Life-table analysis of mortality factors affecting populations of Bemisia tabaci on cassava in Uganda

A study was set up to determine the sources and rates of mortality of Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) on field-grown cassava in Uganda. Using a cohort-based approach, daily direct observations were used to construct partial life tables for 12 generations of egg and nymph populations which were studied over a 1-year period. Mortality was categorized as dislodgement, predation, parasitism (for nymphs only), unknown death, and inviability (for eggs only). The highest mean rate of marginal mortality across all stages was attributed to parasitism, with dislodgement and predation following, respectively. Across all factors, the highest mean rate of marginal mortality was observed in the fourth instar followed by the eggs, first-, second-, and third-instars, respectively. Key factor analysis revealed that dislodgement was the major mortality factor contributing to generational mortality in eggs while for nymphs, parasitism in the fourth instar was the main driving force behind the observed generational mortality. Highest irreplaceable mortality in both the egg and nymph stages was attributed to dislodgement followed by parasitism and predation, and least was due to unknown death. Across stages, highest irreplaceable mortality rates were observed in the eggs and the fourth-instar nymphs. The other stages had relatively low rates of irreplaceable mortality. Rain-protection experiments revealed no significant differences in marginal mortality rates when compared to the open field situation. © 2006 The Authors.