Using geometric wing morphometrics to distinguish Aedes japonicus japonicus and Aedes koreicus

BACKGROUND

Aedes japonicus japonicus (Theobald, 1901) and Aedes koreicus (Edwards, 1917) have rapidly spread in Europe over the last decades. Both species are very closely related and occur in sympatry. Females and males are difficult to distinguish. However, the accurate species discrimination is important as both species may differ in their vectorial capacity and spreading behaviour. In this study, we assessed the potential of geometric wing morphometrics as alternative to distinguish the two species.

METHODS

A total of 147 Ae. j. japonicus specimens (77 females and 70 males) and 124 Ae. koreicus specimens (67 females and 57 males) were collected in southwest Germany. The left wing of each specimen was removed, mounted and photographed. The coordinates of 18 landmarks on the vein crosses were digitalised by a single observer. The resulting two-dimensional dataset was used to analyse the differences in the wing size (i.e. centroid size) and wing shape between Ae. j. japonicus and Ae. koreicus using geometric morphometrics. To analyse the reproducibility of the analysis, the landmark collection was repeated for 20 specimens per sex and species by two additional observers.

RESULTS

The wing size in female Ae. koreicus was significantly greater than in Ae. j. japonicus but did not differ significantly for males. However, the strong overlap in wing size also for the females would not allow to discriminate the two species. In contrast, the wing shape clustering was species specific and a leave-one-out validation resulted in a reclassification accuracy of 96.5% for the females and 91.3% for the males. The data collected by different observers resulted in a similar accuracy, indicating a low observer bias for the landmark collection.

CONCLUSIONS

Geometric wing morphometrics provide a reliable and robust tool to distinguish female and male specimens of Ae. j. japonicus and Ae. koreicus.

Invasive mosquito vectors in Europe: From bioecology to surveillance and management

Invasive mosquitoes (Diptera: Culicidae) play a key role in the spread of a number of mosquito-borne diseases worldwide. Anthropogenic changes play a significant role in affecting their distribution. Invasive mosquitoes usually take advantage from biotic homogenization and biodiversity reduction, therefore expanding in their distribution range and abundance. In Europe, climate warming and increasing urbanization are boosting the spread of several mosquito species of high public health importance. The present article contains a literature review focused on the biology and ecology of Aedes albopictus, Ae. aegypti, Ae. japonicus japonicus, Ae. koreicus, Ae. atropalpus and Ae. triseriatus, outlining their distribution and public health relevance in Europe. Bioecology insights were tightly connected with vector surveillance and control programs targeting these species. In the final section, a research agenda aiming for the effective and sustainable monitoring and control of invasive mosquitoes in the framework of Integrated Vector Management and One Health is presented. The WHO Vector Control Advisory Group recommends priority should be given to vector control tools with proven epidemiological impact.

Complete mitogenome sequence of Aedes (Hulecoeteomyia) japonicus japonicus from Hawai'i Island

We report the first complete mitogenome (Mt) sequence of Aedes japonicus japonicus (Diptera: Culicidae). The sequence was extracted from one adult from the Big Island of Hawai'i Island. The length of the Ae. japonicus japonicus Mt was 16,528bp with 78.1% AT content. Its sequence is most similar to the Mt sequence of Aedes koreicus with 90.81% sequence identity. This is the first full Mt sequence available for this species and provides important genetic resource for studying population genetics and dynamics of this important invasive mosquito species.

An unwanted companion reaches the country: the first record of the alien mosquito Aedes japonicus japonicus (Theobald, 1901) in Slovakia

BACKGROUND

Invasive mosquitoes of the genus Aedes are quickly spreading around the world. The presence of these alien species is concerning for both their impact on the native biodiversity and their high vector competence. The surveillance of Aedes invasive mosquito (AIM) species is one of the most important steps in vector-borne disease control and prevention.

