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

doi:10.21203/rs.3.rs-2003272/v1

Background: Shanghai has a large number of high-rise apartment and office buildings, but the effects of these high-rise spaces on the vertical dispersal, oviposition and blood feeding behaviour of Aedes albopictus are unknown.

Methods: In six multi-storey 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 1^st to 6^th floors. Collections were made for 4 months. Human landing catch (HLC) for Ae. albopictus monitoring was also conducted on 6 consecutive days on six floors of two buildings among those mentioned above to study the feeding behaviour of Ae. albopictus at different heights.

Results: MOT and HLC collected Ae. albopictus at all monitored heights. The vertical distribution, oviposition pattern and biting behaviour varied significantly among the seven heights (1^st–6^th floor and 8^th floor) (mosq-ovitrap index (MOI): F = 5.640, P < 0.001; HLC: F = 15.111, P < 0.001). The MOI at low heights (1^st + 2^nd floors) was significantly higher than that at medium (3^rd + 4^th floor, P < 0.001) and high heights (5^th + 6^th floors, P < 0.001), and there was no significant difference in the MOI for the 3^rd–6^th floors. The outdoor MOIs were significantly higher than indoor MOIs at all heights (outdoor 23.09% vs. indoor 9.58%, t = −5.139, P < 0.001). Ae. 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: Ae. albopictus is more common near the ground level, but it can easily disperse to higher floors in the multi-storey buildings of urban Shanghai. No significant differences in Ae. albopictus density were detected within the 3^rd–6^th floors by MOT or HLC. This suggests that Ae. albopictus might also disperse to areas higher than the 6^th floor and seek hosts there. Ae. albopictus prefers to oviposit outdoors; however, Ae. albopictus was also able to inhabit, oviposit and engage in blood feeding behaviour 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.

Aedes albopictus

vertical dispersal

mosq-ovitrap

human landing catch

multi-storey building

Shanghai downtown

Aedes albopictus, the Asian tiger mosquito, is the most medically important insect vector species in Shanghai, China. It is especially important for its role in the transmission of diseases like dengue [1, 2]. Ae. albopictus is an aggressive human biting species and a public health threat. Ae. albopictus is currently a major priority for control efforts, especially after the first autochthonous dengue case was reported in Shanghai [3].

To improve surveillance, control and management practices, we must increase our understanding of mosquito biology [4]. An important aspect of mosquito biology is its vertical flight dispersal range. Knowledge of the vertical movements of mosquito vectors aids understanding of disease transmission dynamics and helps determine the controls needed to interrupt pathogen transmission [5]. This is particularly important in highly urbanized areas like Shanghai. Many studies have evaluated the vertical oviposition and distribution patterns of mosquitoes occupying equatorial forests and savannahs [6], but relatively little attention has been paid to mosquito problems in urban areas.

Ae. albopictus is the dominant, and most important, vector in Shanghai. This species prefers to oviposit at the ground level, but Ae. albopictus can alter their movements to adapt to decreased habitat availability, leading to more opportunistic oviposition behaviour [7]. Ae. albopictus can oviposit at 3.5 m, 6 m and 7 m in Sri Lanka and Louisiana (USA) [8, 9]. Another study showed that Ae. albopictus could move vertically through a 60-m-high apartment building and oviposit at all heights [10]. In Shanghai, and other locations in China, knowledge of Ae. albopictus oviposition at different heights is lacking.

In addition to resting or seeking oviposition sites, the active vertical dispersal of Ae. albopictus can also be triggered by the search for mates and blood meal hosts. Many structural changes have occurred within the downtown area of Shanghai including an increase in the number of high-rise apartment and office buildings. Over 80% of the population lived in high-rise apartments in downtown Shanghai, and Ae. albopictus often occurs in residences located in high-rise apartments. However, no study has evaluated the impact of these construction change on the behaviour or adaptation of Ae. albopictus in Shanghai. There is little information about the vertical dispersal of Ae. albopictus when it seeks hosts for blood meals.

