Vertical dispersal of Aedes albopictus within multi-story buildings in downtown Shanghai, China.

BACKGROUND: Shanghai has numerous high-rise apartment and office buildings, but the effects of these high-rise spaces on the vertical dispersal, oviposition and blood feeding behavior of Aedes albopictus are unknown. METHODS: In six multi-story building blocks in downtown Shanghai, 174 mosq-ovitraps (MOT) were placed both indoors and outdoors for Ae. albopictus collection at different vertical heights from the 1st to 6th floors and a terrace on the 8th floor. Collections were made for 4 months. The human landing catch (HLC) method for Ae. albopictus monitoring was also conducted on 6 consecutive days on six floors of two of the six buildings to study the feeding behavior of Ae. albopictus at different heights. RESULTS: Both MOTs and HLCs collected Ae. albopictus at all monitored heights. The vertical distribution, oviposition pattern and biting behavior varied significantly among the seven heights (1st-6th floors and 8th floor) (mosq-ovitrap index (MOI): X2 = 140.616, df = 6, P < 0.001; HLC: F (5, 138) = 15.111, P < 0.001). The MOI at low heights (1st + 2nd floors) was significantly higher than that at medium (3rd + 4th floor, P < 0.001) and high heights (5th + 6th floors, P < 0.001), and there was no significant difference in the MOI for the 3rd-6th floors. The outdoor MOIs were significantly higher than indoor MOIs at all heights (outdoor 23.09% vs. indoor 9.58%, X2 = 74.121, df = 1, P < 0.001). Aedes albopictus HLC density on the ground floor was significantly higher than that on all other heights (5.04 vs. 0.13, 0.29, 0.58, 0.79 and 1.50 per half hour, P < 0.05), while no difference was detected among the heights above the ground floor (P > 0.05). CONCLUSIONS: Aedes albopictus is more common near the ground level, but it can easily disperse to higher floors in the multi-story buildings of urban Shanghai. No significant differences in Ae. albopictus density were detected within the 3rd-6th floors using MOT or HLC. This suggests that Ae. albopictus might also disperse to areas above the 6th floor and seek hosts there. Aedes albopictus prefers to oviposit outdoors; however, Ae. albopictus was also able to inhabit, oviposit and engage in blood-feeding behavior indoors on different floors. The three-dimensional dispersal pattern of Ae. albopictus in urban areas could facilitate arbovirus transmission and increase the difficulty of dengue control.

High frequency of Voltage-gated sodium channel (VGSC) gene mutations in Aedes albopictus (Diptera: Culicidae) suggest rapid insecticide resistance evolution in Shanghai, China.

BACKGROUND: Dengue fever is an infectious disease that is imported into Shanghai, China and requires prevention and control measures. Controlling the vector Aedes albopictus through insecticide use is a key approach to dengue control. However, the rapid evolution of insecticide resistance in Ae. albopictus has raised concerns about the failure of dengue control efforts. Knockdown resistance (kdr) caused by point mutations in the voltage-gated sodium channel (VGSC) gene is a primary mechanism of pyrethroid resistance. In this study, we investigated the kdr mutations of Ae. albopictus in Shanghai and evaluated the trend in its evolution. METHODOLOGY/PRINCIPAL FINDINGS: We collected 17 populations of Ae. albopictus from 15 districts in Shanghai in 2020, extracted genomic DNA from individual mosquitoes, and amplified Domain II, III, and IV in VGSC using PCR. Following sequencing, we obtained 658 VGSC sequences. We detected the nonsynonymous mutations V1016G, I1532T, and F1534S/C/I, among which V1016G and F1534C/I were reported in Shanghai for the first time and F1534I was a novel mutant allele in Ae. albopictus. The overall mutation frequency was 84.65%, with individual mutation frequencies ranging from 46.81% to 100%, excluding the Fengxian District population, which had a frequency of 0%. The V1016G and I1532T mutation types accounted for 7.14% and 3.42%, respectively. The mutant allele at codon 1534 accounted for 63.98% of all mutations, including TCC/S (62.77%), TGC/C (1.06%), and ATC/I (0.15%). We identified and classified five intron types in Domain III by length, including A (83 bp, 12.07%), B (68 bp, 87.30%), C (80 bp, 0.16%), D (72 bp, 0.16%), and E (70 bp, 0.31%). Individuals with intron B had a significant mutation tendency at codon 1534 relative to intron A (chi-square test, p < 0.0001). We found no correlation between mutation frequency and the amount of pyrethroid used (Pearson correlation, p = 0.4755). CONCLUSIONS/SIGNIFICANCE: In recent years, kdr mutations in the Ae. albopictus population in Shanghai have rapidly evolved, as evidenced by an increase in mutation types and significantly increased mutation frequency. The F1534I/ATC mutant allele was found to be a novel mutation, F1534C/TGC was reported for the first time in Shanghai, and intron B in Domain III was significantly associated with mutation frequency at codon 1534. Continuous monitoring of resistance changes and strict regulation of insecticide use are required.

