Development and Evaluation of an Attractive Self-Marking Ovitrap to Measure Dispersal and Determine Skip Oviposition in Aedes albopictus (Diptera: Culicidae) Field Populations

Towards the invasion of wild and rural forested areas in Gabon (Central Africa) by the Asian tiger mosquito Aedes albopictus: Potential risks from the one health perspective

BACKGROUND

Since its first record in urban areas of Central-Africa in the 2000s, the invasive mosquito, Aedes albopictus, has spread throughout the region, including in remote villages in forested areas, causing outbreaks of Aedes-borne diseases, such as dengue and chikungunya. Such invasion might enhance Ae. albopictus interactions with wild animals in forest ecosystems and favor the spillover of zoonotic arboviruses to humans. The aim of this study was to monitor Ae. albopictus spread in the wildlife reserve of La Lopé National Park (Gabon), and evaluate the magnitude of the rainforest ecosystem colonization.

METHODOLOGY

From 2014 to 2018, we used ovitraps, larval surveys, BG-Sentinel traps, and human landing catches along an anthropization gradient from La Lopé village to the natural forest in the Park.

CONCLUSIONS

We detected Ae. albopictus in gallery forest up to 15 km away from La Lopé village. However, Ae. albopictus was significantly more abundant at anthropogenic sites than in less anthropized areas. The number of eggs laid by Ae. albopictus decreased progressively with the distance from the forest fringe up to 200m inside the forest. Our results suggested that in forest ecosystems, high Ae. albopictus density is mainly observed at interfaces between anthropized and natural forested environments. Additionally, our data suggested that Ae. albopictus may act as a bridge vector of zoonotic pathogens between wild and anthropogenic compartments.

Aedes aegypti and Aedes albopictus (Diptera: Culicidae) Oviposition Activity and the Associated Socio-environmental Factors in the New Orleans Area

The transmission of Aedes-borne viruses is on the rise globally. Their mosquito vectors, Aedes aegypti (Linnaeus, Diptera: Culicidae) and Ae. albopictus (Skuse, Diptera: Culicidae), are focally abundant in the Southern United States. Mosquito surveillance is an important component of a mosquito control program. However, there is a lack of long-term surveillance data and an incomplete understanding of the factors influencing vector populations in the Southern United States. Our surveillance program monitored Ae. aegypti and Ae. albopictus oviposition intensity in the New Orleans area using ovicups in a total of 75 sites from 2009 to 2016. We found both Aedes spp. throughout the study period and sites. The average number of Ae. aegypti and Ae. albopictus hatched from collected eggs per site per week was 34.1 (SD = 57.7) and 29.0 (SD = 46.5), respectively. Based on current literature, we formed multiple hypotheses on how environmental variables influence Aedes oviposition intensity, and constructed Generalized Linear Mixed Effect models with a negative binomial distribution and an autocorrelation structure to test these hypotheses. We found significant associations between housing unit density and Ae. aegypti and Ae. albopictus oviposition intensity, and between median household income and Ae. albopictus oviposition intensity. Temperature, relative humidity, and accumulated rainfall had either a lagged or an immediate significant association with oviposition. This study provides the first long-term record of Aedes spp. distribution in the New Orleans area, and sheds light on factors associated with their oviposition activity. This information is vital for the control of potential Aedes-borne virus transmission in this area.

Multiple introductions and overwintering shape the progressive invasion of Aedes albopictus beyond the Alps

Aedes albopictus originates from Southeast Asia and is considered one of the most invasive species globally. This mosquito is a nuisance and a disease vector of significant public health relevance. In Europe, Ae. albopictus is firmly established and widespread south of the Alps, a mountain range that forms a formidable biogeographic barrier to many organisms. Recent reports of Ae. albopictus north of the Alps raise questions of (1) the origins of its recent invasion, and (2) if this mosquito has established overwintering populations north of the Alps. To answer these questions, we analyzed population genomic data from >4000 genome-wide SNPs obtained through double-digest restriction site-associated DNA sequencing. We collected SNP data from specimens from six sites in Switzerland, north and south of the Alps, and analyzed them together with specimens from other 33 European sites, five from the Americas, and five from its Asian native range. At a global level, we detected four genetic clusters with specimens from Indonesia, Brazil, and Japan as the most differentiated, whereas specimens from Europe, Hong Kong, and USA largely overlapped. Across the Alps, we detected a weak genetic structure and high levels of genetic admixture, supporting a scenario of rapid and human-aided dispersal along transportation routes. While the genetic pattern suggests frequent re-introductions into Switzerland from Italian sources, the recovery of a pair of full siblings in two consecutive years in Strasbourg, France, suggests the presence of an overwintering population north of the Alps. The suggestion of overwintering populations of Ae. albopictus north of the Alps and the expansion patterns identified points to an increased risk of further northward expansion and the need for increased surveillance of mosquito populations in Northern Europe.

