Biology

According to the species, mosquito larvae can develop in practically all possible types of habitats: freshwater to brackish water; clean water to heavy polluted one; stagnant water to running water; natural habitats to man-made breeding sites; small habitats (puddles, footprints, artificial containers, etc.) to large ones (rice plantations, lakes, etc.). Information on larval ecology is crucial in order to carry out appropriate vector control programs targeting specific mosquitoes. Three keys elements concern the larvae: (1) they feed, therefore it is possible to use insecticides of ingestion like Bacillus thuringiensis or Bacillus sphaericus (useless against pupae which do not feed); (2) they moult, therefore growth regulators like juvenoids or ecdysteroids can be used; and (3) they breathe at the water surface, therefore it is possible to use methods aiming at asphyxiating them, like monolayers, oils or polystyrene chips for example.

Only females bite to take a blood meal for egg maturation, but males and females feed on flower nectar from which they get their energy necessary for flight. Fecundation occurs two to three days after adult emergence, with generally only one fecundation, although several can take place. The female's life is conditioned by the succession of blood meals and the development of the ovaries, this is the gonotrophic cycle, which starts with the unfed female, then after blood-feeding, it becomes half-gravid, and gravid. This cycle must be known for each species or each situation considered in vector control programs, as its duration conditions, the frequency of the contacts host/mosquito, and the ingestion (from man to mosquito) or the transmission (from mosquito to man) of pathogens responsible of the disease considered. After egg laying the female seeks another blood meal, and the ''gonotrophic cycle'' repeats itself every two to three days. In tropical regions, the blood meal is accompanied by a maturation of the ovaries; this is the ''trophogonic concordance". On the other hand, in temperate regions during a cold period there can be a ''trophogonic dissociation'' for which the blood meal is not followed by the development of the ovaries; the females can even enter into complete diapause, allowing hibernation. Mosquitoes can take their blood meal from humans (anthro-pophilic) or animals (zoophilic), or both. The trophic preferences of species are very important to know, the more anthropophilic a mosquito, the higher its vectorial role. The blood meal can be taken indoors (endophagic) or outside (exophagic). Some species bite essentially during the night (nocturnal) like Anopheles, others during the day (diurnal) like Aedes, and others during the morning or at sundown. After the blood meal, the mosquitoes have a phase of digestion which lasts approximately 48 hours, during which they rest either indoors (endophilic) or outdoors (exophilic). All these behaviours are very important to know in the definition of vector control strategies. It is clear that indoor spraying with remanent insecticides will be particularly efficient against anthropophilic, endophagic and endophilic mosquitoes, but effects against the exophagic and exophilic mosquitoes will be quite reduced, and the addition of insecticide could even increase exophilic behaviour and greatly reduce the impact. Some products have effects known as "deterrent" (the mosquito avoids entering the treated house), "excito-repulsive" or "irritant", where the mosquito avoids contact with a treated surface or remains in contact with the product for only a short period of time and so the insecticide does not have a lethal effect. The mosquitoes can thus survive outdoors and continue to bite the human population in spite of indoor treatment. Mosquitoes generally have a lifespan of about one month in tropical areas, although in temperate areas, mosquitoes can survive during the winter in diapause or semi-diapause. Their range of active flight is generally rather weak (active dispersion), three kilometres for Anopheles and Aedes hardly move away from their larval habitat, therefore vector control can target areas near breeding sites. Mosquitoes can, however, be transported by the wind (passive dispersion) and by modern means of transportation (e.g. "airport malaria'' cases can occur as a result of Anopheles vectors travelling by plane from an endemic zone into a malaria-free area and inoculating Plasmodium parasites).

1.2.3.1 Culicinae

The subfamily of Culicinae includes 33 genera; the most important ones in medical entomology are the Culex, Aedes, Mansonia, Sabethes and Haemago-gus. Aedes, Culex and Mansonia are found in the temperate and tropical regions; the genera Sabethes and Haemagogus are found only in Central and South America. Culicinae are easily distinguishable from Anopheles at the larval and adult stages (see Table 1.1).

1.2.3.1.1 Culex. Culex species are widespread in the whole World, except the most northern zones of temperate regions and the poles. There are thought to be some 800 species divided into 21 sub-genera.

The eggs, brown, long and cylindrical, are deposited on the surface of water and bound to form a "raft" composed of some 300 eggs which are laid in a large variety of aquatic habitats: small puddles, pools, permanent or temporary ponds, flooded marshes, borrow pits, ditches, rice plantations, as well as

Table 1.1 Morphological differences between Anophelinae and Culicinae in relation to the stages.

Stages

Anophelinae

Culicinae

Larvae

Adult females Adult males

Eggs

Visible lateral floaters

No respiratory siphon Highly visible siphon

Palps as long as proboscis Palps smaller than proboscis

Palps with rounded extremities Palps with tapered extremities

Adult resting position Oblique on the surface

Parallel to the surface

Figure 1.2 Adult Culex quinquefasciatus (Photo courtesy of CDC/James Gathany).

anthropogenic sites such as cans, cisterns, and even sewage drains with polluted water. The most important species, Culex quinquefasciatus, is strongly associated with anarchistic urbanization, with poor hygiene conditions and worn-out water drainage systems containing organic matter where the larvae can develop (e.g. polluted stagnant water, gutters, septic tanks, sewage drains, etc.). The density of mosquito populations can be very high under such conditions and constitutes a major cause of nuisance for the people affected.

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