Spatial and temporal analyses of the influences of meteorological and environmental factors on Aedes albopictus (Diptera: Culicidae) population dynamics during the peak abundance period at a city scale.

Aedes albopictus (Diptera: Culicidae) is a major vector of multiple diseases. While vaccines have been developed, preventing these Aedes-borne diseases continues to primarily depend on monitoring and controlling the vector population. Despite increasing research on the impacts of various factors on Ae. albopictus population dynamics, there is still no consensus on how meteorological or environmental factors affect vector distribution. In this study, the relationships between mosquito abundance and meteorological and environmental indicators were examined at the town level based on data collected from July to September, the peak abundance period of 2019 in Shanghai. In addition to performing Poisson regression, we employed the geographically weighted Poisson regression model to account for spatial dependency and heterogeneity. The result showed that the environmental factors (notably human population density, the Normalized Difference Vegetation Index (NDVI), socioeconomic deprivation, and road density) had more significant impacts than the meteorological variables in accounting for the spatial variation of mosquito abundance at a city scale. The dominant environmental variable differed in urban and rural places. Furthermore, our findings indicated that deprived townships are more susceptible to higher vector densities compared to non-deprived townships. Therefore, it is crucial not only to allocate more resources but also to increase attention towards controlling the vectors responsible for their transmission in these townships.

Analysis of the spatial distribution of Aedes albopictus in an urban area of Shanghai, China.

BACKGROUND: Aedes albopictus is a vector of major arboviral diseases and a primary pest in tropical and temperate regions of China. In most cities of China, the current monitoring system for the spread of Ae. albopictus is based on the subdistrict scale and does not consider spatial distribution for analysis of species density. Thus, the system is not sufficiently accurate for epidemic investigations, especially in large cities. METHODS: This study used an improved surveillance program, with the mosquito oviposition trap (MOT) method, integrating the actual monitoring locations to investigate the temporal and spatial distribution of Ae. albopictus abundance in an urban area of Shanghai, China from 2018 to 2019. A total of 133 monitoring units were selected for surveillance of Ae. albopictus density in the study area, which was composed of 14 subdistricts. The vector abundance and spatial structure of Ae. albopictus were predicted using a binomial areal kriging model based on eight MOTs in each unit. Results were compared to the light trap (LT) method of the traditional monitoring scheme. RESULTS: A total of 8,192 MOTs were placed in the study area in 2018, and 7917 (96.6%) were retrieved, with a positive rate of 6.45%. In 2019, 22,715 (97.0%) of 23,408 MOTs were recovered, with a positive rate of 5.44%. Using the LT method, 273 (93.5%) and 312 (94.5%) adult female Ae. albopictus were gathered in 2018 and 2019, respectively. The Ae. albopictus populations increased slowly from May, reached a peak in July, and declined gradually from September. The MOT positivity index (MPI) showed significant positive spatial autocorrelation across the study area, whereas LT collections indicated a nonsignificant spatial autocorrelation. The MPI was suitable for spatial interpolation using the binomial areal kriging model and showed different hot spots in different years. CONCLUSIONS: The improved surveillance system integrated with a geographical information system (GIS) can improve our understanding of the spatial and temporal distribution of Ae. albopictus in urban areas and provide a practical method for decision-makers to implement vector control and mosquito management.

Coagulation-flocculation: a potential application for mosquito Larval Source Management (LSM).

Larval mosquitoes have a more limited home range and lower resistance to adverse environment than adults, thus can be ideal targets for vector control in some cases. Coagulation-flocculation technology, which could be used for water treatment in breeding sites of several vector mosquito species, can significantly change both the distribution of organic particles and surface sediment characteristics in water environment. The aim of this study was to explore the effect, principle and possibility of using coagulation-flocculation technology in immature mosquitoes killing. In this study, dechlorinated water was treated with Poly Aluminum Chloride (PACl, sewage treatment using), and we observed the impacts of PACl treatment on the development and survival of immature Culex pipiens pallens mosquitoes. When exposed to PACl treatment, physical effect is believed to be a main reason of coagulation-flocculation caused high larvae mortality: Ⅰ) alum floc layer increases the difficulty of larvae foraging, leads larvae starving to death; (Ⅱ) the little floc particles could attach to the lateral hair of larvae, which impede floatation process and then surface respiration by larval mosquitoes. The alum floc layer had a good killing effect on the mosquito larvae, presented the half lethal time (LT50) of 2d, the 90% lethal time (LT90) of 8.7±7.3 ∼ 14±4.5 d, and the pupation rate of 0 ∼ (6.5±0.5)%, respectively. Our results indicates alum floc, produced by PACl coagulation-flocculation, was shown to be highly active against 1st∼2nd instar larvae, the high mortality rate of immature mosquitoes as a result of physical effect. The observations suggest that coagulation-flocculation technology offers a novel potential approach to a sustainable and low-impact mosquito control method.

