Repelling Pests - Mosquitoes, Moths and Weeds

Using nature's own tricks, we find out how to avoid mosquito bites, grow more crops and keep the weeds away in this week's Naked Scientists. We find out why mosquitoes...
11 May 2008
Presented by Chris Smith, Dave Ansell

Mosquito biting

A mosquito biting a finger


Using nature's own tricks, we find out how to avoid mosquito bites, grow more crops and keep the weeds away in this week's Naked Scientists. We find out why mosquitoes prefer some people to others, and how to use pheromones to turn an insect's sex-drive against it! Also, why dentists could use plasma to cut down on drilling, why breast is best for bigger brains and why Popeye was right, spinach really does make you stronger. Plus, in Kitchen Science, we brave a box of biting mozzies to see if insect repellent really does repel!

In this episode

Cinchona pubescens - fruit (close-up)

- The Royal College of Pathologists at the Chelsea Flower Show

The link between plants, pathogens, diseases and remedies...

The Royal College of Pathologists at the Chelsea Flower Show
with Dr Tim Wreghitt, Royal College of Pathologists

Dr Tim Wreghitt joined the Naked Scientists to discuss a display he has created on behalf of the Royal College of Pathologists for the RHS Chelsea Flower Show 2008. The aim is to explain the link between plants, diseases and remedies...

Chris - James (Logan) was saying, luckily no diseases in England at the moment are spread by mosquitoes but that could change and that’s the thrust of what you’re doing at Chelsea this year.

Tim - Yes it is, it could change. We have the mosquitoes in this country which can transmit malaria. What we don’t have is a lot of malaria for the mosquitoes to transmit. But it could happen and it happened in 1918 when the people came back from the Great War from Thessalonica. There was quite a sizeable outbreak in Kent. In previous centuries there’d been outbreaks in the fens, East of England, Southern England so it could happen.

Chris - What are you trying to achieve with your stand? You’re putting a garden into the Chelsea Flower Show from the Royal College of Pathologists. What’s this aiming to achieve?

Tim - 2 things really. One is that we’re trying to tell people basically that the 2000 people that get malaria each year, travelling abroad to malarious areas, needn’t get it if they take proper precaution.

Chris - That’s people coming back to this country?

Tim - Yes, people coming back to this country. If they took proper precautions then they probably wouldn’t, well not very many of them would get infected. There are 20 people a year that die of malaria.

Chris - In the UK?

Tim - In the UK, yes and the second thing is that we’re trying to educate the people at the Chelsea Flower Show what plants are associated with malaria in terms of treatment.  The cinchona plant that you get quinine from.

Chris - Oh right, that’s where it comes from.

Tim -   The cinchona tree, in 1630 was where it was first discovered and the countess of Cinchon and all that stuff. Artemisia annua is a Chinese wormwood and that’s another plant that we grow to produce artemisin in. We’re also showing plants that can repel mosquitoes, James. Hopefully there won’t be many around.

Chris - I understand James is lending you some mosquitoes to show at Chelsea.

Tim - He is. He’s very kindly leant me some. I think there are some in water and some flipping around in a box. I hope the boxes are very securely fastened because I don’t want them flapping around me, thank you very much.

A dental officer and his assistant remove the wisdom tooth of a crew member of the nuclear-powered aircraft carrier USS DWIGHT D. EISENHOWER (CVN-69) during Fleet Ex 1990.

Plasmas in your mouth

Many people dread the sound of the dentist's drill, part of the reason that it has to be used so much is that the dentist has to be absolutely sure that they have got rid of all the bacteria underneath a filling.

DentistResearchers in china are working on an answer, they are developing a plasma needle. Plasmas are gasses where some of the electrons have been ripped off their atoms, or molecules. They are found in stars, fluorescent lights etc. Plasmas have lots of energetic ions and free radials, which are very reactive chemically this means that they will interfere with the workings of a bacterial cell, killing them efficiently. However a plasma is normally extremely hot so you wouldn't want to put one anywhere near your mouth.

