One is very agile, reproduces in clean water, attacks in broad daylight and is responsible for transmitting dengue fever in Brazil. The other prefers dawn, lays its eggs in dirty water rich in decomposing organic matter and torments sleepless nights with its buzzing. With the arrival of summer, the reproductive and development cycle of the two most urban mosquitoes in the world accelerates: the Aedes aegypti and Culex quinquefaciatus, the domestic mosquito.

Researcher Jos� Bento Pereira Lima, from the Laboratory of Physiology and Control of Vector Arthropods at the Oswaldo Cruz Institute (IOC/Fiocruz), presents the differences between the two species and points out the main measures to eliminate their breeding sites.

It is very common during the rainy season to find mosquito larvae in dirty water, full of decomposing material, such as sewers and cesspools. These larvae can be A. aegypti?
No. It is important to remind the population that the A. aegypti it only deposits its eggs preferably in clean waters. Its larvae cannot survive in polluted reservoirs, with waste and a lot of organic matter. When larvae are found in puddles with contaminated water, very muddy, in open sewers, in ditches or other similar breeding sites, it is certainly not larvae of A. aegypti. Most likely, these are larvae of Culex quinquefaciatus, the domestic mosquito. Unlike the A. aegypti, Culex it prefers to lay its eggs in highly polluted breeding grounds, with a lot of decomposing organic matter.

What are the preferred breeding sites for the two mosquitoes?
O A. aegypti it only lays its eggs in clean water, not necessarily potable, but necessarily with little decomposition material. Therefore, it is important to reinforce to the population that some basic precautions regarding water reservoirs are fundamental for the control of the dengue vector: cover boxes and barrels of water, unclog drains that could accumulate water, throw away old tires, avoid leaving bottles and containers that could accumulate rainwater in an uncovered area and turn them upside down, eliminate small dishes with water under the potted plants.

Deposits of dirty and contaminated water, sewers, ditches, cesspools and all water reservoirs with a lot of organic material are the preferred breeding grounds for the Culex. Therefore, accumulated contaminated water and open sewers are of great importance for public health, as they can cause various problems to the population, but do not constitute potential mosquito breeding sites. dengue transmitter.

Beside the A. aegypti, Culex It is considered one of the most important urban pests in the world. What are the differences between the two mosquitoes?
O Culex It is considered a cosmopolitan species, that is, present in almost the entire world, and it is even more urban than the A. aegypti. Both have a preference for feeding on human blood and are closely associated with the presence of men. In Brazil, when we move away from cities and urban areas, it is difficult to verify the presence of either one. Their entire life cycle, mating and egg laying, takes place in or close to homes. Despite this, the two mosquitoes have many differences between them. In addition to preferring different breeding sites, the eggs, larvae and adult individuals of the two species are very different. Inside houses, it's easy to tell them apart: as an adult, the Culex has a brown color and the legs do not have clear markings, while the A. aegypti It is darker and has white markings on the body and legs.

Gustavo Resende/IOC

O A. aegypti It is dark and has white markings on the legs and body, while the Culex has a brown color. In addition to these differences, the two mosquitoes have very different habits in terms of their preferred breeding sites and times of greatest activity.

It is very common to find both species inside houses, in practically all regions of Brazil. How do they relate in the home environment?
O A. aegypti It is much more active during the day, especially in the early morning and late afternoon, feeding on blood, usually lower body parts, such as feet and shins. It is an essentially diurnal mosquito. It's good to remember that this doesn't mean he doesn't bite at night. It is an opportunistic mosquito: if the resident leaves a leg or arm exposed near the shelter of the A. aegypti, it will probably be stung even at night. O Culex, in turn, is a strictly nocturnal mosquito, which prefers to feed when people are resting. At night, in the dark, it is attracted by the carbonic gas emitted in human breathing, flying close to the face, and only then chooses a place to sting. That's why we usually hear buzzing so characteristic of its approach. O A. aegypti It is a discreet mosquito, rarely noticed when it feeds on blood, and very skittish, fleeing with any sudden movement. O Culex it makes a noise close to the ear and is not as difficult to catch as the other. Inside homes, the two coexist well and are usually found in the same shelters: under tables, behind furniture, between curtains and in niches on shelves, for example.

