Can mosquitoes learn to love DEET?
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But research published
yesterday in the Journal of Experimental Biology suggests that
mosquitoes may learn to associate the smell of DEET with dinner—and start
gravitating toward it instead of away from it. The findings challenge long-held
assumptions about how DEET works and what mosquitoes may be capable of
learning.
Training changed how mosquitoes react to DEET
For the study, researchers from the University of Tours in
France and Virginia Tech examined whether female Aedes aegypti mosquitoes,
the species that spreads dengue, Zika, yellow fever, and chikungunya, could
learn to associate DEET with a food reward.
The team used a form of Pavlovian conditioning in which
mosquitoes feed on warm blood through an artificial membrane. Twenty seconds
into their meal, the researchers released DEET into the feeding enclosure—a
process they repeated three more times before exposing the mosquitos to DEET
but no food reward.
When the trained mosquitos caught a whiff of DEET alone, more than 60% of them tried to feed again, displaying what researchers termed a “biting attempt response” (BAR). That’s compared with roughly 20% of untrained mosquitoes who performed BAR when exposed to DEET alone.
In another experiment, mosquitoes were given a choice
between two human hands. One hand was treated with DEET, and one was untreated.
All of the untrained mosquitoes avoided the DEET-treated hand. Trained
mosquitoes, however, were significantly more likely to orient toward the
treated hand.
Reward learning not limited to blood
In a final experiment, the researchers exposed mosquitos to
DEET while the insects fed on sugar. Later, when the trained mosquitos were
exposed to DEET alone (no sugar), most of them performed a BAR. The findings
suggest that the mosquitoes’ learned behavior was not limited to blood
alone.
“What this result suggests is that the neural circuits that
are processing the detection and interpretation of a ‘reward,’ or positive
reinforcement, are not specific to a given type of reward,” senior author
Clement Vinauger, PhD, Virginia Tech associate professor of biochemistry, tells
CIDRAP News.
“Practically, this implies that mosquitoes could learn the
association between smells and sugary rewards beyond the time window during
which they feed on our blood,” he says. “In other words, mosquitoes’ learning
abilities could influence many more aspects of their daily lives than we
currently think.”
DEET is dose-dependent
At commercial concentrations, DEET is highly effective at
repelling mosquitoes. “This is why, in our experiments, we had to give
mosquitoes the opportunity to start feeding before introducing the smell of
DEET,” says Vinauger. “The other way around, mosquitoes would simply refuse to
feed.”
“However, that repellency is dose-dependent,” he adds. “And
there is a risk that lower doses might still be detected by the mosquito’s
olfactory system, without effectively repelling it. In this case, one could
imagine that the mosquito experiencing this could learn that DEET isn’t so bad
after all.”
What the insect has learned matters just as much as
what the chemical does. That, I think, is a paradigm shift.
A common assumption about DEET is that it works because of
its chemistry—either it blocks mosquitos from smelling us or it simply smells
terrible to mosquitoes, sending them scurrying away. “But what we are showing
is that the mosquito’s brain can rewrite that response based on experience,”
Vinauger says in a Virginia Tech news release. “What the insect has
learned matters just as much as what the chemical does. That, I think, is a
paradigm shift.”
Proper use key to maintaining effectiveness
DEET was first developed by the US Department of Agriculture
in the 1940s and was originally used by the US Army during jungle combat. Today
it is the cheapest and most effective insect repellent in the world.
People should not stop using DEET, says Vinauger,
particularly in parts of the world where mosquito-borne illnesses are common.
But the findings suggest that, as concentrations of DEET on skin or clothing
decline over time, the repellent may become less effective—and concentration
may matter more than expected.
“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, but still strong enough for the insect to smell it, the
mosquito may be more likely to bite people who smell of DEET,” lead author
Claudio Lazzari, PhD, a professor at the University of Tours, says in a Company
of Biologists news release.
Our study highlights the importance of following the
manufacturer's recommendations for dosage and reapplication frequency.
The authors say more research is needed to determine if, in
the real world, mosquitoes could learn that DEET means dinner. “The laboratory
experiments we conducted represent a very particular scenario,” says Vinauger.
“They don’t translate directly into a real-world situation that we expect to
occur.”
At the same time, the data offer guidance on the most
effective way to use insect repellents. “Our study highlights the importance of
following the manufacturer's recommendations for dosage and reapplication
frequency with the specific repellents you use to reduce the risk of mosquitoes
learning to outsmart our control tools,” he adds.
