Mosquitoes Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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Two thousand five hundred species and 1,000 subspecies of mosquitoes thrive across diverse ecological niches from the arid desert to the subarctic tundra. The life cycle, habitat preference, and temporal behavior of mosquitoes is therefore highly variable and species dependent. Over its life cycle, a mosquito hatches from an egg, develops through aquatic larval stages into a pupae and finally an airborne adult. Larval mosquitoes inhabit almost all temporary natural, human altered or anthropogenic water holding bodies on land. Extraordinary larval habitats include salty water, hot springs, tree holes, inside of plants (e.g., pitcherplants, bromeliads) and even water in between plant cellular tissue. Mosquitoes that transfer diseases to humans preferentially breed in disturbed environments (e.g., burrow pits, hoof prints) or containers (e.g., potable water jars, tires) close to or inside human dwellings.

Only a small fraction of eggs survive environmental hazards, resource scarcity, predation, disease, and competition to develop into adults. Reproductive strategies vary from laying individual eggs or egg clusters across multiple locations to depositing all the eggs in one location as a mosquito raft. Depending on the species, eggs are either deposited into a water body and rapidly develop or are deposited on a surface previously submerged in water. Subsequently re-submerging the eggs combined with other environmental cues initiates hatching of the mosquito into a suitable habitat. The cold-blooded mosquito’s rate of development is strongly influenced by the temperature and moisture characteristics of the ambient environment. In general, warm conditions expedite mosquito development while hot or cold conditions stymie mosquito maturation.

Nonpredatory larvae extract and/or browse for food and nutrients from the aqueous environment and store the food for future development. Larvae mature, grow, and shed their exoskeletons three times before metamorphosis accelerates in the pupae stage. Pupae larval muscles and the midgut are completely reconstructed into adult body parts. Pupae are morphologically distinct from larvae, cannot consume food, float at the water’s surface, and can survive outside of water. Adult mosquitoes consume nectar, fruit, and honeydew for sustenance and can be important pollinators of plants. Mature males swarm together and attract viable females to copulate and complete the mosquito life cycle. Inseminated females store the sperm and only fertilize their eggs prior to deposition. Of the mosquito species that extract blood, only the females actively seek out blood meals to provide protein for the development of the eggs. Each species has a preferred time of the day to vigorously seek blood and this circadian rhythm informs strategies to minimize mosquito and human contact. Favorable host seeking times of the day are either the nighttime, daytime, or during the twilight dusk and dawn hours. The degree to which a mosquito hunts for human (anthropophilic) or other vertebrate blood (zoophilic) sources is largely species dependent; however, significant intraspecies variation exists.

Female mosquitoes employ a diverse array of tools to identify appropriate sources of blood and extract plasma without alerting the unwitting victim. She is attracted to the heat our bodies continually emit, carbon dioxide and water vapor we exhale, lactic acid excreted in our sweat, and our body movements. Mosquito repellant masks the body’s excretions and makes us less appealing blood sources. Upon landing, the mosquito finds the optimal extraction site by touching and chemically “smelling” the skin with her antennae and mouthparts. She proceeds to find blood, insert her mouthparts, and inject chemical filled saliva similar to a drug cocktail administered to a patient before an operation. Saliva chemicals stop blood clot formation, keep the blood vessels enlarged, and thwart any host inflammatory immune response.

Infectious viruses, protozoa, or worms (nematodes) in the biting mosquito may be transferred with the saliva to another host to complete or start a new reproductive cycle.

Agents responsible for a large proportion of the global disease burden such as malaria, dengue, Rift Valley fever, yellow fever, elephantitis, and encephalitides are transmitted during this exchange. The female mosquito ingests two to three times her weight in blood and finds a safe habitat to process the blood.

Extermination or Control

Mosquito management historically combines source reduction, surveillance, targeted biological and chemical control, public education, and legislative action at the region, village, household, and individual levels of organization. Multiple remediation activities target the life cycle specific ecological associations of medically important and nuisance mosquitoes. Insecticide application-Dichlorodiphenyltrichloroethane (DDT)-became the dominant method of mosquito control from World War II until it was banned in Western nations in the 1970s.

Top down military eradication efforts of the mid-20th century are in the process of transitioning into more participatory, community supported, and holistic management activities. Technical “magic bullets” singularly focused on a component of disease transmission have proven costly, inflexible, and detrimental to other management strategies. Divergent conceptions of society’s relationship to the mosquito underpin historical shifts from mosquito and disease extermination back toward mosquito mitigation, management, and control. Integrated mosquito management currently attempts to reduce the level of disease transmission by decreasing mosquito abundance.

Source reduction eliminates standing water required for larval development by draining wetlands and marshes or emptying water in man-made containers. Effective source reduction also includes the active management, engineering, and alteration of mosquito habitats.

Channeling, stabilizing, and diverting waterways, controlling the height and motion of standing water, removing invasive or marginal groundcover, and planting trees along waterways historically controlled malaria vectors.

Biological mosquito control intentionally introduces a mosquito predator, parasite, or organic toxin to reduce larval populations in aquatic environments. Predatory mosquito fish (e.g., Cambusia, Tilapia) consume larval mosquitoes as well as change the habitat vegetation structure to be inhospitable to certain mosquito species. Microcrustacean predatory copepods, carnivorous Toxorhynchites mosquitoes, parasitic nematodes, and pathogenic protista are other biological control methods of varying effectiveness.

The mosquito’s rapid reproductive cycle and large number of offspring naturally promote genetic resistance against insecticides. Mosquitoes are adapted to the current generation of pesticides such as organophosphates, carbamates, and pyrethroids and all medically important species are resistant to at least one pesticide. Entirely novel pesticides have not been developed in 15 years and significant financial and safety barriers to chemical generation and introduction exist. Indoor residual spraying is a neighborhood level intervention that applies pesticides inside households on surfaces where medically important mosquitoes rest. This strategy targets the infectious females who may transmit a disease to other household members.

Successful household and individual level interventions focus on mosquito-proofing households, personal protection, and behavioral changes. Households that actively replace standing water, have indoor plumbing, door and window screens, and access to running water decrease their interaction with mosquitoes. To avoid mosquitoes that blood-feed nocturnally or during the twilight hours, individuals can shift their outdoor activities to the daytime hours. Changes in activity patterns may be mandated by state policies like the historic nighttime curfew imposed during malaria transmission season in Israel. Distributing insecticide-coated bed nets similarly capitalizes on the nocturnal feeding preference of Anopheles gambiae (malaria vector) to provide personal protection. For mosquitoborne diseases (yellow fever, Japanese encephalitis) with effective vaccines, disease management focuses on vaccine manufacturing and distribution, and building and maintaining public health infrastructure instead of mosquito control. Transmission of diseases with vaccines persists in environmentally disturbed and politically and economically disenfranchised or disadvantaged regions.

Bibliography:

D. Gillet, Mosquitos (Weidenfeld and Nicolson, 1971);
Uriel Kitron and Andrew Spielman, “Suppression of Transmission of Malaria through Source Reduction; Antianopheline Measures Applied in Israel, the United States and Italy,” Reviews of Infectious Diseases (v.11);
William Marquardt, , Biology of Disease Vectors (Academic Press, 2004);
Timothy Mitchell, Rule of Experts: Egypt, Techno-politics, Modernity (University of California Press, 2002);
Jerry Spiegel et , “Barriers and Bridges to Prevention and Control of Dengue: The Need for a Social-Ecological Approach,” EcoHealth (v.4);
Andrew Spielman and Michael D’Antonio, Mosquito: A Natural History of Our Most Persistent and Deadly Foe (Hyperion, 2001).