Fooling mosquitoes

Imagine that somewhere in your vicinity right now there is a mosquito looking for its next meal of blood. Run for it? Oh no my friend, because unfortunately for you the cheeky little fellow will have already picked up the scent of the carbon dioxide (CO2) in your breath. Mosquitoes are capable of detecting CO2 trails and following them to their hosts over long distances you see. But recently we may have found a way to fell a metaphorical tree over the mosquitoes’ gasseous tracks.

Frankly, it would be ideal if mosquitoes didn’t keep biting people. Mosquitoes carry the parasites and viruses that causes diseases like Malaria and Dengue fever and the WHO estimate that for every 5 children that die in Africa, 1 of their deaths can be attributed to Malaria. As I’ve written before, there are lots of different strategies scientists use to try to fight pests like mosquitoes, but one effective method is to use hormones or strong smells to confuse them. This has the benefit of not actually killing the insects, so unlike situations where pesticides are used, the species isn’t able to build up resistance by the process of natural selection.

It is possible to use decoy CO2 chambers as traps for mosquitoes (positioned near a house for example, to lure the mosquitoes away from it). This sounds like a great idea, but the practicalities prove tricky. Providing a source of CO2 in the kind of places where mosquitoes are a problem – developing countries for the most part, that is –  is not usually easy. CO2 canisters or handfuls of dry ice  are in understandably short supply in these locales.

I put some dry ice on my desk and gave it a squirt of water to get the classic hollywood style bubbling chemical effect. Of course the ‘smoke’ you can see above the ‘ice’ is not CO2 (which is invisible) but condensing water vapour.

I was intrigued recently then, when I discovered some new research [1] conducted by  scientists from the University of California which showed that simple, inexpensive [2] organic molecules can also trigger the mosquitoes CO2 smell receptors. In fact when the researchers measured the mosquitoes brain activity when they smelled the organic compounds, identical parts of their brains lit up as did when they smelled CO2. But when they tested 2,3-butadione (below) something strange happened. The mosquitoes’ neurones started to fire like crazy and didn’t stop. In fact, a 1 second pulse of butadione induced a firing of the neurones for 5.5 minutes [3]. With the CO2 signaling neurone gone haywire, it seems likely that the mosquito would no longer be able to sense delicious CO2 trails.

Three simple organic molecules: 2,3-butadione (top), butanal (middle) and hexanol (bottom).

But the scientists needed proof that the effect they had seen the simple molecules have on the mosquito brain really did translate into a real life difficulty in locating  CO2 emmitting hosts. To do this they stuck the mosquito in a plastic chamber with a CO2 source at one end. If the researchers let the mosquito alone it was easily able to find its way to the delightful CO2 treat. If, however, they gave the critter a squirt of a mixture of the organic compounds shown above the mosquito had no idea what was going on and flew around and around the chamber for several minutes; never making it to the CO2 [4].

This is great news and could lead to effective and cheap new traps and mosquito repellents. It would be a lot like using citronella sticks on a Scottish campsite to dissuade midges from crawling into your sleeping bag.

The problem with the study comes in the form of B.O. That’s right: unfortunately the crop of volatile chemicals produced by humans is not limited to CO2, but includes a complex mix of sweaty odorants. Mosquitoes use these too, not just CO2 to find their hosts. This means it’s too early to celebrate; we can’t do that until we see some proof that mosquitoes are so confused by the 2,3-butadione cocktail that they can’t track down real humans. So like too many articles about science I sadly have to finish by saying ‘we need more experiments!’ But this time the experiments are simple and have well-defined outcomes; if this is going to work we can expect the benefits very soon.

Photo credit: James Jordan, Flickr.

References

[1] S. Turner et al., Nature, 2011, 474, 87-91 (pdf here).

[2]2,3-butadione for example costs £20.30 per 1 mL from Sigma Aldrich for example (and Sigma are generally understood among chemists to charge a slight premium for the fact that they deliver the next day!)

[3] Which was the duration of the test period – the neurone was activated for at least this long.

[4] There’s a video of one of the confused mosquitoes here.

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