METHODS

In 2020, the monitoring of AIM species was conducted in five areas (Bratislava, Zvolen, Banská Bystrica, Prešov, Košice) of Slovakia. The sites were located at points of entry (border crossings with Austria and Hungary) and in the urban and rural zones of cities and their surroundings. Ovitraps were used at the majority of sites as a standard method of monitoring. The collected specimens were identified morphologically, with subsequent molecular identification by conventional PCR (cox1) and Sanger sequencing. The phylogenetic relatedness of the obtained sequences was inferred by the maximum likelihood (ML) method. The nucleotide heterogeneity of the Slovak sequences was analysed by the index of disparity.

RESULTS

A bush mosquito, Aedes japonicus japonicus, was found and confirmed by molecular methods in three geographically distant areas of Slovakia-Bratislava, Zvolen and Prešov. The presence of AIM species is also likely in Košice; however, the material was not subjected to molecular identification. The nucleotide sequences of some Slovak strains confirm their significant heterogeneity. They were placed in several clusters on the ML phylogenetic tree. Moreover, Ae. j. japonicus was discovered in regions of Slovakia that are not close to a point of entry, where the mosquitoes could find favourable habitats in dendrothelms in city parks or forests.

CONCLUSION

Despite being a first record of the Ae. j. japonicus in Slovakia, our study indicates that the established populations already exist across the country, underlining the urgent need for intensified surveillance of AIM species as well as mosquito-borne pathogens.

Diapause characterisation and seasonality of Aedes japonicus japonicus (Diptera, Culicidae) in the northeast of France

The invasive mosquito Aedes japonicus japonicus (Theobald, 1901) settled in 2013 in the Alsace region, in the northeast of France. In this temperate area, some mosquito species use diapause to survive cold winter temperatures and thereby foster settlement and dispersal. This study reports diapause and its seasonality in a field population of Ae. japonicus in the northeast of France. For two years, eggs were collected from May to the beginning of November. They were most abundant in summer and became sparse in late October. Diapause eggs were determined by the presence of a fully developed embryo in unhatched eggs after repeated immersions. Our study showed effective diapause of Ae. japonicus in this part of France. At the start of the egg-laying period (week 20), we found up to 10% of eggs under diapause, and this rate reached 100% in October. The 50% cut-off of diapause incidence was determined by the end of summer, leading to an average calculated maternal critical photoperiod of 13 h 23 min. Interestingly, diapause was shown to occur in part of the eggs even at the earliest period of the two seasons, i.e. in May of each year. Even though we observed that the size of eggs was positively correlated with diapause incidence, morphology cannot be used as the unique predictive indicator of diapause status due to overlapping measurements between diapausing and non-diapausing eggs. This study provides new knowledge on diapause characterisation and invasive traits of Ae. japonicus.

Population genetic structure of the Asian bush mosquito, Aedes japonicus (Diptera, Culicidae), in Belgium suggests multiple introductions

BACKGROUND

Aedes japonicus japonicus has expanded beyond its native range and has established in multiple European countries, including Belgium. In addition to the population located at Natoye, Belgium, locally established since 2002, specimens were recently collected along the Belgian border. The first objective of this study was therefore to investigate the origin of these new introductions, which were assumed to be related to the expansion of the nearby population in western Germany. Also, an intensive elimination campaign was undertaken at Natoye between 2012 and 2015, after which the species was declared to be eradicated. This species was re-detected in 2017, and thus the second objective was to investigate if these specimens resulted from a new introduction event and/or from a few undetected specimens that escaped the elimination campaign.

METHODS

Population genetic variation at nad4 and seven microsatellite loci was surveyed in 224 and 68 specimens collected in Belgium and Germany, respectively. German samples were included as reference to investigate putative introduction source(s). At Natoye, 52 and 135 specimens were collected before and after the elimination campaign, respectively, to investigate temporal changes in the genetic composition and diversity.

