The pest control in inundative strategies often relies on the released individuals, rather than their progeny ( De Bach, 1964). As a result, most biological control programmes rely on inundative release strategies, where natural enemies are periodically introduced in large numbers to control pest problems ( Stinner, 1977 Van Lenteren et al., 2003 Collier and Van Steenwyk, 2004). However, many cropping systems lack these floral resources ( WГ¤ckers et al., 2005). Thirty years ago, Ramakers (1990) proposed the concept of вЂњpredator-in-firstвЂќ and stated that the availability of supplementary foods, such as pollen and nectar, is essential for early establishment of generalist natural enemies. The release of generalists, that can feed on a range of prey, but may also exploit non-prey food, made it possible to maintain populations of natural enemies in crops in absence of the target pest, thus facilitating the preventative use of natural enemies. Biological control in greenhouses has since been extended by the addition of generalist biocontrol agents to complement the specialist beneficials ( Janssen and Sabelis, 2015).

This example was followed in the fifties by the use of the natural enemies against mealybugs ( Doutt, 1951) and in the sixties, by the introductions of the specialist predatory mite Phytoseiulus persimilis Athias-Henriot for the control of spider mites ( Bravenboer and Dosse, 1962). Biological control in greenhouses dates back almost 100 years, when Speyer (1927) at the Cheshunt Experimental Station first described the control of greenhouse whitefly Trialeurodes vaporariorum (Westwood) by the specialist parasitoid Encarsia formosa Gahan ( Hussey et al., 1969).

Biocontrol agents greenhouse