Mosquitoes (Insects)

Mosquitoes are small flying insects and are related to other members of the order Diptera, the “two-winged flies.” The immature stages of larvae are aquatic and live in stagnant water sources in every biogeographic region of the world. Adult female mosquitoes of most species feed on blood of vertebrates, including humans, and this habit has resulted in great economic and public health significance for this group of insects.
There are well over 3000 species and subspecies of mosquitoes in the world. They occur in a variety of habitats, ranging from deserts at or below sea level to high mountain meadows at elevations of 3000 m or more. Adult mosquitoes are terrestrial flying insects; immature stages are aquatic. Larvae and pupae of the various species can be found in ponds, ditches, puddles, swamps, marshes, water-filled rot holes of trees, rock pools, axils of plants, pools of melted snow, water in discarded tires, tin cans, and other artificial containers. Some species are most active in the warmest part of the year, whereas others are adapted to cool temperatures. Many species of mosquitoes are rarely encountered and seldom pose a threat to the health or well-being of humans and domestic animals. However, other species are abundant, frequently encountered, and readily attack people, their pets, and their livestock. Some of these species transmit microbial organisms that cause malaria and encephalitis, and other severe diseases of humans and other vertebrates. Mosquitoes are also commonly used as research objects in a wide range of biological studies.


CLASSIFICATION AND IDENTIFICATION

Mosquitoes are classified into three subfamilies, each with different characteristics in all of their life cycle stages. The species of importance from the standpoint of public health are contained in the subfamilies Anophelinae (referred to as anophelines) and Culicinae (referred to as culicines). Females of species in a third subfamily, Toxorhynchitinae, lack mouthparts adapted for sucking blood from vertebrates. The larvae of this subfamily are predaceous on other aquatic organisms and have been proposed as biological control agents of mosquito larvae.
Common genera of the Culicinae include Culex, Aedes, Psorophora, Mansonia, Haemagogus, Sabethes, Coquilletidia, and Culiseta. Most species in the Anophelinae are contained in the genus
Adult female Aedes aegypti, the yellow-fever mosquito.
FIGURE 1 Adult female Aedes aegypti, the yellow-fever mosquito.
(A) Female and (B) male heads of adult Aedes mosquitoes.
FIGURE 2 (A) Female and (B) male heads of adult Aedes mosquitoes.
Anopheles. The subfamily Toxorhynchitinae contains only the genus Toxorhynchites.
Mosquito adults are small flying midge-like insects. Most female mosquitoes can be differentiated from similar insects by the presence of a long slender proboscis that is adapted for piercing skin and sucking blood, and long slender wings that are covered with small scales ( Figs. 1 and 2A ). Male mosquitoes have scale-covered wings, but their probosces are adapted for sucking plant juices and other sources of sugars (Fig. 2B). Most male mosquitoes can also be differentiated from females of the same species by their generally smaller size and by the presence of much longer and hairier maxillary palps. The immature stages of mosquitoes, the larvae (Fig. 3 ) and pupae (Fig. 4), vary in color from yellowish tan to black. Most mosquito larvae have a distinctive siphon, or air tube, at the rear of their bodies (Fig. 3B), but some species lack this tube (Fig. 3A).
Culicine larvae have an air tube extending from the posterior section of their body and in most species hang at rest from water surfaces at an angle of approximately 45°. Larvae of Coquilletidia and Mansonia have air tubes adapted for piercing submerged plants to obtain air for breathing. They are rarely found at water surfaces. Anopheline larvae lack an air tube and consequently rest parallel to water surfaces.
Larvae of mosquitoes. (A) An anopheline larva (Anopheles quadrimaculatus). (B) A culicine larva (Culex quinque-fasciatus).
FIGURE 3 Larvae of mosquitoes. (A) An anopheline larva (Anopheles quadrimaculatus). (B) A culicine larva (Culex quinque-fasciatus).
 A mosquito pupa.
FIGURE 4 A mosquito pupa.
Culicine adult females have probosces developed for piercing the skin of vertebrates and sucking their blood. While feeding, their bodies are usually arranged somewhat parallel to the skin surface of their hosts. Anopheline adult females also have probosces adapted for piercing vertebrate skin, but they orient themselves at about a 45° angle while blood feeding.
Eggs of mosquitoes. (A) Egg raft of Culex mosquito. (B) Single egg of Aedes mosquito. (C) Single egg of anopheline mosquito.
FIGURE 5 Eggs of mosquitoes. (A) Egg raft of Culex mosquito. (B) Single egg of Aedes mosquito. (C) Single egg of anopheline mosquito.
The eggs of mosquitoes also vary (Fig. 5) . Females of culicine species deposit single eggs (Aedes, Psorophora), boat-shaped rafts of 100 or more eggs (Culex, Culiseta), or clusters of eggs attached to floating plants (Mansonia, Coquilletidia-. Anopheline eggs are also laid singly, but have elaborate floats extending to the sides of the eggs. Anopheline eggs are often found in clusters on water surfaces, forming interesting geometric patterns. Toxorhynchites eggs are also laid singly, usually on water surfaces.

