Mosquito repellents are key to protect ourselves from mosquito bites and the pathogens they might carry. The most widely used active ingredient in insect repellents is N,N-diethyl-meta-toluamide, commonly known as DEET.

Highly effective, long-lasting (approximately five hours) and cheap to make, DEET is a gold-standard insect repellent. But even though it was developed more than 80 years ago, there are important gaps in our understanding of how DEET actually works.

A new paper in the Journal of Experimental Biology led by Claudio Lazzari from the University of Tours, France, now shows mosquitoes can be conditioned to be attracted to DEET.

This provides an important piece of the puzzle in our understanding of how DEET works, and hints that this important mozzie repellent could have a vulnerability.

A vital tool that’s not fully understood
Insect repellents are a major method of protection against mosquito-borne diseases including malaria, dengue, chikungunya, Ross River virus, Japanese encephalitis virus and more. Many of these diseases are expanding on a global scale due to travel, urbanisation and climate change.

Female mosquitoes transmit parasites and viruses when they feed on vertebrate blood, which they need to provide proteins for egg development. To find their next blood meal, mosquitoes are strongly attracted to odours and physical cues emitted by warm-blooded “hosts”, including humans.

These include carbon dioxide we exhale, lactic acid in our sweat, and a complex combination of other chemicals that varies between people. Mosquitoes detect all these with sensory organs located in their antennae, proboscis (the pointy mouth part they use to suck blood) and the maxillary palps that flank it.

DEET has been in widespread commercial use since the 1950s, but there’s a lot of scientific debate over how exactly it works as a mozzie repellent. Is it blocking the odour of the host, is it toxic to the mosquito, or something else?

In 2008, groundbreaking research showed DEET blocks the response of sensory neurons to host odours in mosquitoes and vinegar flies. This means DEET is likely “confusing” the mosquito rather than repelling it. A couple of years later, scientists found a small portion of mosquitoes exposed to DEET are insensitive to it, and it’s a heritable trait.

This means mosquitoes do have a physiological response to DEET. But there are also signs some of the mozzie reactions are behavioural. In one study, mosquitoes exposed to DEET were less sensitive to it if exposed again within three hours. This hints they can temporarily get used to the chemical.

What did the new study find?
The new study shows it’s possible to condition mosquitoes to bite more if they’re repeatedly exposed to DEET during a blood meal. Not only does this tell us more about how it repels mosquitoes, but it raises the prospect mosquitoes may actually be attracted towards DEET in some cases.

First, the researchers developed a behavioural test. They kept mosquitoes in tiny cages and moved a food target (a warm bag of blood) towards them, recording proboscis movements when they sensed the target. This was the “biting attempt response”.

To test things further, the team ran a classical conditioning experiment. Mosquitoes were run through one of five “training programs” exposing them to various combinations of an unconditioned stimulus (heat), a conditioned stimulus (short exposure to DEET in a plume of air) and a reward (a short opportunity to feed on blood).

Here’s where it gets surprising. The mosquitoes whose training program included a squirt of DEET while they were already feeding on blood, afterwards had a significantly higher biting response when exposed to DEET again.

If the mosquitoes were exposed to DEET before being offered the blood bag, none of them tried to bite it.

Then, one of the researchers boldly offered her hands up for testing. One of the hands was treated with DEET. About 50% of the mosquitoes who went through the DEET-blood meal training program tried to bite the hand coated in DEET. By contrast, 100% of untrained mozzies avoided the hand covered in DEET and went for the clean one instead.

What does all this mean?
It’s well established mosquitoes can learn and retain information. What they learn about hosts and their environment can in turn have an impact on disease transmission.

This study indicates DEET doesn’t just affect mosquitoes physiologically. There’s a cognitive response as well, which could be an important part of how it works.

The authors raise the possibility – if the concentration of DEET is not high enough to repel mosquitoes but they still sense it during a blood meal, would these mosquitoes then be more likely to bite people who smell of DEET?

It’s important to note the study happened in highly controlled lab conditions, and the training program the mozzies underwent may not reflect everyday scenarios. Future studies should try and come up with test conditions that better represent real-world situations to see if these results hold up.

At a time when mosquito-borne diseases are on the rise, DEET still provides highly effective protection. What this study contributes is an improved understanding of how DEET works – and how we might improve insect repellents in the future.