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Vector control is any method used to limit or eradicate the organisms that transmit disease-causing agents (pathogens) to humans, animals, or plants. In public health, the term mainly refers to the control of blood-feeding arthropods, such as mosquitoes, ticks, sandflies, tsetse flies, black flies, triatomine bugs, fleas, and lice, and also to some snails that host schistosome parasites and to rodents that carry pathogens or host fleas. Diseases spread by vectors include malaria, dengue fever, Zika fever, chikungunya, yellow fever, West Nile fever, lymphatic filariasis, onchocerciasis, leishmaniasis, Chagas disease, Lyme disease, plague, and African trypanosomiasis. According to the World Health Organization (WHO), vector-borne diseases account for about 17 percent of the global burden of infectious disease and cause more than 700,000 deaths each year, most of them from malaria.

Vector control has a long record of success. The elimination of urban yellow fever from Havana and the Panama Canal Zone in the early 1900s, the near-eradication of malaria from Europe and North America in the mid-twentieth century, and the large decline of malaria in Africa from 2000 to 2015 all relied heavily on it. It is also under pressure from insecticide resistance, funding gaps, urbanization, climate change, and the spread of invasive vectors such as Aedes albopictus.

Vectors and transmission

In vector control, a vector acquires a pathogen when it feeds on an infected host, the pathogen develops or multiplies in the vector, and the vector later transmits it by bite, by feces, or by other contact. In biological transmission, the interval between acquisition and the ability to transmit is the extrinsic incubation period (EIP), during which the vector must survive. Control methods can therefore act on the vector's numbers, its survival, its contact with people, or its ability to carry the pathogen.

Quantifying the effect of control

The Ross-Macdonald model of vector-borne transmission shows why some vector control measures work better than others. In its classical form:

where m is the number of vectors per host, a is the biting rate, b and c are the transmission probabilities from vector to host and from host to vector, μ is the daily vector mortality, n is the EIP in days, and r is the host recovery rate. The vector-specific part is the vectorial capacity:

Three features follow. The biting rate is squared, so reducing contact between vectors and people has a large effect. Vector density appears to the first power in V, and only as a square root in R₀, so halving density reduces R₀ by only about 29 percent. Mortality appears in an exponent. For example, if daily survival falls so that μ rises from 0.1 to 0.2 and the EIP is 10 days, the survival through the EIP falls from e⁻¹ ≈ 0.37 to e⁻² ≈ 0.14, and V falls by a factor of about 5 once the change in the denominator is included. This is the reason that measures that kill adult vectors, especially older ones, are effective for pathogens with a long EIP, such as malaria parasites. The herd-level target for a transmission-blocking measure is the reduction needed to bring R₀ below 1, so that a measure with efficacy E needs E > 1 − 1/_R_₀ in a simple model.

Control of vectors began before the vectors were understood. Drainage of marshes was used in antiquity to reduce "bad air" (mal aria), and bed nets and smoke were used in many cultures. The proof that mosquitoes transmit malaria (Ronald Ross and Giovanni Battista Grassi, 1897 to 1898) and yellow fever (Carlos Finlay and the Walter Reed commission, 1881 to 1901) led to targeted campaigns. William C. Gorgas controlled mosquitoes in Havana and the Panama Canal Zone by eliminating breeding sites, using larvicides, and screening houses. Fred Soper and the Rockefeller Foundation ran campaigns against Aedes aegypti in the Americas in the 1930s to 1960s.

The synthesis of DDT, whose insecticidal properties were discovered by Paul Müller in 1939, made large-scale control of malaria vectors by indoor spraying possible. The WHO launched a Global Malaria Eradication Programme in 1955. It eliminated malaria in many countries but did not succeed globally, partly because of resistance to DDT and to other insecticides, and because of operational and financial problems. Rachel Carson's Silent Spring (1962) drew attention to the environmental effects of DDT, and agricultural use was banned in many countries in the 1970s. The Stockholm Convention (2001) restricts DDT but permits its use for disease vector control under WHO guidance. From the 1980s, insecticide-treated nets, and from the 2000s, artemisinin-based drugs and rapid tests, were added, and the funding that came with the Global Fund and the President's Malaria Initiative expanded coverage. The Global Vector Control Response 2017 to 2030 of the WHO promotes integrated approaches.

Quantitative comparison of major vector control modalities

Environmental management

Environmental management changes the environment to reduce vector breeding or contact with people.

