Anopheles

Climate and urbanization drive mosquitoes’ preference for humans

The yellow fever mosquito (Aedes aegypti) is known throughout the world for inhabiting urban areas, biting humans and thereby spreading diseases such as dengue, Zika, chikungunya and the yellow fever that gives it its name. The WHO estimates that this species causes 50 million infections and 25,000 deaths a year. Studies carried out in America and Asia found that 95% of the blood consumed by the mosquito is human, demonstrating its preference for us.

However, in Africa, where the mosquito originates from, most of the populations of this mosquito do not have a preference for humans, but for other animals, mainly mammals such as primates and rodents. But this trend could change in the coming years, a new study warns. The increasing urbanization that the continent is experiencing and climate change, will favor the spread of mosquitoes with a preference for humans. And this will increase your ability to spread disease.

The yellow fever mosquito is native to Africa, where most of its populations have no preference for biting humans, unlike their populations in America and Asia

To reach these conclusions, the study authors analyzed the preference of various populations of African Aedes aegypti and the climatic and environmental variables that could explain the differences between them. They collected eggs from 27 locations in sub-Saharan Africa (Fig. 1), which varied in their climatic conditions, ranging from the semi-arid regions of the Sahel, to seasonal forests and rain forests. The other variable they analyzed was the density of the human population present in the area. The mosquitoes were bred in a laboratory where their biting preference was studied, exposing them to human odors and odors from other non-human animals at the same time. In this way they could observe which of them they were heading and calculate the preference of the different populations.

Fig. 1. Location of the populations used in the study. The size of the circle represents the density of human population present in the area. The color the mosquito’s preference for biting humans (reddish) or other animals (bluish). The figure below shows the preference index obtained by giving them a choice between humans or other animals for each of the populations. The point is the mean and the bar is the 95% confidence interval. Source: Mosquito Alert CC-BY from the original by Rose et al. 2020. bioRxiv 939041

Urbanization and climate determine mosquito preference

Most of the populations preferred the smell of the other animals (blue), some did not show preference for one or the other (violets), and only a few of them showed a clear predilection for human smell (red) (Fig. 1) . The taste for human blood was present in those regions with a higher human density. Suggesting that when the human population is large in an area, the most favored mosquitoes are those that tend to feed on people’s blood.

Another factor that explained the distribution of mosquitoes with this preference for human odor was the climate. These mosquitoes are more abundant in dry regions. In them the natural habitats where they usually deposit eggs, tree holes or concavities in the rocks with rainwater, are scarce. Instead, human containers and tanks proliferate to store water. Humans provide mosquitoes with places to breed in environments that are initially unfavorable to them.

Both morphology and genetics show that mosquitoes with a preference for humans, which today have invaded the American and Asian continents, are related to the populations of the Sahel, the region that makes the transition between the Sahara desert to the north, and the savannah to the south. The term of Arab origin literally means “coast”, in reference to the appearance of the vegetation delimiting the sandy sea of ​​the Sahara. The yellow fever mosquito populations that inhabit northern Senegal, in addition to showing a preference for humans, is the one that is genetically related to the mosquitoes that we find today in America and Asia.

The yellow fever mosquito tamed itself

In these semi-arid environments, mosquitoes depend on human habitats to reproduce. Human-made tanks and water drums allow them to inhabit a region where they find few natural habitats to reproduce. Being forced to live together with human populations, where the most abundant prey is humans, natural selection ended up generating mosquito populations with a preference for feeding on humans. Individuals with the mutations in sensory systems that determine their preference for human odors are the most abundant in these populations.

A drier climate and greater urbanization foresees an increase in mosquitoes with a preference for humans in Africa in the coming decades

The combination of dry seasons and dependence on humanized spaces has not only been important in the evolution of the yellow fever mosquito, but has also shaped the evolution of Anopheles mosquitoes that transmit malaria in Africa.

In Africa, the climate and density of human populations are changing, and it is feared that the change in rainfall and the increase in urbanized areas will favor mosquitoes that have adapted to live and feed on humans. That would mean a considerable increase in human populations on the continent exposed to the diseases that this species can transmit.

