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This content is taken from the London School of Hygiene & Tropical Medicine & ARCTEC's online course, Preventing the Zika Virus: Understanding and Controlling the Aedes Mosquito. Join the course to learn more.

Skip to 0 minutes and 13 seconds An important first step in controlling a vector-borne disease like Zika or dengue is to understand the biology of the vector. When the biology of the vector is fully understood, one can try to think of interventions. For example, controlling the population. I will tell you more about the basic biology and life cycle of Aedes aegypti. The mosquito undergoes four distinct stages of development– the egg; the larvae, of which there are four different stages; the pupa; and adult stage. The life cycle starts with a female Aedes aegypti mosquito taking a blood meal. Only females bite. The females need blood to provide nutrients for the production of eggs. The females can produce an average 100 to 200 eggs per batch.

Skip to 1 minute and 3 seconds And they can do so five times during their lifetime, every three to four days. The number of eggs they produce is dependent on the size of blood meal they have taken. The eggs are smooth, oval shaped, and about a millimetre long. When freshly laid, they appear to be white, but they turned black within minutes. Unlike some mosquito species, Aedes aegypti lays its eggs separately, not in rafts. Not all eggs have to be laid at once. It can be spread out over hours or days, depending on the availability of the suitable substrates. Unlike other mosquitoes, Aedes aegypti will not usually lay its entire batch at a single site, but spread the eggs over several sites.

Skip to 1 minute and 47 seconds This behaviour is often referred to as skip oviposition behaviour.

Skip to 1 minute and 56 seconds Unlike some other mosquitoes, Aedes aegypti eggs are not laid directly in water. They are laid on damp surfaces in areas that are likely to flood, such as tree holes, barrels, jars, pots, buckets, flower faces, tanks, discarded bottles, and tyres, and many more other places where rainwater comes together or is stored. The eggs are most often placed at different distances above the water line. The development time of the eggs is dependent on temperature. In warm climates, the eggs may develop as little as two days. But in colder climates, it can take up to a week. It is notable that laid eggs can survive for fairly long periods in a dry state, often more than a year.

Skip to 2 minutes and 42 seconds The eggs will immediately hatch when submerged in water. This ability to survive the drying process has facilitated the long distance transport of mosquitoes across the world and provides an ongoing challenge for control and monitoring efforts. Once the larvae hatch out from the eggs, they begin to feed. Larvae will start feeding on algae and other small organisms using their mouth parts which resemble brushes. The larvae spent most time at the water surface where they can breathe air through tubes at the ends of their bodies called syphons, but they will swim to the bottom when disturbed, for example, by a predator or even by a shadow. And they will also swim to the bottom when they’re feeding.

Skip to 3 minutes and 25 seconds The larvae pass through four stages of development calls instars. At the end of each stage, the larvae shed their skin, a process called moulting, enabling them to grow in size. They spend a short amount of time in the first three instars, and up to three days in the fourth instar, which are approximately eight millimetres long. The development of larvae is also temperature dependent. When temperatures are cool, Aedes aegypti can stay in larval stage for months. On average, males develop faster than females and will undergo change in the next stage of development, the pupae, earlier. After the fourth instar, the larvae develop into pupae. Mosquito pupae are mobile and they respond to stimuli in the water.

Skip to 4 minutes and 14 seconds For example, movements or shadow, enabling them to escape from potential predators. Pupae don’t feed and they need approximately two days to develop into adults. Adult mosquitoes emerge from the pupae. The process of adult emergence from the pupa is risky for the mosquito as their exoskeleton is soft and they’re at risk of drowning in the water. Adults emerge by ingesting air to expand the abdomen, which causes the pupal case to split open and the mosquito can emerge head first. The adult mosquito itself is approximately four to seven millimetres, dark with white markings on the legs and a marking on top of the thorax, which makes it easily recognisable.

Skip to 5 minutes and 1 second The males of all species of mosquitoes do not bite humans or animals of any species. They live on nectar. The female of Aedes aegypti feed not only on nectar are but also on blood. In total, it takes one to two weeks from an egg to reach adulthood. Following emergence, an adult female mosquito will search for a blood meal from nearby hosts, frequently humans, in order to provide the nutrients for egg production. A female that has laid one egg batch may seek to take another blood meal and then the cycle will begin again.

The Aedes lifecycle

To control the spread of a vector-borne disease like Zika, we first need to understand the biology of its vector. In this step, Tessa Visser explores the basic biology of Aedes aegypti, highlighting the four separate and distinct stages of its life cycle: egg, larva, pupa, and adult.

While the total length of a mosquito’s life can vary between species and is dependent on a number of environmental conditions, all mosquitoes pass through these stages, and understanding what occurs during each of them can enable us to better plan and target control interventions.

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Preventing the Zika Virus: Understanding and Controlling the Aedes Mosquito

London School of Hygiene & Tropical Medicine

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