• Environmental modification is a lasting change, such as drainage, filling of ponds, lining of canals, piped water supply and sanitation, and the covering of water storage containers. Improved piped water reduces the need to store water in containers that support Aedes aegypti.
• Environmental manipulation is a temporary change, such as the flushing of streams, the intermittent irrigation of rice fields (alternate wetting and drying), and clearing of vegetation. Alternate wetting and drying of rice can reduce the breeding of Culex tritaeniorhynchus, and it also reduces methane emissions.
• Changing human habitation or behavior includes housing improvements, such as screening of windows and doors and the closing of eaves, which reduces entry of mosquitoes and has been associated with lower malaria incidence. Siting of settlements and livestock away from breeding areas is also used.
• Source reduction for container-breeding mosquitoes, in which people remove, cover, or empty containers such as tires, buckets, and flower vases, is the basis of control for Aedes aegypti. Its effect depends on how thoroughly it is done, and community participation is needed.

Biological control

Biological vector control uses natural enemies or competitors.

• Larvivorous fish, such as Gambusia affinis and Poecilia reticulata (guppies), are used in water bodies and in domestic water tanks. Gambusia has had negative effects on native fauna where introduced, so its use is now discouraged in many places.
• Bacterial larvicides are made from Bacillus thuringiensis israelensis (Bti) and Lysinibacillus sphaericus. They produce protein toxins that act on the larval midgut and are highly selective for mosquitoes and black flies. Resistance to Bti is rare because several toxins act together.
• Copepods of the genus Mesocyclops feed on mosquito larvae and have been used for Aedes control in container habitats, with success in Vietnam.
• Entomopathogenic fungi such as Beauveria bassiana and Metarhizium anisopliae infect adult mosquitoes through the cuticle. They kill slowly, and this slowness is useful because it allows the mosquito to transmit less before death and to have less selection pressure for resistance.
• Predatory mosquito larvae (Toxorhynchites) and other predators have been tested.
• Symbiont-based methods use the bacterium Wolbachia to block virus transmission or to suppress populations by cytoplasmic incompatibility (described below).

Chemical control

Insecticides are central to many vector control programs. The main classes are organochlorines (DDT), organophosphates (such as temephos and pirimiphos-methyl), carbamates (such as bendiocarb), pyrethroids (such as permethrin and deltamethrin), neonicotinoids (such as clothianidin), and insect growth regulators (such as methoprene and pyriproxyfen), and the biological product spinosad. They are used in several ways:

• Indoor residual spraying (IRS) applies a long-lasting insecticide to the walls and roofs of houses, where resting mosquitoes pick it up. It is an effective measure against malaria vectors that rest indoors, and it needs high coverage (usually at least 80 percent of houses) and repeated application, typically every 3 to 12 months depending on the product.
• Space spraying uses ultra-low-volume (ULV) cold fogging or thermal fogging from vehicles, backpacks, or aircraft to kill flying adults. It acts only while droplets are in the air, so it is used mainly to reduce transmission quickly in epidemics, and its effect on dengue incidence is not well established except in some settings, where it was targeted and timed with surveillance.
• Larviciding treats breeding sites with larvicides (Bti, insect growth regulators, temephos) and is used when sites are few, fixed, and findable. It is common for urban Aedes and Culex control and in catch basins.
• Insecticide-treated clothing, materials, and spatial repellents are used in some settings. Spatial repellents release volatile chemicals (such as transfluthrin) that reduce biting in a space. Evidence of effect on disease from trials is mixed, and a large trial of one such product in Indonesia found that it reduced the incidence of dengue.
• Acaricides on animals (dips, sprays, and collars) are used against ticks.

Personal protection

Personal protection measures reduce direct human-vector contact:

• Insecticide-treated nets (ITNs) and long-lasting insecticidal nets (LLINs) are the most widely used method against malaria in Africa. They protect users by forming a physical barrier and kill mosquitoes that land on them, and at high coverage they protect non-users by reducing the mosquito population and its age. Trials in the 1990s showed that they reduced child mortality by about 17 percent. Pyrethroid resistance has reduced the effect of standard nets in some areas, and nets that contain the synergist piperonyl butoxide (PBO) or a second insecticide (such as chlorfenapyr, with a different mode of action) have shown better effects in trials.
• Repellents that contain DEET, picaridin (icaridin), IR3535, or oil of lemon eucalyptus reduce bites from mosquitoes and ticks.
• Clothing and screening. Long clothing, permethrin-treated clothing, window screens, and air conditioning reduce contact.