 


References:

Crawford JE, Alves JM, Palmer WJ, Day JP, Sylla M, Ramasamy R, Surendran SN, Black WC, Pain A, Jiggins FM. 2017. Population genomics reveals that an anthrophilic population of Aedes aegypti mosquitoes in West Africa recently gave rise to American and Asian populations of this major disease vector. BMC Biology 15: 16

Dao A, Yaro AS, Diallo M, Timbiné S, Huestis DL, Kassogué Y, Traoré AI, Sanogo ZL, Samaké D, Lehmann T. 2014. Signatures of aestivation and migration in Sahelian malaria mosquito populations. Nature 516: 387-390

Kotsakiozi P, Evans BR, Gloria-Soria A, Kamgang B, Mayanja M, Lutwama J, Le Goff G, Ayala D, Paupy C, Badolo A, Pinto J, Sousa CA, Troco AD, Powell JR. 2018. Population structure of a vector of human diseases: Aedes aegypti in its ancestral range, Africa. Ecology and Evolution 8: 7835-7848

McBride CS. 2016. Genes and odors underlying the recent evolution of mosquito preference for humans. Current Biology 26: R41-R46

Rose NH, Sylla M, Badolo A, Lutomiah J, Ayala D, Aribodor OB, Ibn N, Akorli J, Otoo S, Mutebi JP, Kriete AL, Ewing EG, Sang R, Gloria-Soria A, Powell JR, Baker RE, White BJ, Crawford JE, McBride C. 2020. Climate and urbanization drive mosquito preference for humans. bioRxiv: dos.org/10.1101/2020.02.12.939041

Takken W, Verhulst NO. 2013. Host preference of blood-feeding mosquitoes. Annual Review of Entomology 58: 433-453

 

Why mosquitoes appear in spring

Most of us like spring and summer months. We feel better when we have left behind the cold and the gray cloud accumulation of winter. Spring is the season of colors, nature is full of flowers and greenery. Life seems to manifest again after months in which it seems to have been dormant. With spring, mosquitoes also reappear. From where? How? What had happened to them during the winter?

For a long time it was believed that the origin of many living beings was found in dead matter and its putrefaction. A phenomenon that was known as spontaneous generation. Aristotle (384-322 BC) used this phenomenon to explain the sudden appearance of worms, mosquitoes, eels and mice, among other animals. They all reemerged from the most diverse materials. Mosquitoes poured out of the mud, just like that. The idea of ​​spontaneous generation, in which the inert can give rise to life, persisted for centuries, until an experiment dismantled it in the early seventeenth century.

Do mosquitoes appear by spontaneous generation?

It was the Medici court physician, Francesco Redi, who in 1668 published his book Experiences around the Generation of Insects, attacking the doctrine of spontaneous generation. Redi himself relates that the idea to carry out his experiments came to him after reading Homer’s Iliad, where it is described that Thetis, mother of Achilles, covers Patroclus’s corpse to protect it from worms and flies that “corrupt the bodies of men killed in battle.” Why cover the bodies if, according to Aristotle, worms and flies arose directly from decomposing bodies?

Fig. 1. Francesco Redi’s experiment to deny the theory of spontaneous generation. In the cans sealed with a stopper or gauze, no fly larvae appeared in the rotting meat, because the flies were unable to enter to lay their eggs. Source: Mosquito Alert (CC-BY-NC-2.0)

 

Motivated by doubt, Redi carried out a series of experiments in which she placed pieces of meat, both in uncapped jars, in gauze-covered jars, and in hermetically sealed jars (Fig. 1). With this elegant experiment, he demonstrated that spontaneous generation did not exist. Only fly larvae appeared in the uncovered jars where flies had been able to enter to lay the eggs. His experiment made it clear, life comes from life, not the inert. So what about mosquitoes in spring. How do they arise if there were no mosquitoes in winter?

In winter we don’t see them but they are still there

The answer is that mosquitoes never disappeared in winter. In one way or another they have been there. Latent, but present. Like seeds waiting for the good time to bloom. Mosquitoes, like the vast majority of insects, have a programmed latency program known as diapause.

Diapause not only occurs in winter in temperate zones, it also occurs in dry seasons in tropical zones. When the environmental conditions are bad for the development of the mosquitoes they go into diapause. Understanding that it promotes this state of latency is very important to understand the dynamics of the diseases they transmit, since sometimes this phase serves as a reservoir for pathogens that can re-emerge with the reactivation of mosquitoes.

While most mosquitoes diapause when conditions are poor, the yellow fever mosquito does not

Although one or another type of diapause has been described in most mosquitoes to survive adverse environmental conditions, some species, such as the yellow fever mosquito (Aedes aegypti), seem to lack it.

Diapause is a hormonally programmed latency stage in response to environmental conditions. It allows animals to advance in bad weather and “encapsulate” before the cold kills them. To predict the arrival of winter, mosquitoes consist of several mechanisms. On the one hand, the photoperiod, that is, the number of daylight hours. That the days become shorter is surely the most evident sign of the approach of winter. Much more than the change in temperature which is much more variable. But how does a mosquito know that time passes? With the help of an internal clock present in all animals. This internal clock regulates two different temporal systems. On the one hand the circadian cycles, which are the rhythms of activity of day and night, and another that would allow the identification of seasonal cycles, something like an internal calendar. The latter would be in charge of recognizing that the days become longer or shorter, and as a result, activate a hormonal and physiological response in the mosquito (Fig. 2).

