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Stumped by bed nets, mosquitoes turn midnight snack into breakfast

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Anopheles mosquito (unknown source)

Anopheles mosquito (unknown source)

One of the most effective methods for the control of spread of malaria is the use of bed nets infused with insecticides. Most species of mosquitoes (the Anopheles genus) that carry the malarial parasite (Plasmodium falciparum) are considered to be strictly nocturnal – they are active only during the night.

Thus, sleeping under the net provides protection against getting bitten by the insect vectors of the disease. The net does it in two ways – by providing a mechanical barrier between the mosquito and the human, and by killing mosquitoes that get in contact with the infused insecticide.

As we have learned many times, often the hard way, evolution tends to find a way around such tricks. A number of Anopheles species or local populations have evolved resistance to pyrethroid insecticides usually used in the nets. Yet, the mechanical protection of the net should still be effective, right?

Not so fast! A new study published in September 21 in the Journal of Infectious Diseases documents a behavioral change in a local mosquito population that effectively works around the safety protection of bed nets. What do they do that’s new? They changed the time of day when they bite!

Malaria-carrying mosquitoes are thought to all be strictly nocturnal. Recently, this dogma has started to be questioned, mainly because the rates of malaria did not significantly diminish in areas where bed nets have been implemented. Perhaps they fly and bite during the day, yet nobody bothered to test that hypothesis yet? A previous study noticed a shift in timing of activity and biting from middle of the night into early night. This study was quite systematic – repeating the experiment in two locations in Benin at three time-points: before, during and after the full implementation of bed nets in both locations.

Chronogram of the experiment. Translation for humans: 'chronogram' is an "aren't we sophisticated in our clever use of silly, opaque, uneccessary jargon" version of 'timeline'.

Chronogram of the experiment. Translation for humans: 'chronogram' is an "aren't we sophisticated in our clever use of silly, opaque, uneccessary jargon" version of 'timeline'.

What did they do? They collected mosquitoes in large numbers and recorded the time of day they caught mosquitos. In addition, they used morphology to identify the genus, and PCR to identify the species. Every single mosquito was Anopheles funestus. They tested the caught mosquitoes for pyrethrin resistance and did not detect any – every single mosquito died. Thus all the changes were strictly behavioral.

How did they collect them? They placed humans in strategic places as living targets. It looks pretty much like this video, except they actually captured the insects into vials, then transferred them into small bags:

During the period of just a few years as the bed nets got implemented in the two villages, local mosquitoes dramatically shifted the timing of activity. Instead of 2 or 3am, they now predominantly bit humans around 5am:

Shift in timing in mosquito activity in two locations over three sampling periods.

Shift in timing in mosquito activity in two locations over three sampling periods.

What does that mean?

First, we don’t know yet if this was an evolutionary (i.e., genetic) change or a purely behavioral change. It is possible that there was quite a lot of genetic variation in timing of activity in the population a few years ago and that the bed nets provided a selective regimen that skewed the population to consist mainly of late night and dawn-active individuals. It is also possible that there is sufficient behavioral plasticity in the mosquito allowing it to learn the new best time of day to go out foraging. I’d love to see the mosquitoes placed in isolation chambers to monitor purely genetic patterns of circadian rhythms of activity.

But let’s think more in ecological terms. There are several players here: the Plasmodium parasite, the Anopheles vector, the human host, and predators that eat mosquitoes, notably bats. I have written at length about this a few years ago. Here’s a simple schematic of how the system works when undisturbed:

A crude schematic of possible timing of activity in this ecological system

A crude schematic of possible timing of activity in this ecological system

If the mosquito shifts to almost dawn, what happens?

First, the humans are up and about, outside of nets, readily available to bite. If the humans are healthy but mosquitos are carriers, this is a good way to transmit malaria to them.

But the humans who are sick, the sources of malaria, are still not available. At the times when they undergo “quaternary fevers”, which are the times when malarial parasites are present in their blood (I explained this in great detail before), they are safely hidden by the nets in the middle of the night and they are not bitten by late-biting mosquitos.

Second, a mosquito that bites a human around dawn is much more likely to get detected by that human and be swiftly turned into a small, bloody mush.

Third, while a mosquito that flies around dawn may be able to avoid some of the bats (though not all of them – many bats hunt until the break of dawn), they are now increasingly vulnerable to other predators – frogs, lizards and birds – that tend to hunt at dawn.

As it often happens, there are pros and cons when it comes to evolving new adaptations. The bed nets are now selecting for new adaptations in mosquitoes. It is hard to predict what will be the pros and cons of those adaptations for human health, or the pros and cons of those adaptations for mosquitoes and their survival, or pros and cons of these adaptations to insects’ predators. Future research on this will be both very interesting to watch and very useful for control of malaria.


Nicolas Moiroux, Marinely B. Gomez, Cédric Pennetier, Emmanuel Elanga, Armel Djènontin, Fabrice Chandre, Innocent Djègbé, Hélène Guis and Vincent Corbel, Changes in Anopheles funestus Biting Behavior Following Universal Coverage of Long-Lasting Insecticidal Nets in Benin, J Infect Dis.(2012) doi: 10.1093/infdis/jis565

Comments 14 Comments

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  1. 1. aimee w 8:58 pm 10/3/2012

    A beautiful example of natural selection :)

    One thing, though – when we were in Russia recently, we noticed that the GIANT Siberian mosquitoes were most active (i.e. insanely voracious and hunting in packs) at dawn and dusk…

    I know that cases that far north haven’t been reported since the ’50s, but I’ve also read that malaria is spreading back into Russia.