Do mosquito larvae also show morphological and behavioral differences?
There are some differences between the larvae of the two mosquitoes, such as the size of the head and thorax, which are larger in the Culex, and the shape of the respiratory siphon, smaller and thicker in the A. aegypti. Another difference is light sensitivity. The larvae of the two mosquitoes have negative phototropism, which means that they do not live well with excess light and, therefore, seek the darkest parts of the breeding sites. However, the aversion to light is much more pronounced in the A. aegypti. The larvae tend to accumulate in the darkest corner of the foci, while those of Culex they are more spread throughout the breeding site. If we bring a beam of light closer to a focus, the larvae of the two mosquitoes move away, but not A. aegypti this flight movement is much more accentuated, clearer.

As for the development cycle, are the differences significant?
In general, the A. aegypti and Culex they have a similar development time - from the hatching of the larva to the appearance of the adult - which varies according to the temperature, taking about 8 to 10 days in the summer. In winter, both have a slower development rate.

One of the main difficulties in controlling the population of A. aegypti It is the great resistance of their eggs to drying out. The same goes for the Culex?
No. The eggs of the two species are very different. O A.aegypti place the eggs in the wet part next to the water slide and not directly in the water. They are able to stay up to a year in the dry and remain viable, capable of originating adult mosquitoes when they find the right conditions to hatch. Together, these characteristics are very important for the dispersion of the mosquito and for the epidemiology of dengue, since the eggs can be carried to other regions by human action and resist even the rains. of the next summer, making control actions difficult. Another important characteristic for the epidemiology of dengue is that the female A. aegypti usually deposit eggs in different breeding sites in the same posture.

O Culex it lays its eggs directly in the water, always all together, in the same breeding place. Involving each of the eggs there is a viscous substance that holds them together, forming a "raft", composed of dozens of eggs, stuck together, floating on the surface of the water. the eggs of Culex they do not have resistance to desiccation and wither when removed from the water, no longer being viable.

How many eggs do these species lay in each posture?
The amount of eggs laid by the two mosquitoes depends a lot on the amount of blood ingested by the female, which is necessary for the maturation of the eggs. In general, the two species usually lay around one hundred eggs per laying, but this number can reach 150 or 200. Aedes You can repeat the feeding cycle and lay eggs within three to four days. But it is important to note that not everyone makes it to adulthood.

What is the importance of Culex for public health?
O Culex does not transmit dengue virus. However, in addition to the inconvenience it generates for the population, in some regions of Brazil it is responsible for transmitting filariasis and some arboviruses. O Culex It is the main vector of filariasis, popularly known as elephantiasis. The disease is caused by nematode worms, known as filariae, which are lodged in the lymphatic vessels of the host, which can lead, in the chronic phase, to swelling and excessive growth of the lower limbs. The predilection of Culex by human blood and its nocturnal habit facilitate the transmission of the disease.

At night, when the infected individual is at rest, the microfilariae move to the peripheral blood vessels, getting closer to the surface of the skin, which facilitates the infection. that of the mosquito when it feeds on your blood. Although the incidence of the disease has decreased a lot in Brazil in recent decades, filariasis still represents a serious problem in some regions of the country. O Culex It is capable of transmitting arboviruses, especially encephalitis and severe hemorrhagic fevers, such as the one caused by the Oropouche virus, which in Brazil has already occurred in Pará and Rondônia. In addition, the accumulation of contaminated water and the existence of valleys and open-air sewers, preferred breeding grounds for the Culex, are demonstrations of the lack of infrastructure in some regions of the country and may be related to other important diseases that threaten the population.

*All contents were reviewed by researchers from Instituto Oswaldo Cruz (IOC/Fiocroz)