Mosquitoes: Biology, Repellents & Bite Relief

A mosquito bites a person. — Mosquito bites aren't just a nuisance, they can also carry diseases.

(Image credit: Mosquito photo via Shutterstock)

Nothing ruins a perfect summer evening like the whine of a mosquito in search of a warm meal. There are 176 species of these annoying insects in the United States and more than 3,000 species throughout the world, according to the American Mosquito Control Association (AMCA) — enough to ruin any number of picnics by the pool.

These little pests are considered the most deadly animals in the world and contribute to close to a million deaths per year, according to an article in Smithsonian magazine. This is not because of the itchy bite, but mainly because of the diseases transmitted by the bite.

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Egg. Female mosquitoes lay eggs either on the surface of water or in soil that periodically floods, according to the AMCA. Fish and other predators keep large ponds and streams relatively mosquito-free; instead, mosquito hotspots include temporary puddles, tree holes, discarded car and truck tires and potted plant saucers, according to the U.S. Environmental Protection Agency (EPA). Some mosquitoes lay single eggs, while others lay eggs that float together in "rafts" of as many as 200.
Larva. Out of the eggs hatch squirmy larvae, which live in the water. Some species hang upside down from the water surface, like stalactites. Others attach to vegetation. Breathing through siphon-like tubes and eating bacteria and algae, the larvae grow and molt four times over.
Pupa. After the fourth molt, the mosquito is in the pupa stage. It remains in the water, resting and developing. This takes a couple of days in the common Culex mosquito species in the southern United States, according to the AMCA. In general, the water phases of a mosquito's life play out in a few days to a few weeks.
Adult. After the adult emerges from the pupal stage, it spends a few days drying its wings and resting. Female adults are to blame for itchy summer bites — males do not feed on blood. Mosquitoes don't bite you for food, since they feed off plant nectar, Joseph Conlon, a medical entomologist and the technical advisor to the AMCA, told NBC News. Females suck your blood to get a protein needed to develop their eggs.

In a 2018 study, published in the journal PLOS Neglected Tropical Diseases, researchers from the Baylor College of Medicine in Texas injected baby mice with human hematopoietic stem cells — which later turn into various types of blood cells, including immune system cells — that were taken from umbilical cords. When the mice grew up and had a well-established "human" immune system, the researchers held an open vial of mosquitoes against the footpads of each mouse. The insects bit each mouse around four times. By analyzing blood bone marrow, skin and spleen cells from the mice, the researchers found that a number of immune cells remained active even seven days after the mice were bitten. [Here's Why Mosquito Bites Itch for Such a Long Time]

Calamine, a mixture of zinc and iron oxides, helps relieve itching. Hydrocortisone is a steroid that reduces redness, itching and inflammation. Low-dose treatments containing this steroid can be bought over the counter and are generally safe for most people to use without a prescription. However, you should consult with your physician before using products containing hydrocortisone if you are pregnant or nursing or if you plan to use the product on a child under 2 years of age, according to the National Eczema Association.

People who travel outside the country and return home infected with a mosquito-borne disease can also spread these diseases in the United States. In recent years, this phenomenon has been witnessed in places like Florida and the Northeast United States, where returned travelers have played a role in the spread of Chikungunya virus, which is endemic to areas of the Caribbean, according to the Centers for Disease Control (CDC).

Zika virus is also a recent concern. It is related to West Nile and is spread through mosquito bites. "Those who do feel ill may experience fever, rash, joint pain and red eyes (conjunctivitis)," Dr. Robert Amler, vice president for government affairs, dean and professor of public health and professor of pediatrics and environmental health science at New York Medical College in Valhalla, New York, told Live Science. "These symptoms are usually mild and last only a few days or up to one week."

This disease can be passed from mother to developing infant. "The greatest risk of Zika complications might be to newborn babies," Amler said. "Although a definite cause has not been proven, a sizeable increase in newborns with microcephaly, a serious birth defect, has occurred in Brazil in babies born to mothers who were infected during their pregnancies with Zika virus. Zika virus infection also may be associated with Guillain-Barre syndrome in adults, a serious form of paralysis."

Many mosquito-borne diseases can cause serious illnesses and even death, which is why mosquito-prevention measures are so important. Those who become ill with a mosquito-borne disease should avoid mosquito bites, especially during the early stages of their illness, according to the CDC. This will help limit the spread of the disease to others. Each mosquito-borne disease is different and causes unique symptoms, but some common symptoms of such diseases include fever, fatigue, swollen glands, rash, headache and body ache.

Mosquitoes are more attracted to some people than others. There are a number of reasons for this, and there are a number of steps you can take to keep mosquitoes at bay. The first step is easy: try to relax. Mosquitoes are more attracted to people who are expending a lot of energy, according to Day. The increased production of carbon dioxide (from breathing heavily) and lactic acid (a byproduct of increased physical activity) draw mosquitoes to you, Day said.

Aside from covering your skin, you can also try to avoid mosquito habitats — like wetlands and forests — when mosquitoes are most active, which for many species is at dawn and dusk, according to the AMCA. But if you need to be outside when mosquitoes are looking for a meal, you may want to use insect repellant to deter the insects from biting you. Day recommends using a repellant that contains 5 to 7 percent DEET, a chemical compound that wards off mosquitoes. At this low concentration, DEET repellants typically need to be reapplied every 90 minutes or so, according to the AMCA. DEET is effective when sprayed directly onto skin or clothing. Higher concentrations of DEET can also provide protection against ticks.

The EPA lists several larvae-killing pesticides that are effective at controlling mosquitoes. Insect growth inhibitors such as methoprene prevent larvae from developing into adulthood. Temephos, a chemical that affects the nervous system of insects, can be applied on standing water where mosquitoes breed. Oils applied to puddles can also break up the surface tension that keeps eggs and larvae afloat. Finally, bacteria of the genus Bacillus disrupt the digestive system of insects, ultimately starving them to death.

Mosquito traps often use carbon dioxide or heat to lure the annoying insects in, but scientific studies on their effectiveness are rare, according to the AMCA. A 1996 study in the Journal of the American Mosquito Control Association found that citronella candles and incense did reduce bites under natural conditions, but only by 42 percent and 24 percent, respectively. A 2011 study in the journal of Tropical Medicine & International Health found that citronella candles in indoor tests repelled only 14 percent of mosquitoes. Citronella diffusers (which release more concentrated oil) performed slightly better, repelling 22 percent of female mosquitoes up to 20 feet away.

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