Mosquitoes locate a host primarily using one specific chemical cue: carbon dioxide (CO2). This gas is exhaled by all mammals and birds and serves as the primary signal alerting the mosquito to a potential blood meal. The insect has evolved a highly sensitive olfactory system to detect changes in CO2 concentration. This allows it to navigate toward a host from a significant distance, making the detection of exhaled breath an immediate trigger for host-seeking behavior.
Carbon Dioxide as the Primary Long-Range Signal
The mosquito’s ability to detect CO2 is centered on a pair of specialized sensory organs called the maxillary palps, which are small appendages located near the mouthparts. Within these palps are club-shaped sensory hairs known as capitate peg sensilla. Each sensillum houses three olfactory receptor neurons, with one neuron, known as cpA, being uniquely specialized for CO2 detection.
The cpA neuron expresses a complex of gustatory receptors, often referred to as the Gr family, which bind to the CO2 molecules. This receptor is formed by Gr1, Gr2, and Gr3 proteins. Researchers have found that the CO2-sensitive cpA neuron possesses an outer dendritic surface area that is dramatically enlarged, up to 8 to 12 times greater than its neighboring neurons. This massive surface expansion consists of flattened, leaf-like projections, or lamellae, which maximize the cell’s capacity to capture and sense the gas molecules.
This highly tuned system allows the mosquito to register even minute increases in CO2 concentration against the background atmospheric level. While the ambient air contains about 400 parts per million (ppm) of CO2, a human breath plume can easily exceed this. The initial detection of this plume acts as an “activating” signal, switching the mosquito from a resting state to a focused, host-seeking flight pattern.
The mosquito uses the trail of CO2, which forms a plume carried by the wind, as a navigational cue to fly upwind toward the source. Following the concentration gradient guides the mosquito to the general vicinity of the breathing host. This signal can be perceived from many feet away. Once the mosquito is closer, however, the broad CO2 signal becomes less informative, requiring the insect to integrate other sensory data to confirm its target.
The Role of Secondary Attractants in Close-Range Targeting
While the CO2 plume is crucial for initial, long-range navigation, it is not sufficient for a successful bite, as many non-host CO2 sources exist. Once the mosquito enters the general vicinity of the host, it switches to a combination of secondary cues for final targeting. The CO2 acts as a catalyst, making the mosquito more receptive to these other sensory inputs.
One primary secondary cue is body heat, which the mosquito detects using specialized thermoreceptors. The warmth radiating from a host’s skin confirms the presence of a warm-blooded target. This thermal detection is often paired with the sensation of moisture, as the humidity from exhaled breath and skin perspiration provides another tell-tale sign of a living animal.
Alongside heat and moisture, the mosquito relies on a complex cocktail of volatile organic compounds (VOCs) emitted from the skin and breath. These chemicals are the true signature of a host, as they distinguish a living animal from a non-living CO2 source. Specific compounds found in human sweat, such as lactic acid, ammonia, and 1-octen-3-ol (octenol), are potent attractants. This blend allows the mosquito to confirm the identity and location of its blood source at close range.
As the final step, visual cues come into play when the mosquito is very close to the host. Mosquitoes are attracted to movement, which indicates a live target, and they also tend to be drawn to dark colors. This combination of long-range CO2 signals, followed by thermal, chemical, and visual cues, guides the mosquito to its blood meal.
Applying CO2 Detection Knowledge in Mosquito Control
The scientific understanding of the mosquito’s reliance on CO2 has been directly applied to the design of commercial mosquito control devices. Many outdoor traps use CO2 as the primary lure to draw mosquitoes away from people. This gas is typically generated either by burning propane, which produces CO2 as a byproduct, or by releasing compressed CO2 from a tank.
To maximize the trap’s effectiveness, the CO2 release is often combined with secondary attractants to mimic a host more completely. For example, many traps integrate chemical lures like octenol, which can increase the catch rate significantly. Some advanced lures contain even more complex blends, including lactic acid and ammonium bicarbonate, to replicate the full spectrum of human odor.
These CO2-baited traps are effective at capturing mosquitoes. However, the efficacy of these devices in reducing the overall number of mosquito bites in a given area is a subject of ongoing discussion. While they are successful at luring mosquitoes, some studies suggest that a single backyard trap may not meaningfully reduce the biting rate for humans nearby. The technology remains an evolving tool that requires strategic placement and continuous operation to be an effective part of a broader mosquito management plan.