Director, Centre for Environmental Studies,
Institute of Science
E- mail: email@example.com
LIFE-CYCLE AND CHARCTER:
Among mosquitoes, the females only suck the blood of animals and human beings because of its protein needs for laying the eggs. Most of the flying and biting occurs at night,although Aedes species bite during day time. Anopheles spreads malaria while Aedes transmits dengue,Gunya and viral fevers.Aedes do not prefer to enter into houses and likes to bite only during day time and preferably humans.Culex prefer to bite at dusk and after dark and enters into dwellings for blood meals.The average flight range of a mosquito is 1 to 5 miles, and its life cycle takes about 10 days to 30 days based on environmental conditions.. Most females lay eggs from 100 to300 eggs at a time amounting to 1,000 to 3,000 eggs during their life time. The eggs hatch in one or two days into larvae which are free-swimming and feed by straining minute plants, animals or particles of debris from the surrounding water. The larvae are aquatic and breathe air and after 6 to 8 days of growth, they enter pupal stage from which the adult emerges in about 2 days. Although the life span of a mosquito is about a month, some males were observed to live for about a week only.
Although mosquitoes breed in water, different species prefer to lay their eggs and breed in different localities. Anopheles stephensi which causes urban malaria has a predilection to breed in wells, cisterns, fountains, cess-pools, drains, grass-fields and sewage mixed with tidal wates. Culex fatigans which spreads filaria breeds in highly polluted waters, sullage, street drains, cess-pits etc., Mansonia mosquitoes which transmit filaria breed in polluted water collections that contain aquatic vegetation such as water-lettuces and water-hyacinth. While some mosquitoes breed in temporary pools formed during floods or rains, others breed in container habitats such as tree-holes, leaf-axils, fallen-leaves, water-barrels, discarded tin-cans,drums,barrels, bottles, pots, tyres, flower-pots and ash-trays. Weeds, tall-grass, shrubbery and ill -ventilated huts sheltered buildings, and sheds offer harborage to the adults.
Mosquito Larval and Adult sampling for Surveys
To collect larval samples,a plastic cupdipper attached to the end of a wooden pole is used and the cup is immersed in water to collect the larvae.Dippers can be made or purchased .Depress one end of the dipper under the water surface quickly but smoothly to scoop out the larvae.Take 5 dips from open water and 5 from the water's edge near the vegetation.collect samples from tree holes also.To collect the adult mosquitoes,CDC Light traps baited with Dry Ice[solid Carbon-di-oxide]are used as they attract adult mosquitoes and catch them in a net.Light traps can be run with batteries and they should be hung about 6 ft.off the ground in an open area near the trees or shrubs.
Mosquito Control measures must be taken up when the surveys show that the Dipper contents exceed 5 larvae per dip,if female mosquitoes caught in light traps exceed 25 per night and if the landing rates are more than 5 mosquitoes in 10 minutes or if an outbreak of disease occurs.These general indicators can be modified
II. CHEMICAL CONTROL METHODS AND THEIR LIMITATIONS:
The mosquito larvae which come to the water-surface for air supply are destroyed by treating the breeding grounds with either anti-malarial oils or larval poisons like D.D.T., B.H.C, dieldrin and paris green every week before they emerge out as adults. Even the wells are treated with pure lead-free petrol and other chemicals. For killing the adults, space and residual insecticidal chemical spraying techniques are adopted. For continuing the antilarval and adulticidal spraying programmes, insecticidal spraying would have to be undertaken at regular intervals for an indefinite period. Apart from the recurring cost, the mosquitoes are becoming resistant to chemicals, like DDT and BHC which also kill their predators and natural enemies like birds and fish resulting in an unprecedented population-explosion of insects, by what is known as insecticide induced infestation In this chemical warfare man strategy has miserably failed against that of the mosquito. Most of these chemicals enter into the food chains and food webs in nature and will ultimately poison man and various forms of life useful to man. Hence if the mosquitoes are still breeding fast in our towns inspite of such national control projects, it must be only due to the interim and ineffective methods of control resulting from the ecological illiteracy prevalent among the educated and the uneducated. Perhaps it is time for us to tackle the mosquito problem on a permanent basis by using the civil engineering and bio-engineering methods.