LIFE CYCLE

Egg Stage

The egg-laying habits of female mosquitoes vary widely from species to species. Some female mosquitoes lay eggs on water surfaces (e.g., Anopheles-; others lay single eggs on moist soil (e.g., Aedes). From eggs deposited on water surfaces, larvae usually hatch within a day or so, but eggs laid on soil surfaces do not hatch until the surfaces are flooded, which may occur months, or even years, later. The environmental cues female mosquitoes use to find suitable sites for oviposition remain only partially known. Color, moisture, and volatile chemical stimulants appear to play a role in certain species. Efforts to explain the occurrence of various mosquito species in different aquatic habitats based strictly on oviposition cues have been unsuccessful.

Larval Stages

Small larvae that are nearly invisible to the naked eye hatch from eggs. Larvae molt three times to become fourth-stage larvae. Several days later, this larval form molts again to become a pupa. The time required for development of the larval stages depends on several factors, the most important of which is water temperature. Availability of food and larval density are also other factors. Water temperature and food are inversely related to time of development, whereas larval density is directly related.
Most mosquito species have larvae that are restricted to freshwater. However, larvae of a few species can develop under other conditions, for example, brackish or saltwater or water polluted with organic solids. Species with larvae adapted to saltwater can maintain osmotic pressure within their bodies by drinking substantial amounts of water and by removing ions from their hemolymph through their Malpighian tubules and rectum. Generally, saline species can also develop in freshwater, but do not compete well with freshwater species. The inverse is not true and consequently, various kinds of water usually have a characteristic mosquito fauna.

Adults

Adult mosquitoes emerge 1-2 days after the appearance of pupae, with males emerging first. In the summer, the entire life cycle, from egg to adult, may be completed in 10 days or less. Females feed on vertebrate blood for the development of eggs. This behavior by females is the single most important characteristic of mosquitoes from the human standpoint. Blood feeding in insects is believed to have evolved several times independently from ancestral forms adapted for sucking plant juices or for preying on other insects. The means by which female mosquitoes locate suitable hosts for blood feeding has been studied for many years, but there are still many unknown features to this behavior. The best explanation is that females are attracted by warmth, moisture, and carbon dioxide from hosts, but other factors are involved. Several studies have suggested that substances such as lactic acid, a component of human sweat, also serve as attractants.
Ordinarily, a female mosquito cannot develop a batch of eggs unless she has taken a blood meal to obtain nourishment for ovarian development. However, some strains or individuals of several species can develop eggs without a blood meal, which is called autogeny. The nourishment for egg development is carried over from the larval stages, and consequently, only the first batch of eggs can develop in this way. The usual situation in which a blood meal is required for the development of all batches of eggs in an individual female is called anautogeny.
Some mosquitoes take blood only from certain groups of vertebrate animals. For example, Culex pipiens, the northern house mosquito, feeds almost entirely on birds. Aedes sierrensis, the western treehole mosquito, feeds only on mammals. C. tarsalis, the encephalitis mosquito, feeds on birds and mammals (this dual host preference is one characteristic of an effective vector of disease pathogens). In the past, this relative host specificity has been called host preference. However, this term is not appropriate, because it ignores availability of hosts, host defensive behavior, and other factors unrelated to the mosquitoes themselves. The blood-feeding drive is controlled by neurohormones and can be induced artificially by treatment with juvenile hormone or one of its analogs. This hormonal influence is the reason why mosquitoes that have recently had a blood meal and are developing a batch of eggs do not usually seek another blood meal. However, multiple blood meals (more than one blood meal in a single gonotropic cycle) do occur at times in nature in some species.
Blood feeding by mosquitoes is a complex process. It is facilitated by the injection of saliva into the feeding wound of the vertebrate host. Saliva comes from organs in the thorax of mosquitoes called salivary glands. Saliva may contain a variety of substances, including chemicals that reduce clotting of vertebrate blood. Digestion of a blood meal usually takes 2-3 days, depending on the ambient temperature. The uptake of blood is accomplished by the action of muscular pumps in the head of female mosquitoes. Blood travels through the digestive tract of the mosquito into a structure called the midgut. After blood reaches the midgut it is soon surrounded by a thin sheath, the peritrophic membrane, which is secreted by cells at the front of the midgut. Digestion of the blood takes place within this structure.

