CHAPTER 1
 GENERAL INTRODUCTION

 Biological control may be defined as the use ofparasitoid, predator, pathogen, or

competitor populations to suppress a pest population, making it less abundant and

damaging than it would otherwise be. All living species are attacked by natural enemies-

parasites, predators, or pathogens- which feed on them in one way or another and in

many cases regulate their population densities (Van Driesche and Bellows 2001). Many

potentially injurious pests are kept at very low levels and never reach economic pest

proportions due to the effective action of naturally-occurring natural enemies, without

deliberate intervention by man.

 Natural enemies can be utilized in three maj or ways: (1) importation of exotic

species and their establishment in a new habitat, (2) augmentation of established species

through direct manipulation of their populations, as by insect mass production and

periodic colonization, and (3) their conservation through manipulation of the

environment. When successful, the utilization of natural enemies is an inexpensive, non-

hazardous means of reducing pest populations and maintaining them below economic

injury levels (Van Driesche and Bellows 2001). Success in biological control is often

dependent on a thorough understanding of the organisms involved, both injurious and

beneficial, and their intricate interactions.

 The modern history of biological control can be dated from the control of the

cottony cushion scale, Icerya purcha~si Maskell, by introduced natural enemies on citrus

in California in 1888 (Van Driesche and Bellows 2001). Ever since then, hundreds of