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Parasitism rates of Bemisia tabaci (Gennadius) and searching and oviposition behaviours of its parasitoid Eretmocerus mundus Mercet were compared on two cassava varieties: a glabrous variety, Nase 4 and a hirsute variety, MM97/0245 with c. 88 leaf hairs/cm². Parasitism was assessed after potted plants of both varieties were exposed in open fields to natural infestation by B. tabaci and its natural enemy. For the behavioural studies, naive, less than 24-h-old females were individually observed on infested cassava leaflets under a microscope for a maximum of 1 h each. The different foraging behaviours were recorded using the computer software 'The Observer 5.0' (Noldus Ltd, Wageningen, The Netherlands). Total per cent parasitism and parasitism by E. mundus did not differ significantly between varieties. Upon encounter with leaf hairs, the parasitoids stopped and groomed before resuming the host search. The frequency of repeat probing, host feeding and antennation after probing and host feeding were higher on the glabrous than on the hirsute variety, while the converse was observed when feeding on liquids on the leaf. The duration of host assessment, initial probing, grooming and resting on the leaf was higher on the glabrous than on the hirsute variety. Leaf hairiness at the density investigated caused some changes in the behaviour of the parasitoids, but did not have an overall effect on field parasitism. Since cassava is generally considered to have glabrous leaves and the variety MM97/0245 is one of the most hirsute varieties, we discount leaf hairiness as a factor in determining levels of parasitoid activity. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]