It is an occasion for wonder. Why is it that some people go through life completely ignored by mosquitoes, while others seem to exist as walking dinettes for the irritating insects? As it turns out mosquitoes are actually selecting their targets based on a combination of sensory cues. Scientists at The Rockefeller University examined the information that mosquitoes process when selecting targets, and discovered that the insects require at least two of three factors – heat, carbon dioxide, and odor – in combination before they bite.
Scientists were already aware the CO2 draws mosquitoes to their human prey. Dr. Leslie Vosshall, at The Rockefeller genetically engineered a mutant Aedes aegypti mosquito (the culprit in the spread of yellow fever). This version of the mosquito lacked a gene called Gr3, coding for a carbon dioxide receptor. Without the receptor, mosquitoes were unable to sense the presence of the CO2 gas.
Dr. Conor McMeniman, a postdoc in the Vosshall lab then tested the behavior of the mutant mosquitoes around cues they were supposed to be able to attend to. First, he looked at the mosquitoes in a chamber with a hot plate at the temperature of human skin. Normal mosquitoes didn’t show an interest in the plate unless it was emitting CO2 – and the mutants couldn’t be bothered with it at all. In a similar chamber, McMeniman found that normal mosquitoes were uninterested in a source of lactic acid (present in human breath and skin aroma), unless it coincided with CO2. The mutants who lacked the ability to perceive the CO2 were entirely uninterested in the lactic acid source.
The researchers then looked at the behavior of mosquitoes in a more natural setting. Here, the mutants seemed to be comparatively less impaired than in the more artificial situation. The insects were studied in an enclosure like a greenhouse in Australia; human volunteers sat in the enclosure counting the insects that landed on them. The mosquitoes lacking the CO2 receptor were somewhat less impaired than in the previous, less physiological experimental situation. Apparently, the mutant mosquitoes unable to perceive CO2 were selecting their targets based on a combination of sensory factors, such as size, shape, heat and odor cues coming off the human subjects.
Mosquitoes require blood to survive. Female mosquitoes collect blood because it is necessary for the production of fertilized eggs. McMeniman concluded that the mosquitoes used multiple sensory cues to make practical (if unconscious) decisions about behaviors such has who to go chew on. Evolutionarily speaking, this makes sense. All creatures are required to expend a certain amount of energy in pursuit of whatever it is they do. At the same time, they have to budget the energy they can afford to expend in order to take in the energy they need for other things. It makes no sense for an animal (even a mosquito) to go bite anything that’s warm if the blood thus harvested is nutritionally inadequate. By relying on several factors, the insect has a better chance of a really juicy bite when given a choice of targets.
This research translates into practical information when designing chemical repellents to block mosquitoes from spreading malaria. McMeniman’s research clearly indicates that multimodal combination of sensory cues such as heat, CO2, and human odor, guides the mosquito’s selection of human targets, therefore formulating repellants that block more than one of the mosquitoes sensory modalities (for example, CO2 and heat, or CO2 and lactic acid), are going to be more effective than repellants targeting a single sense.
By Laura Prendergast