Seasonal Development

Some species of mosquito have but a single generation per year (univoltine), whereas others have many (multivoltine), depending upon the length of the season favoring the activity of the adult stages. To avoid seasons of the year not favorable to adult activity (usually the winter), mosquitoes may have some kind of diapause mechanism. In Aedes and related genera, the diapause mechanism usually involves the egg stage. In temperate and subarctic zone Aedes, populations may survive winters as desiccation-resistant eggs, sometimes under the surface of snow or along river flood plains. The larvae then hatch in the spring after the eggs are flooded from melted snow or after flooding of the riverbanks.
Culex and Anopheles females usually survive unfavorable periods as diapausing or quiescent adult females. Male mosquitoes usually do not survive unfavorable periods, so it is necessary for insemination to occur before the onset of diapause.
Some mosquito species survive unfavorable periods as diapausing larvae (e.g., species of Aedes, Anopheles, Culiseta). Diapause can be variable in some species, depending upon the latitude at which they occur, with diapause occurring in the larval stage at warmer latitudes and in the egg stage at cooler ones.
There is considerable variation in the environmental and physiological control of diapause. In nearly all diapausing mosquitoes studied, diapause is triggered by exposure of one or more of the life cycle stages to daylength. In Culex species, and other mosquitoes that overwinter as adults, exposure of late-stage larvae and of pupae to short daily photophases occurring in autumn results in diapause in adult females. This diapause is manifested by lowered activity levels, inhibition of blood-feeding drive, and the arrest of follicle development in ovaries. In some Aedes species, the short autumn days experienced by females result in deposition of eggs that are in a diapause state. The larvae in these eggs do not hatch until after a period of exposure to near-freezing temperatures lasting several months. In other species of Aedes, diapause results from exposure of the eggs themselves to short daylengths. Still other Aedes species have larvae that enter diapause triggered by their exposure to short daylengths.
As with other aspects of reproduction and development, diapause is controlled directly by neurohormones. Diapause can be induced in most diapausing species by exposure to juvenile hormone or one of its analogs.
Many tropical and subtropical species, such as A. aegypti, the yellow-fever mosquito, do not have a diapause mechanism. Still other tropical species have mechanisms for avoidance of hot, dry seasons, but these mechanisms have been little studied.

