Science Interviews


Sun, 11th May 2008

Kitchen Science - How Attractive are You (to Mosquitoes)?

Dr James Logan, Rothamstead Research

Listen Now    Download as mp3 Part 1,2 from the show Repelling Mosquitoes

Ben -   Hello and welcome to Kitchen Science!  This week, I've come to Rothamstead research institute here I've met up with James Logan, Hi James...

James -   Hello there.

Ben -   I'm also here with two volunteers from Sir John Law's School - Jason, Hi Jason...

Jason -   Hello.

Ben -   And Emma, hello Emma...

Emma -   Hello.

The mosquito boxBen - We have a rather unusual experiment here today, usually we do something that you can try out at home, but I would hope that you wouldn't want to try this out!  Could you just describe what we have on the desk?

Jason -   We just have a box with a lot of mosquitoes in!

Emma -   I hope none of them are going to get out, really.  We're not going to get bitten.

Ben -   What are we actually doing with them?

James -   Well what we're going to do is look at how attractive people are to mosquitoes.  The mosquitoes actually respond to chemicals given off in our body odour, so if we place our hands on top of the cage there's a mesh screen, the mosquitoes respond to the odours, fly up to the mesh screen, and then they think they've landed on you so they start probing through the mesh.  We can count the number of mosquitoes that are attracted to a person.

Ben -   So do people vary in attractiveness?  I would have thought that we're all a good meal for a mosquito so they would bite any of us equally.

James -   Yeah, they certainly do.  There's a lot of anecdotal evidence that people differ in their attractiveness to mosquitoes, and we've shown it scientifically.  And what we've shown is that it is, in fact, down to the odours that we produce.  So some people produce different odours which affects the mosquitoes behaviour.

Ben -   Seeing as it's my fault that we're all here with the mosquitoes, I guess I should go first!  So What should I do to find out how attractive I am?

James -   Well, what I want you to do is place your hand above the cage, not touching the mesh - there's a cradle there to put your hand on.  Just hold it there for thirty seconds and I'm going to count how many mosquitoes fly up towards it.

How attractive is BenBen -   So I shall just put my hand above and if you wouldn't mind starting counting...

James -   Okay, so we've got one mosquito up there already, you can see them now sort of buzzing around inside the cage, that's because they're being activated by the chemicals.  One of the chemicals that activates them to fly is carbon dioxide, which is given off in your breath, obviously, but they can't smell that from here.  What they're smelling is the carbon dioxide given off through the skin on the hand.  There are other chemicals as well, and those are the chemicals that draw them in at a short range.  There's about 30 mosquitoes in that cage, and there's only about 2 or 3 that have flown up to your hand, so it looks as if you're not actually that attractive.

Ben -   Well I'm relieved to hear that I'm not too attractive, who would like to go next?

Emma -   I'll go...

Ben -   So they're all still definitely buzzing around, I'm guessing they're still active from when I had my hand there?

James -   Yeah, they are indeed.  That will clear quite soon and they'll start to react to Emma's odours.  So Emma, do you normally get bitten by mosquitoes?

Emma -   I just get bitten on my feet normally, nowhere else.

Ben -   It must be said that people's feet often smell, does this make the feet more attractive?

James -   It does to some mosquito species, certainly the Anopheles mosquitoes, which are the malaria vectors.  The other reason that people get bitten on their feet is that when the odour ploom of your body odour comes from your body, it falls away to the ground.  Mosquitoes locate you by flying along, quite close to the ground and that's the first part of the body that they hit.

Ben -   So it's more that your feet are the first bit of food that you offer?

James -   That's right, but the chemicals on the feet certainly are different.  Bacteria can have a big effect on the types of chemicals that are produced by the body, certainly there's a lot of bacteria on some people's feet and that's why they smell differently.

Ben -   Well Emma's had her hand there for a while now...

James -   They're certainly buzzing around, so they're definitely activated, and we've actually had about 7 or 8 landing just in the last few seconds there, so actually I think she's slightly more attractive than you are.

Ben -   So I guess we should see how attractive Jason is now...

Ben -   Okay, well Jason's had his hand there for a little while, how many have we seen landing there?

James -   We've got about 2 there, so not a very attractive person.

Ben -   Is there anything that we can do, if there anything we eat that might help repel them?

A mosquitoJames -   There's a lot of anecdotal evidence that some food you eat repels mosquitoes.  Garlic is one of them, marmite is another one, and the reason for that is that it contains vitamin b12, which apparently repels mosquitoes.  But in actual fact there's no scientific proof to support that, and most scientific evidence actually suggests the opposite.  There were some experiments done at the London School of Hygiene and Tropical Medicine, and they actually showed that if you eat vitamin b12 it makes absolutely no difference to your level of attractiveness.  So I would never recommend that you use food as a way of protecting yourself against mosquitoes.  In fact, last night I actually ate a lot of garlic in preparation for you coming just to show you that it doesn't actually work, if I stick my hand in the cage, I should get bitten.

Ben -   It seems strange that garlic would repel them, when you consider how if you eat lots of garlic you smell so much more.  You'd think they could smell you from miles away?!

