Introduction to mosquitoes

Introduction to mosquitoes

About 3200 species (37 genera)

Worldwide distribution (5500m high  1250m depth) Important species :

Anopheles ^瘧蚊 : malaria, filariasis ( 班氏、馬來絲蟲 ),arboviruses Culex 家蚊 : 班氏絲蟲 , arboviruses

Aedes ^斑蚊 : yellow fever, dengue, encephalitis virus, filariasis Mansonia ^曼蚊 : 馬來絲蟲

Haemagogus, Sabethes (Central and South America) : yellow fever Psorophora (North and South America) : pest

External Morphology :

Mosquitoes are slender and relatively small insects, the body divided into head, thorax and abdomen.

About 3-6 mm in length, some species, can be as small as 2 mm while others may be as long as 19 mm.

One pair of functional wings : fore-wings

hind-wings ( 退化成 knob-like halteres) 平衡翅 Diptera 雙翅目 : 二對各在中胸及後胸

觸鬚 觸角

M

Head :

The head has a pair of kidney-shaped compound eyes 複眼 . Between the eyes arises a pair of filamentous and segmented antennae 觸角 . In females the antennae have whorls of short hairs (pilose 多毛的 ).

In males (a few exceptions in genera of no medical important) the antennae have long hairs (feathery or plumose 羽狀的 ).

Palp 觸鬚 : below the antennae

依雌雄、種類而不同 Proboscis 吻部 : mouthpart

Thorax :

The thorax is covered, dorsally and laterally, with scales which may be dull or shiny, white, brown, black or almost any color.

It is the arrangement of black and white, or coloured, scales on the dorsal surface of the thorax that gives many species (especially those of the Aedes) distinctive patterns.

Wings :

The wing veins 翅脈 are virtually the same for all mosquito species, and are covered with scales (brown, black, white, creamy yellow, brightly).

The shape of the scales and pattern they form differs between genera and species.

Rest : across each other (closed pair of scissors) Fringe : along the posterior border

Mate attraction

翅室

wing cell

Legs :

The legs are slender and covered with scales (usually brown, black or white and arranged in patterns, often in the form of ring).

The tarsus ( 跗節 ) terminates in a pair of simple claws.

Some genera, such as Culex, have a pair of pulvilli ( 褥盤 ).

爪間體

Abdomen :

Compose of 10 segments, but only the first seven or eight are visible. (scales)

The last segment of the female terminates in a pair of finger-like

cerci ( 尾毛 ), whereas in the males a pair of prominent claspers ( 把握器 ), comprising part of the male external genitalia, are present.

In unfed female, the abdomen is thin and slender, but after a blood-meal, the abdomen becomes distended (red oval ballon).

When the abdomen is full of developing eggs it also dilated, but is whitish and not red in appearance.

Mouthparts and salivary glands :

The proboscis is elongate and projects forwards in both sexes.

Labium 下唇 : flexible, gutter-shaped ( 水溝 ),

terminates in a pair of labella ( 唇瓣 )

encircle all the other components of mouthparts

(protective sheath)

All the components are hold close together in life and only become partially separated during blood-feeding.

大顎 小顎

下唇

下咽頭 上唇 (U 型食管 )

When a female bites a host, the labellum are placed on the skin and the labium.

(curved backwards) This allow the paired mandibles, paired maxillae, labrum and hypopharynx to penetrate the host’s skin.

Saliva from a pair of salivary glands ( 位 胸部前端之腹側 ) is pumped down the hypopharynx.

Contents of saliva :

Anticoagulant : prevent clotting and obstructing of mouthparts.

Antihaemostatic enzymes : produce haematomas in the skin.

Anaesthetic substances : reduce the pain and host defensive reactions.

In the male, the maxillae and mandibles are usually reduced in size or the mandibles are absent, so males cannot bite. 不傳播疾病

They feed on nectar of flowers, and other naturally occurring sugary secretions.

Female also feed on sugary substances to obtain energy for flight and dispersal.

Only in a few species (autogenous), this type of food is sufficient for egg development.

Differences between mosquitoes and flies

1. Forward-projecting proboscis

2. Scales on the thorax, legs, abdomen and wing veins

3. A fringe of scales along the posterior margin of the wing

Life cycle

1. Blood-feeding and the gonotrophic cycle :

Most mosquitoes mate shortly after emergence from the pupa.