III. ENGINEERING WROKS AND MOSQUITO BREEDING:
About 50% of all malaria is man-made and results from such engineering activities which are carried out without paying much attention to the mosquito breeding possibilities. It is therefore necessary to discuss here the inherent conditions of some such engineering works so as to take appropriate remedial action.
1. For instance, in the case of highway and rail road construction, the embankments frequently run across lines of natural drainage with the result that temporary pools of water are formed during rains. As no effort is made to refill the borrow-pits made during construction stage, the rain water gets stagnated for considerable periods and these pools become ideal breeding places for mosquitoes. Even the culverts provided may not always ensure free drainage due to insufficient capacities, less numbers or poor levels.
2. Similarly pits near stone quarries receive seepage and storm-water and provide good breeding places.
3. In the case of irrigation canals, sub-soil water-level rises due to percolation and water-logged areas in the vicinity give rise to formation of a number of water pools that breed mosquitoes. When canals are under repair, supply is cut off and the main canals become breeding places and as soon as the water is released into the distributaries the mosquito-larvae will be carried into the inhabited areas. The chain of borrow-pits along the canals receive the seepage and rain-water and become breeding grounds.
4. In the case of slow-moving and unlined streams in a flat country, during periods of low summer-flows,the water stagnates on both sides of the streams in small pockets that provide mosquito breeding centres..
5. In the case of lakes and dams, conditions such as minimum wave-action, almost constant water-level, flatting of debris, twigs and trees, along with the growth of aquatic vegetation, offer protection to larvae and become congenial breeding centers.
6. In the case of drinking water supply schemes, leaky sluice valves, hydrants and public-fountains which are not provided with neat platforms and connecting drains give rise to mosquito breeding places..
7. In the case of rural and urban areas without underground drainage, old and faulty surface-drains empty the sullage and surface wash into ponds of the low lying areas where the waste-water remains stagnant and septic and such pools again become massive breeding centers.
8. In the case of house drainage, if the water-closets, gully traps and cisterns remain out of use for some time, mosquito nuisance will occur, unless these fixures are made mosquito- proof and as the pipes are of small bore and short length, mosquitoes proliferate unless the vent pipes are screened with mosquito-proof mesh.
9. In many urban areas, improper sewage-farms encourage breeding of mosquitoes.
10. In rural areas, unless the rice-fields are periodically drained at such intervals of about 5 days that a mosquito cannot complete its life cycle, the fields become congenial mosquito breeding centers.
IV. MOSQUITO CONTROL BY CIVIL ENGINEERING METHODS:
The abatement programme should start with a sanitary survey to locatealmost all the breeding places, followed by the collection and identification of larvae,pupa and adult mosquitoes. Spotting the data on a map of disease-bearing and pestiferous mosquitoes and their concentrations indices will show the location of the problem areas, their relationship to habitation and feasibility of biological control and other methods like draining, filling, diversion of water, relocation of channels, dusting or spraying with ecologically acceptable chemicals, such of those areas which cannot otherwise be effectively tackled.
The principle involved in drainage is to remove all surface water collections from a given area by the use of surface-drains, subsurface-drains ,pumps, filling operations etc.
(1) The open surface drains which carry storm water and sullage from individual houses and plots, empty the same into rivers, canals, lakes and oceans. If these drains are not lined with concrete, brick or stone, scouring and silting and formation of stagnant pools will occur and weeding and cleaning becomes difficult. They must be so designed as to accomplish drainage of the whole area within a week after the maximum rainfall so as to prevent any brood of mosquitoes to develop. To minimize the deposit of silt at the junctions, the branch-drains must join the main-drains at an acute angle.
(2) Syphons, in place of open culverts, should be avoided as they provide the silting up of flow with resulting stagnation of water.
(3) The dangerous conditions in a meandering length of a sluggish stream are eliminated by straightening the stream and shortening the distance connecting any two points of the stream.
(4) Sub-surface drains are used to lower the ground water table so that the seepage or other water-collections can be more easily absorbed by the soil below.
(5) Pumps are used to drain off low-lying areas which otherwise cannot be drained, as the adjacent water-course happens to be at a higher level.
(6) In case of low-lying areas, that cannot be drained properly the filling of such areas with the town-refuse by the Sanitary land fill methodeliminates the mosquito breeding centers.
(7) Some times pools and swamps with impervious-stratum immediately below the ground surface, may be drained by sinking one or more wells to the porous-stratum below.