RESULTS

At Natoye, the genotypic microsatellite make-up showed a clear difference before and after the elimination campaign. Also, the population after 2017 displayed an increased allelic richness and number of private alleles, indicative of new introduction(s). However, the Natoye population present before the elimination programme is believed to have survived at low density. At the Belgian border, clustering results suggest a relation with the western German population. Whether the introduction(s) occur via passive human-mediated ground transport or, alternatively, by natural spread cannot be determined yet from the dataset.

CONCLUSION

Further introductions within Belgium are expected to occur in the near future, especially along the eastern Belgian border, which is at the front of the invasion of Ae. japonicus towards the west. Our results also point to the complexity of controlling invasive species, since 4 years of intense control measures were found to be not completely successful at eliminating this exotic at Natoye.

American Aedes japonicus japonicus, Culex pipiens pipiens, and Culex restuans mosquitoes have limited transmission capacity for a recent isolate of Usutu virus

Usutu virus (USUV; Flavivirus) has caused massive die-offs in birds across Europe since the 1950s. Although rare, severe neurologic disease in humans has been reported. USUV is genetically related to West Nile virus (WNV) and shares an ecological niche, suggesting it could spread from Europe to the Americas. USUV's risk of transmission within the United States is currently unknown. To this end, we exposed field-caught Aedes japonicus, Culex pipiens pipiens, and Culex restuans-competent vectors for WNV-to a recent European isolate of USUV. While infection rates for each species varied from 7%-21%, no dissemination or transmission was observed. These results differed from a 2018 report by Cook and colleagues, who found high dissemination rates and evidence of transmission potential using a different USUV strain, U.S. mosquito populations, temperature, and extrinsic incubation period. Future studies should evaluate the impact of these experimental conditions on USUV transmission by North American mosquitoes.

Oviposition of Aedes japonicus japonicus (Diptera: Culicidae) and associated native species in relation to season, temperature and land use in western Germany

BACKGROUND

Aedes japonicus japonicus, first detected in Europe in 2000 and considered established in Germany 10 years later, is of medical importance due to its opportunistic biting behaviour and its potential to transmit pathogenic viruses. Its seasonal phenology, temperature and land use preference related to oviposition in newly colonised regions remain unclear, especially in the context of co-occurring native mosquito species.

METHODS

Focussing on regions in Germany known to be infested by Ae. japonicus japonicus, we installed ovitraps in different landscapes and their transition zones and recorded the oviposition activity of mosquitoes in relation to season, temperature and land use (arable land, forest, settlement) in two field seasons (May-August 2017, April-November 2018).

RESULTS

Ae. japonicus japonicus eggs and larvae were encountered in 2017 from June to August and in 2018 from May to November, with a markedly high abundance from June to September in rural transition zones between forest and settlement, limited to water temperatures below 30 °C. Of the three native mosquito taxa using the ovitraps, the most frequent was Culex pipiens s.l., whose offspring was found in high numbers from June to August at water temperatures of up to 35 °C. The third recorded species, Anopheles plumbeus, rarely occurred in ovitraps positioned in settlements and on arable land, but was often associated with Ae. japonicus japonicus. The least frequent species, Aedes geniculatus, was mostly found in ovitraps located in the forest.

CONCLUSIONS

The transition zone between forest and settlement was demonstrated to be the preferred oviposition habitat of Ae. japonicus japonicus, where it was also the most frequent container-inhabiting mosquito species in this study. Compared to native taxa, Ae. japonicus japonicus showed an extended seasonal activity period, presumably due to tolerance of colder water temperatures. Higher water temperatures and arable land represent distribution barriers to this species. The frequently co-occurring native species An. plumbeus might be useful as an indicator for potentially suitable oviposition habitats of Ae. japonicus japonicus in hitherto uncolonised regions. The results contribute to a better understanding of mosquito ecology and provide a basis for more targeted monitoring, distribution modelling and risk management of mosquitoes.