Usually fertilize all eggs laid during a female lifetime.

(only one mating is required) Anautogenous development : blood-meal is necessary for the

development of the eggs in the ovaries.

Autogenous development : can develop the first batch of eggs without blood-meal.

The speed of digestion of the blood-meal : depend on the temperature.

Most tropical species : 2-3 days

Colder, temperate countries : 7-14 days

After oviposition the female takes another blood-meal and after 2-3 days (tropics) a further batch of eggs is matured.

Gonotrophic cycle :

blood meal  eggs maturation  oviposition whitish

2. Oviposition and biology of the eggs :

Female lays about 30-300 eggs at any one oviposition.

Eggs are brown or blackish and 1mm or less long.

None of these eggs (C. & Ano.) can survive if they become dry.

In tropics, eggs hatch within 2-3 days, in cooler temperate countries they may hatch until after 7-14 days, or longer.

terminal filament : glue to the underside of floating plants boat-shaped ovid

Anopheles

hold together by surface tension (egg raft)

(lay the eggs directly on the water)

Aedes do not lay eggs on the water surface, but just above the water line on damp substrates, such as mud and leaf litter, or on the inside walls of tree-holes or water-storage pots.

Eggs of Aedes can withstand desiccation : remain dry for months or even years but still viable and hatch when soaked in water.

When flooded, hatching may extend over long periods because the eggs hatch in instalments.

The eggs may enter a state of either quiescence, hatching in suitable conditions, or diapause and will not hatch until some specific stimulus.

(environmental stimuli : change in daylength and/or temperature) In termerate regions the many Aedes overwinter as diapausing eggs.

3. Larval biology :

No larvae can withstand desiccation, although they may be able to survive short periods among wet mud.

具觸角、複眼

2 pairs (sausage-shaped) osmoregulation

氣孔

Siphon 吸管 pointed at tip (serrated cutting structure), insert into the roots or stems of aquatic plants, thus oxygen for respiration is obtained. (Anopheles do not have)

2 pairs

須到水面呼吸

The larvae feed on yeasts, bacteria, protozoans and numerous other plant and animal micro-organisms found in the water.

Some species are carnivorous or cannibalistic.

In tropics, the time from egg hatching to pupation can be short as 5-7 days, but many species require about 7-14 days.

In temperate areas the larval period may last several weeks or months, and several species overwinter as larvae.

Larval habitats

Freshwater :

permanent : swamp, marshes, ricefields, etc.

temporary collections : pool, puddles, etc.

natural container : tree holes, rock pool, bamoo stumps, snail shell, etc.

man made container Brackish or salt water Shaded

Usually absent from expanses of uninterrupted water : lakes (fish and other predators)

large rivers and fast-flowing water

4. Pupal biology :

All mosquito pupa are aquatic and common-shaped.

Pupae do not feed but spent most of their time at the water surface.

If disturbed they swim up and down in a jerky fashion.

In tropics, the pupal period last only 2-3 days. In temperate regions, the period my be extended over 9-12 days, or longer.

5. Adult biology and behavior

Females require a blood-meal before or after mating.

Anthropophagic : feed on human

Zoophagic : mainly feed on non-human hosts Ornithophagic : feed on birds

Females are attracted to the hosts by various stimuli emanating from them, such as body odours, CO² and heat.

Vision usually plays only a minor role in host orientation.

Some species are attracted by the silhouette or movement of hosts.

Some species feed more or less indiscriminately at any time of the day or night; others are mainly diurnal or nocturnal.

Endophagic : enter the house to feed Exophagic : outside house

Endophilic : blood digestion and maturation of the ovaries inside house Exophilic

Aedes aegypti ^埃及斑蚊 : anthropophagic, exophagic and exophilic Anopheles gambiae : anthropophagic, endophagic and endophilic

The feeding behavior may be change.

For example, in certain areas and seasons a species may be

anthropophagic, endophagic and exophilic, whereas at other times, especially if there are few people but many animals in the area, the species may become zoophagic, exophagic and exophilic.

The biting behavior may be important in the epidemiology of disease transmission.

For example, mosquitoes that feed out of doors and late at night, will not bite young children.

During hot and dry periods, people may sleep out of doors. (exophagic) Mosquitoes bite within forest or wooded areas.

