Entomology and Plant Pathology

General Mosquito Biology

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 The mosquito has four distinct stages in its life cycle: egg, larva, pupa, and adult. The adult is an active flying insect, while the larvae and pupae are aquatic and occur only in water. Depending on the species, eggs are laid either on the surface of water or are deposited on moist soil or other objects that will often be flooded. 

Eggs

One factor common to all mosquito species is that eggs are laid in association with free water or on a moist surface. Eggs are white when first deposited, darkening to a black or dark brown within 12-24 hours. Single eggs are about 1/50 inch (0.5mm) long, and those of most species appear similar when seen by the naked eye (one exception is the Anopheles spp. whose eggs have floats attached to each side of the egg). Eggs are laid singly by some species, and others lay eggs together to form rafts. The incubation period (time between when eggs are laid and when they hatch) may vary considerably among species. Eggs of permanent-water mosquitoes where eggs are deposited on the water surface may hatch in 1-3 days depending on temperature. Floodwater species deposit their eggs on moist soil or another wet substrate and have a wide variation in incubation periods. These eggs will not hatch until submerged by rising water caused by rainfall, melting snow in the spring, or other floodwater. Depending on the species and conditions these eggs may hatch the next time they are flooded, as soon as ten days, or may not hatch until they are flooded a year or more later.

eggs 
Aedes albopictus (top)Culex spp. (middle), Anopheles spp. (bottom)  

Larvae

The larvae (wigglers or wrigglers) of all mosquitoes live in water and have four developmental periods or instars. These are called 1st, 2nd, 3rd, and 4th instars with each succeeding stage larger than the last. At the end of each instar, the larva sheds its skin by a process called molting. The larva is an active feeding stage. Larvae feed on particulate organic material in the water. The larvae of most species have a breathing tube (click here for larval anatomy) and must occasionally come to the surface of the water to get oxygen. The total length of time that larvae spend in the larval stage depends on the species and the water temperature. Some can develop in as little as 5 or 6 days. Upon maturity the 4th instar larvae molts into the pupal stage.

larvae

Aedes albopictus (left)Culex spp.(right) 

Pupae

Unlike most other insects, the mosquito pupa is very active, and, like the larva, lives in water. It differs greatly from the larva in shape and appearance. The pupa has a comma-shaped body divisible into two distinct regions. The front region consists of the head and thorax (cephalothorax) and is greatly enlarged. It bears a pair of respiratory trumpets on the upper surface. It must periodically come to the surface to get oxygen. The second region is the abdomen which has freely-movable segments with a pair of paddle-like appendages at the tip. Feeding does not take place during the pupal stage. The pupal stage only lasts for a few days and is the stage when all the larval tissues change into the adult tissues. The adult emerges directly from the pupal case on the surface of the water.

pupae

Aedes albopictus (left)Anopheles spp. (right) 

Adult

The adult mosquito is entirely terrestrial and is capable of flying long distances. Both females and males feed on nectars which they use for energy. Males and females mate during the first 3 to 5 days after they have emerged. Females mate only once. Males generally live for only a week. Only the females feed on blood, which is what is occurring when they are biting. Females evidently gain little nourishment from blood meals but need them in order to develop eggs. Many mosquitoes feed on any warm-blooded bird or mammal. However, some prefer cold-blooded animals. Some species also prefer birds and seldom feed on mammals, which is the case with Culex spp. mosquitoes which are known to transmit the West Nile virus (WNV). Unfortunately many species feed on a wide range of warm-blooded mammals and humans are often attacked. Once a female has completely engorged she flies to a shaded environment until her eggs are completely developed, usually 3 to 5 days. Once the eggs are developed the female is called a gravid female and she begins to search for a desirable place to lay her eggs. If a female survives her egg laying activities, she will very soon start searching for another blood meal after which she will lay another batch of eggs. She does not need to mate a second time. Generally a female will only live long enough to lay 1 to 3 batches of eggs.

Most mosquito species are actively searching for a blood meal in the evening hours from just before dark until 2 to 3 hours after dark. During the daytime the females normally rest in cooler vegetated areas where the humidity is higher and they are protected from drying out. Females will often bite in the daytime if humans or animals invade the wooded areas where they are resting. However, Aedes albopictus is an aggressive biter which prefers to feed during the daylight hours and is often a nuisance in urban areas.

antennae 

Aedes albopictus female (left), Aedes albopictus male (right)

aa&cq

Aedes albopictus, Asian Tiger Mosquito (left), Culex quinquefasciatus, Southern House Mosquito (right)

 

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