Ted's teaching tips

19th May 2000 at 01:00
How to use The Big Picture

What would you do if you saw this ghoulish monster bearing down on you? This magnified aphid-carrying ant looks like a still from some horror film that children would not be allowed to watch. Yet the tiny world under our feet is as intricate as the one we can see better.

Aphids We can see ants clearly, but how big do you think the tiny aphid in the picture is (size of a pinhead)? What do aphids do (deform plants by sucking their sap and transmitting viruses)? How can we recognise their effects (look for plants such as roses and fruit trees with shrivelled brown leaves, and you may see numerous little white aphids)? How are they usually dealt with (a chemical spray bought from garden centres)?

Seeing the small How can we see things that are very small (under a magnifying glass or microscope)? What is magnification (making something look bigger, eg x10 means the object looks 10 times larger)? How is it achieved (one or more lenses, as in spectacles or a magnifying glass, can magnify up to several hundred times; an electron microscope, by bombardingthe object with electrons, can magnify a thousand or a million times)?

Symbiosis The Big Picture has shown other symbiotic relationships, such as animals that tolerate certain insects that keep their skin clean,but what is symbiosis (two or more lifeforms benefiting from each other;parasitism is where one organism livesoff another, like mistletoe, or the louse)? Children are fascinated by how antsand aphis live in symbiosis (ants carry aphids from dying plants to freshones and kill off the eggs of their predators; aphids excrete a sweet fluid, called honeydew, which ants feed off by stroking them - a process similar to us milking cows).

Writing You are shrunk down to the size of a pinhead, what do you see (make it factually accurate)?

* Ted's talking points

Aphids are pests we should wipe out. But is it that simple? Ants manage to live in harmony with them, so should we simply eradicate anything we don't like with a chemical spray?

For Uncontrolled natural forces, like locusts, can devastate a whole area. Aphids kill off or blight beautiful plants such as roses, or destroy our vital food supplies. Modern pesticides are safe. People are hypocritical about pest control. Those who oppose it would not want to buy spotty apples in the supermarket. High quality fruit has often been produced by safe use of insecticides.

Against There are natural ways of controlling pests, such as using ladybirds, which eat aphids. Although some pesticides are better than others, they are still noxious and substances such as arsenic can get into our food chain. Organic foods are growing in popularity because people do not want to eat poisons, in however diluted a form. We do not always understand the delicate balance of nature, and chemical interference can have disastrous consequences, only apparent years later.

Ted Wragg is professor of education at the University of Exeter

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