Adulticidal Efficacy and Sublethal Effects of Metofluthrin in Residual Insecticide Blends Against Wild Aedes albopictus (Diptera: Culicidae)

Field development for optimizing the use patterns of spatial repellents are an ongoing research need. To investigate volatile pyrethroids in blends containing multiple active ingredients, metofluthrin was formulated into a vegetation spray for application to foliage and mosquito harborage. Metofluthrin was compared by itself and in combination with Sector (permethrin), Hyperion (sumithrin), Onslaught Fast Cap (s-fenvalerate, prallethrin), and against a benchmark for comparison, OneGuard (λ-cyhalothrin, prallethrin, pyriproxyfen). Field sites with established Aedes albopictus (Skuse) (Diptera: Culicidae) populations were allocated as control and treatment and surveilled with both adult traps and oviposition cups before and after treatment. Adult mosquito reductions were consistently higher over 4 wk in the combination treatments and were comparable to OneGuard. In contrast, efficacy provided by metofluthrin alone or the standalone existing product (Sector, Hyperion, Onslaught) by were consistently less effective than OneGuard and the comparative blends. Additionally, poorly melanized and collapsing eggs were identified from mosquito adults collected at treated field sites. A 20-50% significant decrease in nonviable mosquito eggs, a 50% or more reduction in total eggs and an 80% or more reduction in adult mosquitoes collected at the local field sites was demonstrated across all metofluthrin-containing treatments. Metofluthrin applications both supplemented the adulticidal control of the selected pyrethroid-containing products and provided significant reduction in egg laying frequency and viability during monitoring of areas adjacent to the treated vegetation. Metofluthrin, and possibly other volatile ingredients, appears to have complex interactions for mosquito control in the field and could be unique as a supporting ingredient to other insecticides.

Spatial Bet Hedging in Sand Fly Oviposition: Factors Affecting Skip Oviposition in Phlebotomus papatasi Sand Flies

Skip oviposition is considered as an example of spatial bet hedging and involves a tradeoff between the benefit of reducing the risk of complete reproductive failure due to stochastic loss of a breeding site and the energetic and mortality costs associated with dispersal across several oviposition sites. Skip oviposition has been studied extensively for container-breeding mosquitoes but has never been studied with sand flies. By conducting a series of bioassays using solitary gravid females exposed to varying numbers of oviposition sites and to oviposition sites of variable quality at small (oviposition jar) and medium (free-flight cage) scales, we showed that sand flies exhibited skip oviposit at both scales. Specifically, with low-quality oviposition sites, females spread their eggs across several oviposition sites with total egg clutch size remaining constant and number of eggs per oviposition site decreasing with increasing number of oviposition sites. With variable quality sites, sand flies biased their oviposition to sites containing increasing levels of organic matter (OM), but also laid eggs in poor-quality sites; a behavior consistent with spatial bet hedging. We also demonstrated that the presence of OM stimulated larger egg clutch size and increased percent skip oviposition. Skip oviposition was less frequent at the free flight cage scale. But, at this scale, females were shown to be stimulated to lay more eggs when in the presence of other females than when alone.

Consolidation of Aedes albopictus Surveillance Program in the Autonomous Community of the Region of Murcia, Spain

BACKGROUND

Due to the Spanish legal framework, the national program for vector-borne diseases results from the agreement between national and regional governments, and it is the basis for the development of the regional programs, which should include the regional entomological surveillance program. Aedes albopictus was recorded for the first time in the Region of Murcia, in 2011. It gave rise to a new epidemiological scenario due to the presence of a competent vector of several arboviruses, which resulted in autochthonous cases of dengue in 2018.

