthormyxoviridae). Whole or partial genomes for the partiti-like virus, toti-like virus, and one orthomyxo-like virus were assembled and analyzed phylogenetically. Multigenerational maintenance of these MAVs was confirmed by RT-PCR, indicating vertical transmission as a mechanism for persistence. This study provides fundamental information regarding MAV ecology and variability in A. aegypti and the potential for vertically maintained core viromes at the population level.

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.

Evidence of Local Extinction and Reintroduction of Aedes aegypti in Exeter, California

Established populations of Aedes aegypti, a mosquito vector of multiple major arthropod-borne viruses, were first found in three California (CA) cities in 2013. From 2013 to April 2021, Ae. aegypti thwarted almost all control efforts to stop its spread and expanded its range to 308 cities, including Exeter, in 22 counties in CA. Population genomic analyses have suggested that multiple genetically distinct Ae. aegypti populations were introduced into CA. However Ae. aegypti collected for the first time in 2014 in Exeter, appeared to be different from three major genetic clusters found elsewhere in CA. Due to intense control efforts by the Delta Vector Control District (DVCD), Ae. aegypti was thought to have been eliminated from Exeter in 2015. Unfortunately, it was recollected in 2018. It was not clear if the reemergence of Ae. aegypti in Exeter was derived from the bottlenecked remnants of the original 2014 Exeter population or from an independent invasion from a different population derived from surrounding areas. The goal of this work was to determine which of these scenarios occurred (recovery after bottleneck or reintroduction after elimination) and if elimination and reintroduction occurred to identify the origin of the invading population using a population genomic approach. Our results support the reintroduction after elimination hypothesis. The source of reintroduction, however, was unexpectedly from the southern CA cluster rather than from other two geographically closer central CA genetic clusters. We also conducted a knockdown resistance mutation profile, which showed Exeter 2014 had the lowest level of resistant alleles compared to the other populations, could have contributed towards DVCD’s ability to locally eliminate Ae. aegypti in 2014.

A Magnetic-Bead-Based Mosquito DNA Extraction Protocol for Next-Generation Sequencing

A recently published DNA extraction protocol using magnetic beads and an automated DNA extraction instrument suggested that it is possible to extract high quality and quantity DNA from a well-preserved individual mosquito sufficient for downstream whole genome sequencing. However, reliance on an expensive automated DNA extraction instrument can be prohibitive for many laboratories. Here, the study provides a budget-friendly magnetic-bead-based DNA extraction protocol, which is suitable for low to medium throughput. The protocol described here was successfully tested using individual Aedes aegypti mosquito samples. The reduced costs associated with high quality DNA extraction will increase the application of high throughput sequencing to resource limited labs and studies.

Under-the-Radar Dengue Virus Infections in Natural Populations of Aedes aegypti Mosquitoes

The incidence of locally acquired dengue infections increased during the last decade in the United States, compelling a sustained research effort concerning the dengue mosquito vector, Aedes aegypti, and its microbiome, which has been shown to influence virus transmission success. We examined the "metavirome" of four populations of Aedes aegypti mosquitoes collected in 2016 to 2017 in Manatee County, FL. Unexpectedly, we discovered that dengue virus serotype 4 (DENV4) was circulating in these mosquito populations, representing the first documented case of such a phenomenon in the absence of a local DENV4 human case in this county over a 2-year period. We confirmed that all of the mosquito populations carried the same DENV4 strain, assembled its full genome, validated infection orthogonally by reverse transcriptase PCR, traced the virus origin, estimated the time period of its introduction to the Caribbean region, and explored the viral genetic signatures and mosquito-specific virome associations that potentially mediated DENV4 persistence in mosquitoes. We discuss the significance of prolonged maintenance of the DENV4 infections in A. aegypti that occurred in the absence of a DENV4 human index case in Manatee County with respect to the inability of current surveillance paradigms to detect mosquito vector infections prior to a potential local outbreak.IMPORTANCE Since 1999, dengue outbreaks in the continental United States involving local transmission have occurred only episodically and only in Florida and Texas. In Florida, these episodes appear to be coincident with increased introductions of dengue virus into the region through human travel and migration from countries where the disease is endemic. To date, the U.S. public health response to dengue outbreaks has been largely reactive, and implementation of comprehensive arbovirus surveillance in advance of predictable transmission seasons, which would enable proactive preventative efforts, remains unsupported. The significance of our finding is that it is the first documented report of DENV4 transmission to and maintenance within a local mosquito vector population in the continental United States in the absence of a human case during two consecutive years. Our data suggest that molecular surveillance of mosquito populations in high-risk, high-tourism areas of the United States may enable proactive, targeted vector control before potential arbovirus outbreaks.