 

Fig. 2. Mosquitoes can perceive the change of season thanks to having an internal clock with which to compare external light periods. The red curves represent the daily oscillations of the internal clock. The light and dark areas, the light and dark hours of each day. As winter approaches, daylight hours become shorter. This signal induces diapause in the mosquito. Source: Mosquito Alert (CC-BY-NC-2.0).

 

To the stimulation of the hours of light, the one of the temperature is added. Although more variable than light, temperature allows you to refine the animal’s response. Its effect when initiating diapause has been observed in the tiger mosquito (Aedes albopictus), in other Aedes species, as well as in the common mosquito (Culex pipiens).

Diapause can occur in embryos, larvae, or adults

The change in daylight hours, temperature, and humidity perceived by female mosquitoes causes their embryos to go into diapause. So it is with the tiger mosquito. It is not the adult mosquito that goes into diapause, but the embryos, which remain dormant during the winter in their eggs without hatching. These eggs are different from normal eggs. They are larger and contain a higher amount of lipids to protect and nourish the embryos for months. They have to hold on until spring returns.

The tiger mosquito survives winters in their embryonic phase, protected inside the eggs waiting for the return of good weather

The diapause of the common mosquito (Culex pipiens) is different. The change in daylight hours and temperature alter the behavior of their females. With the eminent arrival of winter, females stop feeding on blood, instead of being attracted to humans and other vertebrates, they are attracted to the nectar of flowers. They also start the search for a dark and humid place in which to take refuge during winter. The common mosquito does not spend the winter as an egg, as the tiger mosquito does, but as adult mosquitoes that will reactivate with the arrival of good weather.

The common mosquito overcomes winters in its adult phase, sheltered in dark and humid places that protect it from low temperatures

In other mosquito species, diapause does not occur during the embryonic phase, nor during the adult phase, but during the larval phase (Fig. 3). Whether in the embryonic, larval or adult phase, what is known is that diapause is something much more complex than a simple stop in the development of the animal. Diapause involves molecular, physiological, developmental and behavioral changes before and during the diapause, which affects the development and reproduction of animals after the diapause is over.

Fig. 3. Phylogenetic relationship of different genus of mosquitoes and the phases of the biological cycle in which diapause has been described for different species. The numbers refer to the number of species of the genus with diapause, for example, in Aedes mosquitoes, 18 species with embryonic diapause have been described, including the tiger mosquito, and 6 species with diapause in the larval phase, but no species with diapusa in the adult phase. In Anopheles and Culex mosquitoes it is the other way around, most species have diapause in the adult phase. Data from Denlinger & Armbruster 2005. Annual Review of Entomology 59: 73-93. Source: Mosquito Alert (CC-BY-NC-2.0).

Invasive mosquitoes adapt their diapause to new climatic conditions

The study of invasive species such as the tiger mosquito has shown that the timing of the diapause can evolve rapidly. During their expansion along different climatic gradients, mosquito populations have been adapting to the signals that trigger the diapause, advancing or delaying it. Not all populations respond the same amount of daily light hours. The answer depends on the latitude at which they are. This suggests that they will also be able to respond to climate change. Understanding how mosquitoes respond to environmental changes is essential to predict the spread of the diseases they transmit.

The molecular basis of the diapause of species of sanitary interest is increasingly known. Advances in genetics and changes in gene expression may lead to the development of new ways to control their populations, based on genetic or chemical disruption of diapause.

At the moment, we know that with the arrival of good weather, mosquitoes come out of their diapause state and are active again. They have never disappeared. They have always been here. Egg-shaped, larvae or adult sheltered in some dark corner, they have endured winter to return once again in spring. It is time to remember to take the necessary measures so that our home is not a place where they can raise. Avoid having containers that store standing water, and have the Mosquito Alert application prepared to report the presence of mosquitoes.


References

Armbruster PA. 2016. Photoperiodic diapause and the establishment of Aedes albopictus (Diptera: Culicidae) in North America. Journal of Medical Entomology 53: 1013-1023

Bova J, Soghigian J, Paulson S. 2019. The prediapause stage of Aedes japonicus japonicus and the evolution of embryonic diapause in Aedini. Insects 10: 222

Chang V, Meuti M. 2020.Circadian transcription factors differentially regulate features of the adult overwintering diapause in the Northern house mosquito, Culex pipiens. Insect Biochemistry and Molecular Biology: doi.org/10.1016/j.ibmb.2020.103365

Denlinger DL, Armbruster PA. 2014. Mosquito diapause. Annual Review of Entomology 59: 73-93

