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.

While most fruit flies are known for their attraction to fermenting fruit, one species has evolved to hunt in fast-moving streams in Africa, taking on the role of a predator.

A team of researchers from Lund University has mapped the genome of Drosophila enhydrobia, a fruit fly with a unique life cycle. Its larvae develop underwater in fast-flowing streams, where they prey on black fly and midge larvae. The study, published in Current Biology, reveals how a lineage that was once considered a household nuisance transitioned into a new ecological world and identifies the genetic changes that supported this shift.

“We’re talking about a fruit fly that has completely turned its lifestyle upside down,” said Marcus Stensmyr, biology researcher at Lund University and lead author of the study. “From feeding on yeast and rotting fruit, it has become a specialized predator in running water.”

Museomics Provides an Answer

D. enhydrobia has not been observed in the wild since 1981. To obtain genetic material, the research team located a single pinned specimen in a natural history museum in Zurich and used modern DNA techniques to extract an almost complete genome without damaging the specimen.

This method, called museomics, is part of a wider effort to recover genetic information from museum collections that wasn’t accessible when the specimens were first collected. The Zurich specimen, preserved for about 40 years, still contained enough intact DNA for the researchers to conduct both phylogenetic and comparative genomic studies. Earlier technology could not have achieved this result.

Not an Evolutionary Loner

One of the main findings is that D. enhydrobia is not as biologically isolated as once believed. Genomic analysis shows it belongs to a group of flies linked to water-adjacent habitats, mostly in South Asia. Its relatives already possess traits that have evolved into an extreme aquatic lifestyle in this species.

“What at first looked like an evolutionary mystery turned out to be an extreme elaboration of something that already existed,” Stensmyr said. “That makes the story both more understandable and, in a way, even more fascinating.”

A Genome Trimmed for a Different Life

Genomic analysis reveals evidence of genetic trade-offs associated with adaptation to an aquatic environment. The analysis shows that the species has lost several gene families involved in smell, taste, and metabolism, which fruit flies that feed on fermenting food typically rely on. For a species whose relatives rely on chemosensory detection to find food and mates, these losses are significant. The remaining sensory genes display signs of intensified selection, suggesting adaptation to new ecological pressures.

“It’s as if it has fewer tools in the toolbox, but the tools that remain are all the more finely tuned for this particular environment,” said Hamid Ghanavi, a biology researcher at Lund University and co-author of the study.

The findings suggest that major evolutionary shifts can involve losing functions that no longer serve a species, while refining those that do.

The Potential of Museum Collections

In addition to its evolutionary findings, the study is a prime example of the value of natural history collections worldwide. Specimens collected many years ago can now provide new genetic insights thanks to modern sequencing technology.

For species that have disappeared from the wild or gone unobserved for years, museum archives may offer the only source of available biological material. The D. enhydrobia specimen examined in this study serves as an example of this; without it, the genetic history of this unusual fruit fly would have remained unknown.

Stensmyr said his team has only scratched the surface of what those collections might contain. Continued advances in ancient DNA recovery could make museum archives a significant resource for tracking how species have evolved over time and how they might respond to future environmental shifts.

Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.