Mortality factors acting on field populations of Bemisia tabaci (Hemiptera: Aleyrodidae) SSA1 on cassava in Uganda

Natural death is a key determinant of a species population dynamics. Thus, a clear understanding of natural mortality factors aids the development of appropriate management strategies for insect pests. Cohort-based life tables were constructed to determine the sources and rates of mortality of fi eld populations of the pest, Bemisia tabaci Sub-Saharan Africa 1 (SSA1) on cassava in Uganda. Monthly cohorts (10 in total) were established separately for eggs and nymphs on two cassava genotypes with known levels of resistance to B. tabaci infestation (Alado alado and NAROCASS 1). Mortality was recorded using daily observations for the eggs and the different nymphal instars. The recorded mortality sources were disappearance (total removal of egg or nymph from the leaf), predation, parasitism (nymphs only), unknown death and inviability (eggs only). Median marginal mortality rate was highest for disappearance (0.355) followed by parasitism (0.058). The highest level of mortality occurred during the third nymph stage (55% on Alado alado) and only 12% of nymphs reached the adult stage. Irreplaceable mortality (Ic) was highest for disappearance followed by third instar parasitism. Key-factor analysis revealed a close resemblance of the curve for disappearance to that of total mortality coupled with the highest regression slopes: 0.896 for eggs and 0.725 for nymphs on NAROCASS 1. From these results, we conclude that disappearance and parasitism are the major mortality factors controlling B. tabaci SSA1 populations. Therefore, the development of interventions that focus on enhancing the levels of disappearance and third instar parasitism may lead to population-level reductions in B. tabaci SSA1. Further studies need to be conducted to understand the factors that contribute to the high mortality associated with disappearance.