Lacour G, Vernichon F, Cadilhac N, Boyer S, Lagneau C, Hance T. 2014. When mothers anticipate: effects of the prediapause stage on embryo development time and of maternal photoperiod on eggs of a temperate and tropical strains of Aedes albopictus. Journal of Insect Physiology 71: 87-96

Lacour G, Chanaud L, L’Ambert G, Hance T. 2015. Seasonal synchronization of diapause phases in Aedes albopictus. PLoS One 10: e0145311

Medley KA, Westby KM, Jenkins DG. 2019. Rapid local adaptation to northern winters in the invasive Asian tiger mosquito Aedes albopictus: a moving target. Journal of Applied Ecology 56: 2518-2527

Robich RM, Denlinger DL. 2005. Diapause in the mosquito Culex pipiens evokes a metabolic switch from blood feeding to sugar gluttony. Proceedings of the Natural Academy of Science of the United States of America 103: 15912-15917

 

International demand for goods contributes to malaria risk

Previous studies have shown that deforestation and disturbance of tropical forests lead to increased transmission of malaria. The destruction of the jungle generates new environmental and ecological conditions in which mosquitoes thrive better. A new study links global demand for goods with deforestation and the risk of malaria.

The work is the first to directly relate global consumption to the increase in malaria cases in some regions. It is estimated that in these regions where cases are increasing, a fifth of deforestation is driven by international trade. Coffee, wood, soy, cocoa, palm oil, tobacco, calf meat and cotton are the main products demanded globally that stimulate deforestation.

Despite the fact that globally it seems that the number of cases has been decreasing since 2000, its increase is worrying in some regions. In 2018 there were 228 million cases worldwide and 405,000 deaths, of which seven out of ten were children under the age of five.

7 out of 10 deaths from malaria worldwide are children under the age of 5

90% of malaria cases occur in three tropical forest regions: the Congo Basin, the Amazon Basin, and the Greater Mekong Basin in Asia (Fig. 1). In these regions, the main mosquitoes that transmit malaria are: Anopheles gambiae, Anopheles funestus, Anopheles dirus, Anopheles minimus and Nyssorhynchus darlingi. All of them associated with deforestation, land use changes and human migration.

Distribución malaria Africa America asia

Fig. 1. Map of the regions with populations at risk of contracting malaria in yellow. Striped areas represent the areas that are suffering the most deforestation. Simplified map of Chaves et al. 2020. Nature Communications 11: 1258. Source: Mosquito Alert (CC-BY-NC-2.0)

Demand for basic products from rich countries increases the risk of malaria in other countries

The link between deforestation and the increase in malaria cases has been demonstrated in Indonesia, Nigeria and the Amazon. A work carried out in 795 municipalities of the Amazon during 13 years, concluded that clearing the forests by 10% leads to a 3.3% increase in malaria cases.

The new study reconnects the incidence of malaria with deforestation, but also establish a relationship between deforestation with the consumption of products globally. For this they used a detailed international database with the entry and exit of products by country. With it they were able to establish the global network of supplies, from the deforested area to produce, to the countries that consume the product.

Countries with an increased risk of malaria associated with deforestation are Nigeria, Tanzania, Cameroon, Uganda, the Democratic Republic of the Congo, India, Zambia, Burma, the Central African Republic and Burundi. Part of the increased risk in these areas is due to the demand for products from the richest countries, with Germany, the United States, Japan, China, the United Kingdom, France, Italy, Spain, the Netherlands and Belgium leading the list. The demand for wood, cocoa, tobacco and cotton are among the main causes of deforestation.

Populations most exposed to malaria are those that benefit the least economically from forest exploitation

The work is new evidence of the close relationship between the health of human populations and the environment, and shows, with numbers, that the problem is global. The authors hope that their results can be used to mitigate malaria cases, either by regulating supply chains or by labeling and certifying products.

Communities that are facing a greater transformation of their landscape to meet international demands are at the same time the most at risk as they are more exposed to mosquitoes that transmit malaria. In addition, the populations most exposed to the disease are those that benefit least from the exploitation of forests.


References

Austin K, Bellinger M, Rana P. 2017. Anthropogenic forest loss and malaria prevalence: a comparative examination of the causes and disease consequences of deforestation in developing nations. AIMS Environmental Science 4: 217-231

Berazneva J, Byker TS. 2017. Does forest loss increase human disease? Evidence from Nigeria. American Economic Review 107: 516-521

Chaves LSM, Fry J, Malik A, Geschke A, Salud MAM, Lenzen M. 2020. Global consumption and international trade in deforestation-associated commodities could influence malaria risk. Nature Communications 11: 1258

Garg T. 2019. Ecosystems and human health: the local benefits of forest cover in Indonesia. Journal of Environmental Economics and Management 98: 102271

MacDonald AJ, Mordecai EA. 2019. Amazon deforestation drives malaria transmission, and malaria burden reduces forest clearing. PNAS 116: 22212-22218

 

Who eats mosquitoes?

