Repelling Pests!

08 September 2008
Presented by Chris Smith, Helen Scales.

In this first episode of the Naked Scientists in Africa we find out why mosquitoes are the most dangerous insect in the world and why they find some of us more tasty than others. We also look into a new trial in Kenya to use plants as natural pesticides, find out about a scanning technique that spots cancers as small as a pinhead and hear about new scientific methods being used in South Africa to monitor the ivory trade and stop poachers in their tracks! Plus, in Question of the Week we find out if it's possible for our memory to fill up.

In this episode

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, Rothamstead 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.

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.

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