Plant Science, Composting and Mosquito Repellents

Scientists at the University of Wisconsin have discovered how a species of leaf cutter ant produces its own home-made antibiotics to keep fungal infections at bay in the nest. Leaf cutters are nature's gardeners. They crop green plants and carry the pieces back to their nests where they infect them with a fungus which breaks down the plant matter and is then, in turn, itself eaten by the ants. But just as weeds grow in even the most carefully tended gardens, the ants nests are threatened with invasion by a poisonous fungus, called Escovopsis, which the ants have to keep at bay. They do this by coating their body surface with a species of filamentous bacterium, called pseudonocardia, which provides the ants with an antibiotic chemical weapon with which to combact the invading fungus. Until now it wasn't known how the ants carried or nurtured their bacterial charges, but writing in this week's edition of the journal Science, Cameron Currie and his colleagues at the university of Wisconsin have discovered that the ants have evolved a special system of pouches, called crypts, each of which is fed by the ant-equivalent of a sweat gland. These provide the ideal home for the bacteria which take up residence and turn themselves into portable pharmacies, producing a cocktail of antibiotics which the ants can dispense around their nests. So it looks as though Alexander Fleming may have to surrender the crown for discovering antibiotics - these ants must have been using them for millions of years!

Chris - Tell us about your research. How did you actually get into this and what have you found?

John - Well there are a lot of stories about people being differently attractive to mosquitoes and other nuisance flies. The perceived wisdom is that the people who aren't attacked lack some attractant material. In fact what we've found is quite the opposite. People who aren't attacked produce some extra chemistry and put off the mosquitoes and other flies. The way we found this was to look first of all at the emanations that come out of people's skin. We chose a number of volunteers and looked at the way the mosquitoes would fly towards their arms in a device called a wind tunnel.

Chris - Presumably you had to pay people quite a lot to take part in this research!

John - No, they volunteered, and we didn't actually allow them to be attacked by the insects. We stopped them before they reached the arms so we could count them as they approached. What we did then was to take the volatile chemicals coming out of the skin of the volunteers. We did this by putting the volunteers into a kind of bag, which we kept a positive pressure into, so we could draw air out of it and take out the chemicals that the volunteers were putting into the air. By that means we could show that the same people that were not attacked by the mosquitoes also produced a range of chemicals that had the same effect.

Chris - How did you actually identify what the chemicals were?

John - Well that's even more clever. We use a lot of chemical analysis in this kind of work, but we can also use the insect itself. We can put small microelectrodes into the antennae of the mosquito and find out which chemicals in the complicated mixture given off by the skin were actually repelling the mosquitoes.

Chris - So essentially you attach a wire onto a nerve fibre coming from the antennae and record from that when you present different chemicals to the mosquito and see what the response is electrically.

John - Yes. We can either record from individual neurons or we can record from the whole antenna. In fact, we used a preparation that used the whole antenna in this work.

Chris - And if a mosquito is particularly put off by a smell, does you still get a positive response from the antennae or does it switch off?

John - The antennae is just a recording device. It's the central nervous system in the animal that decides whether an animal is going to approach the source of the material, or whether it goes away. What we've done is looked at the chemicals that are produced by those people who are attractive, and then we've tried to find the extra ones that the mosquitoes respond to from people who are not attractive. These have then been tried out in this wind tunnel device to see if they will put mosquitoes off from going to the attractive people's hands. That works.

Chris - So you can confer unattractiveness on individuals by decorating them with this smell.

John - Yes, and in the field. With help from our colleagues at the University of Aberdeen where they have the Scottish biting midge, we've shown that you can reduce almost to nothing the normal attacks that you get from the Scottish biting midge.

Chris - Why do some people have these chemicals, but not others? You would have thought that it would be a huge advantage if you think of places like Africa where they have malaria. If you have gene that means that you have components in your sweat that put mosquitoes off, it would be so advantageous that everyone should have it.

John - Yes that's right. The close relationship between some animals and these biting flies have meant that the mosquitoes are always keeping up with how we try to evolve away from them. But the kind of chemistry we're seeing sees to relate to stress. Of course, mosquitoes have to put in quite an investment to get a blood meal from somebody. They have to inject I chemicals that dilate the blood vessels and stop the blood coagulating. That's quite an expense for the mosquito, so they're very keen not to go to somebody who's stressed. We think that might be the reason for it, but we're not certain that this is true at the moment.

Kat - Are these pheromones going to be commercially available?

John - Well it's creating a lot of interest at the moment. We've been able to use one of the main commercial repellents, deet, in our comparative work in Scotland and we can certainly beat that in the short term. So there's a lot of interest. What we're doing at the moment is writing up the work and patenting it. That will of course facilitate commercial development.

Chris - So tell us, what actually is Wiggly Wigglers?

Heather - We're a natural gardening company, but we started our business worm composting, so supplying kits to individuals so that they can turn their kitchen waste into lovely worm casts. I think you had one in Australia!

Chris - Yes, when I was in Australia I did have one on my balcony because we lived in a flat and we had a roof garden. We found this thing could consume kitchen waste faster than you could put the kitchen waste in there. The worms turned it into this wonderful juice.

Heather - Yes, but not only juice, but they also make a cast as well. They like dog hair and bananas too!

Chris - You've structured your company around trying to get people to use science to get their garden to perform a bit better. How long have you been going?