(8) In the case of dams, frequent water level fluctuations re effected along with drift removal, marginal drainage maintenance, and plant growth control, with the result hat micro-habitat normally favourable for mosquito is destroyed and the mosquito cannot develop beyond the larval stage.
(9) Intermittent sluicing or flushing from a dam, causes a sudden rise in the water level of the stream and destroys mosquito eggs, larvae and pupae by drowning them, burying them in silt, or stranding them in positions where they cannot survive.
V. MOSQUITO CONTROL BY BIO-ENGINEERING METHODS:When the civil engineering works fail to achieve the desired results, biological methods will have to be adopted as mosquito populations are somewhat controlled by the depredations of predators and parasites. Such environmental techniques which alter the balance by favouring the adverse ecological factors, the parasites and predators against the pest may be termed as bio-engineering methods. These methods aim at modifying the water to render it unsuitable for mosquito-breeding by utilizing mosquito-limiting factors like pollution, sun-light, shade and natural enemies. For instance sun-loving mosquitoes are controlled by introducing shade producing trees into breeding places and shade-loving ones are controlled by eliminating the vegetation. Similarly, Anopheline larvae which breed in fairly clean-water are destroyed by dumping organic and industrial waste matter or cut-green vegetation into breeding places. Fresh water species are controlled by flooding their habitats with sea-water. Brakish- water mosquitoes are controlled by altering the salt content by dilution with fresh-water or by salinification with sea water. For meaningful eradication, environmental techniques will be so manipulated that the biological control against the different stages in the life-cycle of a mosquito will be accomplished very effectively. In this connection, the part played by various parasites, predators and natural enemies of the mosquito will have to be considered in detail.
1. For instance, many ducks and geese are reported to consume egg-rafts of culex mosquitoes. While some fishes ingest mosquito eggs, the snail Bulinus pectorosus feeds on individual eggs and rafts as well.
2. In case of larval control, plants such as Lemna and Azolla are known to cover the water surface so thickly that they interfere with the respiration of larvae or even cause death from asphyxiation. While the bladder-wort Utricularia is known to kill larvae byh entrapping them, plants like brasenia and chara are known to be poisonous to the larvae. Parasites like Vorticella protozoans and Coelomomyces fungi are known to adversely effect the larva. Many vertebrates and invertebrates are voracious feeders of mosquito larva. For instance, the toads and newts among Amphibia, Gambusia, Lebistes reticulates, gold-fish and Mexican minnows among fishes, small turtles and water-tortoises among reptiles, shore-birds and ducks among birds, the Hydra among coelenterta, water-spiders and insects like may-fly larva aquatic-bugs and beetles among arthropods are known to attack the mosquito lava. Even larva of some mosquitoes like culex Eretrapodites and Toxorhynohites are known to prey on other mosquito larvae.
3. At the pupal stage, while anthomyid flies like Lipsa attack pupae at the water surface the tiger beetle Cicindela Octoguttata is repsorted to prey over the papae at the edge of water ponds. Among fish, although Ctenogobius Nebulosus and Lardina-Hepletupus prefer the larvae, they also attack the pupae.One mosquito-fish eats 50 larvae in 30 minutes and 168 in 8 hours.One bat eats after dark 600 to 1000 insects in one hour including mosquitoes.
4. During their flight, the adults are attacked by birds, bats and dragon-flies, while resting on walls, ceelings and plants, they are deavoured by lizards, bugs and spiders. While ovipositing on water they are attacked by frogs and toads and by ponds-skaters, when just emerging as adults. As mosquitoes reach the peak of their flying at dusk, some birds and bats habitually attack them and kill them on a large scale.
5. For control of aquatic vegetation predators like flea beetles, marissa snails, monatees or the white amur-fish are introduced into stagnant water. Special variety of ducks and dugongs are reported to be very useful.
6. If the above methods fail to yield the desired results, control of mosquito must be made by releasing parasitic viruses, use of less toxic chemicals, hormones sex-attractants and male-sterilisation techniques.
7. To supplement the above measures mosquito nets and mosquito repellents like citronella, camphor odomoss, door and window screens, nets and chemically treated clothing are used to prevent contact between man and the mosquito and even these methods are not going to strike at the root cause of the problem.
It must be once again remembered that the development works planned by man and the insecticide and pesticides largely used by him for other purposes should not disturb natures balance with the result that the mosquito eradication programme will suffering the long run.
