control

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

Coordination between epidemiologists, public health professionals, entomologists and microbiologists: key for minimizing the risk of Zika transmission in Barcelona

  • The Barcelona Public Health Agency (ASPB) has published a sociodemographic, epidemiological, clinical study which also recounts mosquito control efforts carried out following detections of Zika in the city since 2016.
  • To date, no cases of Zika transmission have been documented in Barcelona, but this does not mean that the risk does not exist. With this in mind, the ASPB has made improvements to the response protocol to further reduce any risk of transmission.
Mosquit tigre. Foto: Pixabay CC0 PD

Tiger mosquito. Photo: Pixabay CCO PD

 

Since the first detection of Zika in Spain in December 2015, the Barcelona Public Health Agency has included vigilance for this virus in its Tiger Mosquito Control and Monitoring Program, and carries out comprehensive studies of each detected case. As explained in the study published in the journal Frontiers in Microbiology, monitoring for the disease and the insect that transmits it are essential for preventing the local transmission of arboviruses – the group of viruses transmitted by arthropods – helping to avoid dire impacts on public health. This is made possible by good coordination among epidemiologists, clinicians, entomologists and microbiologists.

In February 2016, the World Health Organization (WHO) declared Zika as an emerging disease threatening public health worldwide as a result of its rapid expansion and associated illnesses which include Guillain-Barré syndrome and microcephaly in newborn babies. In fact, Spain is one of the European countries with the highest risk of local transmission of Zika; in a city like Barcelona, ​​where the tiger mosquito is established and there are large fluxes of human migration, tourism and global trade, the risk continues to increase.

More travelers from Zika-affected countries

Since 2000, the number of people of Latin American origin residing in Spain has increased considerably. Countries including the Dominican Republic, Venezuela, Nicaragua and Colombia are those most visited by travelers originating in Barcelona, whatever the motives for the visit (to visit family, work, for tourism), and these are precisely the places where the Zika virus is endemic. With this significant movement of humans, it is only more likely that the disease will be imported.

In Spain, all cases of Zika, Chikungunya and Dengue infections are reported to the public health authorities and to the main epidemiology units. In 2016, there were 118 cases of Zika in Barcelona, ​​all of which were due to infections which occurred outside of Spain. Zika cases are classified as probable, confirmed, imported or local depending on the patient’s symptoms in addition to other clinical and socio-demographic data (age, sex, country of origin, etc.).

Esquema dels passos protocolaris que es segueixen quan es detecten casos importats de Zika. Font: ASPB

Scheme of the protocolary steps followed when imported Zika cases are detected. Source: ASPB

 

The most effective measure to avoid local transmission of Zika is to control the vector

Zika is a virus that is transmitted between infected and healthy people through the bite of the yellow fever mosquito – currently not present in Spain – or the tiger mosquito. The Mediterranean is vulnerable to local transmission since the tiger mosquito is present or established in many areas. It is therefore essential to set up surveillance and prevention protocols to avoid local transmission, using individual and community prevention measures to prevent mosquito breeding and biting. In fact, the published study shows that the periods of year with the most mosquitos – April, August and September – coincide with number of imported Zika cases and therefore a greater risk of local transmission.

In the case of an infection, the city’s epidemiology service instructs the patient on the use of preventive measures to reduce additional infections. The goal is to minimize the risk of Zika transmission through mosquito bites in the area where the infected person lives. At the same time, inspections are carried out to detect if there are mosquitoes or mosquito breeding sites either in the infected person’s home or in the surrounding neighborhood. In most cases, insecticidal treatments are applied in areas with standing water in order to kill any larvae.

Mapa de la ciudad de Barcelona con la localización de los diferentes casos de arboviosis y los resultados de las inspecciones entomológicas. "Risk zonas" (azul): inspecciones mensuales. "Citizen incidencia" (verde): avisos hechos por la ciudadanía que se han hecho durante el estudio de 2016.

Map of the city of Barcelona, with the location of different cases of arbovirosis and entomology inspeccions. “Risk zones” (blue): monthly inspections. “Citizen incidences” (green): reports made by citizen during 2016.

 

In relation to such preventive measures, new technologies have sweep areas for tiger mosquito presence. Specifically, citizen participation with the Mosquito Alert app has led to the detection of breeding sites close to the homes of infected persons, and this technology has facilitated vector surveillance and control in the city. During 2016, the Barcelona Public Health Agency made entomological inspections at 19 homes of people infected by Zika, 34 public spaces, and carried out 134 follow-up and control tasks associated with cases of imported Zika.

In short, the Barcelona Arbovirus Surveillance Program is an example of how reducing the risk transmission arboviruses such as Zika should be tackled in a multidisciplinary manner. The study was carried out by the principal Spanish public health entities as well the Public Health Agency of Barcelona, ​​CIBER Epidemiology and Public Health, Lokímica Laboratories, Doñana Biological Station, The Microbiology Department of Vall de Hebron Hospital, Hospital Clínico of Barcelona, ​​ISGlobal, and the Mosquito Alert community as part of the Barcelona Zika Working Group.

Renferenced article:

Millet, J. P., Montalvo, T., Bueno, R., Romero-Tamarit, A., Prats-Uribe, A., Fernandez, L., … & Zika Working Group in Barcelona (2017). Imported Zika Virus in a European city: how to prevent local transmission?. Frontiers in Microbiology, 8, 1319.

http://journal.frontiersin.org/article/10.3389/fmicb.2017.01319/full

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