The Asian bush mosquito Aedes japonicus japonicus (Diptera: Culicidae) in Europe, 17 years after its first detection, with a focus on monitoring methods

After the first detection of the Asian bush mosquito Aedes japonicus japonicus in the year 2000 in France, its invasive nature was revealed in 2008 in Switzerland and Germany. In the following years, accumulating reports have shown that Ae. j. japonicus succeeded in establishing in several European countries. Surveillance efforts suggest that there are currently four populations in Europe, with the largest one, formed by the recent fusion of several smaller populations, ranging from West Germany, with extensions to Luxembourg and French Alsace, southwards to Switzerland and continuing westwards through Liechtenstein to western Austria. This paper summarises the present distribution of Ae. j. japonicus in Europe, based on published literature and hitherto unpublished findings by the authors, and critically reviews the monitoring strategies applied. A proposal for a more standardised monitoring approach is provided, aiming at the harmonisation of future data collections for improving the comparability between studies and the suitability of collected data for further research purposes, e.g. predictive modelling approaches.

Targeted surveillance reveals native and invasive mosquito species infected with Usutu virus

Background

The emergence of Usutu virus (USUV) in Europe was first reported in Austria, 2001, and the virus has since spread to many European countries. Initial outbreaks are marked by a mass die-off of European blackbirds (Turdus merula) and other bird species. During outbreaks, the virus has been detected in pools of Culex pipiens mosquitoes, and these mosquitoes are probably the most important enzootic vectors. Beginning in 2017, a second wave of blackbird deaths associated with USUV was observed in eastern Austria; the affected areas expanded to the Austrian federal states of Styria in the south and to Upper Austria in the west in 2018. We sampled the potential vector population at selected sites of bird deaths in 2018 in order to identify infected mosquitoes.

Results

We detected USUV RNA in 16 out of 19 pools of Cx. pipiens/Cx. torrentium mosquitoes at sites of USUV-linked blackbird mortality in Linz and Graz, Austria. A disseminated virus infection was detected in individuals from selected pools, suggesting that Cx. pipiens form pipiens was the principal vector. In addition to a high rate of infected Cx. pipiens collected from Graz, a disseminated virus infection was detected in a pool of Aedes japonicus japonicus.

Conclusions

We show herein that naturally-infected mosquitoes at foci of USUV activity are primarily Cx. pipiens form pipiens. In addition, we report the first natural infection of Ae. j. japonicus with USUV, suggesting that it may be involved in the epizootic transmission of USUV in Europe. Ae. j. japonicus is an invasive mosquito whose range is expanding in Europe.

Cache Valley Virus in Aedes japonicus japonicus Mosquitoes, Appalachian Region, United States

We detected Cache Valley virus in Aedes japonicus, a widely distributed invasive mosquito species, in an Appalachian forest in the United States. The forest contained abundant white-tailed deer, a major host of the mosquito and virus. Vector competence trials indicated that Ae. j. japonicus mosquitoes can transmit this virus in this region.

Vector competence of field populations of the mosquito species Aedes japonicus japonicus and Culex pipiens from Switzerland for two West Nile virus strains

Aedes (Hulecoeteomyia) japonicus japonicus (Diptera: Culicidae) (Theobald) is an invasive mosquito species in Central Europe, where it has colonized several areas. In this study, field-collected specimens of Ae. japonicus and Culex pipiens (Diptera: Culicidae) (Linnaeus) from Zürich (Switzerland) were orally exposed to two strains (NY99 and FIN) of the avian zoonotic pathogen West Nile virus (WNV) (family Flaviviridae, genus Flavivirus). Dissemination and transmission of the viruses after incubation for 12-15 days under a fluctuating Central European midsummer temperature regime (24 ± 7 °C) was investigated by detection of viral RNA in homogenates of pools of both head/thorax and saliva by reverse transcription real-time polymerase chain reaction (PCR). Culex pipiens was susceptible to WNV NY99 only, whereas both virus strains could be detected in Ae. japonicus, with the additional isolation of WNV NY99 in Vero cell culture from one saliva pool. Given the high abundances of Ae. japonicus in many newly colonized areas, its recently demonstrated broad host range, including mammalian and avian blood hosts, and its vector competence, this species is a potential key bridge vector of WNV in Central Europe.