There is no summer without mosquitoes. There are many summer nights in which as soon we turn off the light we hear the unmistakable hum of the common mosquito. It is the announcement of a bite, taking over from species like the tiger mosquito that bite us during the day. Mosquitoes are not only annoying but have a great impact on the health and economy of societies by transmitting a large number of diseases. Malaria, dengue fever, yellow fever, Zika, chikungunya or West Nile fever are some of the diseases that affect millions of people in the world and cause more than half a million deaths each year.

From a human perspective, mosquitoes are nothing more than a plague that feed on our blood, but there are other animals in nature that do not see them that way, but as part of their diet. Although the idea of ​​using these mosquito predators to control their populations is attractive, in reality, they rarely help to control them effectively.

Mosquitoes have many natural enemies, but do they help us control them effectively?

Throughout their life cycle, mosquitoes are exposed to a large number of predators, those that prey on larvae, pupae or adults. The natural enemies of mosquitoes change from one habitat to another, from aquatic to terrestrial environments. Many groups of animals feed on mosquitoes, among which we find arachnids, crustaceans, fish, amphibians, birds and even mammals. Despite the large number of species that include them in their diets, it is very difficult to assess their true impact on mosquito populations.

Something understandable if we consider that there is no predator that completely eliminates its prey. If he did, he would be putting his own livelihood at risk. Predators and prey usually establish a dynamic balance between the two. To this we must add the enormous demographic capacity of mosquitoes that allows them to compensate for the losses suffered by predators. The growth of their populations is such that if we eliminate 50 of 100 mosquitoes we would not notice any change. Possibly we would not perceive changes in the abundance of mosquitoes if we were able to kill 90 of them, it is estimated that to perceive a change in the number of mosquitoes it is necessary to suppress more than 95% of them.

 

Predators of their larvae

In aquatic environments it is where they find a greater number of predators. Among them are mosquito fish, both Gambusia affinis and Gambusia holbrooki, both species native to North America. These fish have been introduced worldwide with the idea of ​​controlling mosquito pests. In part they did, at the beginning of the 20th century they were used in the Mediterranean rice fields where they managed to eliminate many mosquitoes from the Anopheles group, thus controlling malaria. However, studies on the diet of these fish carried out in Catalonia show that in environments of high diversity, mosquitoes are a small part of their diet and are therefore not very effective in their control. In fact, its great voracity and its reproductive potential have generated an ecological disaster by displacing native fish species and preying on native amphibians and insects, thus destroying the trophic networks of aquatic environments. Today, both species are included in the list of the 100 most harmful invasive alien species in the world and, in Spain, their possession, commercialization or manipulation by the Law of Protection of Natural Heritage and Biodiversity is prohibited.

 

Beyond fish, mosquito larvae and pupae find large predators among insects. Among them are the Notonectidae, popularly known as backswimmers, and the aquatic beetles (Ditiscidae). Larvae of dragonflies and damselflies also hunt mosquito larvae. In more eutrophized environments, copepods, small crustaceans that can occupy freshwater environments, include mosquito larvae in their diets. The use of copepods as a biological control worked in a region of Vietnam to reduce populations of the yellow fever mosquito, Aedes aegypti, and thus the transmission of dengue.

mosquito alert depredadores naturales mosquitos

Fig. 1. [1] Larva of Toxorhynchites sp. that predates on larvae of Aedes aegypti. [2] Fish, some species of fish reduce the number of mosquito larvae. [3] Heteroptera, such as notonectidae, are small predators of mosquito larvae. [4] Dragonfly larva, another predator of mosquito larvae in some environments. [5] Amphibian tadpoles, some species also prey on mosquito larvae. Source: Mosquito Alert CC-BY

 

Other works have studied the potential of amphibians as predators of mosquitoes, particularly frogs and toads. It has been seen that tadpoles of various species of Sri Lanka fed on the larvae of Aedes aegypti and were able to reduce their populations. However, similar experiments carried out in Thailand with local species did not produce satisfactory results. Even demonstrating its effectiveness, control with amphibians, as with fish, is impossible in urban and home environments where Aedes mosquitoes occupy small water spaces.

Mosquitoes eating mosquitoes

A highly researched group is the Toxorhynchites mosquito larvae, known as the elephant mosquito, large mosquito larvae that consume larvae from other mosquitoes. The good thing is that the Toxorhynchites are non-hematophagous mosquitoes, that is, they do not feed on blood and, on the other hand, they do kill mosquito larvae of sanitary interest such as the Aedes. Under laboratory conditions, carried out in the Philippines, it has been observed that Toxorhynchites can consume half of the mosquito larvae of Aedes aegypti, Aedes albopictus and Culex quinquefasciatus. But the continuous release of Toxorhynchites larvae in bamboo areas in Indonesia have failed to reduce Aedes aegypti populations.