The ovitrap or mosq-ovitrap (MOT) is an efficient and sensitive device for detecting the presence/absence of dengue vectors, even at low population densities [11]. Human landing catch (HLC) is a controversial method, which may pose a risk to monitoring participants. However, HLC is the most effective method for adult Aedes sampling. In the present study, we used both MOT and HLC to evaluate the vertical dispersal of Ae. albopictus host seeking and ovipositing behaviour in six high-rise apartments with ≥ 6 storeys in an urban environment. This is the first study in China to describe the vertical dispersal and distribution of Ae. albopictus in multi-storey buildings.

Study sites

The study was conducted within the downtown areas of Shanghai China (31°13′N, 121°27′E, el 3.5 m). Six building blocks ≥ 6 storeys located in five residential neighbourhoods and one enterprise campus were selected for mosquito sampling. Geographical and ecological descriptions of the study sites are given in Table 1.

Table 1

Geographical and ecological information of the six building blocks
Site ID	Type of environment	Building heights	Target floors with traps	No. of MOTs^a placed	Coordinate
Site 1	Residential neighbourhood	> 20 floors	1–6F	26	31°12'30.52"N, 121°28'28.41"E
Site 2	Residential neighbourhood	8 floors	1–6F, 8F^b	16	31°12'58.66"N, 121°27'21.21"E
Site 3	Residential neighbourhood	10 floors	1–6F	24	31°12'21.48"N, 121°28'56.92"E
Site 4	Residential neighbourhood	> 20 floors	1–6F	48	31°13'27.99"N, 121°29'41.63"E
Site 5	Residential neighbourhood	> 20 floors	1–6F	48	31°12'55.88"N, 121°28'55.00"E
Site 6	Enterprise and institution	12 floors	1–6F	12	31°12'12.97"N, 121°27'23.76"E
^a MOTs: mosq-ovitraps;
^b In site 2, I addition to 1st -6th floors, a terrace on the 8th floor was also included for MOT monitoring.

Mosquito sampling

Mosquito monitoring was conducted using two sampling methods: MOT and HLC. MOT was carried out during 4 months from June 21st to October 20th of 2019, and HLC was conducted for 6 consecutive days in August 2019.

Mosq-ovitrap (MOT)

The MOT (Tianpai, Kaiqi Co. Ltd, Shanghai, China) as described by Lin[12] and Gao [11] was used in this study. It is a modified ovitrap with a design for easy mosquito entry but a difficult exit. It consisted of a transparent cylindrical plastic jar and a black top cover with three conical holes. A white circular filter paper (7.5 cm in diameter) is placed at the bottom of the jar as an oviposition substrate, and 20 mL of dechlorinated tap water is added to the jar to keep the filter moist but not submerged. MOT can collect both adult mosquitoes seeking oviposition sites and eggs produced by trapped females.

In this study, MOT sampling was conducted for 4 months, with 10 d as a sampling interval. A fixed number of MOTs were placed on each floor covering 1^st–6th floors of the six buildings (total 174 traps × 12 sampling intervals = 2088 trap times). The locations of MOTs on each floor were divided into indoor and outdoor environments according to the straight line distance from a window or outside. On each floor, “outdoor” in this study refers to balconies projecting outward, open windowsill or areas with a distance < 0.5 m from windows. “Indoor” refers to other interior spaces of the apartment block such as corridors, stairways as well as under shoe racks located next to the door. To maximize accessibility and operability, MOTs were not placed inside the homes of the occupants.

For each 10 d sampling interval, the number of adult mosquitoes trapped and the number of eggs produced and trap conditions (i.e., tipped, dried, missing, flooded or broken) were recorded. Collections were made between 09:00 and 12:00 h, which is the daily time of lowest mosquito activity [13]. A positive MOT is defined as a trap containing at least one adult, egg or larva.

Trapped mosquitoes were collected and stored at − 80℃.

