Is polyphagy of a specific cryptic Bemisia tabaci species driving the high whitefly populations on cassava in eastern Africa?

There have been increasingly high whitefly ( Bemisia tabaci ) populations and associated vector-borne viral disease pandemics since the 1990s on cassava, one of the key staple food crops in eastern Africa. The increased whitefly numbers have also been observed on other crops and uncultivated plants. Previous studies, mainly based on adult whitefly surveys have associated the upsurges on cassava with two B . tabaci populations, sub-Saharan Africa 1 (SSA1) and SSA2, yet factors behind the population increases on other plants and their relations are unclear. In addition, other species of B . tabaci including East Africa1 (EA1), Indian Ocean (IO), MED, SSA9 and SSA10 have also been reported to occur on cassava in low numbers. In this study fourth-instar nymphs were collected from cassava and 20 other common host plants to establish the actual host plants of B . tabaci for their growth and development in Uganda, a centre of diversity of this insect pest complex. Host-transfer experiments were also conducted to establish the ability of seven B . tabaci species: EA1, MEAM1, MED-ASL, SSA1-subgroup1, SSA1-Hoslundia, SSA6 and SSA12 to develop on cassava. Identities of fourth instar nymphs were determined using the partial mitochondrial cytochrome oxidase 1 ( mtCO1 ) sequences (657 bp) in the 3′ barcode region used for B . tabac i systematics. Twelve B . tabaci of which one is a proposed novel B . tabaci putative species, B . afer and one novel whitefly species were identified based on the 3.5% threshold criterion in nucleotide sequence divergence. Cassava was colonised by B . tabaci populations; SSA1-SG1, SSA1-SG2, and SSA2. The most prevalent whiteflies in this study were B . tabaci , SSA1-SG1 (26.8%), MED-ASL (14.5%), and SSA13 (12.9%). These B . tabaci species were also the most polyphagous colonising twelve, eight and six plant species, respectively. Several (≥ 3) whitefly species colonised specific weeds; Aspilia Africana , Commelina benghalensis , Hoslundia opposita , Pavonia urens and Sida acuta . The ability to colonise several plants provides (i) continuous habitat leading to the increased whitefly populations and (ii) reservoirs for mixed infections of whitefly-vectored viruses. Management of whitefly pest populations in eastern Africa will require an integrated approach that takes into consideration their degree of polyphagy and an environment that enables the continuous presence of crop and uncultivated plant hosts.