Press the start button then select the most appropriate word to fill each gap from the drop-down list. Each word in the list is used only once. Hint: read ahead and think about grammar to narrow down the choice of possible answers. When you think that you have it right, press the check sequence button. There is a 5 second penalty if you are wrong!
Don't forget to login if you want your time to be recorded.
Pesticides are chemicals that are designed to kill organisms that with us for food. are pesticides that are designed to kill insect pests that feed on our , reducing the final yield of food.
DDT is an example of a very powerful insecticide. Although it is safe for other plants and animals in small amounts, DDT does not easily - we say that it is a persistent insecticide. DDT was widely used to kill the mosquitoes that carry the parasite (Plasmodium) that causes .
Unfortunately, because DDT lasts a long time in the , it gets into rivers and lakes where it is taken up by microscopic . These plant plankton are eaten in turn by animal plankton (), which are then eaten by small fish. The small fish are eaten by larger fish, which in turn are eaten by top such as birds. In this way, DDT can be passed up the .
Because the DDT is soluble in tissue and is not easily broken down, at every in the food chain the DDT becomes more . Eventually it builds up to toxic levels in the top predators, causing and a collapse in their ability to breed.
The tendency for pesticides to become more concentrated in organisms the higher in the food chain they go is termed .
This problem with modern agricultural chemicals was famously highlighted by Rachel Carson in her book 'Silent ' in 1962. More modern pesticides, such as Malathion, are designed to break down more easily in the environment, reducing this problem. We say that they are .
Organic farming avoids the use of , but usually results in lower yields. One way of avoiding the use of pesticides is to use other to control the pests. This approach is called .