It is rare to attend an outdoor party in hot weather without hearing people complaining about mosquitoes. They walk away, sit in the smoke of a campfire, cover themselves with blankets and eventually give up and go inside. On the other end of the spectrum, there are plenty of people who don’t seem bothered by mosquitoes at all.
As a medical entomologist who has worked with mosquitoes for over 40 years, I am often asked why some people seem to be mosquito magnets while others are oblivious to these blood-sucking parasites buzzing all around them.
Most species of mosquitoes, as well as a host of other arthropods – including ticks, fleas, bedbugs, black flies, horseflies and biting midges – need for protein in the blood develop a batch of eggs. Only the female mosquito feeds on blood. Males feed on plant nectarwhich they convert into energy to fly.
Blood feeding is an extremely important part of the mosquito’s reproductive cycle. For this reason, enormous evolutionary pressure has been exerted on female mosquitoes to identify potential sources of blood, quickly and efficiently get a full blood meal, then sneak out of the unlucky victim. If you check some or all of the mosquito search boxes, you may find that you are a mosquito magnet.
Detection of CO2 and olfactory signals
Depending on the time of day they are active, mosquitoes use visual, sound and olfactory cues to identify a potential blood source. Most nocturnal species rely on olfactory signals or receptors. The most important chemical signal is carbon dioxide, which all vertebrates, including humans, release with each breath and through their skin.
Mosquitoes are very sensitive to CO2 and can detect a CO2 source several meters away. Receptor cells in the mosquito’s antennae and legs bind CO2 molecules and send an electrical signal to the brain. When more molecules hit their receptors, the higher the CO2 concentration and the closer they are to the host.
However, there are many non-living sources of carbon dioxide such as cars, boats, planes and trains. To separate living and non-living sources of CO2, mosquitoes rely on secondary olfactory signals produced by living animals. Metabolic processes like breathing and moving generate these olfactory cuesincluding lactic acid, ammonia and fatty acids which act as additional olfactory cues that help female mosquitoes focus on their next blood meal.
Thus, the production of carbon dioxide is the first mark of a mosquito magnet. Since the production of CO2 and secondary attractants is related to metabolic rate, the higher the metabolic rate, the more attractants are produced. Metabolic rate can be genetically determined, but it increases with physical activity.
The human mosquito magnets you may spot at summer parties may have a genetically high metabolic rate or may be more physically active than other attendees. They may also engage in other activities that increase their metabolic rate, such as drinking alcohol. The increased metabolic rate is why runners attract more mosquitoes during their recovery stretching exercises. Pregnant women, perhaps due to their increased metabolic rate, attract a disproportionate number of mosquitoes as well.
Natural body odors are also important cues used by mosquitoes to select a host. For example, some species of Anopheles mosquitoes are attracted to specific components of foot odor. These mosquitoes transmit human malaria and feed indoors in the middle of the night. By feeding on the feet of a sleeping person, mosquitoes avoid the head, where most of the CO2 is produced, and reduces the risk of waking the victim.
Mosquitoes active during the day and at dawn and dusk also use visual cues to identify a host. Mosquitoes generally fly close to the ground. From this point of view, they see their potential hosts against the horizon. Dark colors pop and light colors blend in, so how a person is dressed will determine how many mosquitoes they attract. Wearing lighter colors can not only help you stay cool, but also help you escape the notice of a mosquito.
Mosquitoes can detect movement visually, again by contrasting a silhouette with the horizon. This is why people who walk near a salt marsh in the middle of the day after a large emergence of salt marsh mosquitoes are inundated with mosquitoes that visually detect their presence.
There is also psychological component to mosquito activity. Some people just don’t notice the mosquitoes around them. A single mosquito flying around some people will elicit a strong reaction – you’ve probably seen someone go crazy trying to track down a buzzing mosquito in order to finish off the little bloodsucker.
Other individuals are unbothered and do not notice mosquitoes that are attracted to them, even when the insects are feasting on their blood. Some mosquitoes specialize in feeding on hard-to-see and hard-to-crush body parts. For instance, Aedes aegypti is a species of mosquito that prefers to feed on humans, especially around the ankles.
Whether or not you’re a mosquito magnet, their bites itch just as much!