Systemic Insecticides

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Ripper.W. E.
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A fundamental advance has been made in recent years in the use of chemicals for controlling certain insect pests of plants. Hitherto, insecticides have been of two classes. One of these poisons the insect when it comes into external contact with it; the other works only when the insect eats the poison. So the contact insecticides are sprayed or dusted on the plants in the hope that particles will actually fall on the insects, or that the insects will pick up particles of it in moving over the plant. Obviously this insecticide cannot work if the insect is in such a position that spray and dust cannot reach it, or fail to reach it by accident. The stomach poisons are sprayed on to leaves in the expectation that insects eating these leaves will ingest enough poison to kill them. Insects that do not eat the leaf as a whole, but suck out its contents only through a puncture in its surface, will not be affected by a layer of the poison confined to the leaf's surface.
East African Agricultural And Forestry Journal, 19, p. 59-60