The resting behavior of adult mosquitoes may be an important consideration in planning control measures.

For example : malaria vs. DDT (wall)

control the endophilic mosquitoes

Most mosquitoes probably disperse only a few hundred meters (<2 km) from the emergence sites. (control program)

100 km or more? wind ? Airportmalaria ?

In tropics, female mosquitoes live on 1-2 weeks, and 3-4 weeks in temperate.

Male adults usually have a shorter life-span.

Species that hibernate live much longer.

For example, in Europe some fertilized females of Culex pipiens survive in hibernation from August until May.

Mosquito control :

1. Immature stages :

Biological control (naturalistic control) : slow effect and also eat harmless insects

A. Predators :

Larvivorous fish : mostly wide used

1. Gambusia affinis and G. affinis holbrooki (warm-water fish, 原生於 southern USA) have been introduced to over 60 countries.

They are cannibalistic and aggressive fish which have sometimes destroy native fish.

They should not be introduced into new areas.

2. South African guppy (Poecilia reticulata) : not so voracious, but can tolerate organic pollution better and more heat tolerant.

3. Others :

 carp 鯉魚 (Cyprinus carpio) in Chinese ricefield.

 edible catfish 鯰魚 (Clarias fuscus) in water-storage tanks in Myanmar to control Aedes aegypti.

Tilapia in Africa

 Aplocheilus in Europe and Asia

Some predatory fish, such as Aphanius dispar and Fundulus species, breed in saline water, can be introduced into salt-water habitats.

Fish are unsuitable for the control of mosquitoes which breed in small container, pools and puddles that rapidly dry out.

Some fish, such as Nothobranchius and Cynolebias, which are the

so-called instant or annual fish, have drought-resistant eggs and these are more suitable for introducing into small temporary habitats that repeatedly dry out.

Although fish have sometimes greatly reduced the numbers of larvae in certain habitats, such as pit, ponds and ricefields, they have rarely proved effective in reducing the size of mosquito populations over large areas.

Nor is there usually much convincing evidence that they have

significantly decreased the incidence of mosquito-borne diseases.

Other predators : rarely effective as control agents

tadpoles of frogs and toads, various aquatic insect larvae A few mosquitoes have predacious larvae (Toxorhychites).

These have been introduced into container-habitats in certain

areas (e.g. Fiji, Samoa and Hawaii) to control other mosquitoes but results have not been very encouraging.

B. Pathogens and parasites : harmless to humans

virus, bacteria (Bacillus thuringiensis subsp. israelensis, etc.) protozoan (Nosema vavraia, etc.), fungi (Coelmomyces, etc.) nematodes (Romanomermis – commerically mass-produced)

B. t. i. (serotype H-14) cannot be routinely advocated, especially in tropical countries.

It’s undoubtedly the most useful pathogen

can be mass-produced, toxicologically safe to humans and wildlife

 formulated as powder, can be sprayed on the water surface and kills larvae when it is ingested. ( 破壞消化道 )

 delta-endotoxin : recombinant technique (transfer the gene into other inactive bacteria)

Genetic control :

A. Sterile male mosquitoes (release in the field, compete)

 Can be achieved by ionizing radiation, crossing closely related species to produce infertile hybrid males.

 Introducing chemosterilants into breeding trays containing the larvae which make the emerging adult (both sexes) sterile.

The eggs will be sterile and fail to hatch.

In El Salvador the release in the 1970s of some 4.36 million

chemosterilized male Anopheles albimanus (malaria vector that had

developed resistance to most insecticides) over 4.5 months in an isolated coastal region (about 15 km²) caused a more than 97% reduction in the biting population.

Enormous numbers of mosquitoes had to be reared to obtain control over a very limited area, the later expanded trial failed.

B. Translocation, lethal genes, etc. : excessive males

undesirable genes spray among natural populations rapidly genetically altered (incapable of transmission parasites)

Physical control (mechanical or environmental control) :

Filling in (rubble, earth or sand) : tree-holes, ponds

Drainage

Source reduction : containers, piped water supply

Cover : water-storage pots

Environmental manipulation 環境操控 : Increase water flow.

Periodic opening of sluice gate.

Removal of overhaning vegetation for shade-loving larvae.

Planting vegetation along reservoirs and steams for sun-loving larvae.

Intermittent flooding of ricefields to allow drying out every 3-5 days.