PUBLIC HEALTH AND VETERINARY

IMPORTANCE

As discussed earlier, female mosquitoes of nearly all species require blood from vertebrate animals to develop their eggs, and many species bite people, their pets, and livestock for this purpose. The most important result of this behavior is the transmission of microorganisms that cause diseases such as malaria, filariasis, yellow fever, and dengue. These and other mosquitoborne diseases can have serious and sometimes fatal consequences in people. These diseases can also have an impact on livestock, pets, and wildlife. Even when no infectious disease pathogens are transmitted by mosquitoes, they can be a serious health problem to people and livestock. Biting of people by mosquitoes can result in secondary infections, allergic reactions, pain, irritation, redness, and itching. Mosquito biting in beef cattle can result in reductions in weight gains and in dairy cows, reduction in milk production.
The interactions between mosquito hosts and the pathogens they transmit are highly variable. Three basic types of transmission mechanisms are involved: (1) propagative transmission, in which the pathogen multiplies within the mosquito but does not undergo any changes in developmental form; (2) developmental transmission, in which the pathogen undergoes developmental changes but does not multiply; and (3) propagative-developmental transmission (also called cyclodevelopmental transmission), in which the pathogen multiplies and undergoes changes in developmental forms. Transmission of the yellow-fever virus by the yellow-fever mosquito is an example of propagative transmission. The virus is taken up by a female mosquito from a viremic host during blood feeding, multiplies many times, and eventually infects the salivary glands of the host. When the female mosquito takes another blood meal, she may infect a new host by the injection of saliva.
Some pathogens are transmitted to the offspring of female mosquitoes via infected eggs. This type of transmission is known as tran-sovarial transmission.
Filarial worms, the cause of the disease filariasis (a type of which is called elephantiasis) in humans and other vertebrates, are transmitted by developmental transmission. In this example, very small immature forms of the worms, called microfilariae, occur in the blood of infected vertebrate hosts and are taken up by female mosquitoes in a blood meal. Within the mosquito, the filariae molt several times until they eventually become infectious larvae. These larvae migrate down the proboscis of the mosquito and enter the feeding wound caused by the mosquito during a subsequent blood feeding. Within the vertebrate host, these larvae may eventually develop into adult male and female worms that mate and produce microfilariae. It is the presence of large numbers of adult worms that results in the symptoms of filariasis.
Malarial parasites have a very complex life cycle, involving both multiplication of parasites and development of life cycle stages. Anopheline mosquitoes are the vectors of human malaria, and because the sexual stages and fertilization occur within mosquitoes, by definition they are the definitive hosts. Parasite forms called microgametocytes (male sex cells) and macrogametocytes (female sex cells) occur in the peripheral blood of humans and are taken up by mosquitoes. Fertilization of the female cells by the male cells occurs within the gut of the vector mosquito. After several life cycle changes, and multiplication of forms within cysts on the gut wall, forms of the parasite called sporozoites enter salivary glands of the mosquito and infect new hosts during blood feeding.
There are hundreds of types of microorganisms that are transmitted by mosquitoes to vertebrates that result in diseases. A few are extremely important worldwide because of their high incidence and the severity of their symptoms in humans.
Malaria is one of the most important diseases in the world. Several hundred million people are infected with malarial parasites, resulting in over 2 million fatalities annually, mostly in tropical countries in Africa and Asia. Malaria is especially serious in pregnant women and young children. Typically, more than a million children die every year from this disease. The economic development of a number of tropical countries is badly hindered by malaria because of the burden of chronic malaria infections in working-age men and women.
The virus disease known as dengue, transmitted mostly by the yellow-fever mosquito, is a rapidly expanding problem in the world and is now considered second in importance only to malaria among mosquitoborne diseases. The increase in global human travel resulting from expanded rapid air transportation has been paralleled by the increase in the number of viral strains causing dengue and the increase in the number of cases of a particularly serious form of the disease called dengue hemorrhagic fever. This form of the disease is most serious in children and is a significant cause of mortality.
Filariasis is a general term applied to infection of vertebrate animals by many different species of parasitic worms belonging to the superfamily Filaroidea. A form of mosquitoborne filariasis is called lymphatic filariasis because this infection can cause impairment of the lymphatic system. Lymphatic filariasis is a chronic disease that can lead to the well-known disfigurement of humans called elephantiasis. Another type of filariasis called dog heartworm occurs in dogs, other canids (e.g., wolves and coyotes), and felids (e.g., domestic cats). Heavy infections can result in large buildups of adult worms in the cardiopulmonary system and can be fatal.
Yellow fever, a virus disease, has virtually disappeared from the United States because of the availability of an extremely effective vaccine. This vaccine may provide lifelong immunity from a single inoculation. Unfortunately, the availability of the vaccine is limited on a worldwide basis and there are many unvaccinated people living in areas where the mosquito vector, A. aegypti. is common. Yellow fever is an extremely serious disease. There is no available treatment, and infections in humans are frequently fatal. Periodic epidemics continue to occur in various tropical countries. A. aegypti is common in urban and suburban areas of the tropics and subtropics. The larvae of this species occur in water in various types of artificial containers such as shallow wells, water urns, discarded containers, and tires. It is very difficult to control.
There are many other mosquitoborne diseases, several of them caused by viruses. Some of these viral diseases, such as Japanese encephalitis, La Crosse encephalitis, West Nile fever, Ross River disease, and Rift Valley fever, affect large numbers of people in parts of the world where they occur.

