Fourth International Bemisia Workshop International Whitefly Genomics Workshop
Changes in the Biology of Bemisia tabaci on Cassava in Africa and Their Impact on Virus Disease Pandemics
1 International Institute of Tropical Agriculture-Tanzania, Dar es Salaam, Tanzania. Correspondence: j.legg@cgiar.org
2 Natural Resources Institute, Chatham Maritime, Kent, UK
3 International Institute of Tropical Agriculture-Uganda, Kampala, Uganda
4 Kawanda Agricultural Research Institute, Kampala, Uganda
5 University of Arizona, Tucson, Arizona, USA
Bemisia tabaci has been recognized as the vector of cassava mosaic geminiviruses (CMG) for many years. More recently, it has also been shown to transmit cassava brown streak virus (CBSV), which causes cassava brown streak disease (CBSD). Super-abundant populations of B. tabaci and the recombinant CMG, East African cassava mosaic virus-Uganda, have been associated with the expansion through nine countries of East and Central Africa of the African pandemic of severe cassava mosaic disease (CMD). The pandemic currently affects over 100 million people in an area greater than 2.6 million square kilometers and continues to expand, primarily southwards and westwards. Experiments to investigate the biology and population genetics of super-abundant B. tabaci have revealed a number of novel features of this pest and its relationship with its cassava host. Yield loss studies aimed at quantifying physical damage have shown that losses solely attributable to physical feeding are as high as 45%. Local cultivars are highly susceptible to CMD but relatively unaffected by whitefly feeding damage and the associated sooty mould. Improved varieties currently being promoted in view of their resistance to CMD have variable levels of yield loss resulting from whitefly physical damage (12.5–44.6%), but host consistently higher populations of B. tabaci and therefore have more feeding and sooty mould damage. Pesticide regimes required to protect cassava crops from the effects of whitefly physical damage consisted of two soil-drench applications of imidacloprid (at one and four months after planting), and weekly foliar applications of cypermethrin. Comparisons of whitefly abundance in these chemically controlled plots (2002–4) with unprotected plots of similar varieties prior to the passage of the pandemic (1992–4) revealed a similar average value over the first eight months after planting. Efforts to determine the cause of the change in Bemisia populations on cassava have focused both on interactions with virus-infected cassava host plants and the genetic characteristics of B. tabaci populations. Preliminary evidence based on MtCO1 sequence comparisons suggested the occurrence of pandemic-associated genotype clusters. More recent work using nuclear markers seems to confirm the hypothesis that genetically distinct ‘invasive’ populations are linked with the super-abundance phenotype and concomitant role as a key determinant of pandemic spread. Recent increases in CBSD incidence in Uganda and unconfirmed reports of the disease in Democratic Republic of Congo and western Kenya raise the worrying possibility that super-abundant B. tabaci are now driving a dual pandemic.