Mosquitoes study to keep away from 5 widespread pesticides after a single non-lethal dose, research finds
They say what doesn’t kill you only makes you stronger, but if you’re a mosquito, it might endow you with street smarts too.
- Some species of mosquito transmit diseases that kill hundreds of thousands of people every year
- Pesticides have been used to control them, but mosquitoes can quickly become resistant
- Mosquitoes can learn to avoid a pesticide if they survived a previous exposure, a new study shows
Lab experiments found most female mosquitoes who survived a dose of pesticide learned to avoid that chemical when they later encountered it on a net — even if a meal of blood was waiting for them on the other side.
These evasive maneuvers were consistent across five pesticides commonly used to control mosquitoes across the world, according to Frederic Tripet, a medical entomologist at Keele University, and co-author of a study published today in the journal Scientific Reports.
“We found a similar response that indicates [mosquitoes] can learn very quickly from their misfortunes.”
The study is the first experimental evidence that two widely spread species of mosquito can change their behavior to avoid contact with pesticides, he added.
“This was a black box. We knew it was happening, but we don’t really know the process.”
And understanding how mosquitoes behave could help tailor programs to better curb the spread of diseases, such as malaria, without over-relying on pesticides.
Chemical arms race
For some, mozzies are a mere pest to swat at barbecues, but for billions of people, being begged by an infected female mosquito can mean hospitalization or death.
Aedes aegypti mosquitoes can carry Viruses such as dengue, zika and chikungunya, and nearly half of the world’s population is at risk of malaria, caused by a parasite spread by infected Anapholes mosquitoes.
Some regions are home to clouds of mosquitoes so dense they can literally choke animals to death.
“At one point, I was doing postdoctoral studies in Galveston, Texas, and that’s one area of the world where humans living there really depend on mosquito control,” Professor Tripet said.
“There, cattle can die of mosquito inhalation — their lungs get filled with mosquitoes, and they die of that. It’s extreme.”
Since the first modern insecticides were developed in the 1930s, people have smeared and sprayed them around — sometimes too liberally — to keep mosquitoes under control.
For a pesticide to do damage to an adult mosquito, it must first penetrate its tough outer shell, or cuticle.
Once inside the body, the chemical slots into receptors on their cells, like keys in locks, and disables the insect’s nervous system.
But in the arms race between mosquitoes and insecticides, the humble mozzie has evolved some crafty ways to avoid being killed off by chemicals.
Some of these are physical and cellular changes, such as growing a thicker cuticle that’s impervious to insecticides, changing the receptor “lock” the chemical latches onto, and more efficiently mopping up toxins.
But there’s also the lesser-studied “behavioural resistance”, when mosquitoes change what they do, and where and when they do it, to dodge a pesticide’s deadly effects.
And this is what Professor Tripet and his colleagues at Keele University and the Vector Control Unit of University Sains Malaysia wanted to explore.
Into the mind of a mozzie
The researchers investigated how mosquitoes behave after exposure to five commonly used pesticides: malathion, propoxur, deltamethrin, permethrin and lambda-cyhalothrin.
Of the thousands of mosquito species out there, they chose two prolific disease transmitters: A. aegypti, as well as Culex quinquefasciatus, which can spread Diseases such as West Nile virus and a parasitic worm that can cause elephantiasis.
Only female mosquitoes were used because they’re the blood-suckers, needing protein found in blood to develop their eggs.
Professor Tripet and his crew first exposed female mosquitoes to a pesticide — enough to knock them out and even kill off a few, but not so much that they all died.
When the surviving mosquitoes came to, they were placed in one end of a tunnel. At the other end was bait: a rat.
Insecticide-treated nets have helped bring malaria transmission down over the past couple of decades.(Getty Images: Luis Robayo/AFP)
Separating them was netting, with some holes poked in it, treated with the same pesticide the mosquitoes had been exposed to previously.
Only 15 per cent of pre-exposed A. aegypti and 12 per cent of C. quinquefasciatus mosquitoes flew through the pesticide-laden net to get at the rat.
