DDT and Food Chains

DDT and Food Chains

Introduction

DDT is an insecticide—a substance that kills insects. It has been banned in many countries for some years, but is still used in other parts of the world. Now there is a big campaign to have it banned completely. Why is this?

Why DDT is used

DDT was first made in 1873. However, it was not until 1939 that is was realized that it was an extremely effective insecticide. The person who made this discovery, a Swiss scientist called Paul Muller, was awarded the Nobel Prize in 1948.

The first big use of DDT was in 1943, during World War I. There was a big problem with a very unpleasant disease called typhus, which was spreading through Naples. The U.S. army used DDT to kill the body lice whose bites were spreading typhus.

After the war, it was realized that DDT was excellent for killing mosquitoes. This was very important in many parts of the world, because some kinds of mosquitoes are responsible for spreading malaria. Malaria is a dangerous disease and even today around 2 million people die from it each year. The World Health Organization (WHO) estimates that the use of DDT has saved about 25 million human lives.

DDT was seen as the ideal insecticide. It is cheap to make, so it is possible for developing countries, who don't have much money to spend, to use it. It is very toxic to insects, so quite low concentrations will kill them, but it is not very toxic to mammals, including humans. Moreover, it is a very stable compound. This means that it lasts a long time without breaking down. So you can spray a house with just a small amount of DDT and this will kill all the mosquitoes in it without harming the people who live there. And there will be no mosquitoes for quite a long time afterward.

Problems with DDT

Unfortunately, in the 1950s and 1960s people began to realize that DDT might be causing some problems.

What seemed to be happening was that DDT was building up in very high concentrations in the bodies of some animals, especially birds of prey. This could happen because, when an insect or other animal absorbs DDT, the DDT just stays in its body. It dissolves in fat in the body and—as you will remember—it doesn't break down quickly.

When a bird eats an insect, it eats the DDT as well. If it eats lots of insects, it eats lots of DDT. And all of this DDT stays in its body. Over a period of time, it collects more and more DDT.

And then a bird of prey might eats lots of birds with DDT inside their bodies. So the concentration of DDT that builds up in the bird of prey can get very high indeed. Even though DDT is not very toxic to birds, at these very high concentrations it can begin to do serious harm.

There is, for example, some evidence that these high concentrations of DDT caused birds of prey—such as sparrowhawks and peregrine falcons—to lay eggs with much thinner shells than usual, so they broke before the young birds could hatch out. The adults seemed to be dying, too. When their bodies were analyzed, they were found to contain huge amounts of DDT.

Banning DDT

As a result of these fears, the use of DDT was completely banned in the United States in 1973, and since then has been banned in many other countries, including Britain. However, it is still used in many other countries, mainly in the fight against malaria. Now the World Wide Fund for Nature, the WWF, is pressing for a world-wide ban on its use. It says that DDT sprayed in Africa can end up in the fat of polar bears in the Arctic. So long as DDT is used anywhere at all in the world, then nothing is safe.

However, the World Health Organization disagrees. It says that if DDT is banned, then millions more people will die each year from insect-borne diseases such as malaria. The malaria problem is a very serious one. For example, there is quite strong evidence that, as less DDT is used in some South American countries, the rate of infection with malaria goes up. And it is difficult to find any other insecticides that can do the same job, and almost impossible to find one that is cheap enough for developing countries to use.

It is a very difficult decision to make. Some scientists suggest that perhaps we could ban the use of DDT in agriculture, but still allow it to be used for killing mosquitoes. For example, in the South American country of Guyana, the amount of DDT needed to spray only 4 km<sup>2</sup>/1.5 mi<sup>2</sup> of a cotton crop to kill insect pests would be the same as that needed to spray enough houses to control malaria in the whole country.

However, the decisions are very difficult ones to make. Perhaps one day there will be a cheap vaccine for malaria, so there will no longer be any need to kill the mosquitoes that carry it. However, any effective vaccine is still a long way off. In the meantime, we have to decide what is the best thing to do about the use of DDT.

Question & Answer

Question: When was DDT first used?

Answer: ______

Question: How can DDT help to control the spread of typhus?

Answer: ______

Question: How can DDT help to control the spread of malaria?

Answer: ______

Question: Why is DDT an especially useful insecticide for developing countries?

Answer: ______

Question: DDT is a very stable compound. What does this mean?

Answer: ______

Question: What kinds of animals have been found to have very high concentrations of DDT in their bodies?

Answer: ______.

Question: In which parts of an animal's body does the DDT accumulate?

Answer: ______

Question: How does DDT harm birds?

Answer: ______

Question: Which organization wants a global ban on the use of the DDT?

Answer: ______

Question: Which organization is arguing against such a ban?

Answer: ______