While mosquitoes don’t smell the way we humans do, they do have senses that enable them to find their prey by detecting CO2 emissions as well as heat, so even using the best mosquito coils won’t help all the time. Some mosquitoes also have a receptor called Ir8a that can detect and track the smell of human sweat.
Why Is This Happening?
On beautiful summer evenings spent outside on the porch or even camping out in the woods, we try our best to avoid pesky mosquito bites by dousing our skin with bug repellents and even lighting citronella candles in an attempt to bewitch them into submission.
While these efforts might be successful in keeping them at bay for a while, no solution is really perfect in the long run. Truth be told, these insects have learned to adapt and have evolved their hunting game to use a triple threat combination of visual, olfactory and thermal cues that allow them to zone in on their human targets.
A study published by researchers in the online version of the journal called ‘’Current Biology’’ has tried to shed light on the exact methods these insects use when going out hunting.
First things first, we should clear up the myth about mosquito feeding. The truth is both male and female mosquitoes actually feed on nectar, not blood. However, the females of the species need the protein from the blood in order to be able to lay and develop their eggs. While males are basically only there for reproduction, the ladies have a much more developed role.
When they need a blood meal, they will naturally search for a host which will typically be a human. Many insects, mosquitoes included, are attracted by the odor of carbon dioxide that is naturally exhaled from our bodies and the animal bodies. However, compared to others, mosquitoes can also use their vision and thermal sensory information to find a new meal.
How Are They Doing It?
When it comes to this process, your over-friendly buzzing insects are really complex creatures. To find a human host, they face the challenging task of integrating sensory cues which are separated in time and space.
As we said, this process happens as a result of their multi-pronged strategy which begins with using their smell to track a plume of carbon dioxide upwind. This is where we check the number one box on the answer ‘’Yes, mosquitoes can smell’’.
Furthermore, the research from the online journal has shown that when coming into contact with the CO2, they are also exploring visual features they would otherwise ignore when not looking for prey. This behavior will eventually lead them toward potential hosts where they will adapt and track the body heat in order to locate a landing site.
How Do Mosquitoes Use Their Smell?
To better understand the way these insects use their smell to find their next target, researchers put together a well-thought plan:
They released hungry, mated female mosquitoes into a wind tunnel where they could experiment and control different sensory cues and see how they react to them. In one set of attempts, they used a high-concentration CO2 plume to mimic the signal created by the breath of a human.
During another, they introduced the same type of plume but this time with a low concentration of carbon dioxide and watched the events unfold. As expected, the mosquitoes followed the high-concentration plume within the tunnel without paying any interest to the control plume consisting of low-CO2 background air.
As we said before, mosquitoes also resort to their visual senses once they get close enough to the target by using their smell and the researchers tested this hypothesis by placing a dark object on the floor of the wind tunnel.
What they found was that in the area with a high concentration of carbon dioxide, mosquitoes were extremely attracted to the dark high-contrast object while the control group showed no interest whatsoever to the same object when CO2 was missing from the equation.
While it was no surprise that mosquitoes tracked a target using carbon dioxide, the observations have shown that not only do they track the gas but whenever they sense it they are actually bound to fly toward that potential host, even if the CO2 is missing by the time they get there.
How Do Mosquitoes Track Heat?
Since we know mosquitoes use smell and heat-tracking in some semblance of order when looking for prey, it was difficult to merely test their heat attraction without engaging their olfactive senses.
What researchers chose to do was to construct two glass objects coated with a clear chemical substance that made it possible to heat them to any desired temperature. They got one of the objects to a temperature of about 98.6 degrees, which is approximately the human body temperature while allowing the other one to remain at room temperature.
Placing both of them on the floor of the wind tunnel with and without CO2 plumes they found that mosquitoes showed a clear preference for the warm object but that this time it was not related to the presence of carbon dioxide.
Therefore, it’s quite easy to observe that while mosquitoes use their smell and their heat tracking senses in order, they actually can use both of them separately in order to hunt us down and feed.
Using This Knowledge For Repellents
We now know that female mosquitoes rely on an array of sensory information when looking for prey and that the smell, while just the beginning of the process, can also be used on its own should the need arise.
The next question was how exactly they sense our body temperature when they get close enough. Researchers for the same ‘’Current Biology’’ journal managed to find out how the insects pick up acidic volatiles found in human sweat to track us down.
What they understood was that the key is a smell receptor called ‘’Ir8a’’. Apparently, mosquitoes with this gene missing from their anatomy were much less attracted to people. These findings, therefore, suggest new approaches and possibilities for improved mosquito repellents in the future.
Since it seems that a missing Ir8a removes about 50 percent of the entire mosquito host-seeking activity with any of the three senses, creating an odor that is able to mask it could prove to be the key to a solution which has eluded mankind for generations: being free from mosquito bites.
This could prove to be an important first step in blocking the transmission of diseases like dengue, yellow fever, Zika, and malaria. The approach the researchers use seems to be understanding the molecular basis of mosquito behavior in order to find new solutions to prevent their bites and therefore stop the diseases from spreading.
For this, understanding the way mosquitoes smell is a tall and important order because apparently activating the Ir8a requires a perfectly functioning sense of smell from the insect. In plain English, they need to be able to smell you in order to understand they no longer want to feed on you.
All this information enabled the researchers to create a model of how the mosquito manages to find its prey over different distances. The hypothesis is that from 30 to 150 feet away it detects a potential host using its sense of smell. As it flies closer to something between 16 to 52 feet, the mosquito begins to see the target.
Then, guided by visual cues, it draws even closer and manages to sense the body heat to determine the best landing spot. This seems to show that the female mosquitoes conduct this entire process in a rather elegant way, only paying attention to visual features after their smell confirms a potential host is nearby.