Researchers have discovered that mosquitoes, particularly the species Aedes aegypti, can detect infrared radiation (IR) from human skin, enhancing their ability to find hosts. This newly identified sense doubles their host-seeking behavior when combined with other cues like CO2 and human odor.
Key Takeaways
- Mosquitoes detect IR radiation from human skin, guiding them to their hosts.
- The IR detection is most effective within 2.5 feet, enhancing mosquito targeting.
- Understanding this IR detection could improve mosquito control methods.
Guided by Thermal Infrared
It is well established that mosquitoes like Aedes aegypti use multiple cues to home in on hosts from a distance. These include CO2 from our exhaled breath, odors, vision, heat from our skin, and humidity from our bodies. However, each of these cues has limitations. The insects have poor vision, and a strong wind or rapid movement of the human host can throw off their tracking of the chemical senses. So the authors wondered if mosquitoes could detect a more reliable directional cue, like infrared radiation.
Within about 10 cm, these insects can detect the heat rising from our skin. And they can directly sense the temperature of our skin once they land. These two senses correspond to two of the three kinds of heat transfer: convection, heat carried away by a medium like air, and conduction, heat via direct touch.
But energy from heat can also travel longer distances when converted into electromagnetic waves, generally in the infrared (IR) range of the spectrum. The IR can then heat whatever it hits. Animals like pit vipers can sense thermal IR from warm prey, and the team wondered whether mosquitoes, like Aedes aegypti, could as well.
A Trick for Sensing Infrared
It isn’t possible for mosquitoes to detect thermal infrared radiation the same way they would detect visible light. The energy of IR is far too low to activate the rhodopsin proteins that detect visible light in animal eyes. Electromagnetic radiation with a wavelength longer than about 700 nanometers won’t activate rhodopsin, and IR generated from body heat is around 9,300 nm. In fact, no known protein is activated by radiation with such long wavelengths. But there is another way to detect IR.
Consider heat emitted by the sun. The heat is converted into IR, which streams through empty space. When the IR reaches Earth, it hits atoms in the atmosphere, transferring energy and warming the planet. The authors thought that perhaps our body heat, which generates IR, might then hit certain neurons in the mosquito, activating them by heating them up. That would enable the mosquitoes to detect the radiation indirectly.
Diving into the Biochemistry
The activity of the heat-activated TRPA1 channel alone might not fully explain the range over which mosquitoes were able to detect IR. A sensor that exclusively relied on this protein may not be useful at the 70 cm range the team had observed. At this distance, there likely isn’t sufficient IR collected by the peg-in-pit structure to heat it enough to activate TRPA1.
Fortunately, Montell’s group thought there might be more sensitive temperature receptors based on their previous work on fruit flies in 2011. They had found a few proteins in the rhodopsin family that were quite sensitive to small increases in temperature. Although rhodopsins were originally thought of exclusively as light detectors, Montell’s group found that certain rhodopsins can be triggered by a variety of stimuli. They discovered that proteins in this group are quite versatile, involved not just in vision, but also in taste and temperature sensing.
Upon further investigation, the researchers discovered that two of the 10 rhodopsins found in mosquitoes are expressed in the same antennal neurons as TRPA1. Knocking out TRPA1 eliminated the mosquito’s sensitivity to IR. But insects with faults in either of the rhodopsins, Op1 or Op2, were unaffected. Even knocking out both the rhodopsins together didn’t entirely eliminate the animal’s sensitivity to IR, although it significantly weakened the sense.
A Tactical Advantage
Half the world’s population is at risk for mosquito-borne diseases, and about a billion people get infected every year. What’s more, climate change and worldwide travel have extended the ranges of Aedes aegypti beyond tropical and subtropical countries. These mosquitoes are now present in places in the US where they were never found just a few years ago, including California.
The team’s discovery could provide a way to improve methods for suppressing mosquito populations. For instance, incorporating thermal IR from sources around skin temperature could make mosquito traps more effective. The findings also help explain why loose-fitting clothing is particularly good at preventing bites. Not only does it block the mosquito from reaching our skin, but it also allows the IR to dissipate between our skin and the clothing so the mosquitoes cannot detect it.
Despite their diminutive size, mosquitoes are responsible for more human deaths than any other animal. Our research enhances the understanding of how mosquitoes target humans and offers new possibilities for controlling the transmission of mosquito-borne diseases.
