Your Health

Deflecting mosquitoes during bite season

Courtesy of Center for Disease Control and Protection
The best way to fend off mosquitoes is to apply a repellent with 15-50 percent DEET, 20 percent picaridin or 30 percent oil of lemon eucalyptus. Repellents with lower percentages are not as effective.

Mosquitoes are incredibly well adapted for living on Earth in extreme conditions. They exist at 8,000 feet in the Himalayas and below sea level in the California desert. The eggs of mosquitoes can survive months to decades in desert, frozen tundra and even on dried flowers.

When it rains, the eggs hatch immediately, releasing mosquito larvae. Water reduces the amount of oxygen available to the eggs, which triggers hatching. Mosquitoes’ normal diet is nectar and aphid excrement. Blood is ingested only to fulfill reproductive needs. Mosquitoes become sexually mature at 2 days old and mate in swarms at dusk or dawn.

Different species feed at characteristic times of day. For example, Aedes aegypti, the mosquito responsible for spreading Zika, yellow fever, dengue fever and chikungunya, prefers to feed at dusk and has a proclivity for ankles and feet. Culex pipiens, the common house mosquito, feeds after dark.

Carbon dioxide and lactic acid in your breath attract mosquitoes because they have special sensors on their antennae for these specific molecules. The human scent plume is heavier than air, so it sinks toward the ground. The mosquito flies low to intersect the plume at its widest part, using antennae to orient itself toward the odor gradient.

Before feeding, the mosquito probes your skin 20 times or more with its proboscis, which consists of six stylets. With each insertion of its proboscis, the mosquito searches for an arteriole or venule from which to draw blood. Saliva is injected into the skin to prevent clotting. The proteins in the saliva can cause itching and swelling.

Feeding takes approximately 90 seconds, then the mosquito will fly to the nearest vertical surface, such as a wall, a tree or a post, to digest the blood.

When the process is complete, the mosquito flies away and searches for an appropriate site for laying eggs. Mosquitoes prefer to breed in areas of stagnant water. Certain chemicals produced from bacteria in dirty water stimulate the female to lay eggs. Immediately after sucking your blood, the mosquito has a difficult time flying away, laden down by a full belly, and is an easy target to kill.

Protective measures

Mosquitoes have evolved to have a preference for human odor because of the presence of odoran receptor Aaeg0R4, which recognizes sulcatone, a volatile compound produced in large quantities by humans. Interestingly, sulcatone is found in citrus, cashews, citronella oil, lemon grass oil, plums cherries and is also used as a flavoring ingredient. Perhaps altering one’s diet could reduce sulfactone production? Other attractants:

• Clothing. Mosquitoes are attracted to dark clothing, can bite through loose weave and light fabrics, and some can pierce even denim. Wearing clothing made of fabric with a tight weave and heavier weight provides better protection against bites. Mosquito netting clothing can also be effective, as long as the netting is not in direct contact with your skin.

• Scent. When we exhale, we release a unique cocktail of chemicals into the air such as carbon dioxide, lactic acid, octenol, uric acid and fatty acids. CO2 production is higher in pregnant women and in individuals with a high body mass index.

All mosquito repellents work by the same mechanism: masking the scent of CO2, lactic acid and other volatile fatty acids in sweat composition. This makes it more difficult for the mosquito to sense you. How effective repellent is depends on the particular agent and its concentration.

The three most effective ingredients are 15-50 percent DEET (diethyltoluamide), 20 percent picaridin and 30 percent oil of lemon eucalyptus. Products with lower concentrations of these ingredients are not as effective. DEET at 40-50 percent concentration is much more effective than lower concentrations. Studies have shown that applying a concentration greater than 50 percent DEET does not provide higher protection.

Product testing

I went hiking in Alaska in June – mosquito season – and tested these three mosquito repellents. They all were effective, but I had to reapply the lemon eucalyptus oil more often to keep the mosquitoes away. I also wore mosquito netting with repellent. Citronella candles are not effective.

Botanical agents are not required to be registered with the U.S. Environmental Protection Agency, and manufacturers are not required to prove these products work. Independent research testing confirms that botanicals do not work as well as DEET or picaridin.

Bacteria breaks down sweat into chemicals that generate body odor, which attracts mosquitoes. Showering can reduce body odor. Wearing freshly laundered clothing will decrease body scent. Fragrant perfumes can also attract mosquitoes.

Blood type O is preferred by mosquitoes. Blood type A is the least popular.

Moisture and movement also attract mosquitoes’ highly sensitive receptors. Thus, waving one’s arms around to shoo the flying pests away only causes more arousal on the part of the mosquito.

For best mosquito-repelling results, wear light-colored clothing that is clean (no residual body odor from previous wearings), don’t wear perfume, spray your clothing and exposed skin generously using one of the three proven effective repellents in the higher concentrations, wear clothing with a tight weave, avoid stagnant pools of water and don’t plant corn in your garden.

Dr. Patricia Wong is a Palo Alto-based dermatologist. For more information, visit

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