A plasma needle on the other hand is much cooler, it consists of a thin tube with a wire running down the middle. you pump gas through the tube and pulse high voltages on the wire. these break some gas molecules apart without heating the gas much. Up till now the largest needles produced have been a few mm long or too hot, but XinPei Lu at Huazhong University of Science and Technology has worked out a way of studying how the electrons are moving in the plasma and so has managed to make a 4cm long needle which is safe to touch.

He is mostly looking at dental hygiene applications reducing the amount of that dreaded drilling, but it could also be used for steralising medical aparatus sensitive to heat.

Breastfeeding an infant

Breast is best - for a big brain

A large study carried out by scientists in Canada strongly suggests that breast feeding boosts IQ, by about 6 points.

Breastfeeding an infantMichael Kramer and his team at McGill University and the Montreal Children's Hospital enrolled over 17,000 infants in a study designed to overcome one of the main criticisms of previous breast-feeding research, that babies who are breast-fed also tend to be born to parents who are themselves better educated, thus accounting for the apparent increase in intelligence.  But in this study, dubbed PROBIT (short for Promotion of Breastfeeding Intervention Trial) and published this month in the Archives of General Psychiatry, a group of clinics were randomly assigned either to adopt a programme supporting and promoting breast feeding, or to continue with existing policies.  Infants brought to one of these facilities were followed up for 6.5 years.

Using this approach, there were no significant differences between the educational levels of the mothers visiting either the "control" (non-intervention) clinics, or those promoting breast-feeding.

The impact of the intervention was very marked.  About 45% of mothers visiting a centre promoting breast feeding were feeding their babies breast milk between 3 months and 1 year of age, compared with only 6% of mothers attending the "control" clinics.  When the children were assessed at the age of 6.5 the breast-fed children scored, on average, 7.5 points higher on verbal tests, 2.9 points higher on non-verbal tests and 5.9 points higher on measures of overall intelligence.

Despite this clear relationship, the reason for the effect is still not altogether clear, say the study authors. "Even though the treatment different appears causal it remains unclear whether the cognitive effects of breast feeding are due to some constituent of breast milk, or are related to the physical and social interactions inherent in breast feeding." There are a number of chemicals and nutrients in breast milk could be responsible, including long chain fatty acids and growth factors such as insulin like growth factor 1 (IGF1), but breast feeding could also lead to better verbal interaction between mother and child, which could improve children's cognitive development.

The eFuel100 fermentation and distilling plant.

A fuel station in your garage

The rush towards biofuels to run your car on keeps going even if the mathematics behind it are possibly dubious. Now an American company has built an ethanol generation system for people to keep at home.

E fuel 100They call it the Efuel100 microfueler, and it is basically a brewery and a distillery in one easy to use package. You feed it a mixture of sugar water and yeast in a 750 litre tank, the yeast then converts the sugar into ethanol, in the same way that the sugar in barley is converted to alcohol when you make beer.

The machine then concentrates the alcohol to about 95% pure by distillation - heating the mixture, causing the alcohol to vapourise more quickly than the water, and condensing it in another tank.

This is still not concentrated enough to run a car on, because the small amount of water stops it mixing with petrol, so the machine pumps the mixture through a special membrane separating the last of the water from the alcohol and collecting the resulting fuel in a 140l tank. Apparently after a party you can also concentrate any unused alcoholic drinks and run your car on them.

It will apparently produce fuel costing about 30p a litre, although of course in this country it would be illegal on several grounds, for both not paying road or alcohol taxes, and it isn't really a green solution as sugar takes almost as much energy to grow and refine as it produces, but they are working on versions that break down cellulose, but they could take a while.

Still frame from the animated cartoon \Little Swee' Pea\ (1936)

Popeye was right, it seems - Spinach really does make you stronger

When Popeye underwent a muscle-busting transformation whenever he scented a whiff of spinach most people laughed at the artistic license, but it looks like the effect was based on fact, not fiction.  A team at Rutgers University led by Ilya Raskin has found that spinach contains steroid-like compounds that boost the growth of muscle cells.