In most of the cases cited, the experienced predatory species are tropical, exotic species for Europe, so it would be irrational and illegal to introduce them, however effective they might be: mosquito fish have already caused major ecological disasters to generate one again by introducing new exotic species. Here we see the biggest drawback of the so-called “biological control.” The effective species, generally, are not native species but species that we have to introduce, since the native species have been living with the mosquitoes for centuries or thousands of years without having extinguished them. A predator ending its prey would face a big problem. It would be as if we ate all the chickens on the farm without leaving layers … bread for today, hunger for tomorrow.

Much of the species that effectively prey on mosquitoes are tropical, exotic species in Europe, where it is illegal to introduce them

 

Swifts, swallows and bats: do they eat many mosquitoes?

When mosquitoes fly they also expose themselves to several predators, including dragonflies that feed on all kinds of flying insects not focusing on mosquitoes. Even so, it has been estimated that some dragonflies are capable of hunting between 30 and hundreds of daily mosquitoes. Everything and this ability to hunt, dragonflies can reduce their populations a bit but not solve, much less, the problem. We are talking, perhaps, of many thousands of mosquitoes flying in our garden.

Among the birds we also find mosquito predators. The species that eat the greatest number of mosquitoes are: the common house martin (Delichon urbica), the meadow pipit (Anthus pratensis), the European pied flycatcher (Ficedula hypoleuca), the swallows (Hirundo rustica) or the common swift (Apus apus).

If a swift or swallow had to feed only on mosquitoes, it would need to hunt 14,000 mosquitoes daily

It is quite common to think that having these birds near you frees you from mosquitoes, given their ability to hunt them, the reality is different. If a swift, common house martin or swallow had to feed only on mosquitoes it would require ingesting about 14,000 mosquitoes daily. The same amount of energy can be obtained by capturing a dozen beetles. Pursuing and hunting mosquitoes is a great expense of energy and time that is not very important to them, so that mosquitoes are occasional prey but not the basis of their diet.

Fig. 2. A spider male, Icius hamatus, capturing a tiger mosquito in a garden in Barcelona. Photographed sent to Mosquito Alert by Antonio Piñera. Source: Antonio Piñera CC-BY

 

Something similar happens with bats despite the popular belief that they are large predators of mosquitoes. The reality is very different and especially complex. Bat prey varies depending on the size of the bats themselves. Large species feed on large insects, while small species do on smaller insects, including some mosquitoes. The main and most important prey for all bats are moths. If you are a bat that is spending energy flying after the prey, what would you choose: a moth, equivalent to a 500-grilled steak, or a mosquito, which would become a 5-gr minihamburger.

 Spiders stalk mosquitoes in the shade

Beyond the active flight period, adult mosquitoes spend much of their time resting among the vegetation. While resting they are exposed to a large number of predators, where we find spiders among them the most effective. A large number of spider species include mosquitoes in their diets, both those that build cobwebs and those that don’t.

mosquito alert depredadores naturales mosquitos

Fig. 3. [1] Bats, some species can actively prey on adult mosquitoes. [2] Swallows and swifts are also natural predators of mosquitoes. [3] Geckos, also occasionally prey on mosquitoes. [4] Spiders prey on mosquitoes in their shelters. [5] Other types of reptiles can sometimes prey on mosquitoes. Source: Mosquito Alert CC-BY

 

Recent studies show that predators not only have a direct effect reducing the number of mosquitoes, but altering their behavior, is what biologists call: landscape of fear. Thus it has been seen that aquatic environments with a greater number of potential predators are avoided by females to deposit eggs.

The most effective method to reduce the number of mosquitoes at home is still to prevent them from having to reproduce

Mosquito populations depend on numerous factors, including landscape (urban or rural), abiotic or climatic factors such as rain or temperature, and biotics such as trophic networks with the predators that have been described. All these factors are in turn related, so that the landscape affects the species that could prey on mosquitoes. In urban environments, the natural enemies of mosquitoes often do not survive, making them a space free of enemies for mosquitoes.

Despite having a large number of animals capable of preying on mosquitoes, the most effective method to control them on our properties is still to avoid favorable habitats. Mainly, avoid providing small water points that can be used to reproduce. If you do not want to contribute to a new generation of mosquitoes, avoid the accumulation of water in your spaces, because in the water of your vase there is no predator that lives. Just mosquitoes.

