3D tracking of mosquito flight paths reveals how the malaria-transmitting Anopheles gambiae makes use of odor and heat to locate its target. Knowing this, more effective traps could be designed to lessen the spread of this disease.
This article also exists in French ("Suivre les vols de moustiques en 3D pour mieux combattre le paludisme"), translated by Timothée Froelich.
Female Anopheles gambiae mosquito feeding. (Source: CDC / Jim Gathany)
It’s a dreadful moment when you’re drifting off to sleep on a warm summer’s night and a tiny noise jolts you awake. Mosquito! In and out of your ear it seems to bob; up, down, advancing, retreating, and evading all attempts to slap it and put an end to that infernal buzzing. Though you may not have thought about it, you are likely aware of such flight behavior when certain mosquitoes are looking for a meal.
A team at Wageningen University in the Netherlands developed a system to track the flight of the Anopheles gambiae mosquito in 3D, and tested them under different conditions in a climate controlled wind tunnel. The results published May 2 in PLOS ONE (available on MyScienceWork), show how human odor and heat interact to draw these mosquitoes to their target. When it comes to malaria-spreading mosquitoes, this information can provide important clues for the development of cheaper, more effective traps.
Jeroen Spitzen, a research associate in the lab of Willem Takken, explains what makes An. gambiae – the group’s mosquito of choice – so special. Unlike other species, it has a marked preference for human blood over other animals’. This means that it is extremely effective in spreading malaria from one person to another. “We have found elements of human odor that attract them, so we can now synthesize the molecules and put blends of these in traps. We also need to know how a mosquito uses odor cues to get close to the source. And what it does when it loses the source.”
The researchers developed a system to track the insects’ highly complex flight in three dimensions. A wind tunnel, consisting of a box 60 cm in height and width, by 160 cm long, was fitted with a cone at one end, through which air was passed at a rate of 20 cm/sec. Depending on the condition being tested, a heat source and/or a worn nylon sock was inserted into the cone. (The human odor variable was provided by a sock worn by a person for 24 hours, and frozen between experimental uses.) At the opposite end of the tunnel, infrared light shone into the space and two cameras captured its reflection off a single mosquito’s wings. From this information, a full 3D flight path was constructed.
Quantifying mosquito flight
“This was the first time we could quantify their flight,” says Jeroen Spitzen. The resulting video shows significant differences between mosquitoes flying idly, and those on the trail of a meal. All mosquitoes take off flying in an upwind direction, which they sense via mechanoreceptors on their antennae. But, without any odor or heat cues, the insects fly straight on and land on the opposite wall. Mosquitoes tested under odor and heat conditions, however, zigzag along their flight route, seeming to scan the air for more information as they home in on their target (clearly visible in the video and figure below.)
The principal cues for the strictly nocturnal An. gambiae are the odor produced by bacteria on a person’s skin and his or her body heat. From the current study, it is not clear whether heat contributes to the mosquito’s targeting of a meal by enhancing the movement of odor in the air, or if it is an attractive factor in itself. Jeroen Spitzen leans toward the latter: “I do think heat tells the mosquito that it’s close to the source. Without heat, they zigzag a lot, too, but don’t land near the source. Odor cues only help them get most of the way there.”
All of these details about a tiny insect’s flight could have big implications for the design of better anti-malaria measures. Current traps rely on a fan to physically draw mosquitoes in. This means they need a power source to run. “Ideally, we will have traps that will do without power,” Spitzen says. The goal is to have an apparatus that not only uses synthesized molecules mimicking human odor, but absorbs heat during the day and releases it slowly at night when the malaria mosquitoes bite. On-the-ground tests of new traps are the next step planned, in a large field trial in Kenya, for instance. “What we do in the lab is nice and fancy, but we need to go to the field as soon as possible, because that’s where the problem is.”
Figure 3. Examples of flight tracks of Anopheles gambiae s.s. for each treatment viewed from different angles. (Source: PLOS ONE)