Human landing catch (HLC)

HLC sampling was conducted for 6 consecutive days in early August of 2019, on heights from the 1st floor to the 6th floor in two buildings located in Site 4 and Site 5. A total of 12 volunteers aged between 31 and 59 years were recruited and trained for HLC after we obtained their informed content. The participants were divided into two groups, with six volunteers designated for each building. Within each group, one participant occupied each floor from the 1st floor to the 6th floor. The participants stood still and collected mosquitoes landing on their exposed arms or legs using a portable, battery-powered aspirator. The catches period had a duration of 30 min and was performed twice a day, including the early morning (7:30–8:00 h) and the pre-sunset late afternoon (16:30–17:00 h). To eliminate human-bait attraction bias caused by individual differences among the volunteers, a Latin-square design was adopted for HLC sampling in each building. For each of the 6 consecutive days, each individual volunteer occupied a different floor on different day (Fig. 1).

The HLCs were carried out in areas without reported dengue or Zika cases. According to report of local center for disease control and prevention(CDC), there were no other Aedes-vectored disease cases during the study period.

Mosquito processing

Adult mosquitoes trapped by MOTs and HLCs were killed by freezing and then counted and identified using taxonomic keys (Becker et al., 2003)[14]. Eggs or eclosed larvae collected by MOTs after 10 d were returned to the laboratory. For species identification, we randomly selected 20% of the trapped eggs or larvae and then hatched and reared these until adult emergence. Adults were identified and the results were extrapolated to the rest of the unhatched eggs.

In the laboratory, the eggs were hatched by immersing them in transparent glass pans filled with MiliQ water. Larvae were reared using standard techniques, and adults were identified by taxonomic keys(Becker et al., 2003)[14] in the same manner as for MOTs and HLCs.

Statistical methods

All data obtained from this study were analysed as follows:

1. Mosq-ovitrap index (MOI), the percentage of positive MOTs against the total number of MOTs recovered from each monitoring site;

2. Adult Ae. albopictus density, mean number of Ae. albopictus per recovered positive MOT or sampled by HLC per 30 min;

3. Eggs density, mean number of eggs per recovered positive MOT.

Data were analysed using SPSS version 13.0 (SPSS, Inc., Chicago, IL, USA) statistical package. Quantitative data, such as MOI, mosquito density or egg density, were not normally distributed. After logarithmic transformation of the data, an independent t-test or one-way analysis of variation (one-way ANOVA) was used for comparison, and Tukey’s test was used for pairwise comparison. Pearson correlation analysis was used for vertical distribution of mosquito sampling yields of MOT and HLC. A value of P < 0.05 was considered to represent a statistically significant difference.

Mosquito population structure

A total of 9,245 eggs were collected by the MOT. A randomized 20% sample of these eggs were reared to adults, and all were identified as Ae. albopictus. A total of 481 adult Ae. albopictus (♀: ♂ = 355: 15), 2 adult female Culex pipiens complex and 1 adult female Culex tritaeniorhynchus were collected by the MOT. Two species were collected by HLCs, including 200 Ae. albopictus (♀: ♂= 180: 20) and 2 female Cx. pipiens complex. The female Culex mosquitoes were mostly collected at the ground level (only one was collected on the 6th floor) and were scattered in different sites. It is not within the scope of this paper to present the results for other mosquito species; so, the results are limited to Ae. albopictus.

Vertical and temporal distribution of Ae. albopictus with MOT

Among the 2088 MOTs, the number of invalid MOT caused by loss or other reasons was small (n = 11) and was ignored.

Adult female Ae. albopictus, or its eggs, were found in MOTs placed at all heights from the ground floor to the 6th floor in all six buildings. The overall number of positive MOTs was 349 (MOI = 16.71%). Among the positive MOTs, the mean densities of adult Ae. albopictus and eggs were 1.38 and 26.49 per trap, respectively.

The Ae. albopictus density and oviposition pattern monitored by the MOT in the six study sites are shown in Table 2. No significant differences were detected in the three parameters of mosq-ovitrap index (MOI), adult Ae. albopictus density and eggs density among the 6 sites (F = 1.897, P = 0.107; F = 1.140, P = 0.349; F = 0.679, P = 0.641).

