Analyze This! Mosquito repellents that work
Summer is here. And it’s time to choose a repellent to avoid itchy and potentially dangerous mosquito bites. There are plenty of deterrents to choose from. These include sprays, citronella candles and wearable devices. But which is best? We look to data for the answer.
Earlier this year, scientists tested the effectiveness of several mosquito repellents. Those treatments included the OFF! clip-on fogger; Cutter Lemon Eucalyptus spray; a 98 percent solution of DEET, which stands for diethyltoluamide (Dy-ETH-ul-TAAL-yu-ah-myde); a spray made from 40 percent DEET; an herbal spray; and Avon Skin-So-Soft.
The researchers built a mosquito cage with three sections and placed it in a wind tunnel. Each of the volunteers then spent time becoming bait for the Aedes aegypti (AY-deez Eh-JIP-tye) mosquitoes. One by one, the recruits stood at the end of the tunnel that was furthest from a high-powered fan that was pulling air out of the test chamber. Sometimes the recruits were 1 meter (a bit more than 3 feet) in front of the cage, other times 3 meters from it. (Because the mosquitoes were screened in, the volunteers never got bit).
The fan pulled the scent of the volunteer and the repellent through the mosquito cage. The researchers then measured the share of mosquitoes (in percent) that flew toward the person wearing or placed near each repellent. And for each repellent, the scientists repeated the bait test four times.
The wearable OFF! Battery-powered clip-on fogger performed best in the wind tunnel. It actually killed mosquitoes. But the researchers note that those bugs were all in a screened cage. That meant they couldn’t escape the repellent’s chemical.
Although the fogger is very effective in such a controlled environment, it might not work best outdoors. If someone using the fogger was sitting still and there wasn’t much wind, the repellent vapor would stay where she was and should prevent mosquito bites. But for activities like hiking, biking or playing outside, the fogger may work less well. Moving around a lot could prevent the repellent vapor cloud from surrounding the wearer completely enough to repel many mosquitoes.
The advantage of sprayed-on repellents is that they would move with you, whatever your activity. Moreover, some of the sprays were nearly as effective as the fogger, even in the wind-tunnel tests. The best performing sprays contained either DEET or oil from the lemon eucalyptus plant.
Bracelets, the citronella candle and a wearable device that makes high-pitched noises (meant to drive mosquitoes away) all performed poorly in the tests. Each was little better than using no repellent at all.
This table summarizes the results of the experiments:
1. Examine the experimental design depicted in the image above. Identify three potential variables in this experiment, and explain how the researchers may have controlled each.
2.. The researchers noted that the OFF! Clip-On repellent killed every mosquito in the cage during this experiment. This was likely because the mosquitoes were exposed to a high dose of the active ingredient, metofluthrin. It is a common, plant-based neurotoxin used in mosquito repellents. Explain how you might redesign the experiment to potentially prevent killing the mosquitoes during the trial.
3, Explain what is meant by average attraction rate (percentage). Can you hypothesize why the researchers chose to measure repellent effectiveness in terms of average attraction rate, as opposed to average repellent rate?
4. How would you design an experiment to measure average repellent rate? How would your experimental design be similar to and/or different from the design used for the study described by this article? Explain your answer.
1. DEET, or diethyltoluamide, is an active ingredient in many mosquito repellents. According to the chart, what percent (by mass) of Ben’s Tick & Insect Repellent spray is DEET? If you spray a 2-milliliter dose, how many grams of DEET would that include? Assume the density of the sprayed solution is approximately 0.998 grams/milliliter.
2. According to the chart, what percent (by mass) of Repel Sportsmen Max Formula is DEET? If you spray a 2-milliliter dose of it, how many grams of DEET would that include? Assume the density of the sprayed solution is approximately 0.998 grams/milliliter.
3. If both sprays cost the same amount, which would you prefer to use? Explain your answer.
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Aedes aegypti A species of mosquito that can transmit the viruses responsible for several tropical diseases, including dengue fever, yellow fever and West Nile disease.
average (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
citronella An insect repellent made from oil from types of lemongrass.
DEET The abbreviation for the chemical diethyltoluamide. It is one of the most common active ingredients in insect repellents.
eucalyptus Several species of tall and aromatic trees found naturally only in Australia. Their wood is valued for timber. The oil found in the leaves has been used in medicine. And these trees are perhaps best known as the only thing adult koalas will eat.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
density The measure of how condensed some object is, found by dividing its mass by its volume.
hypothesis (v. hypothesize) A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.
insect A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.
metofluthrin A chemical used as an insect repellent. Because it is a neurotoxic chemical, meaning a poison that can target the nervous system, it is only used in mosquito-repelling devices that use a fan to blow the chemical's vapors away from the wearer.
variable (in experiments) A factor that can be changed, especially one allowed to change in a scientific experiment. For instance, when researchers measure how much insecticide it might take to kill a fly, they might change the dose or the age at which the insect is exposed. Both the dose and age would be variables in this experiment.
wind tunnel A facility used to study the effects of air moving past solid objects, which often are scale models of real-size items such as airplanes and rockets. The objects typically are covered with sensors that measure aerodynamic forces like lift and drag. Also, sometimes engineers inject tiny streams of smoke into the wind tunnel so that airflow past the object is made visible.
JOURNAL: S. Rodriguez et al. Efficacy of some wearable devices compared with spray-on insect repellents for the yellow fever mosquito. Journal of Insect Science. Published online February 16, 2017. doi: 10.1093/jisesa/iew117.
Source Story (Science News): Spray-on mosquito repellents are more effective than other devices