Abstract:

The whitefly pest complex Bemisia tabaci (Gennadius. 1889) is one of the most devastating
agricultural pests globally. Over the past two decades, increased abundance of B. tabaci
populations throughout the cassava-growing regions of East and Central Africa has been reported.
The recent outbreak of large B. tabaci populations intensifies its significance as a direct pest of
cassava and also increases the prevalence of B. tabaci transmitted diseases. However, there are no
practical B. tabaci management options available for smallholder farmers in the region. This is
largely because previous research efforts focused on the development of virus disease resistant
cassava genotypes with no special emphasis on the vector. The main aim of this study therefore
was to understand the factors that influence B. tabaci population dynamics and fitness.
A controlled study to evaluate the development duration, survival, and population parameters for
B. tabaci was conducted at five constant temperatures (16, 20. 24, 28 and 32 °C) on three cassava
genotypes (Alado alado. NAROCASS 1 and NASE 14) in a randomized complete block design in
the laboratory. High temperatures significantly reduced development time of all stages and female
longevity across all genotypes; the total life cycle was 63.8 days at 16 °C and 17.9 days at 32 °C
on NAROCASS 1. However, there was no significant effect of temperature on development times
To investigate the effect of cassava morphological traits, temperature, rainfall and relative
humidity (RH) on the abundance of B. tabaci. five cassava genotypes (Alado alado. Njule Red,
NAROCASS I. NASE 14 and NASE 3) with varying levels of resistance to B. tabaci vectored
diseases and B. tabaci infestation were planted in three Ugandan agro-ecological zones in 2016
and 2017 in a randomized complete block design. Results have shown that B. tabaci Sub-Saharan
Africa I (SSA 1) was the species on cassava in Uganda. The tallest genotype Alado alado supported
the lowest number of B. tabaci adults. In areas with high B. tabaci prevalence, leaf area, leaf lobe
width and leaf lobe number exhibited significant positive effects (/?<0.001) on B. tabaci adult
count across locations. Positive effects of relative humidity, and negative effects of temperature
and rainfall, on B. tabaci adult and nymph counts were observed in 2016 and 2017. hence the low
populations in the North Eastern Savanna grassland. Temperatures of 28-30 °C, rainfall of 30-
150 mm and RH of 55-70% and deployment of cassava genotypes of short stature with large leaf
area and lobe width were significantly associated with increased B. tabaci populations.
xx i
Cohort-based life tables were constructed to determine the sources and rates of mortality of B.
tabaci field populations in Uganda. Monthly cohorts (10 in total) were established separately for
eggs and nymphs on two cassava genotypes (Alado alado and NAROCASS 1) in a randomized
complete block design. Results revealed that disappearance followed by parasitism were the key
mortality factors influencing B. tabaci populations in the field. This study has also shown that the
highest mortality occurred during the third nymphal stage (55% on Alado alado). and only 12% of
nymphs reached the adult stage. Therefore, management interventions that focus on enhancing the
levels of disappearance and parasitism may lead to desirable B. tabaci population suppression.
Cassava genotype had no effect on B. tabaci mortality.
for all nymph instars except the third instar stage at 28 °C and 32 °C. Survival of each growth stage
and for the entire life cycle, and fecundity increased with temperature, with highest values recorded
at 32 °C. However, survival at 28 °C and 32 °C was remarkably similar. For all genotypes, the
intrinsic rate of increase (rm), finite rate of increase (X) and net reproductive rate (Ro) increased
with temperature, while mean generation time (T) reduced with temperature. At 32 °C. rm, Ro, X
and T were 0.2. 48.7. 1.2 and 22.6 days respectively, compared to 0.01, 1.9. 1.0 and 71.2 days at
16 °C on Alado alado. Therefore, the ideal development temperature for B. tabaci SSA1 was
determined to be 32 °C. This study thus predicts possible expansion of B. tabaci populations in
parts of the world with a hot and humid climatic condition.
Efficacy studies for the two common B. tabaci parasitoids: Encarsia sophia Girault and Dodd and
Eretmocerus mundus Mercet (Hymenoptera: Aphelinidae) at five B. tabaci SSA1 host densities
(10.40,60, 100 and 140) on three cassava genotypes (Alado alado. NAROCASS 1 and NASE 14)
were conducted in the screenhouse. The results have shown that host density and parasitoid species
(/?<0.007) and the genotype by parasitoid species interactions (p<0.001 top<0.05) had significant
effects on host feeding and parasitism of B. tabaci SSA1. The parasitic and host feeding capacity
of both E. mundus and E. sophia was highly density dependent with levels of nymph death
increasing with host density: E. mundus parasitised 1.5 and 40.2 nymphs at host density of 10 and
140 nymphs, respectively. At all host densities. E. sophia killed more host by feeding while E.
mundus parasitized more hosts. However, E. mundus caused more total nymph death than E.
sophia at all host densities. At a host density of 100 nymphs, E. mundus killed 38.9 nymphs
xxi i
compared to the 22.4 nymphs killed by E. sophia. Thus, E. mundus is a more effective biological
control agent. Thus, augmentative release of these hymenopteran parasitoids in areas with high B.
tabaci populations would offer impressive pest population suppression. Further, it was observed
that interaction of cassava genotype with parasitoid species improved bio-control efficiency.
Glabrous cassava genotypes used in this study enhanced parasitoid effectiveness.
This is the first study in Uganda to investigate factors driving the B. tabaci population explosion
on cassava, the findings will critically contribute to design of effective strategies for the
management of this global pest for African farmers.