Abstract
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 field 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.
Highlights
Several abiotic and biotic mortality forces including predators, parasitoids, pathogens, and weather have been reported to act on Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) pest complex populations (Naranjo, 2001; Naranjo & Ellsworth, 2005)
When pooled over all mortality factors, sizeable variation in marginal mortality rates (MMR) was observed in each development stage and the highest median rate of Marginal mortality rate of interest (MA) Inviability Predation Disappearance Parasitism Unknown death
This study identified and quantified the impact of different mortality factors acting on immature populations of B. tabaci Sub-Saharan Africa 1 (SSA1) species in a controlled field experiment on cassava
Summary
Several abiotic and biotic mortality forces including predators, parasitoids, pathogens, and weather have been reported to act on Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) pest complex populations (Naranjo, 2001; Naranjo & Ellsworth, 2005). There are currently over 40 cryptic species in the Bemisia tabaci complex (Hu et al, 2017; Jiu et al, 2017; Dinsdale et al, 2010) and they are all polyphagous phloem-feeders (Polston et al, 2014). Their populations have reportedly increased in abundance throughout the cassava-growing regions of East and Central Africa over the past two decades (Colvin et al, 2004; Legg et al, 2014a; Ally et al, 2019). The current outbreaks of B. tabaci populations heighten its importance as a direct pest of cassava and intensify its role as a vector of two damaging viral diseases of cassava: Cassava Mosaic Disease (CMD) and Cassava Brown Streak Disease (CBSD) (Legg et al, 2014b; Maruthi et al, 2017), undermining food security and livelihood benefits of cassava production. In Sub-Saharan Africa, cassava is considered a staple crop that is highly resilient to climate change (Nweke et al, 2002; Jarvis et al, 2012; Maruthi et al, 2017)
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