Biology of mosquito

Characteristics of mosquito:

• slender and long mouth parts
• scaly wings
• complete head for larvae
• a pair of spiracles on the dorsal part of the 8^th abdominal segment of the larvae

Life cycle: complete metamorphosis with 4 stages (egg, larva, pupa and adult)

Egg:

• Adult mosquito generally lays eggs on water surface or its adjacent sites
• Development of eggs takes 2 to 3 days

Larva:

• Aquatic animal without leg
• With four stages of development
• Comes up to the water surface to take atmospheric air with its spiracle at the 8^th segment

Pupa:

• Non-feeding aquatic form
• Has to come up to the water surface to breathe usually
• Life span is 2 to 3 days

Adult:

• Body can be divided as head, thorax and abdomen
• A pair of compound eyes and antennae as well as mouthparts on its head
• A pair of jointed legs on each segment of the thorax and a pair of wings
• The abdomen is composed of ten segments

Behaviour :

1. Mating: Female mosquito of many species mate 1 to 2 days after hatching. Female adults normally mate only once in its life.
2. Feeding: Normally only female adults feed on animal blood. Male adults feed on plant juice. Some species are anthropophilic and some are zoophilic.
3. Egg laying: Female mosquito of most species has to get blood meal for the development of eggs. Breeding place can be permanent stagnant water, flowing water, temporary stagnant water or containers.
4. Resting: Adult mosquito rests near breeding site for a few hours after hatching. Exophilic mosquito also rests indoors for a short period of time before and after feeding.
5. Dispersion: Adult mosquito disperses for feeding, finding suitable resting sites, mating and laying eggs. It can disperse through flight, air current or vehicles.
6. Hibernation: Some mosquitoes (for example most species of Anopheles and Culex) hibernate in winter at adult stage. Aedes over winter at egg stage.
7. Longevity: Generally male mosquitoes only survive one week but the females can live for two to three weeks.

Mosquitoes commonly found in Hong Kong

Among the more than 3000 species found around the World, more than 70 species have been found locally. Some 10 species such as Anopheles sinensis, Anopheles maculatus, Aedes albopictus Culex quinquefasciatus and Culex tritaeniorhynchus are commonly found. The mosquito-borne disease vectors found locally are:

1. Anopheles jeyporiensis (Photo)
   • Breed in paddy field, waterlogged field and stream
   • Female adults are most active during 23.00p.m. to 2.00a.m.
   • Anthropophilic although it also feeds on animals
   • Local malaria vector
2. Anopheles minimus (Photo)
   • Breed along slow running, unpolluted stream with diffused sunlight and marginal vegetation
   • Greater larval density in periods from March to April and September to October
   • Feed on human and animals at night but viciously at midnight
   • Adults stay indoors before dawn after feeding
   • Local malaria vector
3. Aedes togoi (Photo)
   • Breed in large number in pools with semi-saline water along seacoasts
   • Vicious night biter
   • Strong flier
   • Vector of filariasis
4. Aedes albopictus (Photo)
   • Eggs can withstand drought very well
   • Breed in small containers such as cans, discarded tyres, tree holes, rock pools and bamboo ends holding small amount of water
   • Adults rest in shrubby area
   • Adult has a white stripe on the dorsal surface of thorax and bands on legs. Although exophilic, adults may also enter into houses
   • Day biter
   • Weak flier (about 100m)
   • Vector of dengue fever
5. Culex quinquefasciatus (Photo)
   • Breed in large number in practically all types of water collections from rainwater holding artificial containers to polluted river
   • Always found in construction sites
   • Each egg raft has about 300 eggs
   • Female adults feed viciously on animals or human at night
   • Strong flier
   • Vector of filariasis
6. Culex tritaeniorhynchus (Photo)
   • Breed in pool, fishpond, water lodged grass field, slow running stream and ditch
   • Adults are active at night with peak of activities one hour after dark
   • Exophilic but stay indoors before and after feeding on blood
   • Mainly zoophagous but also feed on human
   • Vector of Japanese encephalitis and filariasis

Common local mosquito-borne diseases

• Malaria
  • caused by plasmodium
  • a disease transmitted by a plasmodium-infected mosquito feeding on human
  • the local vectors responsible for the transmission of malaria are Anopheles minimus and Anopheles jeyporiensis
• Dengue fever
  • caused by dengue virus
  • a disease transmitted by a dengue virus-infected mosquito feeding on human
  • the local vector responsible for the transmission of dengue fever is Aedes albopictus
  • the infected mosquito can pass the infection to their progeny by transovarial transmission

New information on Gravidtrap Index for Aedes albopictus

• Japanese encephalitis
  • caused by Japanese encephalitis virus (JE)
  • a disease transmitted by a JE-infected mosquito feeding on human
  • the local vectors responsible for the transmission of dengue fever are Culex tritaeniorhynchus, Culex gelidus and Culex fuscocephala
  • the infected mosquito can also pass the infection to their progeny by transovarial transmission
  • click here for more information on Japanese Encephalitis

Mosquito survey

Malaria vector

To study the distribution of Anopheles. A larval and an adult survey have been conducted locally in order to have detailed information about the growth patterns and distribution of the local malaria vectors.