Field evaluation of Mosq-ovitrap, Ovitrap and a CO2-light trap for Aedes albopictus sampling in Shanghai, China.

BACKGROUND: The Mosq-ovitrap (MOT) is currently used for routine surveillance of container-breeding Aedes in China. However, the effectiveness of monitoring Aedes albopictus using the MOT and other mosquito monitoring methods, such as the Ovitrap (OT) and the CO2-light trap (CLT), have not been extensively compared. Moreover, little is known about the spatial-temporal correlations of eggs with adult Ae. albopictus abundance among these three types of traps. METHODS: Comparative field evaluation of MOT, OT and CLT for Ae. albopictus monitoring was conducted simultaneously at two city parks and three residential neighborhoods in downtown Shanghai for 8 months from April 21 to December 21, 2017. RESULTS: Significantly more Ae. albopictus eggs were collected from both MOTs and OTs when traps remained in the field for 10 d or 7 d compared with 3 d (MOT: 50.16, 34.15 vs. 12.38 per trap, P < 0.001; OT: 3.98, 2.92 vs. 0.63 per trap, P < 0.001). Egg collections of MOTs were significantly greater than OTs for all three exposure durations (Percent positive: X 2 = 72.251, 52.420 and 51.429, P value all < 0.001; egg collections: t = 8.068, 8.517 and 10.021, P value all <0.001). Significant temporal correlations were observed between yields of MOT and CLT in all sampling locations and 3 different MOT exposure durations (correlation coefficient r ranged from 0.439 to 0.850, P values all < 0.05). However, great variation was found in the spatial distributions of Ae. albopictus density between MOT and CLT. MOT considerably underestimated Ae. albopictus abundances in areas with high Ae. albopictus density (>25.56 per day ⋅ trap by CLT). CONCLUSION: The MOT was more efficient than the OT in percent positive scores and egg collections of Ae. albopictus. The minimum length of time that MOTs are deployed in the field should not be less than 7 d, as Ae. albopictus collections during this period were much greater than for 3 d of monitoring. MOT considerably underestimated Ae. albopictus abundance in areas with high Aedes albopictus density compared to CLT. In areas with moderate Aedes albopictus densities, MOT results were significantly correlated with CLT catches.

Autogeny, Fecundity, and Other Life History Traits of Culex pipiens molestus (Diptera: Culicidae) in Shanghai, China.

Culex pipiens molestus Forskål is common in subterranean and aboveground environments in Shanghai, China. However, little is known about its autogeny, fecundity, and other life history traits. In this study, Cx. p. molestus larvae were collected from four different types of underground parking lots and artificially maintained, without blood feeding, in a laboratory. Life history traits parameters were recorded from the F0 to F9 generation. Culex pipiens molestus from Shanghai can autogenously persist for at least 10 generations, and some females can develop more than one egg batch without blood feeding (oviposition rate in Lot C was >100% in some generations). The average number of eggs per raft produced autogenously ranged from 36.94 to 42.19. The average generation time (GT) ranged from 20.06 to 21.30 d, and the pre-oviposition time (POT) ranged from 2.17 to 12.68 d among the generations. Culex pipiens molestus from the four groups had different autogeny, fecundity, and longevity patterns. This variability may result from genetic polymorphism caused by co-occurrence with other Culex pipiens subspecies (Diptera: Culicidae).

Comparison of the human-baited double net trap with the human landing catch for Aedes albopictus monitoring in Shanghai, China.

