Saturday, 18 May 2013

Biological control


 Reading Comprehension
 Biological Control
Biological pest. Control uses a natural enemy of the pest to keep its numbers down. A good example occurred in Australia in the 1920s and 1930s. A cactus called prickly pear was brought to Australia from America in the 1920s. It quickly established itself in Australia and became a weed pest. It grew so fast that large areas became thickly covered with the cactus and could not be used for farming. Removing the cactus with weed killers or by digging it up would have been very expensive. Scientists, therefore, began looking for ways of controlling the cactus biologically. Eventually they discovered a moth, Cactoblastis cactorum, which lays its eggs on the prickly pear cactus. When the caterpillars hatch out, they eat away at the cactus. If there are enough caterpillars, they eventually destroy the cactus. Millions of eggs of the moth were released onto the cacti by farm workers. The operation was so successful that within five years, most of the cacti had been destroyed. After all the cacti had disappeared, the caterpillars died out.
This was important, because they might have turned to another source of food, and perhaps started eating food crops. In many ways biological control is safer-than chemical control. It does not involve introducing toxic chemicals into the environment. Nevertheless, biological control can go wrong.
After reading the above passage , complete your answers in the spaces provided :
a: Due to the spread of cactus the disadvantage was that ___________________
b: It is safer to use biological control than chemical control because__________________
c: Alternative methods to solve the problems would have been______________________
d: The menace of cactus took a period of____________________
e: The food web was not interfered because____________________

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