The Hole Story: How Mosquitoes Seek Out Gaps in Nets or Screens
By Leslie Mertz, Ph.D.
It’s a hot and humid summer night, the window is open, a breeze filters in, and you begin to drift off to sleep when you hear the maddening buzz of a mosquito at your ear. After several unsuccessful slaps to the side of your head, you flip on the lights and see a little hole in your window screen. While chasing your tormentor for the next 15 minutes, you ask yourself how that mosquito was able to find such a tiny gap and sneak into your house.
For Andrew Dickerson, Ph.D., an assistant professor of mechanical engineering at the University of Central Florida, the question turned into a research project: How do mosquitoes identify miniscule voids, especially holes in the bed netting that is used in tropical areas of the world to exclude dangerous disease-causing species? “We know bed nets get torn, but we don’t know at what level they become ineffective,” he says. “And while we’re not the first ones to look into this, we are the first ones to look at it in the way we have.”
The surprising results were published last week in the Journal of Insect Science.
An Unusual Approach
Dickerson and his students put their expertise to work to discover the gap-finding capabilities of the yellowfever mosquito (Aedes aegypti). “What engineers can bring to these sorts of problems is a physical understanding of what’s going on, and that includes using physics and engineering tools to describe what insects do, because, after all, they are like little robots or little aircraft, if you will,” Dickerson says.
Their experimental setup included a 2-foot segment of 6.5-inch-diameter acrylic tubing in which the mosquitoes could fly. Before adding the mosquitoes, they fitted bed netting to the last third of the tube and put an attractant behind it.
The attractant posed a bit of a dilemma. “If you look at mosquito literature, there’s not really a consistent way in which people have attracted mosquitoes, and part of that is because different species respond to different attractants,” Dickerson says. He settled on the method outlined in one of the scientific papers he found in his research: a dirty sock. (For consistency in the experiment, the same undergraduate student wore the sock for at least 12 hours before each trial.) He also consulted a mosquito specialist at UCF who noted that heat and moisture are strong attractants for yellowfever mosquitoes, so they added a beaker of heated water next to the sock. Dickerson says, “With the heat and the humidity on top of the scents of the human sock, we thought the mosquitoes would not be able to resist that.”
The “Hole” Story
The researchers conducted trials with meticulously cut holes—ranging from 8 millimeters (mm) up to 20 mm—and found out some surprising things about mosquito strategy, Dickerson says. First, a mosquito will fly quickly toward the attractant, but it doesn’t find the hole through sight. Rather, it comes upon the holes by sheer chance. It flies at the net, bounces off, and either reorients to fly at the net again or takes a break and mostly just stands there. In fact, Dickerson noted that mosquitoes seemed to strongly prefer to travel via flight and covered very little distance by crawling, and thus they were much more likely to fly through holes rather than crawl through. For gaps smaller than the mosquito width in flight, landing at the gap and crawling through was occasionally witnessed.
Second, the mosquito sticks to the same area of the net. “Every time that they approach the net, the location of that approach is influenced by where they were in the previous approach,” he explains. In other words, if the mosquito is not close to the hole on its first try, it may never find the hole. Third, hole size is important. Mosquitoes don’t sneak through 8 mm gaps even though they physically can, and they prefer the 10 mm voids even more so than the larger holes.
There’s much still left to learn about how mosquitoes pass through bed netting, and Dickerson says he hopes to one day study a number of related topics, including why 10 mm is the sweet spot for hole size and how insecticide-treated nets influence passage. “I do think this is important work, but I just have to find somebody to fund it,” he says.
On a personal level, Dickerson admits that the study results, especially seeing what a big role chance plays in allowing a mosquito to find a void, have given him a little more peace when he is relaxing on his screened porch (or lanai) in hot and humid Florida: “This study does make me feel a little bit more at ease. Now if I get a hole in my lanai, I don’t fret about it as much as I would have otherwise.”
Journal of Insect Science
Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.