PopeyeWriting in the Journal of Agricultural and Food Chemistry, the team explain how they extracted chemicals called phytoecdysteroids from the plant and then treated muscle cells in the dish and even live rats with them.  The muscle cells speeded up their growth by about 20%, and the rats became stronger.

Although a human would have to eat over a kilo of spinach each day to obtain enough of the chemical to achieve any kind of body-building effect, these plant-steroids are exciting for another reason: they seem to be able to exert their growth-promoting effects but without binding to the chemical docking stations normally exploited by anabolic steroids.  This means that they could be able to achieve beneficial effects in the body but without the side effects associated with anabolic steroid use - including voice changes, acne, hairiness, mood disturbances and an increased risk of cardiovascular disease.So one can expect Olympics officials to now be on the lookout for athletes who have developed a voracious appetite for greenery...

What causes ice ages?

Dave: There's various things we call ice ages. Strictly speaking we're actually in an ice age now. Over the last few tens of millions of years we've had periods when the north and south poles have been frozen but there've been other periods in the Earth's history when they haven't been frozen at all. Over the long period we are in an ice age at the moment. People aren't quite sure why that's been happening. It's been suggested it might be something to do with the Himalayas growing and altering he climate patterns in the world. I'm not entirely sure of the reasons for that. On the shorter scale there's various other things that will affect the climate: one of them is we get cycles of exactly where the Earth's pole is pointing. Sometimes it's almost vertical at which point the seasons are very weak. Some times it's keeled over more. Chris: It's the Earth's tilt, isn't it? It's about 23.5? and it wobbles a bit, doesn't it? The planet sort of wobbles backwards and forwards a little bit on its orbit. I think it's over 30,000 years or something like that.Dave: Yeah, there's various different cycles on different forms of vibration. Chris: Why should that make an ice age?Dave: If you suddenly have stronger seasons that will affect it, the way water and air flows around the world. Subtle changes like that can affect the temperature of the whole world. That could be creating ice ages.Chris: It's a sort of positive feedback loop too, isn't it? There's this thing called a Milankovic cycle because where the Earth doesn't go round the sun in a perfect circle it's an ellipse over time you get more of an ellipse than other times. This means that more energy reaches the Earth sometimes from the sun than other times and this means the planet goes into phases of cooling and warming but once you're into cooling it makes it easier for ice to form. Once you have ice forming it means you can have more ice. More ice reflects more light back, because it's very white, back into space so the Earth cools a bit more and it goes into a positive feedback loop: making it very cold. When something breaks that cycle we warm up again. Dave: That's right and that's why we should be so worried about human-led climate change because if we make a small effect you never know, there might be a load of other positive feedback. You might find that there's less snow on the ground so the world heats up even more than it would do just with carbon dioxide. Chris: It's not just as simple as carbon dioxide in the air?Dave: Yeah, basically it's all just horribly complicated.

Why do women develop cravings at some points in the month?

Chris: Yes, my wife does this: chocolate! Lots of women say their intakes of certain foods change menstrually. Favourites are chocolate around day 14 and also around day 28 when you're about to start your period. It could be that the reason for this is similar to a woman who's pregnant craves things, allegedly. The reason that's suggested is that if you have certain cravings it could be that your body is deficient in certain things. When you're going to get pregnant you must make sure that you have a ready supply with lots to spare of every single kind of micronutrient you might need in the body. So if you have a craving it makes sure that you binge on lots of things at the end of the month so that when you're about to fall pregnant your body's already replete with everything you might need. Dave: So it's a bit like if you're short of water you get thirsty?Chris: I guess so but what you're basically doing is overeating in preparation for a pregnancy because that way you eat lots of green leafy vegetables and you have lots of folate; you have lots of B vitamins and iron. Basically you need lots of things because the foetus is like a parasite, it's going to steal lots of things from your body and if you haven't got enough to spare the baby's health can suffer. But then, so could yours.

The main transmitter of the West Nile virus in the southeast is the mosquito species C. quinquefasciatus

16:20 - Mosquitoes - Why do they find you so attractive?

