Repelling Pests - Mosquitoes, Moths and Weeds

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.

Researchers 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.

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.

They 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.

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.

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?

James - 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.

Chris - 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.

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?

Tony - 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.

Chris - 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.

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?

Andrew - 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.

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.

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.

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.