Removal of rooted or floating vegetation for Mansonia.

Impoundment : relatively deep permanent water.

(Aedes ^{尤其不適合生存 )}

sometimes stocked with fish and ducks (also helpful) When a larval habitat has been modified to reduce breeding of

mosquitoes, but not eliminated, may now favor of other mosquito species.

Chemical control :

A. Oils : the oldest control method

( 可經陽光及微生物分解，對蚊幼蟲具毒性或使窒息 ) interfere with the interface of air-water and cause larvae, pupae and emerging or ovipositing adult to drown.

Mineral oil

Diesel oil ( 柴油 ) and kerosene ( 煤油 ) : 140-190 litres/ha

Addition of detergents, such as 0.5% octoxinol 辛苯昔醇 , or 1-2.5%

vegetable oils (e.g. castor oil 蓖麻油 , coconut oil) increase the spreading power allowing the application rates to be reduced to 18-50 L/ha.

Commercial oils (e.g. malariol 瘧蚊殺 ) have improved spreading properties so only 10-20L/ha need be used.

The addition of organophosphate or carbamate insecticides to high-spreading oils greatly enhances their effectiveness.

In tropics, oils have to be sprayed on breeding place about every 7-10 days. Less frequent applications in cooler temperate areas.

B. Paris green (cooper acetoarsenite 銅醋酸亞砷酸物 ) Pellet : float on the water surface

When ingested by surface-feeding larvae, such as Anopheles, it acts as a stomach poison.

Granule : sink

against bottom-feeders (Culex, Mansoni and Aedes) Repeated applications are usually needed.

因毒性強，雖無抗藥性，又有替代藥，已少用

C. Insecticides :

Because their persistence in the environment and accumulation in

food-chains, DDT and other organochlorine insecticides should not be used.

Less persistent and biodegradable insecticides should be used, such as organophosphates and carbamates.

Organophosphates : malathion 馬拉松 , fenitrothion 撲滅松 (Sumithion), pirimiphos methyl 亞特松 (Actellic) ( 極毒致癌、已禁用 )

temephos 亞培松 (Abate) : 使幼蟲神經中毒，但也傷害水生之無脊椎動物。

對哺乳類毒性低。作成球狀、顆粒或膠囊式，慢慢釋入水中，可達數週。

加入飲用水源？

Carbamates : propoxur 安丹 (Baygon) ( 可滅蚤 )

Pyrethroids 除蟲菊 : permethrin 百滅寧 , deltamethrin 第滅寧 但同時殺死其他水生昆蟲及甲殼類，甚至是魚類，必要時才使用。

Low in toxicity to mammals (but at high doses can affect the nervous system) and are practically nontoxic to birds.

Permethrin is registered and sold in a number of products such as

household insect foggers and sprays, tick and flea sprays for yards, flea dips and sprays for cats and dogs, termite treatments, agricultural and livestock products, and mosquito abatement products.

In organically polluted water insecticides are less effective and either higher dosage must be used or the more effective

organophosphates such as fenthion 芬殺松 (Baytex) or chlorpyrifos 陶斯松 (Dursban) applied.

Chlorpyrifos : more toxic to larvae than most other insecticides

cause high mortalities among fish and other aquatic organisms not so widely used

In tropic, insecticides usually have to sprayed on water every 10-14 days, and more frequently on highly polluted water.

Mansonia larvae can be killed by spraying herbicides, such as diquat 大刈 and 2,4-D 二氯苯氧機乙酸 .

Larvicides are usually applied as emulsions or oil solutions,

but granules (0.25-0.6 mm) or pellets (0.6-2 mm) are better for penetrating dense growths of aquatic vegetation.

Slow-release granules or pellets can be scattered over marshy areas when they are relatively dry.

When they become flooded, the granules release toxicants into water.

Sprayers : operators, machines on truck, aerial spray

Insect growth regulators (IGRs)

high coast, low toxicity to human, 對特定發育階段非常有效

I. 擬青春激素模擬物質 (juvenile hormone mimics)

arrest larval development 或成蟲不孕

Methoprene 美賜平 (Altosid) : safe for use in drinking water. (WHO)

 with an attractive bait that foraging fire ant workers take back to the

queen.

 affects the queen's ability to reproduce, so when existing workers die normally within a 3 week period), no new workers are created to replace them.

