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Weeds play an important role in human affairs over much of the earth. The major characteristics of weeds are their unwanted occurrence, undesirable features and ability to adopt to a disturbed environment (Combellack, 1992). Despite measures adopted for their control, weeds are estimated to reduce world food supplies by about 11.5% annually ((Combellack, 1992Parker and Fryer, 1975). Many of our problem weeds are of exotic origin, having been introduced accidentally or deliberately as ornamental plants, etc. They flourish in the new environment as they have escaped from the natural enemies, which suppress their vigour and aggressiveness in their native lands.