Biological control of greenhouse whitefly (Trialeurodes vaporariorum) with the parasitoid Encarsia formosa: how does it work?

Commercial biological control of greenhouse whitefly, Trialeurodes vaporariorum (Westwood) through releases of the parasitoid Encarsia formosa Gahan is used at present on about 5000 ha in most countries with an important greenhouse industry. Although other types of natural enemies of greenhouse whitefly are known (predators and entomopathogenic fungi), it is mainly introductions of parasitoids that have led to economically feasible control. Fundamental research on the relationship among E. formosa, greenhouse whitefly, and host plants, has provided information on how the parasitoid locates and attacks its hosts and how greenhouse climate and plant architecture influence finding of the hosts and parasitization efficiency. The parasitoid is not able to locate infested plants from a distance. Searching is random on all levels, and after a host has been found the search pattern does not alter. The only important change in foraging behavior which was observed is that, in comparison with search times on an uninfected leaf, a parasitoid keeps searching considerably longer (2-10 times) on a leaf once a whitefly larva has been found or when other indicators of whitefly presence were discovered (e.g., honeydew, exuviae, dead hosts). On a number of important crops, a single E. formosa or her offspring is able to kill more whiteflies per unit of time than an individual whitefly female can produce. On other plant species, the development of whitefly is so fast that seasonal inoculative releases of E. formosa are not sufficient for reliable control and inundative releases have to be made. A stochastic simulation model, which includes (a) the detailed search behavior of the parasitoid and (b) the demographics and distribution of whitefly and parasitoid in relation to host plant and greenhouse climate, is developed to be able (1) to esplain the capability of E. formosa to reduce whiteflies in large commercial greenhouses on crops like tomato, (2) to improve introduction schemes of parasitoids for crops where control was difficult, and (3) to predict effects of changes in cropping practices (e.g., greenhouse climate, choice of cultivars) on the reliability of biological control.