Survey on whiteflies and their parasitoids in cassava mosaic pandemic areas of Tanzania using morphological and molecular techniques.

BACKGROUND: Bemisia tabaci (Gennadius) is the vector of cassava mosaic geminiviruses (CMGs) and cassava brown streak viruses (CBSVs) in Africa, which cause devastating yield losses. As a prerequisite to developing biological control methods and enhancing knowledge of the fauna of whitefly parasitoids in sub-Saharan Africa, endemic parasitoids were surveyed in the cassava-growing regions of Tanzania and analysed using both morphological and molecular methods. An attempt was made to corroborate the identification of the parasitoid species on the basis of consideration of their morphology and sequence analyses of three DNA fragments, namely partial cytochrome oxidase I (COI), the D2 expansion segment of the 28S rRNA and the internal transcribed spacer I (ITS1). RESULTS: Eight whitefly species colonising cassava and twelve species of parasitoids were detected. A species in the Encarsia strenua group and a species in the Eretmocerus mundus group were the most common parasitoids. Molecular systematics indicated the occurrence of two new species of Eretmocerus Haldeman parasitising B. tabaci. CONCLUSION: The accurate identification of natural enemies is an essential first step in developing effective biological control solutions for B. tabaci in Tanzania and the wider cassava-growing environments of Africa. The new data provided here represent an important contribution to this goal.

Biology and management of Bemisia whitefly vectors of cassava virus pandemics in Africa.

Cassava mosaic disease and cassava brown streak disease are caused by viruses transmitted by Bemisia tabaci and affect approximately half of all cassava plants in Africa, resulting in annual production losses of more than $US 1 billion. A historical and current bias towards virus rather than vector control means that these diseases continue to spread, and high Bemisia populations threaten future virus spread even if the extant strains and species are controlled. Progress has been made in parts of Africa in replicating some of the successes of integrated Bemisia control programmes in the south-western United States. However, these management efforts, which utilise chemical insecticides that conserve the Bemisia natural enemy fauna, are only suitable for commercial agriculture, which presently excludes most cassava cultivation in Africa. Initiatives to strengthen the control of B. tabaci on cassava in Africa need to be aware of this limitation, and to focus primarily on control methods that are cheap, effective, sustainable and readily disseminated, such as host-plant resistance and biological control. A framework based on the application of force multipliers is proposed as a means of prioritising elements of future Bemisia control strategies for cassava in Africa.