Characterisation Of Bemisia Tabaci (Gennadius) (Homoptera: Aleyrodidae) Biotypes Associated With Some Selected Crops In Southern Ghana.

ABSTRACT Intraspecific variation and biological heterogeneity in the whitefly Bemisia tabaci make it a major pest in diverse agricultural systems worldwide. The economic burden of this insect pest owes to its apparent polyphagy, vectorial capacity for viruses, and propensity for insecticide resistance development. Ecological specialisation has been observed in the species, which compounded with the variability, and adaptability pose serious impediments in its control. Effective description of local populations is, therefore, an essential basis of integrated management. Populations of B. tabaci infesting cassava, okra, garden egg and tomato were characterised in three locations in southern Ghana. Descriptions were based on ecological, taxonomic and agricultural parameters. Morphological and molecular characterisation were done by morphometric analysis and RAPD PCR, respectively. Host preference was assayed by two-choice and multiple-choice landing and oviposition preference assays. The host races putatively identified were compared on the rearing suitability on eight hosts and insecticide tolerance. Finally, mating compatibility between them was assessed. There were no distinct morphometric differences between the populations as adults, but nymphal classification was less overlapping. Two genetic clusters (associated with okra and cassava) with 45% similarity were revealed. Host affiliation contributed significantly to total variability, but geographical isolation did not. Whiteflies generally chose their rearing host for landing and oviposition, but clear demarcation on cassava and okra was evident. Okra populations survived on all hosts except cassava, while cassava biotype survived on cassava, tomato, garden egg, eggplant and cowpea but not on okra, pepper or cabbage. The okra populations were more tolerant to chlorpyrifos and lambda cyhalothrin than were the cassava populations. Mating and fertilisation were successful but reproductive isolation was evident in male-dominant sex ratios of progeny. Inter biotype mating produced 2.3: 1 (male: female) sex ratio compared to  0.6 in intra biotype mating, while unmated whiteflies produced only males. Female crosses were normal, and able to oviposit. The existence of two morphologically similar biotypes demarcated by cassava and okra was confirmed. Host interactions and RAPD PCR consistently identified the two biotypes and are suggested for use in their identification. Random primers OPA 02, OPB 08, OPC 05 and OPD 16 are recommended for this. The host range of the cassava biotype seems wider than earlier reported. Solanaceous plants acted as common hosts and their role as potential reservoir hosts was speculated. Toxicological reactions were attributed to different insecticide pressure at the sites. Partial reproductive isolation between the two biotypes was revealed but the stability of the crosses in nature is not clarified.