RESTORING NATURE'S BALANCES FOR MOSQUITO CONTROL
wetland restoration is one of the most effective measure for mosquito control because it provides for restoration of habitat for the natural enemies and predators of mosquitoes like guupies, minnows and gambusia fishes, frogs, turtles, Cyclobes damself flies, dragon flies and provides habitats for mosquito eating insects and birds. In fact when the Essex country mosquito control project restored 1500 acre wet land in Massachusetts the mosquito population dropped by 90 percent. According to Audubon magazine of December 1996 the US experts consider wet land restoration as a potent weapon to fight the mosquitos.The mosquito control in New Jersy proved that the Economics of Marsh Water Management by using the environmentally friendly way as compared with the traditional larvisidal methods resulted in about 90 percent savings in cost. If the fall of Roman empire is said to be due to rise of malaria, India should not be made to register a steep economic downfall due to the present mosquito born epidemics consequent to the growing indifference among the officials and politicians. Unless Governmental agencies and educational institutions from primary school to the University level honestly rise up to the occasion to identify the mosquito breeding places and identify the problem areas and implement appropriate mosquito control measures in time,posterity will not forgive the present generation for these man-made health hazards that are likely to ruin the health and wealth of the great Indian nation.
It is by now clear that for mosquito control no single method can be applied everywhere. Measures adopted against one species may become ineffective against another or even against the same species in a different locality. Consequently the more complete our knowledge about the biology of the species, the more correct will be our choice of control methods and the more successful the results obtained. Hence it is essential that inter-disciplinary research programmes are taken up by many universities in Japan and USA in their fight against the mosquito and if these countries have succeeded in getting rid of the mosquito menace, there is no reason why similar investigations cannot be undertaken by our universities for safe-guarding the health and welfare of the suffering millions.
To summarise, it may be stated that inspite of decades of efforts partments to control malaria today about 500 million new cases of malaria occur every year and about 3 million people are killed annually while about 8,000 people die every day. Since mosquitoes have become resistant to insecticides used to control them and the parasite to the drugs used in treatment, paradoxically malaria today is fast increasing as a result of the very efforts made to control the disease.
Most luckily today researchers thanks to the collaboration efforts of 160 researchers in 10 countries for about 6 years the researchers have got into their hands recently the complete genetic sequence of the disease parasite, (Plasmodium falciparum) the insect (Anopheles gambiae) that transmits it and the human host who is suffering a hell by the chilling fevers. The new genetic information on malaria may lead to products of new insecticides, vaccinations, better medicines and genetically manipulated mosquitoes that break the cycle of disease transmission.
Researchers identified 5 potential targets on parasite for new medications. If the receptors that mosquitoes use to smell humans are found one day the scientists may design new repellents that can drastically reduce the incidence of malaria, filaria, encephalitis, dengue and yellow fevers and other mosquito borne diseases. More genetic research work needs to be done in respect of not only Anopheles but also other species of mosquitoes like culex and Aedes
For more information ,please see the following web sites:
http://www.epa.gov/pesticides/health/mosquitoes/mosquito.htm [ EPA methods]
http://okaloosa.ifas.ufl.edu/mosquitomgmt.htm [Mosquito Management]http://www.who.int/health-topics/malaria.htm [WHO Fact Sheets]http://www.plymouthmosquito.com [Mosquito Control Project ,USA]http://www.rci.rutgers.edu/~insects/njmos.htm [Biology of Mosquito]
http://edis.ifas.ufl.edu/pdffiles/IN/IN06100.pdf [Bio-controls test results]http://www.arcatapet.com/resource8.cfm [Bio-control application packs]http://www.ecodev.gr/en/opek/opekanagk.htm [Greek project]http://www.suffolkmosquitocontrolplan.org/control.html [Mosquito Control]
http://www.getipm.com/thebestcontrol/chapter-23/23.htm [ Mosquito Bio-control,Texas]
http://lionel.suz.free.fr/index.php?id=run&sub=chikungunya [CHIKUGUNYA ]
http://www.gcrio.org/CONSEQUENCES/vol2no2/index.html [ Adverse impacts of BIO-CONTROLS]
http://www.mrcindia.org/MRC_profile/alternate_strategy/fish.pdf [ Exotic fishes ,larvivorous]