Table 2

Spatial comparison of *Aedes albopictus* collections by MOT at six study sites
Site	MOT trap·time	Traps of Positive		adult Ae. albopictus		Egg of Ae. albopictus
Site	MOT trap·time	No. of positive trap·time	Mosq-ovitrap index/% (95%CI)	No. of Ae. albopictus collection	density (per trap) (95%CI)	No. of eggs collection	density (per trap) (95%CI)
1	312	45	14.42 (6.71, 22.14)	67	1.49 (0.75, 2.23)	1274	28.31 (12.81, 43.81)
2	192	19	9.9 (0.89, 18.9)	21	1.11 (0.88, 1.33)	400	21.05 (-2.54, 44.64)
3	288	51	17.71 (8.69, 26.73)	56	1.1 (0.44, 1.75)	1150	22.55 (9.42, 35.68)
4	576	84	14.58 (5.11, 24.05)	120	1.43 (0.85, 2.01)	2494	29.69 (3.51, 55.87)
5	576	129	22.4 (7.56, 37.23)	192	1.49 (1.18, 1.79)	3369	26.12 (17.05, 35.18)
6	144	21	14.58 (6.16, 23)	25	1.19 (0.39, 1.99)	558	26.57 (8.62, 44.52)
Sum	2088	349	16.71 (6.97, 26.46)	481	1.38 (0.83, 1.93)	9245	26.49 (8.92, 44.06)
One way ANOVA			F = 1.897 P = 0.107		F = 1.140 P = 0.349		F = 0.679 P = 0.641

The Ae. albopictus vertical distribution and oviposition pattern varied significantly among the seven floors (1st − 6th floor and 8th floor) (MOI: F = 5.640, P < 0.001) (Table 3). Pairwise comparison showed that the MOI values at the ground level (1st floor) were significantly higher than the MOIs of the 3rd, 4th, 5th and 6th floors (Fig. 2A). After regrouping, the MOI values at low heights (1st + 2nd floors) were significantly higher than those at medium heights (3rd + 4th floor, P < 0.001) and high heights (5th + 6th floors, P < 0.001). There was no significant difference in the MOI values between the medium and high floors (P = 0.802) (Fig. 2B).

Table 3

Vertical distribution of *Ae. albopictus* sampled by MOT at different floor heights
Vertical heights/ floors (m)	MOT trap·time	Traps of Positive		adult Ae. albopictus		Egg of Ae. albopictus
Vertical heights/ floors (m)	MOT trap·time	No. of positive trap·time	Mosq-ovitrap index/% (95%CI)	No. of Ae. albopictus collection	density (per trap) (95%CI)	No. of eggs collection	density (per trap) (95%CI)
1st F (0 ~ m)	360	115	31.94 (17.26, 46.63)	177	1.54 (0.99, 2.09)	3570	31.04 (18.16, 43.92)
2nd F (3 ~ m)	336	89	26.49 (11.73, 41.25)	126	1.42 (1.01, 1.82)	2551	28.66 (16.04, 41.28)
3rd F (6 ~ m)	336	48	14.29 (2.99, 25.58)	69	1.44 (1.00, 1.87)	910	18.96 (9.63, 28.29)
4th F(9 ~ m)	336	25	7.44 (-0.34, 15.22)	29	1.16 (0.87, 1.45)	473	18.92 (9.68, 28.16)
5th F (12 ~ m)	336	35	10.42 (5.27, 15.56)	39	1.11 (0.94, 1.28)	859	24.54 (-3.15, 52.23)
6th F (15 ~ m)	336	24	7.14 (-0.15, 14.43)	26	1.08 (0.74, 1.43)	610	25.42 (10.44, 40.4)
8th F (21 ~ m)	48	13	27.08 (-0.79, 54.96)	15	1.15 (0.78, 1.53)	272	20.92 (2.03, 39.81)
Sum	2088	349	16.71 (4.03, 29.4)	481	1.38 (1.01, 1.74)	9245	26.49 (11.4, 41.58)
One way ANOVA			F = 1.897 P = 0.107		F = 1.140 P = 0.349		F = 0.679 P = 0.641