Heather - We've been going for sixteen years now. When we started it was just myself , and now there's fourteen of us - thirteen women and one man!

Kat - And that's the man we've got here! I've got a quick question for you. My dad is really into composting, and he likes to pee on the compost heap because he heard it was good for it. Why is that?

Richard - In conventional composting it's probably quite a good idea because of the urea involved. If you're looking to compost with worms, it's probably worth avoiding because generally, worms don't like acidic conditions. Some worms have a calciferous gland for excreting and producing calcium. This is why you don't tend to find worms in places where you find acidic soil and rhododendrons for instance.

Chris - So you encourage people to add a dose of worms to their garden to improve the quality of the soil. But does it actually work?

Richard - It does. Worms are the most amazing improvers of soil. When we set up a worm kit, what you're doing is harnessing a natural phenomenon really. You're introducing organic material and the worms are consuming it. The casts that they produce are something like five times higher in nitrogen, seven times higher in phosphorus and a thousand times higher in the number of bacteria in the casts than the original soil.

Chris - So if you've just moved into a house on a housing estate that's just been built and you have that wonderful clay that they give you for your garden, if you sling some of these worms on there, does it make a difference to the soil?

Richard - Absolutely. Worms are obviously going to improve to soil by increasing the aeration and allow oxygen to diffuse into the soil. They also increase the soil permeability. Interestingly enough, they also increase soil water retention, especially around plant roots.

Kat - Where do worms go when they die? Do they get eaten by other worms?

Richard - Probably! Indirectly at least! There are species of flatworms that exclusively feed on our indigenous earthworms. Generally if a worm dies, there'll probably be eaten by nematode worms.

Kat - So if I was to set up a wormery, what sort of thing should I do? Is it hard to look after worms?

Heather - They're not too fussy actually. We sell all different sorts of wormeries and you can just compost with worms on their own. The easiest way to do it is to buy a ready-made thing. Thatw ay you can get the compost out so much easier.

Chris - they're also not huge are they Heather? They're not too big and in the way. It doesn't smell and is quite compact.

Heather - They're about 50 centimetres wide and about 75 centimetres tall, so they can fit in most places.

Chris - How much does it cost to set up?

Heather - You can start with worms and bedding from about £13 or you can have the whole thing for around £89. So there's a full range of prices.

Kat - Is there anything you shouldn't give to your worms?

Heather - You should go easy on citrus peel, and if you do put a lot of it on, you should add extra egg shells or calcified seaweed or something. Other than that, they're pretty cool. They like things like cardboard, cooked waste and raw vegetables. Potato peelings take a little bit longer to break down, but it's really one of those things where you use the natural resource to its best ability. If it goes really cold, they slow down a little bit.

Chris - Are they going to last forever? They would hopefully breed in your wormery, or would they need replacing from time to time.

Heather - It's bad news if they need replacing. I had a customer once who insisted on feeding his son porridge every day but his son didn't like it. It ended up on the compost heap and it did cause a bit of a film over the top! However, it was easily fixed with a bit of cardboard and a bit of variation in their diet. If you do that, the worms will stick anything!

It's all down to the momentum of the thing concerned. The Stardust mission comet Kat spoke about earlier was actually travelling thousands and thousands of miles and hour faster than the probe was, and the tiny particles it was throwing off were capable of destroying that probe. And this is all down to momentum. The momentum of something is the mass of something multiplied by how fast it's going. That's how hard something slams into you. So it doesn't actually matter how heavy or big something is because the faster it hits you, the harder it hits you. If you drive into a fence at 60 miles per hour, the slamming effect will be the weight of your body multiplied by the speed that you're going. If you're hit by a bullet, although the mass of the bullet is much less than you hitting the fence post, it's going a lot faster. The average bullet coming out of a pistol is going at around 700 miles per hour, which is a lot faster than someone in a racing car. As a result, the momentum of a bullet is much higher. The reason bullets cause such a problem is because although they go in and make a very small entry wound, the shock wave of the bullet hitting your tissues rips you apart internally. That's why if you look at someone who's been shot from the front, they don't look too bad until you look at them from the back where there's often a very big exit wound. This is because as the bullet goes through, it compresses all the tissues it passes through and pulls them to pieces. It's like being hit with a fence post but much much harder.

Many flies, which includes the mosquito and the biting midge, use carbon dioxide, heat and various other chemicals that are quite commonly produced. But they will also use specific compounds to see human beings. For example, the malaria mosquito can find a human being in the middle of a very large group of cows, which are oozing carbon dioxide and heat. It uses specific chemicals to find that human subject. It will be the same for the flies that are annoying the friend in Melbourne. These insects are able to perceive what kind of potential host they've got, even if it's not blood they're after, just the mucus around our eyes and mouths.

That's a very good question, and in fact some work has been done on that quite recently. It turns out that the Venus fly trap has two disc-shaped leaves that invert themselves when an insect touches them. This happens very quickly, and they discovered this by doing time-lapse photography. The scientists who discovered it painted ultraviolet dots on the Venus fly trap so they would show up when they shone UV light on to the plant. They took 400 photographs per second while triggering a Venus fly trap and could map what happened. The analogy they used is that if you take a tennis ball, cut it in half and then turn it inside out, it will sit there quite happily. However, if you come along and just touch it very lightly, it pings back again.