Evaluation of Amitus fuscipennis as biological control agent of Trialeurodes vaporariorum on bean in Colombia

The research described in this thesis concerns the study of a natural enemy of whiteflies, Amitus fuscipennis MacGown & Nebeker under Colombian field and laboratory conditions. The general aim of the project was to study whether biological control of Trialeurodes vaporariorum (Westwood) with A. fuscipennis can be included into the guided control program for pests and diseases in bean. In Chapter 1 , natural enemies of whiteflies are reviewed (especially the biology and use of some species of Amitus ), the whitefly problem in Colombian bean crops is described and the aims of the study are given.When the present research project started, few biological data about A. fuscipennis were known. In Chapter 2 data about longevity, sex ratio and developmental time of A. fuscipennis are presented. The immature developmental time and longevity of fed and unfed adult parasitoids were compared under different temperature and relative humidity combinations. Combinations of moderate temperature and low relative humidity negatively affected parasitoid longevity. Results suggest that A. fuscipennis could be a potentially good biological control agent of T. vaporariorum in environments that are not overly dry or warm. Sex ratios of populations collected in the field and reared in the laboratory were strongly female-biased. Reproduction of the parasitoid is through thelytokous parthenogenesis. The species carries Wolbachia; the rickettsia that induces thelytoky but it was not tested if it induces thelytoky in A. fuscipennis . Because of the presence of the Andes mountain range in Colombia, cropping of dry and snap beans is possible at different altitudes as tropical highlands (altitudes above 1000 m) and inter-Andean valleys (altitudes around 1000 m). This range implies as well that there are large climatic variations, going from warm, dry areas to cooler, more humid ones. Therefore, it is important to determine how certain climatic conditions influence the biology and performance of A. fuscipennis . In Chapters 3 and 4 , the reproductive capacity of the pest insect T. vaporariorum and the parasitoid A. fuscipennis were measured as the intrinsic rate of population increase r m . These parameters were measured on bean cultivars 'Chocho' and 'ICA-Pijao', under climatic conditions that simulated those of highland and midland areas. The r m values of A. fuscipennis were higher than those of T. vaporariorum for both kinds of simulated conditions. This result permits us to conclude that A. fuscipennis may therefore be a suitable candidate for use in biological control programs against T. vaporariorum in Colombia. An interesting additional result was the finding that the r m of T. vaporariorum was much lower on bean cv. Chocho than on cv. 'ICA-Pijao'. Cv. 'Chocho' should preferably be used in IPM programs in Colombia to slow down whitefly development and ease biological control.A high r m value is often not the only criterion an effective parasitoid should meet. After parasitoids have reduced pest numbers, their searching efficiency becomes critical in host finding, and in keeping pest numbers low. Therefore, the foraging behavior of A. fuscipennis on bean was studied ( Chapter 5 ). A. fuscipennis is a fast walking parasitoid that probed and oviposited most of the hosts encountered, which may be related to its pro-ovigenic way of reproduction. The percentage of unparasitized hosts accepted for oviposition was high (60%), which combined with large numbers of eggs laid over a short period resulted in high percentages parasitism. A. fuscipennis showed area-restricted searching, which is an adaptation to searching for hosts that are distributed in a clumped manner as T. vaporariorum does.Long-range searching in A. fuscipennis had not been studied yet. The flight response of A. fuscipennis to uninfested and whitefly infested leaves was tested in the laboratory ( Chapter 6) . A. fuscipennis was not attracted to clean or infested leaves or plants, even not over a short distance. A. fuscipennis flew upwards, apparently attracted by the light. Once A. fuscipennis had arrived on a plant, its searching behavior was influenced by the 'quality' of the plant ( Chapter 6 ). Residence time and walking activity of A. fuscipennis were higher on unsprayed infested leaves than on uninfested fungicide sprayed or uninfested unsprayed leaves. A. fuscipennis preferred the leaflet's underside while searching and the lower center when hosts were present. A. fuscipennis was arrested on the plant as a result of both encounters with hosts and feeding from plant secretions.In addition to laboratory experiments, fieldwork was carried out in Colombia ( Chapter 7 ). A. fuscipennis was distributed from middle to high altitudes but it was most commonly found on hillside areas above 1400 m. To determine regional population trends of the whiteflies T. vaporariorum and Bemisia tabaci , and their parasitoids, experiments were performed in sprayed and unsprayed fields during a cropping season of snap beans. Results suggested that, while whiteflies on snap beans cannot be exclusively controlled by natural occurring parasitoids, they do significantly contribute to whitefly mortality. Based on these results, I suggest for whitefly control, (1) to reduce the whitefly population at the beginning of the cropping season e.g. by applying entomopathogenic fungi, and 2) to carry out releases of parasitoids after fungus treatment. As an additional aspect to reduce whitefly populations, the use of partially whitefly resistant bean plants and habitat manipulation tactics to build up a natural enemy population in the surrounding crops or wild vegetation, are suggested.In Chapter 8 , the research findings are discussed in a broader context. Information collected during this Ph.D. project strongly indicates that A. fuscipennis might be an effective natural enemy of T. vaporariorum in beans under Colombian conditions. Field testing of releases of A. fuscipennis within the guided control program for bean will give the final proof of the parasitoid usefulness.