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.

Evaluating the Vector Control Potential of the In2Care® Mosquito Trap Against Aedes aegypti and Aedes albopictus Under Semifield Conditions in Manatee County, Florida

Successful integrated vector management programs may need new strategies in addition to conventional larviciding and adulticiding strategies to target Aedes aegypti and Ae. albopictus, which can develop in small, often cryptic, artificial and natural containers. The In2Care® mosquito trap was recently developed to target and kill larval and adult stages of these invasive container-inhabiting Aedes mosquitoes by utilizing autodissemination. Gravid females that visit the trap pick up pyriproxyfen (PPF) that they later transfer to nearby larval habitats as well as Beauveria bassiana spores that slowly kill them. We assessed the efficacy of the In2Care mosquito trap in a semifield setting against locally sourced strains of Ae. aegypti and Ae. albopictus. We found that the In2Care mosquito trap is attractive to gravid Ae. aegypti and Ae. albopictus females and serves as an egg sink, preventing any adult emergence from the trap (P = 0.0053 for both species). Adult females successfully autodisseminated PPF to surrounding water-filled containers, leading to a statistically significant reduction in new mosquito emergence (P ≤ 0.0002 for both species). Additionally, we found effective contamination with Beauveria bassiana spores, which significantly reduced the survivorship of exposed Ae. aegypti and Ae. albopictus (P ≤ 0.008 for both species in all experimental setups). In summary, the In2Care mosquito trap successfully killed multiple life stages of 2 main mosquito vector species found in Florida under semifield conditions.

Larval Temperature-Food Effects on Adult Mosquito Infection and Vertical Transmission of Dengue-1 Virus

Temperature-food interactions in the larval environment can affect life history and population growth of container mosquitoes Aedes aegypti (L.) and Aedes albopictus Skuse, the primary vectors of chikungunya and dengue viruses. We used Ae. aegypti, Ae. albopictus, and dengue-1 virus (DENV-1) from Florida to investigate whether larval rearing temperature can alter the effects of larval food levels on Ae. aegypti and Ae. albopictus life history and DENV-1 infection and vertical transmission. Although we found no effect of larval treatments on survivorship to adulthood, DENV-1 titer, or DENV-1 vertical transmission, rates of vertical transmission up to 16-24% were observed in Ae. albopictus and Ae. aegypti, which may contribute to maintenance of this virus in nature. Larval treatments had no effect on number of progeny and DENV-1 infection in Ae. aegypti, but the interaction between temperature and food affected number of progeny and DENV-1 infection of the female Ae. albopictus parent. The cooler temperature (24°C) yielded the most progeny and this effect was accentuated by high food relative to the other conditions. Low and high food led to the highest (∼90%) and lowest (∼65%) parental infection at the cooler temperature, respectively, whereas intermediate infection rates (∼75-80%) were observed for all food conditions at the elevated temperature. These results suggest that temperature and food availability have minimal influence on rate of vertical transmission and a stronger influence on adults of Ae. albopictus than of Ae. aegypti, which could have consequences for dengue virus epidemiology.

Weather and landscape factors associated with adult mosquito abundance in southwestern Georgia, U.S.A

Mosquito community composition and population dynamics were compared to weather variables and land use/cover data during 2008 to determine which variables affected population dynamics at the J.W. Jones Ecological Research Center in southwestern Georgia. Models relating adult mosquito distributions to weather variables and time of year were compared using Akaike's Information Criterion (AIC) model selection. Precipitation, temperature, humidity, and Keetch-Byram Drought Index were important factors correlated with mosquito abundance or presence/absence for the species considered. A cluster analysis, which grouped eight sites based on the percentages of land use/cover and hydric soils located in a 1-km radius surrounding collection sites, and an indicator species analysis were used to investigate the associations among 11 mosquito species and sites with similar land use/cover. Aedes albopictus (Skuse), Culex coronator Dyar & Knab, Culex quinquefasciatus Say, and Culex salinarius Coquillett were associated with sites that had the most anthropogenic influence, while Coquillettidia perturbans (Walker) and Psorophora ferox (von Humboldt) were associated with natural land cover such as wetlands and forested land. This study demonstrates that regional climate and land use/cover data can be predictive of the population dynamics of certain mosquito populations and is the first to examine how the distribution of Cx. coronator adults relate to land use/cover in the southeastern United States.