Biological control of whitefly on Gerbera: success or failure ? Tritrophic interactions between Gerbera jamesonii, Trialeurodes vaporariorum and Encarsia formosa

In this thesis fundamental and applied research is described that was initiated to develop biological control of whitefly with the parasitoid Encarsia formosa in the ornamental Gerbera jamesonii Hook (Campanulales: Compositae).To test the hypothesis that host plant architecture of G. jamesonii results in a different whitefly distribution pattern when compared with vegetables such as tomato and cucumber, we started studying whitefly dispersal behaviour and the choice of oviposition and feeding sites of the greenhouse whitefly within a plant. The behaviour of the herbivore was compared on two cultivars of G. jamesonii , differing in hairiness.The dispersal process of whiteflies was directed to the centre of the plant. This leads to adult and egg aggregation on young leaves on the hairy as well as on the less-hairy cultivar. Three parameters (development time, mortality and fecundity) to measure performance of T. vaporariorum were investigated, to determine a possible link with preference for certain leaves or cultivars by whitefly adults. Development duration of immatures is the same on leaves of different ages. Mortality of whiteflies is much lower and fecundity is higher on young Gerbera leaves. The dispersal and aggregation of whitefly adults between plants was investigated next. Whitefly populations were strongly aggregated on Gerbera , which seems typical for this whitefly species. Travel distance and dispersal speed were similar on very different crops such as Gerbera and tomato. Leaf selection behaviour and the resulting distribution of greenhouse whitefly on Gerbera was similar to that on vegetables. The first hypothesis that differences in host plant architecture result in different whitefly distributions is, thus, rejected.The second hypothesis that plant characteristics such as shape and leaf hairiness of G. jamesonii negatively influence the searching behaviour of the parasitoid E. formosa was tested next. Trichome density of Gerbera cultivars appeared to influence the walking behaviour (walking speed, walking activity and straightness of walkng path) and searching efficiency of E. formosa only slightly. Gerbera leaves of all age classes were found and searched by the parasitoid. Number of landings was the same on infested and uninfested Gerbera leaves; parasitoids were observed more often on the abaxial side of the leaves, where hosts are found normally. The foraging behaviour of E. formosa on leaves of a range of Gerbera cultivars is comparable. The searching efficiency of E. formosa on Gerbera is as good as on the vegetable tomato, so also the second hypothesis that plant characteristics negatively influence the searching behaviour of Encarsia on Gerbera , is rejected.The third hypothesis that plant architecture and leaf characteristics of G. jamesonii lead to failure of whitefly biological control on Gerbera was tested in glasshouses. Glasshouse studies are essential to validate the conclusions based on small population experiments and laboratory experiments. One central release of on average three parasitoids per plant, three weeks after whitefly introduction resulted in successful pest control in a small glasshouse of 60 m ² . Sufficient control was not achieved in a larger glasshouse (300 m ² ).Possible explanations for the failure of biological control in Gerbera were evaluated with a simulation model of G. jamesonii , T. vaporariorum and E. formosa . Surprisingly, and as a result of integrating all parameters for crop, pest, natural enemy and glasshouse temperature, we found that it is mainly the glasshouse temperature that determines success or failure of biological control in Gerbera .With an adjustment of the release strategy of parasitoids, the right choice of (partially) resistant Gerbera cultivars, and a slight increase of glasshouse temperature in early spring, biological control of T. vaporariorum on G. jamesonii will be successful