Starvation and elimination of the entire colony.

Pyriproxyfen 百利普芬 : inhibits flea eggs from hatching and affects all developing stages of the flea life cycle.

12 months period of activity

其他藥物 : fenoxycarb 、 hydroprene

II. 幾丁質合成抑制劑 (chitin synthesis inhibitors)

Diflubenzuron 二福隆 (Dimilin) : immature stage 蛻皮時或稍後即死亡 Integrated control : biological + insecdicides

2. Control directed at adults :

A. Personal protection :

Mosquito screening : strong plastic or non-corrosive metal 6-8 meshes/cm (exclude most mosquitoes)

Mosquito net : 9-10 meshes/cm (against night-biting mosquitoes) should be placed over beds before sunset

can be impregnated with pyrethroid (permethrin)

Small spray-guns : permethrin dissolved in kerosene or in an equql mix of kerosene and white spirit

pressurized aerosol canisters

commonly used to spray bedrooms early in the evening

Mosquito coils : pyreythroid

Insecticide-impregnated tablets + electric mini-heater Insect repellents : can provide protection for 6-10 hrs care avoid the eyes

diethyltoluamide (deet)

N,N-diethylphenylacetamide (depa) dimethylphthalate (dimp)

Eucalyptus 尤加利

Citronella oil 香茅 (about 1 hr)

Sweating and rubbing reduce the period of effectiveness.

Repellent soap (20% deet and 0.5% premethrin) ; greasy feeling Repellent clothing (premethrin) : wide-mesh jackets, sock

hoods (as used by military personnel)  longer protection than repellents applied to the skin

If treated clothing is kept in plastic bags when not in use it should

remain effective for many months before re-impregnation is needed.

Aerosols, mists and fogs

Hand-held machines that generate insecticidal mist (51-100 m), aerosol (<50) or fogs (<15) can be used to kill outdoor resting (exophilic) adult.

Several insecticides can be used, including malathion, bendiocarb 免敵克 (Ficam), fenitrothion pirimiphos methyl and the synthetic pyrethroid.

Repeated application are needed to sustain control.

(rapidly invaded by newly emerging adult)

Application of aerosols and mists are best made in calm weather, usually in the evenings or early mornings (fewer thermals rising and less turbulance)

Aerial applications from helicopters or aircraft usually give better coverage and more effect than ground-base operations.

Ultra-low-volume applications (ULV)

225-500 ml/ha as against 5-25 L/ha with conventional spraying.

The trucks or aircraft can spray much larger areas before they have to return to base.

Insecticides : malathion, pirimiphos methyl, fenitrothion, propoxur, chlopyrifos and pyrethroids.

With aerial applications, droplet size is bigger (150-200 m) than

ground-base applications (50-100), because they decrease in size, due to evaporation.

In emergency situations, aerial spraying gives the fastest and most effective control of vectors, and has been used to stop transmission of hemorrhagic dengue, Japanese encephalitis and in North America the encephalomyelitis viruses.

House-spraying :

(interior surface of walls) DDT : cheap, safe to human,

irritant to mosquito (cause leave the house, before they have picked up a lethal dose) dose : 2g/m², 6 months intervals (highly endemic)

若疾病傳播季節性 ( 如雨季 ) 明顯，下雨前噴一次即效果良好

Others : 2-3 months intervals, expensive, toxic

organophosphates : malathion, fenthion, fenitrothion carbamates : carbaryl, bendiocrab, propoxur

permethrin (0.5g/m²)

House-spraying also kills bed-bugs, house-flies and cockroaches.

Population changes : endophilic vs. exophilic

殺蟲劑之不良效應

1. 篩選出具抗藥基因之昆蟲 : resistance, tolerance 2. 殺滅非標的生物 : 如傳粉者、天敵、土壤節肢動物 3. 害蟲復發 (pest resurgence)

4. 次要害蟲猖獗 5. 惡化環境

6. 危及人類健康 抗藥性的機制 1. 增加行為逃避

2. 生理改變 : 如隔離 (sequestration ，將毒性物質堆積

　　　　　　在特化組織 ) 、減少表皮通透性、加速排泄

3. 生化解毒 ( 代謝性抗藥性 metabolic resistance) 4. 減少作用位置之敏感度

( 作用位置抗藥性 target-site resistance)