James -   Yeah, eating garlic you can smell it yourself.  The types of chemicals you can smell are sulphur containing compounds, really nasty, eggy smells.  And the key question really is, does the mosquito respond to those key chemicals, because the body actually produces somewhere in the region of 400 different volatile chemicals and the mosquito doesn't respond to all of those, it only responds to a handful of those chemicals.  That's the question - do those chemicals that they respond to change when you eat something like garlic, and the answer to that, really, is no.

Ben -   So Jason and I are equally as attractive to mosquitoes, so it would seem, and Emma is currently 'winning' if you could call it that, would you like to have a go yourself?

James -   Sure, yeah.  I'm usually fairly attractive to mosquitoes, so I'll give it a go.

Ben -   I've heard that female mosquitoes are the ones that bite you, is that true?

James -   It certainly is, yeah.  If you actually look inside this cage you can see that we've got males and females, so the ones that are sitting around the edge there, and not moving at all, these are the males.

Ben -   So if the males don't bite, then where do they get their sustainence from?

James -   Well all mosquitoes actually feed on nectar, that's their 'energy supply' if you like.  Females need blood, and the reason they need blood is to produce eggs.

Ben -   So obviously a male mosquito is a more friendly mosquito.  They are very active with your hand on there, how many have you seen land so far?

James -   Well at the moment there's about nine up there, and they're probing through the mesh trying to get to my skin, so they think they've actually landed.  As you can see, there's a whole swarm below my hand, so they're certainly very attracted to me, unfortunately.

Ben -   I'm guessing that working with mosquitoes is not, in fact, the best line of work for someone who is clearly very, very attractive to them!

James -   That's true, but it is a good incentive to find a control!

Ben -   Well that sounds like very good encouragement to me.  Later on in the show, James will be putting his bare arm right into the box of mosquitoes to see if insect repellents really do repel.

Ben -   Welcome back to Kitchen Science.  So far we've proved that different people are differently attractive to mosquitoes and that eating a load of garlic for your dinner wont keep them away.  But is there anything we can do to avoid being bitten?

James -   Certainly we can cover up when you go out, and dawn and dusk times are the times when mosquitoes and other biting insects are most active, so stay in doors.  If you have to go out, cover up, wear long sleeves and trousers.  Also you can use insect repellants, and a repellant which contains DEET, which is a synthetic compound made by the US military back in the 1950s is actually the best thing on the market.  It works really, really well, but there are some  adverse side effects, for example it melts plastics and it's absorbed into the body, so I would follow the guidelines if you do use that product.

Ben -   Do you have any DEET that we can test out?

James -   I certainly do, I have a pot of DEET in a formulation which we can put on our arms and see whether we get bitten.  I'm so confident that it's going to work very well that I'm going to put my arm right inside the cage this time.

Ben -   So this time they can actually get to you and bite you.

James -   They can indeed, but hopefully this should work and they wont even land on me.

Ben -   So you obviously have a great deal of confidence in DEET.  There's a net on the side of the box, and so far we've only been exposing ourselves through a small gauze, but you're actually going to undo the net and plunge your hand into the box full of mosquitoes.

James -   That's right, and the DEET should work really well and keep them off my arm once I put it into the cage.

Ben -   Let's go for it, it's the moment of truth!

Arm in box of MosquitosBen - Okay, so your arm is clearly in the cage now, and all the motion of the box has kicked them up even more.  But none of them are landing on your arm yet, in fact, if anything they seem to be trying to stay away.  They're staying near the gauze on the top.  They do all seem to be staying away from your DEET covered arm.  But does DEET work on all mosquitoes, are there some that will still be attracted regardless of the DEET?

James -   DEET works on most mosquito species very well, but in our experiments what we have found is that, just occasionally, you get the odd one or two which does land and begins to feed, which ic quite interesting.  What is it about those individuals which means they're not responding to the DEET?

Ben -   What would have happened if you had put your arm in without DEET?

James -   I would most certainly have been bitten, probably about 20 or 30 times by now.  And you would have seen a big, horrible reaction on my arm.

Inside the boxBen -   So Emma, you turned out to be very attractive to mosquitoes, how do you feel about that?

Emma -   Slightly worried, I hope I don't get bitten too many times when I go away to different places.

Ben -   And have you had much experience with mosquitoes, do you get bitten on holiday?

Emma -   Like I said before, I tend to get bitten more on my feet, but I do get bitten quite a bit.

Ben -   Well obviously you'll need to wear some DEET to be careful.

Emma - Yeah, I'll take some DEET with me next time I go away.

Ben -   And Jason, you and I seem to be equally as attractive, and actually not very attractive to mosquitoes.  You must be pleased...

Jason -   Yeah, I was very happy!

Ben -   Again, do you find that this experiment bears truth in the real world?  Do you get bitten much when you're on holiday and that sort of thing?

Jason -   Yeah, I don't really remember getting bitten very often by mosquitoes, so I guess it shows that the experiment was right.

Ben -   Well thank you both very much for joining us.

Jason -   That's alright, thanks for having us.

Emma -   Yep, thank you for having us.

Ben -   And James, thank you for bringing your 'pet' mosquitoes along to show us, and for braving your arm with the DEET.

James -   You're very welcome.

Ben -   That's it for Kitchen Science this week, we'll be back with more very soon.


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