It is generally accepted that biological control is a broad concept which encompasses a range of control strategies including cultural practices, host plant resistance, and the introduction or encouragement of antagonistic organisms. Since some of these topics are beyond the scope of this book, this discussion will concentrate on the nematode control achieved by natural enemies as a result of parasitism, predation, competition, or antibiosis. Baker and Cook's1 definition of biological control has been adopted and modified for nematodes as follows: reduction in nematode damage by organisms antagonistic to nematodes through the regulation of nematode populations and/or a reduction in the capacity of nematodes to cause damage, which occurs naturally or is accomplished through the manipulation of the environment or by the mass introduction of antagonists. Such a definition recognizes that reduction in nematode damage is the primary aim of any control measure and that it need not necessarily be accomplished by reducing nematode numbers. It embraces situations, for example, where control is achieved with antagonists which restrict the movement or invasion of nematodes rather than killing them,

Nematologists have been interested in using natural enemies to control nematodes since the 1920s and 1930s, when Cobb2 showed an interest in the predatory nematodes, and Linford and Yap3 conducted experiments with the nematode-trapping fungi. Despite a considerable amount of interest and many optimistic pronouncements during the last 50 years, only a few examples of natural biological control of nematodes have been reported and there are no examples of the widespread commercial use of introduced biological control agents. This situation is in marked contrast to fields such as entomology and weed science where there are many well documented examples of biological control.4 Even in the related field of plant pathology, where interest in biocontrol has quickened during the last decade or so, biological control now offers answers to many serious disease problems in modern agriculture.5 The relatively recent recognition of the importance of plant-parasitic nematodes, the need to develop basic information on their taxonomy, physiology, biology, and ecology, the availability of cheap, effective nematicides, and the complexity of the soil environment are perhaps some of the reasons why developments in biological control of nematodes have lagged behind those in other fields.