BIOLOGICAL-CONTROL OF GREENHOUSE-WHITEFLY (TRIALEURODES-VAPORARIORUM)WITH THE PARASITOID ENCARSIA-FORMOSA - HOW DOES IT WORK

Commercial biological control of greenhouse whitefly, Trialeurodes vap orariorum (Westwood) through releases of the parasitoid Encarsia formo sa Gahan is used at present on about 5000 ha in most countries with an important greenhouse industry. Although other types of natural enemie s of greenhouse whitefly are known (predators and entomopathogenic fun gi), it is mainly introductions of parasitoids that have led to econom ically feasible control. Fundamental research on the relationship amon g E. formosa, greenhouse whitefly, and host plants, has provided infor mation on how the parasitoid locates and attacks its hosts and how gre enhouse climate and plant architecture influence finding of the hosts and parasitization efficiency. The parasitoid is not able to locate in fested plants from a distance. Searching is random on all levels, and after a host has been found the search pattern does not alter. The onl y important change in foraging behavior which was observed is that, in comparison with search times on an uninfected leaf, a parasitoid keep s searching considerably longer (2-10 times) on a leaf once a whitefly larva has been found or when other indicators of whitefly presence we re discovered (e.g., honeydew, exuviae, dead hosts). On a number of im portant crops, a single E. formosa or her offspring is able to kill mo re whiteflies per unit of time than an individual whitefly female can produce. On other plant species, the development of whitefly is so fas t that seasonal inoculative releases of E. formosa are not sufficient for reliable control and inundative releases have to be made. A stocha stic simulation model, which includes (a) the detailed search behavior of the parasitoid and (b) the demographics and distribution of whitef ly and parasitoid in relation to host plant and greenhouse climate, is developed to be able (1) to explain the capability of E. formosa to r educe whiteflies in large commercial green-houses on crops like tomato , (2) to improve introduction schemes of parasitoids for crops where c ontrol was difficult, and (3) to predict effects of changes in croppin g practices (e.g., greenhouse climate, choice of cultivars) on the rel iability of biological control. (C) 1996 Academic Press, Inc.