CONTROL OF MOSQUITOES

In most industrialized nations, mosquito control is done by government-supported agencies that are either components of health agencies or separate agencies organized specifically for that purpose. In the United States, states that have the most serious mosquito problems (e.g., New Jersey, Florida, Texas, Louisiana, California) have many such agencies. Some are small and have responsibility for mosquito abatement in a few hundred square kilometers, whereas the activities of others may encompass one or more entire counties. However, even in states that have many mosquito abatement districts, many people live in areas with no organized mosquito control. In underdeveloped areas of the world, organized mosquito control is rare except for scattered programs aimed at specific diseases such as malaria.
Most organized mosquito control is accomplished by searching out mosquito larvae in standing water and then treating the water with some kind of material that kills the larvae. Modern materials are highly specific for mosquitoes and ordinarily have little or no effect on other organisms. One such material is a bacterial product called Bti (Bacillus thuringiensis israelensis) which produces a toxin that kills only larvae of mosquitoes, black flies, and certain midges. Mosquito abatement agencies may also apply chemical pesticides to kill adult mosquitoes, but ordinarily only when adult populations become so high that they cause extreme annoyance to many people or when the threat of disease transmission to people is high. Therefore, the most common method for this is known as ultralow volume (ULV). This approach involves using special equipment to spray extremely small volumes of small particles of highly concentrated insecticides. When used properly, it is a safe and highly specific method of mosquito control.
Control of irrigation water in agricultural areas to avoid excess runoff is an important mosquito control method, but in recent years elimination of small bodies of water that can serve as wildlife habitat has ceased to be a mosquito control option.
In the first half of the past century, elimination of bodies of temporary and permanent water (swamps, marshes, vernal pools) was an accepted form of mosquito control. Recent years have seen the realization that such habitats are valuable and irreplaceable components of the environment and that a variety of activities have resulted in the permanent loss of many of these wetland habitats. This loss has resulted in the development of mosquito management strategies that are much more ecologically sound. Considerable research has been conducted on management strategies that enhance wetland habitats while minimizing problems from mosquito breeding.

Biological Control

A method that is a preferred alternative to chemical control is the use of live organisms to control mosquitoes, either by predation or by infection. Mosquitofish (the common guppy) have been used for many years for this purpose, often with effective results. However, because mosquitofish are generalist predators, they must be used with great care to avoid damage to other aquatic organisms. Many other forms of biological control for mosquitoes have been tried, including other types of fishes, fungi, bacteria, nematode worms, flat worms, protozoan parasites, and predaceous insects (including some mosquitoes). Some of these organisms have been effective under special circumstances, but few of them have been adopted widely. Microbial organisms such as Bti and Bacillus sphaericus may be considered biological control agents, and these are used to great advantage in a variety of aquatic habitats.

Insecticides

At one time there were dozens of insecticides available for killing both adults and immature stages of mosquitoes. However, because of economics, primarily the costs involved in developing, testing, and registering new materials, and the development of resistance to insecticides by mosquitoes, the number of available materials is now down to a handful. A class of insecticides known as insect growth regulators has been highly effective and specific for mosquitoes, but the development of resistance to even these materials has clouded the future of these so-called third-generation pesticides. The best hope for circumventing resistance to pesticides is the use of a combination of approaches referred to as pesticide resistance management. Frequent testing for susceptibility in mosquito populations, alternation of pesticides, and avoidance of methods that result in the persistence of low dosages of pesticides are examples of this approach. Insecticide resistance is under genetic control, and the goal of insecticide resistance management is preservation of genes in mosquitoes associated with susceptibility.