References:

Benelli G, Jeffries CL, Walker T. 2016. Biological control of mosquito vectors: past, present, and future. Insects 7: 52-70

Bowatte G, Perera P, Senevirathne G, Meegaskumbura S, Meegaskumbura M. 2013. Tadpoles as dengue mosquito (Aedes aegypti) egg predators. Biological Control 67: 469-474

Cirinio E. 2016. Can birds survive without mosquitoes? Audobon News March 10, 2016

Digma JR, Sumalde AC, Salibay CC. 2019. Laboratory evaluation of predation of Toxorhynchites amboinensis (Diptera: Culicidae) on three mosquito vectors of arboviruses in the Philippines. Biological Control 137: 104009

García-Berthou E. 1999. Food of introduced mosquitofish: ontogenic diet shift and prey selection. Journal of Fish Biology 55: 135-147

Gonsalves L, Bicknell B, Law B, Webb C, Monamy V. 2013. Mosquito consumption by insectivorous bats: does size matter? PLoS One 8: e77183

Kumar R, Hwang JS. 2006. Larvicidal efficiency of aquatic predators: a perspective for mosquito biocontrol. Zoological Studies 45: 447-466

Nam VS, Yen NT, Phong TV, Ninh TU, Mai LQ, Lo LV, Nghia LT, Bektas A, Briscombre A, Aaskov JG, Ryan PA, Kay BH. 2005. Elimination of dengue by community programs using Mesocyclops (Copepoda) against Aedes aegypti in central Vietnam. American Journal og Tropical Medicine and Higiene 72: 67-73

Ndavas J, Llera SD, Manyanga P. 2018. The future of mosquito control: the role of spiders as biological control agents: a review. International Journal of Mosquito Research 5: 6-11

Shaukat MA, Ali S, Saddiq B, Hassan MW, Ahmad A, Kamran M. 2019. Effective mechanisms to control mosquito borne diseases: a review. American Journal of Clinical Neurology and Neurosurgery 4: 21-30

Staats EG, Agosta SJ, Vonesh JR. 2016. Predator diversity reduces habitat colonization by mosquitoes and midges. Biology Letters 12: 20160580

Weterings R, Umponstira C, Buckely HL. 2018. Landscape variation influences trophic cascades in dengue vector food webs. Science Advances 4: eaap9534

Bloodthirsty: the other reason why a mosquito bites

Mosquito females are the only ones that bite, they do it to acquire protein to the development of their eggs. Yes, our blood serves to give rise to a new generation of mosquitoes. But recent work suggests that blood can also act as a snack for mosquitoes in dry and warm periods.

The study, published in Scientific Reports, has found that mosquitoes exposed to dry environments, with a higher level of dehydration, are more aggressive than those that are well hydrated. Dehydrated mosquitoes venture more to land on a host, bite and feed on blood more often than the others. Scientists believe that during periods of drought the risk of disease transmission can increase. More bites more risk of transmission.

The idea that in the dry periods there may be more cases of infection seems contradictory given the great dependence of mosquitoes on water. Most of them lay their eggs in wetlands where the larvae develop. Thus, the number of mosquitoes depends on weather, increasing their number after rains, once there is stagnant water in abundance where they can reproduce. This relationship between climatic conditions and the number of mosquitoes has also been established among diseases transmitted by mosquitoes. One expects that more rain implicates more mosquitoes and more diseases transmitted. But the data don’t always confirm these expectations.

Droughts cause major epidemic episodes of West Nile fever

In the case of West Nile fever, it has been seen that epidemic episodes are greater in the years of drought. The authors believe that their observations may explain why sometimes epidemics occur during periods of drought.

To study how droughts and dehydration alter the physiology and behavior of mosquitoes, they designed an experiment that they tested on three mosquito species. The species evaluated were: (1) Culex pipiens, the common mosquito that could transmit the West Nile virus in the United States (2) Aedes aegypti or yellow fever mosquito, which can also transmit dengue, chikungunya and fever from Zika, and (3) Anopheles quadrimaculatus, a mosquito from the North American Atlantic coast capable of transmitting malaria.

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Fig. 1. Mosquitoes were offered an artificial host made with a collagen membrane, which contained chicken blood to study the biting rate at different levels of dehidration. Figure based on the original: Hagan et al. (2018) Scientific Reports 8: 6804. Source: Mosquit Alert (CC-BY-NC-2.0)

 

The researchers exposed hundreds of mosquitoes of each species to different temperature and humidity conditions to generate individuals with different levels of dehydration. In the experiment they included another factor, as some mosquitoes had access to water or nectar with which to hydrate while others had no access to any type of liquid. After some time in these conditions, its effect on the mosquito behavior was analyzed. For this, the mosquitoes were offered an artificial host made with a collagen membrane at a temperature of 37 °C, covered with artificial sweat to attract mosquitoes, which contained chicken blood (Fig. 1).