Newly discovered population of Aedes japonicus japonicus (Diptera: Culicidae) in Upper Bavaria, Germany, and Salzburg, Austria, is closely related to the Austrian/Slovenian bush mosquito population

Background

The German mosquito surveillance instrument ‘Mueckenatlas’ requests the general public to collect and submit mosquito specimens. Among these, increasing numbers of individuals of invasive species have been registered. Specimens of the Asian bush mosquito Aedes japonicus japonicus submitted from German Upper Bavaria, where this species had not previously been recorded, triggered regional monitoring in mid-2015.

Methods

The search for Ae. j. japonicus breeding sites and developmental stages concentrated on cemeteries in the municipality of origin of the submitted specimens and, subsequently, in the whole region. A virtual grid consisting of 10 × 10 km² cells in which up to three cemeteries were checked, was laid over the region. A cell was considered positive as soon as Ae. j. japonicus larvae were detected, and regarded negative when no larvae could be found in any of the cemeteries inspected. All cells surrounding a positive cell were screened accordingly. A subset of collected Aedes j. japonicus specimens was subjected to microsatellite and nad4 sequence analyses, and obtained data were compared to individuals from previously discovered European populations.

Results

Based on the grid cells, an area of approximately 900 km² was populated by Ae. j. japonicus in Upper Bavaria and neighbouring Austria. Genetic analyses of microsatellites and nad4 gene sequences generated one genotype out of two previously described for Europe and three haplotypes, one of which had previously been found in Europe only in Ae. j. japonicus samples from a population in East Austria and Slovenia. The genetic analysis suggests the new population is closely related to the Austrian/Slovenian population.

Conclusion

As Ae. j. japonicus is well adapted to temperate climates, it has a strong tendency to expand and to colonise new territories in Central Europe, which is facilitated by human-mediated, passive transportation. The new population in Upper Bavaria/Austria is the seventh separate population described in Europe. According to our data, it originated from a previously detected population in eastern Austria/Slovenia and not from an introduction event from abroad. The dispersal and population dynamics of Ae. j. japonicus should be thoroughly surveyed, as this species is a potential vector of disease agents.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1447-z) contains supplementary material, which is available to authorized users.

Field evaluation of baited traps for surveillance of Aedes japonicus japonicus in Switzerland

The efficacy of Centers for Disease Control (CDC) miniature light traps and ovitraps was tested in the outskirts of the city of Zurich in Switzerland for their use in the surveillance of Aedes (Hulecoeteomyia) japonicus japonicus (Theobald) (Diptera: Culicidae), the invasive Asian bush mosquito. Sets of single CDC traps were run overnight (n = 18) in three different environments (forest, suburban and urban) in 3 × 3 Latin square experimental designs. Traps were baited with: (a) carbon dioxide (CO2 ); (b) CO2 plus light, or (c) CO2 plus lure blend [Combi FRC 3003 (iGu® )]. At the same locations, mosquito eggs were collected weekly using standard ovitraps baited with different infusions (oak, hay or tap water) and equipped with different oviposition substrates (a block of extruded polystyrene, a germination paper strip or a wooden stick). Data were analysed using Poisson and negative binomial general linear models. The use of light (P < 0.001) or lure (P < 0.001) significantly increased the attractiveness of CDC traps baited with CO2 . Oak and hay infusions did not increase the attractiveness of ovitraps compared with standing tap water (P > 0.05), and extruded polystyrene blocks were preferred as an oviposition substrate over wooden sticks (P < 0.05) and seed germination paper (P < 0.05). Carbon dioxide-baited CDC miniature light traps complemented with light or iGu® lure and ovitraps containing standing tap water and polystyrene oviposition blocks can be considered as efficient and simple tools for use in Ae. j. japonicus surveillance programmes.