Protection from Mosquito Bites

People living in areas lacking organized mosquito control must protect themselves from bites of mosquitoes by using a variety of strategies. Probably the most effective method of personal protection from mosquito bites is to avoid places where mosquito densities are high and to avoid being out-of-doors at times of the day when mosquito activity is at its highest. In mountainous areas, most species of mosquitoes bite during the morning and afternoon, and often, they do not bite at all during periods of darkness. In low-elevation areas, some mosquitoes tend to bite at night, whereas others bite during the day. The species of mosquito present in a given area varies from place to place, and it is necessary to learn the activity patterns of the mosquitoes present to develop avoidance strategies.
If exposure to biting mosquitoes cannot be avoided, there are several ways to minimize discomfort. The most important of these is to reduce exposed skin surfaces by wearing hats, long trousers or slacks, and long-sleeved shirts or blouses. Some mosquitoes bite through light clothing, but the number of bites received is definitely reduced if most areas are covered. When mosquito densities become very high, application of a mosquito repellent may be needed to avoid bites. For many years the only really effective repellents on the market were those containing the material DEET (N,N-diethyl-meta-toluamide). However, in recent years newer materials have been discovered that are equally effective, and there are now alternatives available to DEET-based repellents. Skin repellents have some drawbacks. After application, they are effective for only a few hours. Factors such as wind, high temperature, high humidity, and sweating can reduce time of effectiveness even further. When applying skin repellents, the material must be thoroughly applied to all exposed skin, including behind the ears. In recent years, longer-lasting repellent formulations have been developed by the incorporation of various additives such as lotions and polymers.
Many people have tried gadgets such as ultrasonic emitters, electric grids, aromatic plants, and even vitamins for mosquito protection. Research has shown that most such methods are of little or no value in repelling mosquitoes, but such devices continue to appear on the market. In some areas of the world incense coils are sold for avoidance of mosquitoes. They may afford protection within a short distance of the burning coils.
Bednets can provide excellent protection from mosquito bites at night if used properly. The use of bednets treated with insecticides has been shown to afford excellent protection from attack by malaria mosquitoes. Insecticide-treated window and door nets also have been proven to help reduce mosquitoborne diseases in endemic areas. When they are available, vaccines may protect humans from mosquitoborne disease (e.g., yellow fever) and prophylactic drugs may be used to avoid some diseases (e.g., malaria).

MOSQUITOES AS SUBJECTS OF RESEARCH

Mosquitoes have been studied for their role as vectors of diseases of humans and other animals for well over a century. However, they have also been popular test animals for research on fundamental biological processes, both in the laboratory and in the field. Because mosquitoes occur in so many habitats all over the world, they present opportunities for a wide variety of ecological studies such as predator-prey interactions involving larvae. Further, many species can be colonized in laboratories. Because of their short life cycle intervals (many species can complete an entire life cycle from egg to adult in 7-10 days), large numbers of live mosquitoes can be produced for studies of physiological and biochemical processes common to many types of invertebrate animals. A. aegypti and C. pipiens are two species that can be colonized readily and have been studied extensively both for their importance as disease pathogen vectors and as test animals for biological phenomena. Genomics (the field of study involving the determination of the sequence of the entire genome of an organism) is an exciting new field that has great promise in understanding disease processes and their prevention and control. The genomic sequence has been determined for A. aegypti and C. pipiens, as well as for the important malaria vector Anopheles gambiae. Other arthropod species of medical importance, such as ticks and tsetse, are already the subject of genomics analysis. Fine-scale genetic studies of important mosquito disease vectors also open the door to the possibility of altering their genetic makeup, resulting in transgenic individuals with lethal traits or diminished vector capacity.

Next post:

Previous post: