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.
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.
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.
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