Dengue fever vector

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.

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.

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.

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.

Please click here for the phased Gravidtrap Index in March 2023.

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

Remarks:
Gravidtrap Index figures in green:                Level I : Gravidtrap Index <5.0% (indicates that the distribution of Aedes albopictus in the survey area is not extensive)
Gravidtrap Index figures in yellow:               Level II : 5.0% ≤ Gravidtrap Index <20.0% (indicates that the distribution of Aedes albopictus in the survey area is fairly extensive)
Gravidtrap Index figures in red:                      Level III : 20.0%≤  Gravidtap Index <40.0% ( indicates that the distribution of Aedes albopictus in the survey area is extensive)
Gravidtrap Index figures in purple:                Level IV : Gravidtrap Index ≥40.0% (indicates that the distribution of Aedes albopictus in the survey area is very extensive)

Density Index figures in green:                      Level I : 1.0 ≤ Density Index ≤ 1.5 (Aedes albopictus is not abundant in the locations with positive Gravidtraps)
Density Index figures in yellow:                     Level II : 1.6 ≤ Density Index ≤ 2.4 (Aedes albopictus is slightly abundant in the locations with positive Gravidtraps)
Density Index figures in red:                          Level III : Density Index ≥ 2.5 (Aedes albopictus is abundant in the locations with positive Gravidtraps)

Remarks:
Gravidtrap Index figures in green:                Level I : Gravidtrap Index <5.0% (indicates that the distribution of Aedes albopictus in the survey area is not extensive)
Gravidtrap Index figures in yellow:               Level II : 5.0% ≤ Gravidtrap Index <20.0% (indicates that the distribution of Aedes albopictus in the survey area is fairly extensive)
Gravidtrap Index figures in red:                      Level III : 20.0%≤  Gravidtap Index <40.0% ( indicates that the distribution of Aedes albopictus in the survey area is extensive)
Gravidtrap Index figures in purple:                Level IV : Gravidtrap Index ≥40.0% (indicates that the distribution of Aedes albopictus in the survey area is very extensive)

Density Index figures in green:                      Level I : 1.0 ≤ Density Index ≤ 1.5 (Aedes albopictus is not abundant in the locations with positive Gravidtraps)
Density Index figures in yellow:                     Level II : 1.6 ≤ Density Index ≤ 2.4 (Aedes albopictus is slightly abundant in the locations with positive Gravidtraps)
Density Index figures in red:                          Level III : Density Index ≥ 2.5 (Aedes albopictus is abundant in the locations with positive Gravidtraps)

Mosquito prevention

Advice

To the public

The public is advised:

• To tidy up their premises and check for any accumulation of water inside their premises;
• To remove all unnecessary water collection and eliminate the sources;
• To change at least every week the water in flower vases and saucers of potted plants to prevent breeding of mosquitoes. The use of saucers should be avoided whenever possible;
• To properly cover all containers that hold water to prevent mosquito from accessing the water;
• To properly dispose articles that are able to contain water such as empty lunch boxes, cans and tyres;
• To stop storing water along morning walk trails or Government land for irrigation;
• To make large holes on tyres used as anti-bumping measure in garage to prevent water trapping or use mark(s) on the parking space to prevent bumping instead.
• To contact the district pest control offices or PCAS of FEHD, or pest control companies for assistance in mosquito control or prevention.
• To install mosquito screen on windows and doors.
• To use mosquito net for sleeping if necessary.
• To apply mosquito repellent on the clothes or skin according to label instructions.
• To wear light-coloured long-sleeved clothes and long trousers.
• To avoid applying odour-producing cosmetics such as perfume/body lotion during outdoor activities.

To the management of construction sites

The management of construction sites is advised:

• To assign a staff for the control and prevention of mosquito in the site;
• To tidy up the construction site regularly, at least weekly and check for any accumulation of water inside the site;
• To remove all unnecessary water collection and eliminate the source(s);
• To cover all containers that hold water to prevent mosquito from accessing the water;
• To render the vertical poles in scaffolding unable to hold water by making holes at the ends or filling them up with sand;
• To dispose articles that are able to contain water such as empty lunch boxes, cans, disused articles and tyres into covered container(s) and remove these articles from the site regularly, at least weekly;
• To carry out larviciding against mosquito breeding where the breeding sources or potential breeding grounds are inaccessible or could not be eliminated.
• To contact the district pest control offices or PCAS of FEHD, or pest control companies for assistance in mosquito control or prevention.

Mosquito control

Adult Control

• to use mosquito trap for trapping the mosquito
• to use mosquito coil or aerosol to kill the mosquito directly
• to kill mosquito with electrical device designed for the purpose (Observe the safety precautions in using the device)

Larval Control

• to remove containers with water
• to clear stagnant water
• to pave/fill uneven ground and/or holes
• to apply larvicidal oil or pesticide to kill the larvae
• to keep fish which feeds on mosquito larvae