Table 4

Spatial and temporal distribution of *Ae. albopictus* monitored by HLC at different vertical heights at site 4 and site 5
Level of vertical height/ floor	Early morning (7:30 − 8:00 h)		Pre sunset late afternoon (16:30 − 17:00 h)
Level of vertical height/ floor	No. of adult Ae. albopictus collection	Density/ per 30 min (95% CI)	No. of adult Ae. albopictus collection	Density/ per 30 min (95% CI)
Site 4
1	19	3.17 (-0.55, 6.89)	35	5.83 (0.72, 10.95)
2	2	0.33 (-0.21, 0.88)	27	4.50 (1.33, 7.67)
3	3	0.50 (-0.38, 1.38)	15	2.50 (0.22, 4.78)
4	5	0.83 (-0.85, 2.51)	9	1.50 (-0.57, 3.57)
5	1	0.17 (-0.26, 0.60)	6	1.00 (-0.33, 2.33)
6	0	0 (0, 0)	2	0.33 (-0.21, 0.88)
Sum	30	0.83 (0.20, 1.47)	94	2.61 (1.53, 3.69)
Site 5
1	16	2.67 (-0.42, 5.76)	51	8.50 (2.50, 14.50)
2	3	0.50 (-0.07, 1.07)	4	0.67 (-0.19, 1.52)
3	0	0 (0, 0)	1	0.17 (-0.26, 0.60)
4	0	0 (0, 0)	0	0 (0, 0)
5	0	0 (0, 0)	0	0 (0, 0)
6	0	0 (0, 0)	1	0.17 (-0.26, 0.60)
Sum	19	0.53 (0.02, 1.04)	57	1.58 (0.29, 2.88)

For positive MOTs, the density of adult Ae. albopictus and eggs was relatively balanced and no significant difference was detected among different floors (adult Ae. albopictus: F = 1.030, P = 0.415; egg density: F = 1.278, P = 0.281).

After removing the data for the 8th floor, there was a significant negative correlation between the values of MOI and vertical height levels (Pearson r = -0.605, P < 0.001). However, after removing the data for 8th, 1st and 2nd floors, no significant correlation was observed anymore (Pearson r = − 0.164, P < 0.265). This shows that the Ae. albopictus density on the lower floors was significantly higher than that on medium and high floors, while there was no significant difference in Ae. Albopictus density between the medium and high floors.

For the indoor/outdoor effect on Ae. albopictus dispersal and oviposition behaviour at vertical heights, we observed that Ae. albopictus prefer to oviposit outdoors. The MOI values of outdoors were significantly higher than that of indoors at all heights (1st floor to 6th floor) (outdoor 23.09% vs. indoor 9.58%, t = − 5.139, P < 0.001) (Fig. 2E and 2F). There were no significant differences in adult Ae. albopictus and egg densities between indoors and outdoors (Ae. albopictus: outdoor 1.28 vs. indoor 1.25 per trap, t = − 0.165, P = 0.870; eggs: outdoor 23.78 vs. indoor 29.32, t = 1.004, P = 0.259).

The temporal distributions of Ae. albopictus at all seven 7 heights monitored by MOTs are shown in Fig. 3. The temporal variances of adult Ae. albopictus and egg density were not significant (F = 1.657, P = 0.109; F = 0.822, P = 0.619). The MOI value fluctuated greatly in different months. MOI peaked in mid-July and early-August (29.31% and 29.31%, respectively), while in late June and Mid October, the MOIs were < 10%.

Human landing catch

In sites 4 and site 5, a total of 144 HLC · time (6 participants × 2 buildings × 2 30-min periods per day × 6 days) were performed over 6 consecutive days. The mean density of adult Ae. albopictus in the late afternoon was significantly higher than the adult density during early morning (2.10 vs. 0.68 per 30 min, t = − 3.065, P = 0.003).