Why do mosquitoes find people so attractive? What can we learn from those that never seem to get bitten?

Mosquitoes - Why do they find you so attractive?
with Dr James Logan, Rothamsted Research

Chris - James, let's talk abut your work on mosquitoes. It sounds a bit frivolous talking about mosquito bites but in the grand scheme of things mosquitoes are probably the most dangerous animals, aren't they?

Mosquito species C. quinquefasciatusJames - Quite possibly, yes. It's not actually the mosquitoes that cause the problem although they bite and people react differently to their bites. You can have quite a severe allergic reaction to the bites. It's actually the pathogens that they carry; the diseases they carry such as malaria, dengue fever, filariasis. There's a whole number of diseases that they carry and, of course, those are the diseases that cause the problem.

Chris - There's something like 300,000,000 cases of malaria every year, 3,000,000 deaths. It's a huge number.

James - That's right. Exactly, almost 2,000,000 people die every year.

Chris - So understanding what lures them to us is crucial in being able to tackle that problem head-on because we know that just spraying them doesn't work.

James - Yes, the more we understand about finding them in the first place the better we can develop control methods to stop that from happening.

Chris - Those few hundred chemicals that you were telling Ben ooze out of our skin and that mosquitoes are sensitive to (at least to some of them): what actually are those chemicals and what are they doing?

James - Well, there's a whole range of chemicals that mosquitoes respond to. The main chemical they respond to is carbon dioxide which is mainly given off by our breath but is also released through our body as well. Other chemicals such as 1-octen-3-ol which is also given off as an alcohol, it's given off in our breath, and certain acids as well which are found in quite high amounts on your feet. These types of chemicals are very attractive to mosquitoes. Ammonia is another one as well.

Chris - Are they just naturally produced by cells in the skin and the mosquitoes have learned that this equals lunch so they home in on them?

James - Absolutely, yeah. The mosquitoes have learned that some chemicals are released through the skin. Other chemicals are actually produced by the bacteria on the skin. So bacteria do play a very big role and they sort of convert the chemicals into more volatile chemicals that the mosquitoes find attractive.

Chris - How do you do the research where you take a person and then look at what's coming out of them to work out how they're attractive, whether they're attractive or not and also what those chemicals are that are doing that?

James - We use quite a bizarre technique. We place people inside large silver bags, thermal survival bags that are commonly used for mountaineering. They lie in there for two hours and we extract their body odours from the bag.

Chris - Could be bad!

James - Yeah. We trap the chemicals onto a filter and strip the chemicals off the filter which then gives us a liquid extract that we can analyse. We have all sorts of weird techniques whereby we can actually look at the response of the receptors on the antenna of the mosquito, which is its nose, to detect which chemicals the mosquito responds to in this complex mixture.

Mosquito LarvaeChris - So, the chemicals that mosquitoes respond to: are they only attracted to us because some of the people that were in Kitchen Science with Ben were not attractive. You were very attractive, he wasn't attractive. Is it just that he's got less of these chemicals or is he making something else that's in fact making mosquitoes go away?

James - It might be quite logical to assume that if you weren't attractive to mosquitoes then you just simply lack the attractive chemicals but of course we all breathe. We all release carbon dioxide and these other very attractive chemicals.  There is something special about people who seemingly never get bitten and what we've found is that those people are producing certain chemicals in much higher concentration. When tested those chemicals have a repellent effect so it's almost as if your body's got a natural defence systems against these insects.

Chris - If you test people who live in areas where there are more mosquitoes that spread diseases (I'm thinking of places like Africa where malaria's endemic and other diseases like that) do you find that the population naturally make more of these chemicals?

James - That's a really good question and you probably would expect that in a place such as Africa where the selection pressure would be quite high. At the moment we don't actually know. There was a study that was done fairly recently that showed 20% of the population in a township were most susceptible to contracting malaria. The other 80% seemed to be fairly protected. The authors suggested that this could be to do with those people producing repellent chemicals but nobody actually knows. We haven't done a big enough study to be able to tell that.

