Introduction
In an interesting study published by Nature, researchers at the University of Washington have found that mosquitoes use several cues to pinpoint their targets and are especially drawn to the color red.
The study, led by Jeffrey A. Riffell, involved using a 3D tracking system and a wind tunnel to control the olfactory and visual environment of mosquitoes. Over 1.3 million mosquitoes were tracked in this system leading to a very comprehensive set of data.
The basis of the study
Insects have a behavioral preference for certain bands in the visible spectrum that they use for various important activities such as plant-insect/predator-prey interactions and disease transmission. Vision is important for biting insects such as mosquitoes and tsetse flies for behaviors such as flight control, locating host or nectar, object tracking, etc. Scent and heat also play an important role along with visual cues.
It has long been known that mosquitoes are attracted to dark, high-contrast objects, leading to the development of black traps. Recent work has shown that if CO2 (mosquitoes can smell CO2 unlike humans) is added to the environment, Aedes aegypti mosquitoes get highly attracted to black objects and ignore these objects if CO2 is removed.
However, despite the knowledge of the above, details regarding which wavelengths attract mosquitoes and which odors sensitize them remained unclear. The study by Dr. Riffel’s group aimed to provide us with more details on mosquito behavior.
The experiments
In a series of ingenious experiments, the team tracked the behavior of female (only female mosquitoes drink blood) yellow fever mosquitoes, Aedes aegypti, as they were presented with various visual and olfactory baits or cues in test chambers. Various odors were sprayed into the chamber and a visual cue was present at the bottom of it. This helped test whether the mosquito was attracted to a specific visual cue in the presence of a specific olfactory cue.
Without any odors, the mosquitoes mostly ignored the visual cues. However, with the addition of CO2, mosquitoes were more willing to visit the visual cue if they were red, orange, black, or cyan. Green, blue, or purple colors were largely ignored.
The experiments were also repeated with human skin tones as visual cues. Mosquitoes flew to the visual cue only if CO2 was sprayed in the chamber. If color filters removed long-wavelength signals, the CO2-primed mosquitoes were no longer flying toward the cue.
What could be driving these mosquitoes?
Dr. Riffel’s team hypothesized that the Aedes aegypti are strongly influenced by the opsin-1 gene, which helps recognize CO2, and the opsin-2 gene, which helps in recognizing the green to orange band of the visual spectrum. They tested this by using opsin-1 and opsin-2 mutants in the same test chambers.
Mosquitoes with mutated opsin-1 showed no color preference while opsin-2 mutated variants were unable to see the visual cues (almost colorblind) even in the presence of CO2.
Further explorations
The investigators say that the influence of other visual and odor cues, such as skin secretions, need to be studied. In addition, other mosquito species might be influenced by different visual and olfactory cues, which also need to be explored in the near future.
Reference
- Alonso San Alberto D, Rusch C, Zhan Y, Straw AD, Montell C, Riffell JA. The olfactory gating of visual preferences to human skin and visible spectra in mosquitoes. Nat Commun. 2022 Dec;13(1):555.
