Mosquitoes may have surprisingly overcome one of humanity’s best defenses against them, coming to associate the smell of DEET with a nearby meal, an international team of researchers says.
In a recent paper published in the Journal of Experimental Biology, the researchers identified that repeated exposure reduced DEET’s repellent effect on mosquitoes. However, their findings don’t end there; the team also discovered that under certain conditions, DEET may actually begin to attract mosquitoes rather than repel them, offering a strange glimpse into nature’s adaptive qualities.
DEET and Mosquitoes
DEET, the common name for diethyltoluamide, is a clear or slightly yellow liquid used to ward off insects, such as ticks, fleas, and mosquitoes. It has been used by the US military since 1949 and by civilians since 1957.
Claudio Lazzari of France’s University of Tours and Clément Vinauger of Virginia Tech led the international study, rooted in Ivan Pavlov’s famous 1890 study of conditioning, in which he noted that any indication that a dog was about to be fed, such as the ringing of a bell, would cause it to salivate, even without the sight of food.
Yellow fever mosquitoes (Aedes aegypti) were the subjects of the team’s research. This particular species is known to infect millions of humans with deadly diseases such as dengue fever, Zika, yellow fever, and chikungunya every year.
Feeding Them Blood
Since the insects feed on blood, the team first tested their attraction by placing a bag of warm blood on the other side of a fabric mesh restraining the mosquitoes, to observe how much effort the creatures would expend attempting to stab through to the meal. They found that insects were extremely enthusiastic, yet backed off when the smell of DEET was introduced.
They next devised an experiment to see if that could produce Pavlovian conditioning in the mosquitoes, getting them to associate the smell of DEET with feeding time. In a remarkably short time, the researchers observed a positive result. They began the experiment with 30-second feeding periods, during which the last 10 seconds introduced DEET. After a mere four repetitions of this tactic, the team found that 60% of the mosquitoes attempted to feed solely in response to the smell of DEET.
Lending further confirmation to the finding, the team offered one of their colleagues, Ayelén Nally, from the University of Buenos Aires, Argentina, a free meal to the insects. One of Nally’s hands was sprayed with DEET, while the other was clean. Surprisingly, the mosquitoes showed a strong preference for the DEET-covered hand over the clean one, once they had been conditioned to associate the scent with food.
DEET Remains Useful
The team repeated the process, next training the mosquitoes to associate DEET with receiving a sugary treat, producing the same effect. The team says their findings indicate that, in the right scenario, DEET may shift from a repellent to an attractant for pests.
“If a mosquito bites someone who applied DEET to their skin several hours earlier and the concentration of the repellent is too low to repel the mosquito,” Lazzari said, “but still strong enough for the insect to smell it, the mosquito may be more likely to bite people who smell of DEET.”
The researchers say that their work is only the beginning of efforts to better understand how insect repellents work, demonstrating that learned behavior may play a role. Despite their findings, they note that DEET generally works and saves lives by reducing insect-borne illnesses.
“If someone applies DEET and the concentration fades over time, but a mosquito still manages to feed, the insect may begin associating that smell with a reward,” Vinauger concluded. “That’s a possibility we should take seriously when we think about how repellents are used in the real world.”
The paper, “Associative Learning Switches DEET Valence from Aversive to Appetitive in Aedes Aegypti,” appeared in The Journal of Experimental Biology on May 28, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.