Chris - There was a paper, I think it was published in Plos a couple of years ago. It was very interesting because they took children from the local school that had malaria and they put them at one end of a piece of apparatus and they had children who were malaria-free and put them at the other end of the apparatus. Then they put the mosquitoes in the middle and counted where the mosquitoes went. When the child was highly infectious for malaria all the mosquitoes flocked towards the malaria-ridden child. When they repeated the experiment with mosquitoes that were themselves malaria positive they avoided the child with malaria and went to the people that didn't have malaria. How are they doing that?

James - Again that was a really interesting study. What it showed was that it wasn't to do with things like body temperature and things like that. We know that mosquitoes are attracted to heat and to moisture. When you have malaria you do have a fever and so you're very hot. Some people might think that's the reason. These scientists actually showed that it was all to do with body odours.  So your body odour was changing and the point at which the parasite was most transmissible between the human and the mosquito was the point at which they were most attractive. Quite incredible.

Chris - It is incredible.

James Logan with his arm in a box of mosquitoes, only protected by some DEET

17:09 - Kitchen Science - How Attractive are You (to Mosquitoes)?

Are mosquitoes attracted to some people more than others? We find out by exposing ourselves to a box of biting mozzies...

Kitchen Science - How Attractive are You (to Mosquitoes)?
with Dr James Logan, Rothamstead Research

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.

A Cornfield

21:17 - Push-Pull - Using Plants as Natural Pesticides

How can we use a plants natural defence chemicals to ward off pests, such as insects and weeds?

Push-Pull - Using Plants as Natural Pesticides
with Dr Tony Hooper, Rothamstead Research

Chris - Tony: You're working at Rothamstead as well. You're looking at how plants actually produce chemicals which themselves can repel pests.

Tony - Yes, that's right. As James was talking about just a minute ago, plants themselves produce 2-300 compounds which are volatile chemicals on the surface of the plant. It's what gives them the smell to us but it's also what gives them the smell to the insects which try to colonise that crop and cause damage to it.

Chris - So the insects are sniffing out lunch by following the odour of the plant?

Tony - Yes.

Chris - And can the reverse also be true because James was saying that you can get humans that are naturally repellent to mosquitoes? Are there plants that are naturally repellent to plant pests?

AphidsTony - Well, in this country aphids are, in particular, a nuisance pest for our crops. They've designed their olfactory system to go to the crop and find the specific one that they want to have for lunch., as it were. So there are compounds given out by that crop which that particular aphid really wants to go for. In the same way it's not going to waste time going around trying to find crops where they're not there. So there are some chemicals given off by none-host plants that the aphid doesn't want to go to. They can repel the aphid.

Chris - So by planting those near to a plant which you want to have as a crop you could mask the smell of the attractive with the nasty one and then you repel the pest?

Tony - Yes, we have a project going on at the moment in Kenya in East Africa and in that region the people there are subsistence farmers. They want to grow maize. Maize is a very important crop there. It's subsistence farmed but the maize is attacked by a stem borer pest. That's a moth which lays eggs on the maize and the eggs hatch, create caterpillars, the caterpillars eat out the centre of the maize plants and they fall over and die so there's no food. We put together a push-pull system in that part of the world where we're using plants to control what those moths are doing. Round the outside of the field we have a plant which is very attractive to the moths so the moths will want to go there rather than go to where the maize is. The maize is protected from those moths. At the same time, in between the maize which is growing in the field, there are some plants planted which give off a chemical signal to the moth that would come in that says these plants are already damaged. The volatile chemicals that they give off simulate or are the same chemicals as what would be produced by a maize plant if it was damaged. Any insects that are inside the field looking for a good place to lay their eggs, they're going to think that's not a good place for me.

Chris - Because it's already been half damaged? They think someone else has got there first and eaten all the food?

Tony - That's right the signals tell them that this is not a good place.

Chris - Ok, so it sounds good on paper but does it work? If you do objective studies on this and count numbers does it work?

