Introduction

Biological control has had many remarkable successes since a ‘classical’ introduction saved the Californian citrus industry from the ravages of cottony cushion scale at the end of the nineteenth century (Caltagirone & Doutt, 1989; Doutt, 1958; van Driesche & Bellows, 1996). The technique has since been developed and extensively deployed, often with good economic return (e.g. DeBach & Rosen, 1991; Greathead, 1986, 1994). The rationale that underpins biocontrol is the impression that many species in their native environment are limited in numbers primarily by consumers, whether predators, parasitoids, pathogens or herbivores, and which are called ‘natural enemies’ or, within the biocontrol context, ‘beneficial species’ (van Driesche & Bellows, 1996). Biocontrol is therefore aimed at restoring the original dynamic, usually in situations in which the pest species has invaded a new area without its natural enemies. The aim is not to exterminate pests, but to maintain them at lower non-pestiferous densities.