    Link to this
  2. 2. Bora Zivkovic 9:30 pm 10/3/2012

    Anopheles mosquitoes are supposedly strictly nocturnal (night-active). The other species of mosquitoes, including the ones we normally have here in USA, are crepuscular (dawn and dusk), while Asian Tiger mosquitoes are completely diurnal (day-active).

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  3. 3. papadick 10:29 pm 10/3/2012

    Plasmodium falciparum is just one of the four “type” species of malarial parasite that is capable of infecting humans: P. falciparum, P. malariae, P. vivax, P. ovale – and there may be more. refer to the following from:

    Four distinct Plasmodium species infect humans: P. falciparum, P. vivax, P. malariae, and P. ovale. However, molecular methods have revealed the possible existence of other species or morphological variants (see box). For example, sequencing of the gene for the circumsporozoite surface protein (CSP) revealed that some individuals diagnosed with P. vivax infections were actually infected with a distinct species more closely related to P. simiovale, a simian malaria parasite which is morphologically identical to P. vivax. In addition, molecular analysis indicates that P. ovale consists of two clades that are as divergent as distinct species and Sutherland et al (2010) have proposed to designate them as sub-species: P.o. curtisi and P.o. wallikeri.

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  4. 4. Bora Zivkovic 10:40 pm 10/3/2012

    That is correct, @papadick. I did not want to make this post too complicated. I did mention that in the old, much longer post I linked here (twice).

    Link to this
  5. 5. rajsecret 12:04 am 10/4/2012

    so in India, we have neem tree( abundance and if you eat(or swallow) its 10-15 leave (its so bitter) everyday in morning then even if these malaria mosquitos bite u ..u survive. And if u r taking it since childhood then u survive a poisionous snake bite too. So, when they bite me they fell down unconsious and when they bite my wife she feel the pinch. :)
    But in the age of ipads who wants thess natural wonders.

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  6. 6. Bora Zivkovic 12:17 am 10/4/2012

    Barks of several trees contain quinine, which is a traditional medication to control malarial fever. But if a mamba bites you, you are dead no matter what you just ate.

    Link to this
  7. 7. Tiqueo10 6:21 am 10/4/2012

    Since some populations of mosquitoes’ habitats are in places where people can provide vital “blood-feasts” it would seem logical the ones with the ‘morning breakfast’ gene would dominate.

    Link to this
  8. 8. Eliezer 3:00 pm 10/4/2012

    l like how you had all those charts. They give out really good information.

    Link to this
  9. 9. love cakes 3:04 pm 10/4/2012

    i’ve heard alot about this topic and i think this blog really makes it even more clear for me now i know that mosquitoes spread alot of things not just leave bumps on your body

    Link to this
  10. 10. Bill_Crofut 6:09 pm 10/4/2012

    Re: “The bed nets are now selecting for new adaptations in mosquitoes.”

    Bed nets, put in place by humans with a purpose, would not seem to constitute natural selection.

    Rather, the situation would seem to have been summed up by the following observation:

    “…[W]e don’t know yet if this was an evolutionary (i.e., genetic) change or a purely behavioral change.”

    It would seem far more likely to me the second option is the correct one.

    Link to this
  11. 11. Bora Zivkovic 1:56 am 10/5/2012

    Artificial selection. I was careful not to use the term “natural selection” there. And we don’t know yet (if genetic or phenotypic change so far), so I don’t want to state if I “feel” one alternative is more likely than the other – let people collect data and find out.

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  12. 12. sykosys 6:35 pm 10/9/2012

    Far better to use a technology that’s not able to be adapted around. Working on a tech being tested in Guatemala, Canada, Benin… look up MosquitoDMZ (fb/t/www)

    Link to this
  13. 13. 2008RealityCheck 11:59 pm 10/18/2012

    Mosquito nets are coated with pyrethrins and piperonyl butoxide. The UN WHOPES website doesn’t seem to have an issue with the chemical hazards.

    The pyrethrin MSDS claims it to be ‘highly toxic’, ‘do not breathe’, ‘skin sensitizer’, etc. Putting nearly an entire continent’s children (UN expanding to all age groups) into nets impregnated with such chemicals where they breathe these chemical vapors, and come into skin contact with the net surface about 1/3 of their lives could be problematic.

    Piperonyl butoxide has additional detriments by lowering the IQ of children born to women sleeping in the nets.

    Link to this
  14. 14. DrSatyan 6:45 am 04/1/2014

    Malaria transmission by mosquito is a great concern. Use insecticide impregnated mosquito net is very useful as proved by WHO.Increasing resistance to insecticide and associated side effects can greatly handled by a newly invented mosquito Trapping net. It catches and kills mosquitoes all most all the mosquitoes come to bite sleeping person. No insecticide or chemical or electricity used. For more detail please visit.

    Link to this

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