Tony - Yes. It works beautifully in East Africa at the moment. Because there is such a lot of damage caused by these stem borer pests and the expense of having pesticides in there is just not possible for subsistence farmers. While their yields are quite low because they haven't got chemical inputs like we have in Europe the yields are very low. If you can increase those by using this push-pull approach where you're pulling the insects out in to those trap crops, pushing them away with the inter-crops away from that maize then you get significantly more yield with it. 2-4 times the yield is common in these areas.

A CornfieldChris - What about other kinds of pests because it's not just insects which are a pest. Elephants are as well in Africa and I don't expect you to do anything about that but plants themselves can be a pest. I've got bindweed galore in my garden at home and I know there are certain forms of that which can be a real nuisance. Is there any way of dealing with the plants themselves?

Tony - Plants themselves are also producing chemicals through the roots. We've talked about volatile chemicals that insects locate or avoid their host plant. Plants also produce materials in the soil. That's a battleground as well so they'll be trying to compete with other plants.

Chris - So they can literally fend off another plant?

Tony - Yes.

Chris - If they decide they don't like it by secreting something into the soil?

Tony - That's right.

Chris - And how can you use that?

Tony - It can be from the roots or the folia or material above the ground which then falls to the earth.

Chris - And how can you use that? Do you have plants that are friendly to crops but unfriendly to weeds then?

Tony - Well, this project in Africa that I was talking about just now. One of the crops that we put in between the maize to repel the insects: when that was used in the field situations we found that it completely stopped a parasitic weed which normally attacks maize as well. This weed's called Striga hermonthica. It's a Striga plant, it's a witch weed and causes massive damage. We found that the intercropping that we use - the intercrop that we use in particular is called Desmodium (silverleaf is the common name). This was producing chemicals in the roots, naturally occurring chemicals in the roots which it uses in its own ecology. This was affecting the Striga seeds and preventing them from germinating in the soil and then attacking the maize plant.

Is your reaction to mosquito bites and your attractiveness to mosquitoes linked?

It's all down to an immune response. People think that when you react it's something that the mosquito's done to you: a bit like a sting. That's not the case. It's in fact what your immune systems doing that's making the mosquito's bite unpleasant. It's only the females because they need the blood when they want to lay eggs, they need a high protein meal: that means you. When a mosquito bites you what it does is to insert the proboscis, their mouth parts, into the skin and they probe around until they find a blood vessel. Usually a capillary which they get into and then they can start drawing blood. To stop the blood from clotting and also to ward off your immune system while they're doing that they inject a cocktail of about 20 proteins that go into the local tissue. They keep the mosquito having a nice dinner without harm coming to it and it also means you don't know it's there, immunologically speaking. The problem is that it leaves the vestiges of its saliva in the wound site. This means that when the mosquito's flown away your immune system flocks in and begins to react to it. It's reacting to that foreign protein and that means as you get more bites over your lifetime you get better and better at responding. That means some people get quite vigorous reactions to mosquitoes; other people the immune system tends to damp down its effect. It's a bit like having injections to damp down allergies, for example. It just depends on how your immune system is rigged up and some people have a form of their immune system that means they react more and more vigorously, sometimes until it gets really quite severe as James was saying earlier. In other people it tends to become less severe. Maybe you're of the latter type and your wife is of the former. I hope that answers your question.

Sesiidae, Chamaesphecia empiformis (det. Andreas Lange) on pheromone trap

37:28 - Pheromone Traps - Using Sex as Bait

Pheromones are chemicals used for communication - but can they be used to trap pests?

Pheromone Traps - Using Sex as Bait
with Dr Andrew Salisbury, Royal Horticultural Society

Meera - This week, I've come down to the fruit gardens here at the Royal Horticultural Society's site in Wisley, Surrey to find out how the chemical signals that insects use to communicate with one another can also be used to stop them from damaging plants and crops. Apple growers, for instance, often find their profits quite literally being eaten into by the larvae of coddling moths. It's possible to solve the problem by spraying regularly but an alternative, more environmentally friendly approach is to use a device known as a pheromone trap. RHS entomologist, Dr Andrew Salisbury, is with me now to explain how this works. So Andrew, what is a pheromone?