METHODS

40 out of 45 municipalities participated in the regional entomological surveillance program, and 266 sampling points were established, with two ovitraps at each site as pseudo-replications. The study period was from April 16th to November 26th, with bi-weekly sample collections: 16 regional samplings were carried out.

RESULTS

Regional participation was high, and data loss was low (1.26%). Ae. albopictus was detected in 4.9% of samples and 89.4% of points, located in 39 of the 40 municipalities. The intensity of the presence of Ae. albopictus was estimated by a positivity index, that is, the percentage of positive samples over time. The vector phenology was obtained at a regional level, using the number of eggs as estimation of population density and the positivity values of points and municipalities. Every two weeks, real-time results were provided to the municipalities, which could use them as a vector management tool.

CONCLUSION

The regional entomological surveillance program for Ae. albopictus in the Region of Murcia was consolidated in 2019, with standardized and comparable methods. Almost all the municipalities of the region have observed the presence of Ae. albopictus, although intensity and spatial and temporal cover vary among them.

Not Just from Blood: Mosquito Nutrient Acquisition from Nectar Sources

Anthropophilic female mosquitoes are well known for their strong attraction to human hosts, but plant nectar is a common energy source in their diets. When sugar sources are scarce, female mosquitoes of some species can compensate by taking larger and more frequent blood meals. Male mosquitoes are exclusively dependent on plant nectar or alternative sugar sources. Plant preference is likely driven by an innate attraction that may be enhanced by experience, as mosquitoes learn to recognize available sugar rewards. Nectar-seeking involves the integration of at least three sensory systems: olfaction, vision and taste. The prevention of vector-borne illnesses, the determination of the mosquitoes' ecological role, and the design of efficient sugar-baited traps will all benefit from understanding the molecular basis of nectar-seeking.

Active dispersal of Aedes albopictus: a mark-release-recapture study using self-marking units

BACKGROUND

Understanding the dispersal dynamics of invasive mosquito species is fundamental to improve vector surveillance and to target control efforts. Aedes albopictus has been deemed a poor flyer as its range of active dispersal is generally assumed to be limited to a few hundred metres, while laboratory studies suggest this mosquito could actually fly several kilometres. The discrepancy may be due to differences in the local environment or to the methodological approach. In Switzerland, Ae. albopictus has been present since 2003 and has since then expanded its range. While passive dispersal is a key driver, it remains unclear how far this mosquito spreads through active flight and what the age structure and size of the local population are, all important parameters for vector surveillance and control.

METHOD

We investigated the active dispersal, daily survival rate and population size of Ae. albopictus in mark-release-recapture studies in Coldrerio and Lugano, two areas of intensive control in Switzerland. To mark mosquitoes emerging from breeding sites, we used self-marking units with fluorescent pigment that have minimal impact on mosquito survival and behaviour. We recaptured the adult mosquitoes with BG-Sentinel traps within a radius of 1 km from the marking units over 22 consecutive days.

RESULTS

We found that 77.5% of the recaptured Ae. albopictus individuals flew further than 250 m, the limit that is usually deemed sufficient for vector containment. The average age of females and males was 8.6 days and 7.8 days in Coldrerio and Lugano, respectively, while the estimated mosquito population densities were 134 mosquitoes/ha in Coldrerio and 767 mosquitoes/ha in Lugano.

CONCLUSIONS

Self-marking units are an effective tool to mark wild mosquitoes. Using this approach, we found that mosquitoes survive long enough to potentially transmit arboviral disease in our study area and that host-seeking Ae. albopictus females may travel further than previously assumed for European mosquito populations. This finding has direct implications for vector control as emergency treatments around positive cases, as well as surveillance and control around detections of new infestations, might need to be extended beyond the usual recommended range of just a few hundred metres.

