General Introduction

In a parallel and competitive evolution, under ecological pressure, it is not surprising to expect at least some plants to emerge with chemical defence against specific phytophagous insects. Empirical knowledge of plants useful for combating insect pests has also accumulated over millennia in different cultures in different parts of the world. For example, Indian neem tree (Azadirachta indica A. Juss.) leaves have been used in India and the neighbouring countries for control of insect infestation in food grains, clothing, etc. since Vedic times (1500 B.C.) ¹ . The dried seed powder of custard apple (Annona squamosa Linn.) has been used as an insecticide in India and many tropical countries since ancient times. ² Pyrethrum (Chrysynthemum cinerarifolium Vis.) flower heads have been used as an insecticide since the nineteenth century, if not earlier, in Iran and Yugoslavia. Insecticidal properties of tobacco (Nicotiana tabacum Linn.) had been known to American Indians, and European farmers started using tobacco extract as a plant spray in the 17th century. Plants of the genera Derris and Lanchocarpus have been used as piscicides and later as insecticides by people in Southeast Asia and South America for several centuries. There are many other plant species recognized as containing insecticidal principles and used at one time or the other by different people in different regions of the world. ^{3 – 6} .