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Throughout human history, a tiny insect has influenced wars, shaped settlement patterns, halted major engineering projects, and caused more deaths than any other animal.
For most of history, no one understood why these creatures were so dangerous, or even that they were the things responsible for spreading disease.
Eventually, we learned how to fight back, but our efforts were middling at best.
Learn more about mosquitoes, how they became one of humanity’s greatest biological enemies, and how we learned to fight them, on this episode of Everything Everywhere Daily.
The deadliest animal in history hasn’t been sharks, tigers, or venomous snakes. It has been the tiny mosquito. Mosquitoes seldom kill directly. Their lethality comes from their role as disease vectors.
Mosquitoes can be found almost everywhere, from the tropics to the Arctic. Some species are remarkably cold-tolerant and survive harsh winters as eggs, larvae, or dormant adults. Others thrive in tropical climates.
Biologically, mosquitoes are insects belonging to the order Diptera, the group of true flies. Diptera literally means “two wings,” because members of the group possess a single functional pair of wings.
Mosquitoes belong to the family Culicidae, which contains more than 3,500 known species. Within Culicidae, mosquitoes are divided into several subfamilies and many genera. The overwhelming majority of species important to human health belong to three genera: Anopheles, Aedes, and Culex.
Anopheles mosquitoes are best known for transmitting malaria. Aedes mosquitoes include species such as Aedes aegypti and Aedes albopictus, which can transmit yellow fever, dengue, and Zika. Culex mosquitoes are important vectors of West Nile virus and Japanese encephalitis virus.
Despite their reputation as bloodsuckers, mosquitoes primarily survive on plant sugars. Both males and females normally feed on nectar and other sugary plant fluids.
Male mosquitoes do not drink blood at all. In most blood-feeding species, females take blood meals because they need proteins and other nutrients to produce eggs.
There are exceptions, and some mosquito species can reproduce without taking blood, but the familiar mosquito bite is almost always produced by a female preparing to reproduce.
The itching and swelling associated with mosquito bites are not primarily caused by the physical puncture of the skin. They are an immune reaction to proteins in the mosquito’s saliva. The body recognizes these proteins as foreign and releases compounds, including histamine, that produce the familiar redness, swelling, and itching.
Mosquitoes have a very short life cycle. Male mosquitoes typically live shorter lives than females, sometimes only a week or two, while females of some species can survive for several weeks or considerably longer under favorable conditions.
The mosquito life cycle consists of four stages: egg, larva, pupa, and adult. The first three stages normally occur in water.
After mating and, for many species, obtaining a blood meal, a female searches for an appropriate place to lay her eggs. Different mosquito species have evolved dramatically different preferences. Some deposit eggs directly on standing water. Others place eggs on damp soil or vegetation where flooding will later cause them to hatch.
Basically, almost anywhere there is standing water has the potential for mosquito eggs to be laid.
When the eggs hatch, mosquito larvae emerge. The larvae are sometimes called wrigglers because of the way they move through water. They feed on microorganisms, algae, bacteria, and bits of organic matter.
Many mosquito larvae breathe atmospheric oxygen forcing them to return frequently to the surface.
As larvae grow, they molt several times and they eventually enter the pupal stage. Mosquito pupae, sometimes called tumblers, do not feed, but unlike many other insects, they are highly mobile. They remain aquatic while the adult mosquito develops inside.
The word mosquito comes from the Spanish mosca, which means fly, and the suffix -ito, which means little.
The relationship between humans and mosquitoes goes back as far as history itself, even if for much of that time they were unaware that it was mosquitoes that were causing a problem.
For most of history, humans believed epidemics arose from environmental corruption, divine punishment, imbalances within the body, or poisonous vapors known as miasmas. Ironically, attempts to avoid swamp air sometimes reduced mosquito exposure without anyone understanding why.
The ancient Greeks recognized an association between fevers and swampy environments, although they did not understand mosquitoes or parasites. Medical writers described recurring fevers that closely resembled malaria.
Malaria profoundly affected some ancient societies. Marshy regions around Rome were notorious for fevers. Seasonal outbreaks reduced agricultural productivity, killed travellers, and contributed to the abandonment of some low-lying areas.
The disease was so strongly associated with marshes and foul-smelling air that Italians eventually used the term mal-aria, which was usually interpreted to mean “bad air.”
Mosquitoes have repeatedly influenced warfare. Historically, armies entering tropical or malarial environments sometimes lost enormous numbers of soldiers to disease. Military commanders might win battles yet lose campaigns because mosquito-borne illness debilitated their forces.
The first major breakthrough came in 1880 when the French military physician Alphonse Laveran observed malaria parasites in the blood of infected patients. This, for the first time, demonstrated that malaria was caused by a living organism.
The next crucial discovery came through the work of the British physician Ronald Ross. In 1897, while working in India, Ross showed that mosquitoes transmitted malaria parasites. Italian researchers, including Giovanni Battista Grassi, subsequently established that human malaria was specifically transmitted by Anopheles mosquitoes.
At roughly the same time, researchers were solving another mosquito mystery involving yellow fever.
Yellow fever had repeatedly devastated cities and armies in tropical and subtropical regions. Victims could suffer fever, internal bleeding, and vomiting of blood.
