Mosquito, (Culex, Aedes and Anopheles)

(Diptera: Culicidae)

There are over 3000 species of mosquitoes in the world which are distributed from the tropics to the altitude of 4300 meters. They are also found 1160 meters below the sea level in the gold mines in south India. They are virtually distributed everywhere except in Iceland and poles.

Disease transmission

Identification characters

Males of all species have rudimentary maxillae and mandibles so that they cannot suck blood but can suck fluids and nectar from flowers. They also possess very bushy whorl plumose antennae and tip of abdomen with characteristic male genitalia. On the other hand females have short hairs on the antennae and needle-like maxillae and mandibles for piercing the skin of host for sucking blood. Other characteristics are given below according to the species.

LIFE CYCLE

Eggs are laid on the surface of water and have some device to keep them afloat for oxygenation. Blood meal is essential for the development of ovaries and maturation of ova and hence female sucks blood. Incubation period in all species is about 2 days.

Larva has head, thorax and a long abdomen. Thorax is the bulkiest part of the body. There are paired eyes, antennae and feeding brush that drives a current of water along with food particles towards the mouth. Abdomen is 10-segmented, 8-9 segments are fused to form a complex spiracular apparatus or respiratory apparatus. Anal gills are located on the tenth segment. Respiratory siphon is located on the 9^th segment and internally divided by a septum into two chambers. Thorax and abdomen have long hairs on the lateral sides. There are 4 larval instars and larval period ranges between 6 and 8 days.

Pupa is comma shaped, having cephalothorax and abdomen. Cephalothorax has a pair of respiratory tubes called respiratory trumpets and a pair of eyes. Abdomen is 8 segmented and has caudal paddles and hairs at the tip. Pupa does not feed but moves about actively. Pupal period is two days. During adult emergence, the cephalothorax of pupa breaks on the outer surface and imago comes out, sits on the pupal case for sometime to dry up its wings and then flies away. Total life cycle takes 10-12 days.

Culex pipiens fatigans. It is dull whitish mosquito having unspotted wings and makes humming sound when flying. There are overlapping scales and six transverse whitish bands on the abdomen. Scutellum in dorsal view looks trilobed and each lobe has a bunch of long hairs. Thorax has no markings on the dorsal side. Maxillary palps of female are short, 3-5 segmented, while in male they are long and equal to or longer than proboscis. While resting it sits parallel to the ground. There are about 240 species in India out of which 4-5 are vectors of diseases. It breeds in cesspools, drains, disused wells and stagnated water. Polluted water is preferred for breeding.

Eggs are long cigar-shaped, whitish in colour and deposited on the surface of water in a raft of 50-100 eggs, which help the eggs to float. Egg incubation period is 2 days. Larva is aquatic, with head, thorax and 10-segmented abdomen and bunches of long hairs on the thorax and abdomen. Respiratory siphon on the 9th abdominal segment is long and narrow and is thrust out of water for air breathing, while the body hangs at an angle of about 45 degrees.

Head bears a pair of eyes, antennae, maxillary palps and feeding brush. There are long bunches of hairs and anal gills on the tip of abdomen but the gills are inadequate respiratory organs. Larva is also called wriggler. There are 4 larval instars with a total larval period of 6-8 days. Pupa is coma-shaped, with long and narrow respiratory trumpets on the cephalothorax. Abdomen is curved with caudal paddles on the last segment for swimming. It does not feed but swims actively. Pupa is also called tumbler. Pupal period is two days and pupa comes to the surface before adult emergence.

Aedes egypti: This is called zebra mosquito as it has black and white bands on the abdomen and legs. It belongs to the same subfamily as Culex and hence has structural similarities with it. Thorax is black with a pair of sickle like white markings on the dorsal side. Scutellum is trilobed with three bunches of long hairs on the posterior margin. Maxillary palps of female are small but those of male equal to proboscis or longer.

The species breeds in tree holes, broken containers, flower pot, puddles, coolers and other small water collections. Eggs are laid singly on water surface. They are blackish with small pits on the surface which help them to float on water. They hatch in two days. Eggs can also be laid in moist soil where they can remain dormant for months. Larva is also black in colour and has a short and barrel shaped spiracle. It is a bottom feeder and has structural similarities with Culex larvae.

Pupa is deep coma shaped, black with white markings and having three abdominal segments attached to cephalothorax. Respiratory trumpet is funnel-shaped, narrow at base and gradually broadening at the apex. Pupal period is only two days.

Anopheles spp.: There are 44 species out of which six species are known vectors of malaria in India.

Adults are dull whitish in colour having wings with blackish spots and dark veins. They make no noise while flying. There may be scattered scales on the abdomen. Thorax without any markings on the dorsal side and scutellum not lobed and has uniformly distributed hairs on its posterior margin. Maxillary palps in both sexes are equal to proboscis but in male they are clubbed at the tip. Adult in resting position makes an angle of 45 degrees against the surface.

Eggs are laid singly on the surface of water. They possess a pair of floats which prevent them from drowning. Larva rests parallel to the water surface and has palmate hairs on the sides of abdomen for that. Respiratory siphons are absent and there are a pair of respiratory openings instead. Larval period is about a week.

Pupa is deep coma shaped in which 5 abdominal segments are attached to cephalothorax. Abdomen is 8 segmented. Respiratory trumpet wine-glass shaped, with a stalk and parallel-sided body. Adults emerge in 2-3 days.

MOSQUITO CONTROL

Mosquito control efforts have not been successful because of the ability of mosquitoes to develop resistance against insecticides very quickly and their capacity to inhabit a variety of environmental conditions. The following measures are generally adopted to reduce mosquito populations.

Personal preventive measures

Use of mosquito nets is an effective method to prevent adults from biting and transmitting malaria. Application of mosquito repellent chemicals, such as citronella oil, dimethylphthalate, odomos cream or pyrethrum cream also prevent mosquitoes from sucking blood. Mosquito repelling fumigants, e.g. tortoise mosquito coil contains pyrethrum in it and Goodknight mats or Allout liquids contain synthetic pyrethroids such as deltamethrin, decamethrin and allethrin. They effectively repel and confuse mosquitoes.

Anti-larval measures

Removal of breeding places effectively reduces mosquito population. Broken pots, old tyres, tins and other containers should be removed from the surroundings as they serve as breeding places for Aedes. Coolers and overhead water tanks should be periodically cleaned or treated with potassium permagnate to kill the larvae. Small water bodies, ditches and ponds that cannot be filled should be sprayed with light diesel or petroleum oil that makes a thin film on the water surface and clogs respiratory siphons of larvae.

Use of Paris green (copper aceto-arsenite) also kills larval and pupal stages. Biological control of larvae and pupae in ponds has been achieved by releasing larvivorous fishes, such as the native Gambusia and Nothobranchius guntheri introduced by CIBC from Africa. These fishes actively feed on the larvae and can aestivate in mud when ponds dry up in the summer months. Naiads of dragonflies and damselflies are also effective predators of mosquito larvae and pupae.

Anti-adult measures

Trapping of adults by hanging black cloths to serve as hiding places during day time and then killing the adults by spraying insecticide should be done daily. UV electrocuting traps should be used to attract and kill adults. Destruction of tall grasses and bushes serve as resting places for mosquitoes in day time and hence should be removed from the surrounding areas.

Use of malathion and endosulfan aerosols in the colonies periodically has been effective in reducing their populations. Aerial sprays of pyrethrum, carbaryl, carbofuran, arprocarb mixed with mineral oil are still effective in killing adults. Synthetic pyrethroids are quite effective and new chemicals used against mosquitoes but are prohibitively expensive.