BACKGROUND: Human landing catch (HLC) is the most efficient method for Aedes monitoring, but it is not ethical due to its high risk of human exposure to pathogens. We designed trials to assess the performance of an alternative human-baited double net trap (HDN) for field Aedes albopictus monitoring compared with the standard HLC. METHODS: Outdoor HDN and HLC catches were conducted simultaneously at 15 field sites on two sunny days in mid-July and August. The tests were performed 3 h apart: an early morning period (7:30-8:30 h), a pre-sunset period (16:30-17:30 h) and a post-sunset period (18:30-19:30 h). A total of 90 comparisons were made between the two methods. Field comparisons were designed to minimize half-hour bias and human-bait attraction bias. RESULTS: Two mosquito species were collected by HDN and HLC, with the predominated species being Ae. albopictus (HDN: n = 1325, 97.35% of total; HLC: n = 531, 92.51% of total). A small proportion were adults of the Culex pipiens complex (HDN: n = 36, 2.65% of total; HLC: n = 43, 7.49% of total). Although the mean Ae. albopictus catch per hour of HLC was significantly higher than HDN (14.72 vs 5.90 per h, t(178) = 3.151, P = 0.003), there were significant positive spatial and temporal correlations between HLC and HDN for Ae. albopictus sampling among different sites and hours (r(90) = 0.785, P < 0.001; r(90) = 0.785, P < 0.001). Both methods proved that Ae. albopictus was most active during the hours before sunset and least active after sunset. No significant variation was observed in Ae. albopictus catch size of HDN between groups of more attractive and less attractive humans (3.38 vs 2.51 per 30 min, t(88) = 1.283, P = 0.201). CONCLUSIONS: With moderate sampling efficiency, significantly positive spatial correlation with HLC, and less human-bait attraction bias, HDN appears to be a safer alternative to HLC for Ae. albopictus monitoring in Shanghai. With mosquito activity peaking in the pre-sunset hours, Ae. albopictus catches of HDN should be performed in the hours before dark. The trap design could be improved to make it more portable and easier for field operation.

Aedes albopictus production in urban stormwater catch basins and manhole chambers of downtown Shanghai, China.

BACKGROUND: The near-surface urban drainage system in Shanghai is highly complex, with hundreds of thousands of catch basins (CBs) and manhole chambers (MCs). Comparatively little is known about the breeding of mosquitoes in this vast system, especially for the locally predominant species Aedes albopictus. A cross-sectional mosquito sampling study was conducted from late July to early August of 2017 using 539 CBs and 309 MCs located in 10 communities of downtown Shanghai. We measured the water-holding status of the drainage systems and density of mosquito larvae. Mosquito species were examined on site and in the laboratory later. RESULTS: The CBs were characterized by a lower percentage of standing water compared to MCs (47.0% vs. 79.9%, respectively; X2 = 76.407, P<0.001), but CBs contained a higher percentage of stagnant water percent than MCs (45.2% vs. 35.3%, respectively; X2 = 11.465, P = 0.001). There were exclusively two species of mosquito larvae found in the drainage systems, Ae. albopictus and Culex pipiens complex. Compared with MCs, the structures of CBs were more conducive to larval production and yielded more larvae-positive samples (43.4% vs 14.2%, X2 = 53.136, P<0.001) and higher larval density (8.23 vs. 4.09 per dipper, t = 3.287, P = 0.001). Aedes albopictus was the predominant species in CBs, with a constituent ratio of 71.7%. Regarding structures with different features in different locations, CBs that had a vertical grate with an unsealed lid and MCs with plastic composite covers were the most favorable types for mosquito breeding, and residential neighborhoods yielded the highest number of Ae. albopictus. CONCLUSION: Aedes albopictus was the predominant species in both CBs and stormwater MCs, especially in residential neighborhoods. CBs, particularly those with vertical grates, were a major source of mosquito production in downtown Shanghai. MCs featured more running water and fewer larvae by percentage, and few larvae were found in Sewage MCs. However, due to the tremendous baseline amount, MCs were still an important breeding source of mosquitoes. We suggest that Aedes control in Shanghai should focus on CBs or other potential larvae habitats in and around residential neighborhoods. The use of permeable materials and completely sealed covers should be adopted in the construction of CBs and MCs henceforth.

Abundance and distribution of immature mosquitoes in urban rivers proximate to their larval habitats.

Whether ecological restoration of polluted urban rivers would provide suitable breeding habitats for some mosquitoes was not clear yet. It was therefore important to determine how altered river conditions influence mosquito ecology. Monthly data on water quality and larval density were obtained to determine the effects of river systems on the distribution and abundance of immature mosquitoes in two coastal cities in Eastern China. In total, 5 species within two genera of mosquitoes were collected and identified in habitat with vegetation from three positive rivers. Culex pipiens pallens was the most abundant and widely distributed species. A new species (Culex fuscanus) was reported in certain districts. Physico-chemical parameters of river water were important, but not the only, set of influences on immature mosquito breeding. Aquatic vegetation could increase the likelihood of mosquito breeding while artificial aeration might prevent the approach of mosquitoes. Slow-moving water might be a new potential marginal habitat type for some Culex and Aedes albopictus. Variation of river system with ecological restoration might influence the abundance and distribution of immature mosquitoes.