Local Vector Biology

Aedes albopictus

1. Eggs can withstand drought very well
2. Breed in small containers such as cans, discarded tyres, tree holes, rock pools and bamboo ends holding small amount of water
3. Adults rest in shrubby area
4. Adult has a white stripe on the dorsal surface of thorax and bands on legs
5. Although exophilic, adults may also enter into houses
6. Day biter (Peak : within 2 hours after dawn and before sunset)
7. Weak flier (about 100m)
8. Vector of dengue fever

Vector Surveillance

Gravidtrap

Since 2000, the Food and Environmental Hygiene Department (FEHD) has been using Oviposition Trap (Ovitrap) to detect the presence of adult Aedine mosquitoes. Tools and methodology of dengue vector surveillance is reviewed from time to time and starting from April 2020, a new tool, Gravidtrap has been adopted to replace ovitrap for monitoring the dengue vector of Aedine mosquitoes. The Gravidtrap is a simple device made of a black plastic container of approximately 600ml with a glue board placed inside. It is covered by a black lid with openings and a raised grey cover.

Gravidtrap Index

To study the distribution of Aedes albopictus, Gravidtraps are set in selected areas throughout the territory for monitoring the breeding of these mosquitoes. The percentage of the Gravidtraps found positive with breeding of these mosquitoes gives the Gravidtrap Index for Aedes albopictus. The value of Gravidtrap Index for Aedes albopictus indicates the extensiveness of the breeding of the vectors.

Scope of Survey

At present, there are a total of 64 survey areas selected for the vector surveillance throughout the territory and the following four different indices will be enumerated:

1. Area Gravidtrap Index for Aedes albopictus (AGI),
2. Area Density Index for Aedes albopictus (ADI),
3. Monthly Gravidtrap Index for Aedes albopictus (MGI), and
4. Monthly Density Index for Aedes albopictus (MDI).

The AGI and ADI indicate the extensiveness of the distribution and density of Aedine mosquitoes respectively in that particular survey area, while the MGI and MDI are enumerated by pooling together all AGIs and ADIs of the same month reflecting the territory-wide situation of Aedes albopictus.

In addition to the areas in the community, surveys of Aedes albopictus are also carried out to cover all major port areas. The following two different groups of indices will be enumerated:

1. Port / Port Monthly Gravidtrap Index for Aedes albopictus (PGI/PMGI), and
2. Port / Port Monthly Density Index for Aedes albopictus (PDI/PMDI).

The Port Gravidtrap Index for Aedes albopictus (PGI) indicates the extensiveness of the distribution of Aedine mosquitoes in a particular group of port areas. The Port Monthly Gravidtrap Index for Aedes albopictus (PMGI) is enumerated by pooling together all PGIs of the same month, which reflects the overall degree of occurrence of Aedes albopictus in port areas. Another index, the Port Density Index for Aedes albopictus (PDI) indicates the density of Aedes albopictus in positive traps in a particular group of port areas. The Port Monthly Density Index for Aedes albopictus (PMDI) is enumerated by pooling together all PDIs of the same month, which denotes the overall density of Aedes albopictus in locales detected with Aedine mosquito activities in port areas.

Gravidtrap only detects activities of Aedine mosquitoes in the area where it is set. It cannot be used for detecting activities of other species of mosquitoes (such as Culex and Anopheline mosquitoes). The Gravidtrap Index for Aedes albopictus of an area reflects the extensiveness of the distribution of Aedine mosquitoes and the Density Index for Aedes albopictus of an area reflects the density of Aedes albopictus in the relevant surveillance area during the two weeks of surveillance. Apart from making reference to the relevant Gravidtrap Index and Density Index for Aedes albopictus, parties concerned should take into account other factors (such as the findings and reports on mosquito problem received) in planning and implementing their mosquito prevention and control work.

Survey Mechanism

The period for setting Gravidtraps in the survey areas is two weeks. During the survey period, the Gravidtraps are collected on a weekly basis. At the end of the first week of surveillance, all the Gravidtraps are retrieved and another batch of Gravidtraps is set at the same location for surveillance for the second week. The glue boards inside the retrieved Gravidtraps are immediately examined for the presence of adult Aedine mosquitoes (“Aedes-positive”) for the purpose of compiling the Gravidtrap Index (First Phase) and Density Index (First Phase). At the end of the second week of surveillance, all the Gravidtraps are retrieved and instantly checked for the presence of adult Aedine mosquitoes (“Aedes-positive”) on the glue boards. Data of the two weeks’ surveillance will be combined to obtain the Area Gravidtrap Index (AGI) and Area Density Index (ADI).

FEHD will notify its District Environmental Hygiene Offices and other relevant government departments of the Gravidtrap Indices and Density Indices of various phases once the figures are available, so that they can promptly carry out targeted mosquito prevention and control work. To facilitate public access to the latest survey data, FEHD will announce the phased indices for Aedes albopictus for each of the surveyed areas on the FEHD webpage. Corresponding mosquito prevention and control measures can be taken by the public in a more expeditious manner so as to prevent the spread of dengue fever.

The Gravidtrap Index and Density Index for Aedes albopictus of each surveyed area are released in two phases. Figures released in the first phases are phased indices and the second phase indices represent the AGI and ADI of the surveyed area in that month.