This was markedly different to cohorts of mosquitoes that had not been exposed to pesticides before the test.
More than half of those (58 per cent and 54 per cent of A. aegypti and C. quinquefasciatus respectively) chose to flit through the net.
In the end, mosquitoes who’d encountered the pesticide before were more than twice as likely to survive than their naive counterparts.
In another set of experiments, the researchers gave mosquitoes a choice of two boxes in which to rest. One smelled slightly of pesticide, while the other contained hints of silicone oil or olive oil.
Mosquitoes who’d been pre-exposed to the pesticide overwhelmingly chose the oil box, Professor Tripet said.
“They just didn’t want to rest in the compartment with the pesticide smell.”
Overall, those who’d not had that pre-exposure experience were as likely to choose either box.
Small insect, huge behavioral shifts
James Cook University’s Tanya Russell, who was not involved in the study, says the new research backs up what she and other medical entomologists have seen in the field for years.
“I’m really quite happy to see papers like this starting to break down how this [behavioural change] could be occurring,” she said.
Dr Russell and colleagues tracked how malaria-carrying mosquitoes in Solomon Islands shifted their feeding habits to circumvent insecticides applied on mosquito nets and on walls inside houses.
Indoor residual spraying, which kills mosquitoes that settle on the inside walls of houses, is another commonly used mosquito control tool.(Getty Images: Christina Aldehuela/AFP)
“But instead of taking blood meals from people during the night and inside houses when people are asleep, they started feeding on people outdoors and very early in the evening when they’re cooking — the kitchen is often outdoors,” Dr Russell said.
It’s not just a few mosquitoes that started dining earlier and al fresco, either. The practice spread through the entire local mosquito population.
These sweeping changes in behavior can happen quickly too.
Take, for instance, the case of DDT, a chemical widely used to control malaria mosquitoes in the middle of last century.
“They could see population-level differences in the behavior of mosquitoes within 12 months,” Dr Russell said.
“It was very fast, very, very quick.”
How can we better control mosquitoes?
The new study showed some mosquitoes could learn from their own experience and change their behavior accordingly.
Could this be passed down to their offspring?
“Of course, maybe there is an inherited component due to selection over time,” Professor Tripet said.
He speculated that mosquitoes in West Africa — “in areas where there is resistance to absolutely every pesticide you throw at them” — could have behavioral resistance written in their genes.
“So they never go inside, and they don’t need to learn it — they now have this innate behavior.”
Sprayed pesticides and “mosquito foggers” are sometimes deployed to kill adult mosquitoes if there is a disease outbreak.(Getty Images: Joe Raedle)
So mosquitoes can learn to stay away from insecticide-treated bed nets. That’s a good thing, right?
It is — to an extent, Professor Tripet said.
“But if you look at why [insecticide-treated] bed nets were designed, they’re really designed to have a human baiting the mosquitoes into touching the net and killing the mosquito.
“Therefore, that leads to a decrease in the population size of the mosquitoes, and that feeds into a decrease in disease transmission.”
If manufacturers added a substance to a pesticide that mosquitoes were drawn to, and exposure to a pesticide wasn’t a completely negative experience, they might not be as easily conditioned to avoid it should they survive, he added.
Professor Tripet is also keen to see more studies being conducted that expose mosquitoes to one pesticide and test them on another, or examine the behavior of other mosquito species — particularly Anopheles species, which transmit malaria.
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This study highlights the importance of mosquito surveillance in the tropics and sub-tropics, Dr Russell said — not only looking out for physiological variations, but behavioral ones too.
And there are tools and techniques that help keep a lid on mosquito populations that don’t depend on the behavior of the adult females.
For instance, killing mosquitoes during their larval stage, while they’re still wriggling around in water.
Some mosquito species, including A. aegypti, prefer to lay eggs in small water containers, so cleaning up plant pot bases and the like can help control local populations too.
Unfortunately, there’s no one-size-fits-all fix, Dr Russell said.
“It’s really just a matter of understanding your target mosquitoes and their behaviors, and then building up those effective control products, and tailoring programs for each area.”
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