Genetic and biotechnological approaches

Modern genetic and microbial vector control techniques modify or suppress target populations:

• Sterile insect technique (SIT) releases males that were sterilized, usually by radiation, so that wild females that mate with them produce no offspring. It requires repeated releases and sex separation. It eradicated the New World screwworm from North and Central America and the tsetse fly from the island of Zanzibar, and it is now used for mosquitoes in pilot programs.
• Incompatible insect technique (IIT) releases males that carry a Wolbachia strain that is incompatible with the wild females. It is combined with SIT to remove accidentally released females.
• Population replacement with Wolbachia releases mosquitoes of Aedes aegypti that carry strains (_w_Mel or _w_AlbB) that reduce dengue transmission and spread by cytoplasmic incompatibility. A randomized trial in Yogyakarta, Indonesia, reported a 77 percent reduction in symptomatic dengue in treated areas.
• Genetically modified mosquitoes. Release of male Aedes aegypti that carry a self-limiting gene whose offspring die before adulthood (OX513A and later versions) has reduced populations in field trials in Brazil, the Cayman Islands, and elsewhere. Gene drives, in which a genetic element spreads in a population at a higher rate than Mendelian inheritance would give, have been developed in the laboratory for Anopheles gambiae to suppress populations or to make them unable to carry Plasmodium parasites. They have not been released, and they raise questions of safety, regulation, and international governance. The Target Malaria program is working toward field trials.

Vaccination and animal measures

Vaccination of people and animals is a complement to vector control. Livestock treated with insecticides reduce the biting of vectors that prefer cattle, such as zoophilic Anopheles and tsetse flies. For tsetse control, odor-baited traps and insecticide-treated "tiny targets" have reduced the number of flies and the incidence of human African trypanosomiasis.

Integrated vector management (IVM) is a rational decision-making process promoted by the WHO to use resources for vector control in an optimal way. Its principles include evidence-based decisions that use local data, the use of several methods together, collaboration among health and other sectors (such as agriculture and urban planning), community participation, and capacity building. A program typically has these steps:

1. Mapping of the vector species, their behavior (when and where they bite and rest), and the disease burden.
2. Selection of methods that suit the local ecology. For example, nets and IRS are effective for indoor-biting, indoor-resting malaria vectors, whereas they are less suited to outdoor-biting vectors, and larval source management is suited to places with few breeding sites.
3. Monitoring of coverage, entomological indicators, insecticide susceptibility, and disease, and adaptation of the program.

Examples of successful integrated programs include the malaria elimination efforts of Sri Lanka (certified malaria-free in 2016) and the Maldives, and the control of onchocerciasis by a combination of larviciding of black fly breeding sites and drug treatment in West Africa.

Entomological surveillance in vector control measures the abundance, behavior, and infection of vectors. Methods include human landing catches (the reference method for biting rates, which has ethical limits), CDC light traps, gravid traps, BG-Sentinel traps, ovitraps (for Aedes eggs), larval dipping, and pyrethrum spray catches of resting mosquitoes. Standard indices for Aedes include the house index, the container index, and the Breteau index, but they correlate poorly with human risk. The entomological inoculation rate (EIR) for malaria is the number of infectious bites per person per unit time:

where m is the number of mosquitoes per person, a is the human biting rate per mosquito, and s is the proportion of mosquitoes with sporozoites in their salivary glands.

For pooled testing of vectors for pathogens by RT-PCR, with M pools of s mosquitoes and Y positive pools, the maximum likelihood estimate of infection prevalence is:

Evaluation of control measures includes randomized trials (individually randomized or cluster-randomized), which provide the strongest evidence, and the monitoring of epidemiological outcomes. Susceptibility of vectors to insecticides is tested with WHO tube tests and CDC bottle bioassays.

In vector control, resistance is a heritable change that reduces the effect of an insecticide, and it is a major threat. Insecticides that were used widely in agriculture and in public health select for resistance, and many vector populations are resistant to pyrethroids.

• Target-site resistance involves a mutation in the target protein. In the voltage-gated sodium channel, kdr mutations (for example L1014F) reduce the binding of pyrethroids and DDT. A mutation in acetylcholinesterase (ace-1 G119S) confers resistance to organophosphates and carbamates.
• Metabolic resistance is the increased detoxification by enzymes, in particular cytochrome P450 monooxygenases, esterases, and glutathione S-transferases. The synergist piperonyl butoxide inhibits P450s and restores some susceptibility.
• Cuticular resistance (a thicker cuticle that slows uptake) and behavioral resistance (avoidance of treated surfaces) also occur.