Pheromone trap in actionAndrew - The technical definition is a chemical that mediates a response between individuals of the same species. There are several different types. The main one people have heard about is the sex pheromones where a male or a female releases a chemical and attracts the opposite sex. There are also aggregation pheromones, which attract both sexes, and even alarm pheromones which cause insects to move away from each other.

Meera - The pheromone traps actually use sex pheromones to trap the insects chemically work in order to attract a male mate?

Andrew - Basically, the female releases the pheromone and the molecules are picked up by specific receptors on the male's antennae. The actual number of molecules they can pick up can be very small. This causes a response in the male to follow the pheromone stream upwind until they find their potential mate.

Meera - Knowing this, how do pheromone traps exploit it?

Andrew - Pheromone traps basically exploit it by using a synthetic version of the same pheromone to which the males are attracted. Males fly to the trap, hopefully in preference to their female mates.

Meera - How are the pheromones actually found and known about in order for them to be used in these traps?

Andrew - Basically, entomologists and other scientists olfactometers. These are pieces of equipment that are choice chambers. In one side of the equipment you have a stimulus which, if you are looking for a pheromone, may be a female moth. A male moth is released into the equipment and he can't see the female. If he moves towards the female and significant numbers move towards the female then you can say there probably is a sex pheromone.

Meera - Ok then. How do you find the actual chemical?

Andrew - First of all, you have to collect the odour of females and this is basically done in glass vessels and air is drawn over a polymer that picks up volatile organic chemicals. You then take the volatile organic chemicals. Basically, you use things like gas chromatographs, mass spectrometers to come up with identities of chemicals in the mix that you've collected. Of course, you don't really know which of the chemicals you've collected affects male behaviour. The next stage is to actually cut off male antennae, stick them in an electroantennogram and pass various chemicals over the antennae. If you get a peak you know the antennae response to it. You've made progress in identifying the pheromone. Then you take it out to the field and do field trials. If all goes well eventually you'll have a product.

Meera - We've got a pheromone trap here in front of us. It's hanging on an apple tree and it's kind of like a tent with a base attached to it. Why do they have this particular structure?

Andrew - The open edge of the tent structure is partly to keep the weather out and larger animals such as birds actually getting into the trap. The base actually contains a sticky substance, a non-drying glue to which the insects you're trying to attract get trapped on.

Meera - How does this help pest control?

Andrew - There is obviously going to be some mating disruption here. Those males which get trapped don't mate with females. The main way that these sticky traps work is that it can tell you when the peak emergent or peak flying period time of this particular moth is which can tell you when or even if it's worth using a chemical. Coddling moths can be around for two months and laying eggs for two months and spraying several times during that period is uneconomic and not very environmentally friendly.

Meera - So you say this sticky trap is more associated with amateur gardeners. We've got another one hanging on a tree over on this side of the garden. This uses another method in order to affect mating, does it?

Andrew - Yes, this has no sticky base on this one. It's just a tray with little dimples full of powder. The powder in this case contains pheromone and the idea with this one is males fly in, get covered in the powder, which contains the female sex pheromone, fly off and they now smell like a female. This causes massive mating disruption and hopefully eliminates the need for chemicals.

Meera - I love that. So there's going to be a male with loads of other males following him around?

Andrew - That's right. That's the sort of thing we hope will happen with this sort of control.

Meera - What other sorts of traps are available out there? What other species can be targeted?

Andrew - One quite exciting development which is going on at Rothampstead research is that they have found the aphid alarm pheromone (greenfly alarm) which makes them get up and walk away. It's a very similar chemical. It's found in catmint. They've been growing catmint, extracting the chemical from the plant and this process has the added advantage that cat toys can be made out of the product as well.

Meera - So a dual purpose?

Andrew - Yes, indeed. Much more is in development. Many species of insects we've investigated produce a pheromone of some type. Watch this space for further developments.

Ear Plugs and Internal Noise?

