insect pest may recover faster than the predator, and so cause more damage than if the
insecticide had not been used in the first place. This is known as insect resurgence.
Secondary Pest Outbreak
Sometimes the use of an insecticide enables a normally harmless insect to become a
pest. An example is the White Wax Scale insect which lives on citrus trees and which is
normally kept under control by other natural insect predators. However if the citrus trees
are sprayed with insecticides such as parathion to control the Red Scale insect then the
predators of the White Wax Scale insect are also killed so that its population increases
to damaging levels.
Another example is from West Africa, where the Cocoa Shield Bug developed into a
serious pest in cocoa plantations where there had been intensive use of chlorinated
hydrocarbon insecticides to control the Mirid Pest (Sahlbergella singularis). The
change in status of the Shield Bug, from harmless to harmful, is due to the elimination
of its parasites and predators by the indiscriminate use of insecticides.
Resistance to Insecticides
Resistance can build up if one insecticide is used very frequently, and is another good
reason for minimising the use of insecticides. As resistance to a particular chemical
develops, the insect population returns to the numbers where damage occurs more and
more rapidly. The chemical then has to be applied more often and/or the dose rate
increased, or a different chemical has to be used.
Insecticide Treadmill
The three examples quoted above indicate the less obvious but possible dangers of using
insecticides. If these chemicals are used indiscriminately and without any understanding
of the ecosystem they can create more problems than they solve. They can leave food
growers on an insecticide treadmill that is very difficult to get off.
1K. DISEASES
In fact most plants of cultivated crops develop some form of minor disease symptoms at
some stage during their life but these symptoms are very often below the level of
economic interest, and so pass almost without notice.
Diseases can usually be kept relatively under control by using appropriate, disease
resistant varieties and good husbandry techniques such as rotations, maintenance of soil
fertility and destruction of diseased plants. So it is unusual for a crop to be entirely
wiped out by disease, though this does occasionally happen.
Some crops cannot be grown economically in certain regions because one or more
diseases are prevalent there. For example, rubber cannot be grown profitably in South
America because of the fungal disease Leaf Blight, even though rubber as a species
originated in South America.
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In global terms plant diseases are estimated to cause almost 10% loss in crop yields.