Adult Ae. albopictus were collected at every vertical height from the ground floor to the 6th floor at site 4, while Ae. albopictus were only collected on the 1st, 2nd, 3rd and 6th floors at site 5. The vertical distribution of Ae. albopictus varied significantly among the different levels (F = 15.111, P < 0.001), and the highest density of Ae. albopictus was found on the ground floor at Site 4 and Site 5. There was a significant correlation between the mean number of Ae. albopictus collected and the height of the buildings (Site 4: r = − 0.514, P < 0.001; Site 5: r = − 0.628, P < 0.001), indicating that Ae. albopictus density decreased with the increase of vertical height. Pairwise comparison using the Tukey method showed that the density of Ae. albopictus on the 1st floor was significantly greater on the 1st floor than on the 2nd − 6th floors (5.04 vs. 0.13, 0.29, 0.58, 0.79 and 1.50 per 30 min, P < 0.05). No significant difference in Ae. albopictus density were detected among the heights above the ground floor (P > 0.05) (Fig. 4).

People living or working in high-rises buildings of Shanghai often complain about being bitten by mosquitoes. There is a need for up-to-date information on Ae. albopictus vertical dispersal in urban areas to guide control methods[10].

Based on the filed monitoring used in this study, we confirmed that Ae. albopictus were present, ovipositing and seeking blood meal hosts on 1^st–6^th floor heights in all of the monitored buildings in downtown Shanghai. Many Ae. albopictus were also found breeding and flying on an 8^th floor terrace. Ae. albopictus are typically more common in areas near the ground, like the ground floor and the 2^nd floor in this study. However, it appears that they can readily disperse to the higher floors of multi-storey buildings. In areas between the height range of 3^rd–6^th floors, there was no significant difference in Ae. albopictus density, as monitored by MOI. As an exophilic species, Ae. albopictus preferred to oviposit in the outdoor areas on different floors.The MOI in outdoor environments was significantly higher than that in indoor environments. However, Ae. albopictus can also occur, and oviposit indoors. The biting rate of Ae. albopictus in Shanghai was higher in the evening before sunset than in the morning.

Each mosquito population has a different range of dispersal as a consequence of its intrinsic flight capacity and the ecological setting [5], and Ae. albopictus appear to conforms to this rule. For vertical dispersal, previous studies showed that Ae. albopictus prefer to fly at ground level or at heights < 3m [15, 16]. This may be because: a). Ae. albopictus prefer to oviposit in small pockets of container water, and these breeding habitats are more common on the ground; b). Ae. albopictus prefer to bite humans. In contrast to ornithophilic mosquito species that usually fly higher, anthropophilic mosquito species usually fly lower, since their sugar sources and blood meal hosts are mostly distributed and active on the ground; c). Ae. albopictus appear to have relatively weak flight capacity and their maximum horizontal flight range is only 100–200m[5, 17, 18].

We found that Ae. albopictus in urban Shanghai prefer to be active at ground level or on low building floors, but we also confirmed that they can disperse to higher areas, like floors 3–6 or floor 8, and engage in blood feeding and oviposition on the high floors. A report of Ae. albopictus invading higher spaces was previously documented. Amerisinghe and Alagoda(1984) demonstrated that Ae. albopictus could oviposit at heights of 3.5 and 6 m in Sri Lanka[8]. In Louisiana, Ae. Albopictus were found to oviposit at heights up to 7 m [9]. In Singapore, Liew and Curtis found that Ae. albopictus could move vertically through a 60-m-high apartment building and oviposit at all heights[10]. In Malaysia, Ae. albopictus were found up to the sixth floor (16.1–18.0m), and this indicated that Aedes could be found at even higher levels of high-rise apartments [19]. Lau et al. reported Ae. albopictus in Malaysia breeding up to the 14^thfloor (39.1–42.0m). This indicated that Aedes could breed at every level of the apartment building and were not restricted by the height of the apartment [20]. This dispersal to higher areas is not limited to Ae. albopictus, but has also been found in populations of Aedes aegypti, which were found breeding on the highest floors (16^th floor, 45.1–48.0m) in some studies [20].

