Landscape factors and how they influence whitefly pests in cassava fields across East Africa

Abstract

Context African production landscapes are diverse, with multiple cassava cultivars grown in small patches amongst a diversity of other crops. Studies on how diverse smallholder landscapes impact herbivore pest outbreak risk have not been carried out in sub-Saharan Africa. Objectives Bemisia tabaci is a cryptic pest species complex that cause damage to cassava through feeding and vectoring plant-virus diseases and are known to reach very high densities in certain contexts. However, the factors driving this phenomenon are unclear. Methods Bemisia density data in cassava across a large number of sites representing a geographic gradient across Uganda, Tanzania and Malawi were collected. We tested whether in-field or landscape factors associated with land-use patterns underpinned Bemisia density variability and parasitism. Results We found the B. tabaci SSA1 species dominated our study sites, although other species were also common in some cassava fields. Factors associated with the surrounding landscape were unimportant for explaining variability in adult density, but the in-field variables of cassava age and cultivar were very important. The density of nymphs and the parasitism of nymphs was heavily influenced by a diversity of landscape factors surrounding the field, including the size of focal cassava field, and area of cassava in the landscape. However, unlike the trend from many other studies on drivers of natural enemy populations, this pattern was not solely related to the amount of non-crop vegetation, or the diversity of crops grown in the landscape. Conclusions Our findings provide management options to reduce whitefly abundance, including describing the characteristics of landscapes with high parasitism. The choice of cassava cultivar by the farmer is critical to reduce whitefly outbreak risk at the landscape-scale.

Genetic diversity of whitefly (Bemisia spp.) on crop and uncultivated plants in Uganda: implications for the control of this devastating pest species complex in Africa

“…Hierarchical clustering revealed clear host preferences of B. tabaci species for MED-ASL (pumpkin, S. acuta and sweet potato), MED-Q1 (tobacco), SSA1 and SSA2 (cassava), SSA6 (African basil) and B. Uganda1 (sweet potato) assisting in predicting probable identity of whiteflies on these plant species. Specific whitefly-host associations were revealed most markedly for cassava where all 98 whiteflies were SSA1-SSA2 supporting previous reports (Legg 1996;Sseruwagi et al 2006;MacFadyen et al, 2020). It is noteworthy that even for the most abundant non-cassava populations, namely MED-ASL (n = 265) and B. Uganda1 (n = 105), not a single whitefly was collected from cassava.…”

“…pumpkin (55/70 pumpkin whiteflies), sweet potato (38/79) and tomato (14/51) but not at all to cassava (0/98). The strong association with sweet potato in the field has been noted previously (Sseruwagi et al 2006;Misaka et al 2019), and recent studies have verified that sweet potato is a preferred host for MED-ASL under laboratory as well as field conditions (Vyskočilová et al 2019;Macfadyen et al 2020). These data support the proposal of Vyskočilová et al (2018Vyskočilová et al ( , 2019 to classify MED-ASL as a distinct species from MED-Q1, due to these populations failing to interbreed, showing a distinct mtCO1 phylogenetic placement, as well as marked differences in their preferred host ranges.…”

“…In this study, these plants would not have been indicated to be potential hosts with either none or only a single SSA1-SG3 whitefly found to be present. The presence of large numbers of SSA1 flies on common bean (17/41) and cowpea (5/6) is therefore tentatively considered as an indication of these being alternate hosts, recently verified for SSA1 on cowpea by Macfadyen et al (2020). Furthermore, if plant hosts from which ≥ 3 whiteflies are considered as reliable indicators, this study suggests pumpkin, tomato, eggplant, as well as the weeds Erythrina abyssinica and S. acuta are additional possible alternate hosts for SSA1 'cassava' whiteflies.…”

“…A number of suggestions have been made as to the causes of the dramatic cassava whitefly population increases over the past few decades, which have included (a) the presence of highly fecund invasive B. tabaci species on cassava (Legg et al 2002(Legg et al , 2014bSseruwagi et al 2005;Boykin et al 2018) and (b) an increase in the cultivation of virus-resistant but whitefly-susceptible cassava varieties in Uganda that were reported to attract high populations of whitefly (Omongo et al 2004(Omongo et al , 2012. It is apparent that there are many factors contributing to the increased whitefly abundance and that singling out any with higher impact is complex (Macfadyen et al 2018(Macfadyen et al , 2020. There are many knowledge gaps that still need to be addressed and one of these is to what extent other crops and uncultivated plants contribute to the population dynamics of the abundant whitefly populations on cassava.…”

Improving climate suitability for Bemisia tabaci in East Africa is correlated with increased prevalence of whiteflies and cassava diseases

“…We run the B. tabaci MEAM1 CLIMEX model of Kriticos et al 64 with the CLIMEX Compare Locations/Years module, using a set of gridded monthly climate time series to test the hypothesis that the modelled annual climate suitability for B. tabaci MEAM1 is correlated with a 13-year time series of abundance of B. tabaci in Uganda 71 , and that the correlation is stable through time. As a further check of model relevance, we compare the CLIMEX model outputs for long-term average climate with data on B. tabaci abundance collected from cassava fields across Uganda, Tanzania and Malawi 72 . Finally, we use the time series of climate suitability to test whether the climate suitability for B. tabaci in East and Central Africa has been increasing when and where B. tabaci SSA, CMD and CBSD have been observed to increase in prevalence 28 , 30 .…”