Mosquitoes use infrared vision to track and bite humans

While a mosquito bite may often be just a temporary nuisance, it can be a serious concern in many parts of the world. Some mosquitoes, such as Aedes aegypti, are responsible for spreading viruses that cause over 100 million cases of diseases like dengue, yellow fever, and Zika each year. 

Another species, Anopheles gambiae, spreads the parasite that causes malaria, leading to more than 400,000 deaths annually, according to the World Health Organization. These figures have earned mosquitoes the title of the deadliest animals.

How mosquitoes locate their hosts

Male mosquitoes are harmless, but female mosquitoes require blood for egg development. Given this, scientists have dedicated over a century to understanding how mosquitoes locate their hosts.

Through this research, experts have discovered that mosquitoes don’t rely on a single cue; instead, they use a combination of different senses at various distances to find their targets.

A team led by researchers at the University of California, Santa Barbara (UCSB) has now added a new sense to the mosquito’s known capabilities: infrared detection.

They found that infrared radiation from a source at roughly the temperature of human skin doubles the mosquitoes’ host-seeking behavior when combined with CO2 and human odor. 

Strong preference for the infrared source

The mosquitoes showed a strong preference for the infrared source during their host-seeking activities. The researchers also identified where this infrared detection occurs and uncovered how it functions at the morphological and biochemical levels. 

“The mosquito we study, Aedes aegypti, is exceptionally skilled at finding human hosts,” said Nicolas DeBeaubien, co-lead author of the study and a former graduate student and postdoctoral researcher at UCSB in Professor Craig Montell’s laboratory. “This work sheds new light on how they achieve this.”

Detecting new hosts from a distance 

It is well known that mosquitoes like Aedes aegypti use multiple cues to home in on their hosts from a distance.

“These include CO2 from our exhaled breath, odors, vision, (convection) heat from our skin, and humidity from our bodies,” explained Avinash Chandel, co-lead author and current postdoc at UCSB in Montell’s group. “However, each of these cues have limitations.” 

The insects have poor vision, and strong winds or rapid movements can disrupt their ability to track chemical signals.

Given these limitations, the researchers wondered if mosquitoes could detect a more reliable directional cue, like infrared radiation.

Within about 10 centimeters, mosquitoes can detect the heat radiating from our skin. They can also sense the temperature of our skin once they land on it.

These abilities correspond to two of the three types of heat transfer: convection (heat carried away by air) and conduction (heat through direct contact). 

However, heat energy can also travel longer distances when converted into electromagnetic waves, typically in the infrared (IR) spectrum.

Animals like pit vipers can detect thermal IR from warm prey, so the team wondered if mosquitoes, like Aedes aegypti, could do the same.

Focus of the study

The researchers placed female mosquitoes in a controlled environment and measured their host-seeking behavior in two zones. 

Both zones were exposed to human odors and CO2 at concentrations similar to what we exhale. However, only one zone was also exposed to IR from a source at skin temperature. 

A barrier was used to prevent heat exchange through conduction and convection. The researchers then counted how many mosquitoes began probing as if searching for a vein.

Infrared radiation and mosquito behavior 

Adding thermal infrared radiation from a 34º Celsius source (about skin temperature) doubled the mosquitoes’ host-seeking activity, identifying infrared radiation as a newly documented sense mosquitoes use to locate humans. The team discovered that this sense remains effective up to about 70 centimeters (2.5 feet).

“What struck me most about this work was just how strong of a cue IR ended up being,” DeBeaubien said. “Once we got all the parameters just right, the results were undeniably clear.”

Previous studies didn’t observe any effect of thermal infrared on mosquito behavior, but senior author Craig Montell suspects this is due to differences in methodology. 

“Any single cue alone doesn’t stimulate host-seeking activity. It’s only in the context of other cues, such as elevated CO2 and human odor, that IR makes a difference,” Montell said. Indeed, his team found that infrared alone has no impact.

How do mosquitoes detect infrared radiation?

It’s impossible for mosquitoes to detect thermal infrared radiation the same way they detect visible light because 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 the IR generated from body heat is around 9,300 nm.

No known protein is activated by radiation with such long wavelengths, Montell said. However, there is another way to detect infrared radiation.

Consider heat emitted by the sun: it’s converted into IR, which streams through space and, when reaching Earth, warms the planet by interacting with atoms in the atmosphere. 

“You have heat converted into electromagnetic waves, which is being converted back into heat,” Montell explained, noting that the IR coming from the sun has a different wavelength from the IR generated by our body heat, as the wavelength depends on the temperature of the source.

Human body heat and mosquito neurons 

The researchers hypothesized that our body heat, which generates IR, might hit certain neurons in the mosquito, activating them by heating them up. This would enable mosquitoes to detect the radiation indirectly.

Scientists have known that the tips of a mosquito’s antennae contain heat-sensing neurons. The team found that removing these tips eliminated the mosquitoes’ ability to detect infrared radiation.

Another lab had identified the temperature-sensitive protein TRPA1 at the end of the antenna. The UCSB team observed that mosquitoes without a functional TRPA1 gene, which codes for the protein, couldn’t detect infrared radiation.

The tip of each antenna has peg-in-pit structures well-suited for sensing radiation. The pit shields the peg from conductive and convective heat, allowing highly directional IR radiation to enter and warm up the structure. The mosquito then uses TRPA1 – a temperature sensor – to detect infrared radiation.

Mosquito IR detection range

The heat-activated TRPA1 channel alone might not fully explain the range over which mosquitoes can detect IR. A sensor that exclusively relied on this protein may not be useful at the 70 cm range the team 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 had previously discovered in fruit flies that some rhodopsin proteins are quite sensitive to small increases in temperature. 

Although rhodopsins were originally thought to be 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 versatile, involved not only 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.

Mosquito sensitivity to infrared radiation 

Knocking out TRPA1 eliminated the mosquito’s sensitivity to infrared radiation. But mosquitoes with defects in either of the rhodopsins, Op1 or Op2, were unaffected. 

Even knocking out both rhodopsins together didn’t entirely eliminate the mosquitoes’ sensitivity to IR, although it significantly weakened it.

The results indicated that more intense thermal IR – like what a mosquito would experience at closer range (about one foot) – directly activates TRPA1. 

Meanwhile, Op1 and Op2 can be activated by lower levels of thermal IR and then indirectly trigger TRPA1. Since our skin temperature is constant, extending the sensitivity of TRPA1 effectively extends the range of the mosquito’s IR sensor to around 2.5 feet.

Broader implications of the study 

Half the world’s population is at risk for mosquito-borne diseases, and about a billion people are infected each year, Chandel said. 

Moreover, climate change and global travel have expanded the range of Aedes aegypti beyond tropical and subtropical countries, bringing these mosquitoes to areas in the U.S., such as California, where they were not found just a few years ago.

The team’s discovery could help improve methods for controlling mosquito populations. For example, 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. Loose clothing not only blocks mosquitoes from reaching our skin but also allows the IR to dissipate between our skin and the clothing, making it harder for mosquitoes to detect.

“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,” DeBeaubien concluded.

The study is published in the journal Nature.

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