 

Thirsty mosquitoes’ bite more

When quantifying the times, they landed on the artificial host and fed on it; they saw that the number of bites was higher among dehydrated mosquitoes that also had no access to water or nectar. Only 10 percent of the mosquitoes with access to water landed and bite the host, while among the mosquitoes deprived of water was 30 percent. Mosquitoes in conditions that recreated a drought bite much more than the other mosquitoes (Fig. 2).

The sensors available to mosquitoes to detect a host, make it easier for them to detect a host than to locate a water point with which to hydrate when they are thirsty. The highest rates of West Nile virus transmission observed during droughts could be due to mosquitoes using blood to replace the water they lose.

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Fig. 2. Mosquitoes in condition that recreated a drough with higher levels fo dehidration bite much more than well hidrated mosquitoes. Figure based on the original: Hagan et al. (2018) Scientific Reports 8: 6804. Source: Mosquit Alert (CC-BY-NC-2.0)

 

Knowing the climatic conditions that alter mosquito bite behavior has practical applications as it can be incorporated into epidemiological mathematical models. The abundance of mosquitoes is an important factor to consider in these models, but studies like that one show that environmental factors alter mosquito-human interaction so that the risk of transmission can vary with weather.

The increase in bites to quench thirst is not the only mechanism that can explain the relationship between drought periods and epidemics. Other studies have found that during droughts there are fewer wetlands and resources, which helps animals and mosquitoes come into contact more easily, favoring disease transmission. In addition, in the few existing wetlands there is a greater concentration of nutrients necessary for larval development. In these temporary and ephemeral ponds the proliferation of mosquitoes is favored by the absence of large predators, which inhabit permanent aquatic environments. The periods of heat and drought not only affect predators of aquatic environments but also the potential predators of adult mosquitoes, allowing their populations to be larger.

In short, that periods of drought can favor the proliferation of some species of mosquitoes, and make that mosquitoes become more aggressive. The increase in temperatures caused by global warming could lead to more frequent and longer periods of drought in areas where mosquitoes are a threat to human health.

 

References:

Barnard DR, Dickerson CZ, Murugan K, Xue RD, Kline DL, Bernier UR. 2014. Measurement of landing mosquito density on humans. Acta Tropica 136: 58-67

Chase JM, Knight TM. 2003. Drought-induced mosquito outbreaks in wetlands. Ecology Letters 6: 1017-1024

Hagan RW, Didion EM, Rosselot AE, Holmes CJ, Siler SC, Rosenlade AJ, Herdershot JM, Elliot KSB, Jennings EC, Nine GA, Perez PL, Rizlallah AE, Watanabe M, Romick.Rosendale LE, Xiao Y, Rasgon JL, Benoit JB. 2018. Dehydration prompts increased activity and blood feeding by mosquitoes. Scientific Reports 8: 6804

Paull SH, Horton DE, Ashfaq M, Rastogi D, Kramer LD, Diffenbaugh NS, Kilpatrick AM. 2017. Drought and immunity determine the intensity of West Nile virus epidemics and climate change impacts. Proceedings of the Royal Society B 284: 20162078

Paz S. 2015. Climate Change impacts on West Nile virus transmission in a global context. Philosophical Transactions Royal Society B 370: 20130561

Paz S, Malkinson D, Green MS, Tsioni G, Papa A, Danis K, Sirbu A, Ceianu C, Katalin K, Ferenczi E, Zeller H, Semenza JC. 2013. Permissive summer temperatures of the 2010 European West Nile fever upsurge. PLoS One 8: e56398

Romano D, Stefanini C, Canale A, Benelli G. 2018. Artificial blood feeders for mosquitoes and ticks – where from, where to? Acta Tropica 183: 43-56

Shaman J, Day JF, Stieglitz M. 2005. Drought-induced amplification and epidemic transmission of West Nile virus in southern Florida. Journal of Medical Entomology 42: 134-141

Wang G, Minnis RB, Belant JL, Wax CL. 2010. Dry weather introduces outbreaks of human West Nile virus infection. BMC Infectious Diseases 10: 38

 

How a mosquito sneaks after it steal your blood

Imagine you sneak into a banquet. A huge one. Where there is plenty of food everywhere. You eat, eat and eat. You eat so much that without realizing it you have doubled or tripled your weight. You can’t anymore It’s time to sneak out, you think, but do you think you could do it? You have doubled your weight, you will be slow, you would move dragging the belly, the most possible is that you collapsed. But if you were a mosquito, things would be completely different. If you were a mosquito, you would take off without problems after the feast, and most important of all, you would fly without being detected.

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