Reference: Emily J. Dennis, Olivia V. Goldman, Leslie B. Vosshall. 2019. Aedes aegypti mosquitoes use their legs to sense DEET on contact. Current Biology, 29: 1551-1556; DOI: 10.1016/j.cub.2019.04.004
It’s summertime, so you know what that means: long days in the sun slathered in sunscreen and warm nights coated in bug spray.
That’s right, with warm weather comes a population increase of everyone’s least-favorite summertime creepy-crawly, the mosquito. Luckily there is a wide variety of products on the market to keep these blood-sucking fiends at bay. The vast majority of these products contain the chemical compound DEET.
A team of researchers at Rockefeller University lead by neurobiologist Emily Dennis recently cracked the final code in understanding what makes DEET a superior insect repellent.
So, what is DEET’s secret? The study proves it is DEET’s three-pronged attack on the mosquito’s nose, mouth and feet.
A Deep Dive Into DEET
DEET is the common name for the chemical N,N-Diethyl-3-methylbenzamide. The US Army developed the synthetic compound in the 1940s to protect military personal from disease carrying insects. Soon after DEET’s debut, the oily compound proved to be the most effective insect repellent on the market.
Scientists at Rockefeller University have studied the effects of DEET for over ten years. A previous study conducted in 2008 by team member Leslie Vosshall showed DEET deters insects by confusing their olfactory (scent) receptors. While this was a breakthrough in understanding the effects of DEET, Vosshall wanted to know more.
In 2013 Vosshall partnered with Dennis to create mutant mosquitoes with impaired olfactory receptors. The mutant mosquitoes lacked the gene orco, which builds the proteins needed to detect certain smells. The orco-lacking mosquitoes could no longer smell DEET; however, when they landed on a human arm slathered in the chemical, they quickly flew away. This finding prompted Dennis and Vosshall to conduct their most recent study.
In order to fully understand how DEET works, Dennis and her team had to separately analyze a mosquito’s sensory receptors: smell, taste and touch. Based on previous research, the team knew how DEET affects the mosquito’s olfactory receptors. As a result, Dennis used mutated mosquitoes without the ability to smell DEET, allowing them to focus on taste and touch.
First researchers offered mosquitoes three solutions: sugar water; sugar water mixed with DEET; and sugar water mixed with a similar bitter chemical (bitters). Unsurprisingly, the mosquitoes preferred the untainted sugar water. However, they did not make a distinction between the two chemical laden samples.
The researchers then spread high concentrations of bitters on their arms. To their surprise the mosquitoes happily fed. This proved that a bitter mouth taste was not a significant deterrent to the insects. Next the researchers offered the subjects droplets of blood under a thin membrane covered in either DEET or bitters. The mosquitoes fed on the sample smeared in bitters, but avoided the one covered in DEET.
Dennis hypothesized that the mosquitoes did not want to touch the membrane coated in DEET because of the tiny hairs that cover their legs. Each tiny hair can sense molecules meaning that mosquitoes can taste with their feet! To test this theory, Dennis offered the mosquitoes DEET covered blood that they did not have to land on to consume. When touch was removed from the equation, the mosquitoes happily enjoyed the meal.
This showed that unlike other insect repellents, DEET affects a mosquito’s sense of touch, which is now understood to be a crucial component in their feeding habits.
Knowing the Enemy
The study was conducted on Aedes aegypti, commonly known as the yellow fever mosquito.
This species can be identified by its brown body and white stripped legs. Like their common name suggests, Aedes aegypti can transmit viruses such as yellow fever, dengue fever, and Zika. Female Aedes aegypti transmit these viruses when feeding on human blood.
Aedes aegypti use three sensory receptors, smell, taste and touch to find a meal. A mosquito’s keen sense of smell allows it to detect carbon dioxide and body odor. These scents tell the mosquito there is a living human around to eat.
Why it Matters
DEET is highly effective but must be reapplied every few hours to keep bugs at bay. For some bug spray is a convenience, for others a necessity. Longer-lasting insect repellents could reduce the spread of viruses and save lives. By breaking down how DEET affects a mosquito’s sensory systems, scientists can work towards creating even more effective insect repellents.