Trevor Cox, Professor of Acoustic Engineering, University of Salford: Why is it that when you listen to yourself when you've got earplugs you sound very different and all the sounds of munching and crunching sound very different if you eat something? When I'm speaking I'm not just hearing the sound coming out of my mouth, going round and down the side of my head and into my ear canal. I'm also hearing my bones vibrate. It's called bone conduction. When my vocal fold are opening and closing, when the air in my mouth is resonating the sound is also passing through my head to my ears and being picked up as sound. When you block up the outside path by sticking in things like earplugs all you can hear is the bone conduction. Actually you've got something also happening called the occlusion effect. Because you've got an ear plug in you've got a little resonant cavity in the ear canal so you actually get a little boost towards the top of speech around 2000Hz. That also means that certain sounds are amplified as well. Daniel Rowan, Audiologist, Institute of Sound and Vibration Research, University of Southampton: It's probably related to a well-known phenomenon called the occlusion effect. All sounds generated within the body such as mastication or talking are transmitted all around the body or the body tissues, particularly the bone. That sort of sound escapes, if you will, into the air-filled cavities of the body including the ear canals. The sound in the ear canals usually follows the line of least resistance and comes out of the ear canals rather than going into the ear drum and into the inner ear. When you plug your ear canal up with your finger then that sound goes into the eardrum, into the cochlear making things sound louder. In acoustical terms the amount of sound that gets into your inner ear can be as much as 1000 times. In terms of how loud we perceive it, it can be 5 times as loud or even more. This is a particular problem with people who wear hearing aids because having a hearing aid in the ear canal can make the sound of their own voice uncomfortable. That's one problem that occurs through it.

How can you get blood poisoning from a mosquito?

I think there what was happening was the mosquito was providing a route of entry to a bacterial infection. It was probably Staphylococcus aureus or strep can sometimes do that.

Are mosquitoes a problem all over the world?

We put this question to James Logan:There's lots of different species of mosquitoes and they are found right across the world. Whether they're bothersome or not: I think they are bothersome because they all bite. If they bite humans then clearly there's a problem but in terms of diseases they don't spread diseases everywhere. For example, in the UK we don't currently have diseases spread by mosquitoes.

Does Listerine repell mosquitoes?

We put this question to James Logan:There are some compounds in Listerine such as menthol which does have a repellent effect against quite a lot of insects, actually. Perhaps it does have some sort of effect. I'm not sure if they're saying it's repellent by using it in their mouth or whether it's repellent by putting it on their arms. I'm not sure but it's certainly a new one. I hadn't heard of that before.

Do any plants repel Chiggers?

We put this question to James Logan:Well we have tested these repellent chemicals [those that people produce naturally, see James' interview] against different species of mosquito and also against midges, and currently we have some trials going on in Kenya to see if these chemicals repel ticks. Ticks are probably more related to chiggers than to mosquitoes, because they're actually arachnids. We don't have the results yet, but hopefully these chemicals will repel Chiggers too.

Do you react more to mosquitoes as you age?

This boils down to having an 'adaptive' immune system. A mosquito will inject a cocktail of 20 or so proteins, including something to block platelets, something to block your immune system and something to stop your blood from clotting. The immune system is very good at reacting to foreign proteins, and the more you are exposed, the better your immune system can react. This means your immune system gets 'tuned up' to react, and so you can get a stronger reaction more quickly. But not everyone's immune system will adapt in the same way.

Do women get bitten more than men?

We put this question to James Logan:There is anecdotal evidence to suggest so, however there is also anecdotal evidence to suggest the opposite, or that fatter people are bitten more than thinner or older people more than younger. One thing we do know is that women change in their attractiveness to mosquitoes throughout their menstrual cycle, which we assume is related to hormonal changes. Likewise, pregnant women attract more bites.

Can plants affect one another in the garden?

We put this question to Tony Hooper:You certainly wouldn't want to try to grow anything under a pine tree, as they drop resins which are poisonous to other plants, as do walnut trees.With regards arable crops, rather than gardens, wheat produces chemicals in the soil that are generally toxic both to other plants and to insects that may attempt to colonise them. We're working on getting this respondse 'on demand', to protect crops from attack.


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