The speed at which resistance evolves depends on the selection pressure, the initial frequency of the resistance alleles, and their fitness costs. In a simple model, if resistant homozygotes have a relative survival advantage s under treatment, the allele frequency q changes per generation by about Δ_q_ = s q (1 − q) for a dominant-additive case, so resistance rises slowly when rare and quickly at intermediate frequencies. Resistance management uses rotations or mosaics of insecticides with different modes of action, mixtures, combination nets, and non-chemical methods, and the WHO Global Plan for Insecticide Resistance Management (2012) provides guidance.

Modern vector control programs face several operational and biological constraints:

• Resistance, in particular to pyrethroids, as above.
• Behavioral change in vectors. Increased outdoor and early-evening biting by some Anopheles after the use of nets and IRS reduces the effect of indoor methods ("residual transmission").
• Invasive vectors. Anopheles stephensi, an urban malaria vector from Asia, has been found in the Horn of Africa and parts of Africa since 2012, and Aedes albopictus and Aedes aegypti have spread to new regions, including parts of Europe.
• Urbanization and climate change. Unplanned urban growth with poor water and drainage supports Aedes aegypti. Changes in temperature and rainfall shift the ranges and seasons of vectors and pathogens, but other factors also matter.
• Funding and system capacity. Programs depend on funding, trained entomologists, and supply chains, and gains can reverse quickly when programs lapse, as in the resurgence of malaria in many places after the end of the eradication programme.
• Equity and community acceptance. Coverage is lower in conflict areas and among poor and mobile populations. Acceptance of spraying, nets, and the release of modified mosquitoes needs community engagement.
• Environmental and health effects of insecticides. Non-target effects on beneficial insects, aquatic life, and humans must be balanced against the benefits.

In global vector control, insecticides are regulated nationally, and the WHO Prequalification programme and the WHO Pesticide Evaluation Scheme evaluate products for public health use. The release of genetically modified or _Wolbachia_-carrying mosquitoes is regulated in each country and involves risk assessment and community consent. Gene drives would affect shared ecosystems across borders, which has led to proposals for international governance and for staged testing with community engagement.

Why does targeting adult vector survival reduce transmission more effectively than larval control?

In the Ross-Macdonald vectorial capacity equation, V = m a² e^−_μn_ / μ, mosquito density (m) enters linearly, so reducing larval emergence by 50% reduces V by 50% and R₀ by only 29%. By contrast, adult daily mortality (μ) appears both in the denominator and exponentially in the survival term e^−_μn_ across the extrinsic incubation period (n), meaning that doubling μ from 0.1 to 0.2 per day when n = 10 days reduces vectorial capacity by roughly fivefold.

How do dual-active pyrethroid–chlorfenapyr bed nets overcome metabolic insecticide resistance?

Whereas standard pyrethroids target voltage-gated sodium channels and are detoxified by overexpressed cytochrome P450 monooxygenases in resistant *Anopheles* mosquitoes, chlorfenapyr is a halogenated pyrrole pro-insecticide that requires oxidative removal of its *N*-ethoxymethyl group by those exact P450 enzymes to form the active metabolite tralopyril. Tralopyril then uncouples oxidative phosphorylation in flight muscle mitochondria by disrupting the proton gradient across the inner mitochondrial membrane, turning metabolic resistance into heightened susceptibility.

Why does resistance to *Bacillus thuringiensis israelensis* (Bti) rarely evolve in wild mosquito populations?

During sporulation, *Bacillus thuringiensis israelensis* produces a parasporal crystal containing four distinct protoxins—Cry4Aa, Cry4Ba, Cry11Aa, and the cytolytic protein Cyt1Aa—which are solubilized in the alkaline mosquito larval midgut and activated by digestive proteases. Because Cyt1Aa inserts directly into midgut membrane phospholipids and acts as a membrane-bound receptor that synergizes pore formation by the three Cry toxins, a mosquito larva would require simultaneous mutations across multiple independent receptor pathways to survive.

• Integrated vector management
• Insecticide resistance
• Mosquito control
• Wolbachia
• Arbovirus
• Malaria
• Dengue fever
• Sterile insect technique
• Insecticide-treated net