Marini (2010) suggested that the active dispersal of Ae. albopictus is triggered by the search for mates, sugar sources and resting sites. For females, dispersal helps identify hosts for blood meals and oviposition sites and depends on environmental thresholds, such as light intensity, wind speed and direction, and temperature[5]. Vavassori conducted a mark‑release‑recapture study and found that the active dispersal of host-seeking female Ae. albopictus may be greater than previously reported [21]. Chadee also reported that the adaptation of Aedes mosquitoes to house design evolved from ground floor areas to higher elevation apartment buildings[22].

Shanghai is the largest metropolis in east China, and it has undergone many environmental changes, particularly in urban sanitation, housing types and population density. These factors have favoured an expanded three-dimensional spatial distribution of Ae. albopictus: 1). urban sanitation campaigns have removed many scattered small waste containers and reduced the potential outdoor water breeding sites, which has forced Ae. albopictus to fly further or higher to locate suitable water oviposition sites; 2). the heights of buildings in the urban areas are increasing and a greater proportion of the human population live and work in high-rise spaces. To obtain blood meals, Ae. albopictus also need to disperse and adapt to high-rise areas; 3). The application of pesticides in the ground environment forces Ae. albopictus to fly to higher areas to find a more suitable habitat. Tinker (1974) suggested that the movement of Ae. aegypti above ground level may be due to insecticide treatment at the ground level [23]; 4). Other human activities may also affect the vertical dispersal of mosquitoes. The frequent use of elevators may have carried Ae. albopictus into high-rise areas. The growing popularity of high-rise terrace gardens in urban areas also provides good breeding habitats for mosquitoes like Ae. albopictus.

Ae. albopictus are known as an exophilic species [24]. However, Ae. albopictus are now commonly found in indoor spaces, especially in urban areas like Shanghai. In this study, although Ae. albopictus prefer to oviposit outdoors on different floors, we also confirmed that they can inhabit, bite and oviposit indoors. This may be the first vertical dispersal test of Ae. albopictus population directly evaluated by HLC. The landing Ae. Albopictus, which were detected indoors in high space areas by HLC may have originated from multiple sources: a). an aboriginal Aedes population inhabiting the particular floor that were attracted to the host at certain monitoring time; b). ground inhabiting Aedes mosquitoes, which were attracted by CO2 and human odour and then flew or used elevators to reach the high space with the human host. Chadee (2004) also investigated indoor oviposition and biting behaviours in the West Indies These increased over an extended period and were attributed to artificial lighting and peaks in human activity within homes [22]. As for biting rhythm, the biting rate in the evening before sunset is higher than that in the morning, and we did not study the biting rate over an extended period in this study.

The active dispersal of Ae. albopictus on higher floors and indoor spaces may pose challenges to mosquito control and dengue fever control. Current Aedes mosquito and dengue control measures mainly focus on the control of adult mosquitoes near the ground and the removal of outdoor breeding sites. However, as the urban ecological settings change, certain control measures near the ground may drive more Ae. albopictus to seek suitable habitats at higher elevations. When there is a viremic person infected with dengue living in a high-rise residential building, the female mosquitoes inhabiting the same floor or adjacent floors would be the most problematic. These females would comprise the group most likely to bite the infected person and then spread the pathogen. The implementation of mosquito control measures at the ground level would probably not have a substantial impact on the infected mosquitoes of high-rise buildings. Ae. albopictus living in high-elevation spaces may become a blind spot in the control of mosquitoes and dengue fever. It is therefore necessary to consider three-dimensional mosquito control measures such as residual insecticide spraying in the building where the infected person lives.