During the eighteenth and nineteenth centuries, yellow fever periodically swept through American cities including Philadelphia, New Orleans, Charleston, and Memphis. The Philadelphia epidemic of 1793 killed thousands and caused much of the population, including members of the federal government, to flee the city.
Yellow fever was also one of the greatest obstacles to European military operations in the Caribbean. During Napoleon’s attempt to suppress the Haitian Revolution, disease killed far more French soldiers than combat did. Similar problems confronted workers during early French attempts to build the Panama Canal.
A Cuban physician, Carlos Finlay, proposed in 1881 that mosquitoes transmitted yellow fever. Much of the medical community initially dismissed or ignored his hypothesis. Nearly two decades later, the Yellow Fever Commission led by Walter Reed provided strong evidence supporting mosquito transmission.
Once this mechanism became accepted, disease prevention changed dramatically.
During the American construction of the Panama Canal, sanitary engineer William Gorgas organized a massive mosquito-control program. Workers drained standing water, fumigated buildings, improved drainage, and treated breeding sites with oil or other substances that killed mosquito larvae.
Yellow fever was effectively eliminated from the Canal Zone, and malaria deaths fell sharply as well. This achievement demonstrated that mosquito control could transform areas previously considered almost impossible for large numbers of outsiders to inhabit safely.
The twentieth century introduced chemical mosquito control on an unprecedented scale.
One of the most famous chemicals was DDT, or di-chloro-di-phenyl-trichloro-ethane. Although first synthesized in the nineteenth century, the Swiss chemist Paul Hermann Müller discovered its powerful insecticidal properties in 1939.
DDT is a contact insecticide that disrupts a mosquito’s nervous system by interfering with sodium channels in nerve cells. This causes uncontrolled nerve firing, paralysis, and eventually death.
DDT proved extraordinarily effective against mosquitoes and other insects. During World War II, Allied forces used it extensively to control malaria and insect-borne typhus. Buildings and entire communities were treated with DDT.
However, widespread agricultural use of DDT created major environmental problems. The chemical persists in ecosystems and accumulates through food chains. It became particularly notorious for causing eggshell thinning in predatory birds.
Rachel Carson’s 1962 book Silent Spring drew widespread attention to the environmental effects of indiscriminate pesticide use, including DDT. Many countries subsequently restricted or banned agricultural DDT use.
Modern mosquito control uses a combination of approaches rather than relying on a single weapon.
One strategy is source reduction, meaning eliminating breeding habitat. Communities remove discarded containers, improve drainage, cover water-storage tanks, and reduce unnecessary standing water.
Larvicides can be applied to water that cannot easily be removed. One widely used biological larvicide is produced by the bacterium Bacillus thuringiensis israelensis, commonly abbreviated as Bti.
The bacterium produces toxins that kill mosquito larvae after they ingest them, with relatively limited effects on many other organisms when used appropriately.
Adult mosquitoes can be targeted with insecticides delivered through trucks, aircraft, and handheld sprayers. Modern mosquito-control programs try to use these methods selectively because insecticides can affect other insects and because mosquito populations can evolve resistance.
Insecticide resistance has become one of the greatest challenges in mosquito control. Mosquitoes reproduce quickly, and intense pesticide use creates powerful evolutionary pressure. Individuals carrying mutations that help them survive insecticides reproduce, gradually making the population more resistant.
Modern repellents provide another layer of personal protection. DEET, developed by the United States military after World War II, became one of the most widely used mosquito repellents. Repellents work largely by interfering with the mosquito’s ability to detect or recognize a host rather than simply poisoning the insect.
Another mosquito control technique is the sterile insect method. Large numbers of male insects are sterilized, traditionally using radiation, and released into the wild. Females that mate with sterile males produce no viable offspring, potentially reducing the population.
This works extremely well, but only for short periods. Mosquitoes reproduce quickly, so even if you reduce the population by 99%, it can bounce back to previous levels in just a few months.
An even more powerful and controversial possibility involves gene drives. Normally, an organism has roughly a 50 percent chance of passing a particular version of a gene to its offspring. In particular, the sex of the offspring.
Gene-drive systems can dramatically increase that probability. A mutated or modified mosquito could, in theory, be released that would only create other male mosquitoes, which in turn could only create other male mosquitoes.
As the population of these modified mosquitoes increased, the percentage of female mosquitoes would decrease, eventually eliminating the entire species, or at least within a certain area. Despite how annoying mosquitoes are, there are serious concerns about completely eliminating any species.
If you have ever experienced mosquitoes, you might have found yourself wondering exactly what functions mosquitoes actually serve. Many species do not bite humans, and most do not transmit diseases.
Mosquito larvae form part of aquatic food chains, consuming microorganisms and organic matter while themselves providing food for insects, fish, and other organisms.
Adult mosquitoes can serve as prey for spiders, dragonflies, birds, bats, and other animals. Because both males and females often consume nectar, mosquitoes also contribute to plant pollination.
The benefits of mosquitoes can be hard to appreciate when you are constantly slapping yourself and getting bitten.
Mosquitoes may be small, but few animals have had a greater impact on human history. They have spread diseases that shaped wars, limited settlement, and killed millions, while forcing humans to develop countless techniques to fight them.
The battle against mosquitoes continues and it, quite probably, it will never fully end.