Introduction

Of the 1200 described whitefly species, only some 20 may be considered as potential pests. Until recently most research on control of whiteflies was directed towards the greenhouse whitefly (Trialeurodes vaporariorum (Westwood)) (van Lenteren & Woets, 1988). However, since the mid-1980s, whiteflies of the genus Bemisia have created problems of such a large scale both in the field and in large, commercial greenhouses that research has shifted to this pest (Gerling, 1990; Gerling & Mayer, 1996; van Lenteren et al., 1997). Greenhouse whitefly (Trialeurodes vaporariorum) and silverleaf whitefly (Bemisia argentifolii) are very common, highly polyphagous pest insects worldwide. Biological control of greenhouse whitefly with the parasitoid Encarsia formosa was first applied in 1926 and has since been used with great commercial success.

Usually, in natural ecosystems and agroecosystems where pesticides are not (or selectively) used, an array of natural enemies keeps the number of whiteflies at very low levels: predators, parasitoids, and pathogens all take their toll. Examples from cotton and tomato show that both whitefly species can be kept at densities well below the economic threshold (van Lenteren et al., 1996). If natural control is insufficient, inoculative or inundative releases with natural enemies can be made. Commercial biological control of greenhouse white-fly through releases of the parasitoid Encarsia formosa Gahan is used at present on about 5000 ha of greenhouse crops and in most countries with an important greenhouse industry (van Lenteren, 1995).