This study evaluated the oviposition and host-seeking activity of Ae. albopictus in urban high-rise apartments. This is the first attempt to evaluate the vertical dispersal of Ae. albopictus in multiple-storey buildings in China, and it provides information that could facilitate the dengue vector control programme in this area.

There were some limitations in the research design, which are as follows:

a). As an exploratory study, we limited our attention to the 1^st–6^th floors. The vertical dispersal of Ae. albopictus in areas higher than the 6^th floor should be verified in a future study. We found no significant difference in Ae. albopictus density monitored by MOI in spaces above the 2^nd floor (as 3^rd–6^th floors), indicating that Aedes density may not decrease as the floor level increases. This suggests that Ae. albopictus could be found breeding at every level of the apartment and may not be restricted by the height of the apartment, as was suggested by Lau [20]. There were no significant differences in egg density monitored by MOI among different floor heights, suggesting that the vertical floor heights has no significant impact on the oviposition behaviour of Ae. albopictus. Once Ae. albopictus reach a certain floor, they treat this floor as the ground starting point and then continue to spread to lower or higher floors. We will continue to verify the activities of Ae. albopictus on the 7^th–20^th floors as the next step.

b). Another shortcoming is that the dispersal of Ae. albopictus to higher floors in this study does not prove that Ae. albopictus reach certain floors directly by flying. Its vertical flight capacity or behaviour in urban areas should also be further studied.

Although Ae. albopictus are more common near the ground, they appear to easily disperse to higher floors of urban buildings. We confirmed the presence of Ae. albopictus ovipositing and seeking hosts on the 1st − 6th floors of multi-storey buildings in urban Shanghai. No significant differences in Ae. albopictus density were found in the spaces of 3rd − 6th floors by MOT or HLC, suggesting that Ae. albopictus might also disperse to spaces higher than the 6th floor and seek hosts within these spaces. Ae. albopictus are an exophilic species that prefer to oviposit outdoors on different floors. Ae. albopictus can also inhabit, oviposit and seek blood meals indoors. The extensively three-dimensional dispersed pattern of Ae. albopictus in urban areas could facilitate arbovirus transmission and increase the difficulty of dengue control.

MOT

mosq-ovitrap

MOI

mosq-ovitrap index

HLC

human landing catch

ANOVA

analysis of variance

CO₂

carbon dioxide.

Acknowledgements

Not applicable

Funding

This work was funded by National Natural Science Foundation of China Youth Fund (81903376) to Qiang Gao, Training Program Foundations for Excellent Young Doctors of Shanghai (Public health leader), the Major Public Health Project of Huangpu District (HWZFX201805), and the National Major Scientific and Technological Special Project during the Thirteenth Five-year Plan Period (2017ZX10303404001006).

Availability of data and materials

All relevant data are within the paper.

Authors’ contributions

QG conceived the study and drafted the manuscript. QG, SQJ and JF designed the field work and collected samples. PEL designed and led the field work and collections. HC, JF and ZDZ conducted the experiments, analyzed the data and wrote the manuscript. All authors read and approved the final manuscript.

Ethics approval and consent to participate

For human landing catch used in field Ae. albopictus sampling, informed consent was obtained from all the participants in this study. During the study period, there was no Aedes-borne disease cases, and a follow-up was made at 3 months following the end of the study to ensure that none of the participants were positive for Aedes-borne pathogens. The Shanghai Huangpu Center for Disease Control and Prevention (HPCDC) through the HPCDC Ethical Review Committee granted ethical approval (HPCDC-LL20190302) for this study.

Consent for publication

Written consent forms for publication were obtained from all individuals involved in this study.

Competing interests

The authors declare that they have no competing interests.

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No competing interests reported.

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

Status:

Journal Publication

Version 1

Abstract

Figures

Background

Methods

Study sites

Mosquito sampling

Mosq-ovitrap (MOT)

Human landing catch (HLC)

Mosquito processing

Statistical methods

Results

Mosquito population structure

Human landing catch

Discussion

Conclusions

Abbreviations

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1