DDT

DDT is a pesticide that caused long-term ecosystem damage through biomagnification.

Introduction

Many species of insects are problematic for humans. Some are disease carriers. The Anopheles mosquito (Figure 1) transmits Plasmodium, a protozoan that causes malaria when injected into a human. Other insects feast on crops like corn and cotton. For these reasons, scientists have searched for pesticides, chemicals that can be used to kill troublesome insects. The first-generation pesticides were made of highly toxic chemicals, such as arsenic and hydrogen cyanide.

These either proved ineffective against insects or too dangerous for humans to apply them. Second-generation pesticides were organic compounds synthesized in laboratories. In 1939, Swiss chemist Paul Muller (1899–1965) conducted experiments with 1,1,1-trichloro-2,2-bis-(p-chlorophenyl) ethane (DDT), an odorless white powder that resembles table salt. The chemical structure of DDT is shown in Figure 2. Because the chemical was widely used to kill disease-causing insects, Muller was awarded the Nobel prize in medicine and physiology in 1948.

Figure 1 Anopheles mosquito

Figure 2 Chemical structure of DDT

Although DDT had many advantages as a pesticide, environmentally conscious scientists began to notice some problems related to the chemical. Because DDT is not soluble in water, it persists in the environment for a long time. When organisms take in DDT, the chemical dissolves in the fatty compounds in their tissues. Since animals store fat, the chemical stays with them a long time, building up as it is continuously introduced into the body through food.

Fatty compounds also make up the membranes of cells. The job of cell membranes is to control what enters and leaves cells. Because it is fat, soluble, DDT can become incorporated into cell membranes, but it does not fit perfectly. As a result, the chemical damages membranes, impairing their ability to regulate the flow of ions into and out of the cell.

This is especially problematic for nerve cells, which conduct electrical impulses by closely regulating the movement of sodium ions and potassium ions. When damaged nerve cells stop functioning, the muscles they control are no longer able to contract normally. Because of DDT poisoning, organisms die either of convulsions, erratic muscle contractions, or muscle paralysis. In this experiment, you will learn more about the effects of DDT on organisms and ecosystems.

Time Required

two 55-minute periods

Materials

• access to the Internet

• color printer (optional)

• colored pencils

• 3 sheets of plain white paper

• stapler

• science notebook

Please review and follow the safety guidelines at the beginning of this volume.

Procedure

1. Make a flip book from three sheets of plain white paper. To do so:

a. Stack three sheets of paper so that the bottom edges are staggered with about 1/4 inch (in.) (.64 centimeters [cm]) of paper extending beyond the sheet above it (see Figure 3).

b. Fold the top half of the papers down so that you have six staggered edges.

c. Staple the papers at the top.

Figure 3

2. Access the Internet to find information on DDT and record your findings in your science notebook. This information will form the basis of your flip book on DDT. Also find answers to the Analysis questions.

3. Create your flip book. The top page of the flip book will be the cover page where you will place the title of the book, your name, and an appropriate drawing or photo. On the other five pages, provide information on the following topics:

a. The advantages of DDT as a pesticide.

b. Reasons for banning DDT.

c. The effects of Rachel Carson’s book Silent Spring on the public’s understanding of DDT.

d. The effects of DDT bioaccumulation and biomagnification.

e. The effects of DDT on humans.

Each page of the flip book should include written information and at least one picture. Make the flip book informative, interesting to read, neat, and colorful.

4. Your teacher and classmates will use the rubric to grade your flip book. Before you begin your work, look over the rubric so that you will know exactly what you are supposed to do.

Analysis

1. When was DDT first used as a pesticide?

2. Name two diseases that are spread by insects.

3. In what year was use of DDT banned in the United States?

4. How does DDT reduce bird populations?

5. Why is DDT still in use in some parts of the world?

When DDT was introduced, it was considered to be a wonder chemical because it could get rid of any kind of insect pest. Farmers found that it would eliminate crop-destroying bugs, such as potato beetles, appleeating moths, corn earworms, and cotton boll weevils (Figure 4). The peak year of use in the United States was 1959, when more than 80 million pounds (more than 36 million kilograms [kg]) of DDT were used on crops. In other countries, the use of DDT to kill malaria-causing mosquitoes and typhus-carrying body lice improved the health of millions of people. The World Health Organization (WHO) estimates that at least 25 million lives were saved by the application of this chemical. It seemed an ideal solution to insect problems because it was cheap, easy-to-use, and harmless to mammals.

Figure 4 Boll weevil

However, by the late 1940s, some scientists were beginning to see problems. For one thing, insects were developing resistance to the chemical, so it was losing its effectiveness. In addition, DDT was found to be toxic to fish, and scientists discovered that it was accumulating in fatty tissue of other animals. DDT has a half-life of 8 years, which means that it takes 8 years for half of the DDT in tissue to break down. If an animal continuously eats DDT-laced food, the load of pesticide accumulates over time.

Animals at the tops of food chains, which includes predators, like large birds and carnivorous mammals, can accumulate heavy loads of the toxin. Although the DDT does not kill these animals, it is harmful. In birds, the chemical prevents formation of strong shells on eggs, causing the deaths of all chicks. In mammals, DDT seems to mimic the action of some hormones and disrupts normal reproductive cycles. While the use of DDT was banned in 1973 in the United States and most other developed countries, the chemical is still manufactured and used routinely in some countries.

Connections

Three decades after most governments banned DDT, the chemical is showing up again in the environment. Scientists working in Antarctica found unexpectedly high levels of DDT in the body fat and the egg shells of Adelie penguins (Figure 5). After checking all regions of the penguin’s environment, the source of this DDT was found to be glacial melt water.

Apparently, the chemical had been carried to the poles by air currents during its peak use. Precipitation took the chemical to the surface, where it became part of the glaciers. Now, rapid warming in Antarctica due to global warming is causing the glaciers to melt at an unprecedented rate and releasing the dangerous chemical once again. Krill, tiny, shrimplike crustaceans, live in the melt water and are the primary food of penguins. Currently, scientists do not think that the birds are likely to suffer serious injury based on the amounts of the chemical they see in the melt water. However, no one knows if the melting has already hit the peak deposits of DDT.

Figure 5 Adelie penguin