Estimating Spatio-Temporal Dynamics of Aedes Albopictus Dispersal to Guide Control Interventions in Case of Exotic Arboviruses in Temperate Regions

The increasing number of exotic arbovirus cases imported in Europe and the 2017 chikungunya outbreak in central/southern Italy highlight the urgency of evidence-based outbreak management plans to predict, prevent or interrupt spreading of these arboviruses to non-endemic countries in temperate regions. We here present the results of three mark-release-recapture experiments conducted in a peri-urban area of North-East Italy to estimate the spatio-temporal dynamics of the dispersal of Aedes albopictus females looking for oviposition sites. The Flight Range of 90% of the mosquito population (FR₉₀) was found to exceed 200 m, consistently with data obtained from a previous study conducted in a highly urbanised area in Rome (Central Italy). Modelling results showed that dispersal can be so rapid that insecticide spraying within a 200m-radius around a potential infected case leaves >10% probability that a potentially infected mosquito escapes the treatment, even if this is carried out after only 2–3 days since the importation of a viremic case. These data provide evidence in favour of an update of guidelines for the control of exotic autochthonous arbovirus transmission in temperate areas and highlight the need of effective surveillance approaches and rapid response to contain the risks associated to imported viremic cases.

Sublethal effects of a vapour-active pyrethroid, transfluthrin, on Aedes aegypti and Ae. albopictus (Diptera: Culicidae) fecundity and oviposition behaviour

BACKGROUND

It is assumed that mosquitoes surviving exposure to spatial repellents when attempting to bite a host will not have significant adverse impacts on their downstream biology. Therefore, a critical knowledge gap is understanding the extent to which sublethal exposure to volatile pyrethroids may damage the performance of mosquitoes that survive exposure to vapour-active pyrethroids. To address this, laboratory-reared Aedes aegypti (L.) and Ae. albopictus (Skuse) were exposed to one of three sublethal concentrations of transfluthrin before being offered a blood-meal, after which their survival, fecundity, fertility, and egg-laying behaviour was assessed.

RESULTS

Both species expressed reduced skip-oviposition behaviour at all exposures. Both species also suffered a major reduction in viable eggs (50-75% reduction in viable eggs laid). A phenotype where eggs collapsed after laying was observed in Ae. aegypti, and this response increased with exposure concentrations. Dissected females of both species retained 50% or fewer of their eggs, with Ae. albopictus retaining a significant proportion of melanised oocytes following the highest exposure.

CONCLUSIONS

Our findings suggest that volatile pyrethroids can reduce skip-oviposition, which may improve source reduction outcomes during integrated management. The additional fecundity reduction caused by sublethal exposures to volatile pyrethroids improves our confidence in recommending them for urban vector management. Furthermore, we suggest that volatile pyrethroids should be adapted into delivery methods compatible with mosquito abatement programs.

Effectiveness of autodissemination stations containing pyriproxyfen in reducing immature Aedes albopictus populations

Background

Aedes albopictus, the Asian tiger mosquito, is an aggressive, highly anthropophilic, day-biting mosquito with an expanding geographic range. Suppression of Ae. albopictus is difficult because of the abundance and prevalence of larval habitats within peridomestic environments, particularly cryptic habitats such as corrugated extension spouts, fence post openings, discarded food containers, etc. Because of the challenges of eliminating or treating larval habitats of this species, we tested an autodissemination concept to contaminate these habitats with the insect growth regulator pyriproxyfen.

Methods

Our study was conducted in the City of Trenton (Mercer County), New Jersey, USA (40°12′N, 74°44′W). We selected six hot spots, where five or more Ae. albopictus males or females were collected based on weekly trap surveillance. A trapping unit was a city block, approximately 0.8 ha (hot spot), where we deployed 26 to 28 autodissemination stations per treatment plot. To gauge efficacy, we deployed BGS traps, oviposition cups, and sentinel cups in treatment and control locations.

Results

We found a significant reduction in eggs (P < 0.0001) and larval populations (P < 0.0001) as a result of treatment. Pupal mortality, as determined through bioassays, was also significantly higher in the treatment sites (P < 0.0001).

Conclusion

Our results clearly show the potential and unique use of the autodissemination stations to control immature Ae. albopictus in urban areas. Penetration of larvicides with existing methods are difficult to reach cryptic habitats, but the autodissemination approach, which exploits the oviposition behavior of the target pest, can be integrated into intervention programs. New tools are urgently needed to curb the expansion and public health implications of Ae. albopictus and other container-inhabiting species.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2034-7) contains supplementary material, which is available to authorized users.

The State of the Art of Lethal Oviposition Trap-Based Mass Interventions for Arboviral Control

The intensifying expansion of arboviruses highlights the need for effective invasive Aedes control. While mass-trapping interventions have long been discredited as inefficient compared to insecticide applications, increasing levels of insecticide resistance, and the development of simple affordable traps that target and kill gravid female mosquitoes, show great promise. We summarize the methodologies and outcomes of recent lethal oviposition trap-based mass interventions for suppression of urban Aedes and their associated diseases. The evidence supports the recommendation of mass deployments of oviposition traps to suppress populations of invasive Aedes, although better measures of the effects on disease control are needed. Strategies associated with successful mass-trap deployments include: (1) high coverage (>80%) of the residential areas; (2) pre-intervention and/or parallel source reduction campaigns; (3) direct involvement of community members for economic long-term sustainability; and (4) use of new-generation larger traps (Autocidal Gravid Ovitrap, AGO; Gravid Aedes Trap, GAT) to outcompete remaining water-holding containers. While to the best of our knowledge all published studies so far have been on Ae. aegypti in resource-poor or tropical settings, we propose that mass deployment of lethal oviposition traps can be used for focused cost-effective control of temperate Ae. albopictus pre-empting arboviral epidemics and increasing participation of residents in urban mosquito control.

Targeting a Hidden Enemy: Pyriproxyfen Autodissemination Strategy for the Control of the Container Mosquito Aedes albopictus in Cryptic Habitats

Background

The Asian tiger mosquito, Aedes albopictus, is a vector of dengue, Chikungunya, and Zika viruses. This mosquito inhabits a wide range of artificial water-holding containers in urban and suburban areas making it difficult to control. We tested the hypothesis that female-driven autodissemination of an insect growth regulator could penetrate cryptic oviposition habitats difficult to treat with conventional insecticidal sprays.

Methodology

Oviposition preferences of Ae. albopictus females for open and cryptic cups were tested in semi-field experiments. Two conventional larvicidal sprayers were tested to determine droplet penetration and larvicidal efficacy in open and cryptic habitats using Bacillus thuringiensis var. israelensis (Bti) in the field. Finally, the efficacy of pyriproxyfen autodissemination stations was assessed in cryptic and open cups in residential areas during 2013 and 2014.

Principal Findings

Gravid females strongly preferred cryptic (53.1±12.9 eggs/cup) over open (10.3±4.3 eggs/cup) cups for oviposition. Cryptic cups showed limited droplet penetration and produced 0.1–0.3% larval mortality with a conventional backpack and low-volume sprays of Bti. The autodissemination stations effectively contaminated these cryptic cups (59.3–84.6%) and produced 29.7–40.8% pupal mortality during 2013–2014. Significant pupal mortality was also observed in open cups.

Conclusions

The autodissemination station effectively exploits the oviposition behavior of wild gravid females to deliver pyriproxyfen to targeted oviposition habitats. Although the pupal mortality in cryptic cups was relatively lower than expected for the effective vector control. Autodissemination approach may be a suitable supporting tool to manage Ae. albopictus immatures in the cryptic habitats those are less accessible to conventional larvicidal sprays.

Aedes albopictus is a highly invasive mosquito species and a competent vector of dengue, Chikungunya, Zika and other arboviruses. In the absence of therapeutics, vector control is the only means of controlling these diseases. Larvicides are important tool to limit mosquito populations. Backpack and low volume sprayers are heavily used for ground-based larvicide applications, but their potential to reach hidden or cryptic larval habitats is not well established. In the present study, we found that Ae. albopictus shows a strong oviposition preference for cryptic cups. Moreover, cryptic cups received limited droplet penetration using conventional spray methodologies, resulting in almost negligible larval mortality as compared to open cups in field experiments. We further evaluated the efficacy of an autodissemination station to deliver the insect growth regulator pyriproxyfen to cryptic cups in field experiments. Autodissemination shows good efficacy in transferring toxic concentrations of pyriproxyfen to cryptic cups and may provide a significant advantage over sprayers in delivering insecticide to the cryptic larval habitats strongly preferred by Ae. albopictus.