Level of Index

Gravidtrap Index

The Gravidtrap Index for Aedes albopictus is divided into four levels, reflecting the distribution level of the Aedes albopictus. Level 1 (less than 5 per cent) indicates that distribution of Aedes albopictus is not extensive in the area surveyed. Level 2 (5 per cent to less than 20 per cent) indicates that distribution of Aedes albopictus is fairly extensive in the area surveyed. Level 3 (20 per cent to less than 40 per cent) indicates that distribution of Aedes albopictus is extensive in the area surveyed. Level 4 (40 per cent or above) indicates that distribution of Aedes albopictus is very extensive in the area surveyed. Specific preventive and control measures will be initiated accordingly. Management of housing estates, hospitals, schools, construction sites and other venues or properties are also advised to take specific measures to contain mosquito problems in their own properties. The four levels of Gravidtrap Index for Aedes albopictus and actions to be taken by the management are listed in the following table.

Density Index

With the adoption of Gravidtraps since April 2020, a new index called the Density Index (DI) is also recorded in each survey area.  It is denoted as Area Density Index (ADI) which is enumerated by dividing the number of adult Aedes by the total number of Aedes positive Gravidtraps during the survey period.  The minimum value of the index is one.

The Density Index for Aedes albopictus is divided into three levels, reflecting the abundancy of Aedes albopictus.  Level 1 (1 to 1.5) indicates that mosquito is not abundant in the locations with positive Gravidtraps.  Level 2 (1.6 to 2.4) indicates that mosquito is slightly abundant in the locations with positive Gravidtraps.  Level 3 (2.5 or above) indicates that mosquito is abundant in the locations with positive Gravidtraps.  Specific preventive and control measures will be initiated accordingly.  Management of housing estates, hospitals, schools, construction sites and other venues or properties will also be advised to take specific measures to contain mosquito problems in their own properties.  The three levels of DI and actions to be taken by the management are listed in the following table. 

Public should refer to the “Action to be taken” for AGI as primary reference.  If both indices are of similar levels, their actions are also similar.  However, if the AGI is low, e.g. Level 1 or Level 2, but level of ADI is high, e.g. Level 3, public should carry out actions for corresponding level of AGIs and supplement with actions for higher level of ADI, e.g. fogging, in locations with high mosquito density.

Information of Survey Area in the Community

Latest Index Information

The phased Gravidtrap Indices and Density Indices for Aedes albopictus for each surveyed area in March 2023 are provided below:

Index value remarks：

Colour	Gravidtrap Index	Density Index
Green：Level I	Gravidtrap Index < 5.0% [indicates that the distribution of Aedes albopictus is not extensive]	1.0 ≤ Density Index ≤ 1.5 [Aedes albopictus is not abundant in the locations with positive Gravidtraps]
Yellow：Level II	5.0% ≤ Gravidtrap Index <20 .0% [indicates that the distribution of Aedes albopictus is fairly extensive]	1.6 ≤ Density Index ≤ 2.4 [Aedes albopictus is slightly abundant in the locations with positive Gravidtraps]
Red：Level III	20.0% ≤ Gravidtrap Index  <40 .0% [indicates that the distribution of Aedes albopictus  is extensive]	Density Index ≥ 2.5 [Aedes albopictus is abundant in the locations with positive Gravidtraps]
Purple：Level IV	Gravidtrap Index ≥40.0%  [indicates that the distribution of Aedes albopictus  is very extensive]

Brief of Gravidtrap Indices and Density Indices in February 2023

The monthly Gravidtrap index for Aedes albopictus in February was 0.3 per cent. The index in February remained at Level 1 at its lowest, indicating that distribution of Aedes albopictus in the areas surveyed was not extensive. All the 64 survey areas were recorded with a Gravidtrap index lower than the alert level of 20 per cent. Moreover, the monthly density index for Aedes albopictus in February remained at 1, which represented that an average of one Aedes albopictus adult was found in the Aedes-positive Gravidtraps, indicating that the number of adult Aedes albopictus was not abundant in the survey areas.

Record of Indices in 2023

Area Gravidtrap Indices and Density Indices for Aedes albopictus for the 64 survey areas in 2023 are provided below:

Index value remarks：

Colour	Gravidtrap Index	Density Index
Green：Level I	Gravidtrap Index < 5.0% [indicates that the distribution of Aedes albopictus is not extensive]	1.0 ≤ Density Index ≤ 1.5 [Aedes albopictus is not abundant in the locations with positive Gravidtraps]
Yellow：Level II	5.0% ≤ Gravidtrap Index <20 .0% [indicates that the distribution of Aedes albopictus is fairly extensive]	1.6 ≤ Density Index ≤ 2.4 [Aedes albopictus is slightly abundant in the locations with positive Gravidtraps]
Red：Level III	20.0% ≤ Gravidtrap Index  <40 .0% [indicates that the distribution of Aedes albopictus  is extensive]	Density Index ≥ 2.5 [Aedes albopictus is abundant in the locations with positive Gravidtraps]
Purple：Level IV	Gravidtrap Index ≥40.0%  [indicates that the distribution of Aedes albopictus  is very extensive]

Number of areas in respective levels of Survey Area Gravidtrap Indices in 2023 are provided below: